What is the maximum allowable size for slip-on flanges used in boiler external piping according to ASME B.31.1?

In accordance with ASME B31.1, which governs the Power Piping Code, the maximum allowable size for slip-on flanges used in boiler external piping is indeed 4 inches. This specification is based on ensuring the integrity and safety of the piping system, particularly in environments subject to high pressure and temperature, which are typical in boiler applications.

Slip-on flanges are designed to slide over the pipe and are welded both inside and outside to provide a secure connection. However, their use is limited in size because larger sizes can lead to difficulties in achieving a proper weld and may affect the overall strength and reliability of the joint under operational conditions. The limitation to a maximum of 4 inches helps to standardize practices and ensure that the materials and their installation can adequately handle the stresses encountered in boiler operations without compromising safety or efficiency.

By adhering to these guidelines outlined in ASME B31.1, professionals can maintain the integrity of their systems and follow industry-standard practices for piping installations.