What is the smallest allowable pipe inlet size for a steam boiler pressure relief valve?

The smallest allowable pipe inlet size for a steam boiler pressure relief valve is determined by safety and efficiency standards. A 1/2" NPT pipe size is generally accepted as the minimum to ensure that sufficient flow can be achieved during a pressure relief scenario. This size allows for an adequate discharge capacity to handle sudden pressure fluctuations, which is critical for maintaining safe operations within a steam system.

Choosing a smaller inlet size, such as 1/4" NPT, would not provide enough flow area to effectively divert steam in case of overpressure, potentially leading to system failure or safety hazards. Similarly, while larger sizes like 3/4" NPT or 1" NPT would accommodate higher flow rates, they exceed the minimum requirement and may not be necessary unless dictated by system specifications or other safety considerations.

Overall, 1/2" NPT strikes a balance between meeting the necessary safety standards while ensuring the valve operates efficiently within the designed parameters of steam systems.