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.
|Method Used||Tools Required||Variation of tension from mean value|
|Wrench (No Torque Control)||Spanner or Wrench||±50%|
|Wrench (With Torque Control)||Calibrated Torque Wrench||±30%|
|Hydraulic Tensioner||Multiple Stud Tensioners||±2% to ±5%|
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 % will vary based on grade of bolts.
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.
- As the temperature increases the yield strength of the bolt material decreases.