Bolt Preload for ASME flange 1

[wsmith22]

I've seen bolting preload stress recommendations vary from 50%-90% yield strength (gasket manufacturers & PCC-1) and 75% bolt proof strength (Shigley text book). What is good practice to use for the preload stress to determine the target torque requirement for a low yield strength bolt such as A193 B8M class 1?

Also, for self energizing gasketed flanges, I assume any preload stress greater than the design allowable stress for the bolt would suffice since there is no gasket seating load. (i.e use 30000 psi to determine the target torque for A193 B7 bolts with allowable of 25000 psi) Is this a correct assumption?

Thanks in advance.

 

[ijzer]

Please read ASME VIII-1 appendix S to understand more about this.

 

[wsmith22]

I understand that App. S says that a higher preload stress may be required. I'm trying to calculate torque charts for different types of bolting.

 

[1969grad]

The old rule of thumb for bolt preload stress was 50% of yield. However I think this rule of thumb was applicable to the old compressed asbestos gasket and prior to the popularity of the spiral would gasket.

I have been struggleing with this problem myself and now have come to the conclusion that you calculate the face stress of the gasket and work backwards to the associated bolt load which then gives you the stress which then gives you the required torque. Note that torque varies with the type of lubricate.

For spiral would gaskets the max face stress is a little less than you indicate and is more likely no more like 20,000 to 25,000 psi. More important the minimum stress for sealing is on the order of 10,000 psi.

So my current thinking is to use an intermediate value of the gasket face stress between 10,000 and 20,000 and then calculate the bolt loads.

Using the old rule of thumb of 50% of yield for the bolt load can cause the gasket allowable face stress to be exceeded for some Class 600 flanges.

 

[MalcolmA]

What is meant by face stress? Is it force/area on gasket face. Is the face stress something that has to be considered on flanges made out of a softer material such as brass?

 

[1969grad]

Gasket load (lb)divided by gasket face area (sq in) of the flange. Assume an allowable gasket face stress of say 25,000 psi times the gasket area of the flange gives you the load. Divide this load by the number of bolts and figure the bolt stress.