What Is Flange Alignment?

Flange alignment is the process of positioning two mating flanges so their faces are parallel, concentric, and correctly spaced before gasket installation and bolt-up. Poor alignment is one of the leading causes of flange leaks—even with correct gaskets and proper bolt torque, a misaligned joint will not seal reliably. ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) defines the tolerances.

ASME PCC-1 Alignment Tolerances

Parameter	Tolerance	Measurement Method
Flange face parallelism	0.15 mm per 100 mm of flange OD (max 1.5 mm)	Taper gauge or feeler gauge at 4 points
Bore concentricity	Within 1.5 mm (1/16 in) of pipe bore	Visual or internal alignment pin
Flange gap	Gasket thickness + 3 mm (1/8 in) nominal	Feeler gauge or ruler
Bolt hole alignment	Bolts pass through freely without forcing	Visual inspection
Rotation (bolt hole straddle)	Bolt holes straddle plant north/vertical centerline	Check before welding flange to spool

Types of Flange Misalignment

Type	Description	Consequence
Angular	Flange faces not parallel (tilted)	Uneven gasket compression; leaks
Offset (lateral)	Flanges not concentric	Bolt holes don’t align; gasket off-center
Rotational	Bolt holes do not match	Cannot install bolts without forcing
Excessive gap	Flanges too far apart	Bolts too short; gasket not compressed
Insufficient gap	Flanges too close	Cannot insert gasket; pipe stress

Common Mistake

Never use bolts to “pull” misaligned flanges into position. Forcing bolts through misaligned holes introduces bending stress on the flange, uneven gasket loading, and residual pipe stress. Correct the alignment by adjusting pipe supports, cutting and re-welding spool pieces, or using alignment tools.

Alignment Tools

Tool	Function
Flange alignment pins	Temporary pins inserted in bolt holes to center flanges
Taper gauges / feeler gauges	Measure flange gap and parallelism
Straightedge	Check face flatness across the flange
Laser alignment system	Precision alignment for critical joints
Come-alongs / chain falls	Adjust pipe position for alignment
Flange spreaders	Create gap for gasket insertion

Pro Tip

Check alignment at two stages: (1) after tack-welding the flange to the spool (before final welding), and (2) after installing the spool in the field. Welding distortion and field fit-up variations can change alignment. Verify with a taper gauge at the 12, 3, 6, and 9 o’clock positions around the flange.

Bolt Hole Orientation

Per ASME PCC-1, bolt holes should straddle the vertical and horizontal centerlines of the pipe—never sit directly on them. This “bolt hole straddle” rule applies unless the piping isometric or P&ID specifies otherwise. Correct orientation ensures consistent practices across the project and simplifies bolt-up access.

Read the full guide to flanges