Gate valve Balance stem force

[Hanson Gill_1983]

Guys,

I’m working on gate valve design with balancing stem.To calculate the force or thrust acting on the stem I used
thrust F = F1+F2+F3
F1 - axial force on gate due to inlet
pressure thru valve bore x coefficient of friction between gate/seat (or valve factor)
F2 - stem piston force
F3 - packing frictional drag force considering packing friction factor

Since the stem setup is a ball screw type
I used Torque required to raise stem = F x Stem lead / 2 *PI * 90% ball screw efficiency.

Now, since I have the balance stem below the gate with gland packing, what should I consider to include this in the calc above to get the net force acting on the stem.
Attached is a sample valve pic for Ref.

 

[gerhardl]

The most important and widely varying rest is the 'safety factor' added by experience. For many types of gatevalves and difficult fluids it can be necessary with added force times five or ten.