Marine Watermaker Knowledge Base

A marine watermaker is not a single component performing one isolated task. It is a complete hydraulic system in which every part has a precise role, from bringing seawater onboard to delivering freshwater to the boat’s tank.

In this section, we will follow the desalination cycle in the same order in which water moves through a conventional reverse osmosis system during normal operation.

🌊 Following the Water Through the System

The process begins at the seawater intake, where raw seawater first enters the boat. The water then passes through the sea strainer and reaches the feed pump, which supplies a steady flow to the prefilters.

The prefilters remove suspended particles before the seawater enters the high-pressure section. The high-pressure pump then raises the pressure to the level required by the reverse osmosis membrane.

Inside the membrane vessel, the seawater is divided into two different streams:

• Product water or permeate: The freshwater that passes through the membrane and is sent toward the freshwater tank after its quality has been checked.
• Brine or concentrate: The remaining seawater, now containing a higher concentration of salts, which passes through the pressure-regulating valve and is discharged overboard.

🔍 What We Will Explain for Each Component

Each stage of the system will be covered individually, with attention to both function and practical installation.

• Purpose: What the component does and why it is required.
• Position in the system: Where it belongs in the hydraulic sequence.
• Installation: How and where it should normally be mounted onboard.
• Connections: Which hoses, fittings, electrical supplies or controls are required.
• Maintenance: What should be inspected, cleaned or replaced.
• Common problems: What can reduce performance or cause unreliable operation.

⚙️ A System Designed Around Balance

A reliable watermaker depends on correctly matched components rather than on any single oversized or expensive part. Feed flow, pump capacity, membrane size, operating pressure and freshwater production must all remain in balance.

A larger membrane, for example, will not automatically produce more water if the high-pressure pump cannot supply the required flow. In the same way, increasing pressure cannot compensate indefinitely for poor prefiltration, low feed flow or an unsuitable membrane configuration.

🛠️ Guidance for DIY Builders

Throughout this chapter, we refer to Blue Gold components and configurations because they have been selected for marine use and are available as individual parts, kits or complete systems.

However, the hydraulic principles described here are not limited to Blue Gold products. The same guidance remains useful when building, repairing or upgrading a system with other suitable marine reverse osmosis components.

📚 Components Covered in This Chapter

1. Seawater intake and through-hull fitting
2. Sea strainer
3. Feed pump
4. Prefilter housings and cartridges
5. High-pressure pump and motor
6. Pressure gauge and safety devices
7. Reverse osmosis membrane and pressure vessel
8. Pressure-regulating valve
9. Brine discharge
10. Product-water outlet and flow measurement
11. Salinity monitoring and freshwater diversion
12. Freshwater flushing and preservation circuit

Every watermaker begins at one essential point: bringing seawater onboard in a safe, reliable and efficient way. The through-hull inlet, seacock and sea strainer form the first stage of the desalination cycle.

Even the most advanced watermaker will not operate correctly if the intake does not provide a steady, clean and air-free supply of seawater.

🌊 1. What the Seawater Intake Does

The seawater intake supplies raw seawater to the feed pump and the rest of the system. It must remain reliable under different operating conditions, including when the boat is stationary, sailing, motoring or moving through disturbed water.

The intake should be:

• Securely mounted below the waterline: This ensures that the inlet remains submerged during normal operation.
• Capable of supplying a steady flow: The feed pump must receive enough seawater without restrictions or interruptions.
• Protected from debris and marine growth: A sea strainer prevents larger particles from entering the pump and filter system.
• Resistant to corrosion: Materials must be suitable for continuous exposure to seawater.
• Mechanically strong: The through-hull fitting, seacock and hose connections must withstand vibration and hull movement.

📐 2. Recommended Through-Hull Size

For many conventional BlueGold installations, we recommend a dedicated ¾-inch through-hull fitting combined with a 20 mm hose connection.

The intake should be installed as low as practical on the hull, where it can maintain a stable water column and remain submerged under normal sailing and motoring conditions.

🚫 3. Use a Dedicated Intake

Whenever possible, the watermaker should have its own dedicated seawater intake rather than sharing one with another onboard system.

Avoid sharing the intake with equipment such as:

• Marine toilets
• Engine cooling circuits
• Air-conditioning systems
• Deck-wash pumps
• Other pumps with intermittent or high flow demand

➡️ 4. Intake Grille Orientation

When a scoop or grille-type through-hull fitting is used, it should normally face forward, toward the bow.

While the boat is moving, this orientation creates a small positive pressure at the inlet and helps supply seawater to the system.

🔎 5. Using an Existing Spare Intake

An existing spare through-hull may be suitable, provided that it is correctly positioned, adequately sized and not shared with another system.

Before using it, verify:

• The internal diameter of the through-hull and hose connection
• Its distance below the normal waterline
• The absence of restrictions, sharp bends or undersized valves
• The condition of the fitting and seacock
• That the inlet remains submerged when the boat heels or pitches

🧹 6. Sea Strainer Access

The sea strainer should be installed where its transparent bowl or filter basket can be inspected and removed easily.

Leave enough clearance below and around the strainer to allow routine cleaning without dismantling nearby components.

🔩 7. Materials and Corrosion Resistance

All fittings exposed to seawater must be made from suitable marine materials.

BlueGold-approved intake assemblies may use:

• AISI 316L stainless steel: Suitable for high-quality seawater fittings when correctly installed and electrically isolated where required.
• Marine-grade polymers: Lightweight, corrosion-resistant and widely used for strainers and selected through-hull components.
• Suitable bronze or approved marine alloys: Commonly used for seacocks and through-hull fittings.

✅ 8. Quick Installation Checklist

The feed pump, also called the booster pump or low-pressure pump, is one of the most important components in a marine watermaker. Its purpose is to deliver a steady, air-free and adequately pressurised flow of seawater through the prefilters and toward the high-pressure section.

A watermaker cannot operate reliably if the high-pressure pump is starved of water. Insufficient feed flow, air entering the suction line or excessive restriction before the high-pressure pump can cause unstable pressure, reduced freshwater production, cavitation, noise and premature wear.

🌊 1. What the Feed Pump Does

The feed pump draws or receives seawater from the through-hull intake/strainer and pushes it through the hoses and prefilter cartridges. It must overcome the pressure losses created by these components while still delivering sufficient flow to the high-pressure pump.

A correctly selected feed pump should provide:

• Continuous seawater flow: The high-pressure pump should never be allowed to run without an adequate supply of water.
• Positive inlet pressure: A small positive pressure at the high-pressure pump inlet helps prevent cavitation and unstable operation.
• Enough reserve capacity: The pump should compensate for normal filter loading and reasonable hose losses.
• Stable operation: Flow should remain consistent throughout the desalination cycle.

⚙️ 2. Two Valid Feed-Pump Solutions

BlueGold supports two different feed-pump concepts: magnetic-drive centrifugal pumps and self-priming marine-bronze pumps. Neither solution is universally better. The correct choice depends on installation height, available space, electrical consumption and the operating requirements of the watermaker.

🧲 3. Magnetic-Drive Centrifugal Feed Pumps

Magnetic-drive pumps transmit motor power to the impeller through a magnetic coupling. There is no conventional rotating shaft passing through the pump housing and therefore no shaft seal between the seawater circuit and the motor.

These pumps are particularly suitable when the installation allows the pump to be positioned below the waterline, close to the seawater intake.

⭐ Main Characteristics

• Very low electrical consumption: Depending on the model, consumption may remain below approximately 50 W.
• No conventional shaft seal: This removes a common leakage and maintenance point.
• Quiet operation: Magnetic-drive pumps are generally smooth and produce little vibration.
• Compact size: Their small footprint is useful in machinery lockers and modular installations.
• Low routine maintenance: There are no brushes, belts or shaft couplings in the pump head.
• Good energy efficiency: Particularly useful on battery- and solar-supported boats.

📐 Installation Requirements

• Install the pump below the waterline whenever possible.
• Position it close to the through-hull and sea strainer.
• Keep the suction hose short, straight and free from high loops.
• Avoid restrictions, undersized valves and unnecessary fittings.
• Ensure the pump remains flooded before startup.
• Provide easy access for hose inspection and electrical connections.

⚓ 4. Self-Priming Marine-Bronze Feed Pumps

A self-priming marine-bronze pump is an alternative for installations where it is difficult or impractical to position the feed pump below the waterline.

This can be particularly useful on catamarans, shallow hulls or boats where the available machinery space is located above the normal seawater level.

The BlueGold C.E.M. 020 is a self-priming side-channel pump manufactured with a bronze pump body, bronze impeller and corrosion-resistant marine materials. It must be filled with water during initial commissioning so that the priming cycle can begin correctly.

⭐ Main Characteristics

• Self-priming operation: Once initially filled and correctly installed, the pump can evacuate air from the suction line and restore seawater flow.
• Flexible installation position: It may be installed above the waterline when necessary, within the permitted suction-height limits.
• Marine-bronze construction: Suitable materials are used for seawater service.
• Strong suction capability: Useful where the intake and pump cannot be positioned at the same level.
• Robust mechanical design: Well suited to demanding marine installations.
• Continuous-duty capability: Appropriate for the sustained operating periods required by a watermaker.

⚡ Electrical Consumption

The greater suction capability and more robust mechanical construction require more electrical power than a small magnetic-drive pump.

The BlueGold C.E.M. 020 installation should be planned for approximately 200 W, depending on the exact voltage and motor configuration.

🪣 Initial Priming

Self-priming does not mean that the pump should be started completely dry. During initial commissioning, the pump body and suction circuit must be filled with water according to the installation instructions.

• Fill the pump body before the first startup.
• Check that the suction line and fittings are airtight.
• Open the seacock fully.
• Verify that the strainer bowl and suction hose contain water.
• Do not allow prolonged dry running.

⚖️ 5. Magnetic-Drive or Self-Priming Bronze?

📏 6. Correct Flow and Pressure Matter More Than Pump Type

Regardless of the pump technology, the feed pump must supply the flow required by the high-pressure pump and membrane configuration.

An undersized feed pump can cause:

• Low inlet pressure at the high-pressure pump
• Cavitation and irregular pump noise
• Unstable operating pressure
• Reduced freshwater production
• Premature high-pressure pump wear
• Difficulty compensating for partially loaded prefilters

An unnecessarily oversized pump can increase electrical consumption, noise and bypass requirements without improving membrane performance.

🧱 7. Prefilters and Suction Restrictions

The feed pump must overcome the resistance created by the sea strainer, hoses, valves and filter cartridges.

As filter cartridges collect sediment, their pressure loss increases. The feed-pump selection should therefore include enough reserve to maintain correct flow throughout the normal service interval.

• Use correctly sized suction hoses.
• Avoid narrow fittings and unnecessary reducers.
• Keep suction runs as short as possible.
• Use smooth bends instead of multiple tight elbows.
• Inspect and replace clogged cartridges promptly.
• Check for air leaks on the suction side.

🔧 8. Installation and Maintenance Checklist

🧭 9. Which Solution Should You Choose?

• Choose a magnetic-drive pump when it can be installed below the waterline and minimum electrical consumption, quiet operation and simplicity are the main priorities.
• Choose a self-priming bronze pump when installation geometry makes flooded suction difficult, when the pump must be positioned above the waterline or when stronger suction capability is required.

Freshwater flushing is one of the simplest and most effective ways to protect a marine watermaker between operating cycles. After producing water, the flushing circuit replaces the concentrated seawater remaining inside the prefilters, pumps, membrane vessels and hydraulic lines with clean freshwater from the boat’s tank.

The BlueGold Easy Flushing System performs this operation through an electric solenoid valve controlled by a simple switch. It can be integrated into new installations or added to many existing marine watermakers, regardless of brand.

💧 1. Why Freshwater Flushing Matters

When a watermaker stops, seawater remains trapped inside much of the hydraulic circuit. As this water sits, it can contribute to:

• Salt crystallisation inside valves, pumps and fittings.
• Biological growth in hoses, filters and membrane vessels.
• Sticking check valves and pressure-regulating components.
• Corrosion of metallic pump and valve parts.
• Unpleasant odours after periods of inactivity.
• More frequent membrane cleaning and maintenance.

A freshwater flush greatly reduces the salt concentration and biological load remaining inside the system. It does not replace proper preservation when the watermaker will remain unused for a long period, but it is ideal for routine shutdowns between normal operating cycles.

⚙️ 2. How the Easy Flushing System Works

The flushing line connects the boat’s pressurised freshwater supply to the low-pressure side of the watermaker. When the electric solenoid valve is opened, freshwater enters the feed circuit and follows essentially the same route normally used by seawater.

The two check valves automatically control the direction of flow:

• The seawater-intake check valve prevents pressurised flushing water from flowing backwards and escaping through the intake through-hull.
• The freshwater-line check valve prevents seawater from flowing backwards into the boat’s freshwater plumbing.

This allows seawater and freshwater to share the downstream feed circuit while keeping the two sources hydraulically separated.

🔄 3. The Two Check Valves Are Essential

Although the flushing circuit appears simple, correct check-valve selection is critical. The valves must operate reliably with seawater, create very little flow restriction and open at extremely low pressure.

BlueGold uses check valves manufactured from saltwater-compatible engineering polymer, with internal metallic parts and springs made from AISI 316L stainless steel. In versions specified with PTFE components, the wetted sealing parts are also selected for chemical and saltwater resistance.

Intake-Side Check Valve

The intake-side valve allows seawater to travel from the through-hull toward the feed pump during normal watermaker operation.

During freshwater flushing, it closes automatically and prevents the freshwater supply from being discharged backwards through the hull fitting.

Freshwater-Side Check Valve

The second valve permits freshwater to enter the watermaker when the flushing solenoid is activated, but prevents seawater from moving backwards toward the freshwater tank and domestic plumbing.

🚫 4. Why Domestic Brass Check Valves Must Be Avoided

Ordinary domestic plumbing check valves may look suitable, but they are normally designed for pressurised freshwater systems rather than low-pressure seawater suction circuits.

They can create several problems:

• Excessively stiff spring: The small hydrostatic pressure available at the seawater intake may not be sufficient to open the valve fully.
• Restricted feed flow: A partially opened valve can starve the feed pump and high-pressure pump.
• Priming difficulties: The intake line may fail to flood correctly, particularly when using a small magnetic-drive feed pump.
• Unsuitable brass composition: Domestic brass may deteriorate through dezincification or corrosion in seawater.
• Non-marine spring material: Internal springs may corrode, weaken or seize.
• High pressure loss: A valve designed for mains-water pressure can impose a significant restriction at the low pressures used on the feed side of a watermaker.

🌊 5. Why Low Cracking Pressure Matters

The pressure required to begin opening a check valve is often called its cracking pressure.

In a domestic pressurised-water system, a moderately strong spring may not be a problem because several bars of pressure are available. At a boat’s seawater intake, however, the available natural head may only be a small fraction of one bar.

A check valve with excessive cracking pressure can therefore behave almost like a closed valve, even though it appears mechanically correct.

This is particularly important with compact magnetic-drive pumps, which move water efficiently but are normally not self-priming and have limited suction capability.

🔌 6. Electric Flushing Operation

The electric version uses a 12 V or 24 V solenoid valve connected to a switch or watermaker controller.

A typical manual electric-flush sequence is:

1. Finish the normal freshwater-production cycle.
2. Reduce high pressure completely and stop the high-pressure pump.
3. Leave the low-pressure flushing path open according to the watermaker instructions.
4. Activate the flushing switch.
5. The solenoid valve opens and freshwater enters the feed circuit.
6. The intake check valve closes, preventing water from escaping overboard through the through-hull.
7. Freshwater flows through the watermaker and exits through the normal brine discharge.
8. After the specified flushing time, switch the solenoid valve off.

🕹️ 7. Fully Manual Alternative: A Three-Way Valve

An electric solenoid valve is convenient, but it is not compulsory. Builders who prefer a completely manual and electrically independent system can use a suitable three-way valve to select between seawater and freshwater.

The three-way valve has:

• One inlet connected to the seawater intake.
• One inlet connected to the freshwater flushing supply.
• One common outlet connected to the feed pump and watermaker.

In the normal position, the valve connects the watermaker to the seawater intake. For flushing, it is turned manually to connect the freshwater supply instead.

The manual valve must be suitable for potable water and seawater, have an adequate internal bore and produce minimal restriction in the seawater-feed position.

⚖️ 8. Electric System or Manual Three-Way Valve?

Both arrangements can work reliably. The choice depends on the desired level of automation, available electrical control and the owner’s preferred operating method.

📐 9. Installation Guidelines

• Install both check valves in the correct flow direction.
• Follow the arrows moulded or marked on the valve bodies.
• Keep hoses short and avoid unnecessary restrictions.
• Use hose and fittings with an internal diameter appropriate for the feed flow.
• Mount the solenoid valve in a dry, accessible position.
• Protect electrical connections from moisture.
• Make the valves accessible for inspection and replacement.
• Use only components compatible with seawater and potable freshwater.
• Do not install domestic spring-loaded brass check valves.
• Confirm that the freshwater system can provide the required flushing flow.

🔍 10. Commissioning and Functional Test

Before relying on the system, test both operating modes carefully.

Normal Seawater Operation

• Confirm that seawater floods the intake line freely.
• Check that the intake check valve opens without excessive restriction.
• Verify stable feed-pump flow.
• Confirm that no seawater enters the freshwater flushing line.

Freshwater Flushing Operation

• Activate the solenoid or turn the manual three-way valve.
• Confirm that freshwater flows toward the watermaker.
• Check that no freshwater exits through the intake through-hull.
• Verify flow at the normal brine discharge.
• Inspect every connection for leakage.

🧰 11. Maintenance

The flushing system requires little maintenance, but its operation should be checked periodically.

• Inspect check valves for debris or salt deposits.
• Verify that the valves open freely at low pressure.
• Check that the solenoid valve opens and closes completely.
• Inspect electrical connectors for moisture or corrosion.
• Check hoses for hardening, cracking or leakage.
• Confirm that the freshwater side remains isolated during seawater operation.
• Test the flushing cycle after prolonged inactivity.

✅ 12. Quick Selection Checklist

Pre-filtration is one of the most important parts of a marine watermaker. Before seawater reaches the high-pressure pump, Clark Pump or reverse osmosis membrane, suspended particles, sand, silt and biological material must be removed as effectively as possible.

A correctly designed prefilter system protects the most valuable components of the installation, helps maintain stable flow and reduces membrane fouling. Poor or neglected filtration can affect the entire watermaker, even when the seawater appears visually clean.

🌊 1. Why Pre-Filtration Is Essential

Seawater contains more than visible sand or debris. It may also contain fine sediment, plankton, organic material, algae and microscopic marine growth.

Without effective pre-filtration, these contaminants can:

• Restrict flow to the high-pressure pump or Clark Pump.
• Cause unstable pressure and reduced freshwater production.
• Increase the risk of cavitation at the high-pressure pump.
• Deposit inside valves, hoses and small hydraulic passages.
• Promote biological fouling of the membrane.
• Increase cleaning frequency and operating costs.
• Trigger low-flow or low-pressure alarms in automated systems.

⚙️ 2. Recommended Filter Configurations

The appropriate filtration arrangement depends mainly on the seawater flow required by the high-pressure pump and on the expected water quality.

Systems up to approximately 5 litres per minute

For small systems with relatively low feed flow, a single 5-micron melt-blown cartridge is often sufficient, provided that the seawater intake includes a suitable sea strainer and the operating area is not unusually dirty.

Systems above approximately 5 litres per minute

For larger systems, BlueGold normally recommends a two-stage arrangement:

1. 20-micron cartridge: Captures larger particles and protects the second filter.
2. 5-micron cartridge: Provides finer filtration before the high-pressure section.

🧵 3. Melt-Blown or String-Wound Cartridges?

For marine watermaker pre-filtration, BlueGold generally recommends melt-blown polypropylene cartridges rather than traditional string-wound cartridges.

Advantages of Melt-Blown Cartridges

• Graded-density construction: Larger particles are captured in the outer layers while finer material is retained deeper inside the cartridge.
• High dirt-holding capacity: Contamination is distributed through the depth of the filter rather than being concentrated only on the surface.
• More uniform filtration: The continuous polypropylene structure offers consistent filtration without gaps between wound fibres.
• Good flow behaviour: A correctly sized melt-blown cartridge normally maintains usable flow as it gradually loads.
• No loose winding fibres: There are no strings or threads that can unravel or migrate downstream.
• Low cost and easy replacement: Standard DOE cartridges are widely available and simple to keep onboard as spares.

Limitations of String-Wound Cartridges

String-wound cartridges can work in many industrial applications, but their performance depends strongly on manufacturing quality and winding consistency.

Potential disadvantages include:

• Uneven flow paths between the wound fibres.
• Possible channel formation as the cartridge loads.
• Lower consistency between inexpensive cartridges.
• Possible release of small fibres or winding material.
• Less predictable filtration when exposed to changing flow and pressure.

🧽 4. Why Not Use Washable Filters as the Main Prefilter?

Washable mesh or screen filters can be useful as coarse strainers, but they should not normally replace disposable fine-filter cartridges immediately before the high-pressure section.

Most washable filters are designed to stop relatively large debris. They may not provide the consistent 5-micron filtration required to protect pumps and membranes from fine sediment and biological material.

Common Limitations

• Coarser filtration: Many washable elements stop shells, leaves or larger particles but allow fine sediment to pass through.
• Uncertain performance after cleaning: The element may appear clean while fine material remains trapped inside the mesh.
• Risk of damage: Aggressive brushing or pressure washing can deform or enlarge the filtering surface.
• Biological contamination: Biofilm can remain on the element even after a visual rinse.
• No depth filtration: A screen normally captures particles mainly on its surface rather than throughout the depth of the cartridge.

📏 5. Monitoring Filter Condition with Pressure Gauges

A useful prefilter assembly may include one gauge before the cartridges and another after them. Comparing the two readings indicates how much resistance the cartridges are creating.

• Upstream pressure: Pressure entering the prefilter assembly.
• Downstream pressure: Pressure available after the cartridges and before the high-pressure pump.

The difference between these values is the filter pressure differential.

Typical Interpretation

• Approximately 0.1–0.3 bar: Often normal for clean or lightly loaded cartridges, depending on flow and installation.
• A steadily increasing differential: Indicates that the cartridges are collecting contamination.
• Approximately 0.5 bar or more: The cartridges should normally be inspected and replaced.
• Very low downstream pressure accompanied by poor flow: May indicate heavily blocked filters, an intake restriction or an inadequate feed supply.

🧲 6. Why Small Magnetic Pumps May Show Almost No Gauge Pressure

Compact magnetic-drive feed pumps are designed primarily to move water, not to generate high static pressure. In a small, low-resistance watermaker circuit, the gauges may therefore indicate only a few tenths of a bar or appear to remain close to zero.

This does not necessarily mean that the pump is not working correctly.

Low readings are common when:

• The feed pump has a low-pressure, high-flow characteristic.
• The suction and discharge hoses are short and correctly sized.
• The filter cartridges are clean.
• The high-pressure pump accepts the supplied flow without creating significant backpressure.
• The gauge range is too large to display small pressure changes clearly.

The important operating condition is that the high-pressure pump receives a continuous, air-free supply of seawater with enough positive flow to prevent cavitation.

🔍 7. Pressure Is Not the Same as Flow

Pressure and flow describe two different aspects of the feed circuit. A pump can provide adequate flow while producing very little measurable pressure if the circuit has low resistance.

Conversely, a blocked filter can create pressure before the restriction while providing insufficient flow after it.

When diagnosing the system, consider together:

• Pressure before the filters.
• Pressure after the filters.
• Feed-water flow.
• High-pressure pump noise and stability.
• Freshwater production.
• The age and visible condition of the cartridges.

⚠️ 8. What Happens When Prefilters Are Neglected?

Clogged or biologically contaminated cartridges increase resistance and progressively reduce the flow available to the rest of the system.

Possible consequences include:

• Starvation of the conventional high-pressure pump or Clark Pump.
• Cavitation, irregular operation and pump overheating.
• Unstable high-pressure readings.
• Reduced freshwater output.
• Higher electrical consumption as pumps operate under unsuitable conditions.
• Membrane fouling caused by fine solids or organic material.
• Low-flow or low-pressure shutdowns in automated systems.

🧫 9. Biological Growth and Filter Hygiene

Seawater trapped inside filter housings can support bacterial and biological growth, particularly in warm climates.

Freshwater flushing reduces salt concentration and biological activity in the downstream watermaker circuit, but the intake and prefilter arrangement should still be inspected regularly.

• Do not leave heavily contaminated cartridges installed during long periods of inactivity.
• Replace cartridges that smell unpleasant or show signs of biological growth.
• Clean filter bowls before installing new cartridges.
• Do not handle clean cartridges with dirty or oily hands.
• Keep replacement cartridges dry and sealed until required.

📦 10. Standard 10-Inch or Extended 20-Inch Filters?

BlueGold systems normally use standard 10-inch DOE melt-blown cartridges. They provide a good balance between compact dimensions, availability, filtration capacity and ease of replacement.

10-Inch Cartridges

• Compact and easy to install in confined spaces.
• Widely available from marine and industrial suppliers.
• Easy to carry onboard as spares.
• Suitable for most small and medium watermakers.

20-Inch Cartridges

• Greater filtering surface.
• Higher dirt-holding capacity.
• Lower pressure drop at the same flow rate.
• Longer service intervals.
• Particularly useful for high-flow systems or long-distance cruising.

🔧 11. Cartridge Replacement and Housing Maintenance

Replace cartridges when the pressure differential becomes excessive, when flow decreases noticeably or when contamination is suspected.

Recommended Procedure

1. Stop the watermaker and close the seawater intake if required.
2. Release any pressure from the filter housing.
3. Unscrew the bowl using the correct housing wrench.
4. Remove and discard the used cartridge.
5. Clean the inside of the bowl with fresh water.
6. Inspect the housing O-ring for dirt, flattening or damage.
7. Apply a very light film of silicone grease to the O-ring.
8. Install the new cartridge and refit the bowl.
9. Prime the circuit and inspect carefully for leaks.

📋 12. Quick Pre-Filter Selection Guide

✅ 13. Pre-Filter Maintenance Checklist

• Record clean-filter pressure readings during commissioning.
• Monitor the pressure difference before and after the cartridges.
• Do not wait for complete blockage before replacing the filters.
• Inspect the sea strainer and intake at the same time.
• Keep several replacement cartridges onboard.
• Store spare filters in sealed, dry packaging.
• Leave enough clearance below each housing for easy servicing.
• Lightly lubricate housing O-rings with silicone grease.
• Check for leaks and trapped air after every cartridge change.

The high-pressure pump is the heart of every conventional marine watermaker. Its role is to deliver a controlled flow of seawater at the pressure required by the reverse osmosis membrane to separate freshwater from dissolved salts.

Unlike the feed pump, which supplies the system at low pressure, the high-pressure pump operates in the region normally required for seawater reverse osmosis. Correct pump selection, motor sizing, feed-water supply, pressure regulation and maintenance are all essential for reliable operation.

💧 1. What the High-Pressure Pump Does

The high-pressure pump receives filtered seawater from the feed side and moves a fixed volume of water through the reverse osmosis membrane circuit.

In a conventional system, the pump provides the flow while the operating pressure develops when the concentrate stream is restricted by the pressure-regulating valve.

The high-pressure pump must provide enough flow for the selected membrane configuration while remaining within its rated speed, pressure, temperature and power limits.

📏 2. Normal Seawater RO Operating Pressure

Marine seawater reverse osmosis systems commonly operate at approximately 55–60 bar, although the exact pressure depends on several factors:

• Seawater salinity
• Water temperature
• Membrane size and number
• Desired freshwater production
• Membrane age and condition
• Feed-water flow

Cold or highly saline seawater may require a different operating pressure from warm, lower-salinity water. The objective is not to reach the highest possible pressure, but to obtain the required production while remaining within the limits of the membrane, pump, vessel and fittings.

🎛️ 3. How Pressure Is Regulated

The operating pressure is normally adjusted with a needle valve or purpose-designed pressure-regulating valve installed on the concentrate or brine outlet.

Closing the valve gradually increases resistance and therefore raises the pressure inside the membrane circuit. Opening it reduces pressure and allows more brine to flow overboard.

Never Adjust Pressure Abruptly

• Do not close the regulating valve suddenly.
• Do not start the motor against unnecessarily high residual pressure.
• Increase pressure gradually while observing the high-pressure gauge.
• Confirm that concentrate flow remains present.
• Never exceed the rated pressure of the pump, vessel, membrane or fittings.

🛡️ 4. Pressure Gauge and Safety Devices

Every conventional high-pressure system should include a clearly visible pressure gauge installed in a suitable position on the high-pressure circuit.

Depending on the design, the system should also include a pressure relief valve or another reliable means of preventing accidental overpressure.

Recommended Safety Elements

• High-pressure gauge with an appropriate range
• Pressure relief valve or equivalent protection
• Pressure-rated hoses and fittings
• Pressure vessel rated above the maximum operating pressure
• Securely mounted regulating valve
• Guarding around exposed rotating couplings

🔩 5. Stainless-Steel or Brass Pump Head?

Marine high-pressure plunger pumps are commonly available with either an AISI 316L stainless-steel head or a suitable brass head.

Both solutions can be used successfully in a marine watermaker, but they offer different advantages in terms of cost, corrosion resistance and expected operating conditions.

AISI 316L Stainless-Steel Head

• Maximum corrosion resistance: Particularly suitable for intensive use and long-term exposure to seawater.
• Suitable for warm climates: High seawater temperatures can accelerate corrosion and biological activity.
• Long-term investment: Normally more expensive, but attractive for high-use or professional installations.
• Excellent mechanical durability: Well suited to demanding marine service.

Brass Pump Head

• Lower initial cost: A practical option for many DIY and seasonal installations.
• Widely available: Replacement parts and service expertise are often easy to obtain.
• Suitable for seawater use: Provided that the pump is correctly selected, flushed and maintained.
• Good value: A reliable compromise when maximum corrosion resistance is not the primary requirement.

🌊 6. Correct Feed-Water Supply

A high-pressure plunger pump should not be expected to draw seawater through the complete intake and prefilter circuit by itself. It must receive a continuous and adequately supplied flow from the feed pump.

The inlet side should remain fully flooded and free from air during operation.

Insufficient Feed Supply Can Cause

• Cavitation
• Irregular mechanical noise
• Pressure fluctuations
• Reduced freshwater production
• Overheating
• Damage to seals and valves
• Premature pump wear

Small magnetic-drive feed pumps may show very little pressure on a gauge while still providing adequate flow. The condition of the high-pressure pump should therefore be assessed from feed flow, pressure stability, sound and overall system performance—not from feed pressure alone.

🚫 7. Never Run the Pump Dry

A high-pressure pump must never be allowed to operate without water, even for a short period.

The pumped water contributes to cooling and lubrication of the wet-end components. Dry running can rapidly damage:

• Plunger seals
• Valve seats
• O-rings
• Guides and internal surfaces

🔄 8. Pump Flow and Membrane Matching

Each high-pressure pump has a nominal flow rate, normally expressed in litres per minute. This flow must be matched to the membrane size and number of membrane vessels.

A pump that is too small may not provide enough cross-flow through the membrane. A pump that is unnecessarily large may increase power consumption, brine flow and pressure-control requirements without improving freshwater production proportionally.

Typical BlueGold Pump and Motor Combinations

🧮 9. Calculating Required Mechanical Power

The theoretical hydraulic power required to move water at a given pressure and flow can be estimated using:

Real pumps are not perfectly efficient. Mechanical and volumetric losses mean that the motor must supply more shaft power than the theoretical hydraulic value.

For practical preliminary sizing, BlueGold uses the more conservative empirical formula:

Example: 6 L/min at 60 bar

The estimated mechanical power required is:

This is the approximate minimum mechanical power that must be available at the motor shaft. The selected motor should normally provide at least this value, with a reasonable safety margin.

⚡ 10. Mechanical Power vs. Electrical Consumption

The power printed on an electric motor nameplate normally refers to the mechanical output available at the shaft—not the electrical power drawn from the supply.

Because no motor is 100% efficient, the electrical input is always higher than the mechanical output.

Sources of Motor Losses

• Heat in the windings
• Bearing friction
• Magnetic losses
• Cooling fan losses
• Capacitor and power-factor losses in single-phase motors

A simple approximation of apparent electrical input can be obtained from the nameplate voltage and current:

For AC motors, actual real power also depends on power factor and efficiency, so nameplate data and manufacturer specifications should be used for final electrical design.

🏷️ 11. How to Read a Motor Nameplate

The motor nameplate contains the most important information required for power-system and installation planning.

Look For

• Rated voltage: For example 230 V AC, 400 V three-phase, 12 V DC or 24 V DC.
• Rated mechanical power: Expressed in kW or horsepower.
• Rated current: Important for cable, fuse, contactor and inverter sizing.
• Rotational speed: Often approximately 1450 rpm for a four-pole 50 Hz motor.
• Duty rating: Preferably S1 for continuous operation.
• Frequency: Normally 50 or 60 Hz.
• Protection rating: Indicates resistance to dust and water ingress.
• Insulation class: Relevant to operating temperature and durability.

🔌 12. AC, DC and Three-Phase Motors

Single-Phase AC Motors

• Common on boats using generators or inverters.
• Simple to integrate into a 230 V onboard system.
• Normally less efficient than comparable three-phase motors.
• Require correct capacitor and starting arrangements.

Three-Phase Motors

• Generally more efficient and mechanically smooth.
• Well suited to larger watermakers.
• Can be controlled by a suitable variable-frequency drive where the complete pump system permits it.
• Require appropriate electrical infrastructure.

DC Motors

• Can operate directly from 12 V or 24 V battery systems.
• High-current models require very large cables, fuses and switching devices.
• Voltage drop becomes particularly important at higher powers.
• Continuous-duty thermal performance must be verified carefully.

🔋 13. Inverter and Generator Sizing

The inverter or generator must support both the continuous running demand and the temporary startup load of the motor.

For example, an 1.1 kW single-phase motor may require approximately 1.3–1.4 kW of electrical input under full load, depending on efficiency and power factor.

The inverter should therefore not be selected at exactly the nominal motor rating.

Also consider the other electrical loads that may be running at the same time, such as:

• Feed pump
• Control panel
• Freshwater flushing pump
• Battery charger
• Refrigeration
• Navigation electronics

🧲 14. Start and Run Capacitors

BlueGold single-phase motors may use a dual-capacitor system consisting of a start capacitor and a run capacitor.

Start Capacitor

The start capacitor provides additional starting torque during the first seconds of operation. It is disconnected automatically after the motor reaches operating speed.

Run Capacitor

The run capacitor remains active during operation and helps support:

• Stable motor performance
• Improved power factor
• Reduced vibration
• Smoother operation
• Lower winding stress

🔄 15. Speed, Rotation and Frequency

Positive-displacement plunger pumps are designed to operate within a specified speed range.

Pump flow is approximately proportional to rotational speed. Increasing speed increases flow, power demand and mechanical stress.

Important Checks

• Use the rotational speed specified for the pump.
• Verify the motor frequency: 50 Hz and 60 Hz motors may run at different speeds.
• Confirm the correct direction of rotation before prolonged operation.
• Do not exceed the pump’s maximum rated speed.
• Do not use a variable-frequency drive without checking pump lubrication, minimum speed and cooling requirements.

🔗 16. Pump and Motor Coupling

The motor and pump must be mounted on a rigid base and connected with a correctly aligned coupling.

Misalignment can cause vibration, bearing wear, coupling damage and premature seal failure.

Installation Guidelines

• Use a suitable flexible coupling.
• Align pump and motor shafts carefully.
• Do not force the shafts into alignment by tightening mounting bolts.
• Mount the assembly on a rigid frame.
• Use vibration-damping mounts where appropriate.
• Protect exposed rotating components with a guard.
• Check mounting bolts after the first operating hours.

📐 17. Installation Position

The high-pressure pump may normally be installed above or below the waterline, provided that the feed pump supplies a stable and positive flow.

The high-pressure section should remain fully flooded before startup.

Good Installation Practice

• Keep feed plumbing short and correctly sized.
• Avoid high loops that can trap air.
• Install the pump in a dry and ventilated area.
• Leave access for oil checks, valve servicing and seal replacement.
• Protect the motor from direct seawater spray.
• Allow cooling air to circulate around the motor.

🛢️ 18. Oil and Routine Pump Maintenance

Many high-pressure plunger pumps have an oil-filled crankcase. Oil level and condition must be checked according to the pump manufacturer’s instructions.

Routine Checks

• Check the oil level before operation.
• Use only the specified oil type.
• Perform the first oil change at the recommended interval.
• Continue with regular oil changes according to operating hours.
• Inspect for water contamination or milky oil.
• Check for leakage between the pump head and crankcase.
• Inspect valves and seals if pressure or flow becomes unstable.

🚿 19. Freshwater Flushing

Freshwater flushing removes concentrated seawater from the pump head, valves, membrane and pressure circuit after operation.

Regular flushing helps reduce:

• Salt crystallisation
• Valve sticking
• Corrosion
• Biological growth
• Seal deterioration
• Problems after extended inactivity

🧯 20. Common Problems and Symptoms

✅ 21. High-Pressure Pump Selection Checklist

• Confirm the required pump flow.
• Match the flow to the membrane configuration.
• Calculate the required shaft power.
• Select a motor with suitable reserve capacity.
• Verify voltage, current, frequency and duty rating.
• Check the pump’s maximum pressure and speed.
• Choose brass or AISI 316L according to use and budget.
• Provide a suitable feed pump and prefilter system.
• Install a pressure gauge and overpressure protection.
• Use rated hoses, fittings and pressure vessels.
• Provide safe coupling alignment and guarding.
• Plan ventilation, drainage and maintenance access.
• Follow the manufacturer’s oil and service instructions.
• Include freshwater flushing in the system design.

Assembling the high-pressure pump and motor is a straightforward mechanical operation that can be completed with normal hand tools. The exact procedure depends on the motor shaft configuration: some motors use a hollow shaft for direct coupling, while others use a standard male shaft combined with a flexible coupling and bellhousing.

The photographs accompanying this guide show each stage clearly. Before beginning, place the motor and pump on a clean, stable surface and confirm that all supplied screws, washers, fittings and coupling parts are available.

🧰 1. Tools and Materials Required

• Correctly sized Allen keys
• Open-ended or adjustable spanners
• Suitable assembly grease
• PTFE tape for the low-pressure inlet fitting only
• The supplied high-pressure sealing washer
• Screwdriver suitable for the electrical terminal cover
• All original screws supplied with the pump, motor and bellhousing

⚙️ 2. Identify the Motor Shaft Type

Before starting, identify which of the following motor configurations you have:

• Hollow-shaft motor: The male shaft of the high-pressure pump fits directly inside the hollow motor shaft.
• Male-shaft motor: The pump and motor are connected through a flexible coupling housed inside a bellhousing.

🕳️ 3. Assembly with a Hollow-Shaft Motor

Step 1 — Remove the Motor Front Flange

Place the motor securely on the work surface. Remove the four Allen screws holding the front flange to the motor body, then carefully lift the flange away.

Step 2 — Attach the Flange to the Pump

Position the removed flange against the high-pressure pump and align the four mounting holes. Fasten the flange to the pump using the four supplied screws.

Tighten the screws progressively and evenly rather than fully tightening one screw before the others.

Step 3 — Lubricate the Shaft Connection

Apply a suitable layer of grease to:

• The male shaft of the high-pressure pump
• The internal surface of the hollow motor shaft

The grease helps the shafts slide together smoothly, prevents corrosion between the mating surfaces and makes future disassembly easier.

Step 4 — Insert the Pump Shaft into the Motor

Align the pump shaft carefully with the hollow motor shaft. Slide the pump and flange assembly straight into the motor without forcing or twisting it excessively.

The flange should approach the motor body evenly. If the shaft does not enter easily, remove the pump and check the alignment rather than using the mounting screws to pull the parts together.

Step 5 — Refit the Flange to the Motor

Once the shaft is fully inserted and the flange is seated correctly, reinstall the four Allen screws that secure the flange to the motor.

Tighten them gradually in a cross pattern so that the flange remains evenly seated.

🔗 4. Assembly with a Male-Shaft Motor

Step 1 — Attach the Bellhousing to the Pump

Align the bellhousing with the high-pressure pump and fasten it using the four supplied screws.

Tighten the screws evenly and confirm that the bellhousing is seated flat against the pump body.

Step 2 — Lubricate the Shafts and Flexible Coupling

Apply a suitable layer of grease to:

• The male motor shaft
• The male pump shaft
• The internal coupling surfaces that slide over both shafts

This makes assembly easier and helps prevent corrosion between the shafts and coupling.

Step 3 — Fit the Flexible Coupling to the Motor

Place the motor vertically with the shaft facing upward. Fit the flexible coupling onto the motor shaft and make sure it is fully seated in the correct position.

Step 4 — Lower the Pump and Bellhousing onto the Motor

Hold the assembled pump and bellhousing above the motor. Carefully align the pump shaft with the upper side of the flexible coupling, then lower the assembly vertically.

The pump shaft should enter the coupling smoothly while the bellhousing approaches the motor mounting face.

Step 5 — Secure the Bellhousing to the Motor

Once the coupling is correctly engaged and the bellhousing is fully seated, fasten the bellhousing to the motor body using the supplied screws.

Tighten them gradually in a cross pattern and confirm that there is no visible gap between the mating surfaces.

💧 5. Install the Low-Pressure Feed-Water Fitting

The low-pressure seawater inlet is located on the lower part of the pump head.

1. Confirm that the inlet thread is clean and undamaged.
2. Apply approximately two neat turns of PTFE tape to the male thread of the hose fitting.
3. Keep the first thread reasonably clear so that loose tape cannot enter the pump.
4. Screw the fitting into the lower pump inlet.
5. Tighten it firmly but without excessive force.

🚿 6. Install the High-Pressure Outlet Nipple

The high-pressure outlet is located on the upper part of the pump head.

1. Check that the supplied sealing washer is present and undamaged.
2. Position the washer correctly on the high-pressure nipple.
3. Screw the nipple into the upper outlet.
4. Tighten it according to the supplied installation instructions.

⚡ 7. Connect the Motor Power Cable

Remove the electrical terminal-box cover from the motor. If the capacitor assemblies obstruct access to the terminals, move them carefully aside without disconnecting or straining their wires.

Locate the two designated blade terminals used for the single-phase supply.

1. Route the supply cable through the appropriate cable gland.
2. Connect phase and neutral to the two designated blade terminals.
3. On the specified BlueGold motor, the two supply connections are not polarity-sensitive, so phase and neutral may be connected in either order.
4. Confirm that each terminal is fully inserted and secure.
5. Arrange the wiring so that it cannot touch moving parts or be pinched by the cover.
6. Return the capacitor assemblies to their original position.
7. Refit and secure the terminal-box cover.

🔍 8. Final Mechanical Inspection

Before installing the assembly onboard, complete a final visual and mechanical check.

• All pump, flange and bellhousing screws are fitted and evenly tightened.
• The pump and motor are correctly aligned.
• No visible gap remains between the flange or bellhousing surfaces.
• The low-pressure fitting is installed in the lower inlet.
• The high-pressure nipple and sealing washer are installed in the upper outlet.
• No PTFE tape has been used on the washer-sealed high-pressure connection.
• The electrical cable is securely clamped by the cable gland.
• The terminal-box cover is correctly refitted.
• No tools, screws or packaging remain near the coupling or motor ventilation openings.

🛡️ 9. Before the First Startup

Mechanical assembly does not mean that the unit is ready to run dry on the workbench.

Before starting the motor:

• Mount the complete assembly securely.
• Connect the feed-water and high-pressure circuits.
• Confirm that the pump crankcase contains the correct oil, where applicable.
• Prime the feed circuit completely.
• Verify that seawater reaches the high-pressure pump without air.
• Leave the pressure-regulating valve open.
• Check the electrical supply voltage, fuse and protective devices.
• Confirm the correct rotation direction where required by the pump model.

✅ 10. Assembly Summary

The reverse osmosis membrane and its pressure vessel form the core separation unit of every marine watermaker. This is where pressurised seawater is divided into two streams: freshwater that passes through the membrane and concentrated brine that continues toward the discharge.

Membrane format, active surface area, high-pressure pump flow and vessel configuration must all be selected together. A larger membrane does not automatically guarantee greater production, and adding more membranes does not compensate for insufficient pump flow.

💧 1. What an RO Membrane Does

A seawater reverse osmosis membrane is a spiral-wound element containing multiple layers of semi-permeable material around a central product-water tube.

When seawater flows across the membrane surface under sufficient pressure:

• Water molecules pass through the membrane and enter the central product-water tube.
• Most dissolved salts remain in the feed stream and leave the element as concentrated brine.
• Suspended particles should already have been removed by the seawater strainer and prefilters.

🛡️ 2. What the Pressure Vessel Does

The pressure vessel safely contains the membrane and the high-pressure seawater flowing around it. It also supports the end caps, seals and product-water connections that keep the freshwater and brine streams separated.

A suitable marine pressure vessel must provide:

• Safe operation at the maximum possible system pressure.
• Resistance to seawater corrosion.
• Reliable sealing around the end plugs and product-water tube.
• Correct internal dimensions for the selected membrane format.
• Access for membrane installation, removal and maintenance.
• Strong and secure mounting aboard the vessel.

📐 3. Understanding Membrane Format Numbers

Marine RO membranes are normally identified by a four-digit format number.

• The first two digits indicate the approximate membrane diameter in tenths of an inch.
• The last two digits indicate the approximate membrane length in inches.

📏 4. The 2521 Format

The 2521 membrane is compact and particularly useful where installation space is limited or where several short vessels can be distributed around the boat.

Main Characteristics

• Approximately 2.5 inches in diameter and 21 inches long.
• Easy to handle and replace in confined machinery spaces.
• Suitable for small and medium modular systems.
• Multiple elements can be combined to increase total membrane area.
• Useful where a long 40-inch vessel cannot be installed.

📏 5. The Traditional 2540 Format

The 2540 membrane has the same nominal diameter as a 2521 element but almost twice the length. Its larger active area allows one element to replace several smaller membranes in many configurations.

Main Characteristics

• Approximately 2.5 inches in diameter and 40 inches long.
• High active membrane area in a single element.
• Well suited to medium- and high-output watermakers.
• Uses fewer vessels and interconnecting high-pressure hoses than multiple 2521 elements.
• Requires a long, straight installation space and enough clearance for membrane removal.

🆕 6. The Compact High-Capacity 3021 Format

The 3021 format combines the short length of a 2521 membrane with a larger three-inch diameter. The increased diameter provides substantially more active membrane area without requiring a traditional 40-inch vessel.

The BlueGold 3021 pressure vessel is specifically designed for 3 × 21-inch seawater membranes and offers a modern alternative to conventional 2540 or multiple-2521 arrangements.

Main Advantages

• Compact length: Easier to install in lockers and machinery spaces where a 40-inch vessel will not fit.
• Large active membrane area: Production potential comparable to traditional 2540 configurations, depending on membrane model and operating conditions.
• Simpler installation: One short vessel can replace a longer vessel or a more complex multi-vessel rack.
• Easier service access: The shorter membrane can be removed with less longitudinal clearance.
• Reduced plumbing complexity: Fewer high-pressure interconnections may be required.
• Suitable for modern energy-recovery systems: Particularly attractive where compactness and membrane area are both important.

⚖️ 7. Comparing the Three Formats

🔄 8. Single or Multiple Membranes?

Several smaller membranes can be connected to obtain the total active surface required by the system. The correct arrangement depends on pump flow, desired production, pressure loss and installation space.

Multiple vessels may offer excellent layout flexibility, but they also introduce more:

• High-pressure hoses and fittings.
• Product-water connections.
• End-plug seals and O-rings.
• Mounting brackets.
• Potential leak points.

A single larger membrane or compact 3021 element simplifies plumbing, while multiple 2521 vessels may fit more easily around irregular spaces.

🚰 9. Pump Flow and Membrane Area

The high-pressure pump must provide enough flow across the membrane surface to produce freshwater while carrying rejected salts away in the brine stream.

Too little flow can lead to:

• Insufficient cross-flow along the membrane.
• Excessive recovery from the available feed water.
• Higher salt concentration near the membrane surface.
• Scaling, fouling and reduced membrane life.
• Unstable freshwater production.

Excessive pump flow can increase motor power, pressure loss and brine volume without providing a proportional increase in freshwater output.

📊 10. Typical Conventional BlueGold Configurations

The following values are practical estimates for conventional high-pressure pump systems. Actual output depends on the membrane model, seawater temperature, salinity, operating pressure and membrane condition.

🧱 11. Vessel Materials and Construction

Marine pressure vessels may be manufactured from stainless steel, reinforced composite or other materials specifically rated for seawater reverse osmosis pressure.

Composite Vessels

• Lightweight and corrosion-resistant.
• Easy to handle during installation.
• Well suited to marine environments.
• Require protection from mechanical damage and incorrect clamping.

Stainless-Steel Vessels

• Very strong and mechanically robust.
• Can provide excellent durability when manufactured from suitable alloys.
• Heavier than composite alternatives.
• Require attention to galvanic corrosion and mounting materials.

🛠️ 12. Installation Planning

Before choosing a membrane format, measure the complete space required—not only the length of the vessel.

Allow room for:

• End caps and high-pressure fittings.
• Product-water connections.
• Hose bend radius.
• Mounting brackets.
• Removal of the end plug.
• Withdrawal of the full membrane element.
• Inspection and O-ring replacement.

✅ 13. Selection Checklist

• Determine the freshwater production required.
• Confirm the available high-pressure pump flow.
• Choose the membrane format and active area.
• Select a vessel specifically compatible with that format.
• Check maximum operating-pressure ratings.
• Allow adequate brine flow and avoid excessive recovery.
• Confirm the available installation and membrane-removal space.
• Use pressure-rated hoses and fittings.
• Keep vessels accessible for flushing, cleaning and replacement.
• Follow the actual membrane manufacturer’s data sheet.

The production stated on a membrane data sheet is not a fixed quantity that will be obtained in every sea and season. Freshwater flow and product-water salinity change with feed-water temperature, seawater salinity, operating pressure, membrane age and system recovery.

Understanding these effects prevents unnecessary membrane replacement and helps distinguish normal seasonal variation from a real system problem.

🧪 1. Membrane Test Conditions

Membrane manufacturers publish nominal production and salt-rejection values under specified laboratory test conditions.

These usually define:

• Feed-water salinity.
• Feed-water temperature.
• Applied pressure.
• Recovery rate.
• Feed-water pH.

For small seawater membranes, typical reference conditions are often around 25°C and approximately 32,000–35,000 ppm salinity, but the exact pressure and other parameters depend on the membrane model.

🌡️ 2. Effect of Feed-Water Temperature

Water becomes more viscous as temperature decreases. Cold water therefore passes through the membrane less easily, reducing product-water flow.

As seawater becomes warmer, viscosity decreases and permeate flow increases.

As Temperature Increases

• Freshwater production normally increases.
• Salt passage generally increases.
• Product-water TDS may rise.

As Temperature Decreases

• Freshwater production decreases.
• Salt passage generally decreases.
• Product-water TDS may improve.

Approximate examples often encountered in practice include:

• At around 30–32°C, production may be noticeably above the nominal 25°C value.
• At around 15°C, production may fall by approximately one third.
• At around 10°C, production may approach half of the warm-water value, depending on membrane model and operating conditions.

🧮 3. Temperature Correction Factors

A temperature correction factor can be used to estimate production away from the membrane’s standard reference temperature.

Example

Assume a system is rated at 120 litres per hour at 25°C and the applicable correction factor at 15°C is 1.47.

The expected production at 15°C is therefore approximately 82 litres per hour, even though the membrane and pump are operating normally.

🧂 4. Effect of Feed-Water Salinity

Dissolved salt creates osmotic pressure, which opposes the pressure applied by the high-pressure pump.

As feed-water salinity increases, more of the applied hydraulic pressure is required simply to overcome osmotic pressure. Less effective pressure remains available to produce freshwater.

As Salinity Increases

• Freshwater production decreases.
• Product-water TDS generally increases.
• More pressure may be needed to obtain the same production.
• Motor and pump load may increase if pressure is raised.

As Salinity Decreases

• Freshwater production increases.
• Product-water TDS generally decreases.
• The same output can often be achieved at lower pressure.

Salinity can be checked with a suitable refractometer or conductivity instrument. Measurements should be made on the actual seawater entering the watermaker rather than assumed from a general regional value.

💥 5. Effect of Operating Pressure

Increasing applied pressure raises the net driving force across the membrane. Within the permitted operating range, this generally increases freshwater flow and improves salt rejection.

As Pressure Increases

• Freshwater production normally increases.
• Product-water TDS generally decreases.
• Pump shaft power increases.
• Motor current and heat production increase.
• Mechanical stress on the complete high-pressure circuit increases.

As Pressure Decreases

• Freshwater production decreases.
• Product-water TDS generally increases.
• Motor and pump load decrease.

Increasing operating pressure from 55 to 60 bar raises the hydraulic power requirement by approximately nine percent at the same pump flow. This extra power becomes additional motor current and heat.

⚖️ 6. Net Driving Pressure

Membrane production depends not only on gauge pressure but on the pressure remaining after osmotic resistance and product-side pressure are considered.

In simplified terms:

This explains why two systems operating at the same gauge pressure can produce different quantities of freshwater when salinity or product-side pressure differs.

🚰 7. Product-Water Backpressure

The freshwater outlet should normally discharge freely toward the flow meter, diversion valve and tank.

Restrictions or pressure on the product-water side reduce the effective pressure across the membrane and can damage membrane seals if excessive.

Possible causes include:

• Undersized product-water tubing.
• Blocked flow meters.
• Closed or incorrectly positioned valves.
• Pressurised freshwater tanks connected without suitable separation.
• Check valves with excessive opening pressure.

📈 8. Why TDS Changes

Product-water TDS is influenced by both membrane salt rejection and the quantity of water passing through the membrane.

TDS may increase because of:

• Higher feed-water salinity.
• Higher feed-water temperature.
• Insufficient operating pressure.
• Membrane ageing or chemical damage.
• Damaged O-rings or product-water seals.
• Excessive recovery.
• High product-water backpressure.

🔍 9. Flow Loss Is Not Always Membrane Fouling

Reduced freshwater production may be caused by conditions outside the membrane itself.

Before suspecting fouling, check:

• Feed-water temperature.
• Feed-water salinity.
• High-pressure pump flow.
• Actual operating pressure.
• Prefilter condition.
• Feed-pump performance.
• Air entering the suction line.
• Motor voltage and rotational speed.
• Brine flow and recovery ratio.

🧫 10. Fouling, Scaling and Ageing

Over time, membrane performance may decline because of:

• Suspended-particle fouling.
• Biological growth and biofilm.
• Organic contamination.
• Mineral scaling.
• Oxidant or chlorine damage.
• Incorrect cleaning chemicals.
• Mechanical seal damage.
• Normal membrane ageing.

Fouling commonly reduces product-water flow and increases pressure loss. Chemical damage or mechanical leakage may instead produce an abrupt increase in TDS.

📝 11. Recommended Operating Log

📋 12. Quick Effect Summary

BlueGold 2.5-inch and 3-inch pressure vessels perform the same basic function, but their construction and assembly details are different. For this reason, each vessel type should be assembled according to its own dedicated procedure.

This guide describes the components and assembly sequence of the BlueGold 2.5-inch pressure vessel for standard 2521 or 2540 seawater reverse osmosis membranes.

📦 1. Components Included

A standard single-membrane BlueGold 2.5-inch pressure-vessel assembly includes:

• 1 pressure tube: Supplied in either 21-inch or 40-inch length, depending on the selected membrane.
• 2 Delrin end caps: One for the feed end and one for the brine end.
• 4 external O-rings: These seal the end caps against the internal wall of the pressure tube.
• 2 internal O-rings: These seal the membrane’s central permeate tube inside the end caps.
• 2 nipples, ¼″ NPT × ¼″ BSPP: Used for the high-pressure feed and brine connections.
• 1 permeate-side plug: Used to close one of the two product-water outlets.
• 1 John Guest ¼″ push-fit connector: Used for the product-water tube.
• 2 aluminium closure plates: Positioned outside the end caps to retain the complete assembly.
• 4 stainless-steel tie rods with protective PVC coating.
• 8 stainless-steel washers, 10 × 30 mm.
• 8 stainless-steel M10 nuts.
• 8 stainless-steel M10 cap nuts.
• 2 stainless-steel mounting brackets.
• 1 set of flow-direction and identification labels.

🧰 2. Tools and Materials Required

• Suitable open-ended or adjustable spanners
• Clean silicone grease compatible with potable-water and membrane applications
• PTFE thread-sealing tape
• Clean lint-free cloth
• Marker for temporarily identifying flow direction
• Clean freshwater for rinsing components if required

➡️ 3. Mark the Flow Direction

Before installing any internal component, identify the intended feed and brine ends of the pressure tube.

Mark the direction of seawater flow temporarily on the outside of the tube:

The supplied permanent labels can be applied after assembly has been completed and the final orientation has been confirmed.

🔩 4. Install the High-Pressure Nipples in the End Caps

Each end cap receives one ¼″ NPT × ¼″ BSPP nipple for the high-pressure seawater connection.

1. Identify the tapered ¼″ NPT side of the nipple.
2. Apply approximately two neat turns of PTFE tape to the NPT thread.
3. Keep the first thread reasonably clear so that loose tape cannot enter the hydraulic circuit.
4. Screw the NPT side into the threaded port of the Delrin end cap.
5. Tighten firmly, but do not apply excessive torque.

⭕ 5. Install the End-Cap O-Rings

Prepare both end caps before inserting either of them into the pressure tube.

External O-Rings

Fit the two large external O-rings into their respective grooves around each end cap.

Check that:

• The O-rings are not twisted.
• They sit fully inside their grooves.
• No part of the seal is pinched or stretched.
• The grooves are clean and free from debris.

Internal Permeate-Tube O-Ring

Install the smaller internal O-ring into the central bore of each end cap. This O-ring seals against the permeate tube protruding from the centre of the membrane.

🧴 6. Lubricate the Sealing Surfaces

Apply a light, continuous film of silicone grease to:

• The external end-cap O-rings.
• The internal permeate-tube O-ring.
• The internal wall of the pressure tube near each open end.

The grease should make the surfaces smooth and slippery without leaving large deposits.

🚿 7. Install the Brine-Side End Cap First

Identify the end of the vessel marked as the brine outlet.

Insert the prepared end cap into this end of the pressure tube. Push it inward evenly and keep it square to the tube.

The end cap should enter smoothly as the lubricated external O-rings pass inside the vessel.

🧬 8. Prepare the Membrane

Inspect the seawater membrane before installation.

Confirm that:

• The membrane format matches the vessel length.
• The membrane wrapper is undamaged.
• The central permeate tube is clean.
• The brine seal is correctly seated.
• The flow-direction arrow is clearly visible.

Apply a light film of silicone grease to:

• The membrane brine-seal O-ring or lip seal.
• Both ends of the central permeate tube.
• The internal surface of the open feed end of the pressure vessel.

➡️ 9. Insert the Membrane from the Feed Side

The membrane must be inserted from the end of the vessel that has remained open: the feed side.

Align the flow-direction arrow printed on the membrane with the flow direction previously marked on the pressure tube.

Slide the membrane carefully into the vessel and push it steadily toward the already installed brine-side end cap.

Continue until the membrane’s permeate tube engages fully with the internal O-ring inside the brine-side end cap.

🔒 10. Install the Feed-Side End Cap

Prepare the feed-side end cap in exactly the same way as the brine-side cap:

• Confirm that the two external O-rings are correctly seated.
• Confirm that the internal permeate-tube O-ring is installed.
• Apply a light film of silicone grease to all O-rings.
• Lubricate the internal wall at the feed end of the tube.

Align the central bore of the end cap with the membrane’s permeate tube and insert the cap squarely into the vessel.

Push it inward until it is correctly and evenly seated.

🧱 11. Position the Aluminium Closure Plates

Place one aluminium closure plate at each end of the assembled vessel.

The plates retain the end caps and distribute the axial load generated when the vessel is pressurised.

Check that:

• Both plates are correctly centred.
• The high-pressure fittings pass through the appropriate openings without interference.
• The plates sit flat against the ends of the assembly.
• No hose fitting or permeate connection is trapped behind a plate.

🔗 12. Install the Tie Rods

Pass the four PVC-coated stainless-steel tie rods through the corresponding holes in both closure plates.

Fit the large 10 × 30 mm stainless-steel washers and the standard M10 nuts on both ends.

Tighten the nuts progressively and evenly:

1. Bring all nuts into light contact.
2. Check that both plates remain parallel.
3. Tighten opposite rods alternately.
4. Continue gradually until the assembly is firm and evenly compressed.

🧲 13. Install the Mounting Brackets and Cap Nuts

Once the standard nuts have been tightened and the vessel assembly is stable, position the two stainless-steel mounting brackets on the tie rods.

Install the stainless-steel M10 cap nuts on both ends and tighten them to secure the brackets and finish the retaining assembly.

Confirm that:

• The brackets are aligned with the intended mounting surface.
• The closure plates remain parallel.
• The tie rods are not visibly bent.
• The end caps remain centred in the pressure tube.

🚰 14. Install the Product-Water Connector and Plug

The central product-water passage can normally be accessed from either end of the vessel.

Install the ¼-inch John Guest push-fit connector on the selected permeate outlet and install the supplied plug on the opposite end.

1. Inspect the small sealing O-rings on both fittings.
2. Apply a very light film of silicone grease to the O-rings.
3. Insert or screw the fittings into their respective permeate ports according to their design.
4. Confirm that both fittings are fully seated.

🏷️ 15. Apply the Identification Labels

After final assembly, clean the outside of the pressure tube and apply the supplied labels.

The labels should clearly identify:

• The seawater flow direction.
• The feed end.
• The brine end.
• The product-water connection where appropriate.

🛠️ 16. The Vessel Must Be Securely Mounted

Before the pressure vessel is connected and pressurised, it must be fixed securely to a rigid onboard surface using the two supplied stainless-steel brackets.

The mounting is not merely intended to prevent the vessel from moving around the compartment. It also helps maintain the geometry of the complete closure assembly.

Secure mounting is especially important in multi-membrane assemblies, where the vessel group is longer, heavier and subject to greater cumulative movement.

🔄 17. Multi-Membrane Vessel Connections

In a multi-membrane BlueGold arrangement, two adjacent vessels can be connected directly through a dedicated interconnector nipple fitted with O-rings.

This eliminates the need for an external high-pressure hose between the two intermediate end caps.

Intermediate Connection

• The intermediate end caps do not use the standard threaded high-pressure nipple.
• Each cap contains a smooth receiving bore for the interconnector.
• The interconnector O-rings seal directly inside these bores.
• A dedicated double closure plate retains the two vessel ends together.

Assembly Procedure

1. Inspect the interconnector and its O-rings.
2. Apply a light film of silicone grease to the O-rings.
3. Lightly lubricate the receiving bores in both intermediate end caps.
4. Insert the interconnector into one cap.
5. Align and bring the second vessel into position.
6. Confirm that the nipple enters both caps without pinching an O-ring.
7. Install the dedicated double closure plate.
8. Fit and tighten the complete tie-rod system evenly.

A double-vessel assembly normally uses six tie rods instead of four. The number of washers, standard nuts and cap nuts increases accordingly.

⚠️ 18. The Three Fundamental Assembly Rules

🔍 19. Final Inspection Before Installation

Before connecting the vessel to the watermaker, verify the following:

• The membrane flow direction matches the vessel flow labels.
• The brine-side end cap was installed before the membrane.
• The membrane was inserted from the feed side.
• All external and internal O-rings were installed and lubricated.
• The end caps are fully and evenly seated.
• The closure plates are parallel.
• All tie rods are straight and evenly tightened.
• The mounting brackets are firmly secured.
• The NPT fittings are sealed without excessive tightening.
• The permeate connector and plug are correctly installed.
• No PTFE tape has been used on O-ring-sealed connections.
• The complete vessel is securely mounted before pressure testing.

💧 20. Initial Pressure Test

The first pressurisation should be carried out gradually while every end-cap, high-pressure and permeate connection is observed.

1. Establish stable low-pressure feed-water flow.
2. Confirm that the vessel is completely flooded and free of trapped air.
3. Inspect all low-pressure connections.
4. Start the high-pressure pump with the regulating valve open.
5. Increase pressure slowly.
6. Check both end caps, high-pressure nipples and permeate fittings for leakage.
7. Stop immediately if any component shifts, leaks or becomes misaligned.

The BlueGold 3-inch pressure vessel performs the same basic function as the 2.5-inch version, but its construction is different. The end caps, retaining plates, hydraulic inserts and membrane spacers are specific to the 3-inch design, so the assembly procedure must be followed separately.

This guide describes the components and assembly sequence of the BlueGold 3-inch pressure vessel for a standard 3021 seawater reverse osmosis membrane.

📦 1. Components Included

A standard BlueGold 3-inch pressure-vessel assembly includes:

• 1 pressure tube: 21-inch length for a 3021 seawater membrane.
• 2 Delrin end caps: One for the feed end and one for the brine end.
• 2 round closure plates: These retain the feed, brine and permeate inserts inside the end caps.
• 2 square closure plates: These secure the complete end-cap assemblies to the pressure tube.
• 4 large external O-rings: Two for each end cap, sealing against the internal wall of the pressure tube.
• 2 internal membrane O-rings: These seal the membrane permeate tube inside the end caps.
• 4 O-rings for the ¾-inch feed and brine inserts.
• 4 O-rings for the ¼-inch permeate insert and permeate plug.
• 2 plastic membrane spacers: One fitted at each end of the membrane.
• 2 AISI 316L stainless-steel inserts: One for feed and one for brine.
• 1 Delrin ¼-inch permeate insert.
• 1 permeate-side plug.
• 4 stainless-steel tie rods with protective PVC coating.
• 8 stainless-steel washers, 10 × 30 mm.
• 8 stainless-steel M10 nuts.
• 8 stainless-steel M10 cap nuts.
• 1 set of identification and flow-direction labels.

🧰 2. Tools and Materials Required

• Suitable spanners for the M10 nuts
• Clean silicone grease compatible with membrane and potable-water applications
• Clean lint-free cloth
• Marker for temporary flow-direction identification
• Anaerobic thread sealant for the high-pressure fittings
• PTFE tape for the permeate fitting only

➡️ 3. Mark the Flow Direction

Before installing any internal component, identify the intended feed and brine ends of the pressure tube.

Mark the seawater flow direction temporarily on the outside of the tube:

The permanent labels supplied with the vessel can be applied after final assembly.

⭕ 4. Prepare Both End Caps

Before inserting either end cap into the pressure tube, install all the required O-rings in their correct grooves.

For Each End Cap

• Install the two large external O-rings around the outside of the cap.
• Install the internal O-ring that seals around the membrane permeate tube.
• Install the two O-rings for the ¾-inch feed or brine insert.
• Install the two O-rings for the permeate insert or permeate plug.

Check carefully that every O-ring:

• Sits completely inside its groove.
• Is not twisted.
• Is not stretched or pinched.
• Is free from dirt, cuts or flattening.

🧴 5. Lubricate Only the Seals Needed for End-Cap Insertion

At this stage, apply a light film of silicone grease to:

• The two large external O-rings on the brine-side end cap.
• The internal membrane O-ring on the brine-side end cap.
• The internal wall of the pressure tube near the brine end.

Do not yet lubricate the O-rings for the feed, brine, permeate insert or plug. These will be lubricated immediately before those components are installed later.

🚿 6. Install the Brine-Side End Cap First

Identify the end of the pressure tube marked as the brine outlet.

Align the prepared brine-side end cap with the tube and press it inward evenly.

Because the 3-inch vessel uses large external O-rings and a close mechanical fit, some firm hand pressure may be required.

If the cap does not enter evenly:

• Remove it.
• Inspect the O-rings.
• Confirm that the grooves and vessel wall are clean.
• Apply a little more silicone grease if necessary.
• Try again with the cap correctly aligned.

🧬 7. Prepare the 3021 Membrane

Before inserting the membrane, inspect it carefully.

Confirm that:

• The membrane is the correct 3021 format.
• The external wrapper is undamaged.
• The central permeate tube is clean.
• The membrane seal is correctly seated.
• The flow-direction arrow is clearly visible.

Install the Plastic Membrane Spacers

Fit one supplied plastic spacer at each end of the membrane before insertion.

These spacers position the membrane correctly between the two end caps and maintain the intended internal geometry of the assembled vessel.

🧴 8. Lubricate the Membrane and Feed-Side Tube Entrance

Apply a light film of silicone grease to:

• The membrane seal.
• Both ends of the membrane permeate tube.
• The internal wall of the open feed end of the pressure tube.

➡️ 9. Insert the Membrane from the Feed Side

Insert the membrane from the end of the tube that has remained open: the feed side.

The arrow printed on the membrane must point in the same direction as the flow arrow marked on the vessel.

Slide the membrane carefully into the tube and push it steadily toward the already installed brine-side end cap.

Continue until the membrane permeate tube engages fully with the internal O-ring in the brine-side end cap and the spacer reaches its correct position.

🔒 10. Install the Feed-Side End Cap

Prepare the feed-side end cap in the same way as the brine-side cap.

Apply a light film of silicone grease to:

• The two large external O-rings.
• The internal membrane O-ring.
• The internal wall of the tube near the feed end.

Align the end cap with the membrane permeate tube and press it into the vessel squarely and evenly.

Some firm pressure may again be required because of the large external O-rings.

🔩 11. Install the Feed, Brine and Permeate Inserts

The 3-inch vessel uses separate removable inserts for the feed, brine and permeate connections.

Before installing them, apply a light film of silicone grease to:

• All insert O-rings.
• The conical sealing surfaces of the inserts.
• The corresponding bores in the Delrin end caps.

Feed and Brine Inserts

Insert the two AISI 316L stainless-steel ¾-inch components into the designated feed and brine ports.

Push each insert straight into its bore until it is fully seated.

Permeate Insert and Plug

Install the Delrin ¼-inch permeate insert at the selected product-water outlet and install the permeate plug at the opposite end.

Again, press both components into their bores until they are fully and evenly seated.

⭕ 12. Install the Round Retaining Plates

Place one round closure plate over each end-cap assembly.

The round plates directly retain the feed, brine and permeate inserts inside their respective bores.

Check that:

• Each insert passes correctly through its designated opening.
• The plate sits flat against the end cap.
• No insert is tilted or trapped.
• The permeate plug and connector remain fully seated.

⬛ 13. Install the Square Closure Plates

Position one square closure plate outside each round retaining plate.

The square plates form the structural ends of the complete assembly and transfer the axial pressure load to the tie rods.

Confirm that:

• The square plate is centred on the vessel.
• The four tie-rod holes align correctly.
• The round plate remains flat behind it.
• The hydraulic inserts remain correctly seated.

🔗 14. Install and Tighten the Tie Rods

Pass the four PVC-coated stainless-steel tie rods through the aligned holes in both square closure plates.

Fit the 10 × 30 mm stainless-steel washers and standard M10 nuts on both ends.

Recommended Tightening Sequence

1. Bring all nuts into light contact.
2. Confirm that both square plates are parallel.
3. Tighten diagonally opposite rods in sequence.
4. Increase tension gradually and uniformly.
5. Check repeatedly that the plates and end caps remain centred.

🔘 15. Install the Cap Nuts

Once the standard M10 nuts have been tightened and the vessel geometry has been checked, install the stainless-steel M10 cap nuts on all tie-rod ends.

The cap nuts provide a finished external surface and protect the exposed tie-rod threads.

Complete a final visual check to confirm that:

• Both square plates remain parallel.
• The round plates remain correctly trapped behind them.
• The inserts are centred and fully retained.
• The tie rods are straight.
• No end-cap O-ring is visibly extruded.

🏷️ 16. Apply the Identification Labels

Clean the outer surface of the vessel and apply the supplied labels after assembly.

The labels should clearly indicate:

• The seawater flow direction.
• The feed connection.
• The brine connection.
• The product-water connection where appropriate.
• The 3021 membrane format.

🛠️ 17. Mounting Hardware and Hydraulic Fittings

The standard BlueGold 3-inch pressure-vessel kit does not include mounting brackets or the final external hydraulic fittings.

These items should be selected separately according to the layout and hose system used onboard.

Possible mounting and connection options may include:

• Dedicated vessel clamps or saddles.
• Custom brackets fixed to a rigid bulkhead or equipment frame.
• Flexible high-pressure hoses.
• Stainless-steel elbows or straight fittings.
• Adapters suitable for the selected feed, brine and permeate hose sizes.

🚫 18. Never Lift the Vessel by the Inserts

Do not lift, carry, rotate or position the assembled pressure vessel by holding the feed, brine or permeate inserts.

These parts are designed to provide hydraulic connections and to be retained by the closure plates. They are not handles or structural lifting points.

🧪 19. Install the External Hydraulic Fittings Only After Vessel Assembly

The final high-pressure fittings should be installed on the stainless-steel feed and brine inserts only after the complete vessel has been assembled and tightened.

This prevents the fittings from interfering with the retaining plates and avoids applying unnecessary leverage to loose inserts during assembly.

Recommended High-Pressure Thread Sealant

For the stainless-steel feed and brine fittings, BlueGold recommends a suitable anaerobic thread-sealing compound.

A highly recommended product is:

An anaerobic compound provides reliable sealing without requiring extreme tightening torque.

Advantages

• Excellent resistance to pressure and vibration.
• Uniform sealing around the threaded connection.
• Reduced need for excessive mechanical torque.
• Lower risk of damaging or twisting the removable inserts.
• No loose PTFE fragments entering the high-pressure circuit.

🚰 20. Seal the Permeate Fitting with PTFE Tape Only

The ¼-inch external fitting connected to the Delrin permeate insert should be sealed using approximately two neat turns of PTFE tape.

Apply the tape only to the male thread and keep the first thread reasonably clear.

Tighten the fitting only enough to obtain a secure and correctly oriented connection. Excessive force can damage the plastic thread or load the insert unnecessarily.

⚠️ 21. The Three Fundamental Assembly Rules

🔍 22. Final Inspection Before Mounting

Before installing the vessel onboard, verify the following:

• The membrane flow arrow matches the vessel flow labels.
• The brine-side end cap was installed first.
• The membrane was inserted from the feed side.
• Both plastic membrane spacers are installed.
• All large external O-rings are correctly seated.
• Both internal membrane O-rings are installed.
• All feed, brine, permeate and plug O-rings are present and lubricated.
• All hydraulic inserts are fully seated.
• The round retaining plates sit flat.
• The square closure plates remain parallel.
• The tie rods are straight and evenly tightened.
• The cap nuts are installed.
• No external hydraulic fitting has distorted an insert.
• The vessel will be mounted securely before pressurisation.

💧 23. Initial Pressure Test

The first pressure test should be carried out only after the vessel has been securely mounted and all hydraulic connections have been completed.

1. Establish stable low-pressure feed flow.
2. Allow the vessel to fill completely and purge trapped air.
3. Inspect the feed, brine and permeate connections at low pressure.
4. Start the high-pressure pump with the pressure-regulating valve open.
5. Increase pressure gradually.
6. Inspect both end-cap areas and every insert for leakage.
7. Stop immediately if an insert moves, an O-ring leaks or either closure plate becomes misaligned.

The final filter used in the BlueGold system is an activated carbon filter installed on the freshwater flushing line. Its purpose is to remove any residual chlorine from marina or municipal water before that water reaches the reverse osmosis membranes.

🚿 1. Why Chlorine Must Be Removed

Freshwater supplied from a marina or municipal network may contain chlorine or chloramine used for disinfection.

Although these substances are useful in a drinking-water distribution network, oxidising disinfectants can damage seawater reverse osmosis membranes.

Repeated exposure may reduce salt rejection and shorten membrane life. The damage may be gradual and is normally irreversible.

🧱 2. How the Activated Carbon Filter Works

Activated carbon has a highly porous internal structure. As flushing water passes through the cartridge, residual chlorine is removed before the water enters the watermaker.

The filter is installed only in the freshwater flushing circuit:

Its purpose is membrane protection, not the treatment of seawater or newly produced product water.

📍 3. Correct Installation Position

The carbon filter should be installed upstream of the flushing solenoid valve or manual three-way valve, so that all freshwater used for flushing passes through it.

The housing should remain accessible for cartridge replacement and should be installed with the correct flow direction where indicated.

🔄 4. Cartridge Replacement

Activated carbon has a limited chlorine-removal capacity. Once the medium is exhausted, water may still pass through the cartridge even though chlorine is no longer being removed effectively.

Replacement frequency depends on:

• The quantity of flushing water used.
• The chlorine concentration in the freshwater supply.
• The cartridge size and carbon quality.
• The time the cartridge has remained wet and unused.

Replace the cartridge periodically and whenever its condition or service history is uncertain.

⚠️ 5. Important Limitations

• The carbon filter does not replace correct membrane preservation.
• It should not be used as the main seawater prefilter.
• It does not make contaminated freshwater microbiologically safe.
• An exhausted cartridge may no longer protect the membrane even if flow remains normal.
• The housing and cartridge must be suitable for the pressure of the flushing-water supply.

At the heart of an energy-recovery watermaker is the Clark Pump: a hydraulic pressure intensifier that recovers energy from the pressurised brine stream and transfers it back into the incoming feed water.

This allows seawater reverse osmosis pressure to be achieved with a much smaller electrical input than in a conventional system driven by a large electric high-pressure pump.

🔄 1. How Energy Recovery Changes the System

In a conventional watermaker, an electric motor drives a high-pressure plunger pump continuously. The pump supplies all the mechanical energy needed to raise the seawater to reverse osmosis pressure.

In a Clark Pump system, the electric feed pump operates at a much lower pressure. The Clark Pump then uses the pressure contained in the returning brine stream to intensify that feed pressure and produce the high pressure required by the membrane.

This architecture is particularly attractive for:

• Battery-powered sailing yachts.
• Solar-supported installations.
• Long-distance cruising boats.
• Systems designed to minimise generator or inverter use.
• Installations where low electrical consumption is more important than maximum production per hour.

⚙️ 2. The Clark Pump as a Pressure Intensifier

A Clark Pump contains hydraulically linked pistons with different effective areas. Pressure acting on the larger side of the mechanism produces a higher pressure on the smaller high-pressure side.

The percentage assigned to a Clark Pump—commonly 10%, 15% or 20%—describes the relationship between the effective high-pressure piston area and the larger drive area.

In simplified terms, this percentage determines the approximate pressure multiplication ratio:

📐 3. Why a Lower Percentage Produces More Pressure

A lower percentage corresponds to a greater pressure multiplication ratio.

For example:

• A 20% unit multiplies pressure by approximately five.
• A 15% unit multiplies pressure by approximately 6.7.
• A 10% unit multiplies pressure by approximately ten.
• A theoretical 7% unit would multiply pressure by more than fourteen.

At first glance, a very low percentage may appear attractive because it can achieve high membrane pressure from relatively modest feed-pump pressure.

In practice, however, an excessively high multiplication ratio creates a narrow and potentially difficult operating window.

💥 4. Why BlueGold Avoids Clark Pumps Below 10%

A Clark Pump below 10% can generate very high membrane pressure from a feed pump that appears to be operating at a moderate pressure.

For example, if a feed pump pressure switch allows the system to approach approximately 120 psi, a very low-percentage Clark Pump may theoretically intensify that pressure beyond the safe working range of:

• RO membranes.
• Pressure vessels.
• High-pressure hoses.
• Fittings and connectors.
• Pressure sensors and gauges.
• The Clark Pump itself.

Cold seawater, high salinity, fouling or excessive membrane resistance can increase high-side pressure even further.

🌡️ 5. Why Operating Conditions Affect Clark Pump Pressure

The final high pressure in an energy-recovery system is not established by a conventional manually adjusted needle valve in the same way as a traditional plunger-pump watermaker.

It develops from the interaction between:

• Feed-pump pressure and flow.
• Clark Pump multiplication ratio.
• Membrane resistance.
• Feed-water temperature.
• Feed-water salinity.
• Membrane area and condition.
• Brine and product-water flow.

Conditions That Tend to Increase Pressure

• Colder seawater.
• Higher salinity.
• Reduced membrane permeability.
• Membrane fouling or scaling.
• Excessive feed-pump pressure.
• Insufficient membrane area for the applied flow.
• Restrictions in the high-pressure or brine circuit.

🧬 6. BlueGold Enhanced Clark Pumps

BlueGold systems use third-generation Enhanced Clark Pumps manufactured by ElectroMaax in Canada.

These units are intended for continuous seawater reverse osmosis operation and are selected according to the pressure, flow and membrane requirements of the complete watermaker.

BlueGold configurations use 10%, 15% and 20% versions depending on:

• The feed-pump pressure available.
• The required seawater flow.
• The total membrane surface area.
• The intended freshwater production.
• The expected seawater temperature and salinity.
• The pressure limits of the complete high-pressure circuit.

🔟 7. Typical Use of a 10% Clark Pump

A 10% Clark Pump offers a high pressure-multiplication ratio and can reach normal seawater RO pressure with comparatively modest feed pressure.

It is commonly suited to compact energy-recovery systems using one or two membrane elements, depending on the specific membrane format and feed-pump characteristics.

Possible membrane formats include:

• 2521 membranes.
• 3021 membranes.
• 2540 membranes

1️⃣5️⃣ 8. Typical Use of a 15% Clark Pump

A 15% Clark Pump has a lower multiplication ratio than the 10% version and therefore requires greater feed pressure to reach the same membrane pressure.

It can be advantageous in higher-flow systems with more membrane area, where the lower multiplication ratio creates a broader and more controllable feed-pressure range.

2️⃣0️⃣ 9. Typical Use of a 20% Clark Pump

A 20% Clark Pump produces approximately five times the feed pressure in simplified theoretical terms.

It therefore requires a feed pump capable of supplying a higher pressure, but the reduced multiplication ratio can make the high-pressure side easier to control in systems with substantial membrane area and high feed flow.

⚖️ 10. Comparing 10%, 15% and 20% Versions

🔋 11. Feed Pump Selection Is Critical

The feed pump in a Clark Pump watermaker performs a different role from the low-pressure booster pump used in a conventional high-pressure watermaker.

It must provide:

• The flow required by the Clark Pump and membranes.
• The pressure needed for the selected intensification ratio.
• Stable operation over the full watermaker duty cycle.
• Enough reserve capacity for cold or high-salinity seawater.
• Protection against excessive discharge pressure.

🛡️ 12. Feed-Pump Cutoff Protection

The feed pump should include a correctly selected pressure cutoff or another reliable means of preventing excessive pressure.

The cutoff value must be chosen in relation to the Clark Pump percentage.

A pressure limit that is safe for a 20% unit may be dangerously high when used with a 10% unit.

Pressure protection should consider:

• Maximum normal feed pressure.
• Feed-pump bypass setting.
• Pressure-switch cutoff tolerance.
• Clark Pump ratio.
• Maximum vessel and membrane pressure.
• Possible pressure increase in cold or saline water.

🔁 13. Why BlueGold Prefers Feed Pumps with Adjustable Bypass

BlueGold favours vane feed pumps equipped with an internal bypass where suitable for the system.

The bypass allows the operator or installer to fine-tune the delivered pressure and flow without switching an entire pump on or off.

With an adjustable bypass, pressure and flow can often be reduced by only the amount required—for example 10–15%—to compensate for:

• Colder seawater.
• Higher regional salinity.
• Seasonal changes.
• A different membrane configuration.
• Normal variation in system resistance.

🎚️ 14. Adjusting Pressure Without Sacrificing Excessive Flow

Reducing feed pressure too aggressively also reduces the flow available to the Clark Pump and membranes.

This can lower production and reduce the cross-flow needed to carry rejected salts away from the membrane surface.

The objective is therefore to:

• Maintain the highest safe feed flow.
• Keep intensified pressure below the system limit.
• Preserve adequate brine flow.
• Avoid unnecessary electrical consumption.
• Maintain stable cycling of the Clark Pump.

🔍 15. Pressure Is a Result, Not a Direct Setting

In a conventional system, the operator normally adjusts a needle valve until the desired membrane pressure is reached.

In a Clark Pump system, high pressure emerges from the combination of:

• Feed pressure.
• Intensification ratio.
• Membrane resistance.
• Feed and brine flow.
• Temperature and salinity.

This means that a gradual increase in high pressure may be an early sign of:

• Colder water.
• Higher salinity.
• Membrane fouling.
• Restriction in the brine circuit.
• Excessive feed-pump pressure.

📊 16. Parameters to Monitor

A properly commissioned energy-recovery system should be monitored using several measurements rather than one pressure value alone.

In a Clark Pump watermaker, the vane pump is normally the main feed pump: it draws seawater from the intake and supplies the flow and pressure required by the Clark Pump.

A separate low-pressure booster pump can be added before it, but in most installations it is not strictly necessary.

⚙️ 1. Why a Booster Pump Is Usually Not Required

Rotary vane pumps have good self-priming capability and can create enough suction to lift seawater from the intake, provided that:

• The suction line is correctly sized.
• The hose run is reasonably short.
• There are no air leaks in the intake circuit.
• The through-hull and strainer are unobstructed.
• The pump is in good mechanical condition.

For this reason, a vane feed pump can normally be connected directly to the seawater intake without an additional centrifugal or magnetic-drive booster pump.

🌊 2. Recommended Installation Without a Booster Pump

In the simplest arrangement, seawater follows this path:

This arrangement is compact, efficient and uses fewer electrical and hydraulic components.

For best results:

• Place the feed pump as low as practical.
• Keep the suction hose short and direct.
• Avoid unnecessary elbows and restrictions.
• Use a hose diameter appropriate for the required feed flow.
• Mount the pump where the intake and strainer can be inspected easily.
• Check all suction-side hose clamps and fittings for air leaks.

💨 3. Why Cavitation Must Be Avoided

Although vane pumps are self-priming, they should not be expected to operate continuously with a restricted or air-filled suction line.

If the pump cannot receive enough seawater, local pressure inside the pump can fall sufficiently for vapour bubbles to form. Their repeated formation and collapse is known as cavitation.

Cavitation may cause:

• Irregular or noisy pump operation.
• Reduced feed pressure and flow.
• Excessive vibration.
• Overheating.
• Damage to the internal vane surfaces.
• Chipping, cracking or complete failure of the vanes.

Vane pumps are relatively simple and economical components, but replacing one at sea is still unnecessary trouble. Regular intake inspection is far easier than repairing damage caused by cavitation.

🧽 4. Inspect the Seawater Strainer Regularly

The seawater strainer is the first defence against leaves, seaweed, shells, plastic fragments and other debris entering the feed circuit.

Because the vane pump is capable of producing significant suction, a partially blocked strainer may not immediately stop the system. Instead, the pump may continue operating with insufficient water supply.

Inspect the strainer:

• Before starting the watermaker after a long period of inactivity.
• More frequently in harbours, shallow bays and areas with seaweed.
• Whenever feed pressure or flow becomes unstable.
• Whenever the vane pump becomes noisier than usual.
• After operating in visibly polluted or debris-filled water.

⬆️ 5. What If the Feed Pump Is Above the Waterline?

A vane feed pump can still operate above the waterline because it is self-priming. A booster pump therefore remains optional even in this arrangement.

However, an above-waterline installation is less forgiving.

Greater attention must be paid to:

• The vertical lift from the through-hull to the pump.
• The total length of the suction hose.
• Air leaks at fittings and hose connections.
• Restrictions caused by the strainer or check valves.
• Maintaining the pump’s initial prime.
• Regular inspection of the entire intake path.

➕ 6. When a Booster Pump Can Be Useful

A booster pump may be worth adding when the installation has one or more difficult suction conditions.

Typical examples include:

• The vane pump must be installed well above the waterline.
• The seawater hose run is long.
• The intake circuit contains several bends or valves.
• The through-hull cannot be positioned close to the system.
• The boat’s layout makes reliable self-priming difficult.
• The system is intended to prime automatically after long idle periods.
• Additional protection against suction-side restrictions is desired.

In these cases, the booster pump does not replace the vane pump. It simply provides a small positive inlet pressure to the main feed pump.

🔄 7. Typical Arrangement with a Booster Pump

When a booster pump is installed, the seawater path becomes:

The booster pump should be:

• Suitable for continuous seawater use.
• Correctly sized for the full feed-flow requirement.
• Installed below the waterline whenever it is not self-priming.
• Protected from running dry.
• Wired to operate whenever the vane feed pump is running.

🛡️ 8. Why the Prefilter Is Installed After the Vane Feed Pump

The prefilter position in a Clark Pump system differs from the arrangement commonly used in a conventional high-pressure watermaker.

In the BlueGold energy-recovery layout, the fine prefilter is installed:

There are two important reasons for this arrangement.

Protecting the Clark Pump from External Contamination

The prefilter removes suspended particles from the seawater before they enter the Clark Pump and membrane circuit.

Protecting the Clark Pump from Vane Damage

If an internal vane were ever to chip or break, fragments would travel downstream from the feed pump.

Placing the prefilter after the feed pump allows the filter to capture these fragments before they can enter the Clark Pump.

📈 9. The Prefilter Must Be Rated for Feed-Pump Pressure

A vane feed pump does not operate at the very low pressure normally found upstream of a traditional watermaker’s high-pressure pump. Depending on the selected Clark Pump ratio and system configuration, the filter housing may be exposed to several bars of continuous pressure.

The prefilter housing must therefore be specifically rated for:

• The maximum normal feed-pump pressure.
• The feed-pump bypass setting.
• The pressure-switch cutoff value.
• Possible pressure surges during startup and cycling.
• Continuous marine operation.

🧰 10. Why BlueGold Uses a Cintropur Filter

BlueGold supplies a Cintropur prefilter selected for operation at the pressures generated by the vane feed pump.

This type of housing is used because it provides:

• An appropriate pressure rating for the feed side of the Clark Pump.
• A robust housing and closure design.
• Good internal flow capacity.
• Low pressure loss when correctly sized.
• Easy inspection and maintenance.
• Reliable protection of the downstream Clark Pump.

🚫 11. Avoid Standard Domestic or Low-Cost Watermaker Housings

Below the result of a test using a standard bowl, which is perfectly suitable for systems with high pressure pump, but not with pressures over 7 bars generated by a vane pump

Many transparent filter housings used in conventional watermakers are installed on the low-pressure suction or booster side. They may be perfectly adequate in that position but unsuitable after a pressurised vane feed pump.

Lower-quality housings may be operating close to their structural limit at the pressures required by a Clark Pump system.

Possible failure modes include:

• Cracking of the transparent bowl.
• Deformation of the threaded closure.
• O-ring extrusion.
• Sudden leakage.
• Separation or rupture of the bowl under pressure.

🔍 12. Monitor Pressure Before and After the Filter

Where practical, pressure measurement before and after the prefilter provides useful information about both feed-pump operation and filter condition.

An increasing pressure difference may indicate:

• A clogged filter element.
• Insufficient filter capacity.
• Excessive feed flow.
• A collapsed or incorrectly installed cartridge.

A clogged downstream filter can create several simultaneous problems:

• Higher pressure at the feed-pump outlet.
• Reduced flow to the Clark Pump.
• Unstable Clark Pump cycling.
• Reduced freshwater production.
• Increased risk of reaching the feed-pump cutoff pressure.

⚖️ 13. With or Without a Booster Pump?

✅ 14. Recommended Practice

• Install the vane feed pump below the waterline whenever possible.
• Use a dedicated, correctly sized seawater intake.
• Keep the suction circuit short, airtight and unrestricted.
• Inspect the seawater strainer regularly.
• Stop the system immediately if the vane pump cavitates.
• Add a booster pump only where the installation genuinely benefits from positive inlet pressure.
• Install the fine prefilter after the vane feed pump.
• Use a filter housing rated for the full feed-pump pressure.
• Do not use inexpensive domestic housings on the pressurised feed side.
• Mount every filter housing where leakage can be detected and contained.

After leaving the reverse osmosis membranes, the product water may look perfectly clear even when its salt content is still too high. For this reason, water quality should be checked before the freshwater is allowed to enter the boat’s tank.

In BlueGold systems, TDS is measured immediately downstream of the membrane outlet. The sensor continuously monitors the product water and sends an electrical signal to the touch-screen controller, where the value is processed, displayed and used to control the freshwater diverting sequence.

💧 1. What Does TDS Mean?

TDS stands for Total Dissolved Solids. It is commonly expressed in:

• ppm — parts per million
• mg/L — milligrams per litre

For the low concentrations normally found in freshwater, these two units are often treated as approximately equivalent.

A TDS reading gives an estimate of the total amount of dissolved ionic material in the water, including salts and minerals.

⚡ 2. Why Conductivity Can Be Used to Estimate TDS

Pure water conducts very little electricity. When salts dissolve in water, they separate into electrically charged ions, which increase conductivity.

A TDS sensor measures this conductivity and converts it into a value that is easier for the operator to interpret.

In practical terms:

• Higher conductivity usually means more dissolved salts.
• Lower conductivity usually means less dissolved salt.
• A healthy seawater RO membrane should reduce conductivity dramatically.

📍 3. Where TDS Is Measured in a BlueGold System

The TDS sensor is installed immediately after the product-water outlet from the membrane or membrane group.

The typical sequence is:

The exact order of the TDS sensor and flow meter may vary according to the hydraulic layout, but both are installed before the water is finally accepted into the tank.

Positioning the sensor close to the membrane outlet allows the controller to evaluate the actual product water before it becomes mixed with water already present in the tank.

🖥️ 4. TDS Monitoring in BlueGold Systems

BlueGold systems use a compact electronic TDS interface based on a widely available microcontroller-compatible measurement principle, commonly used in Arduino projects.

The system consists of:

• A conductivity or TDS probe in contact with the product water.
• An electronic interface that converts the probe signal into a stable electrical measurement.
• A connection to the BlueGold touch-screen controller.
• Software that processes, compensates and displays the reading.

The controller receives the sensor signal, converts it into a TDS value and displays the result directly on the touch screen.

Blue Gold non proprietary OPEN SOURCE control panel for watermakers based on ESP32-P4 available on Github for non commercial purposes.

• The product-water quality is visible throughout the operating cycle.
• The controller can detect when the initial high-TDS water has cleared.
• The diverting valve can be operated automatically.
• Unexpected changes in membrane performance can be detected quickly.
• The operator does not need to collect repeated manual samples.

🚿 6. Why TDS Is Normally High at Startup

When a watermaker has been stopped, water remains inside the membrane and product-water lines.

During this period:

• Salt can diffuse across the membrane.
• Residual water may become more concentrated.
• Freshwater used for flushing may mix with the first new product water.
• Preservation or cleaning residues may still be present after maintenance.

As a result, the first product water may not represent normal membrane performance.

The system should therefore discharge this initial water overboard until:

• The TDS value has stabilised.
• The reading is below the chosen acceptance threshold.
• Any cleaning or preservation residue has been fully removed.

🎚️ 7. TDS and the Diverting Valve

In an automated system, the controller compares the measured TDS with a programmed limit.

If the reading is too high:

• The diverting valve sends the product water to discard.
• The freshwater tank remains isolated.

Once the TDS falls below the selected value and remains stable for the required time:

• The controller changes the diverting valve position.
• Accepted product water is sent to the freshwater tank.

🌡️ 8. Temperature Affects TDS Measurement

Electrical conductivity changes with water temperature. The same water sample can therefore produce different conductivity readings at different temperatures.

Many electronic TDS systems use temperature compensation to convert the measurement to a standard reference temperature, commonly 25°C.

Where no automatic compensation is available, readings should be compared under similar temperature conditions.

🧮 9. TDS Conversion Factors

Conductivity and TDS are related, but they are not identical measurements.

Electronic meters normally estimate TDS using a conversion factor:

Different instruments may use different factors, depending on the salts they are intended to approximate.

This means that two correctly functioning meters can show slightly different TDS values when testing the same sample.

🧪 10. Manual TDS Testing

A fully automatic TDS system is convenient, but manual testing remains a valid alternative.

The simplest procedure is:

1. Allow the watermaker to start producing water.
2. Keep the product water directed to discard.
3. Collect a fresh sample in a clean glass or container.
4. Insert a portable TDS meter into the sample.
5. Wait for the reading to stabilise.
6. Rinse the meter after use with clean freshwater.
7. Direct water to the tank only after an acceptable reading has been confirmed.

📱 11. Portable TDS Meters

Many portable TDS instruments are available, ranging from simple pocket meters to professional conductivity instruments.

Basic Pocket Meters

• Compact and inexpensive.
• Useful for occasional checks.
• Easy to keep as an emergency backup.
• Accuracy and repeatability vary significantly.
• Some models cannot be recalibrated.

Better-Quality Portable Instruments

• More stable readings.
• Replaceable or higher-quality probes.
• Calibration functions.
• Automatic temperature compensation.
• Clearer documentation of measurement range and accuracy.

🔍 12. What to Check When Buying a Portable Meter

• Measurement range appropriate for low-TDS product water.
• Published accuracy specification.
• Resolution at low conductivity.
• Automatic temperature compensation.
• Possibility of calibration.
• Availability of calibration solution.
• Replaceable battery.
• Water-resistant construction.
• Stable readings rather than rapidly changing numbers.

🧭 13. Use a Portable Meter as a Reference

Even when continuous electronic monitoring is installed, keeping a separate handheld meter onboard is good practice.

It can be used to:

• Verify the installed sensor.
• Check calibration.
• Test water directly at the membrane outlet.
• Compare tank water with newly produced water.
• Continue operating manually if the fixed sensor fails.

🛠️ 14. Calibration

All conductivity-based TDS sensors should be checked periodically.

Calibration should be carried out using a proper conductivity or TDS reference solution with a known value.

A good calibration procedure includes:

1. Rinse the probe with clean water.
2. Shake off excess water without wiping the sensing surfaces aggressively.
3. Place the probe in fresh calibration solution.
4. Allow the reading and temperature to stabilise.
5. Adjust the meter or controller to the reference value.
6. Rinse the probe after calibration.

🧼 15. Sensor Maintenance

The TDS probe is installed on the freshwater side, so it is generally exposed to a much cleaner environment than seawater sensors. Even so, contamination can affect readings.

Possible causes include:

• Mineral deposits.
• Biofilm inside stagnant product-water tubing.
• Residue from membrane cleaning chemicals.
• Oil or grease contamination.
• Air bubbles trapped around the probe.

Inspect and clean the sensor according to its design and manufacturer instructions.

💨 16. Avoid Air Around the Sensor

The probe must remain properly wetted by product water. Air bubbles around its sensing surfaces can produce unstable or falsely low readings.

For reliable installation:

• Position the sensor where the chamber fills completely.
• Avoid a high point where air can collect.
• Maintain a steady product-water flow.
• Purge the line after maintenance.
• Observe whether readings become stable once air has cleared.

📊 17. What Is an Acceptable TDS Value?

There is no single universal number that applies to every installation, tank and operating condition.

The selected acceptance limit should consider:

• The quality of the feed seawater.
• The membrane manufacturer’s rejection specification.
• Feed-water temperature.
• Operating pressure.
• The intended use of the product water.
• Local drinking-water requirements.
• The accuracy of the installed measuring device.

📈 18. Causes of Increasing Product-Water TDS

A higher reading does not always mean that the membrane must be replaced.

Possible causes include:

• Higher seawater temperature.
• Higher feed-water salinity.
• Insufficient membrane pressure.
• Startup salt passage.
• Incorrect sensor calibration.
• Air bubbles around the probe.
• Contaminated sampling equipment.
• Damaged membrane O-rings or permeate seals.
• Excessive product-water backpressure.
• Membrane fouling, ageing or chemical damage.

🚨 19. Typical Warning Signs

✅ 20. Recommended BlueGold Practice

• Measure product-water TDS immediately after the membranes.
• Keep the water directed to discard during startup.
• Transfer water to the tank only after the TDS has stabilised below the selected limit.
• Use continuous monitoring where automatic control is desired.
• Keep a portable meter onboard as an independent reference.
• Calibrate both fixed and portable instruments periodically.
• Do not rely blindly on extremely inexpensive no-brand meters.
• Investigate trends before replacing membranes.
• Keep the sensor chamber free from air and contamination.
• Record normal operating values for future comparison.

Measuring product-water flow is useful for confirming actual watermaker output, comparing performance over time and detecting changes caused by temperature, salinity, pressure or membrane condition.

BlueGold systems use two different Hall-effect flow sensors, selected according to the expected freshwater production range. Each sensor generates electrical pulses as water passes through it, allowing the touch-screen controller to calculate and display the current flow rate and total water produced.

⚡ 1. Hall-Effect Flow Sensors

A Hall-effect flow sensor contains a small internal rotor. As product water passes through the sensor, the rotor turns and produces a series of electrical pulses.

The controller counts these pulses and converts them into:

• Instantaneous product-water flow.
• Litres produced during the current cycle.
• Total accumulated production, where configured.

Because very small and larger watermakers operate at quite different flow rates, BlueGold uses two sensor sizes rather than forcing one sensor to cover every application.

📊 2. Variable-Area Flow Meters

A variable-area flow meter, often called a rotameter, provides a simple mechanical indication of product-water flow.

Water enters the lower part of the transparent tapered tube and lifts a float. The height of the float corresponds directly to the flow rate shown on the graduated scale.

Main advantages include:

• No electrical supply.
• No electronic calibration.
• Immediate visual reading.
• Very low cost.
• Easy troubleshooting.
• Good reliability in simple manual systems.

🔄 3. Clark Pump Systems and Pulsating Flow

Small Clark Pump watermakers do not always produce a perfectly continuous product-water stream. The reciprocating action of the energy-recovery unit can create visible flow pulsations.

An electronic Hall sensor will still detect this flow, but the displayed instantaneous value may fluctuate unless the controller applies suitable averaging.

A variable-area flow meter naturally dampens these pulsations and gives the operator a clear visual indication of the average production rate.

⚖️ 4. Why BlueGold Often Prefers Mechanical Flow Measurement on Small ERP Systems

In compact energy-recovery watermakers, the Clark Pump automatically establishes membrane pressure according to feed pressure, membrane resistance, temperature and salinity.

The operator therefore does not need a complex electronic flow-control system simply to make the watermaker function correctly.

For these smaller systems, BlueGold often prefers a variable-area flow meter because it:

• Handles pulsating product-water flow well.
• Provides all the information needed during normal operation.
• Avoids unnecessary sensors, wiring and software.
• Reduces the number of possible failure points.
• Keeps the system easier to understand and maintain.
• Avoids increasing the final system price without a meaningful practical benefit.

🧭 5. Which Type Should You Choose?

The diverting valve is installed downstream of the membranes and water-quality monitoring system. Its purpose is simple: it directs product water either to the freshwater tank or to the discard line.

During startup, product-water TDS is normally higher and may take some time to stabilise. Until the measured value falls below the selected limit, the water is sent to discard. Once the water quality is acceptable, the valve redirects the flow toward the freshwater tank.

⚡ 1. The BlueGold Electric Diverting Valve

BlueGold systems use an electrically operated three-way solenoid valve controlled directly by the touch-screen display.

The controller evaluates the TDS reading and selects the correct outlet:

• High or unstable TDS: Product water is sent to discard.
• Acceptable and stable TDS: Product water is sent to the freshwater tank.

The valve can also be operated according to the programmed startup and shutdown sequence of the watermaker.

🧲 2. Solenoid Valve Versus Motorised Valve

A solenoid valve is different from a motorised ball valve.

The BlueGold diverting valve uses an electromagnetic solenoid to change the internal flow path almost immediately when electrical power is applied or removed.

A motorised valve instead uses a small electric motor and gearbox to rotate a ball or other valve element between positions. It normally moves more slowly and contains additional mechanical parts.

🕹️ 3. Manual Alternative for DIY Systems

In a manually operated DIY watermaker, the electric solenoid valve can be replaced by a standard three-way ball valve suitable for potable water.

The operator simply selects between:

• The discard outlet during startup and whenever water quality is uncertain.
• The freshwater tank once an acceptable TDS reading has been confirmed.

✅ 4. Recommended Practice

• Default the system to discard during startup.
• Send water to the tank only after TDS has stabilised below the selected limit.
• Use materials suitable for potable water.
• Select a valve with minimal flow restriction.
• Clearly label the tank and discard positions on manual valves.
• Return the valve to discard before the next startup.

A correctly operating reverse osmosis membrane provides an extremely effective microbiological barrier. Product water leaving an intact membrane therefore has a very low microbiological load, and an ultraviolet lamp is normally unnecessary between the watermaker and the freshwater tank.

🚫 1. Why UV Before the Tank Has Little Value

Installing a UV lamp directly after the watermaker treats water that has already passed through the reverse osmosis membrane.

Once this water enters the freshwater tank, however, it may be exposed again to:

• Microorganisms already present inside the tank.
• Biofilm in hoses and fittings.
• Air entering through the tank vent.
• Water loaded from a marina or shore supply.
• Contamination introduced during maintenance or filling.

✅ 2. The Correct Position for a UV Lamp

If UV disinfection is desired, it should be installed downstream of the freshwater tank, on the water-supply side of the system.

For maximum protection, the lamp should be located as close as practical to the point of use, such as the drinking-water tap.

Positioning the lamp close to the outlet also reduces the length of pipe in which bacteria could grow after treatment.

📍 3. Simple Placement Summary

Reverse osmosis removes not only salts and contaminants, but also most of the natural minerals and alkalinity present in seawater. The resulting product water is very soft and poorly buffered, which means that its pH can change easily and the water may become chemically aggressive toward tanks, fittings and distribution components.

💧 1. Why RO Water May Need Stabilisation

Freshly produced reverse osmosis water contains very little calcium, magnesium and bicarbonate alkalinity. Even when its measured pH appears acceptable, its low buffering capacity means that it can absorb carbon dioxide and become mildly acidic.

Over time, unstabilised RO water may contribute to:

• Corrosion of metallic tanks, fittings and plumbing components.
• Leaching of metals from unsuitable distribution materials.
• An unstable or slightly acidic taste.
• Large pH variations caused by even small chemical changes.

🪨 2. The BlueGold Dolomite Postfilter

BlueGold uses a simple postfilter containing only calcined dolomite.

As the product water passes through the filter medium, it dissolves a small and controlled quantity of calcium and magnesium compounds. This helps:

• Increase mineral content.
• Raise and stabilise pH.
• Increase alkalinity and buffering capacity.
• Reduce the aggressive character of very low-mineral RO water.
• Improve the taste of the final drinking water.

📍 3. Where the Postfilter Is Installed

The remineralisation filter is installed on the product-water line after the membranes and water-quality control components, before the water enters the freshwater tank.

Only water accepted for storage should pass through the remineralisation filter. Startup water being sent to discard does not need to be treated.

⚖️ 4. When Remineralisation Is Most Important

We particularly recommend a dolomite postfilter when the boat’s freshwater tanks are filled almost exclusively with water produced by the watermaker.

In this situation, the stored water receives very little natural mineral content from other sources, so its alkalinity may remain extremely low.

If the tank is also filled periodically with marina or municipal water, the minerals and alkalinity already present in that water may partially stabilise the mixed tank contents. Remineralisation can still be beneficial, but it becomes less critical.

🔍 5. Simple Maintenance

Calcined dolomite is gradually consumed as water passes through the filter. The medium should therefore be inspected and replenished or replaced periodically.

The service interval depends on:

• The quantity of water produced.
• The initial pH and alkalinity of the RO water.
• The size of the filter housing.
• The contact time between the water and the medium.

After replacing or refilling the medium, flush the postfilter until the water runs clear before directing it to the tank.