Does anyone have any experience of ASTM A 193 B7M studbolts relaxing at high temperature and can advise the amount of relaxation we can expect mm/deg C.
Hi Dennis,
Thanks for your reply.
The bolts are 3/4"unc set at 295 ft-lbs.
We are having trouble with the joints leaking at ambient having cooled from 365 deg. F.
Very often, this is a gasket problem. Use the thinnest gasket material that is properly rated for the service. Go back after the line has been up to temperature for about 24 hours and re-torque the bolts. Very often, you don't get anything on them, but sometimes I've seen nuts loose enough that they're not even finger tight.
I've also seen problems at flanges due to thermal expansion and contraction of the line. This is not the flanges themselves expanding, but the line itself moving, and "cocking" the flange, resulting in a gasket leak that showed up every time the line cooled off and contracted. Once the anchor for the expansion joint was repaired, the problem went away.
I agree with TBP. 365 °F is not hot enough to begin to have the bolt material creep. What type of gaskets are you using? Spiral wound (Flexitallic, Lamons, etc.) will be much more successful than any sheet gasketing.
As pointed out above, its probably your gasket not your bolting. Depending on your gasket you may find that it is good policy to retorque after initial hydrotest or dervice to compensate for gasket yielding.
What preload has been applied to the bolts?
What thickness and type of gasket is being used?
How fast is your process cooling from 365°F to ambient?
A very rapid cool down will cause the flanges to shrink incrementally and the bolts will lag behind in temperature, staying elongated. If preload on the bolts is low the gasket may become unloaded during the cool down. The difference in thermal expansion can easily be calculated if you know the temperature profile of the entire bolted joint. Applying Belleville washers very often solves these types of problems.
If you have enough preload to accommodate the thermal cycles but your gasket creeps under load (many materials do this, teflon is one of the worst) a change in gasket material/type may be in order. Bellevilles often help where minor gasket creep is causing loss of preload.
