Technical requirements for tightening during the installation of equipment and pipeline flanges

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Inspection of flange fasteners 1 ● Gaskets: When installing, ensure that the gaskets are new and clean and dry, and check for any defects or damage in them； Old gaskets cannot be reused ； Before installation, ensure that the size and grade of the gasket used match those indicated on the flange. ● Flange: Before installation, check the flange surface for any damage such as scratches, grooves, dirt, corrosion, or burrs. If there are indentations or scratches on the radial sealing surface of the flange that are deeper than 0.2 mm and cover more than half of the width of the gasket’s sealing surface, the flange must be replaced or its sealing surface must be reworked and repaired ； The support surface of the nut on the back of the flange should be parallel and smooth ； Check whether the flanges are aligned; the inspection method shall follow the pipeline installation requirements in 6.2 of SH3501-2011. ● Bolts and nuts: Check whether bolts and nuts are used correctly in accordance with the design requirements of the equipment and pipelines ； The threads and contact surfaces must be free of dirt, rust, scale, scratches, burrs, debris, and other foreign substances that could affect the torque during tightening ； Bolts shall not be repaired by welding or machining methods ； After the flange is securely fastened, at least two threads should be visible outside the nut ； Bolts and nuts must be lubricated before use to achieve a low friction coefficient when tightening them, as well as to enhance their resistance to thread slippage and corrosion ； The stud threads, nut threads, and contact surfaces must be degreased and dried before applying lubricant ； A uniform lubricant should be properly used on the bolt threads, nut threads, nut bearing surface, washers, and the nut support surface on the flange ； Use a high-temperature wear-resistant agent as needed. Bolt tightening methods 2 ● Torque-free allen wrenches or impact wrenches: Suitable for tightening bolts on general equipment and pipe flanges; the choice depends on the bolt size and the pressure rating of the flange. The tightening requirements are as follows: 1) The maintenance team shall develop a tightening plan, tighten the flanges symmetrically, number the tightening sequence, and refer to the numbering in Figures 1 and 2. 2) Use 4 bolts at positions 1, 2, 3, and 4 to position the gasket, ensuring that the center of the wound gasket is within the edge of the flange. 3) Tighten the setting bolts by hand, then insert the other studs and tighten them by hand to balance the load, ensuring that at least 2 threads are exposed on each end of the nut. 4) Based on the equipment and flanges on site, calculate the number of tightening cycles – one cycle per round of tightening – and determine an appropriate number of tightening cycles (at least 3 times) as well as the impact force to be used for each tightening. The impact force should be increased gradually from low to high (e.g., 50%, 80%, 100%); it is not allowed to apply excessive force too quickly, in order to prevent failure of the gasket seal. 5) Sequence for tightening each time using a torque-free plummer block wrench or impact wrench: Tighten the two bolts that are radially opposite to each other until the specified impact load (force) is reached on those bolts ； Tighten the other pair of bolts that are approximately 90 degrees apart circumferentially from the previous two bolts ； Continue tightening until all the remaining bolts are tightened to the specified hammer load. 6) Finally, use a 100% hammering load (force) to tighten all bolts in sequence, either clockwise or counterclockwise. Image Figure 1 Image ● Torque wrench: Suitable for important equipment and pipeline flanges that are exposed to high temperatures and pressures, as well as those that are flammable or explosive. The tightening requirements are as follows: 1) The maintenance unit must develop a tightening plan, determine the appropriate torque, and conduct design reviews based on parameters such as bolt strength, the initial and operating sealing pressures of the gaskets, as well as the medium pressure, in order to prevent bolt breakage and seal failure caused by the gaskets losing their elasticity due to excessive compressive force. 2) Tighten the flanges symmetrically, number the tightening sequence, and refer to the numbering in Figures 1 and 2. 3) Use 4 bolts at positions 1, 2, 3, and 4 to position the gasket, ensuring that the center of the wound gasket is within the edge of the flange. 4) Tighten the setting bolts by hand, then insert the other studs and tighten them by hand to balance the load, ensuring that at least 2 threads are exposed on each end of the nut. 5) Based on the equipment and flanges on site, calculate the number of tightening cycles – taking one cycle as one round of tightening – and determine an appropriate number of tightening cycles (at least 3 times) as well as the torque to be applied each time. The tightening torque should be increased gradually from low to high (e.g., 50%, 80%, 100%). Do not apply excessive loads too quickly, to prevent failure in the gasket sealing. 6) Sequence for tightening with a torque wrench each time: Tighten the two bolts that are radially opposite to each other to the specified torque ； Tighten the other pair of bolts that are approximately 90 degrees apart circumferentially from the previous two bolts ； Continue tightening until all the remaining bolts are tightened to the specified torque. 7) Finally, tighten all bolts in sequence, either clockwise or counterclockwise, to a torque value of 100%. 8) Record the torque value for reference during future maintenance. ● Bolt tensioner: Suitable for flanges of critical equipment and pipelines under high temperature, high pressure, as well as in environments prone to fire and explosion. The tightening requirements are as follows: 1) The maintenance unit shall develop a tightening plan, determine an appropriate tensile force, and conduct design reviews based on parameters such as bolt strength, the initial and operating seal pressure of the gaskets, as well as the medium pressure, in order to prevent bolt fracture and seal failure caused by the gaskets losing their elasticity due to excessive compressive force. 2) When using a bolt tensioner to tension and tighten bolts individually (step by step), follow the principle of uniform bolt tightening, and use a torque wrench to determine the sequence in which the bolts should be tightened. 3) During the bolt tensioning process using a bolt tensioner, it is important to determine an appropriate number of tensioning cycles. The pressure should be increased gradually, step by step (such as 50%, 80%, 100%), with even pressure application. Each time the pressure is increased, it should be maintained at that level for a while before further increase, in order to avoid excessive tensile forces that could affect the bolt’s pre-tightening effect. 4) Record the pressure value for reference during future maintenance.