Flange installation quality is one of the critical factors determining the sealing performance of a piping system. Among the various installation quality indicators, the parallelism of the flange faces and the alignment accuracy of the two flanges are the two most important parameters. In actual engineering practice, however, control of these two indicators often presents numerous challenges.
Regarding flange face parallelism, relevant codes specify clear technical requirements. Flange connections shall be kept parallel, and the deviation shall not exceed 1.5‰ of the flange outside diameter, with an absolute maximum of two millimetres. This requirement means that for larger‑diameter flanges, the allowable parallelism deviation is capped by the 1.5‰ proportion; for smaller diameters, the absolute two‑millimetre limit governs. Misalignment of a flange joint shall not be corrected by force‑tightening the bolts—such forced tightening only causes excessive local stress on the flanges and uneven gasket loading, increasing the risk of leakage.
The perpendicularity of the flange face to the pipe centreline is also strictly required. For pipe nominal diameters not exceeding 300 mm, the allowable deviation is one millimetre; for diameters greater than 300 mm, the allowable deviation is two millimetres. This precision requirement poses a significant challenge for field installation of large‑diameter piping.
In terms of alignment accuracy, codes require that flange connections be kept on the same axis, that the deviation of bolt hole centres should not exceed 5% of the hole diameter, and that bolts should be able to pass freely through the holes. Bolts and bolt holes shall be of matching diameters; bolts shall not be driven into holes by force. Flange bolt holes are generally to be installed in a straddle orientation—that is, the bolt holes are symmetrically positioned on both sides of the pipe centreline.
In actual installation operations, achieving the above precision requires proper construction procedures and quality control measures. For flanges on critical equipment, specialised alignment tools are often used to assist positioning. For example, in the mating of large flanges, alignment dowel pins may be inserted symmetrically as guides—the dowel diameter is generally about 0.5 mm smaller than the bolt hole diameter. During installation, feeler gauges and other tools should be used to measure the clearance between mating flanges to verify that parallelism meets the requirements.
The impact of installation deviations is particularly pronounced in long piping systems. The small deviations at each pair of flanges can accumulate to a considerable magnitude; therefore, adjustment spools should be provided in the piping design phase to compensate for cumulative installation tolerances. In addition, ambient temperature changes can affect flange installation accuracy—under large temperature differences, thermal expansion and contraction of the pipe may induce additional stresses and deformation in already installed flanges.
The core principle of flange installation quality control is “prevention is better than correction.” Strict control at three stages—pre‑installation inspection of the flanges themselves, use of appropriate measuring and positioning tools during installation, and acceptance inspection after installation in accordance with codes—forms the foundation for ensuring long‑term reliable flange connection performance.
All data are sourced from publicly available sources and are provided for learning, communication, and reference purposes only. If there are any errors, please contact for correction. Please make your own judgment, this website assumes no responsibility.