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Chapter A7.10.3 - Tensioning Systems

By Mohinder L. Nayyar, P.E.
From Piping Handbook, 7th Edition

Many of the variables that reduce the control of bolt preload using the torque
process are eliminated using hydraulic tensioning systems.

Hydraulic tensioners are hollow hydraulic compact cylinders that are threaded
onto a protruding section of the studbolt generally using a pulling device. A bridge
supports the hydraulic head straddling the nut and reacting against the flange while
hydraulic pressure is applied to the hydraulic head. Under the applied hydraulic
load, the bolt stretches at the same time as it compresses the flange and gasket.
Residual bolt load equivalent to the desired preload (Fp) is achieved by manually
turning down the nut under the tensioner bridge during the applied hydraulic load.
Applied bolt load is directly proportional to the hydraulic pressure and the area
of the hydraulic cylinder. There are no frictional losses associated with tensioning,
as compared to torquing. A cross section of a hydraulic stud tensioner is illustrated
in Fig. A7.20. The residual load (preload Fp) = Applied Load - Load Loss Factor.

The load loss factor is dependent upon the stud stress realized, bolt diameter,
and effective length of the bolt. For each application its load loss factor can be
precisely calculated to determine the necessary applied load to generate the residual
preload. Development of thorough procedures is essential to maintain the accuracy
of hydraulic stud-tensioning process.

Cross talk is significantly reduced by utilizing multistud tensioning. Generally
50 percent of the studs in a flange are tensioned simultaneously by using multiple

 

tools interconnected with a high-pressure hose tied into a common pump source.
Many flange configurations allow for 100 percent of the studs to be tensioned
simultaneously. This completely eliminates cross talk.

Hydraulic tensioning provides the most controlled tightening method for achieving
specified bolt preload.

 

© 2000
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