Sodium Hypochlorite Dosing Pump: How Hypochlorite Feed Works and Why It Vapor Locks

A sodium hypochlorite dosing pump does a simple job: it injects a measured volume of hypochlorite solution into a pipe or basin in proportion to the water flow. In water, hypochlorite forms hypochlorous acid, which kills bacteria and oxidizes organics and other reducing substances, and the free chlorine left over, the residual, tells you whether the dose was enough. The principle is easy. The chemistry is what causes trouble. Hypochlorite is unstable and slowly breaks down, giving off gas the whole time. When those bubbles reach the pump head, the diaphragm keeps moving but nothing comes out. That is vapor lock, also called gas lock, and it is the most common complaint on chlorination duty. This article explains why it happens. Step-by-step fixes are covered in a separate guide.

How a hypochlorite feed system works

The feed chain has three sections.

Metering. Chemical leaves the storage or day tank through a foot valve and suction line, and the sodium hypochlorite dosing pump discharges a fixed volume per stroke at the set stroke length and rate.

Delivery. The discharge passes a relief valve, back pressure valve and injection valve on the way to the injection point. Lines are usually PVC, and every valve and fitting has to resist oxidation.

Reaction. The chemical mixes with the water. Part of the chlorine is consumed by organics and ammonia, and the remainder is the residual. Control is usually flow-paced, trimmed by an online chlorine analyzer.

Why hypochlorite gives off gas

Sodium hypochlorite solution decomposes slowly into sodium chloride and oxygen. Several things speed that up:

1. Heat. A hot dosing room or a tank in the sun decomposes much faster.
2. Strength. Concentrated product is less stable, which is one reason many plants dilute before dosing.
3. Light. Clear tubing and translucent tanks in daylight lose strength faster.
4. Contaminants. Metal ions catalyze breakdown, so rusty or brass fittings do not belong in hypochlorite service.
5. Age. Old product has lost available chlorine and carries more dissolved gas.

On top of that, the suction line runs under slight vacuum, and dissolved gas comes out of solution more easily at low pressure. That is the direct reason gas collects in the head.

Why bubbles stop a sodium hypochlorite dosing pump

A metering pump works by changing the volume of the pump chamber with each stroke of the diaphragm. When the chamber holds only liquid, which is nearly incompressible, every bit of diaphragm travel pushes out the same volume. Put a pocket of gas in there and the picture changes. On the discharge stroke the diaphragm spends its travel compressing the gas, and the stroke ends before pressure is high enough to open the discharge valve. On the suction stroke the gas expands again and the suction valve never opens.

The diaphragm moves, but the pump delivers nothing. Solenoid pumps have short strokes and a relatively large dead volume, so a small bubble is enough to stall them. That is why vapor lock is so common on a small solenoid sodium hypochlorite dosing pump.

Site conditions that make vapor lock worse

The chemical is only part of the story. These installation details help gas win:

• High suction lift, with the pump well above the tank, so the suction side sees more vacuum.
• Long or small-bore suction tubing, or a high point where gas can sit.
• Long idle periods, during which chemical in the head keeps decomposing; the pump will not prime at the next start.
• No back pressure on the outlet, so the discharge valve does not snap shut and bubbles are not cleared.
• Crystals on the valves after a winter shutdown, which look exactly like a gas problem.

Designing a sodium hypochlorite dosing pump installation to avoid gas lock

Choose a pump head with automatic degassing or at least a manual bleed valve. The Iwaki EHN range, for example, offers auto-degassing heads that continuously vent gas from the chamber. Without that feature, the head still needs a bleed valve that operators use at startup and after every tank change.

Mount the pump below the tank level so it is flooded on the suction side and never sees vacuum. Keep the suction line short and straight with no high points. Fit a back pressure valve on the discharge so bubbles stay small and the valves seat cleanly.

Give the sodium hypochlorite dosing pump PVC or PVDF heads and piping, pick elastomers that suit hypochlorite and avoid metal fittings. Keep tanks shaded and ventilated. Wear goggles and gloves, and never store hypochlorite next to acids, since mixing the two releases chlorine gas.

Conclusion

Hypochlorite feed is proportional dosing checked against a residual, and what sets it apart from other chemicals is constant off-gassing. Once you see how a gas pocket swallows the stroke of a sodium hypochlorite dosing pump, the design rules make sense: a degassing head, flooded suction and some back pressure. A well-specified sodium hypochlorite dosing pump on a flooded suction rarely loses prime. For more detail read fixing hypochlorite gas lock and crystallization, sodium hypochlorite dosing pump selection and the hypochlorite dosing flow diagram. Pumps and analyzers are under solenoid metering pumps and chlorine analyzers. Send your dose rate, concentration and layout and contact our engineers.

FAQ

Should I dilute hypochlorite before dosing?

Diluted product is more stable and gasses less, and it lets you use a larger sodium hypochlorite dosing pump that is easier to control. The trade-off is a bigger day tank and more mixing work.

How do I tell vapor lock from a failed sodium hypochlorite dosing pump?

Open the bleed valve. If gas comes out and the pump delivers normally afterwards, it was vapor lock. If it still does not pump, check the balls, diaphragm and electrics.

What is the point of a clear pump head?

A transparent PVC head lets you see bubbles in the chamber, so you can spot gas lock and confirm that priming is complete.

Can I use stainless steel fittings on hypochlorite?

Generally not. Hypochlorite attacks many stainless grades, and dissolved metal speeds up decomposition. PVC or PVDF is the usual choice.

The pump will not prime after a few days off. What should I do?

Bleed the head first. On systems that stop often, flush the head with water before shutdown or choose an auto-degassing head.