When do we use slip-on flange, a weld neck flange or a lap joint flange?  

The answer to this question is mixed. First, the piping material engineer should have made it very clear which type of flanges are included in (and/or allowed in) each line class specification.The piping material specifications should not include every flange type in every line class. If lap joint flanges are the flanges called for by the line class then you may only use lap joint flanges. For other line classes you would normally use only weld neck flanges for all flange situations for 3” (NPS) and larger. The use of a slip-on flange could be an exception here only when there is a space limitation problem.

Slip-on flange
Weld neck flange
Lap joint flange

Slip-on flange	Weld neck flange	Lap joint flange

This text is an extract from questions gathered from numerous queries of anonymous people in the Piping Engineering and Design profession. The answers or discussion of the topics were provided by James O. Pennock. Images obtained from www.coastalflange.com