It is set at or below the boiler's MAWP, 15 psi on a low-pressure steam boiler, and must be able to discharge all the steam the burner can make. No valve may sit between it and the boiler, and its discharge pipe runs full size to a safe point. Operators lift the try lever periodically, with pressure on, to prove the disc is not stuck to the seat.
Safety valve on number two lifted at 14, that's right where it should be, but the operating control should never let it get there.
Never plug, cap or reduce a safety valve discharge; inspectors shut a boiler down for that alone.
Also heard as
- safety
- steam safety
- pop valve
Do not mix it up with
- Relief valve: Steam boilers use safety valves that pop open fully; hot water boilers use relief valves that open in proportion to overpressure.
Related terms
Relief valve
The pressure-relief valve on a hot water boiler, rated in Btu per hour and set at or below the boiler's maximum pressure.
Pop-off
Shop slang for the safety valve on a steam boiler or the relief valve on a hot water boiler.
MAWP
Maximum allowable working pressure: the highest pressure a boiler is approved to operate at, stamped on its nameplate.
High limit
A safety control that shuts down the burner if pressure or temperature rises above the normal operating range.