Flange alignment
and parallelism —
how misalignment
damages gasket compression
When they are not — when one face is angled relative to the other, or when the flanges are laterally offset — the gasket receives very different compression across its face. Some areas are over-stressed. Others never reach the seating condition needed for reliable sealing. Bolt tightening should not be used to correct this. Alignment must be addressed before assembly begins.
Assembly scope: Alignment should be corrected by pipework, support or equipment adjustment before gasket installation. Bolts should not be used as the primary method for forcing misaligned flanges into position, especially on live, pressurised, hazardous-media or rotating-equipment systems. Follow the applicable standard, site procedure and equipment manufacturer requirements.
What alignment and parallelism mean — the distinction
Two terms are often used interchangeably but describe different geometric conditions in a flanged joint:
- Parallelism refers to whether the two flange faces are in the same plane — parallel to each other across the full face diameter. A lack of parallelism means the faces are angled relative to each other: the gap between them is wider on one side than the other. This is also called angular misalignment.
- Lateral alignment (or concentricity) refers to whether the two flanges are centred on the same axis. Lateral offset — where one flange is shifted sideways relative to the other — displaces the bolt holes and gasket position relative to the mating face. In severe cases, bolt holes do not align and assembly is not possible; in moderate cases, assembly is possible but the gasket is not symmetrically loaded.
Both conditions affect how the gasket is compressed. Parallelism is often the more critical of the two for sealing performance, because angular offset directly determines whether the gasket contact stress is uniform around the circumference.
Angular misalignment — lack of parallelism
Lateral offset — misaligned centres
Why bolt tightening cannot fix alignment
When two flanges are brought together with angular misalignment, tightening the bolts pulls the faces toward each other. On the side where the gap is wider, the bolt must stretch further to close the gap before the gasket is loaded. On the side where the gap is narrower, the bolt reaches the gasket sooner and compresses it first.
The result is that by the time all bolts reach their target torque, the gasket has been compressed very unevenly. The narrow-gap side has been significantly compressed — possibly over-compressed — while the wide-gap side has received much less load. The total bolt load may be correct, but its distribution across the gasket face is not.
Bolt load closes the gap — it does not straighten pipework. Using additional torque to force misaligned flanges together introduces bending stress into the pipe system. This stress is carried by the pipe, the welds, and the connected equipment — pumps, vessels, valves — as a sustained load that was not part of the original design. Many equipment manufacturers specify maximum nozzle loads, and forcing misaligned flanges together may exceed these limits even if the joint itself appears to seal initially.
What uneven compression does to the gasket
Reading the gasket after disassembly — what misalignment leaves behind
The compression pattern on a removed gasket is one of the most direct indicators of joint condition during assembly. A gasket that has been in service in a well-aligned joint shows different characteristics from one that experienced misalignment.
Well-aligned joint
Full-face uniform compression mark. Consistent colour and surface texture change around the full circumference.
Angularly misaligned joint
Non-uniform compression mark. Heavy compression one side, light or absent on the opposite side.
Other patterns that may indicate misalignment or alignment-related problems include a compression mark that is off-centre relative to the gasket OD and ID, a compression mark that does not follow the full annular ring but is partial or broken, and a gasket found displaced from its original installed position.
Where misalignment commonly originates
- Pipe sag and dead weight: long unsupported pipe runs sag under their own weight between supports. At the end of an unsupported run, the flange face may be angled downward relative to the mating connection. This is a common source of angular misalignment in horizontal pipework.
- Thermal expansion and contraction: a pipe that is hot during operation expands; when cooled for maintenance, it contracts and the flange position changes. The cold alignment may be satisfactory but the operating condition can change flange position and may contribute to angular offset if the system is not properly supported or able to expand as intended.
- Support settlement or movement: if a pipe support shifts, settles or is incorrectly positioned, the pipe it supports moves with it. The resulting misalignment at connected flanges may be gradual and not immediately visible.
- Incorrect spool fabrication: a pipe spool fabricated with angular error at the flange end will produce misalignment at every joint where it is installed. The error is in the component, not the installation.
- Equipment nozzle movement: pumps, vessels and heat exchangers may deflect under operating load, hydrostatic test pressure, or temperature. A nozzle flange that is aligned at ambient conditions may be misaligned at operating conditions.
Alignment checks that matter before assembly
These checks are a diagnostic framework for understanding alignment-related gasket failures. They do not replace the applicable site procedure, flange standard or equipment manufacturer instruction.
A joint that leaks repeatedly after correct gasket replacement may be misaligned. If a joint continues to leak after repeated gasket replacement attempts — with gasket size, material and assembly conditions checked — the pipe alignment at that joint should be assessed before fitting another gasket. Misalignment is a recurring root cause rather than a one-off event.
Alignment is a prerequisite for gasket compression — not something bolt tightening can substitute for.
A gasket compressed in a misaligned joint will be over-stressed on one side and under-compressed on the other. The under-compressed sector is a leak path under operating conditions. The over-compressed sector may be permanently damaged. Neither outcome improves with higher torque. Alignment — checking face gap uniformity and bolt hole alignment before assembly — is the correct point of intervention. Gasket analysis after removal is a useful diagnostic indicator of whether alignment may have been satisfactory at the previous assembly.