Iwaki Metering Pump Selection: EHN Auto-Degassing vs High Pressure
On a sodium hypochlorite line, the worst failure isn't a broken pump. It's a pump that clicks away happily while no chemical leaves the head. Open it up and the chamber is full of gas. For exactly this problem, the Iwaki metering pump range in the EHN series includes special versions: NAE for auto-degassing, 55 for high compression ratio, and H for injecting into pressurized boiler lines.
All three are set by position 8 of the model code. They look much alike from the outside but do very different jobs. Pick an Iwaki metering pump on flow alone and it's easy to end up with a high-pressure unit on an open tank, or a standard head on a heavily gassing chemical. Here's how to choose by duty.
Why Gassing Chemicals Make a Pump Run Dry
A solenoid Iwaki metering pump moves a small volume per stroke. Let a little gas collect in the chamber and the diaphragm compresses gas instead of liquid. The check valves never open, nothing is delivered, and you have an air lock. Sodium hypochlorite and hydrogen peroxide keep releasing gas, and it gets worse in hot weather or when the tank sits in the sun.
Standard heads have a vent valve you can turn to let the gas out, but only by hand. On a system running around the clock, nobody wants to send someone out to turn a knob every day. A typical story: the Iwaki metering pump looks fine on the day shift, the tank warms up overnight, gas builds, and by morning the residual chlorine has dropped.
EHN-NAE: The Auto-Degassing Iwaki Metering Pump
The EHN-NAE is built for liquids that gas off easily, such as sodium hypochlorite or hydrogen peroxide, and Iwaki states that air lock does not occur. An automatic vent valve is built into the chamber, and a manual vent valve is also fitted to release pressure in the discharge line.
How the automatic venting works
Gas entering at the suction port travels through the head to the fitting. Because the discharge side is under pressure, the gas heads for the automatic vent valve, where pressure is lower, and leaves gradually with some liquid. Once the air lock clears, the pump returns to normal delivery, and a small amount of liquid continues to come out of the vent valve.
That means the vent port shouldn't be left open. We usually run a line back to the chemical tank so nothing drips on the floor.
Picking a size
The auto-degassing Iwaki metering pump comes in four NAE sizes. The BN11-NAE delivers up to 30 mL/min at 1.0 MPa, the BN16-NAE 55 mL/min at 0.7 MPa, the CN16-NAE 65 mL/min at 1.0 MPa, and the CN21-NAE 110 mL/min at 0.7 MPa. Materials are VC, VC-S6, VC-HC and VS, differing mainly in the separator rod. VC-S6 and VC-HC are only for the B11 size.
Note that liquid and ambient temperature are both limited to 0–40℃ on the NAE, narrower than the standard heads.
EHN-55: A Different Answer for Hypochlorite
The EHN-55 takes another route. Instead of adding a vent valve, it reduces the dead volume in the chamber to raise the compression ratio, so gas bubbles can't eat up as much of the stroke. Iwaki lists it for sodium hypochlorite dosing.
This Iwaki metering pump comes only in VC material, in three sizes: the BN11VC-55 at 38 mL/min and 1.0 MPa, the BN16VC-55 at 65 mL/min and 0.7 MPa, and the BN21VC-55 at 100 mL/min and 0.4 MPa. Stroke length is 50–100%, and liquid temperature is again 0–40℃.
NAE or 55?
If the tank gasses a lot, the lines are long, or the pump stops and restarts often, go with NAE, because it actively removes the gas. If gassing is milder and you'd rather avoid a vent port and return line, the 55 is the simpler option. Still unsure? Iwaki also offers a degassing fitting for the pump inlet that strips bubbles from the suction line first.
EHN-H: Injecting Into Pressurized Boiler Lines
The H version is the high-pressure Iwaki metering pump. Maximum discharge pressure is 1.5 MPa, at up to 360 spm. Two models are offered, the BN11PS-H and CN16PS-H, with maximum outputs of 30 and 60 mL/min.
Iwaki's typical example is boiler chemical injection, including at the outlet of the feed pump, as long as pressure there stays within 1.5 MPa. Because the pump runs accurately at very small flows, undiluted boiler chemicals can be injected directly. The wetted end is PS: a GFRPP head with balls equivalent to SUS316 and EPDM seals, with liquid temperature 0–60℃.
As with any Iwaki metering pump, chemical resistance varies, and Iwaki warns that some chemicals may not be suitable. Boiler chemical formulas differ by supplier, so check materials before ordering.
What Every Special Iwaki Metering Pump Has in Common
All three run on AC100–240V, 50/60Hz single phase, with stroke rate set from 1 to 360 spm. The brochure's maximum output is measured on clean water at maximum discharge pressure, so output rises at lower pressure. At discharge pressures of 0.12 MPa or less, a check valve is required to prevent over-feeding.
That point gets missed most often on open-tank dosing or when injecting into an unpressurized basin. Nothing is wrong with the pump, the flow just runs above setpoint, and it takes hours to find there's no back pressure. The high-pressure Iwaki metering pump is no exception: its pressure rating is a ceiling, not a license to run at low pressure.
Conclusion
Choose a special Iwaki metering pump by duty. Gassing chemicals with frequent air lock point to NAE. Milder gassing and a preference for simplicity point to the 55. Injection into pressurized lines within 1.5 MPa calls for the H version. All three have temperature and material limits, so confirm chemical, temperature and injection pressure before you decide.
FAQ
Can a standard Iwaki metering pump be converted to auto-degassing?
We wouldn't. The NAE head has an automatic vent valve and separator rod and is built differently from a standard head. If you need it, order the NAE model.
Liquid keeps coming out of the auto vent valve. Is that normal?
A small amount of liquid during venting is normal, and Iwaki says so in the brochure. Route the vent back to the tank. If the flow looks clearly excessive, check the valve parts.
Can the H version handle a 2 MPa line?
No. The H version tops out at 1.5 MPa, and the injection point must stay within that.
Which version for hydrogen peroxide?
Iwaki lists hydrogen peroxide as a target liquid for the NAE auto-degassing type. Concentration and material compatibility still need checking.
Can the special versions take the YN controller?
In the model code, the controller is position 7 and the special version is position 8, written separately. The brochure doesn't list every combination, so confirm with Iwaki before ordering.
If you're not sure about your duty, send the chemical, concentration and injection pressure through our contact page. We'll check the Iwaki metering pump model with you and can also suggest a NEWDOSE option for the same conditions.