Linear Misalignment (High-Low)

Linear Misalignment also known in the USA as high-low. This imperfection relates to deviations from the correct position/alignment of the joint.

Liner Misalignment - High-Low

Common causes

This is primarily a result of poor component fit-up before welding, which can be compounded by variations in the shape and thickness of components (eg out of roundness of pipe). Tacks that break during welding may allow the components to move relative to one another, again resulting in misalignment.

Acceptance

The acceptability of this defect is related to the design function of the structure or pipe line either in terms of the ability to take load across the misalignment or because such a step impedes the flow of fluid. Acceptance varies with the application.

BS EN 25817 relates misalignment to wall thickness but sets maximum limits (eg linear misalignment, for moderate limits of imperfections D, = 0.25 x material thickness in mm, with a maximum of 5mm). AWS D1.1 allows 10% of the wall thickness up to a maximum of 3mm. BS2640 gives limits for size and alignment as follows:

Outside diameter		Maximum permissible difference in internal diameter mm	Maximum out of alignment at the bore mm
Over mm	Up to and including mm
-	42.2	1.0	1.0
42.4	114.3	2.0	1.0
114.3	-	3.0	2.0

114.3 - 3.0 2.0

The consequence of linear misalignment can, when welding is carried out from one side, be lack of root or sidewall fusion to give a sharp continuous imperfection along the higher weld face toe. In some situations linear misalignment in the bore of a pipe can lead to in-service problems where turbulence of the carrier fluid in the pipe creates subsequent erosion.