Flange distortion during welding: causes, prevention and on-site remachining
A flange can look perfect after welding in and still leak: weld shrinkage and the heat of welding and PWHT distort the flange face by a few tenths of a millimetre – enough for a gasket no longer to seat evenly. This page explains why this happens, how to avoid it and how a distorted flange face can be restored on site without cutting out the flange.
Why flanges distort
Every weld shrinks as it cools. On a weld neck flange, the circumferential weld pulls the flange neck together unevenly if the heat input is not the same all the way round. The flange ring tilts or dishes slightly – the flange face is then no longer flat and no longer perpendicular to the pipe axis.
Thin flanges of large nominal size and low pressure rating are particularly sensitive, because their ring is weak relative to the diameter. Heavier flanges of higher pressure ratings distort less.
Post-weld heat treatment can also trigger or increase distortion: residual stresses are relieved during PWHT and the component moves. If heating is uneven or the line is not properly supported during PWHT, the flange deforms further.
Prevention during fit-up and welding
- ▸Bolt up a mating flange: On large nominal sizes and low pressure ratings, clamp the flange to a mating or blind flange with all bolts before welding. The assembly is stiffer and distorts less.
- ▸Tack symmetrically: Distribute tack welds evenly around the circumference, placing them opposite each other.
- ▸Plan the welding sequence: Weld passes alternately or with two welders working opposite each other, so that the shrinkage balances out.
- ▸Apply heat evenly: Keep preheat and interpass temperature the same all the way round; during PWHT, arrange heating pads and insulation evenly and support the line.
- ▸Check before closing: Check squareness and flatness after welding or PWHT – before the line is insulated and the pressure test is carried out.
How distortion is measured
What is measured is the flatness of the flange face, its face run-out relative to the pipe axis and the parallelism of the two flanges of a joint. On site this is done with a straightedge and feeler gauge, with a dial gauge on a rotating mount, or with laser measurement on large diameters.
How much deviation is permissible depends on gasket type and code. Soft gaskets compensate more than metallic ones; for flanged joints to ASME, ASME PCC-1 (Appendix D) gives guide values for the permissible flatness deviation of the flange face.
On-site remachining instead of cutting out
If a flange face is distorted, there are two options: cut out the flange, weld in a new one, re-inspect it and heat treat it again if necessary – or machine the flange face flat again on site. The second option is almost always faster, because welding, inspection and heat treatment do not have to be repeated.
For this, a portable flange facing machine is centred on the flange or in the pipe. It faces the flange face flat, restores the raised face or ring groove and produces the required surface finish. Remachining is limited by the minimum flange thickness and the permissible material removal; in case of doubt, the design or the operator decides.
We machine flange faces from small nominal sizes up to large diameters directly in the plant – with a measurement record of flatness and surface finish. On request combined with heat treatment and controlled bolting from a single source.
Frequently asked questions
Can a slightly distorted flange simply be tightened harder?
No. Higher bolt preload compresses the gasket more at the high spots, while at the low spots it remains too weak – the joint does not become reliably leak-tight, and the gasket or bolts can be overloaded. The cause must be corrected at the flange face.
How much material may be removed from a flange face?
As much as still leaves the minimum flange thickness and the shape of the flange face (for example the height of the raised face) intact. The limit is set by the design or the code; we document the initial and final dimensions.
Does distortion also occur as a result of heat treatment?
Yes. During stress relieving, residual stresses are relieved and the component can move. That is why checking the flange faces after heat treatment is part of the sequence – only then is the joint finally closed.
General description based on practical experience, without guarantee. The design, code and operator requirements are authoritative.
Distorted flange face? We make it flat again
Send us the nominal size, pressure rating, facing type and photos – we will tell you whether and how quickly the flange face can be remachined on site.