Strength testing with OFN

Strength testing with OFN


The strength test procedure should comply with the recommendations listed in the European standard EN 378. The test pressure used should be between 1.1 and 1.43 times the allowable pressure  depending on the PED category for that part of the system.

The system may need to be divided into high and low sides for pressure testing purposes. This is required to ensure that the manufacturers test pressure applied to the low side,  eg evaporator etc is not exceeded (particularly important on shell and tube evaporators using water).

On some systems you may apply the same test pressure to the whole system providing that the low side equipment and piping has the same maximum allowable pressure as the high side.

A thorough risk assessment must be performed by a trained and competent service person prior to the pressure testing activity. Ensuring all hazards are identified and sufficient control measures are in place before testing starts.

Care should be taken to ensure that any component that might be damaged by the high pressure is removed or isolated from the system. This may include items such as relief valves, pressure switches, thermostatic expansion valves (TEVs), bursting discs, etc.

Oxygen Free Nitrogen (OFN) should be slowly introduced into the system in order to increase its pressure in a controlled manner. Raise the pressure using calibrated tools at 1bar intervals, leak test the system before any further increase in pressure up to the maximum test pressure and hold at the pressure for a time dependant on the PED for the equipment. Release the pressure in a controlled manner to zero and commence evacuation.

Maximum allowable pressures in the UK are to be at least equivalent to the saturated refrigerant pressures corresponding to the following minimum temperatures:

  • 32oC for the low side of the system

  • 43oC for the high side of a water cooled system

  • 55oC for the high side of an air cooled system.