Dosing Pump Low Pressure: 6 Causes and the Order to Check Them

Dosing pump low pressure is usually caused outside the pump: a back pressure valve set too low, a relief valve lifting early or a leak in the discharge line, with check valves, gas lock and the diaphragm further down the list. Fit a gauge on the discharge, work from the outside in, and most faults are found in about half an hour.

This article covers one situation only: the pump strokes and some chemical comes out, but discharge pressure will not reach its setpoint or the chemical will not enter the process line. If there is no output at all, the causes are different; start with diaphragm metering pump no pressure.

Start with a pressure gauge

Many skids have no gauge on the pump discharge, so "low pressure" is a guess. Fit a damped gauge between the pump and the back pressure valve, ranged at roughly 1.5 to 2 times working pressure.

With a gauge, dosing pump low pressure becomes something you can read:

• Steady needle, but low: usually the back pressure valve or the relief valve.
• Needle swinging hard with each stroke and peaks that stay low: usually leaking valves or gas.
• Needle slowly falling: look for a leak in the discharge line.

Dosing pump low pressure cause 1: back pressure valve set too low

The back pressure valve sets the minimum discharge pressure. If it is set below the pressure at the injection point, chemical cannot get in, and the gauge will not show a high reading.

Reset it against the actual line pressure, typically about 0.5 to 1 bar above it. Turn the adjuster clockwise to load the spring while watching the gauge; see back pressure valve adjustment for the step-by-step. An old valve whose diaphragm has aged will not hold any setting and needs a new diaphragm.

Cause 2: relief valve lifting early

A relief valve set too low, or held partly open by crystals, sends chemical back to the tank and caps dosing pump low pressure at one value.

The test is quick: feel the relief return line. If chemical is flowing there, you have found it. Set the relief pressure above working pressure and below the pump's rated pressure, and clean the seat.

Cause 3: leaking check valves

Valve leakage gives a typical dosing pump low pressure pattern on the gauge. When the suction or discharge valve does not seal, part of every stroke flows back and the pressure peak cannot build. Common reasons:

1. Crystals or particles on the ball or seat.
2. A worn ball or deformed seat.
3. Aged O-rings, or O-rings fitted in the wrong groove.

Remove one valve at a time and note which way the ball and seat face. Clean, refit and test before moving to the next one.

Cause 4: gas in the pump head

Gas lock is the cause of dosing pump low pressure that is most often misdiagnosed; many people assume the diaphragm has failed and find nothing when they open the head. Sodium hypochlorite and hydrogen peroxide release gas, and a loose suction fitting pulls in air. With gas in the chamber, the diaphragm compresses gas and pressure cannot build.

Tighten the suction fittings first, then open the bleed valve, and if the problem returns consider flooded suction. See fixing air in dosing pump lines.

Cause 5: diaphragm or pump head damage

Only after the checks above does the diaphragm come into question as a source of dosing pump low pressure. A ruptured diaphragm usually shows chemical at the leak detection port; hydraulic diaphragm pumps may also show milky oil or a changing oil level. A cracked head typically leaves visible crystal tracks. Both mean replacement parts.

Cause 6: discharge line leaks

Leaking fittings, flanges or valves let pressure escape and cause dosing pump low pressure that slowly worsens. A half-blocked injection check valve can also make the gauge jump around. Walk the line from the pump to the injection point and look for weeping and crystals. Old tubing hardens and cracks, and a split on the inside of a bend is easy to miss, so run a hand along it. Tubing in direct sun ages faster.

Troubleshooting order for dosing pump low pressure

Put together, the recommended order is:

1. Read the gauge: low, swinging or falling.
2. Feel the relief return line to rule out an early lift.
3. Check the back pressure valve setting and adjust if needed.
4. Check suction fittings and bleed the head.
5. Clean the check valves.
6. Inspect the diaphragm and pump head last.

This goes from outside to inside and from easy to awkward, which saves a lot of unnecessary teardown. If the pump's rated pressure is simply lower than the line pressure, dosing pump low pressure is a selection problem; see metering pump pressure adjustment and check models in our diaphragm metering pumps.

Conclusion

Dosing pump low pressure almost always traces back to six places: the back pressure valve, the relief valve, check valves, gas, the diaphragm and the discharge line. Fit a gauge, follow the outside-in order, and most cases are fixed without a new pump. If pressure still will not build, or you want to confirm the pump is the right size, send chemical, flow and injection pressure and contact our engineers.

FAQ

Does dosing pump low pressure mean the pump is broken?

Usually not. Back pressure valves, relief valves and line leaks are more common, so rule those out before opening the head.

How much higher than line pressure should the back pressure valve be set?

Typically about 0.5 to 1 bar higher. Too low and chemical will not enter; too high and the pump works harder than needed.

Is a swinging gauge needle normal?

Some swing is normal because a metering pump delivers pulses. Large swings with low peaks point to leaking valves or gas.

A new pump will not build pressure. Where do I start?

Check the installation. Suction air leaks, an unadjusted back pressure valve and a low relief setting are the three usual causes on new installs.

What are the signs of a failed diaphragm?

Generally chemical at the leak detection port and a drop in both flow and pressure. Hydraulic pumps may also show cloudy oil.