Flange Bolt Torque Sequence

Proper tightening of flange joints is crucial to prevent leaks. The order in which bolts are tightened, known as the bolt tightening sequence or torque sequence, is outlined in the torque tightening procedure.

Flange joints are secured tightly using a torque wrench, which can be manual or hydraulic. The force exerted by the clamp during tightening exceeds 75% of the fastener’s maximum load capacity. To fully benefit from the pre-loading effect, the force applied to the screw must surpass the load required to separate the joint.

When the force needed to tighten bolts exceeds 678 Nm (500 ft-lbs.), it’s recommended to use hydraulic bolt tightening. The amount of torque needed depends on the friction between the threads of the stud bolt and nut head, which can be influenced by the use of a lubricant or any coating applied to the stud threads (such as Cadmium or Zinc).

The bolting standard will specify whether the torque value is for a dry or lubricated stud/bolt thread. If a bolt is tightened instead of the nut, the torque value should be increased to account for the extra friction – bolts should only be tightened if they are installed in clearance holes. Before the bolts are installed, they should be tightened using torque control, applying an anti-seize lubricant like Molykote or a similar product.

The method for tightening bolts is consistent for coated, galvanized, and ungalvanized bolts. Torque bolts and nuts should be tightened in a “CRISS-CROSS” sequence, applying at least three tightening passes, with the maximum bolt stress as defined.

  • PASS 1: Apply torque up to 30% of the final torque value, following the torque sequence. Ensure the gasket is uniformly compressed.
  • PASS 2: Apply torque up to 60% of the final torque value.
  • PASS 3: Apply torque to the final torque value (100%).

    After completing the three initial tightening passes, repeat the tightening of the nuts at least once, using the final torque in a “CRISS-CROSS” manner until no further rotation of the nut is observed.

For easy handling, bolt numbering should be done clockwise around the flange with the following star pattern sequences: