API 579 MAT Problem 1

[Sharik]

I am having a little problem with the API 579 determining the minimum allowable temperatures. In the example problem #3 in API 579, Section 3.11 the formula for S*E* = P((Rt/Tc)+0.6). This is for a vessel shell.

In example problem #4, the formual for S*E* = P/2(Rt/Tc)+0.2). This is for a sphere.

My question is, in example problem #3 where did they get the value 0.6 from for the equation?

Also, for example problem #4 where did they get the value 0.2 for the equation?

Is the value always 0.6 for vessel shells, always 0.2 for spheres, etc.? If so, where would these values be found?

Thanks

 

[JStephen]

I believe the formulas come from ASME Section 8. Not sure about the derivation.

"Formulas for Stress and Strain" gives a thick-walled stress equation for cylinders, that if some t^2/R terms are neglected, will give you stress = P(r/t + 1).

They give another equation, the "mean diameter formula", which will give stress = P(r/t + 0.5). About this equation, they say "the expression for bursting pressure, commonly known as the mean diameter formula, is essentially empirical but agrees reasonably well with experiment for both thin and thick cylindrical vessels."

My guess, lacking better information, is that the equations mentioned are committee compromises between these two results.

 

[jte]

Sharik-

The formulas you are referring to are directly from ASME VIII-1, as stated in API-579 Appendix A.3. In essence, the "nuisance factors" of 0.6P and 0.2P are there to compensate for high pressure / thick wall conditions. For most vessels, they have little effect on the result.

I'm a bit concerned that someone doing an FFS evaluation per API-579 isn't familiar with vessel design per VIII-1. Perhaps you have experience with different codes in which case you have some feel for vessel analysis. If you don't, you really need to get a mentor before playing with FFS evaluations.

jt