The following illustrations show the forces acting on the valve in both actuated and deactuated states. The forces shown are for a Series 27 valve with 1/2″ to 1″ ports. Other valves have similarly proportioned forces.
The only friction to be overcome in the movement of the valve is the U-cup seal on the piston poppet, and the spindle in its guide at the bottom of the valve. The frictional forces at both these points are insignificant in relation to the forces acting on the valve.