Refrigerant pump cavitates or loses pressure

Troubleshooting · Applies to: Ammonia with pump circulation (R717)

An ammonia refrigerant pump that cavitates or loses pressure: too much flow, too little flow, vapour drawn in, low level and a blocked strainer.

Last checked: October 25, 2026 Field mode: large buttons, questions first and the screen kept on.

A refrigerant pump needs more available pressure at its inlet (NPSHA) than it requires (NPSHR); Cornell recommends a margin of at least about 0.6 m (2 ft). Cornell advises troubleshooting in this order: cavitation (too much flow), recirculation (too little flow) and vapour drawn in from the boiling surface in the vessel ^[1]^[2]. With ammonia, pitting damage is rare, but the pump still loses head. Ammonia is toxic: follow the plant safety instructions and use protective equipment when working on the system.

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Read the pump differential pressure and the level in the low pressure receiver, and listen to the pump. Compare the differential pressure with the pump curve and note when the pressure drops: at high flow, at low flow, or at start-up, a new warm load or after hot gas defrost.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1
Flowchart A
Too low a level in the vessel: the level control feeds too little, the submergence is too small or the plant has too little refrigerant.
Remedy: Check the level control, the level sensor and the refrigerant charge, and raise the level setting.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check liquid level control in a vessel
The level control keeps the right amount of liquid in a separator or receiver so the pump has enough suction head and no liquid is carried over to the compressor.
1. Read the actual level. Clean the sight glass or level gauge on the vessel and read the actual liquid level in stable operation. Wear goggles and gloves.
  ✓ Check: The liquid level is clearly visible and noted.
2. Read the sensor signal. Read the level signal in the controller, or measure the 4–20 mA signal with a multimeter at the test terminals or with a mA clamp. Live measurement in the panel only with a CAT III/IV meter and insulated probes.
  ✓ Check: The signal corresponds to the level in the sight glass according to the sensor's range.
3. Calibrate the sensor. If the signal does not match, set zero and span in the controller or transmitter as the manufacturer describes, using the sight glass as reference.
  ✓ Check: The controller shows the same level as the sight glass at two different levels.
4. Check the set points. Check the normal level, and the low level and high level alarms against the design: the low level must give the pump enough suction head, the high level must protect the compressor.
  ✓ Check: The set points agree with the design documentation.
5. Test the alarms. Tell the operator and test the low and high level alarms as the controller allows (simulation in the service menu or by moving a float switch). Check that the pump stops at low level and the liquid feed closes at high level.
  ✓ Check: Each alarm trips at its level and the right output switches.
6. Return to normal. Remove any simulation, check that the feed valve controls the level again and record what was changed.
  ✓ Check: The level is stable at the set point and no alarms remain.
2
Flowchart B
Cavitation: the flow is too large, so the available inlet pressure (NPSHA) falls below what the pump needs.
Remedy: Limit the flow (flow regulators or orifices on the liquid feed) and make sure NPSHA exceeds NPSHR by at least about 0.6 m.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a liquid refrigerant pump
A liquid pump in a pumped recirculation system needs enough liquid head on the suction side and a minimum flow. Too little head gives cavitation, noise and a worn pump.
1. Follow the plant safety rules. Read the plant's safety rules (ammonia or CO2 plants have their own). Wear goggles and gloves: liquid refrigerant and cold pipes cause frostbite. Ventilate low or enclosed spaces before entering.
  ✓ Check: You know the plant rules and have the protective equipment on.
2. Measure the pump pressures. Read the pressure before and after the pump with calibrated gauges and calculate the differential pressure. Compare with the manufacturer's pump curve.
  ✓ Check: The differential pressure lies on the pump curve for the expected flow.
3. Check the liquid level and suction head. Read the liquid level in the separator or vessel and compare its height above the pump inlet with the NPSH the pump manufacturer requires.
  ✓ Check: The liquid level is above the minimum the pump manufacturer requires.
4. Look and listen for cavitation. Listen to the pump and measure the motor current with a clamp meter. Cavitation gives a crackling noise and an unstable differential pressure and current.
  ✓ Check: The pump runs quietly with steady pressure and current.
5. Check the minimum flow bypass. Check that the bypass orifice or valve back to the vessel is open and set as the design requires, so the pump has flow even when all evaporators are closed.
  ✓ Check: There is flow through the bypass (temperature or sight glass), and the pump does not run against closed valves.
6. Clean the suction strainer. If the differential pressure is low or falling, isolate the pump, let the liquid drain back or remove it by the plant procedure, and prove zero pressure before opening the strainer. Clean the screen, refit with a new gasket and leak test.
  ✓ Check: The strainer is clean, the cover is tight and the pump reaches its normal differential pressure.
3
Flowchart C
Recirculation: the flow is too small and the liquid recirculates inside the pump; throttling makes it worse.
Remedy: Open the discharge valve again and fit a minimum flow bypass to the wet return (at least ¾″, globe or needle valve, never a relief valve, about 2.3 m³/h). Set it: bypass fully open, close the discharge, close the bypass slowly until the gauge flutters, then open until steady.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a liquid refrigerant pump
A liquid pump in a pumped recirculation system needs enough liquid head on the suction side and a minimum flow. Too little head gives cavitation, noise and a worn pump.
1. Follow the plant safety rules. Read the plant's safety rules (ammonia or CO2 plants have their own). Wear goggles and gloves: liquid refrigerant and cold pipes cause frostbite. Ventilate low or enclosed spaces before entering.
  ✓ Check: You know the plant rules and have the protective equipment on.
2. Measure the pump pressures. Read the pressure before and after the pump with calibrated gauges and calculate the differential pressure. Compare with the manufacturer's pump curve.
  ✓ Check: The differential pressure lies on the pump curve for the expected flow.
3. Check the liquid level and suction head. Read the liquid level in the separator or vessel and compare its height above the pump inlet with the NPSH the pump manufacturer requires.
  ✓ Check: The liquid level is above the minimum the pump manufacturer requires.
4. Look and listen for cavitation. Listen to the pump and measure the motor current with a clamp meter. Cavitation gives a crackling noise and an unstable differential pressure and current.
  ✓ Check: The pump runs quietly with steady pressure and current.
5. Check the minimum flow bypass. Check that the bypass orifice or valve back to the vessel is open and set as the design requires, so the pump has flow even when all evaporators are closed.
  ✓ Check: There is flow through the bypass (temperature or sight glass), and the pump does not run against closed valves.
6. Clean the suction strainer. If the differential pressure is low or falling, isolate the pump, let the liquid drain back or remove it by the plant procedure, and prove zero pressure before opening the strainer. Clean the screen, refit with a new gasket and leak test.
  ✓ Check: The strainer is clean, the cover is tight and the pump reaches its normal differential pressure.
4
Flowchart D
Vapour drawn in: when the vessel pressure falls fast the liquid boils up to about 0.9 m below the surface and gas follows the liquid into the drop leg. A bypass does not help.
Remedy: Pull the pressure down more slowly, raise the level in the vessel and use a large suction line with a low pressure drop.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check liquid level control in a vessel
The level control keeps the right amount of liquid in a separator or receiver so the pump has enough suction head and no liquid is carried over to the compressor.
1. Read the actual level. Clean the sight glass or level gauge on the vessel and read the actual liquid level in stable operation. Wear goggles and gloves.
  ✓ Check: The liquid level is clearly visible and noted.
2. Read the sensor signal. Read the level signal in the controller, or measure the 4–20 mA signal with a multimeter at the test terminals or with a mA clamp. Live measurement in the panel only with a CAT III/IV meter and insulated probes.
  ✓ Check: The signal corresponds to the level in the sight glass according to the sensor's range.
3. Calibrate the sensor. If the signal does not match, set zero and span in the controller or transmitter as the manufacturer describes, using the sight glass as reference.
  ✓ Check: The controller shows the same level as the sight glass at two different levels.
4. Check the set points. Check the normal level, and the low level and high level alarms against the design: the low level must give the pump enough suction head, the high level must protect the compressor.
  ✓ Check: The set points agree with the design documentation.
5. Test the alarms. Tell the operator and test the low and high level alarms as the controller allows (simulation in the service menu or by moving a float switch). Check that the pump stops at low level and the liquid feed closes at high level.
  ✓ Check: Each alarm trips at its level and the right output switches.
6. Return to normal. Remove any simulation, check that the feed valve controls the level again and record what was changed.
  ✓ Check: The level is stable at the set point and no alarms remain.
5
The level sensor in the vessel reads wrong.
Remedy: Compare with the vessel sight glass and calibrate or replace the sensor.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check liquid level control in a vessel
The level control keeps the right amount of liquid in a separator or receiver so the pump has enough suction head and no liquid is carried over to the compressor.
1. Read the actual level. Clean the sight glass or level gauge on the vessel and read the actual liquid level in stable operation. Wear goggles and gloves.
  ✓ Check: The liquid level is clearly visible and noted.
2. Read the sensor signal. Read the level signal in the controller, or measure the 4–20 mA signal with a multimeter at the test terminals or with a mA clamp. Live measurement in the panel only with a CAT III/IV meter and insulated probes.
  ✓ Check: The signal corresponds to the level in the sight glass according to the sensor's range.
3. Calibrate the sensor. If the signal does not match, set zero and span in the controller or transmitter as the manufacturer describes, using the sight glass as reference.
  ✓ Check: The controller shows the same level as the sight glass at two different levels.
4. Check the set points. Check the normal level, and the low level and high level alarms against the design: the low level must give the pump enough suction head, the high level must protect the compressor.
  ✓ Check: The set points agree with the design documentation.
5. Test the alarms. Tell the operator and test the low and high level alarms as the controller allows (simulation in the service menu or by moving a float switch). Check that the pump stops at low level and the liquid feed closes at high level.
  ✓ Check: Each alarm trips at its level and the right output switches.
6. Return to normal. Remove any simulation, check that the feed valve controls the level again and record what was changed.
  ✓ Check: The level is stable at the set point and no alarms remain.
Check a sensor against a reference
A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.
1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.
  ✓ Check: The reference reading is steady for a few minutes.
2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.
  ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).
3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.
  ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.
4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.
  ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.
5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).
  ✓ Check: The sensor sits firmly at the correct point and is insulated.
6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.
  ✓ Check: Controller and reference agree within the tolerance.
6
Flowchart E
A blocked suction strainer, a valve not fully open or a worn impeller or bearing.
Remedy: Clean the strainer, open the valves and check the pump according to the manufacturer.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a liquid refrigerant pump
A liquid pump in a pumped recirculation system needs enough liquid head on the suction side and a minimum flow. Too little head gives cavitation, noise and a worn pump.
1. Follow the plant safety rules. Read the plant's safety rules (ammonia or CO2 plants have their own). Wear goggles and gloves: liquid refrigerant and cold pipes cause frostbite. Ventilate low or enclosed spaces before entering.
  ✓ Check: You know the plant rules and have the protective equipment on.
2. Measure the pump pressures. Read the pressure before and after the pump with calibrated gauges and calculate the differential pressure. Compare with the manufacturer's pump curve.
  ✓ Check: The differential pressure lies on the pump curve for the expected flow.
3. Check the liquid level and suction head. Read the liquid level in the separator or vessel and compare its height above the pump inlet with the NPSH the pump manufacturer requires.
  ✓ Check: The liquid level is above the minimum the pump manufacturer requires.
4. Look and listen for cavitation. Listen to the pump and measure the motor current with a clamp meter. Cavitation gives a crackling noise and an unstable differential pressure and current.
  ✓ Check: The pump runs quietly with steady pressure and current.
5. Check the minimum flow bypass. Check that the bypass orifice or valve back to the vessel is open and set as the design requires, so the pump has flow even when all evaporators are closed.
  ✓ Check: There is flow through the bypass (temperature or sight glass), and the pump does not run against closed valves.
6. Clean the suction strainer. If the differential pressure is low or falling, isolate the pump, let the liquid drain back or remove it by the plant procedure, and prove zero pressure before opening the strainer. Clean the screen, refit with a new gasket and leak test.
  ✓ Check: The strainer is clean, the cover is tight and the pump reaches its normal differential pressure.
Check valve positions and trapped liquid
A service or shut-off valve left closed or half open gives symptoms like a restriction or a low charge, and liquid shut in between closed valves can burst the pipe when it warms up. Wear goggles and gloves and never exceed the system PS.
1. Go through the circuit with the diagram. Follow the flow diagram and list every service valve, ball valve and king valve from the compressor through condenser, receiver, liquid line, evaporators and suction line.
  ✓ Check: You have a list of all valves and their required position in normal operation.
2. Check the service valves. With a valve key, check that suction, discharge and liquid service valves are fully open (back-seated). Turn slightly off the back seat only while a gauge is connected. Refit the caps afterwards.
  ✓ Check: Spindles are fully open and caps are tight.
3. Look for partly closed valves. With the system running, measure pipe temperature before and after each valve on the liquid line with a clamp thermometer. A clear temperature drop, frost or sweating after a valve means it is partly closed or restricted.
  ✓ Check: There is no noticeable temperature drop across any valve in the liquid line.
4. Check the solenoid valves. Check that solenoid valves open on a call: the coil is energised (magnet tester or voltage on the coil) and there is flow (temperature change after the valve).
  ✓ Check: Each solenoid valve opens when it is called and closes when it is not.
5. Never trap liquid. Before closing two valves on a liquid-filled section, pump the liquid out or make sure the section has a pressure relief path (relief valve or open path to the receiver). Never leave a liquid line closed at both ends.
  ✓ Check: Every section that can be closed in is empty or has a relief path.
6. CO2: check standstill protection. On R744 systems, confirm that the standstill unit, the relief valves and any pressure-holding function are in service before valves are closed, because the pressure of trapped CO2 rises quickly as it warms (see the CO2 standstill guide).
  ✓ Check: The standstill pressure stays well below PS and the relief valves are open to their outlet.
7
Electrical fault: pump motor, inverter or overload protection.
Remedy: Measure voltage and current and read the inverter alarms.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Measure voltage and current under load
Low voltage, phase imbalance or high current make motors overheat and trip. Measure at standstill, during start and in operation and compare with the nameplate. Live measurements in an open panel are for qualified personnel only.
1. Prepare for live measurement. Use a CAT III/IV multimeter with insulated probes and a true-RMS clamp meter with inrush function. Wear safety glasses, no loose clothing, and keep hands clear of fans and belts that can start automatically.
  ✓ Check: Meter and leads are CAT III or CAT IV rated and the meter works on a known source.
2. Measure the supply at standstill. Measure L1–L2, L2–L3 and L3–L1 (and phase to neutral where used) at the incoming terminals with the compressor stopped.
  ✓ Check: All voltages are within the range on the nameplate or data sheet (many compressors allow ±10 % of rated voltage).
3. Measure voltage during start and in operation. Measure the same phases at the compressor contactor while the compressor starts and again after a few minutes of running. A large drop during start points to a weak supply, undersized cables or a loose connection.
  ✓ Check: The voltage stays within the permitted range also during start and in operation.
4. Calculate the voltage imbalance. Imbalance (%) = largest deviation from the average of the three phase-to-phase voltages ÷ average × 100.
  ✓ Check: The imbalance is within the compressor manufacturer's limit (often about 2 %).
5. Measure the current on each phase. Clamp each phase conductor separately: the inrush current at start and the running current after the system has stabilised.
  ✓ Check: The running current does not exceed the rated load amps (RLA) on the nameplate and is about equal on all three phases.
6. Record and act. Write voltages, imbalance and currents in the service report. High voltage drop: tighten and check connections under isolation. Imbalance or high current: trace the supply or the motor before restarting repeatedly.
  ✓ Check: Readings are documented and any fault is traced to supply, connection or motor.
Check an inverter (VFD) and read its alarms
When a speed-controlled compressor, fan or pump will not run or trips, the inverter usually tells you why. Read its alarms first, then check supply, signals and output before you replace anything.
1. Read and note the alarm log. Before you reset or switch off, read the active fault and the fault history on the drive keypad or service tool. Note code, time and the values stored with it (current, DC bus voltage, frequency). Look up each code in the drive manufacturer's manual.
  ✓ Check: You have a written list of codes with their meaning from the manual, and you know which fault occurred last.
2. Check the key parameters. Compare motor data (rated voltage, current, frequency), minimum and maximum speed, ramp times and current limit with the compressor or fan manufacturer's settings list. Compressors often have a minimum speed for oil return and lubrication.
  ✓ Check: The parameters match the manufacturer's list; any differences are noted before you change them.
3. Measure the supply voltage. With a CAT III/IV multimeter and insulated probes, measure phase to phase on L1, L2 and L3 at the drive input, at standstill and in operation. Do not wear loose clothing near open live parts.
  ✓ Check: All three voltages are within the input range in the drive manual and close to each other; a missing or low phase explains supply alarms.
4. Check run, enable and speed signals. At the control terminals, measure the run/enable input and the speed reference (0-10 V or 4-20 mA) while the controller asks for operation. Follow the safety chain (pressure switches, motor protection) if the enable is missing.
  ✓ Check: The drive display shows a run command and the reference value that the controller sends; if not, the fault is upstream of the drive.
5. Measure output voltage and current. With the drive running, measure U-V, V-W and W-U with a meter in low-pass (VFD) mode, and the current in each output phase with a true-RMS clamp meter. Compare with the frequency on the display and the motor nameplate.
  ✓ Check: The three output voltages and currents are balanced, and the current is within the motor rating at that speed.
6. Isolate and wait for the DC bus. Before you disconnect or tighten anything: isolate, lock off and tag. Wait at least the discharge time on the drive label, then measure DC voltage across the DC bus terminals (+/-). Check terminals for heat damage and tighten them. If you megger the motor, disconnect the motor leads from the drive first; never insulation test through the drive.
  ✓ Check: The meter shows no dangerous DC voltage on the bus before you touch anything, and the motor test is done with the drive disconnected.
7. Restart and watch. Reconnect, close the cover, switch on and start the unit. Follow speed, current and alarms during start and a full run cycle.
  ✓ Check: The drive ramps up to the requested speed without new alarms, and the fault log shows no new entries.

Checks from the manufacturers’ manuals (1)

Values and checks the manufacturers give for their own products; they apply to the product named.

Hussmann pumped CO2: the pump stops if its differential stays below about 0.35 bar (5 psi) or above 4.1 bar (60 psi) for 10 s, restarts after 2 minutes and locks out after the fourth stop in 15 minutes.^[3]

📝 Fill in a service report

Refrigerant pump cavitates or loses pressure

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Check liquid level control in a vessel

Check a liquid refrigerant pump

Check a liquid refrigerant pump

Check liquid level control in a vessel

Check liquid level control in a vessel

Check a sensor against a reference

Check a liquid refrigerant pump

Check valve positions and trapped liquid

Measure voltage and current under load

Check an inverter (VFD) and read its alarms

Checks from the manufacturers’ manuals (1)

Service report

Sources