Valve Packing Pressure/Friction

[loilfan]

Hello, I am reviewing a design for a client but can't find a method to check the thickness of a stuffing box. The design is similar to a typical gate valve in that a rod needs to slide through Graphlok packing and is supported by external bolts.

The packing manufacturer has told me that the packing will require 2500-3800 psi of gland pressure to make a seal, but I do not know how to convert that into a pressure to determine the minimum wall thickness of the stuffing box. The conservative approach would be to assume 3800 psi of internal pressure plus the actual MAWP, but that seems like it will be over engineered.

I would also like to calculate the required friction to move the polished rod up and down the packing. The only forum related to that topic that I could find was thread408-252910, but it was closed with no responses.

I would appreciate any advice on this. Thanks!

 

[reverman]

Does the valve need to meet any API standards? for example API 600 lists the required minimum thickness based on stuffing box bore and pressure class.

 

[loilfan]

Reverman, Thanks - API 600 is what I needed.

Any insight on how to determine friction forces?

 

[themad1]

We use an empirical formula as an indication of the final friction force in needle valves. The result is always pessimistic than the output from the prorotipe test.

F = Q * µ

Where:
F = friction
Q = load normal to the contact surface
µ = frictional coefficent - for graphite of polished steel (0.4/0.8 Ra) we consider 0.1

We assume that the load applied on gasket by the packing gland is completely trasferred on the stem as they are perpendicular.

 

[loilfan]

Thanks themad1!