For power boilers, the maximum allowable pressure accumulation with a single safety valve is typically:

• A. 6% above MAWP
• B. 10% above MAWP
• C. 6% above MAWP for single valve; 10% for multiple valves
• D. 8% above MAWP

Answer: (C)

The idea tested here is how much pressure the boiler can tolerate above its rated limit before the safety relief system must act, and how that tolerance changes with the number of relief valves.

Safety valves are there to prevent overpressure by venting steam. Because valves take a moment to lift and because a single valve has limited flow capacity, the code allows only a small upward drift above the MAWP before relief must start. With one valve, that drift is about 6% above MAWP. The pressure should not rise far enough to threaten the MAWP before the valve begins to vent.

If you have two or more safety valves, more total relief capacity is available, so the system can tolerate a slightly larger initial buildup—up to about 10% above MAWP—before needing to relieve. The valves collectively handle the surge, reducing the risk that any single valve’s delay or limited flow would allow the pressure to push past MAWP.

So, the typical rule is a 6% allowance above MAWP for a single valve, and up to 10% for multiple valves.