I. Technical Requirements for Tightening of Flanges in Equipment and Pipeline Installation Process
Inspection of Flange Bolts 1
- Gasket: When installing, ensure that the gasket is new, clean and dry, and check for any defects or damage. Old gaskets cannot be reused. Before installation, confirm that the size and grade of the gasket used are consistent with the markings on the flange.
- Before installation, check whether the flange surface is damaged, such as scratches, grooves, mud, corrosion and burrs. If the depth of the waterline marks and scratches on the flange sealing surface exceeds 0.2mm and the coverage exceeds half of the width of the gasket sealing surface, the flange must be replaced or the sealing surface reprocessed and repaired. The nut support surface on the back of the flange should be parallel and smooth. Check whether the flanges are centered. The inspection method shall be in accordance with the requirements of 6.2 Pipeline Installation in SH3501-2011.
- Bolts and nuts: Check whether bolts and nuts are used correctly in accordance with the design requirements of equipment and pipelines; the threads and contact surfaces must be free of dirt, rust, scale, scratches, burrs, debris and other external substances that may affect torque during tightening; welding or machining methods are not allowed to repair bolts; after the flange installation and tightening are completed, at least two threads should be exposed outside the nut;
Bolts and nuts must be lubricated before use to achieve a low friction coefficient during tightening and improve the anti-slip and anti-corrosion performance of bolts and nuts; the threads of studs, nuts and contact surfaces must be degreased and dried before applying lubricating oil; a uniform lubricating oil should be properly used on the threads of bolts and nuts, the bearing surface of nuts, washers and the nut support surface on flanges; high-temperature anti-seizing agents should be used as needed.
Bolt Tightening Method 2
- Torque-free box wrench or hammer wrench: Applicable for general equipment and pipeline flange fastening. Select based on bolt size and flange pressure rating. The fastening requirements are as follows：
1) The maintenance unit formulates a tightening plan, tightens the flanges symmetrically, and numbers the tightening sequence.
2) Position the gasket with four bolts at locations 1, 2, 3, and 4, ensuring that the center of the spiral wound gasket is within the flange edge.
3) Hold the positioning bolt firmly with your hand, then insert the other stud bolts and hand-tighten them to balance the load. Make sure that at least two threads of each nut are exposed at both ends.
4) Based on the on-site equipment and flanges, calculate the number of tightening rounds as one cycle. Reasonably determine the number of tightening times (at least 3 times) and the hammering load (force) for each tightening. The hammering load (force) should be gradually increased from small to large (such as 50%, 80%, 100% incrementally). Do not apply the load too quickly or too heavily to prevent the gasket seal from failing.
5) For torque-free box wrench or hammer wrench, the tightening sequence for each fastening is as follows: Tighten the two bolts that are radially opposite to each other to the predetermined hammering load (force); then tighten another pair of bolts that are approximately 90 degrees apart from the previous two along the circumference； continue tightening until all the remaining bolts are tightened to the predetermined hammering load.
6) Finally, all the bolts should be tightened in sequence in a clockwise or counterclockwise direction at 100% hammering load (force).
- Torque wrench: Applicable to important equipment and pipeline flanges in high-temperature, high-pressure, flammable and explosive environments. The tightening requirements are as follows:
1) The maintenance and repair unit needs to formulate a tightening plan, determine the appropriate torque, and conduct design verification based on parameters such as bolt strength, the initial sealing specific pressure and working sealing specific pressure of the gasket, and medium pressure, to prevent bolt breakage and gasket loss of elasticity due to excessive clamping force, which could lead to seal failure.
2) Tighten the flanges symmetrically and number the tightening sequence.
3) Position the gasket with four bolts at 1, 2, 3 and 4 to ensure that the center of the spiral wound gasket is within the flange edge.
4) Hold the positioning bolt firmly by hand, then insert the other stud bolts and hand-tighten them to balance the load. Make sure that at least two threads of each nut are exposed at both ends.
5) Based on the on-site equipment and flanges, calculate the number of tightening rounds as one cycle. Reasonably determine the number of tightening times (at least 3 times) and the torque for each tightening. Tighten in sequence from small to large (such as 50%, 80%, 100% increments). Do not apply the load too quickly or too large to prevent the gasket seal from failing.
6) The sequence for each tightening with a torque wrench: Tighten the two bolts that are radially opposite to each other to the specified torque for the bolts; tighten another pair of bolts that are approximately 90 degrees apart along the circumference from the previous two bolts; continue tightening until all the remaining bolts are tightened to the specified torque.
7) Finally, tighten all the bolts in sequence in a clockwise or counterclockwise direction at 100% torque value.
Record the torque value for reference during future maintenance.
- Bolt tensioners: Applicable to flanges of important equipment and pipelines under high temperature, high pressure, flammable and explosive conditions. The tightening requirements are as follows:
1) The maintenance and repair unit formulates a tightening plan, determines the appropriate tensile force, and conducts design verification based on parameters such as bolt strength, the initial sealing specific pressure and working sealing specific pressure of the gasket, and medium pressure, to prevent bolt breakage and gasket loss of elasticity due to excessive clamping force, which could lead to seal failure.
2)When a bolt tensioner is used to tension and fasten bolts one by one (in steps), in accordance with the principle of uniformity in bolt fastening, the tensioning and fastening should be carried out in the same sequence as when using a torque wrench.
3) During the bolt tensioner’s tensioning and fastening process, the number of tensioning and fastening operations should be reasonably determined. Pressure should be applied gradually from small to large (such as 50%, 80%, 100%), and evenly. After each increase in pressure, it should be stabilized before further increasing to avoid excessive impact tension, which may affect the pre-tightening effect of the bolts.
4) Record the pressurization value for reference during future maintenance.
II. Thermal Tightening Requirements for Equipment and Pipelines During Startup Period
Carry out hot-tightening according to the temperatures in Table 1.
Table 1 Hot and Cold State Tightening Temperature of Equipment and Pipelines Unit: ℃
| Operating Temperature | Temperature for Primary Hot Tightening/Cold Tightening | Temperature for Secondary Hot Tightening/Cold Tightening |
| 250~350 | Operating Temperature | － |
| >350 | 350 | Operating Temperature |
| -70~-29 | Operating Temperature | － |
| <-70 | -70 | Operating Temperature |
Note: For working temperatures ranging from -29 to 250℃, thermal tightening and cold tightening are not required.
(2) Hot-tightening or cold-tightening should be carried out after the temperature of the equipment and pipelines has stabilized. Explosion-proof tools should be used, and no operations such as temperature or pressure increase should be conducted during the tightening process.
(3) When tightening, it is recommended to start from the area with the largest gap in the flange and tighten symmetrically. If there is leakage, first deal with the leakage.
(4) During the equipment and pipeline heating-up process in the start-up period, a quality check on the fastening should be conducted. Use a torque wrench to check 100% of the torque value or use an explosion-proof hammering tool to strike the nuts in the tightening direction to check if they are loose.
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