Page 392 - Reciprocating Compressors Operation Maintenance

Safety in Operation and Maintenance 377

To prevent this possibility, noninterchangeable valves are available.
Valve polarization and similar methods have been developed to prevent
accidental interchanges.

FULL-PIN POLARIZATION

In full-pin polarization, the seat and guards are fitted on the circumfer-
ence with locating pins. The pins on the guard are located 180° apart
while those on the seat are located 90° apart. The valve ports are slotted
for insertion of the pins. The discharge port contains these slots at 90°,
and the suction port contains these slots at 180°. Only suction valves can
thus be placed in the suction valve port and discharge valves placed in
the discharge valve port.

SEMI-PIN POLARIZATION

Semi-polarized valves are similar to the polarized valves, except they
have only one locating pin, located on the guard circumference. The suc-
tion valve ports are slotted to accept this pin. Since the discharge valve
ports are not slotted, semi-polarized suction valves will fit only in the
suction ports.

DIAMETRICAL POLARIZATION

Diametrical polarization prevents accidental assembly of discharge
service valves in suction ports, or the reverse. Different concentric diam-
eters, machined on guard and seat, interfere with valve port cavities
when a valve is placed in the wrong port. Valves properly seated in suc-
tion ports will operate as suction valves, and valves seated in discharge
ports will operate as discharge valves (see Figure 7-1).

VALVE COVER BOLTING

Compressor valve covers should be checked to make sure they are
fully bolted rather than short bolted, meaning they have only some of the
threads of the nut in contact with the stud bolt. Full-thread contact is nec-
essary to reduce the risk of the compressor valve cover being blown off.
There is a remote possibility that a longer stud could have been substi-
tuted, which gives the appearance of short bolting. But this is unlikely.