Deviation issues during flange installation are a common yet easily overlooked cause of flange connection leakage. Whether due to insufficient manufacturing precision of the flanges themselves or misalignment caused by improper installation operations, deviations can significantly impair sealing performance.
There are three main reasons for interference or poor sealing during flange mating: angular displacement between adjacent bolt holes exceeding the allowable tolerance, excessive shape error of the bolt holes, and deviation of the centre hole (used as a reference) from the true centre. These deviations may originate during flange manufacturing or may result from improper operation during installation.
Angular displacement beyond tolerance is the most common deviation type in flange mating. When the angular distribution of bolt hole pitches on two mating flanges differs, bolts may still pass through, but the relative positions of the flanges will be offset, preventing the sealing faces from being perfectly parallel and flush. This offset creates uneven compression pressure on the sealing faces—excessive on one side and insufficient on the other—thus forming a leakage path.
Bolt hole shape errors are equally significant. If bolt holes have ovality or position deviation, even if bolts can be inserted, they will impose additional eccentric moments on the flange during tightening, affecting the contact uniformity of the sealing faces.
In the field installation of large flanges, alignment precision is particularly critical. For example, in the mating of upper and lower tower flanges of offshore wind turbine generators, dedicated alignment systems are required for precise positioning. Possible alignment errors include axial deviation, radial offset, and angular tilt. Axial deviation affects the measurement range, radial offset introduces non‑linear errors, and angular tilt produces cosine errors.
The impact of installation deviations also manifests in the cumulative effect of piping systems. In three‑dimensional ship design, gaskets between flanges are often not modelled, leading to length discrepancies between the model data and the actual installed pipe components. Along a long pipeline, small deviations at each pair of flanges can accumulate to a considerable amount, so adjustment spools must be provided to compensate for the accumulated tolerance.
Preventing flange installation deviations requires simultaneous efforts in both manufacturing and installation. During manufacturing, tight control over flange geometry and bolt‑hole positional accuracy is necessary. During installation, proper alignment tools and measuring equipment should be used, following standardised installation procedures. For critical flanges on important equipment, it is advisable to perform a sealing‑face contact check after installation to verify quality.
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