This article provides you with information about the essential variable in welding procedure specification based on requirements of ASME Code Section IX. The code defines this terminology as "Essential variables are conditions in which a change, as described in the specific variables, is considered to affect the mechanical properties of the joint".
It means if you have a welding procedure and one of your essential variable changes out of specified range then you need to re-qualify that WPS because that change affects the mechanical property of the weld.
For example, if you have a WPS with shielded metal arc welding (SMAW) process with post weld heat treatment, and then you need to use this WPS for another job without post weld heat treatment, you need to re-qualify your WPS because post weld heat treatment is an essential variable in SMAW process.
The ASME Code Section IX provides a table for each welding process and identifies essential, nonessential and supplementary essential variables. So if you have a WPS, and you need to use this WPS in another job with the implication of some changes, then you need to refer these tables to see if these changes fall among of essential variables. If they fall, then you need to re-qualify your WPS.
In most of welding process, The P number, F number, A number PWHT, thickness (out of specified range) are essential variable. Based on requirements of ASME Code Section IX all essential variable, nonessential variable and supplementary essential variable (such as Group Number) must be indicated in the welding procedure specification.
When you have a change in the essential variable, you need to write a new welding procedure specification and qualify that with a new procedure qualification record (PQR). To have a PQR you have to prepare a new test specimen that welded based on this new WPS and make two tension test, two root bend test and two face bend test.
This PQR number should be indicated in this new WPS. You need to refer to ASME Code Section IX to find correct size and number of test specimens, test specimen preparation, testing equipment requirements and related acceptance criteria.
The essential variable not limited to the WPS, you have the same variables with the different requirements in the welding performance qualification.