Design acc. to API 6D

[Sweguy]

Hi
I have a question regarding how the drive train in ball valves is allowed to be designed.

I don`t have the standard in front of me right now but I will try to remember.

The design torque is 2x the calculated torque for the valve.

When the design torque is applied on the valve the stresses in the drivetrain is not allowed to be higher then 67% of yield strenght.

I`m currently looking on some valves that shall be designed according to 6D, they has stem dimensions that I thinks is in line with the standard, but the connection between stem and ball seems to be very week, the surface loads between stem and ball must be much higher then 67% of yield strenght.

Can somebody explain this for me, is not surface loads calculated or is there other load limits?

 

[gerhardl]

I do not know API 6D or the details you are mentioning.

Apart from this the necessary torque to operate a ball valve may in practice vary with a factor 1 to 5 or sometimes 1 to 10, depending on fluid (greasing, drying, abrasive, gluing etc... fluids)

Based on this, the real safety factor selected is more important than to hit down as narrow as possible down to any standardized safety factor, which in pratice might be weak for extreme conditions.

And yes: the transfer section of forces to the ball should be dimensioned in my opinion as least as strong (able to take at least the load on the stem with same or better distance to yieldpoint), else you would make this point the weakest and might under circumstances provoke excessive stress here.