Valve stem rating

[corran34]

We have a butterfly valve with 6" 2500# Flanges. A working pressure of 3080 psig, a differential pres of 700 psi. Materials are 316SS. How do I determine the minimum class rating for the stem and bushing inside the valve? If you could point me to the right standard or provide guidance I would appreciate it. The working fluid is Nitrogen.

 

[reverman]

If the valve meets B16.34, which I am guessing it does, the stem arrangement would need to at a minimum meet the pressure rating of the valve.

I'm not sure if that is exactly what you mean.

 

[NGiLuzzu]

As far as I can imagine, a ND 6" butterfly valve disc sized to withstand a class 2500 rating pressure would be hardly feasible or, at least, more or less a... ball (so the relevant stem would have a very large diameter).

Therefore, the butterfly valve internal trim must have been "down-rated" in respect to the flanges (meaning that the differential pressure allowed across the closed valve is much less than the pressure allowed within the open valve as a shell), I guess...

In this case, I believe there are no official standards for disc and stem design (maybe something exists for design verification, once you have chosen dimensions, but probably also for this FEM simulation is better than simple calculation formulas...?), but you should ask the Manufacturer what design pressures they considered.


Hope this helps,
'NGL
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[Redavocado]

Regarding valve stem design, it is always a good practice to do standard thread and / or seat shear calcs using the weakest material in shear. When the valve stem is under internal pressure a shear force is applied to the bonnet arrangement. OTher than that, ASME B16.34 covers bonnet requirements (relating to the valve stem always) which may be computed from materials properties. Always remember to up your design pressure somewhat when doing non-code calculations in order to be conservative.