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Rainman Technology Pty Ltd
49 Orchard Road
Brookvale, NSW 2100


The high pressure pump is the heart of your Rainman watermaker. It creates the high pressure required for the reverse osmosis membranes to extract the fresh water from seawater. We use the best quality piston pump from General Pump, an American subsidiary of Interpump Group in Italy. Although it is a very simple and highly reliable pump, basic maintenance is required to maximise its performance and life span. Many customers may never need to read beyond the Routine Pump Maintenance section of this manual. However, the rest of this manual is included to assist those that may need to perform greater levels of maintenance on more heavily used systems.

Pump Description

Depending on which Rainman watermaker model you own, there will be one of two different high pressure pumps in your system. If you have the older Mk1 electric (either AC or 12VDC) manufactured through January 2019, or any petrol (gasoline) model, your system will have a General Pump WM Series pump. If you own an electric (AC or 12VDC) Mk2 system, manufactured after February 2019, your system will have a General Pump WMR Series pump.

The internals of both pumps are almost identical with only minor changes that are mostly in the manifold (stainless steel section of the pump). The photos and exploded diagrams in this document are for the WMR, but are almost the same for the WM pumps. All procedures described in this document apply equally to the WM and WMR pumps.

general pump wm wmr hp pump

General Pump WM Series (left) and WMR Series (right) high pressure pumps

interpump wmr exploded view power end hp pump

Exploded view of WMR series pump showing pistons, manifold, and relief valve

Routine Pump Maintenance

Very little routine maintenance is required on your General Pump high pressure pump. Required maintenance is in your operations manual, but is also included here for completeness of this document.

Oil Change

When replacing the oil in the pump, use SAE-30 weight oil. General Pump generally recommends an oil change after the first 50 hours, then after each 300 hours of use thereafter. These pumps are designed for significantly higher operating pressure and flow rates, so the oil break down is slower than normal operation for this design. As such, it is not critical to maintain high discipline on this oil change schedule.

The method to change oil in your pump is outlined in the watermaker operations manual.

Preventing Cavitation

The most important maintenance required to maximise the life of your pump is to prevent pump cavitation. Symptoms of cavitation include extra noise, high pressure hose vibration, and significant pressure fluctuation. Causes are anything that restricts flow to the pump. The watermaker operations manual covers more detail, but the two most frequent causes are not changing the prefilter or lift pump impeller frequently enough.

Extended cavitation will prematurely wear the packings.

Replacing the Valves


Each of the three cylinders in the pump has two spring loaded one way valves. One valve allows water to be pushed into the cylinder by the lift pump as the piston retracts. The second valve opens as the piston forces the water under high pressure through the manifold. O-rings are used to seal around the valves.

Only one valve kit (RM01) is necessary to replace all the valves in the pump. The kit includes new O-rings and valve assemblies.

general pump wmr high pressure pump valves

Exploded view of WMR series manifold, showing valve configuration


  1. Using a 22mm spanner or socket, remove the valve cap. Examine threads and O-ring. Replace O-ring (item 1) if there is any evidence of cuts, abrasions, distortion or wear.
rainman high pressure pump valve service
gp interpump wmr hp pump valve
  1. Remove valve assembly (item 2) from valve cavity.
gp interpump wmr pump valve
  1. Remove valve seat O-ring (item 3) from valve cavity.
  2. Inspect manifold for wear or damage.
  3. Install new O-ring (item 3) in valve seat cavity.
general pump high pressure pump valve o ring
  1. Insert valve assembly (item 2) into valve cap as shown.
interpump hp pump valve assembly
  1. Coat the threads of the valve cap with anti-seize grease (such as Renolit ST-80) and reinstall valve cap. Torque to 30 Nm (22 Ft-Lbs).
rainman valve service pump

Replacing the Packings


The purpose of the packings is to allow the pump to push water through at high pressure while also preventing it from leaking out of the pump along the pistons. The hard high pressure seals contain the pressure within the manifold of the pump, but some moisture slides out with the piston on each stroke. The softer low pressure seals then prevent the water from leaking out of the pump. Normal symptoms of worn packings is water dripping from the weep holes underneath the pump.

general pump wmr hp pump packing kit

General Pump WMR packing kit

In the photo to the right are the following seals from top to bottom:

  • Low pressure seal (item 2 in diagram).This is the one with stainless steel spring visible inside .
  • Stainless ring case (item 4 in diagram) with O-ring (item 3). Ensure O-ring is on the case, and is sitting in the correct groove as per photo to the right (the one without holes). If O-rings in the kit came separated from steel casing put them on before proceeding
  • Glyd high pressure seal, (item 5 in diagram).
  • Square high pressure seal, (item 6 in diagram).
rainman gp high pressure pump packing kit

Remove Old Packings

  1. Remove head bolts using a 5mm Allen key and slide manifold away from the crankcase. Remove it as straight as possible so as not to stress the plungers or rods inside the power end of the pump. It is normal for some packing assemblies to remain on the plungers. It may be necessary to rotate the crankshaft and/or use pry bars to separate manifold from crankcase.
gp high pressure pump manifold removal
rainman wmr pump manifold removal
  1. Remove the low pressure seals (item 2) using your fingers. Take careful note of which way seals were facing in order to correctly insert the new seal later.
gp hp pump packing removal
  1. Remove the stainless steel case (item 3 / 4) where low pressure seals (item 2) were installed. If the packing kit was purchased from Rainman, a removal tool is included with your kit. If the kit was sourced from elsewhere, this tool may not be included. Important: take care not to score or mark the manifold’s machined surfaces.
rainman packing extraction tool
general pump packing kit extraction tool
  1. Remove high pressure seals (item 5 / 6) using your finger by gently pulling them upwards. If more force is necessary, try using plastic or timber tools in order to avoid damaging machined surfaces of cavity. Take careful note of the seal’s orientation in order to correctly insert the new seals later.
rainman hp pump packing technical service

Install New Packings

  1. Thoroughly clean and inspect cavity for unusual wear or cracks.
  2. Press the glyd high pressure seal (item 5) into square high pressure seal (item 6).
general pump glyd high pressure seal
  1. Set the high pressure seal insertion guide (item 8) in manifold cylinder. Do not grease any portion of the packing assembly during this step.
interpump high pressure seal insertion
  1. Gently place assembled glyd rings (items 5 / 6) inside insertion guide (item 8) so they are resting evenly
rainman pump glyd ring insertion
  1. Insert pusher tool (item 9), blunt end facing forward and firmly press down to set glyd rings (items 5 / 6) in the manifold.
interpump high pressure pump glyd ring insertion tool
  1. Set low pressure seal guide (item7) over seal casing (item 3 / 4) ensuring recess for seal is facing upwards..
rainman low pressure seal guide
  1. Gently place low pressure seal (item2) into the guide (item7) ensuring the open spring side of the seal is facing toward the casing and resting evenly.
general pump low pressure seal rainman
rainman pump low pressure seal guide
  1. Insert pusher tool (item9) into collar with narrow end facing forward. Firmly press down to set seal in casing.
rainman high pressure pump service tool
  1. Confirm that the O-ring is in the correct groove on casing (item 3 / 4). At this point you should not be able to see the spring in the low pressure seal (item 2) as it should be facing toward the casing itself.
general pump wmr low pressure seal
  1. Check that spacer rings (item1) are on each plunger as per photo. These rings are non-wearing so it is not required to replace them. Apply a very thin coating of silicone grease to each plunger.
rainman wmr high pressure pump power end
  1. Slide the casings (items3/4) on each plunger using low pressure seal guide (item7). Ensure the black low pressure seal (item2) faces towards the power end of the pump, as per photo. Ensure they are straight to prevent damaging new seals.
interpump low pressure seal casing
rainman high pressure pump packing replacement
  1. Reseat the manifold gently pushing it back into place. Install it straight with minimum wiggle.
  2. Once the manifold is properly seated, install head bolts and begin torque sequence. Tighten to 12 Nm or 8.9 Ft-lbs.
rainman hp pump reassembly

Setting Relief Valve

A relief valve is part of every Rainman watermaker as a safety precaution. It prevents build up of excessive pressure in the RO system if the control valve is accidentally closed too far. If the system does get over pressured, the relief valve opens and recirculates water through the pump to ensure no damage or high pressure leaks occur.

In normal use the relief valve remains closed at all times. If the pressure reaches approximately 62 bar (900 psi) the valve starts to open, returning some water to the intake side of the system. At approximately 69 bar (1000 psi) it will be completely open, limiting the maximum output pressure of the PSU to approximately 69 bar (1000 psi). All relief valves are set in the Rainman factory during final testing of each system. In the highly unlikely event that a relief valve needs to be replaced, it will need to be set to the correct opening pressure. This section describes the procedure for setting the correct opening pressure, using the pressure gauge on any of the Rainman RO systems. It should be emphasised that it is highly unlikely you will ever need to replace or adjust the relief valve.

All petrol/gasoline systems and original design, pre-January 2019, Mk1 electric systems utilise the VS100 relief valve, which is separate to the WM series pump itself. On Mk2 electric systems, manufactured after February 2019, the WMR pump has an integrated relief valve built into the manifold. You will need to remove the shell to access the relief valve on all cased electric systems. A separate document exists to remove the shell on Mk1 electric systems.

rainman relief valve mk2

Mk2 Electric PSU WMR Pump

rainman relief valve mk1

Mk1 Electric PSU WM Pump

rainman relief valve petrol

Petrol/gasoline PSU WM Pump

Note that the VS100 relief valve is installed upside down on the Mk1 Electric PSU with the WM pump. Directions here are given as if you are looking at the end hexagonal adjustment cap.

Once you have replaced the relief valve, and before replacing the plastic case of your PSU, reconnect the particle filter housing so that you can perform the following steps.

  1. Loosen the locking nut underneath the large hexagonal part of the relief valve.
  2. Place the pickup hose into a water source and connect the RO unit as during normal operation.
  3. With the pressure control valve on the RO system fully open (counter-clockwise), run the PSU until only clear bubble free water comes out brine hose.
general pump vs100 relief valve
  1. Turn the pressure control valve clockwise until it is fully closed whilst watching the pressure on the gauge. New relief valves from the factory are typically pre-set to a low pressure, so you should be able to close the valve fully before the pressure reaches 55 bar (800 psi). If the pressure reaches 55 bar (800 psi) before the valve is fully clockwise, then the relief valve is already set too high. In this case, turn the hexagonal part of the relief valve anti-clockwise three times and go back to step 3.
  2. With the PSU running and the pressure control valve fully closed (clockwise), turn the adjustment cap on the relief valve clockwise until the pressure gauge reaches 55 bar (800 psi).
relief valve pressure setting
  1. Fully open the pressure control valve on the RO unit (anticlockwise) and turn off the PSU.
  2. Turn the adjustment cap on the relief valve clockwise one complete turn (360°).
  3. The relief valve is now set correctly to 69 bar (1000 psi). Lock the adjustment cap using the locking nut or some other thread locker e.g. Loctite or similar.
interpump vs100 relief valve adjust


The troubleshooting table is highly technical and specific only to the high pressure pump, not your Rainman watermaker in general. In the vast majority of operational troubleshooting issues, you should utilise the table in the general operations manual.

general pump high pressure pump troubleshooting

Maintenance Log

general pump hp pump maintenance log