Threaded and socket weld flanges are used in certain applications due to their ease of installation and lower cost. However, their application has strict boundaries, and misuse can lead to serious leakage and integrity risks. Clearly defining their “rational application domain” is key to responsible engineering practice.
The Inherent Vulnerability of Threaded Flanges: Their strength relies entirely on the engagement of the pipe’s external threads with the flange’s internal threads. This connection has multiple weaknesses: 1) The threads themselves constitute a continuous source of stress concentration and a potential leakage path; 2) The engagement is sensitive to misalignment, leading to uneven load distribution; 3) Under thermal cycling, differential thermal expansion between pipe and flange materials can cause thread “loosening.” Therefore, they are suitable only for low-pressure, ambient temperature, non-cyclic, non-hazardous, and vibration-free auxiliary piping systems. They are strictly prohibited for toxic, flammable, explosive, or potentially fatiguing services.
The “Hidden” Weld Issue of Socket Weld Flanges: Socket welding involves a fillet weld between the pipe’s outer wall and the flange’s socket. The problems are: 1) An inherent gap exists, prone to crevice corrosion, especially in corrosive environments; 2) The root of the fillet weld is susceptible to lack of penetration defects, difficult to clearly identify via standard radiography; 3) The weld area experiences high local stress. They are suitable for small bore lines (typically DN≤50) at moderate pressures but are also not recommended for severe cyclic or highly corrosive services.
Common Engineering Misconceptions:
- “Suitable for all low-pressure applications”: Incorrect. Even at low pressure, vibration or thermal cycling can cause threaded connections to fail.
- “Using a seal weld to fix a leaking threaded connection”: Dangerous practice. A seal weld locks the threads, making future disassembly nearly impossible and potentially worsening damage due to welding thermal stress.
- “Socket weld is easier to install than butt weld, thus more reliable”: Flawed logic. Ease of installation does not equate to long-term reliability. The joint quality of a butt weld is far superior to that of a fillet weld.
Standards like API and ASME B31.3 contain explicit restrictive clauses for the application of these flanges. During design selection, weld neck or integral flanges should be considered first. Threaded or socket weld flanges should only be chosen when they clearly meet the restrictive conditions and the risk is acceptable.