Flange nuts usually need to be retightened after installation, especially under high temperature, high pressure, or vibration conditions. Retightening is a crucial step to ensure connection sealing and safety.
Why is retightening necessary?
1. Gasket creep relaxation: After initial tightening, the gasket undergoes plastic deformation (creep) under continuous pressure, leading to a decrease in bolt preload. Retightening compensates for this loss and maintains the sealing pressure.
2. Effects of hot or cold operation: When equipment heats up or cools down, the different coefficients of thermal expansion of the flange and bolt materials can easily cause stress relaxation. Therefore, hot tightening (high temperature) or cold tightening (low temperature) is necessary after the temperature stabilizes to prevent leakage.
3. Risk of loosening in vibration environments: In vibration scenarios such as mechanical equipment and piping systems, nuts may gradually loosen due to fretting. Retightening can promptly detect and correct loosening problems.
4. Timing and Method of Retightening
Initial Retightening Time: It is recommended to retighten within 24 hours of installation or after 4 hours of equipment operation.
Retightening Torque: Use a torque wrench to retighten to 100% of the original target torque, following the same sequence as the initial tightening (diagonal, step-by-step).
Number of Retightening Rounds: Generally 1-2 rounds are sufficient; for critical equipment, the frequency of checks can be increased depending on the operating conditions.
Special Operating Conditions:
High-Temperature Equipment: Perform hot tightening after heating to the operating temperature and stabilizing.
Low-Temperature Equipment: Perform cold tightening after cooling to the target temperature.
Vibrating Pipelines: It is recommended to retighten after 24 hours of operation and install anti-loosening gaskets.
✅ Note: Retightening is not simply "tightening it again," but should be performed according to the standard procedure to avoid missed checks or over-tightening.
