How does a safety valve on a steam boiler differ from a relief valve on a liquid system?
- It opens slowly as pressure rises
- It pops fully open at its set pressure
- It is opened by an electric solenoid coil
- It closes only when reset by hand
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B. It pops fully open at its set pressure
A steam safety valve has a huddling chamber that makes it pop wide open at the set pressure and stay open until pressure falls. Opening gradually in proportion to overpressure is how a liquid relief valve behaves.
Reference: Boiler fittings: safety valves and relief valves
Under ASME Section I, the safety valves on a power boiler must be able to discharge all the steam the boiler can generate without the pressure rising more than what percentage above the MAWP?
- 3 percent
- 10 percent
- 25 percent
- 6 percent
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D. 6 percent
Section I limits the pressure rise to 6 percent above the MAWP with the safety valves blowing. 3 percent is the allowed tolerance on set pressure for some ranges and is often confused with this limit.
Reference: ASME BPVC Section I, PG-67
Under ASME Section I, a power boiler must have two or more safety valves when its water heating surface exceeds:
- 500 sq ft
- 100 sq ft
- 250 sq ft
- 1,000 sq ft
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A. 500 sq ft
Boilers with more than 500 square feet of bare tube water heating surface need at least two safety valves. The same 500 sq ft figure triggers the requirement for two means of feeding water.
Reference: ASME BPVC Section I, PG-67