Iwaki Pump Head Disassembly: EHN Valves, Seals and the Check Valve

When an EHN stops dosing or the output starts wandering, the trail usually ends at the Iwaki pump head. This guide walks through taking the head and the injection check valve apart on the EHN solenoid pump, what to look at once it is open, and how to put it back so it actually primes.

Honestly, opening an Iwaki pump head is the easy part. Reassembly is where people get caught: a ball dropped in upside down, a twisted O-ring, a gasket left on the bench. The pump then leaks the moment it starts, or clicks away without moving a drop.

Before You Touch the Iwaki Pump Head

The classic mistake is grabbing a wrench and loosening a fitting while the discharge line is still under pressure. Chemical comes straight out. Work in this order:

1. Stop the pump, unplug it, and close the valves on the suction and discharge sides.
2. Release discharge pressure. Opening the manual vent valve lets the residue drain back toward the tank.
3. Put the suction hose in a bucket of clean water, run the pump for a few minutes to flush the chemical out of the Iwaki pump head, then stop and unplug again.
4. Put on gloves and goggles and set a tray under the head to catch drips.

Take a phone photo of the head before you loosen anything. Suction at the bottom, discharge at the top, which way the valve cartridges face, how the tubing runs. Matching a photo beats relying on memory.

What Is Inside an Iwaki Pump Head

According to the structure drawing in the EHN brochure, the wetted parts of a standard Iwaki pump head are the head itself, the diaphragm, gaskets, valve cartridges, valve seats, valve balls and O-rings. Each of the two valves is a ball sitting on a seat inside a cartridge, sealed by O-rings.

Materials follow the liquid end code in the model number. VC and VS have a PVC head and PVC cartridges. PC, PS and PP use GFRPP (glass fiber reinforced polypropylene). Gaskets are PTFE throughout. Balls are alumina ceramic or equivalent to SUS316, seats are FKM or EPDM, and on the PP type they are PCTFE. The diaphragm is PTFE+EPDM, with the EPDM side kept out of contact with the liquid.

Order spare parts by the original material code. VC seals are FKM and VS seals are EPDM. Fit an O-ring of the right size but the wrong rubber and it can swell and deform after a few days in the chemical.

The auto-degassing version has extra parts

On the NAE auto-degassing type the discharge side is built differently, with an automatic vent valve, a separator rod and a manual vent adjusting nut. The rod comes in titanium, equivalent to SUS316, or equivalent to Hastelloy C276. Handle that side gently, lay the parts out in order on a clean cloth, and keep them away from parts from a standard Iwaki pump head.

Valve Cartridges: Order Matters More Than Force

Once the tubing nuts are off, the cartridge comes out along with its seat and ball. Turn plastic parts by hand. Pipe wrenches have no place here: strip a thread in PVC or GFRPP and the whole Iwaki pump head has to be replaced.

Check each valve set as it comes out:

• Is the ball scratched or crusted, and does it still roll true?
• Does the seat sealing face show dents, chips or swelling?
• Are the O-rings and gaskets hard, cracked or flattened?
• Is there scale inside the cartridge that could hold the ball?

With crystallizing chemicals the usual fault is a ball stuck to its seat. A soak in warm water or a suitable cleaning solution normally frees it. Don't pry it off with anything hard.

On reassembly, both valves must face exactly the way they did before. Liquid travels from bottom to top through the Iwaki pump head, so each ball has to lift with the flow and drop back onto its seat. Put one in backwards and the pump will keep clicking without dosing anything.

How to Remove the Iwaki EHN Check Valve

The check valve here is the CAN type fitted at the end of the discharge tubing, at the injection point. It ships as a standard accessory with the EHN. The brochure is clear about its job: preventing overfeeding, backflow and siphoning. If discharge pressure is below 0.12 MPa (0.05 MPa for the BN31 and CN36), a check valve or back pressure valve has to be fitted.

Removing the Iwaki EHN check valve works much like the Iwaki pump head. Stop the pump and release pressure, slip the discharge tubing off the valve, then unscrew the valve from the injection point thread. The CAN type has R3/8 and R1/2 threads, a PVC or GFRPP body, and a spring equivalent to Hastelloy C276.

With it apart, look at two things: whether the spring is corroded or jammed, and whether the O-ring is intact. If the spring is already deformed, replace the whole valve. It saves a lot of fiddling compared with chasing individual parts.

The check valve is not optional

Some sites find the check valve a nuisance and never refit it. With a low pressure injection point and a high tank level, the chemical siphons through and the flow is out of control. A model such as the CAN-1VC-M is set at 0.17 MPa, and that back pressure is what keeps the pump working steadily.

After Reassembly: Vent, Leak Test, Recheck Flow

This last step is the one most often skipped. With everything back together, put the suction hose in clean water and run the pump. Open the manual vent valve to bleed air from the Iwaki pump head, and close it once liquid flows steadily. Then watch every fitting and the edge of the Iwaki pump head for weeping.

Only switch back to chemical once the water test is dry. Calibrate the flow with a measuring cylinder and compare it with the figure before the teardown. A big gap means a valve set is wrong, so open it up and check orientation and seals again.

Conclusion

Servicing an Iwaki pump head comes down to three habits: release pressure, flush and photograph before you start; lay parts out in order and turn plastic by hand; leak test on water and recalibrate before going back to chemical. Valve orientation and seal material cause most of the trouble. Order spares by the original liquid end code, and always refit the check valve after cleaning it.

FAQ

Do I need special tools?

Usually not. Plastic fittings and cartridges normally turn by hand. Add chemical resistant gloves, goggles and a drip tray, and follow the instruction manual for the Iwaki pump head screws.

Can I replace just the balls and seats?

Generally yes. The brochure lists these parts with separate materials. Quote the original model and liquid end code when ordering, and confirm part numbers with Iwaki or the local distributor.

It clicks but won't dose after reassembly. Why?

Most likely a valve went in backwards or there is air in the Iwaki pump head. Open the manual vent valve first. If that doesn't fix it, open the head and compare the valves with your photo.

Can a generic non-return valve replace the Iwaki EHN check valve?

Not recommended. The CAN valve's thread, set pressure and materials are matched to the EHN. A random substitute gives no guarantee on back pressure or chemical resistance.

How often should an Iwaki pump head be opened?

The brochure gives no fixed interval. It depends on the chemical and running hours. Crystallizing or dirty chemicals justify more frequent checks, and a clear drop in output means it is time to look.

Not sure which part fits your pump? Send us a nameplate photo through our contact page and we will check it for you.