Special Requirements for Pre-Tightening Force Control in Welded Flanges

Special Requirements for Pre-Tightening Force Control in Welded Flanges

1. Importance of Proper Pre-Tightening Force

• Correct pre-tightening (bolt preload) ensures a secure flange connection, preventing leaks and maintaining structural integrity under pressure and thermal cycling.

• Insufficient preload can cause gasket blowout or joint leakage; excessive preload risks bolt yielding or flange distortion.

2. Accurate Torque Application

• Use calibrated torque tools or hydraulic tensioners to apply uniform and controlled bolt preload.

• Follow recommended torque values specified in design codes or flange standards (e.g., ASME, API).

• Torque should be applied gradually and evenly in a cross or star pattern to distribute load uniformly.

3. Consideration of Welding Residual Stresses

• Welding introduces residual stresses and potential flange distortion that can affect bolt preload distribution.

• Pre-tightening force should account for these stresses to avoid uneven bolt load and flange misalignment.

4. Temperature and Service Conditions

• Adjust preload values for operating temperature, as thermal expansion affects flange and bolt dimensions.

• For high-temperature service, bolts may relax over time, requiring retightening or use of special bolt materials.

5. Bolt and Gasket Material Compatibility

• Ensure bolts have adequate strength and elongation properties to maintain preload under load and thermal cycles.

• Gasket compressibility affects the required preload; softer gaskets may need higher preload for sealing.

6. Monitoring and Re-Tightening

• For critical applications, monitor bolt preload during service using torque checks or load indicators.

• Implement re-tightening procedures as necessary, especially after initial thermal cycling.

Summary

Controlling pre-tightening force in welded flanges requires precise, uniform application of torque, consideration of welding stresses and thermal effects, and compatibility of bolts and gaskets. Proper preload ensures flange joint reliability and leak prevention under varying operating conditions.