Pressure Relief Valve Setting for Dosing Pumps: Steps and Set Point Logic

The pressure relief valve setting on a dosing system answers one question: at what pressure should the valve open and send chemical back to the tank? The rule is simple. Set it above the highest normal working pressure, and below the lowest pressure rating in the system, whether that is the pump, the pipe, a valve or a gauge. Set it too low and the valve weeps during normal running, so your dose rate drifts. Set it too high and a blocked line breaks the pump head or pipe before the valve ever lifts. This article covers the set point logic first, then a field procedure.

Why a dosing pump needs a relief valve

Metering pumps are positive displacement pumps. If the discharge is blocked, pressure keeps climbing. A motor-driven pump can build very high discharge pressure before its thermal protection trips. The New Dose mechanical diaphragm pump manual says the relief valve belongs on the discharge line between the pump and the nearest isolation valve, to protect the pump, piping and equipment from a blocked outlet or an accidentally closed valve.

A relief valve acts only in abnormal conditions and should stay shut in normal service. That makes it different from a back pressure valve, which works continuously. Our article on relief valve vs back pressure valve explains the split.

How to choose the pressure relief valve setting

Lower limit: maximum working pressure

Add up the injection point pressure, line friction, the back pressure valve setting and the pulsation peak of a reciprocating pump. A gauge shows an average; the peaks are higher, especially without a pulsation damper. If the pressure relief valve setting sits right at the gauge reading, the valve cracks open on every stroke and the return line drips.

Upper limit: the weakest component

List the pump's nameplate maximum, the head and pipe ratings, and valve and gauge ranges. The lowest figure is your ceiling. PVC pipe and plastic pump heads are usually the limit, and they are easy to overlook.

What manufacturers do

Pumps with built-in relief valves often list a maximum setting by pressure class. In the Milton Roy Milroyal B manual, the 2.1–15.4 bar working range has a built-in relief valve maximum of 17.5 bar, and the 31.6–52.5 bar range has 60.2 bar. Same logic: above the top of the working range, within the pump's capability. For an external valve there is no fixed ratio. Pick a pressure relief valve setting between your working pressure and the ceiling, with margin at both ends.

Field procedure for the pressure relief valve setting

1. Prepare. Confirm the valve sits between the pump outlet and the first isolation valve, with its return line piped to the tank or drain and the outlet end visible. Fit a gauge, ideally with a snubber, between pump and relief valve. Use clean water if possible; with chemical, wear goggles and chemical gloves.
2. Back off the spring. Start with the adjusting screw loose so you never jump straight to the ceiling.
3. Run the pump at normal stroke and speed with the outlet valve fully open. Note the normal gauge reading.
4. Slowly close the downstream isolation valve while watching the gauge. Never shut it all at once. Stop near the target pressure.
5. Tighten the screw a small turn at a time until flow just starts in the return line at your target pressure.
6. Verify. Open the isolation valve, let pressure fall, then close it again. Check that the valve lifts at the same pressure and reseats without dripping. Repeat two or three times.
7. Lock and record. Tighten the lock nut, tag the valve with the pressure relief valve setting and date, and log it.

Keep each pressure hold short and never exceed the ceiling. If you have to go above it to see a lift, the pressure relief valve setting you chose is wrong, so stop and rethink the numbers.

A common site trap: the valve is set on water, then the system goes live on concentrated chemical, or cold weather thickens the liquid. Line friction rises, working pressure rises with it, and the old setting now looks low. Recheck against the gauge after a chemical or season change. Also confirm that the body, seat, spring and seals resist the chemical; corrosion makes the lift pressure drift over time.

Troubleshooting a pressure relief valve setting

The return line keeps dripping

Usually the pressure relief valve setting is too close to working pressure and pulsation peaks are lifting it. Crystals or debris on the seat do the same. Clean the seat first, then raise the setting slightly or add a pulsation damper.

The valve does not open on a blocked line

The setting is too high or the poppet is stuck with crystals. This is the dangerous one. Stop the pump, relieve pressure and strip the valve. For pressure checks, see metering pump pressure adjustment and diaphragm pump not building pressure.

Conclusion

There is no universal number for a pressure relief valve setting, only a window: above working pressure including peaks, below the weakest rating in the system. Build pressure slowly with a gauge and the downstream valve, turn the screw in small steps, confirm lift and reseat, then lock and tag. Relief valves and accessories are in our dosing accessory range, with pumps on the New Dose brand page.

FAQ

Can I use the relief valve to control discharge pressure?

No. It should open only in a fault. Use a back pressure valve for steady pressure; otherwise the relief valve leaks chemical all day and dosing goes off.

How often should the pressure relief valve setting be checked?

Check it during routine maintenance, and again after changing the pump, the piping or the injection pressure.

The pump has a built-in relief valve. Do I need another?

If pipe or valve ratings are lower than the pump's, yes. Set the external valve to protect that weakest component.

Why must the return line outlet be visible?

New Dose and KDV manuals both require it, so a leaking relief valve is noticed instead of quietly wasting chemical.

What if I am unsure about the set point?

Send us the pump model, injection pressure, pipe material and valve types, and contact our engineers to work out the pressure relief valve setting.