Sodium Hypochlorite Dosing Skid: Tank, Pumps and Safety Features

A sodium hypochlorite dosing skid puts the metering pumps, valves, instruments and control panel on one base frame, piped and pressure tested at the factory, so the site only connects the chemical supply, the discharge line, power and signals. A skid that runs for years without drama gets three things right: the pump heads do not gas lock, the lines do not scale up, and any leak has somewhere to go. This guide walks through the parts in order. It is written for designers and operations managers at water and wastewater plants, and for buyers comparing skid quotes.

What a sodium hypochlorite dosing skid includes

Between the bulk tank and the injection point there are five sections: tank and fill, suction piping, metering pumps, discharge valve train, and instruments with controls. Small plants often mount the day tank on the frame too; larger plants keep the tank separate and the skid carries only pumps and valves. The safety rules are the same either way.

Tank and suction: where gas lock starts

Sodium hypochlorite slowly decomposes and gives off gas, faster when it is warm or old. That gas collects in the suction line, and the pump loses prime. It is the most common fault on any sodium hypochlorite dosing skid.

Tank placement

Keep the tank out of sunlight and away from heat. Vent it outdoors or to a ventilated point, not into the dosing room. Fit a level gauge, an overflow and a drain, and pipe the overflow to containment rather than onto the floor.

Suction routing

Mount the pumps below the lowest tank level for flooded suction. Keep the suction line short with few bends and no high points where bubbles can sit. Keep the inlet off the tank bottom so sediment stays out of the pump head. Our overview of chemical dosing skid structure covers foot valve and strainer placement.

Sodium hypochlorite dosing skid pumps: heads that clear gas

Small flows usually go to solenoid diaphragm pumps, larger ones to motor-driven mechanical diaphragm pumps. Either way, the question is whether the head can purge gas by itself.

The Iwaki EHN catalog shows two options aimed at hypochlorite. The EHN-NAE auto-degassing head has a built-in vent valve: gas drawn in is pushed toward the lower-pressure degassing valve and leaves with a little liquid, and the catalog says it will not gas lock on easily vaporizing liquids like sodium hypochlorite and hydrogen peroxide. It also has a manual vent. The EHN-55 high-compression head reduces dead volume inside the chamber and is listed for hypochlorite feed. Iwaki also offers a degassing fitting at the pump inlet that strips bubbles from the suction line.

Wetted materials must resist oxidation. Confirm head, ball and seal materials against the manufacturer's resistance chart at your concentration and temperature. Duty and standby pumps are standard on a sodium hypochlorite dosing skid; rotate them regularly so chemical does not crystallize in an idle head.

Discharge valves on a sodium hypochlorite dosing skid

From the pump outward, a typical line-up is:

1. A multifunction or back pressure valve to hold steady discharge pressure and stop siphoning. Iwaki's multifunction valve combines back pressure, air release and pressure relief at fixed settings. Nanfang's CKB solenoid pumps also carry a multifunction outlet valve for priming vent, relief or back pressure and anti-siphon.
2. A pressure relief valve that returns flow to the tank if the line is blocked, set above working pressure and below the pump and piping rating.
3. A pulsation damper on larger mechanical diaphragm pumps, plus a pressure gauge and isolation valves.
4. A calibration column on the suction side to measure actual output.

Pipe relief and vent discharges back to the tank or containment. Do not let them drip onto the frame.

Safety features on a sodium hypochlorite dosing skid

This is where budgets get trimmed, and where failures cost the most.

Spill containment

Build the frame as a bunded tray, or set the whole skid inside a bund, so leaks from heads, flanges and valves stay contained and drain to a sump. Keep joints and flanges to a minimum.

Ventilation and segregation

Ventilate the dosing room. Hypochlorite releases chlorine gas on contact with acid, so if acid is dosed in the same room, keep the two systems' trays, drains and overflow lines completely separate. Many skid proposals skip this point.

Operator protection

Install an eyewash next to the skid. Wear goggles and chemical gloves for tank changes. Before opening a pump head, stop and isolate the pump, close the valves and bleed off line pressure. For room layout, see hypochlorite storage and dosing room safety.

Instruments and controls

On any sodium hypochlorite dosing skid, at minimum a low tank level alarm should interlock and stop the pumps to prevent dry running. For flow-paced feed, send a 4-20 mA flowmeter signal to the pump. For residual control, add a chlorine analyzer. Use a corrosion-resistant panel, keep electrics away from pump heads and valves, and have wiring done by an electrician with power isolated.

Conclusion

A good sodium hypochlorite dosing skid is built from small decisions: flooded suction, gas-tolerant pump heads, back pressure and relief on the discharge, a frame that contains leaks, and a ventilated room kept apart from acid. When you request a quote, send tank size, dose rate, injection pressure, control method and available space in one go. If pumps keep losing prime, read how to clear hypochlorite gas lock and crystals. Packaged systems are listed in our dosing skid range, and degassing heads on the Iwaki brand page.

FAQ

Can a sodium hypochlorite dosing skid sit outdoors?

Yes, under a sunshade. Direct sun and heat speed up decomposition, and more gas means more gas lock.

Do I really need a standby pump?

Disinfection is continuous, and a sodium hypochlorite dosing skid with one stopped pump means non-compliant water, so duty and standby is the usual choice. Run them in rotation.

PVC or PVDF heads?

Choose by concentration and temperature using the manufacturer's chart. Both are common at ambient conditions; at higher temperatures, get the supplier's confirmation.

How big should the bund be?

Large enough for all liquid in the skid piping and heads, and the full tank volume if the tank is on the frame. Follow your local design requirements.

Is frequent gas lock a pump defect?

Usually not. Check for high points in the suction line, a pump mounted above tank level, or old stock. If that is fine, move to an auto-degassing head or add an inlet degassing fitting. Send your dose rate and site photos and contact our engineers for a sodium hypochlorite dosing skid layout.