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Safety Valves
The last line of defense against overpressure. Types, operation, testing, and ASME requirements.
Purpose and Operation
Safety valves are the LAST line of defense against overpressure. They open automatically when pressure reaches the set point to prevent boiler explosion.
- Opens automatically at set pressure
- Relieves steam to prevent pressure from exceeding MAWP
- Must be set at or below the MAWP of the boiler
- Pop-type safety valves open fully and quickly (pop action)
- Must be ASME-stamped and capacity-rated
NEVER gag, block, restrict, or sit on a safety valve for ANY reason.
ASME Code Requirements
ASME Boiler and Pressure Vessel Code sets strict requirements for safety valves.
- Every boiler must have at least one safety valve
- Set pressure: at or below MAWP
- Accumulation: pressure must not exceed MAWP by more than 6% with one valve, 6% with two or more
- Blowdown: the difference between the set pressure (lift) and the closing pressure (reseat)
- Only qualified/authorized personnel may adjust safety valves
- Valves must be tested regularly by lifting the try lever
Testing Safety Valves
Regular testing ensures the safety valve will function when needed.
- Try lever test: manually lift the lever to verify the valve operates freely
- Should be tested when boiler is at 75% or more of operating pressure
- If the valve does not reseat cleanly, it may need repair or replacement
- Never adjust the set pressure unless properly authorized
- Document all safety valve tests in the boiler logbook
Hydrostatic Test
A hydrostatic test uses water pressure (not steam) to verify the integrity of the boiler pressure vessel.
- Typically 1.5x the MAWP
- Done with the boiler filled with water (no steam)
- Water is used because it is incompressible — safer than steam if a failure occurs
- Performed after repairs, new construction, or as required by the inspector