9.2.3 Pressure balance

9.2.3 Pressure balance

The plug force depends on the differential pressure and, therefore, acts as a disturbance variable in the control loop. A high upstream pressure and large seat diameters create considerable plug forces which the actuator must overcome:

By applying special structural measures, this disturbance variable can be almost entirely compensated for.

Fig. 8 shows the version of a valve with a plug balanced by a bellows. The upstream and downstream pressures additionally act on the plug stem via the bellows area , thus creating forces that oppose . If the effective area sizes of and are identical, and if the cross-sectional area of the plug stem is neglected, is compensated for by the forces acting on the bellows.

Fig. 8: Balance of forces in an pressure reducing valve with plug balanced by a bellows

Pressure balanced valves require clearly smaller actuator forces than unbalanced valves (compare Figs. 4 and 8). When calculating the spring reset force that must be overcome, the elasticity of the bellows must be additionally accounted for:

Valves with balanced plugs are used for applications requiring that the control process be as accurate as possible. Balancing systems are always required when high differential pressures are created across the valve, especially with large nominal sizes, which then also necessitates high positioning forces. As will be explained in more detail below, with self-operated regulators the system deviation will become the bigger, the greater the positioning forces they are capable of creating.