The（） is fitted in pair one is the main valve, the other the auxiliary or standby.

A.main stem stop valve
B.auxiliary steam stop valve
C.feed check or control valve
D.blow down valve

A.main steam stop valve
B.auxiliary steam stop valve
C.feed check or control valve
D.blow down valve

A.fuel valves of main engine
B.injector pump of main engine
C.safety valves of marine boiler
D.safety valves of a reciprocating pump