ASME Permitted Misalignment for Piping

What is Misalignment in the Piping Codes?

Internal misalignment in piping, commonly referred to as "hi-lo" or internal offset, is the axial offset or measurable difference between the internal surfaces (inside diameters or bores) of two abutting pipe ends, fittings, or components prepared for a butt weld (girth or circumferential weld).

It occurs when the inner walls of the two pieces are not perfectly flush, creating a step or ridge inside the pipe at the weld joint. This is distinct from external misalignment (offset on the outside surfaces) or angular/centerline misalignment.

Excessive misalignment can lead to internal weld discontinuities or defects such as incomplete fusion on the ID of the root.

Misalignment Correction

• Alignment tooling

• Counterboring

• Tapering

• Building up the thinner wall with weld metal, then blending by grinding or machining

• Re-preparing the ends

• Maintaining minimum wall thickness has to be accomplished regardless of the method of correction

Internal misalignment or “Hi-Lo” can be measured by the use of a “Hi-Lo” gauge.

ASME B31.1 Permitted Misalignment : Power Piping

ASME B31.1 Power Piping : B31.1 specifies the hi-lo to be < 1/16”

• Paragraph 127.3 (C) :

  • The inside diameters of piping components to be butt welded shall be aligned as accurately as is practicable within existing commercial tolerances on diameters, wall thicknesses, and out-of-roundness. Alignment shall be preserved during welding. The internal misalignment of the ends to be joined shall not exceed 1⁄16 in. (2.0 mm) unless the piping design specifically states a different allowable misalignment.

  • When the internal misalignment exceeds the allowable, it is preferred that the component with the wall extending internally be internally trimmed per Figure 127.3. However, trimming shall result in a piping component thickness not less than the minimum design thickness, and the change in contour shall not exceed 30 degrees (see Fig. 127.3).

• Figure 127.3

  • Figure 127.3 depicts the misalignment to be 1/16” (2.0 mm) or less without treatment

  • Figure 127.3 also depicts the internal counterboring which can be used to correct misalignment as long as it does not result in the minimum wall thickness being compromised.

ASME B31.3 Permitted Misalignment : Process Piping

ASME B31.3 Process Piping : B31.3 defers internal hi-lo to the WPS and engineering design without a fixed limit.

• Figure 328.4.3 states “Permitted misalignment. See WPS”.

• The way that B31.3 approaches misalignment requires that:

  • Owner operator: Should have an engineering specification that limits the maximum if they have a preference.

  • Welding Organization: The welding organizations welding procedure (WPS) needs to include a dimension for the permitted misalignment. Regardless of what is placed on the WPS the welded joints are required to meet the VT and RT acceptance criteria.

• B31.3 Paragraph 335.1 (a) : specifies that any distortion of piping to bring it into alignment for joint assembly that introduces detrimental strain is prohibited.

Industry Practices

Common industry practice is to follow the misalignment specified in B31.1 which is 1/16” or (1.6 mm or 2.0 mm)

Misalignment in larger diameters is much more of a challenge especially when it comes to manufactured fittings. Each situation should be evaluated but misalignment can be tolerated up to 1/8” or (3.2 mm or 4.0 mm) for diameters over 16”. This is based on the ability to still be able to consistently make x-ray quality welds, and not impact the internal flow of the piping.