The service valve is the door through which the maintenance technician enters the system. It has a main passage, a stem and one or two service ports. Rotalock is the name of the way this valve is connected to the compressor or receiver: the valve is tightened onto the threaded port on the body with a large nut and a Teflon seal; it can be removed and refitted without brazing. When a compressor is replaced only the nuts are undone, without touching the pipes. On large semi-hermetic compressors the same duty is performed by flanged shut-off valves fastened with bolts.
The stem has three positions. When it is turned fully out (back-seated) the main passage is fully open and the service port is closed; this is the normal operating position. When it is turned fully in (front-seated) the line is closed and the compressor is isolated from the system. In the intermediate position, a turn or two in from the back seat, both the line and the service port are open; the gauge is read in this position. On valves that have a Schrader port, the port is open in every position. The position it is left in is checked after every job.
By closing the valve at the receiver outlet and running the compressor, the refrigerant is collected in the receiver and condenser; in this way the filter drier, expansion valve or evaporator can be replaced without draining the refrigerant. The compressor is never run with the discharge valve closed: the pressure rises to a dangerous level within seconds. Throttling the suction valve to make the compressor pull a deep vacuum also causes damage, especially in scroll compressors. If the compressor is to be removed, both the suction and discharge valves are closed and the gas left in between is taken out with a recovery unit; refrigerant is not released to the air.
Before the stem is turned the packing gland nut is slackened a quarter turn, and after the job it is tightened again; the stem is turned with a ratchet service wrench, not with pliers. When the job is finished the valve is back-seated and the stem cap and port caps are fitted with their seals; a valve left without its caps is one of the most common sources of slow leaks. Rotalock nuts are tightened to the torque given by the manufacturer and the Teflon seal is renewed every time the valve is removed. Finally the area around the stem and nut is scanned once more with a leak detector.