9.2.2 Adjusting the operating point

9.2.2 Adjusting the operating point

The operating point of the regulator is adjusted via spring preloading. Fig. 6 shows the spring forces in travel positions , and , including the resultant spring characteristic. Preloading the spring causes a parallel displacement of the spring characteristic so that at travel position , preloading = is already effective.

Fig. 6: Spring forces and characteristic

Fig. 7: Ideal characteristic of a pressure reducing valve

While the operating point is adjusted, the spring preloading is increased until the process variable to be controlled reaches the required value. The spring force adjusted in this manner results from the balance of forces as illustrated in Fig. 4:

With a small seat area and low differential pressures, only small plug forces are created. Under these conditions, the spring range x which is equivalent to the valve travel changes in proportion to the pressure . The resultant manipulated reaction therefore directly depends on the spring characteristic (see also Fig. 7):

The equation as well as the control characteristic exhibit the proportional-action component of this self-operated regulator:

- The factor / represents the gradient of the characteristic or the proportional-action coefficient of the regulator (see also page 14ff).

- The second summand of the equation ( x / ) describes the displacement of the characteristic into the operating point. If high set points are to be adjusted, this term must increase. For this, either a version with a stiff spring (high ) and a small actuator area must be chosen, or the spring must be of great length so that it can be sufficiently compressed ( will increase accordingly).

As previously mentioned, these correlations are only applicable in cases where the plug force can be neglected. If the seat diameter is large and/or the differential pressures are high, this method is only permissible when the valves are equipped with a so-called pressure balancing system. With self-operated regulators, such balancing systems are already suited alone due to the improved control behavior.