Dealing with Droop

When qualifying valves for a pressure-reduction application, chemical plant engineers must consider several factors. Initially, they must determine whether or not the application requires a control valve to be effective.

Would a self-contained or piloted regulator be sufficient? To make this decision, plant personnel should consider:

The pressure drop, or the difference between P1 and P2.

The set point.

The potential for large flow variations.

The level of importance of regulation/control.

If it meets the design criteria, a regulator will prove a more effective means of pressure reduction in almost all cases. In addition to lower overall costs, a regulator offers other advantages ," most important of which is fast response.

This responsiveness is of great value when applied to noncompressible media, or in applications in which delayed shutoff might blow a safety relief valve. Additionally, regulators are sensitive to load variations commonly experienced in heating or cooling pressure control.

After choosing a regulator over the control valve option, however, the plant must consider the droop effect. Droop is an inherent characteristic of all self-operated and pilot-operated regulators. It is expressed as the deviation of pressure from the set value that occurs when a regulator travels from the minimum-flow position to the full-flow position.

Fig. 1 shows the effect of droop in a regulator set at 40 pounds per square inch (psi) as the valve travels from the minimum-flow position to the full-flow position. The droop is expressed by the difference between the dotted line and solid line. The dotted line represents the actual controlled pressure attained, and the solid line represents the line of perfect regulation.

Figure 1. Droop Effect in a Regulator Set at 40 psi

Consider the application shown in Fig. 2, which indicates a 100-psi water pressure available in a building. To best operate the equipment and taps, it is necessary to reduce the pressure to 40 psi. A pressure-reducing valve can be installed on the service line to handle this reduction.

When there is no demand for water, no flow is required; therefore, the regulator is in the closed position. As demand for water increases to the full capacity of the valve, the regulator moves to the fully open position. However, because the regulator will droop with increasing flows, the set pressure of 40 psi to the building will not be maintained. Why not?

An increase or decrease in the amount of force applied to the spring establishes the set point for a self-contained regulator. In most cases, this is done with an adjusting screw. A clockwise turn of the adjusting screw threads the screw further into the spring housing, compressing the spring and increasing the set point. A counter-clockwise turn of the adjusting screw allows the spring to relax and decreases the set point.

Downstream pressure is transmitted to the diaphragm, either directly or via a downstream tap. When downstream pressure beneath the diaphragm exceeds 40 psi, the spring compresses and the valve closes.

When the pressure beneath the diaphragm decreases, the valve opens once again. In other words, the spring will not expand or contract unless a decrease or an increase in the pressure (force) opposing it occurs.

When downstream demand increases, the valve travels toward the fully open position. This allows the spring to expand, "adjusting" the set point ," but this time from the diaphragm instead of the adjusting screw.

As stated earlier, droop is an inherent characteristic in any self-operated or pilot-operated regulator. However, it can be minimized. The extent of droop is determined by three factors: diaphragm area, spring rate and stroke length.

An increase in the diaphragm area, a decrease in the spring rate and/or a decrease in the length of the valve stroke can reduce the droop. It is important to remember that these factors are interrelated.

Diaphragm area. The diaphragm area is restricted by economic and practical reasons. Larger diaphragms tend to increase the overall cost of the regulator because they require larger spring housings, heavier bolting, etc.

Spring rate. Design engineers typically will use the lowest-rate spring that will allow an adequate range of pressure adjustments (set points) and still retain sensitivity to small changes in pressure. It is possible to reduce the droop with low-rate springs, but plants run the risk of making the regulator too sensitive, which will create instability. Plus, the range of set points with a very light spring might prove too narrow for general industrial use. By lengthening a heavier spring, plants can increase the sensitivity, but this practice often is restricted by economics and valve size.