Over-Torqued Bolts

[strainstress]

We have some SA-193 Gr B7 bolts on flanges that we overtorqued. We torqued them to 450ft-lbs. Some of them streteched upto 3/8" over 8" long bolt rod. The vessel passed the test. However, we want to know whether the bolts need to replaced or over-torquing the bolts OK as long they don't break ?

 

[boo1]

What dia are the bolts?

 

[strainstress]

3/4" bolts

 

[strainstress]

Never mind, I found my answer at AISC's FAQ section

https://www.aisc.org/DynamicQuestion.aspx?Grpid=6&QueId=1782&ste=_Preview&id=2100&type=0

6.6.8. What is the upper limit on the installed tension of high-strength bolts?

As stated in 2004 RCSC Specification Sections 9.2.1 through 9.2.4, installed tensions in excess of those given in RCSC Specification Table 8.1 shall not be cause for rejection. Accordingly, there is no specified upper limit on the installed tension of high-strength bolts. This supports the long-standing rule of thumb that as long as the bolt is not broken during installation, the bolt is adequate for service.

This general rule applies because the bolt is subjected to combined stress (tension and torque) during installation. Once installed, however, the torque is relaxed and the bolt is essentially subject only to a tensile stress that is always less than the combined stress. Thus, even if the bolt were on the verge of failure during installation, it would be subject to a less demanding state of stress (simple pretension) during service.

last modified 1 January 2006

 

[doct9960]

RCSC is for structural connections. For pressure boundary bolted flange joints, you should refer to ASME PCC-1 or ASME Section VIII, Div.1, Nonmandatory Appendix S.

 

[KevinNZ]

I agree with Doct. Do not use structural rules for pressure equipment. With a pressure vessel or pipe flange the loads on the bolts will increase after installation due the internal pressure load and other operating forces and moments.