BOLT LOAD CALCULATION

[Ahsan96]

Hi Guys,

My question may start as weird and weirder as you go along, but would help get rid of the useless thoughts.

I am calculating my bolt load, and minimum pretension based on the worst case of the three cases,

1) minimum seating stress (πbGy)=Wg1
2) working condition (2πbGPw + 0.785G^2Pw)=Wg2 assuming external thermal and mechanical load as zero.
3) Hydrostatic condition (2πbGPt + 0.785G^2Pt)=Wg3

My question is when I have applied pretension based on the maximum force calculated above, let's assume the value is from point # 3 (considering it will be maximum as there is no other load considered for working condition).

When my equipment is under test conditions, there will be forces that will be generated that will be in the opposite direction of the initially created clamping force (Fc). Will my resultant forces(Fr) be
1) Fr=Fc-0.785G^2Pt or
2)Fr=Fc-(2πbGPt + 0.785G^2Pt)----(Which technically would say no clamping force is now present), how would that look as the gasket technically is not seated as per πbGy, isn't that an issue.

If point 2 is true, it means when pressure is applied to the gasket faces exposed to the fluid it will apply force opposite to clamping force.

Another question that bothered me, my pretension also keeps the bolt in tension whereas the external load (other than compressive) also should keep adding the bolt tension, would the load be added for stress calculation or the clamping force generated between joint members will take the hit?

 

[MintJulep]

I guess the answer to your question, and probably questions you haven't thought of yet are in this lecture.

 

[goutam_freelance]

Please clarify the nomenclature.

 

[TGS4]

It appears that you are using the ASME Section VIII, Division 1, Appendix 2 calculations and then trying to use these "bolt loads" for your actual bolt loads. This is WRONG.

The approach that you need to be using, for a gasketed pressure-retaining joint is found in ASME PCC-1.

Try starting there. If you have questions, please return to ask them.

 

[Ahsan96]

Hi TGS4,

Thank you for replying to this, been following your participation in eng-tips for some time now.

I did go through this standard, Appendix O, Assembly Bolt Stress Determination.

Did try to relate it with typical calculation from ASME SEC VIII, Div 1, Appendix 2

Ag x Sgmin-s is = (πbG) x y

Ag x Sgmin-O is = 2πbGPw
Whereas the hydrostatic end load formula remains the same (almost the ASME sec viii used mean dia, whereas the other uses min I.D)
for sake of example if Consider my sbel equal to below equation should it be reasonable assumption

The only thing I see here is the factor of gasket relaxation remaining, this procedure incorporates maximum seating stress and flange rotation.

Had a few questions in regards to the flange joint, are we assuming that the external load will not affect the already preloaded bolts (i.e all the external load will be effecting in decompressing the initial compressive force, if that is the case would'nt there be chances of joint completely decompressing at operating condition due to hydrostatic end load and gasket reaction load.