Sodium Hypochlorite Dosing Pump Selection: Gas-Lock-Resistant Heads and Materials

Sodium hypochlorite is one of the most common disinfectants at waterworks, wastewater plants and swimming pools, and it's also one of the chemicals most likely to make a metering pump misbehave. The pump sounds normal, the light is flashing, and nothing goes in. Nine times out of ten that's a gas lock. Choosing a hypochlorite dosing pump still means working out flow and pressure, but the head design, materials and installation matter just as much.

Why a Hypochlorite Dosing Pump Gas-Locks

Sodium hypochlorite slowly decomposes and gives off gas, and the warmer and older the solution, the more bubbles you get. Once gas collects in the pump head, the diaphragm spends its discharge stroke compressing gas and the stroke ends before the outlet valve opens. On the suction stroke the gas expands and the inlet valve doesn't open either. The pump is moving but just pumping gas back and forth.

Gas builds up even more easily when flow is very small and stroke rate is set very low. So don't buy an oversized hypochlorite dosing pump to save money and then turn it down; aim to run in mid range.

Hypochlorite Dosing Pump Heads: Design Comes First

Heads with a bleed valve

For a hypochlorite dosing pump, choose a head with a bleed valve. The NEWDOSE solenoid manual describes the procedure: start the pump, open the bleed valve half a turn, and chemical quickly fills the head; close the valve as soon as liquid comes out. Run a tube from the bleed valve back to the tank so nothing splashes.

Without a bleed valve you can still purge, it's just a nuisance. Disconnect the discharge tube from the injection valve, wait until chemical reaches the tube, then reconnect it. Operators have to repeat that after every tank change or stop, and they soon get tired of it.

Internal head geometry and valve seats

NEWDOSE documentation states that the head's internal geometry has been proven in service to prevent gas accumulation and cavitation inside the head. The valve seats are PTFE, avoiding the swelling and corrosion that rubber seats suffer over time, which degrade sealing and accuracy. For a chemical that both gasses and corrodes, both points help a hypochlorite dosing pump stay reliable.

For small flows, NEWDOSE Evodose stepper motor pumps are also worth a look. The documentation lists resistance to air locking as a feature, with a PVC head as standard and PVDF or PTFE as options.

Materials for a Hypochlorite Dosing Pump

Hypochlorite is strongly oxidizing and contains chloride, so an SS316 stainless head is generally not recommended. The usual choice is PVC or PVDF. The NEWDOSE PVT set is a PVC head with PTFE diaphragm and seats, ceramic balls and FKM seals; the PVDF set changes only the head material. At higher concentration, warmer solution, or where other chemicals are also present, step up to PVDF or even full PTFE. Seal compatibility should be confirmed with the manufacturer using the actual concentration and temperature.

Don't forget the tubing. Solenoid pumps ship with clear PVC suction tube and translucent PE discharge tube. Outdoors in sunlight, tubing ages, so run it in a duct or shade it.

Installation Matters More Than the Model

For gassing liquids such as sodium hypochlorite and hydrogen peroxide, the NEWDOSE manual recommends mounting the pump below the tank liquid level, with the injection point above the tank level. Chemical then flows into the head on its own, air is pushed out, and the pump primes easily. That's the ideal layout for a hypochlorite dosing pump.

Common traps:

Pump mounted above the tank with a long suction line: bubbles collect in the line and the pump often fails to prime after a tank change;
Tank level higher than the injection point: the line siphons and chemical keeps flowing after the pump stops, so fit a back pressure valve on the outlet and adjust until free flow stops;
The head ports are 1/2 G parallel threads, and the manual specifically warns against wrapping PTFE tape on the head thread and screwing it into a female fitting, because it will leak over time;
Keep the pump and lines out of direct sunlight. The solenoid pump is rated for 0 to 40℃ ambient, and heat also makes hypochlorite decompose faster.

Where flow is large enough for a mechanical pump, its manual likewise recommends a bleed valve at the outlet to help start-up.

Flow Margin and Calibration

When sizing a hypochlorite dosing pump, calculate solution flow at maximum demand and add about 20% operating margin, as the NEWDOSE solenoid manual recommends. Nameplate figures are measured on clean water, and gassing hypochlorite usually gives somewhat lower real output, so calibrate on site after installation. Put the suction in a graduated container, purge the air, then time the run. The manual suggests at least 5 minutes. Work out real flow from the start and end readings and adjust the pump.

Conclusion

Three things decide a sodium hypochlorite dosing pump: a head with a bleed valve and internal geometry that doesn't trap gas; PVC or PVDF wetted parts with stainless avoided and seals checked against concentration; and a flooded installation with a back pressure valve against siphoning, away from sun and heat. Add margin, calibrate on site and dose by the measured figure, and gas lock and corrosion largely stop being a problem.

FAQ

My hypochlorite dosing pump loses prime at every tank change. What can I do?

Air most likely entered the suction tube during the change. Open the bleed valve to purge, and check whether the pump sits above the liquid level. If you can switch to flooded suction, do.

There's white crystal at the injection valve tip. Is that normal?

Hypochlorite tends to leave crystals where it dries at the outlet, more so after long stops. Clean the injection valve and check valves regularly, and before storing a pump long term, circulate clean water for 10 to 20 minutes and then switch it off.

Can I use stainless steel pipe for hypochlorite?

Generally not recommended, since chloride-bearing oxidizers can corrode stainless. Corrosion-resistant plastic pipe and valves are the safer choice.

The pump runs but residual chlorine stays low. Where do I start?

First check the head for trapped gas, then check whether the chemical has been stored too long. Hypochlorite loses available chlorine with age, and no hypochlorite dosing pump can make up for weak product.

Can a hypochlorite dosing pump also handle hydrogen peroxide?

Peroxide also gasses, so the installation rules are the same. Materials need checking for peroxide separately, so don't assume they carry over.

Send us your chemical strength, flow and injection point, and contact us to confirm the head and installation layout.