jabsab,

Does the valve have a rotary actuator that pushes and pulls the valve stem via a rack-and-pinion or similar device?

I would like to see this valve.  I don't know that I've seen a gate valve where the stem friction wasn't due to the interface between the valve stem and the valve packing.  Regardless, unless this is a small valve in low pressure service, you're going to find that the packing friction force is substantially less that the process forces.

Pharo of Thermo
 

The valve is just a 10" 300lb. manually operated rising stem gate valve.  I agree the largest contributor to stem friction is packing, but I'm looking for a way to reduce the operating nut friction without changing too much existing design (maybe PTFE thrust washers).

If you E mail me i have a small xls to work out stem friction from packing, we used for Russian PWR,s

Cheers

I can be reached at brown_ja@nns.com

Hi,jabsab
In one of our applications we have just used conventional thrust bearing on the stem nut and the result was fantastic.The operating torque has come down drastically.Better try it yourself and you will appreceiate.

jabsab, I agree the largest contributor could be packing - and motion could be improved as others describe above, but the D/P across the gate is often what makes a gate valve stick near the shut position (bypass/pressure equilibration could be needed to nullify sometimes enormous break-free friction forces).

I have found values between 0.45 to 0.55 provide reasonably good results. 0.8 and above did not tally with observed values.

However if you would like to experiment and if you have facilities you can measure the frictional force using some simple set up and use the values

Best of luck.

C Ponnu