Nitrogen pressure test
Also known as: pressure test, tightness test, strength test
Short definitionPressurising the circuit with dry nitrogen and holding it before any refrigerant goes in. It proves both tightness and strength; evacuation and charging follow.
Once the pipework is finished, the circuit is filled with dry nitrogen and held under pressure. Nitrogen is cheap, does not burn, carries no moisture and does not react with oil. Oxygen explodes on contact with oil and compressed air brings moisture into the circuit; neither is used. Pressure is raised first to 3–5 bar and, if there is no gross leak, to the value chosen from the maximum allowable pressure (PS) on the nameplate.
The circuit stands for at least a few hours, preferably 24; the start and end pressures are noted together with ambient temperature. At 20 bar each 1 K is worth about 0.07 bar, so a drop of nearly 0.6 bar in pipework that cooled by 8 K overnight is not a leak. Pressure switches and relief valves that cannot take the test pressure are isolated beforehand. Nitrogen is never vented into a closed plant room; there is a risk of asphyxiation.