Bolt load / EN1591-1

[Fran67]

Hello everybody,
I start the flange calculation acc. to EN1591-1 and I have a doubt about the considered max allowable stress for the bolts.
I consider (values given by the software Caesar II):
- For installation and hydro-test, the max allowable stress is 485 MPa (100% yield strength)
- For service, the max allowable stress is 287 MPa (~60% yield strength)
I used to apply a torque corresponding to 75% of the yield strength of the bolt during installation (common practice).
But with this value, in most of studied cases I am out of the limits in service because 75% is > than 60%.
My conclusion is therefore: A initial torque corresponding to 75% of the yield strength of the bolt is not always relevant and in most of cases a lower torque (50% yield strength) should be better.
What do you think according to your experience?
Thank you
F67

 

[desertfox]

Hi Fran67

I don't have access to the EN1591-1 however I agree with your conclusion in that the torque to reach 75% yield strength of a bolt is not always relevant and a lower torque setting can be used in most cases.
That said one as to design the joint with sufficient clamping force to obtain a seal and so in the case of standard pipe flanges the number of bolt holes and size should be such as to achieve a hydro tight seal and the relevant bolt stress quoted. In the case of a non standard flange then this would be designed from scratch to the relevant code the designer is working too.

Apart from the fact that torque is a very inaccurate way of setting bolt stress i.e. 25% error either way, the 75% yield stress figure for bolts is a common value, used if the said bolts are to be re-used after say a maintenance strip down.
Another figure for bolt stress is 95% of yield but this is when the bolts are not to be reused after a strip down.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Krausen]

I believe ASME PCC-1 addresses this exact question & perhaps ASME VIII too somewhat, but your bolt max allowable stress under the pressure design code is completely separate from the bolt stress from torquing them. It’s understood most bolts are torqued to ~60% to ~80% of their yield stress (pre-load) as part of the flange joint install & gasket seating. As you stated, if your pressure code max allowable stresses applied to this situation then all bolts would fail code after torquing them. But the bolt max allowable stresses under the pressure code are only for the bolts load stresses caused by the system’s internal pressure during operation (or other applied loads during operation)

 

[Krausen]

Non-Mandatory Appendix S in ASME VIII addresses the question of preload torque stress on flange bolting vs. design stress on the flange bolting:

The maximum allowable stress values for bolting given
in Section II, Part D, Subpart 1, Table 3 are design values
to be used in determining the minimum amount of bolting
required under the rules. However, a distinction must be
kept carefully in mind between the design value and the
bolt stress that might actually exist or that might be
needed for conditions other than the design pressure.
The initial tightening of the bolts is a prestressing operation,
and the amount of bolt stress developed must be
within proper limits, to ensure, on the one hand, that it
is adequate to provide against all conditions that tend
to produce a leaking joint, and on the other hand, that it
is not so excessive that yielding of the bolts and/or
flanges can produce relaxation that also can result in
leakage. Remember that the torque to overcome thread
friction plus the bolt-head or nut-turning friction constitutes
approximately 90% of the total applied torque, leaving
only 10% to stretch the bolt. This is true even under
the desired contact surface and lubrication conditions;
thus the need to be mindful of these considerations during
joint assembly (see ASME PCC-1, sections 4 and 7).
The use of through-hardened washers may be appropriate
(see ASME PCC-1, Appendix M).

 

[r6155]

Avoid bolts,use studs
Avoid torque, use elongation.

Regards

 

[XL83NL]

Elongation isn’t always possible or meaningful

Huub
- You never get what you expect, you only get what you inspect.

 

[Krausen]

XL83NL is correct. Nor is it practical in industry for most bolted flange installs.

Bolts used in the above are intended to mean stud bolts. But yes, always stud bolts

 

[r6155]

Please, see

https://www.eng-tips.com/viewthread.cfm?qid=223032

Regards

 

[Krausen]

I don’t think many will disagree that tensioning is superior to torquing from a theory standpoint as an engineer sitting behind a desk. The issue is the field practicality of hydraulically tensioning literally thousands of stud bolts needing to be installed in flanges at many sites. The additional training, time, money, specialized equipment, etc. to tension all your studs is simply not worth it in general in most cases. Heaviguy’s claim in that thread about guessing at your K factor (aka nut factor, which he refers to incorrectly as “friction factor”) when torquing is misleading at best. While there are certainly many variables that can never been determined with 100% accurately, K factors can be accepted to be within an accuracy range of +/-30% with appropriate site QA/QA torquing procedures & using published K factors that have been lab tested by stud lube manufacturers, as thoroughly explained in PCC-1. This +/- 30% accuracy range is also why PCC-1 recommends a max stud preload of 70% of stud yield, avoiding the potential to yield any stud assuming you have QA/QC install practices in place & are applying lab-tested K factors.

 

[r6155]

A hydraulic tensioner is not required for most elongation measurements. Use the same procedure for mounting studs and check the required elongation (previous calculation). Lubrication is not required, it is recommended to reduce the effort with the manual wrench.

Regards