ASME PCC-1 Appendix O in addition with WRC 538 can be used to detemine requisite torque for flange bolt during assembly. The appendix provides two approaches to select appropriate bolt stress and hence torque. In the simple approach of ASME PCC-1, bolt stress is selected to achieve a desired target gasket stress per equation O-1. The torque corrseponding to the selected bolt stress can then be easily found by using equation O-2. Though this procedure is very simple and straightforward to implement, it doesn’t check the safety of all the joint components (flange, bolt and gasket). A second more elaborate approach – “Joint Component Approach” is given in para O-4 of ASME PCC-1 to check the safety of all the joint components. In this approach:
- the bolt damage is prevented by keeping the bolt stresses below yield
- minimum gasket stresses are ensured to achieve proper sealing
- maximum gasket stresses are limited to prevent gasket damage
- flange damage is prevented by keeping the flange stresses within limits
- flange rotation is limited to prevent leakage
Flange stresses and rotation can be checked using either elastic closed form analysis or an elastic plastic FEA. An illustration of using the “Joint Component Approach” for weld neck flanges using an elastic closed form analysis based on Appendix 2 of ASME Sec VIII Div 1 and WRC 538 follows.
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