Essential versus nonessential variables for a WPS

Q: I am unclear on the American Petroleum Institute’s Standard 1104 regarding a weld procedure specification (WPS). Section 5.3.2.3 says:

Are they saying that outside diameter is not an essential variable in a WPS but wall thickness is?

A: We're often familiar with essential variables—those variables that, once changed beyond specific limits, require requalification of a WPS. But what is a nonessential variable? These variables may not show up in a table or list telling us what are allowable variations, but they are equally important when writing your WPS. Oftentimes these are not included on a WPS, which only leads to confusion when customers, inspectors, or welders try to interpret or apply your WPS.

When a nonessential variable is changed, no additional testing is required, but the WPS needs to be changed to reflect the new variable.

Welding Variables to Consider

Here are some examples of what I mean:

• Base Materials. Although base material groups can be essential, the individual material types are considered nonessential. You completed a WPS of a specific material and it qualified you to within a group. If you want to change or develop a new WPS to reflect a different material within the group, it is simply a matter of paperwork.
• Joint Type. Once you qualify a specific joint type that test-qualifies a large number of joint types, you only need to change your paperwork to reflect a different joint design. You can now see how a single procedure qualification record (PQR) can qualify a large number of WPSs.
• Electrode Extension. You can vary current by as much as 50 amps simply by changing your stick-out.
• Electrode Type. Are you changing your gas tungsten arc welding (GTAW) electrode?
• Welding Technique. Should you weave or whip? Are you allowed to oscillate?

All of these should be addressed in the WPS.

A Few More Tips

It surprised me that diameter is not an essential variable per Section 5.4 of the standard. But listing it on the WPS is required. According to 5.3 Welding Procedure Specification, 5.3.1 General:

The welding procedure specification shall include the information specified in 5.3.2 where applicable.

So thickness is an essential variable, but diameter is considered a nonessential variable. However, both must be listed on the WPS.