How long is the flange bolt exposed?

How long is the flange bolt exposed?

Too short bolt length directly causes flange matching failure. Too long bolt length has no significance for flange connection strength. On the contrary, it is easy to cause rust, difficult cutting during disassembly and increase maintenance workload. Bolts are purchased in large quantities as common fasteners. If the length is not appropriate, the materials will be scrapped directly, and the serious ones will affect the project construction period. Therefore, the selection of bolt length needs to be paid enough attention and quantified.

20220314220825 47518 - How long is the flange bolt exposed?

Standard provisions

  • ASME B16. 5 the explanation of the bolt length requirements in non mandatory Appendix C is that when the worst-case tolerance occurs in the relevant dimensions of flange connection, it can also ensure that all threads of thick hexagon nuts can be engaged. If all thread engagement can be met, shorter bolts can also be used. The nominal length of American Standard stud does not include the chamfer length at both ends, so it is also exposed after actual American standard installation.
  • SY/T 0460-2018 technical code for construction of equipment and pipeline installation engineering of natural gas purification unit requires that the exposed thread after bolt fastening is 0-3 pitch.
  • GB 50540-2009 (2012 Edition) code for construction of process pipeline engineering in petroleum and natural gas station requires that the flange bolt shall expose 0-3 pitches beyond the nut after tightening.
  • GB 50517-2010 code for acceptance of construction quality of petrochemical metal pipeline engineering requires that the fastened bolts and nuts should be flush or expose 1-2 pitches.
  • GB 50235-2019 code for construction of industrial metal pipeline engineering requires that all nuts should be screwed into the bolts, and the tightened bolts should be flush with the nuts.


Although the installation requirements of bolt fastening are expressed differently in different standards, they are roughly divided into two categories: one is that there are length requirements for the exposed nuts of bolts after fastening; The other is that the bolt and nut should be flush after fastening.
In the actual installation process, due to the difference of bolt fastening methods and the installation deviation of operators, it is difficult to ensure that the fastened bolt is flush with the nut on the premise of ensuring that all threads of the nut can be engaged. For some projects, in order to ensure the beauty after installation, after ensuring that the bolt can be completely screwed into the nut, cut off the excess bolt length exposed at both ends of the nut by mechanical cutting, so as to make both ends of the bolt flush with the nut.
For bolt installation, whether foreign or domestic specifications, it is not a mandatory requirement that both ends of the bolt are flush with the nut after tightening. Secondly, for the method of mechanical cutting, after the threads at both ends of the bolt are damaged, the disassembly will increase the difficulty.
On the contrary, a certain length of threads at both ends of the nut can be exposed after the bolt is tightened: it can ensure that all threads of the nut are engaged and meet the requirements of bolt tightening force; Provide certain adjustable margin during construction and installation to reduce rework rate.

The exposed length given in the specification is generally 0-3 thread pitches. Since the threads close to the chamfering of the two end faces of the bolt generally have no meshing ability in the bolt manufacturing process, it is recommended to expose 2 thread pitches at both ends after the bolt is fastened, so as to specify the appropriate length of the bolt.

Source: China Flange Bolt Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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