Once again, bearing in mind that high pressures are generated whilst a plant is operating, care must be taken not to pollute the atmosphere when gauges are removed. The following procedure will minimize the loss of refrigerant and discharge to atmosphere:
1. Back seat the discharge service valve.
2. Open both shut-off valves on the manifold and allow the pressures to equalize in the gauges.
3. Close high side valve on the manifold and operate the compressor until a zero pressure is indicated on the compound gauge. Close the low side valve on the manifold.
4. Remove the hose from the discharge service valve union, replace the seal cap and tighten.
5. Back seat the suction service valve.
6. Remove the hose from the suction service valve union, replace the seal cap and tighten.
7. Set both service valves to the operating positions (cracked off the back seat).
8. Replace the service valve caps and tighten.
9. Test for leaks at the valve caps and gauge unions
The low side pressure in systems charged with refrigerants R22 and R502 can be much higher than that in systems charged with refrigerant R12. Thus when removing the hose from the low side, loosen the connection and release the pressure slowly. Although the pressure may be high, the volume of vapour in the hose will be small.