Why are boiler safety relief valves required and how are they sized?

• A. To regulate feedwater temperature.
• B. To prevent overpressure; sized to relieve the maximum allowable working pressure with appropriate blowdown and capacity based on steam flow.
• C. To remove moisture from steam after the turbine.
• D. To vent steam to atmosphere.

Answer: (B)

Safety relief valves protect the boiler from overpressure. They open when pressure approaches or exceeds the maximum allowable working pressure and vent steam to keep the system from reaching unsafe levels. They are sized to relieve the maximum allowable working pressure with proper blowdown and enough capacity to handle the worst-case steam flow the boiler could generate in a fault condition. Blowdown ensures the valve reseats after relieving, while the capacity is chosen based on the anticipated worst-case steam flow, so pressure stays within safe limits. Other functions like regulating feedwater temperature, removing moisture after the turbine, or venting to atmosphere are not the purpose of safety relief valves.