The Fixed Torque Tightening Technology of Flange Bolts and Nuts

Complete Analysis of Fixed Torque Tightening Technology of Flange Bolts and Nuts

The flange connection is a common and critical structure in pipelines and equipment. It is used to connect components such as pipe sections, valves, pumps, etc. The sealing performance of flange connections is directly related to the safe and stable operation of the entire system. In flange connections, bolts and nuts play a crucial role in maintaining the integrity of the pipeline system. They must be tightened with sufficient torque to ensure that there is no leakage or failure in the pipeline system due to pressure fluctuations, thermal expansion, or mechanical vibration. The fixed torque tightening technology of flange bolts and nuts is a method of accurately controlling the tightening force through preset torque values, ensuring the uniformity and repeatability of connections of bolts and nuts.

Technical requirements

When tightening flange bolts and nuts with fixed torque, the appropriate torque value should be determined based on the specifications, materials, types of connectors of the bolts and nuts in accordance with relevant standards GB/T 38343-2019. In addition, it is required that operators must calibrate the tools used to ensure the accuracy of the applied torque. In specific operating environments, it is also necessary to consider the impact of temperature changes on material properties, as well as the effect of lubricating oil on friction coefficient, all of which may affect the accurate application of preload.

Operating procedures and acceptance standards

1. A comprehensive inspection before tightening

A comprehensive inspection before the actual tightening operation begins is the key to ensuring the quality of tightening. 

This includes but is not limited to:
● Inspection of flange sealing surface: Ensure that there is no damage, rust, or oil stains, which can affect the sealing effect.
● Shim condition inspection: 
Verify that the gasket specifications match the requirements and check for any damage or deformation.
● Inspection of bolts and nuts:  
Ensure that there is no wear, corrosion, or damage, and the threads should be clear and intact.

2.  Tightening operation steps and sequence
The tightening operation follows strict steps. It is usually carried out in the following order.

● Preliminary tightening: 
Use 30% of the target torque value for preliminary tightening to ensure the correct position of the gasket.

Middle tightening: 
Increase to 70% of the target torque value and further evenly load the pre-tightening force.

● Final tightening: 
Apply 100% of the target torque value to complete the tightening, usually repeating this step several times to ensure even pre-tightening force.

3.​ Quality control

Quality control after tightening is the final step in ensuring installation quality, including:

● Bolt spot check: 
Conduct spot checks on tightened bolts and nuts according to standards, generally not less than 20% of the total number of bolts and nuts.
● Hammer test:
Knock the bolt to hear the sound to determine if it is securely fastened, and a crisp sound indicates that the fastening is qualified.

Gap measurement: 
Use a caliper to measure the distance between flanges and ensure even distribution of force.

4.​ Acceptance​

After completing the fastening operation, acceptance is required to confirm that the quality of the fastening work meets the design requirements. 

The acceptance process includes:

Visual inspection
Confirm that the exposed length of the bolts and nuts is consistent and there are no signs of damage or improper tool marks.
Torque reconfirmation: 

Use a torque tester to test the torque applied to the bolt and confirm if it meets the design requirements.
● Record: 
Detailed record of tightening data for each bolt, including the type of wrench used, the applied torque value, etc., to provide a basis for future maintenance. Every step of the tightening operation process requires careful attention and precise execution, which directly affects the safety and reliability of the pipeline system. The strict implementation of acceptance standards ensures that each fastening point can meet the design and safety requirements.

Effect and economic benefits

Through precise torque tightening, the reliability of bolted connections has been greatly enhanced, reducing the risk of accidents caused by bolt loosening or excessive tightening. The leakage rate of the tightened flange connection is significantly reduced, which directly improves the stability and safety of the entire system, avoiding potential environmental pollution and economic losses. Due to stronger connections, the frequency of equipment maintenance and repair is reduced, thereby reducing maintenance costs and avoiding production losses caused by downtime.
Compared with traditional manual tightening methods, the use of hydraulic torque wrenches can significantly improve tightening efficiency. Skilled workers can use hydraulic wrenches to tighten bolts and nuts of the same specification, reducing time costs by about 30%.

At the same time, the use of hydraulic wrenches reduces the labor intensity of workers, thereby lowering labor costs and reducing occupational injuries and related medical costs caused by high work intensity. Although the initial investment of hydraulic wrenches is relatively high, they can save maintenance and operating costs in the long run due to their improved work efficiency and reduced rework rates.

Technical Trends

With the improvement of industrial automation and intelligence, the fastening technology of flange bolts and nuts is also constantly innovating. The future development trend may integrate more advanced sensors and control systems into tightening tools, making torque application more precise and real-time monitoring of the tightening process.
Through IoT technology, fastening data can be uploaded in real-time to the cloud platform for engineers to analyze and optimize fastening parameters. In some high-risk or space limited environments, automated robots may be used to replace manual fastening operations.


The fixed torque tightening technology of flange bolts and nuts has become an important tool in modern industry to ensure safety and improve efficiency. It significantly improves the reliability and sealing of flange connections by precisely controlling the pre-tightening force of bolts and nuts. With the continuous progress of technology, we have reason to believe that this technology will become more popular in the future. Its advantages will be fully utilized in more industrial fields, bringing greater economic benefits to enterprises.

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