Socket Weld Fittings

What 3000, 6000 and 9000 Mean

The numbers 3000, 6000 and 9000 in ASME B16.11 are not pressure ratings in psi. They are class designations that correspond to a wall thickness and a matching pipe schedule. Class 3000 corresponds approximately to schedule 80 pipe, class 6000 corresponds approximately to schedule 160, and class 9000 corresponds to schedule XXS. The actual working pressure of a fitting depends on its class and the material, and must be evaluated against the applicable ASME B31 code for the design temperature. As a rough guide, a class 3000 carbon steel fitting suits moderate pressure service and a class 6000 fitting is used where higher pressure or more severe cyclic loading is expected. Class 9000 is the heaviest and is used for the most demanding high pressure small bore service.

Installation: The Expansion Gap

A socket weld joint has one specific installation requirement that, if ignored, causes cracking: the pipe end must not bottom out in the socket. There must be a small gap, approximately 1.6 mm for most sizes, between the end of the pipe and the bottom of the socket before welding. The reason is differential expansion: during welding the socket heats more quickly than the pipe, and if the pipe is seated solid against the socket bottom, the pipe cannot move as the joint cools, and residual stress causes the weld to crack, often not immediately but after the line is put into thermal service. The standard practice is to seat the pipe, then back it out by about 1.6 mm, then weld. This is one reason socket weld fittings are not recommended for severe cyclic or high temperature service where a butt weld would be used instead.