Dosing Pump Bleed Valve: Where It Is and How to Use It

Where you find the dosing pump bleed valve depends on the pump. On a solenoid dosing pump it is almost always on the pump head, beside the discharge check valve, with a small knob or adjusting nut and a barb for a thin return tube. A mechanical diaphragm pump usually has no bleed valve on the head, so one is fitted separately on the discharge line close to the pump. Auto-degassing heads have the valve built into the pump chamber. This guide is for operators and maintenance crews: where each valve sits and how to use it. For the full first-start priming routine, see how to purge air from a diaphragm metering pump.

Bleed valve, vent valve, drain valve: same thing?

Not quite, and the names get mixed up on site. A bleed valve (also called a degassing or air release valve) lets air out of the pump head and discharge line so the pump can prime. Some people also call the pressure release valve on the discharge line a "vent." A drain valve, usually at a low point or on a tee by the pump head, empties leftover chemical for cleaning. Most of this article is about the dosing pump bleed valve; draining is covered at the end.

Where is the dosing pump bleed valve on a solenoid pump?

On solenoid pumps the dosing pump bleed valve is integrated in the head. The NEWDOSE solenoid pump manual says that if the head has a bleed valve, start the pump, open the valve about half a turn, and close it once chemical starts coming out. IWAKI EHN heads bleed by turning a knob, the LMI FastPrime liquid end has a bleed port, and SEKO literature notes that a manual bleed valve helps the pump start.

Always run a tube from the bleed port back to the tank or a container. The NEWDOSE manual lists this in its maintenance section. Without it, sodium hypochlorite, acid or caustic sprays onto the pump and the floor.

If the head has no bleed valve, the manual's workaround is to disconnect the discharge tube at the injection valve, wait for chemical to reach the end of the tube, and reconnect it.

Where is the dosing pump bleed valve on an auto-degassing head?

The IWAKI EHN auto-degassing type (NAE) has an automatic degassing valve inside the pump chamber plus a manual valve for releasing pressure in the discharge line. Gas drawn in travels to the fitting; because the discharge side is pressurized, the gas moves toward the lower pressure degassing valve and leaves with a little liquid.

The point that confuses people: once the pump is running normally, a small flow of liquid still comes out of the degassing valve. That is by design, not a leak, and the return tube still has to go back to the tank.

Dosing pump bleed valve on a mechanical diaphragm pump

Mechanical diaphragm heads rarely have a built-in dosing pump bleed valve. Both the NEWDOSE mechanical pump manual and the Cheonsei KD manual recommend a bleed valve at the pump outlet, and the KD manual asks for a fill and bleed valve on the discharge line for startup and maintenance. Place it, like the relief valve, close to the pump and upstream of the discharge isolation valve; otherwise closing that valve leaves you with no way to vent.

Some makers combine functions. Nanfang literature describes an outlet multifunction valve that handles startup venting, pressure relief or back pressure, and anti-siphon in one body.

Do not confuse it with the gearbox vent

The Cheonsei KD pre-start checklist says to confirm oil level and pull out the vent pin on the oil filler cap. That vent is for the gearbox and has nothing to do with bleeding chemical lines.

How to bleed a dosing pump: using the dosing pump bleed valve

The linked guide covers the whole routine. For the valve itself:

1. Connect the return tube and put its end in the tank or a container.
2. Open every valve on the discharge line before starting; the NEWDOSE manual warns against running the pump against a closed outlet.
3. Start the pump and open the bleed valve about half a turn. You can set stroke to 100% first, as the Cheonsei manual does for fast priming.
4. When the return tube carries solid liquid with no bubbles, close the valve and reset flow.

Turn it by hand, not with a wrench; plastic bodies and O-rings do not take much force. If the pump still will not deliver after bleeding, the suction side is probably leaking air. See how to fix air in a dosing pump line.

When a dosing pump bleed valve causes problems

A dosing pump bleed valve left slightly open sends part of each stroke back to the tank, so the real dose runs low and the gauge will not build pressure. Calibration is where this bites: the NEWDOSE calibration steps open the bleed valve to clear bubbles, close it, and only then start timing.

Once the O-ring ages, the valve weeps even when tight. Replace the seal or the valve; see how to replace a NEWDOSE degassing valve.

Draining the pump before maintenance

The NEWDOSE solenoid manual drains the head by turning the pump upside down or removing the head, then flushing with clean water. On mechanical pumps, tees and isolation valves near the head inlet and outlet let you flush the head without breaking the piping.

Before draining, stop the pump, close inlet and outlet isolation valves and open the bleed valve to release pressure. Wear goggles and chemical-resistant gloves.

Conclusion

The dosing pump bleed valve sits on the head beside the discharge valve on solenoid pumps, on the discharge line close to the pump for mechanical diaphragm pumps, and inside the chamber on auto-degassing heads. Route the return tube to the tank, open the outlet, crack the valve half a turn and close it once liquid runs clear. Spare bleed and check valves are in our dosing accessories and solenoid metering pump range; also make sure check valves face the right way, see check valve installation direction. Not sure where the valve is on your pump? Send the model or a photo of the head and contact our engineers.

FAQ

How far should I open the dosing pump bleed valve?

About half a turn is usually enough. Opening it further sends a lot of chemical back to the tank and can damage the O-ring.

Can I leave the dosing pump bleed valve open while running?

No. Part of each stroke returns to the tank and the dose drops. The small steady flow from an auto-degassing head is the one exception.

Can a mechanical diaphragm pump run without an outlet bleed valve?

It can, but priming after startup or a chemical change means breaking the discharge line. Manufacturers recommend fitting one.

Can the bleed port drain straight into a floor drain?

Not recommended. Hypochlorite, acid or caustic in a floor drain is a safety and environmental problem. Return it to the tank or a container.

Why does my dosing pump bleed valve drip when closed?

First check whether it is an auto-degassing type. If not, the O-ring is likely worn or the seat has crystals; clean it or replace the seal.