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Gasket Installation Guide

To achieve a trouble free joint one of the most important aspects to consider is the bolts or studs. Reliability depends on achieving and maintaining an acceptable level of tension in each bolt. The target tension level for most applications can be calculated using Klinger's gasket design system.

However, the method employed to tighten the bolts is very important and can result in significant variations from those anticipated. As a guide the following table shows the variation of bolt tension with the tightening method.

If the initial bolt stress is too low the total amount of strain (stretch in the bolt) is low and under these circumstances any subsequent reduction in thickness of the gasket due to creep will quickly result in loss of bolt strain and subsequent leakage.

Within reason, it is better to approach the maximum bolt stress recommended by the manufacturer. This is often 80% of the yield strength at the operating temperature.

Other points to consider are

• The crushing strength of the gasket.
• That the hydrostatic end thrust will increase the bolt tension at the operating internal pressure.
• Using a bolt stress which represents less than 50% of the yield strength will cause problems and should only be considered under certain circumstances.
• Most flanges are tightened with ordinary wrenching methods and it is often advantageous to have design bolt stresses which require no more than this. The probable bolt stress developed manually is often expressed as
  

  Sb =	1560	N/mm²
  	√db

  Where
    S_b = bolt stress achieved on assembly
    d_b = bolt diameter in mm
  For smaller bolts this results in excessive stress unless care is excercised. For larger bolts, however, it is often impossible to develop the required stress by hand.
• As the temperature increases the yield strength of the bolt material decreases.