Torque loss is inherent in any bolted joint. The combined effects of bolt relaxation, (approximately 10% during the first 24 hours after installation), gasket creep, vibration in the system, thermal expansion, and elastic interaction during bolt tightening contribute to torque loss. When torque loss reaches an extreme, the internal pressure exceeds the compressive force holding the gasket in place and a leak or blow-out occurs.
RELATED: Torque Value Calculator
A key to reducing these effects is proper gasket installation. By bringing the flanges together slowly and parallel when installing a gasket and taking a minimum of four bolt tightening passes, following the correct bolt tightening sequence, there is a payoff in reduced maintenance costs and increased safety. Proper gasket installation technique is covered in the our Durlon® Technical Handbook.
Proper gasket thickness is also important. The thicker the gasket, the higher the gasket creep which in turn can result in torque loss. On standard ANSI raised face flanges a 1/16" thick gasket is normally recommended. Thinner gasket materials can take a higher gasket load and therefore higher internal pressures.
Even when the installation is ideal, where the bolt stress is uniformly applied to each bolt, and the gasket is properly compressed, problems can still arise. Inherently with time, loosening will occur due to the factors already mentioned. If other factors such as cycling, thermal upsets, water hammer or just a piping system with inadequate pipe supports are present, periodic retorquing might be necessary.
For problem areas, high temperature applications or where there is temperature cycling, or where a flange cannot be retorqued, conical spring washers have been found to be very helpful as an aid to torque retention. They act as a spring and help lessen the effects of torque loss.
Other factors affecting torque loss include:
- Rate of heat up
- New vs. used bolts or studs
- Use of hardened steel washers
- Lubrication of bolts, nuts and nut facings
- Method of bolt up. Order of efficiency from least to greatest:
- Wrench and cheater bar or sledge hammer
- Air impact gun
- Torque wrench
- Hydraulic torque wrench
- Hydraulic stud tensioners
Finally, having the torque information for the gasket material is helpful as well. Please refer to the torque data.