Safety valve discharges or leaks

Causes, most likely first

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

1. 1
   Flowchart A

   The pressure rose during standstill as the refrigerant warmed up.

   Remedy: Troubleshoot the standstill pressure: back-up cooling, pressure relief and valve positions during a power failure.

   🛠 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.

   Test CO2 standstill cooling and power failure

   At standstill a CO2 (R744) plant warms up and the pressure rises. An auxiliary cooling unit, often on back-up power, must hold the receiver pressure below the safety valve setting, and the restart must be done without releasing the safety valves.

   1. Prepare and agree the test. Read the manufacturer's procedure, tell the store or plant operator and plan the test at a time when a short stop is acceptable. Use gauges rated for CO2. Never close valves so that liquid is trapped between them, and never isolate a safety valve.

      ✓ Check: The procedure is at hand and the operator knows the test is being done.

   2. Note pressures and settings. Read the receiver pressure, the safety valve set pressure, the PS of the receiver and the set points for starting and stopping the auxiliary unit.

      ✓ Check: The auxiliary unit's start pressure is clearly below the safety valve setting.

   3. Check the auxiliary unit and its supply. Check that the auxiliary condensing unit is in automatic mode, that its supply comes from the back-up source (UPS or generator, if fitted) and that its fuses and contactor are in order.

      ✓ Check: The auxiliary unit is ready and fed from the back-up supply.

   4. Simulate the power failure. Switch off the main supply to the CO2 rack as the procedure describes. Watch the receiver pressure and check that the auxiliary unit starts at its set pressure.

      ✓ Check: The auxiliary unit starts and the receiver pressure stops rising and stays below the start pressure plus a small margin.

   5. Check gas detection and ventilation. Check that the CO2 detector in the machinery room works and that the alarm and ventilation start at the alarm level (test with test gas as the detector maker describes).

      ✓ Check: The alarm and mechanical ventilation start at the detector's alarm level.

   6. Restore power and restart. Restore the main supply and restart the plant by the manufacturer's procedure. Watch the receiver pressure, the high pressure valve and the flash gas valve as the compressors start.

      ✓ Check: The plant starts up smoothly and no safety valve lifts.

   7. Record the test. Record the date, pressures, start pressure of the auxiliary unit and the result in the plant logbook.

      ✓ Check: The test result is recorded and the plant runs normally.

   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.

   Continue with: High standstill pressure in a CO2 system →

2. 2
   Flowchart B

   Operating pressure too high: the high pressure switch or the controls did not stop in time.

   Remedy: Troubleshoot the high pressure and check that the pressure switch cuts out before the safety valve opens. Check with a leak detector that the safety valve has reseated tightly, and replace it if it leaks.

   🛠 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.

   Test a pressure switch or transducer

   High and low pressure switches protect the compressor and the system. Check that they cut out and cut in at their settings against a calibrated gauge, and that transducers send the correct signal. Never bypass a pressure switch.

   1. Connect a calibrated gauge. Connect a calibrated gauge or digital manifold to a service port on the same side and as close as possible to the switch or transducer. Wear goggles and gloves; with A2L/A3 refrigerants keep ignition sources away.

      ✓ Check: The gauge reads a stable pressure close to the switch.

   2. Read the settings. Note cut-out, cut-in (or differential) and reset type (automatic or manual) from the switch scale or the controller. The HP setting must be below the safety valve set pressure and the system PS.

      ✓ Check: You know what each switch should do and the HP setting is below the safety valve pressure.

   3. Test the low pressure switch. Slowly close the liquid line valve (pump down) while watching the gauge and the switch contact or the controller input. Do not let the compressor run into deep vacuum.

      ✓ Check: The switch opens at its cut-out setting within the manufacturer's tolerance and closes again at its cut-in.

   4. Test the high pressure switch. Where it can be done safely, reduce the condenser airflow step by step while watching the gauge. Stop immediately if the pressure reaches the setting without a trip. On CO2 systems, or where raising the pressure is not safe, test the switch off the system with nitrogen and a regulator instead.

      ✓ Check: The switch trips at its setting within tolerance and well below the safety valve pressure.

   5. Check the transducer signal. Measure the output (e.g. 0.5–4.5 V or 4–20 mA) at the controller input and convert it with the transducer range to pressure.

      ✓ Check: The calculated pressure agrees with the reference gauge within the transducer's tolerance.

   6. Restore and record. Open the valves fully, restore airflow, reset any manual reset device and write the measured cut-out and cut-in points in the service report. Replace a switch that does not trip at its setting.

      ✓ Check: The system runs normally and the measured values are documented.

   Check a safety valve

   The safety valve protects the system against pressure above PS. It must have the right setting, close tightly after opening and discharge to a safe place through a correctly sized line.

   1. Check the marking. Read the set pressure and the valve data from the valve body or nameplate and compare with the PS of the protected part. The set pressure must not be higher than PS. Never adjust, block or isolate a safety valve.

      ✓ Check: The set pressure is at or below the PS of the protected part.

   2. Check the valve's condition. Look for corrosion, damage, a broken seal and signs that the valve has opened (frost, oil or dirt at the outlet). Check the date of manufacture or fitting.

      ✓ Check: The valve is undamaged, the seal is intact and the age is noted.

   3. Check for seat leakage. Check the outlet with an electronic leak detector or bubble solution. A valve that has opened often does not close tightly again.

      ✓ Check: The detector gives no signal and no bubbles form at the outlet.

   4. Check the discharge line. Check that the discharge line is sized for the valve (EN 13136; use a relief line calculator), is not blocked, has no shut-off valve, and ends outdoors in a safe place, protected from rain and with drainage.

      ✓ Check: The line size agrees with the calculation and the outlet is open and safe.

   5. Replace when needed. Replace a valve that leaks, has opened, is damaged or has reached the replacement interval in the manufacturer's instructions or national rules. With a change-over valve, switch to the other safety valve first; otherwise the system must be pumped down or recovered (F-gas certification).

      ✓ Check: A new valve with the right setting is fitted, or the existing valve is approved.

   6. Record. Record the check, any discharge and any replacement (set pressure, serial number, date) in the logbook.

      ✓ Check: The logbook shows the valve data and the result of the check.

   Leak test a refrigeration system

   Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).

   1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.

      ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.

   2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.

      ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.

   3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.

      ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.

   4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.

      ✓ Check: A fluorescent trace marks the leak, or no trace is visible.

   5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.

      ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.

   6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.

      ✓ Check: The logbook entry is complete and a follow-up check is booked.

   Continue with: System trips on high pressure →

3. 3

   The high pressure switch did not open because of an electrical fault: bypassed, wrongly wired or faulty.

   Remedy: Check the wiring and the cut-out with a calibrated gauge. A pressure switch must never be bypassed; fix it at once.

   🛠 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.

   Test a pressure switch or transducer

   High and low pressure switches protect the compressor and the system. Check that they cut out and cut in at their settings against a calibrated gauge, and that transducers send the correct signal. Never bypass a pressure switch.

   1. Connect a calibrated gauge. Connect a calibrated gauge or digital manifold to a service port on the same side and as close as possible to the switch or transducer. Wear goggles and gloves; with A2L/A3 refrigerants keep ignition sources away.

      ✓ Check: The gauge reads a stable pressure close to the switch.

   2. Read the settings. Note cut-out, cut-in (or differential) and reset type (automatic or manual) from the switch scale or the controller. The HP setting must be below the safety valve set pressure and the system PS.

      ✓ Check: You know what each switch should do and the HP setting is below the safety valve pressure.

   3. Test the low pressure switch. Slowly close the liquid line valve (pump down) while watching the gauge and the switch contact or the controller input. Do not let the compressor run into deep vacuum.

      ✓ Check: The switch opens at its cut-out setting within the manufacturer's tolerance and closes again at its cut-in.

   4. Test the high pressure switch. Where it can be done safely, reduce the condenser airflow step by step while watching the gauge. Stop immediately if the pressure reaches the setting without a trip. On CO2 systems, or where raising the pressure is not safe, test the switch off the system with nitrogen and a regulator instead.

      ✓ Check: The switch trips at its setting within tolerance and well below the safety valve pressure.

   5. Check the transducer signal. Measure the output (e.g. 0.5–4.5 V or 4–20 mA) at the controller input and convert it with the transducer range to pressure.

      ✓ Check: The calculated pressure agrees with the reference gauge within the transducer's tolerance.

   6. Restore and record. Open the valves fully, restore airflow, reset any manual reset device and write the measured cut-out and cut-in points in the service report. Replace a switch that does not trip at its setting.

      ✓ Check: The system runs normally and the measured values are documented.

   Trace the control and safety chain

   When a compressor or fan does not get a start signal, follow the control voltage through the safety chain in the wiring diagram to the point where it disappears. Live measurements in an open panel are for qualified personnel only.

   1. Get the wiring diagram. Find the diagram for the unit and mark the path from the control supply through fuses, safety devices (HP/LP switches, motor protection, thermal protection) and controller outputs to the contactor coil. Use a CAT III/IV meter with insulated probes.

      ✓ Check: The diagram matches the unit's model and serial plate, and you know the order of the devices.

   2. Check the control supply. Measure the voltage at the secondary of the control transformer or power supply and check the control fuses.

      ✓ Check: The rated control voltage (e.g. 24 V AC or 230 V AC) is present and the fuses are intact.

   3. Follow the chain device by device. Measure from each terminal in the chain to neutral or control common, in the order of the diagram. Never bridge a safety switch to make the system run.

      ✓ Check: You find the point where voltage is present on the input but 0 V on the output: that device is open.

   4. Find out why the device is open. Read the device: is the HP switch tripped and does it have a manual reset, is the motor protection tripped, is a door or flow switch open? Check the cause (pressure, current, temperature) before resetting.

      ✓ Check: The cause of the trip is known, not only the device.

   5. Check the controller output and relays. With a call for operation active, measure the output at the controller relay terminal. A relay that is energised but has no voltage on its output, or that stays closed without a call, is faulty.

      ✓ Check: The output follows the call: voltage when the controller calls and none when it does not.

   6. Repair and test. Isolate and lock off before replacing a part or tightening terminals. Restore power, reset and check that the chain closes and the contactor pulls in.

      ✓ Check: The system starts on call and stops when each safety device opens.

4. 4
   Flowchart C

   The valve does not reseat tightly after discharging, often because of dirt or a damaged seat.

   Remedy: Replace the valve. Never adjust a safety valve yourself, and record the event in the logbook.

   🛠 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 safety valve

   The safety valve protects the system against pressure above PS. It must have the right setting, close tightly after opening and discharge to a safe place through a correctly sized line.

   1. Check the marking. Read the set pressure and the valve data from the valve body or nameplate and compare with the PS of the protected part. The set pressure must not be higher than PS. Never adjust, block or isolate a safety valve.

      ✓ Check: The set pressure is at or below the PS of the protected part.

   2. Check the valve's condition. Look for corrosion, damage, a broken seal and signs that the valve has opened (frost, oil or dirt at the outlet). Check the date of manufacture or fitting.

      ✓ Check: The valve is undamaged, the seal is intact and the age is noted.

   3. Check for seat leakage. Check the outlet with an electronic leak detector or bubble solution. A valve that has opened often does not close tightly again.

      ✓ Check: The detector gives no signal and no bubbles form at the outlet.

   4. Check the discharge line. Check that the discharge line is sized for the valve (EN 13136; use a relief line calculator), is not blocked, has no shut-off valve, and ends outdoors in a safe place, protected from rain and with drainage.

      ✓ Check: The line size agrees with the calculation and the outlet is open and safe.

   5. Replace when needed. Replace a valve that leaks, has opened, is damaged or has reached the replacement interval in the manufacturer's instructions or national rules. With a change-over valve, switch to the other safety valve first; otherwise the system must be pumped down or recovered (F-gas certification).

      ✓ Check: A new valve with the right setting is fitted, or the existing valve is approved.

   6. Record. Record the check, any discharge and any replacement (set pressure, serial number, date) in the logbook.

      ✓ Check: The logbook shows the valve data and the result of the check.

   Record work in the F-gas logbook

   Leak checks, repairs and every kilogram charged or recovered must be recorded. In the EU this follows Regulation (EU) 2024/573; other countries have their own rules (USA: EPA, 40 CFR Part 82 and 84).

   1. Find the right record. Open the logbook or digital register for the specific system and check that the plant ID, location and refrigerant match the nameplate.

      ✓ Check: The record belongs to the system you are working on.

   2. Record who and when. Enter the date, your company's certificate number, your own F-gas certificate number and your name.

      ✓ Check: Date and both certificate numbers are entered.

   3. Describe the work. Enter the type of work (leak check, repair, maintenance, installation, decommissioning), the result of any leak check, where a leak was found and how it was repaired.

      ✓ Check: Someone else can understand from the entry what was done and found.

   4. Record the quantities. Enter the refrigerant type and the quantity added and recovered in kg, read from the scale. For recovered refrigerant, note whether it is to be recycled, reclaimed or destroyed and where it goes. The CO2 equivalent can be added.

      ✓ Check: Quantities in kg match the scale readings.

   5. Plan the follow-up. After a leak repair, the system must be checked again by certified personnel within one month. Note that date and the next periodic leak check.

      ✓ Check: Both dates are written in the logbook.

   6. Sign and keep. Sign the entry and give the operator a copy if the logbook is not kept on site. The operator keeps the records for at least five years.

      ✓ Check: The entry is signed and the operator has access to it.

5. 5

   Liquid trapped between closed valves (for example after service or during pump-down) expands as it warms up and the pressure rises sharply.

   Remedy: Find which valves were closed when it happened. Never shut in a pipe section full of liquid; open the valves or drain the section, and fit a relief valve on sections that can be shut in.

   🛠 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 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.

6. 6
   Flowchart D

   The valve has the wrong set pressure or is too close to the operating pressure, or the discharge line gives too much back pressure.

   Remedy: Compare the valve marking with the system PS and operating pressure. Check the discharge line with the calculator.

   🛠 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 safety valve

   The safety valve protects the system against pressure above PS. It must have the right setting, close tightly after opening and discharge to a safe place through a correctly sized line.

   1. Check the marking. Read the set pressure and the valve data from the valve body or nameplate and compare with the PS of the protected part. The set pressure must not be higher than PS. Never adjust, block or isolate a safety valve.

      ✓ Check: The set pressure is at or below the PS of the protected part.

   2. Check the valve's condition. Look for corrosion, damage, a broken seal and signs that the valve has opened (frost, oil or dirt at the outlet). Check the date of manufacture or fitting.

      ✓ Check: The valve is undamaged, the seal is intact and the age is noted.

   3. Check for seat leakage. Check the outlet with an electronic leak detector or bubble solution. A valve that has opened often does not close tightly again.

      ✓ Check: The detector gives no signal and no bubbles form at the outlet.

   4. Check the discharge line. Check that the discharge line is sized for the valve (EN 13136; use a relief line calculator), is not blocked, has no shut-off valve, and ends outdoors in a safe place, protected from rain and with drainage.

      ✓ Check: The line size agrees with the calculation and the outlet is open and safe.

   5. Replace when needed. Replace a valve that leaks, has opened, is damaged or has reached the replacement interval in the manufacturer's instructions or national rules. With a change-over valve, switch to the other safety valve first; otherwise the system must be pumped down or recovered (F-gas certification).

      ✓ Check: A new valve with the right setting is fitted, or the existing valve is approved.

   6. Record. Record the check, any discharge and any replacement (set pressure, serial number, date) in the logbook.

      ✓ Check: The logbook shows the valve data and the result of the check.

   Relief valve & discharge line (EN 13136 / EN ISO 24664) →