9.2.1 Example: pressure reducing valve

9.2.1 Example: pressure reducing valve

In the pressure regulator, the medium pressure acts directly or, if required, via equalizing tank on the rolling diaphragm of the actuator. Proportional to the diaphragm area , a force is created which is opposed by the force of a spring as well as the flow-related plug force (Fig. 4).

Force acting on the diaphragm

is created due to the pressure difference between the upstream and downstream pressure acting on the surface of the plug:

Force acting on the plug

The spring creates reset forces in proportion to the spring range x and enables the adjustment of the set point or operating point through preloading.

Spring force

Assuming an initial state of equilibrium, as illustrated in Fig. 4, any change of pressure results in a changed balance of forces, thus causing adjustments of travel. This can be clearly seen in the control cycle described in Fig. 5:

Fig. 4: Balance of forces in a pressure reducing valve

Control cycle

- If the operating point is in a state of equilibrium, the spring force and the force acting on the plug are compensated for by the diaphragm force .

- If the consumption increases, the pressure drop across the valve increases so that the downstream pressure decreases.

- The spring opens the valve against the decreasing diaphragm pressure until a balance of forces is reached again with a wider open valve.

- In the new valve position (Fig. 5, phase 4), the spring force as well as the pressure to be controlled are reduced. A steady-state error (offset) remains with a value that depends on the proportional-action coefficient of the regulator.

Fig. 5: Control cycle in self-operated pressure reducing valves