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Tight Seating of Shut-Off Valves
Most shut-off valves in ammonia refrigeration systems are usually kept in an open position but closed when necessary to isolate a component such as a control valve, evaporator, or other piece of equipment for servicing. Unless the shut-off valve contains large particles under the seat disc or the seat is damaged, applying adequate closing torque to the valve stem or to its handwheel will seal the valve seat against flow or leakage.

For shut-off valves usually kept in a closed position, for which accidental opening is not acceptable, it is wise to use a seal cap over the valve rather than a handwheel. An untrained person can easily open the valve seat with the handwheel and when re-closed by the same person, the torque may not be adequate or the pressure may have already been transmitted. Such valves should be tagged "keep closed" or have similar instructions. (Valves to open ended pipes that are left unattended require special precautions not addressed here. Please refer to Techni-Briefs TB5 dated February 1999.)

A "usually closed" valve should be protected against random but very rare piping vibrations which might slightly rotate the stem to crack open the valve seat. This is virtually impossible with a seal cap valve; however, a sturdy handwheel might have such a vibrating rotation effect to slightly turn the stem, as has been proven in laboratory tests. Extra firm tightening of the stem packing nut will prevent the stem from rotating and ensure that the valve seat remains tight.

Tight seating of shut-off valves can be achieved by using adequate closing torque, using seal caps and marking the valve to avoid non-authorized opening, and by firmly tightening the stem packing nut.