by Joel Baulch, FSA
June 20, 2012

Bolt Tightening Control

The use of manpower to tighten the bolts, by sledgehammer, striking wrenches and pieces of pipe on the end of the wrench is not recommended, since this offers no accuracy. Consult the torque or tightening specifications from the gasket manufacturer or the company’s engineering department for guidance. First, contact the gasket using only sufficient force to lightly tighten the fasteners and stabilize the assembly.

Tighten the fasteners in a star or cross-bolt pattern, checking to ensure that the flange remains even at ninety-degree intervals. No significant gasket compression should be occurring at this stage.

Once the assembly is stabilized, apply only a medium tightening force (30 to 50 percent of the target load) using the same star or cross-bolt pattern. In the next pass, increase the force to an ample but restrained force (60 to 70 percent of the target load) using the same star or cross-bolt pattern. Then increase the force to approach full force (90 to 100 percent of the target load) using the same star or cross-bolt pattern. Apply full target load to all nuts in a circular pass, continuing until all the nuts no longer turn.


 For safety reasons, any retightening of bolts must be done with the system off and the gauge pressure at zero.


Compensate for Relaxation

While it may be important to note that the retightening process is contingent on specific installation requirements, compensating for initial load loss in the assembly is a critical strategy to consider. The re-torque of fasteners at 100 percent of the target load helps offset relaxation to a significant degree. Again, consult the gasket manufacturer or the engineering department for recommendations on retightening. Do not retighten any elastomeric based gaskets, though, that have been exposed to elevated temperatures unless otherwise specified. For safety reasons, any retightening of bolts must be done with the system off and the gauge pressure at zero. All pertinent lock out/tag out procedures must be followed.

 

Conclusions

Each component in a bolted flange connection must be designed, selected and assembled within acceptable limits to ensure reliable, long-term system tightness. Failure to consider all these important aspects can lead to serious product loss and environmental impact. Properly applying these six installation principals is an essential part of ensuring leak-free service. Consultation with the gasket manufacturer will help provide guidance for specific application conditions.

 

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