How to Tighten Flange Bolts Scientifically?

Fastening technical requirements for installation of equipment and pipeline flanges

1. Inspection of Flange Fasteners

Gasket: When installing, make sure the gasket is new and clean and dry, and check whether the gasket is defective and damaged; the old gasket cannot be reused; before installation, confirm the size and grade of the gasket used and the flange Identifies the same.

Flange: Before installation, check whether the flange surface is damaged, such as scratches, nicks, mud, corrosion and burrs, dents and scratches with a depth of more than 0.2mm radially passing through the water line of the flange sealing surface, and the covered surface When the width of the gasket sealing surface exceeds half, the flange must be replaced or the sealing surface must be reworked and repaired;

The position of the nut supporting surface on the back of the flange should be parallel and smooth; check whether the flange is centered, and the inspection method should be in accordance with the requirements of 6.2 pipeline installation in SH3501-2011.

Bolts and nuts: Check whether the bolts and nuts are used correctly according to the equipment and pipeline design requirements;

Threads and contact surfaces must be free of dirt, rust, heavy skin, nicks, burrs, chips and other foreign substances that affect torque during tightening;

Repair of bolts by welding or machining is not permitted;

After the flange is installed and tightened, at least two threads are exposed outside the nut;

The bolts and nuts must be lubricated before use, so that the bolts have a low friction coefficient and improve the anti-slip and anti-corrosion properties of the bolts and nuts;

Stud threads, nut threads and contact surfaces must be degreased and dried before using lubricating oil;

Uniform lubricants should be properly used for bolt threads, nut threads, nut bearing surfaces, washers, and nut bearing surfaces on flanges; high temperature anti-seize agents should be used as needed.

2. Bolt tightening method

Torque-free Torx wrench or hammer wrench: suitable for general equipment and pipeline flange tightening, according to the bolt size and flange pressure rating.

The tightening requirements are as follows:

1) The maintenance unit shall formulate a tightening plan, tighten the flange symmetrically, and number the tightening sequence, refer to Figure 1 and Figure 2 for numbering.

2) Position 1, 2, 3, 4 with 4 bolts as gaskets to ensure that the center of the spiral wound gasket is within the flange edge.

3) Hand-tighten the set bolt, then insert the other stud bolts and hand-tighten to balance the load, making sure that at least 2 threads are exposed on each end of the nut.

4) According to the field equipment and flanges, one tightening cycle is calculated as one time, and the number of tightenings (at least 3 times) and the hammering load (strength) for each tightening are reasonably determined. Tighten in order (such as 50%, 80%, 100% increments), and the unloadable load is too fast and too large to prevent gasket seal failure.

5) The sequence of each tightening of the torque-free Torx wrench or hammer wrench: Tighten two diametrically opposite bolts to the predetermined hammer load (strength) of the bolts;

Tighten another pair of bolts approximately 90 degrees circumferentially apart from the previous two bolts; continue tightening until all remaining bolts are tightened to the intended hammer load.

6) Finally, tighten all bolts clockwise or counter-clockwise with 100% hammer load (strength).

Torque wrench: suitable for high temperature and high pressure, flammable and explosive and other important equipment and pipeline flanges.

The tightening requirements are as follows:

1) The maintenance unit needs to formulate a fastening plan, formulate the appropriate torque, and conduct a design review according to the bolt strength, the initial sealing specific pressure of the gasket, the working sealing specific pressure, the medium pressure and other parameters to prevent the bolt from breaking and the gasket due to excessive compression force. Large and lose elasticity leading to seal failure.

2) Tighten the flange symmetrically, and number the tightening sequence, refer to Figure 1 and Figure 2 for numbering.

3) Position the gasket with 4 bolts at positions 1, 2, 3, and 4, making sure that the center of the spiral wound gasket is within the flange edge.

4) Hand-tighten the set bolt, then insert the other stud bolts and hand-tighten to balance the load, making sure that at least 2 threads are exposed on each end of the nut.

5) According to the field equipment and flange, one tightening cycle is calculated as one time, and the number of tightenings (at least 3 times) and the tightening torque for each time are reasonably determined. The tightening torque is tightened in order from small to large (such as 50%, 80% , 100% increments), the unloadable load is too fast and too large to prevent the gasket seal from failing.

6) The sequence of each tightening of the torque wrench: tighten the two diametrically opposite bolts to the specified torque of the bolts; tighten another pair of bolts about 90 degrees circumferentially apart from the previous two bolts; continue to tighten until all remaining bolts are Tighten to the specified torque.

7) Finally, tighten all bolts in a clockwise or counter-clockwise direction at 100% torque value.

8) Record the torque value for reference in future maintenance.

Bolt tensioner: suitable for high temperature and high pressure, flammable and explosive and other important equipment and pipeline flanges

The tightening requirements are as follows:

1) The maintenance unit formulates the fastening plan, formulates the appropriate tensile force, and conducts design review according to the bolt strength, the initial sealing specific pressure of the gasket, the working sealing specific pressure, the medium pressure and other parameters to prevent the bolt from breaking and the gasket due to the compression force. Excessive size and loss of elasticity can lead to seal failure.

2) When the bolt tensioner stretches and tightens the bolts individually (step by step), according to the principle of bolt tightening uniformity, refer to the tightening sequence of the torque wrench.

3) During the stretching and tightening process of the bolt tensioner, the number of times of stretching and tightening should be reasonably determined, and the pressure should be increased in order from small to large (such as 50%, 80%, 100%), and the pressure should be uniform. Then increase the pressure to avoid excessive impact tension and affect the pre-tightening effect of the bolt.

4) Record the pressure value for reference for future maintenance.

Thermal requirements for equipment and pipeline heating during start-up

(1) Hot tightening according to the temperature in Table 1

Table 1 Fastening temperature of equipment and pipelines in hot and cold states:

UNIT: ℃

Note: If the working temperature is between -29~250℃, hot-tightening and cold-tightening are not required.

(2) Hot tightening or cold tightening should be carried out after the temperature of the equipment and pipeline is stable, and explosion-proof tools should be used. During the tightening period, operations such as heating and pressure increase cannot be performed.

(3) When tightening, it is recommended to start from the largest flange clearance and tighten symmetrically. If there is a leak, perform a tight leak first.

(4) During the heating process of equipment and pipelines during the start-up period, it is necessary to check the tightening quality. Use a torque wrench at 100% torque value or use an explosion-proof hammer tool to hammer the nut in the tightening direction to check whether it is loose.