In the installation and maintenance of equipment and pipelines, the tightening of flange bolts, seemingly a basic operation, is actually a crucial step in ensuring the sealing performance of equipment and preventing leakage accidents. Tightening them too loosely can cause media leakage, while tightening them too much can lead to bolt breakage and gasket loss of elasticity. This is especially true for critical equipment subjected to high temperature, high pressure, and flammable or explosive conditions; even the slightest operational deviation can cause safety hazards. So how should flange bolts be tightened correctly? Which tools should be used in different scenarios? How should hot tightening be controlled during the heating process? This article will provide a detailed overview of the complete set of standard operating requirements for flange bolt tightening, from initial inspection to tool use and hot tightening specifications, explaining everything clearly and thoroughly.
Technical Requirements for Tightening Flanges During Equipment and Pipeline Installation
01
(1) Inspection of Flange Fasteners
Gasket Inspection: New, clean, and dry gaskets must be used during installation. Carefully check for damage, defects, and other issues. Old gaskets must not be reused. Before installation, verify the gasket specifications and pressure rating to ensure complete consistency with the flange markings.
Flange Inspection: Before installation, inspect the flange sealing surface for scratches, dents, dirt, corrosion, burrs, or other damage. If a radial dent or scratch penetrating the flange sealing surface exceeds 0.2mm in depth and covers more than half the width of the gasket sealing surface, the flange must be replaced or the sealing surface must be re-machined and repaired. The nut support surface on the back of the flange should be flat and parallel. Flange alignment checks must strictly comply with the relevant requirements for pipeline installation in section 6.2 of the "SH3501-2011 Specification for Construction and Acceptance of Pipelines for Highly Toxic and Flammable Media in Petrochemical Industry".
Bolt and Nut Inspection: Strictly adhere to the equipment and pipeline design requirements, verifying the bolt and nut models and specifications to ensure correct selection. Threads and contact surfaces must be free of dirt, rust, scale, dents, burrs, debris, and other impurities that could affect tightening torque.
Repairing bolts by welding or machining is strictly prohibited. After the flange is tightened, at least two complete threads should be exposed on the outside of the nut. Bolts and nuts must be lubricated before use to reduce the coefficient of friction during tightening and improve their anti-slip and corrosion resistance.
Stud threads, nut threads, and contact surfaces must be degreased and dried before applying lubricant. Lubricant should be applied uniformly and systematically to bolt threads, nut threads, nut bearing surfaces, washers, and flange nut support surfaces. High-temperature anti-seize agents may be used depending on the operating conditions.
Welding or machining methods are not permitted to repair bolts. After flange installation and tightening, at least two threads must be exposed outside the nut. Bolts and nuts must be lubricated before use to ensure a low coefficient of friction during tightening and improve their anti-slip and corrosion resistance. Stud threads, nut threads, and contact surfaces must be degreased and dried before applying lubricant.
(2) Flange Tightening Methods
Using a torque-less box wrench or hammer wrench: Suitable for tightening flanges on conventional equipment and ordinary pipelines. The appropriate tool should be selected based on the bolt specifications and flange pressure rating. Tightening requirements are as follows:
1) The maintenance unit should develop a tightening plan, tightening the flanges symmetrically and numbering them according to the tightening sequence, referring to Figures 1 and 2.
2) Use four bolts at positions 1, 2, 3, and 4 to position the gasket, ensuring the center of the wrapped gasket is within the flange edge.
3) Tighten the positioning bolts by hand, then insert the other stud bolts and tighten them by hand to balance the load, ensuring at least two threads are exposed at each end of the nut.
4) Based on the on-site equipment and flanges, calculate the number of tightening cycles (at least 3) and the hammer load (force) for each tightening cycle, tightening in ascending order (e.g., 50%, 80%, and 100%). Do not apply the load too quickly or too heavily to prevent gasket seal failure. 5) Tightening sequence using a torque wrench or hammer wrench:
Tighten the two radially opposite bolts to their predetermined hammer load (force);
Tighten the next pair of bolts approximately 90 degrees circumferentially away from the previous two;
Continue tightening until all remaining bolts are tightened to their predetermined hammer load.
6) Finally, tighten all bolts sequentially in a clockwise or counterclockwise direction at 100% hammer load (force).
Torque wrench: Suitable for high-temperature, high-pressure, flammable, and explosive equipment and pipe flanges. Tightening requirements are as follows:
1) The maintenance unit must develop a tightening plan, determine the appropriate torque, and conduct a design review based on bolt strength, initial and working sealing pressure of the gasket, and medium pressure, to prevent bolt breakage and gasket loss of elasticity due to excessive tightening force, leading to seal failure.
2) Tighten flanges symmetrically, numbering the tightening sequence as shown in Figures 1 and 2.
3) Position the gasket at positions 1, 2, 3, and 4 using four bolts, ensuring the center of the wound gasket is within the flange edge.
4) Tighten the positioning bolts by hand. Then insert the remaining stud bolts and hand-tighten them to balance the load, ensuring at least two threads are exposed at each end of the nut.
5) Based on the on-site equipment and flange, calculate the number of tightening cycles (at least 3) and the torque for each tightening cycle, increasing the torque sequentially from lowest to highest (e.g., 50%, 80%, 100%). Do not apply the load too quickly or excessively to prevent gasket seal failure.
6) Tightening sequence with a torque wrench:
Tighten the two radially opposite bolts to their predetermined torque;
Tighten the next pair of bolts approximately 90 degrees circumferentially away from the previous two;
Continue tightening until all remaining bolts are tightened to their predetermined torque.
7) Finally, tighten all bolts sequentially in a clockwise or counterclockwise direction to 100% torque value.
8) Record the torque value for future maintenance reference.
Bolt tensioner: Suitable for critical equipment and pipe flanges subjected to high temperature, high pressure, flammable and explosive materials. Tightening requirements are as follows:
1) The maintenance unit should develop a tightening plan, determine an appropriate tension force, and verify the design based on bolt strength, initial and working sealing pressure of the gasket, and medium pressure, to prevent bolt breakage and gasket loss of elasticity due to excessive tightening force, leading to seal failure.
2) When tightening bolts individually (in stages) with the bolt tensioner, follow the principle of uniform bolt tightening and refer to the tightening sequence of the torque wrench.
3) During the bolt tensioning process, the number of tensioning cycles should be reasonably determined, and the pressure should be increased sequentially from small to large (e.g., 50%, 80%, 100%). Pressure should be applied evenly, and after each increase in pressure, it should be stabilized before further increase to avoid excessive impact force affecting the pre-tightening effect of the bolt.
4) Record the pressure values for future maintenance reference.
Hot Tightening Requirements for Equipment and Pipelines During Start-up
02
(1) Hot Tightening according to the temperatures in Table 1
Table 1 Hot and Cold Tightening Temperatures for Equipment and Pipelines Unit: °C
Note: Hot and cold tightening is not required for operating temperatures between -29℃ and 250℃.
(2) Hot or cold tightening should be performed after the equipment and pipeline temperatures have stabilized. Explosion-proof tools should be used, and heating or pressurization operations should not be performed during tightening.
(3) When tightening, it is recommended to start from the flange gap with the largest gap and tighten symmetrically. If there is a leak, tighten the leak first. (4) During the equipment and pipeline heating process at the start of operation, a tightness inspection is required. Use a torque wrench at 100% torque or use an explosion-proof hammer to hammer the nuts in the tightening direction to check for looseness.
