Threaded Flange

A threaded flange, also called a "screwed flange," is used to connect a flange to a pipe with a screw thread. On the end of the pipe, a male thread is cut, and on the inside of the flange, a female thread is cut. The male-threaded pipe is then screwed into the female-threaded flange.

Pipes with thin walls aren't good candidates for threaded flanges because it's hard to cut a thread on them. In some situations, a seal weld may be used to strengthen the flange and make it less likely to leak. Adding a seal weld will make the flange stronger, but it will also make it harder to put the joint together and take it apart (because the weld makes it a permanent joint).

Most engineering groups and associations limit the use of threaded flanges to sizes between 1/2 and 4 inches and pressure classes of 300 or less. Because there is no weld, threaded flanges are more likely to leak than welded flanges, and the seal can be damaged if the flanges aren't lined up right. Because of these things, threaded flanges are not a good choice for dangerous systems where a leak could cause a disaster.

Non-Destructive Testing (NDT) Techniques

Radiography can be used to look for corrosion on the flange face and to make sure that the threads are engaged and in the right place.

ASME Standards

The ASME B1.20.1 and ASME B31.3 standards spell out what needs to be done for threaded flange connections.