A. A vacuum relief valve is usually located on the steam chest or the
live steam passage to the chest and opens when steam is shut off and
engine drifting, allowing atmospheric pressure to pass into the steam
chest, closing when working steam. A cylinder relief valve is a pop
valve screwed into the cylinder head and set at high enough pressure so
it does not open in ordinary service, but will open to allow water to
pass out when the exhaust port is closed by valves; or on compound
engines when the pressure in the low-pressure cylinder gets too high.