How To Bolt Flanges

According to the Fluid Sealing Association (FSA,) incorrect tightness is the leading reason gasketed joints fail. This can be prevented by following good bolting practice.


After installing a new gasket or seal it’s essential to tighten the fasteners with a torque wrench that’s been recently calibrated. Without this it’s impossible to know if the joint has been tightened to the required level.

Friction between the nut, washers, flange faces and thread increases the torque measured at the wrench, possibly resulting in insufficient clamping force being applied to the gasket. Avoid this by applying a thin, uniform coating of high quality lubricant to the underside of bolt heads, nuts and washers and the thread itself. Take care to keep it off the gasket.

Tightening sequence

The gasket must be compressed uniformly to avoid material displacement. It’s also important to avoid deforming the flange faces. There are two aspects to consider: the bolt pattern and the tightening sequence.

Bolt pattern

To bring the joint together, fasteners should be tightened in opposite pairs. Start at 12 o’clock and then move to 6 o’clock. Then halve the angle between them, moving to the 3 and 9 o’clock pair. Halve the angle again, going to the pair closest to 1:30 and 7:30. Keep repeating until every bolt has been tightened.

Tightening sequence

  1. Following the pattern described above, insert the bolts and run up the nuts by hand.
  2. Set the torque wrench to 30% of full torque and, using the pattern, tighten each fastener.
  3. Repeat with the torque wrench at 60%.
  4. Repeat again with the torque wrench at 100%.
  5. Make a final pass, this time in a circumferential direction, ensuring each fastener is at the required torque.

Do the job once

Replacing gaskets and seals can be expensive, so whenever joints are made in pipes and ducting it’s important to ensure they don’t leak. One factor in achieving a good joint is to follow good bolting practice. Control the torque applied, the bolting pattern and the tightening sequence to avoid leaks.

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