Metering Pump Pressure: How to Set the Back Pressure and Relief Valves

Metering pump pressure is not something you dial in on the pump itself. A diaphragm or plunger metering pump is a positive displacement pump, so it pushes out whatever pressure the discharge line resists with. The two valves on the discharge side do the real work: the back pressure valve builds a stable minimum, and the pressure relief valve sets the ceiling. Get the order and the gap between them right and the gauge will sit still.

This article is for commissioning engineers and plant operators working with solenoid and mechanical diaphragm pumps.

What actually decides metering pump pressure

Metering pump pressure on the discharge side is the sum of what the pump has to overcome: the pressure in the main or vessel at the injection point, the friction loss along the dosing line, any height the chemical must be lifted, and the setting of the back pressure valve if one is fitted. The pump simply meets that resistance stroke by stroke, up to its rated limit.

That is why two identical pumps can show very different gauge readings on the same site. One injects into an open tank, the other into a pressurised boiler feed line. Neither is "wrong". What matters is that metering pump pressure stays below the pump's rating and above the minimum the valves need to seal properly.

The back pressure valve: building a stable minimum

When the injection point is at low or negative pressure, the pump's check valves may not seat crisply, and chemical can siphon straight through the head. Output then drifts above the setting. A back pressure valve fixes this by holding the line at a set pressure, so every stroke has to open it deliberately.

How to set metering pump back pressure

Most spring-loaded back pressure valves have an adjusting screw under a cap. With the pump running, turn it in to raise the setting and watch the gauge between the pump and the valve. A common field practice is to set it a little above the highest pressure the injection point will ever see, so the system pressure cannot push the valve open on its own. Our article on how to adjust a dosing pump back pressure valve covers the procedure in more detail.

The relief valve: capping metering pump pressure

If someone closes a downstream valve or the injection quill blocks with scale, a positive displacement pump keeps pushing until something breaks: the diaphragm, the pump head, the pipe, or the gearbox. The pressure relief valve is there to open before that happens and return chemical to the tank or suction side.

Relief valve setting rules

Set the relief valve above the normal working metering pump pressure but below the pump's maximum rated pressure and the rating of the weakest part in the line. Leave a clear margin between the back pressure setting and the relief setting. If the two are too close, pressure peaks on each stroke can lift the relief valve, chemical returns to the tank, and the delivered dose falls without any alarm. The difference between the two valves is explained in back pressure valve vs relief valve.

Which to set first, and how to read metering pump pressure on the gauge

Set the back pressure valve first, with the relief valve wound up toward its upper limit so it cannot interfere. Then bring the relief valve down to its working setting, and test it by briefly closing the discharge isolation valve while watching the gauge. It should open at the intended point and stop the climb. Restore the line straight away.

Reading the gauge is easier said than done. A diaphragm pump produces a pulsing flow, so a bare gauge needle swings with every stroke. A pulsation dampener on the discharge line, or a glycerine-filled gauge with a snubber, gives a steadier reading. Read the average metering pump pressure, not the peak. If the needle swings wildly, check the dampener precharge before blaming the valves.

Safety first: relieve line pressure before you remove any valve cap or fitting. Treat every dosing line as charged until the gauge shows zero, and wear eye protection when working with acids, caustic or hypochlorite.

Diaphragm pump low pressure: what to check

When a diaphragm pump cannot build metering pump pressure, work through the causes in this order:

1. The relief valve is set too low or is passing, so chemical returns to the tank.
2. The back pressure valve is set too low or its diaphragm is damaged.
3. Air in the suction line or pump head, often from a loose suction fitting or gas-forming chemicals such as hypochlorite.
4. Worn or dirty check valve balls and seats, so liquid slips back on each stroke.
5. A ruptured or stretched pump diaphragm.

Most cases are solved at steps 1 to 3 without opening the pump head. Our diaphragm metering pump no-pressure troubleshooting guide goes through each check. If you need replacement back pressure or relief valves, see our dosing accessories.

Conclusion

Metering pump pressure is set by the line, not the pump. Build a stable minimum with the back pressure valve, cap it with the relief valve, keep a clear gap between the two, and read the gauge as an average with a dampener fitted. If pressure will not build, check the valves and the suction side before opening the pump. Send us your injection pressure, line length and chemical, and contact our engineers to confirm valve settings for your system.

FAQ

Can I raise metering pump pressure by turning up the stroke?

No. Stroke length and frequency change flow, not metering pump pressure. Pressure follows the resistance in the line. A longer stroke only makes each pulse larger.

Do I need a back pressure valve if the main already gives enough metering pump pressure?

Often you still do, if the main pressure varies or can fall near zero. If it is always well above atmospheric, the system itself acts as back pressure, but many designers fit one anyway to stop siphoning during shutdowns.

Where should the relief valve discharge go when metering pump pressure gets too high?

Back to the chemical tank or the suction side, through a line that cannot be isolated. Never route it to a drain where corrosive chemical could splash.

Why does my gauge swing so much?

Diaphragm pumps deliver in pulses. Fit a pulsation dampener and a snubbed or liquid-filled gauge. A swinging needle with a dampener installed often means the dampener precharge is wrong.

How high can I set the relief valve?

Below both the pump's maximum rated pressure and the rating of the weakest component in the discharge line. Check the nameplate and the pipework rating before setting it.