Exposed length of flange bolt after tightening
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The explanation given in ASME B16.5 Nonmandatory Appendix C for the requirements regarding bolt length is that it ensures that all of the threads on the thick hex nut can engage, even when the relevant dimensions of the flange connection are at their worst-case tolerances. If full thread engagement can be achieved, it is also possible to use shorter bolts. ·SY/T 0460-2018 \"Technical Specifications for the Construction of Equipment and Pipeline Installation Projects in Natural Gas Purification Plants\" requires that after the bolts are tightened, 0 to 3 pitch lengths of the exposed threads should remain. ·GB 50540-2009 (2012 edition) \"Code for Construction of Process Piping in Oil and Gas Stations\" requires that after the flange bolts are tightened, 0 to 3 thread pitches should protrude from the nuts. ·GB 50517-2010 \"Code for Acceptance of Construction Quality of Metal Piping Projects in Petrochemical Industry\" requires that the bolts and nuts after tightening should be level or protrude by 1 to 2 pitches. ·GB 50235-2019 \"Code for Construction of Industrial Metal Piping Projects\" requires that all nuts be tightened onto the bolts, and the bolts and nuts after tightening should be at the same level. ·GB 50231-2009 \"General Specifications for Construction and Acceptance of Mechanical Equipment Installation Projects\" stipulates in Article 5.2.1 that when tightening connections using screws, the following requirements must be met: 1) When tightening bolts, it is advisable to use a ratchet wrench; impact methods as well as forces exceeding the allowable value for the bolts shall not be used ； 2) When multiple bolts or screws are used to fasten the same assembly, they should be tightened in a cross, symmetrical, and even manner ； When there are positioning pins, start tightening the bolts or screws evenly from those closest to the pins ； 3) The contact between the bolt head, nut, and the connected component should be tight ； When there are special requirements for the contact area and contact gap, inspections shall still be carried out in accordance with the specified requirements ； 4) After the bolt and nut are tightened, the bolt should protrude from the nut by 2–3 thread pitches, and its bearing surface should make contact with the part being fastened ； After countersunk screws are tightened, the countersunk head should be flush with the surface of the component. Lm – Length after bolt tightening (mm) ； Original length of the Ls-bolt when the gap with the connected component is zero (mm) ； P0 – Pre-tension force (N); the stiffness of the CL bolt (N/mm) can be calculated in accordance with the provisions of Appendix F of this specification. ·Article 4.3.15, point 5 of CJJ94-2009 \"Code for Construction and Quality Inspection of Indoor Gas Engineering in Urban Areas\" stipulates that when roof pipes are connected using flanges, metal wires with a cross-sectional area of not less than 6 mm2 should be used for bridging at both ends of the connection ； When a threaded connection is used, metal wires should be used for bridging. Section 4.3.18 stipulates that the flange should be installed in a location that facilitates maintenance, and it must not be positioned right against walls, floors, or pipe supports ； The flange connection should be concentric with the pipeline; the flange screw holes should align properly. The flange surfaces of the pipeline and the equipment or valves should be parallel to each other, and bolts should not be used to force them into alignment ； Bolts of the same specification should be used; their installation direction must be consistent, and the nuts should be tightened symmetrically and evenly ； After the nut is tightened, the exposed threads of the bolt should be 1–3 turns, and anti-corrosion treatment (applying grease) should be carried out. ·Article 5.3.5 of CJJ 33-2005 \"Code for Construction and Acceptance of Urban Gas Transmission and Distribution Projects\" stipulates that flanges should be kept parallel to each other; the deviation shall not exceed 1.5% of the outer diameter of the flange, nor shall it exceed 2 mm. The use of tightening bolts to correct such deviations is not allowed ； Flange connections must remain coaxial; the deviation of the centers of their screw holes should not exceed 5% of the hole diameter, and the bolts must be able to pass through freely ； The diameters of the bolts and screw holes should be compatible; bolts of the same specification should be used, with consistent installation orientation. After tightening, the length of the bolt protruding from the hole should not exceed one pitch, and it must not be shorter than the nut ； After tightening, the bolts should be in close contact with the flange; there should be no gaps. When gaskets are required, no more than one gasket per side should be used for each bolt ； The distance between the flange and the edge of the support or the wall should not be less than 200 mm. Analysis: Although different standards specify varying requirements for bolt tightening, they can generally be divided into two categories: One category involves requirements regarding the length of the exposed nuts on the bolts after tightening ； § Another category is where, after tightening, the bolt and nut should be level.
According to Appendix C, which is not mandatory under ASME B16.5, even when the relevant dimensions of flange connections are subject to worst-case tolerances, it is still possible to ensure that all threads of the thick hex nuts engage properly. Therefore, it is also possible to use shorter bolts as long as all thread engagements are satisfied. Other standards generally require that the bolts and nuts after tightening should be level with each other or protrude by 0–3 pitch lengths; alternatively, they may protrude by 1–2 pitch lengths. The requirements regarding the extent of this protrusion can vary among different standards, and the appropriate choice should be made in accordance with the relevant standard specifications. .
Following the teacher’s advice: Neither GB/T 20801 nor the ASME piping codes and ASME PCC-1 require bolts to be exposed; on the contrary, the relevant ASME technical documents seem to stipulate only that after the bolts are tightened, all of the threads inside the nuts must be engaged. It seems that ASME PCC-1 also recommends allowing the bolt to be slightly below the nut, with one or two teeth not in contact. Since the threads of exposed bolts are prone to corrosion, this will make disassembly difficult. However, from the perspective of ensuring construction quality and a neat appearance, domestic specifications regarding pipeline construction once stipulated that bolt heads must be exposed, with the exposure limited to one or two threads. To this end, when formulating the 1997 edition of the standard for HG pipe flanges, the formula for calculating bolt length specified in ASME B16.5 was mechanically applied. All limiting conditions—such as the positive tolerance for flange thickness, the negative tolerance for bolt length, and the chamfering at both ends of the bolts—were taken into account, thereby determining the final bolt length. In reality, for the flanges supplied domestically, the nominal tolerance for their thickness after machining is usually zero; as a result, after the bolts are installed, three to four threads, or even more, are typically exposed. Subsequently, it was found that foreign engineering companies often made adjustments when developing databases for bolt lengths. Therefore, adjustments were also made during the formulation of the 2009 version of the HG standard for pipe flanges. Regarding the issue of whether washers are needed and how to adjust the bolt length after using washers, ASME PCC-1 as well as various foreign technical documents on determining bolt tightening torque generally consider that the function of flat washers is to reduce the friction coefficient between the lower surface of the nut and the surface of the flat washer when the bolt is tightened – whereas in the absence of washers, it is the lower surface of the nut that comes into contact with the upper surface of the flange. The harder material of the washer, along with its smoother surface, contributes to this reduction in friction, allowing the tightening torque to be converted more effectively and steadily into compressive force on the bolt. Moreover, spherical washers also help to adjust and reduce the bending stress on the bolts caused by various factors during bolt installation. Therefore, using washers when installing flange bolts helps improve the quality of installation, but whether it is necessary depends on the actual circumstances as well as the level of concern shown by the owner or the designers. Therefore, no provisions are made in the standards; it is usually limited to additional design requirements when necessary. Indeed, if washers are added, they should be taken into account when calculating the bolt length. It should also be noted that the length of fasteners with metric threads includes the chamfers at both ends, whereas for fasteners with imperial threads, the length is generally not inclusive of the chamfers at either end – it refers only to the length of the thread itself. The two marking methods are different.
GB 600 General Technical Requirements for Valves and Fittings in Ship Pipelines, Clause 4.10.2: After the valves are assembled, the length protruding from the bolts or studs should be 1 to 3 pitches
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