High standstill pressure in a CO2 system

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 rises as the refrigerant warms to ambient temperature. Without standstill cooling the safety valve eventually opens.

   Remedy: Check that the auxiliary cooling unit is running and has backup power if needed. Start the system according to the manufacturer's instructions. Never close valves so that liquid CO2 is trapped between them, and never isolate the safety valve.

   🛠 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.

2. 2

   The back-up cooling unit or its supply (UPS, fuse, contactor) does not work.

   Remedy: Test run the back-up cooling and check supply, fuses and control signal.

   🛠 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.

3. 3
   Flowchart B

   The flash gas valve or its control is not working, so the receiver pressure rises.

   Remedy: Check the valve's driver, transducer and setpoint, and that the MT compressors can take the flash gas.

   🛠 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 CO2 high pressure and flash gas valves

   On a transcritical CO2 (R744) system the high pressure valve controls the gas cooler pressure and the flash gas bypass valve controls the receiver pressure. A faulty valve, driver or transducer gives poor capacity, high pressure trips or a safety valve that opens.

   1. Prepare for CO2 pressures. Use gauges and hoses rated above the PS of the high pressure side (see the nameplate; often above 100 bar), goggles and gloves. Never close valves so that liquid CO2 is trapped between them: the pressure rises quickly when it warms. CO2 displaces oxygen in low and enclosed spaces, so check that the gas detector and ventilation work.

      ✓ Check: Gauges are rated above the system PS and the machine room gas detector shows normal.

   2. Compare transducers with gauges. Connect the controller service tool and read gas cooler pressure, receiver pressure and gas cooler outlet temperature. Compare each pressure transducer with a calibrated gauge at the same point.

      ✓ Check: Each transducer reading agrees with the gauge within the accuracy stated in the transducer data sheet.

   3. Check the high pressure valve. Watch the valve opening (%) and the gas cooler pressure in the service tool while the gas cooler outlet temperature changes. Compare the pressure with the setpoint the controller calculates. Check the driver for alarms and, if the manual allows, drive the valve manually and listen or feel for movement.

      ✓ Check: The gas cooler pressure follows the controller setpoint and the valve opening changes smoothly, without hunting or alarms.

   4. Check the flash gas bypass valve. Compare the receiver pressure with the setpoint in the commissioning documents. Watch the valve opening as the load changes.

      ✓ Check: The receiver pressure stays close to its setpoint and below the receiver safety valve setting.

   5. Check the gas cooler fan control. Check that the gas cooler fans speed up as the outlet temperature rises and that the control sensor reads correctly (compare with a contact thermometer).

      ✓ Check: Fan speed follows the gas cooler outlet temperature and the sensor agrees with the reference thermometer.

   6. Check relief paths and record. Check that every pipe section that can be closed off by valves has a pressure relief device, and that safety valve outlets are open and unblocked. Record readings, alarms and any corrections.

      ✓ Check: No section can trap liquid without relief, and the readings are recorded.

4. 4
   Flowchart C

   High gas cooler pressure because of a fault in the high-pressure valve or the gas cooler fans, or high outdoor temperature.

   Remedy: Check the fans, cleanliness and the high-pressure valve control. Follow the high-pressure troubleshooting.

   🛠 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.

   Clean a condenser or gas cooler

   A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

   1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

   2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

   3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

   4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

   5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

   6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

   Check a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

   Check CO2 high pressure and flash gas valves

   On a transcritical CO2 (R744) system the high pressure valve controls the gas cooler pressure and the flash gas bypass valve controls the receiver pressure. A faulty valve, driver or transducer gives poor capacity, high pressure trips or a safety valve that opens.

   1. Prepare for CO2 pressures. Use gauges and hoses rated above the PS of the high pressure side (see the nameplate; often above 100 bar), goggles and gloves. Never close valves so that liquid CO2 is trapped between them: the pressure rises quickly when it warms. CO2 displaces oxygen in low and enclosed spaces, so check that the gas detector and ventilation work.

      ✓ Check: Gauges are rated above the system PS and the machine room gas detector shows normal.

   2. Compare transducers with gauges. Connect the controller service tool and read gas cooler pressure, receiver pressure and gas cooler outlet temperature. Compare each pressure transducer with a calibrated gauge at the same point.

      ✓ Check: Each transducer reading agrees with the gauge within the accuracy stated in the transducer data sheet.

   3. Check the high pressure valve. Watch the valve opening (%) and the gas cooler pressure in the service tool while the gas cooler outlet temperature changes. Compare the pressure with the setpoint the controller calculates. Check the driver for alarms and, if the manual allows, drive the valve manually and listen or feel for movement.

      ✓ Check: The gas cooler pressure follows the controller setpoint and the valve opening changes smoothly, without hunting or alarms.

   4. Check the flash gas bypass valve. Compare the receiver pressure with the setpoint in the commissioning documents. Watch the valve opening as the load changes.

      ✓ Check: The receiver pressure stays close to its setpoint and below the receiver safety valve setting.

   5. Check the gas cooler fan control. Check that the gas cooler fans speed up as the outlet temperature rises and that the control sensor reads correctly (compare with a contact thermometer).

      ✓ Check: Fan speed follows the gas cooler outlet temperature and the sensor agrees with the reference thermometer.

   6. Check relief paths and record. Check that every pipe section that can be closed off by valves has a pressure relief device, and that safety valve outlets are open and unblocked. Record readings, alarms and any corrections.

      ✓ Check: No section can trap liquid without relief, and the readings are recorded.

   Continue with: System trips on high pressure →

5. 5
   Flowchart D

   The safety valve opens too early: wrong set pressure, a damaged valve or a valve that has already lifted and no longer seals.

   Remedy: Compare the valve's set pressure with the system's PS. Replace a valve that has lifted or is leaking, and check the discharge line according to EN 13136 / EN ISO 24664.

   🛠 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) →