A PX pressure exchanger that is performing badly is not always a device that needs parts. This guide sets out the symptoms, their likely causes and the checks to make first, based on Energy Recovery’s own manuals. If it does come down to parts, we source them.
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In short: Most PX faults come from operating conditions: unbalanced flows, flow above the rated maximum, too little back pressure on the brine outlet, or air in the system. Parts are needed when the rotor has stalled on debris or the ceramic is damaged. Check flows and pressures against the limits in your manual before ordering anything.
Energy Recovery states that PX devices contain no wear parts and need no routine maintenance, and gives them a design life of 25 to 30 years. A PX that has been running correctly for years does not usually fail on its own. When performance changes, something around the device has normally changed first.
This table follows the troubleshooting guide in Energy Recovery’s 4S Series manual. The same logic applies to other PX models, but valve numbers and limits differ, so use it alongside the manual for your device.
| Symptom | Probable causes | First action |
|---|---|---|
| Excessive noise | Flow above the rated maximum on the low-pressure or high-pressure side. Brine discharge pressure below the minimum. Air in the system. Unit installed the wrong way round. Damaged ceramic. | Reduce flow at once using the booster pump and low-pressure control valve. Raise the back pressure. Bleed air. Check orientation. If none of these clears it, the ceramic may be damaged. |
| High salinity in the membrane feed | Unbalanced low-pressure and high-pressure flows. A rotor that has stopped. A booster pump fault. | Balance the flows. Listen for rotation. Check booster pump rotation, flow and pressure. |
| Excessive pressure in the RO system | High-pressure pump delivering too much flow. Recovery set too high. Low-pressure flow less than high-pressure flow. A stuck rotor. | Check pump flow against membrane capacity. Reduce recovery by increasing and balancing flows. |
| Stalled rotor, no sound of rotation | Operation above rated pressure or below the minimum flow. Debris lodged in the device. | Check pressures and flows against the limits. If they are correct, the device needs inspection. |
| Low permeate flow | High-pressure pump fault. High lubrication or leakage flow. | Verify high-pressure pump flow and pressure. Confirm every rotor is turning. |
| Low reject flow | High pressure loss through the RO system. Booster pump fault. | Check the booster pump. Refer pressure loss to the system supplier. |
Source: Energy Recovery 4S Series installation, operation and maintenance manual, troubleshooting guide.
The figures below are the limits Energy Recovery publishes for its 4S Series devices. They show the kind of limits every PX has. The values for your model are in its own manual.
| Parameter | Limit, 4S Series |
|---|---|
| Maximum high pressure | 82.7 bar (1,200 psig) |
| Maximum feed water inlet pressure | 20.7 bar (300 psig) |
| Minimum feed water inlet pressure | 1.9 bar (27 psig) |
| Minimum brine discharge pressure | 1.0 bar (15 psig) |
| Filtration | 5 micron |
| Temperature | 1–49 °C (33–120 °F) |
| pH | 1–12, short term at the limits |
Unbalanced flows. A PX works properly when the low-pressure seawater flowing in matches the high-pressure brine flowing in. If they do not match, brine mixes into the feed. Feed salinity rises, the membranes need more pressure, and permeate quality drops. Even a correctly balanced device mixes a little: Energy Recovery gives volumetric mixing of about 5% for the PX Q300 and under 3% for the PX Q400 at balanced flow.
Overflow. Energy Recovery’s manual states that the maximum feed flow to the device must never be exceeded, and that damage may occur if it is. Overflow produces noise first. On shutdown, the brine outlet valve has to be throttled so that the low-pressure side does not overflow when the high-pressure pump stops.
Low back pressure. The brine leaving the device must stay above a minimum pressure. Energy Recovery’s manual warns that without back pressure the noise level rises and destructive cavitation may occur, damaging the device.
Air and debris. All air has to be purged before the system is pressurised. Pipework has to be flushed with filtered water before a PX is installed, because foreign material can damage it. Lodged debris is one of the two causes the guide gives for a stalled rotor.
A small part of the high-pressure water passes through the rotor’s hydrodynamic bearing to the low-pressure side. This is the lubrication flow. Energy Recovery’s manual gives a simple way to measure it: the low-pressure brine flow leaving the device minus the low-pressure feed flow entering it. Energy Recovery describes monitoring it as a good way to check the integrity of an operating unit. A lubrication flow that has risen over time can point to internal leakage. Trend it, and compare it with the performance curves for your model.
Parts by model: PX-180, PX-220 and PX-260, PX Q300, PX Q400 and PX Q650. Kit numbers are in the ERI part number reference.
Many faults are created during start-up or a shutdown. Energy Recovery’s 4S manual gives this order for start-up: start the seawater supply pump, bleed air, start the booster pump after five to ten minutes and bleed again, balance the brine flow, start the high-pressure pump after a further five to ten minutes, then verify the flows are balanced and the brine reject pressure is above its minimum.
For a shutdown of more than two weeks the manual requires the high-pressure and low-pressure sides to be flushed separately with fresh water.
Fault you cannot clear, or parts needed after an inspection? Send the symptom and serial number. Plant-down enquiries get a reply within 4 hours, standard quotations within 48 hours.
Page reviewed October 2026 · Meliora FZCO, Dubai Silicon Oasis, UAE
The most common cause is unbalanced flow through the PX: the low-pressure seawater flow does not match the high-pressure brine flow, so brine mixes into the feed. Other causes in Energy Recovery's troubleshooting guide are a rotor that has stopped turning and a booster pump fault.
Energy Recovery lists five causes: operating above the rated flow, operating below the minimum back pressure on the brine outlet, air in the system, a unit installed the wrong way round, and damaged ceramic. Reduce flow and check back pressure and air first.
Either the device is operating outside its limits, above rated pressure or below minimum flow, or foreign debris has lodged in it. If pressures and flows are within limits and the rotor is still not turning, the device needs inspection.
It is the small amount of high-pressure water that passes through the rotor's hydrodynamic bearing to the low-pressure side. It is measured as the low-pressure brine flow leaving the device minus the low-pressure feed flow entering it, and a rising value can indicate internal leakage.
Energy Recovery gives volumetric mixing at balanced flow of about 5% for the PX Q300 and less than 3% for the PX Q400. Mixing above the normal level for the model usually indicates unbalanced flows.
Energy Recovery states that PX devices contain no wear parts and require no routine maintenance. Parts are normally needed only after debris, overflow or cavitation damage, or when a device has been opened and its seals need replacing.