stem MAST calculations per API 6D 6.17.1

[MuShoe]

Hello,

When verifying stem MAST calculations against API 6D 6.17.1, should the ASME BPVC II pD allowable stress at temperature be used to get the "design torque" or should the yield limit of the material be used?

Any thoughts?

 

[bcd]

If you want to be able to find an actuator size that will turn the valve without exceeding the MAST, you had better use yield.

 

[MuShoe]

Yes, I agree that stresses should always be below yield. However, to my experience, the ASME allowable stresses are always less than yield, so I'm looking for some interpretation on what value to use for the MAST.

 

[bcd]

Before you pick a stress limit, you should understand why MAST exists and how it will be used. Also understand that calculating the torque capacity using typical formulas gives you the limit when a point or points in the cross section reach your stress limit. 99+% of the remainder of the cross section is still well below your stress limit. So you are nowhere near deforming the stem. Theoretically, yes, you have defomatin in the stem, but nothing you will notice or measure.

There is no reason to use ASME material limits for this analysis unless for some reason, someone could argue that the weak portion of the stem is pressure retaining. In other words, if the portion of the stem that would break due to excessive torsion would result in uncontrollable release of product outside the valve.

Do your calculations based on various parameters, build a prototype and test it. Compare the results and decide which method is best. Can't argue the results if you back them with test data.