How to Tighten Flange Bolts and Nuts Scientifically?
Technical requirements for fastening during the installation process of equipment and pipeline flanges
Inspection of flange fasteners
● Gaskets: During installation, 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 flange markings.
● Flanges: Before installation, check the flange surface for damage such as scratches, dents, mud, corrosion, and burrs. If there are dents or scratches deeper than 0.2mm that pass radially through the water lines on the flange sealing surface, and the coverage exceeds half of the width of the gasket sealing surface, the flange must be replaced or the sealing surface must be reprocessed and repaired.
The position of the nut support surface on the back of the flange should be parallel and smooth. Check if the flange is aligned, and follow the pipeline installation requirements in section 6.2 of SH3501-2011 for the inspection method.
● Bolts and nuts: Check whether the bolts and nuts are used correctly according to the equipment and pipeline design requirements. The threads and contact surfaces must be free of dirt, rust, heavy skin, scratches, burrs, debris, and other external substances that may affect torque during the tightening process.
Welding or machining methods are not allowed to repair bolts. After the flange installation is tightened, at least two threads are exposed outside the nut. Before use, bolts and nuts must be lubricated to ensure a low coefficient of friction and improve their resistance to slipping and corrosion. Before using lubricating oil, the thread of the screw, the thread of the nut, and the contact surface must be degreased and dried.
Uniform lubricating oil should be used properly for bolt threads, nut threads, nut bearing surfaces, washers, and nut support surfaces on flanges. Use high-temperature anti-bite compound as needed.
Bolt tightening methods
Non-torque ring wrench or hammering wrench: it is suitable for general equipment and pipeline flange fastening, selected according to bolt size and flange pressure rating. The fastening requirements are as follows.
1) The maintenance unit shall develop a tightening plan, symmetrically tighten the flanges, and number them in the tightening sequence, referring to Figure 1 and Figure 2 for numbering.
2) Use 4 bolts and nuts as washers to position at positions 1, 2, 3, and 4, ensuring that the center of the wrapped washer is within the edge of the flange.
3) Tighten the positioning bolt by hand, then insert other stud bolts and nuts 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 on-site equipment and flanges, calculate the number of tightening cycles (at least 3 times) and the impact load (force) for each tightening cycle. Tighten the impact load (force) in ascending order from small to large (such as 50%, 80%, and 100% increasing), and do not load too quickly or excessively to prevent gasket sealing failure.
5) The sequence of tightening each time with a torque free plum blossom wrench or impact wrench: tighten the two radially opposite bolts and nuts to the predetermined impact load (force) of the bolts and nuts. Tighten another pair of bolts and nuts that are approximately 90 degrees apart along the circumference from the previous two bolts and nuts. Continue tightening until all other bolts and nuts are tightened to the predetermined hammering load.
6) Finally, tighten all bolts and nuts in a clockwise or counterclockwise direction with 100% hammering load (force).
Torque wrench: it is suitable for high temperature, high pressure, flammable and explosive important equipment and pipeline flanges. The fastening requirements are as follows.
1) The maintenance unit needs to develop a tightening plan, set appropriate torque, and conduct design review based on parameters such as bolt strength, initial sealing pressure and working sealing pressure of the gasket, medium pressure, etc., to prevent bolt fracture and gasket loss of elasticity due to excessive clamping force, leading to seal failure.
2) Tighten the flanges symmetrically and number them in the tightening sequence
3) Use 4 bolts and nuts as washers to position at positions 1, 2, 3, and 4, ensuring that the center of the wrapped washer is within the edge of the flange.
4) Tighten the positioning bolt by hand, then insert other stud bolts and nuts 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 on-site equipment and flanges, calculate the number of tightening cycles (at least 3 times) and tightening torque for each cycle. Tighten the torque in ascending order from small to large (such as 50%, 80%, and 100% increasing), and do not load too quickly or excessively to prevent gasket sealing failure.
6) The sequence of tightening each time with a torque wrench: tighten the two radially opposite bolts and nuts to the predetermined torque of the bolts and nuts. Tighten another pair of bolts and nuts that are approximately 90 degrees apart along the circumference from the previous two bolts and nuts. Continue tightening until all other bolts and nuts are tightened to the predetermined torque.
7) Finally, tighten all bolts and nuts in a clockwise or counterclockwise direction according to the 100% torque value.
8) Record the torque value as a reference for future maintenance.
Bolt tensioner: suitable for high temperature, high pressure, flammable and explosive important equipment and pipeline flanges. The fastening requirements are as follows.
1) The maintenance unit shall develop a tightening plan, establish appropriate tensile force, and conduct design review based on parameters such as bolt strength, initial sealing pressure and working sealing pressure of the gasket, medium pressure, etc., to prevent bolt fracture and gasket loss of elasticity due to excessive clamping force, resulting in seal failure.
2) When using a bolt tensioner to individually (step by step) tighten bolts and nuts, follow the principle of uniform bolt tightening and refer to the tightening sequence of a torque wrench for tightening.
3) The process of bolt tensioner stretching and tightening should be reasonably determined by setting the number of stretching and tightening cycles. The pressure should be gradually increased from small to large (such as 50%, 80%, 100%), and evenly applied. After increasing the pressure to a certain level, it should be stabilized and then increased again to avoid excessive impact tension that may affect the pre-tightening effect of the bolt.
4) Record the pressurization value as a reference for future maintenance.
2. Thermal tightness requirements for equipment and pipeline heating during the start-up period
(1) Perform hot fastening according to the temperature in Table 1
Table 1 Hot and Cold Tightening Temperature of Equipment and Pipelines Unit: ℃
Working temperature: primary hot tightening, secondary hot tightening, secondary cold tightening. Working temperature: 250~350. Working temperature: ->350. Working temperature: -70~-29. Working temperature:<-70~70. Working temperature:
Notes: Hot or cold tightening is not required for working temperatures between -29 and 250 ℃.
(2) Hot or cold fastening should be carried out after the temperature of the equipment and pipeline has stabilized. Explosion proof tools should be used, and heating or pressure boosting operations should not be performed during the fastening period.
(3) When tightening, it is recommended to start from the point with the largest flange gap and tighten symmetrically. If there is a leak, perform a tight leak first.
(4) During the start-up period, a quality inspection of fastening is required in the heating process of equipment and pipelines. Use a torque wrench to perform a 100% torque value or use explosion-proof hammering tools to hammer the nuts in the tightening direction to check for looseness.
