The Tightening Sequence of Flange Bolts and Nuts
Flange connections are widely used in industrial pipeline systems, pressure vessels, and other equipment. The tightening sequence of flange bolts and nuts is crucial for ensuring the sealing reliability and safe operation of the equipment. A proper tightening sequence ensures uniform force distribution across the flange surface, preventing localized over-compression of the gasket, which could lead to damage or leakage.
The primary goal of the tightening sequence of flange bolts and nuts is to ensure the sealing integrity and flatness of the flange connection. Improper tightening can result in uneven force distribution on the flange surface, causing localized over-compression of the gasket, flange misalignment, and ultimately, leakage.
01 Tightening Principles
(1) Uniform Force Distribution Principle
The key to a flange connection lies in achieving uniform compression of the gasket through the tightening force of the bolts, thereby ensuring a proper seal. Therefore, during tightening, it is essential to ensure that the pressure is evenly distributed across the flange surface, avoiding localized over- or under-compression.
Uneven force distribution can lead to gasket extrusion, flange warping, or bolt overload, all of which can compromise the seal and potentially cause leakage.
(2) Symmetrical Tightening Principle
Whether dealing with circular or square flanges, the symmetrical tightening principle must be followed. This prevents flange misalignment during tightening and ensures that the flange surfaces remain parallel.
For circular flanges, symmetry is centered around the circle's center. For square flanges, symmetry is based on the central axis or diagonal. Tightening bolts in symmetrical positions simultaneously helps balance the forces acting on the flange.
02 Tightening Sequences of Common Flange Bolts and Nuts
Circular Flanges
For circular flanges, the "cross-pattern method" or "symmetrical tightening method" is typically used. Taking the cross-pattern method as an example, the bolts are divided into four groups, resembling a "cross" shape.
1. Initial Tightening Stage:
Divide the bolts into four groups (e.g., top, bottom, left, right for an 8-bolt flange).
Use a manual or torque wrench to lightly tighten the bolts at the cross points (e.g., top, bottom, left, right).
Initial torque should be 30% to 50% of the final torque. This stage ensures the gasket is properly positioned and prevents shifting during further tightening.
2. Intermediate Tightening Stage:
Tighten the remaining bolts in a diagonal sequence (e.g., top-left and bottom-right, then top-right and bottom-left).
Increase torque to 60% to 80% of the final torque. This stage further adjusts the flange flatness and gasket compression, ensuring even force distribution.
3. Final Tightening Stage:
Tighten all bolts to the specified final torque in a diagonal sequence.
Final torque values should comply with design requirements or relevant standards (e.g., ASME, GB).
Use appropriate torque measurement tools (e.g., torque wrench or electric wrench with torque sensors) to ensure consistency.
Circular sequential method (for flanges with a large number of bolts):
1. Initial Tightening Stage:
Divide the bolts into groups along the circumference (e.g., 4 groups of 4 bolts for a 16-bolt flange).
Tighten the first bolt in each group sequentially along the circumference.
2. Intermediate Tightening Stage:
Tighten the second bolt in each group sequentially along the circumference.
Repeat for subsequent bolts in each group.
3. Final Tightening Stage:
Tighten all bolts to the final torque sequentially along the circumference.
Maintain uniform tightening rhythm and force to avoid over- or under-tightening.
Square Flanges
For square flanges, the tightening sequence generally starts from the center and proceeds symmetrically toward the edges.
1. Initial Tightening Stage:
Tighten the center bolt to ensure the gasket is initially seated.
Initial torque should be 30% to 50% of the final torque.
2. Intermediate Tightening Stage:
Tighten bolts symmetrically around the center (e.g., middle bolts on opposite sides, then corner bolts).
Increase torque to 60% to 80% of the final torque.
3. Final Tightening Stage:
Tighten all bolts to the final torque in a symmetrical sequence.
Monitor flange flatness to ensure uniformity across all edges and corners.
Large-Sized Flanges
For large-sized or high-pressure flanges, the tightening process requires greater precision.
Use specialized tools such as hydraulic wrenches or torque multipliers.
Divide the flange into multiple zones and tighten bolts within each zone symmetrically before coordinating between zones.
Monitor flange deformation using tools like dial indicators or laser measurement devices.
High-Temperature Environments
In high-temperature environments, material thermal expansion affects bolt tightening.
Increase bolt preload during design to account for thermal expansion.
Perform hot tightening during operation to compensate for reduced preload due to thermal expansion.
Vibration-Prone Environments
In environments with vibration, flange connections are prone to bolt loosening.
Use anti-loosening devices such as spring washers, double nuts, or nylon-insert lock nuts.
Regularly inspect bolts for loosening using marking methods or anti-loosening monitoring devices.
