Key Takeaways
Flange fastening requires strict pre‑assembly inspection of gaskets, flange sealing faces, bolts‑nuts and thread lubrication. Select appropriate tightening tools according to application severity. Adopt numbered cross‑symmetrical multi‑pass incremental‑stress tightening sequence, prohibit heavy hammer striking, and ensure proper exposed‑thread length to achieve reliable sealing of flange joints.
| Inspection Item | Key Technical Points |
|---|---|
| Gasket | Must use brand‑new intact gasket; reuse is forbidden. Size‑grade must match flange identification. |
| Flange sealing face | Radial scratch deeper than 0.2 mm & covering over half gasket‑sealing width: re‑machine or replace flange. Nut supporting face shall be flat and parallel. |
| Bolts & nuts | Verify diameter/length per piping drawing; threads & contact surfaces clean, free of burrs/debris. B8 / B8M bolts cannot be reused; bolt repair via welding/machining prohibited. Min 2‑2.5 thread turns exposed past nut after tightening. |
| Lubrication | Degrease & dry threads / nut bearing surfaces before applying uniform lubricant; reduce friction coefficient for consistent torque and anti‑corrosion. |
Tightening‑method selection
- General‑grade conditions: Uncontrolled hammer‑wrench or controlled‑type (torque wrench / hydraulic bolt tensioner) are both acceptable.
- Strict critical service conditions: Only controlled‑type tools (torque wrench or hydraulic bolt tensioner) are permitted; tool choice depends on bolt size and flange class.
Flange alignment requirements
- Align flanges by adjusting connected pipes only; do not shift equipment for alignment.
- Measure flange‑face parallelism gap at prescribed circumferential intervals based on bolt‑quantity to verify centering.
Multi‑pass cross‑symmetrical tightening workflow
- Number bolt positions; position gasket using four hand‑tightened locating bolts. Install remaining studs and hand‑tighten all.
- Multi‑pass cross‑diagonal tightening in sequence: 50 % → 80 % → 100 % → final 100 % plus one additional clock‑wise round for load balancing.
- Inspect uniform bolt stress and re‑tighten as‑needed.
- Forbidden practice: heavy hammer blows on wrenches, which risk damaging bolts, flanges and gaskets.
Final check: 2.5 full thread turns shall protrude beyond each nut upon completion.
Flange fasteners have a series of technical requirements and precautions during operation. Following a scientific procedure for operation can enable the fasteners to achieve their best performance and ensure the safe operation of the equipment.
1. Inspection of flange fasteners:
I. Gaskets:
When installing, ensure that the gasket is new, clean and dry, and check for any defects or damage to the gasket.
Under no circumstances can the gasket be reused.
Before installation, make sure that the size and grade of the gasket used are consistent with the identification on the flange.
II. Flange Surface:
Before installation, check whether the flange surface is damaged, such as scratches, grooves, dirt, corrosion and burrs. If the depth of the concave and scratch lines passing through the flange sealing surface radially exceeds 0.2mm and the coverage area exceeds half of the gasket sealing surface width, the flange must be replaced or the sealing surface reprocessed.
The support surface of the nut on the back of the flange should be parallel and smooth.
III. Bolts and Nuts:
1. Check the correctness of the bolt diameter and length according to the pipeline grade diagram.
2. The threads and contact surfaces must be free of dirt, rust, scale, scratches, burrs, debris and other external substances that may affect torque during the fastening process.
3. All grades of bolts of type B8 and B8M are not allowed to be reused.
4. 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.
IV. Lubrication of Stud Bolts and Nuts:
1. Before use, bolts and nuts must be lubricated to ensure a low friction coefficient during tightening and to enhance their anti-slip and anti-corrosion properties.
2. The stud threads, nut threads and contact surfaces must be degreased and dried before applying lubricating oil for use.
3. Uniform lubricating oil should be properly applied to the threads of bolts and nuts, the bearing surface of nuts, washers, and the nut support surface on flanges.
2. Selection of bolt tightening methods:
1. General Applications: Under general application conditions, the flange bolt tightening can be carried out by an uncontrollable method using a tightening wrench or a hammer wrench based on the bolt size and flange grade, or by a controllable method using a torque wrench or a hydraulic bolt tensioner.
2. Strict Application: Under strict application conditions, flange bolt tightening can only be carried out through controlled methods using a torque wrench or a hydraulic bolt tensioner. The bolt size and flange grade determine whether to use a torque wrench or a hydraulic bolt tensioner.
3. Fastening technical requirements:
I. Inspection of Flange Alignment:
1. For standard flanges, it is considered centered when the bolts can freely pass through the bolt holes.
2. During the installation of pipes and pipe flanges, loosen the adjacent pipe supports and adjust to the correct alignment. When installing pipes to equipment, only adjust the pipes.
3. Under no circumstances should the equipment be adjusted to achieve alignment.
4. Confirm the parallelism error of the flange surfaces by measuring the gap between the two mating flanges of the pre-joint. For flanges with 8 bolts, measure the gap at 4 approximately equal intervals; for flanges with 8 to 32 bolts, measure the gap at every other bolt; for flanges with more than 32 bolts, measure the gap at 16 approximately equal intervals.
II. Bolt Connection and Tightening Steps:
1. In the bolt-tightening procedure, each bolt hole is sequentially numbered using a bolt-tightening sequence diagram.
2. 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.
3. Tighten these four bolts 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 each end.
4. Tighten the two radially opposite bolts to 50% of the specified bolt stress.
5. Tighten the other pair of bolts that are approximately 90 degrees apart from the previous two bolts along the circumference.
6. Continue tightening until all bolts are tightened to 50% of the specified stress.
7. Repeat steps 2, 3 and 4 to tighten to 80% of the specified bolt stress.
8. Repeat steps 2, 3 and 4 to tighten to 100% of the specified bolt stress.
9. Repeat steps 2, 3 and 4 to tighten to 100% of the specified bolt stress, and then turn clockwise one more turn to balance the bolt load.
10. Check whether the stress on all bolts is uniform. Retighten them if necessary.
After the installation and tightening are completed, the bolt thread should protrude 2.5 turns beyond the nut.
Under normal circumstances, it is not allowed to tighten bolts by heavy hammering. Heavy hammering of the wrench increases the risk of damage to the bolts, flanges or gaskets.
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