Flange joints require proper tightening to avoid leakage. Bolt tightening sequence, or torque sequence, is defined in the torque tightening procedure.
Flange joints are torqued tight with the help of a torque wrench (manual or hydraulic). The clamp load produced during tightening is higher than 75% of the fastener’s proof load. To achieve the benefits of the pre-loading, the clamping force in the screw must be higher than the joint separation load.
When the bolt loading requires a torque higher than 678 Nm (500 ft-lbs.) to be applied, hydraulic bolt torqueing is recommended.
The torque value is dependent on the friction between the threads of the stud bolt and nut head, this friction can be affected by the application of a lubricant or any plating (e.g. Cadmium or Zinc) applied to the stud threads. The bolting standard will define whether the torque value is for a dry or lubricated stud/bolt thread. If a bolt is torqued rather than the nut then the torque value should be increased to compensate for the additional friction – bolts should only be torqued if they are fitted in clearance holes.
Before installation, the bolts should be tightened by torque control, using an anti-seize lubricant such as Molykote or equivalent. The specified method of bolt tightening is equally applicable to coated, galvanized and ungalvanized bolts.
Torque bolts and nuts in a “CRISS-CROSS” sequence using a minimum of three torquing passes and the maximum bolt stress as defined.
- PASS 1: Torque to a maximum of 30% of the final torque value in accordance with the torque sequence. Check that gasket is getting compressed uniformly.
- PASS 2: Torque to a maximum of 60% of the final torque value.
- PASS 3: Torque to the final torque value (100%).
After the three basic torque passes are completed, repeat torquing 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:
- 4 Bolt Flange
- 8 Bolt Flange
- 12 Bolt Flange
- 16 Bolt Flange
- 20 Bolt Flange
- 24 Bolt Flange
- 28 Bolt Flange
- 32 Bolt Flange