Flange Bolt Tightening: Master the Correct Method Now

Technical requirements for fastening in the process of equipment and pipeline flange installation

(1) Inspection of flange fasteners

● Gasket: When installing, make sure that the gasket is new and clean, and check whether the gasket has defects or damage.

Old gaskets cannot be reused. Before installation, confirm that the size and grade of the gasket used are consistent with the identification of the flange.

● Flange: Check whether the flange surface is damaged before installation, such as scratches, inscriptions, mud, corrosion, and burrs.

When the depth of dents or scratches that pass through the radial sealing surface watermark line of the flange exceeds 0.2mm and the covering surface exceeds half the sealing surface width of the gasket, the flange must be replaced or the sealing surface needs to be re-machined and repaired.

The back nut support surface position of the flange should be parallel and smooth. Check whether the flange is centered, and the inspection method should follow the pipeline installation requirements in SH3501-2011.

● 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 shall not have external substances such as dirt, rust, heavy skin, inscriptions, burrs, debris, and others that affect torque during tightening.

Welding or machining methods are not allowed to repair bolts. After the flange is installed and tightened, at least two threads should be exposed outside the nut.

Bolts and nuts must be lubricated before use to reduce the friction coefficient during bolt tightening, and to improve the anti-slip and anti-corrosion properties of bolts and nuts.

The threads of stud bolts, nut threads, and contact surfaces must be degreased and dried before using lubricating oil.

The bolt threads, nut threads, nut bearing surfaces, washers, and nut support surfaces on flanges should use the same lubricating oil properly. High-temperature anti-seizing agents should be used as needed.

(2) Bolt fastening method

● Non-torque spanner or striking spanner: Suitable for general equipment and pipeline flange tightening. Select according to bolt size and flange pressure level.

The fastening requirements are as follows:

• The maintenance unit formulates a fastening plan and tightens the flange symmetrically, numbering the tightening sequence, and referring to Figures 1 and 2.
• For positioning at positions 1, 2, 3, and 4, use 4 bolts as gaskets to ensure that the center of the wound gasket is within the edge of the protrusion.
• Tighten the positioning bolts by hand, and then insert other bolts and balance the load by hand. Ensure that at least two threads of the nut are exposed on each end.
• According to the equipment and flange on-site, calculate one tightening cycle as one time, and reasonably formulate the tightening times (at least 3 times) and the hammering load (strength) for each tightening. The hammering load (strength) should be tightened in sequence from small to large (such as 50%, 80%, 100%) to prevent the gasket from losing sealing due to too fast or too large loading.
• The order of each tightening of the non-torque spanner or striking spanner:

Tighten the two bolts that are radially opposite to the predetermined hammering load of the bolts.

Tighten another pair of bolts that are about 90 degrees from the previous two bolts along the circumference.

Continue to tighten until all remaining bolts are tightened to the predetermined hammering load.

• Finally, tighten all bolts in a clockwise or counterclockwise direction according to the 100% hammering load.

● Torque wrench: Suitable for important equipment and pipeline flanges with high temperature, high pressure, flammable and explosive properties, etc.

The fastening requirements are as follows:

• The maintenance unit needs to formulate a fastening plan, determine the appropriate torque, and design and check it according to parameters such as the bolt strength, the initial sealing compression ratio and the working sealing compression ratio of the gasket, and the medium pressure to prevent bolt fracture and the gasket from losing elasticity due to excessive tightening force, leading to sealing failure.
• Tighten the flange symmetrically, numbering the tightening sequence, and referring to Figures 1 and 2.
• For positioning at positions 1, 2, 3, and 4, use 4 bolts as gaskets to ensure that the center of the wound gasket is within the edge of the protrusion.
• Tighten the positioning bolts by hand, and then insert other bolts and balance the load by hand. Ensure that at least two threads of the nut are exposed on each end.
• Calculate one tightening cycle as one time according to the equipment and flange on-site, reasonably formulate the tightening times (at least 3 times) and the tightening torque for each time. The tightening torque is tightened in sequence from small to large (such as 50%, 80%, and 100%), to prevent the gasket from losing sealing due to too fast or too large loading.
• The order of each tightening of the torque wrench:

Tighten the two bolts that are radially opposite to the predetermined torque of the bolts.

Tighten another pair of bolts that are about 90 degrees from the previous two bolts along the circumference.

Continue to tighten until all remaining bolts are tightened to the predetermined torque.

• Finally, tighten all bolts in a clockwise or counterclockwise direction according to the 100% torque value.
• Record the torque value as a reference for future maintenance.

● Bolt tensioner: Suitable for important equipment and pipeline flanges with high temperature, high pressure, flammable and explosive properties, etc.

The fastening requirements are as follows:

• The maintenance unit formulates a fastening plan, determines the appropriate tensile force, and designs and checks it according to parameters such as the bolt strength, the initial sealing compression ratio and the working sealing compression ratio of the gasket, and the medium pressure to prevent bolt fracture and the gasket from losing elasticity due to excessive tightening force, leading to sealing failure.
• When a single (step) bolt is tightened by the bolt tensioner, it should be carried out according to the principle of uniform bolt tightening, referring to the tightening sequence of the torque wrench.
• During the bolt tensioning process, reasonably formulate the tightening times, gradually increase the pressure uniformly from small to large (such as 50%, 80%, and 100%), and stabilize the pressure before increasing the pressure to avoid excessive impact tensile force on bolts, affecting the pre-tightening effect of the bolts.
• Record the pressure value as a reference for future maintenance.

Requirements for thermal tightening during equipment and pipeline heating during construction period.

(1) Perform thermal tightening according to the temperature in Table 1.

Table 1: Temperature for Thermal and Cold Tightening of Equipment and Pipeline, Unit: ℃

Note: The hot and cold tightening cannot be carried out for working temperature between -29~250℃.

(2) Hot or cold tightening should be carried out after the equipment and pipeline temperature has stabilized. Use explosion-proof tools and do not perform operations such as heating and pressurizing during tightening.

(3) For tightening, it is recommended to start from the largest gap between flanges and tighten symmetrically. If there is leakage, first tighten the leak.

(4) During the equipment and pipeline temperature rising process, it is necessary to carry out quality inspection for tightening. Use torque wrench according to 100% torque value or use explosion-proof hammer to check whether the nut is loose in the tightening direction.