Maintaining Constant Pressure in Your Fluid System
A regulator’s main purpose is to maintain a constant pressure on one side of the regulator even if the pressure on the other side of the regulator fluctuates. In the case of a pressure-reducing regulator, pressure is controlled downstream of the regulator. All pressure-reducing regulators will see a drop in outlet (downstream) pressure as flow demand increases. This phenomenon is referred to as droop ( Figure 1 ). Regulator manufacturer’s provide flow curves to illustrate a regulator’s performance in terms of outlet pressure (Y axis) and flow rate (X axis). For many regulator applications, this flow curve allows a regulator user to predict and determine if a regulators performance will meet their application’s requirement. Remember- regulators control pressure, not flow!
For many regulator applications, droop is not critical. If flow is at a constant rate or if there is no flow, droop will be unimportant. However, for those applications where flow will vary, droop may be important. Figure 1 shows at a given set point what the outlet pressure will be as flow varies. Remember the shape of the flow curve is nearly the same regardless of set point: that is, we can shift the curve up or down for any given set point and the flow curve will be the same shape.
What causes droop? There are several causes. The largest is the range spring. Eliminating the spring through the use of a dome loaded regulator will greatly reduce droop. The curve in Figure 2 is for a regulator with a spring control. The curve in the middle (Figure 3) is for a dome-loaded regulator, which has no spring. The curve in Figure 4 is a dome-loaded regulator with feedback, which gives our best performance.
The next easiest way to reduce droop is to specify a regulator’s input and output as closely as possible to actual inlet/outlet pressures. Figure 5 shows the effect of using a regulator specified 3600 psig inlet when there is actually only 100 psig inlet pressure. Matching the regulator specification to actual conditions will provide a substantial improvement in regulator performance.
Making the regulator perform better in your application can be accomplished by taking some time to review your application and expectations of how you would like the regulator to perform.