Bolt Pre-Load 1

[KllrWolf]

I have a question on bolt preload and have been getting confused. I am trying to calculate the bolt preload for an end plate so we are able to pressure test the motor. Do I set the bolt pre-load to what will hold the end plate in place without fluid leaking so the bolt does not pass its yield point when under pressure? Or is the expected pressure seen per bolt also required to be taken up by the preload.

I believe it is the first method, so the bolt does not stretch and will return to its starting dimensions, thus maintianing the seal. However, some of the mechanics believe they should tighten it to near yield point.

Thanks for your help.

 

[gr2vessels]

The bolts are actually sized for the larger of gasket seating force or end hydraylic force. Depending on the application, for one off use like hydrotest, you can use 90% of the yield, otherwise use the calculation method in ASME VIII.

 

[KllrWolf]

The bolts have been sized to handle the pressure the test will create on each one. The part I am get confused on is if the bolts are tightned enough to securely hold the cover in place with no leaks at no pressure, or equal to the force they will encounter when under pressure? I believe it to be the first method, but the mechanics are questioning it (which causes everyone else to panic).

 

[Duwe6]

Your mechanics are [partially] right. The preload [times # of bolts] needs to exceed the test load [times sq-inches of wetted endplate], by just a little.

What I have seen many mechanics do is over-torlque bolts "if 55 is good, 100 is twice as good" and cause them to yield. A bolt that has started to yield is starting to fail. It is necking down, and providing much less preload. And when there is a leak, the mechanic will tell you that you gave him "bad bolts".

 

[KllrWolf]

Thank you Duwe6, I learned something new. I guess as I am not used to calculating Torque at about 1500 ft-lbs for a 1.5" bolt, I had figured something was wrong and I was doing it wrong. I am also glad our mechanics are good enough to point out something when my brain gets confused.

 

[jamesl]

ASME PCC-1 have guidelines on how to tighten the flange bolts. It tells you to preload B7 bolts to 50 ksi prestress, which is about 50% yield. It also has tarque values to get to this stress level.