On a pressurised network, the weld is a singular point: it cannot be checked by eye. QMOS and DMOS qualifications exist to make that point verifiable, before and after assembly.
DMOS and QMOS: two documents, two roles
The DMOS (welding procedure specification) describes how to weld: material, process, filler metal, positions, heat input, preheating. The QMOS (welding procedure qualification record) is the report proving that this procedure, carried out to that specification, produces a compliant joint once tested. One prescribes, the other validates.
Welder qualification is a separate matter
A qualified procedure is not enough: the operator must be qualified in their own right, within a defined range of validity (process, material, thickness, position, diameter). These individual qualifications have a period of validity and must be kept current: a check that is frequently overlooked at handover.
What to require in the file
Ask for the DMOS applicable to the joints on your network, the corresponding QMOS, the welder certificates covering the range concerned, and the inspection plan (dye penetrant, radiography or ultrasonics depending on criticality). A documented network is signed off quickly; an undocumented one has to be inspected all over again.
Technical information article. It does not constitute a contractual commitment or a certification statement.
