August 2022

Special Focus: Refining Technology

Choosing the proper regulator configuration can reduce droop

Many factors go into selecting the proper regulator for your system. One of those factors is sensitivity to changes in outlet pressure as downstream flow fluctuates, which is often characterized as “droop” as illustrated within a regulator’s flow curve.

Arumpanayil, S., Swagelok Company

Many factors go into selecting the proper regulator for your system. One of those factors is sensitivity to changes in outlet pressure as downstream flow fluctuates, which is often characterized as “droop” as illustrated within a regulator’s flow curve.

What is droop?

Droop is defined as the reduction of outlet pressure experienced by pressure-reducing regulators as the downstream flowrate increases. While every pressure-reducing regulator will exhibit some droop, external components can be added to the regulator configuration to help maintain reliable downstream pressure and minimize droop.

Flow curves are a useful tool in understanding the range of outlet pressures a regulator will maintain based on various system flowrates. They are created through product testing and represent the real performance of a regulator for a given set of system parameters.

 FIG. 1. This chart illustrates different flow curves using different pressure regulation configurations: a simple spring-loaded regulator (Option A—Baseline); a dome-loaded regulator and a pilot regulator (Option B); a dome-loaded regulator and a pilot regulator with an added feedback line to the dome-loaded regulator (Option C); and a dome-loaded regulator and a pilot regulator with an added feedback line to the pilot regulator (Option D).
FIG. 1. This chart illustrates different flow curves using different pressure regulation configurations: a simple spring-loaded regulator (Option A—Baseline); a dome-loaded regulator and a pilot regulator (Option B); a dome-loaded regulator and a pilot regulator with an added feedback line to the dome-loaded regulator (Option C); and a dome-loaded regulator and a pilot regulator with an added feedback line to the pilot regulator (Option D).

FIG. 1 is an example of a flow curve with four different curves shown to demonstrate how different pressure regulation configurations can help to reduce droop. The vertical axis displays outlet pressure, with the horizontal axis showing downstream flowrate. The flatter or more horizontal the curve, the more the regulator preserves consistent pressure. For example, the purple curve—which is a result of a configuration using a dome-loaded regulator and a pilot regulator with an added feedback line to the pilot regulator—shows the best performance. The far right of each curve indicates that the pressure will quickly decline toward zero when the regulator is completely open. Under these circumstances, the poppet is pushed open toward its stroke limit. At this point, the regulator acts more like a restricting orifice than a pressure control device.

No matter how good your regulator is, some droop will be experienced. However, the goal is to flatten the curve as much as possible. Selecting the proper regulator configuration for your system will enable you to achieve this goal.

Option A: A simple spring-loaded regulator

The most common type of pressure-reducing regulator is a spring-loaded regulator (FIG. 2A). In this design, a spring applies force on a sensing element—either a diaphragm or a piston—which moves the poppet closer to (FIG. 2B) or farther away from the orifice. This controls the downstream pressure. The spring-loaded regulator is then used for purposes of comparison as the baseline (Option A) in FIG. 1.

FIG. 2. A spring-loaded regulator (A), which uses a spring to control flow and pressure by moving the poppet either closer to or farther away from the orifice (B), provides a baseline (Option A) for the flow curve comparison.
FIG. 2. A spring-loaded regulator (A), which uses a spring to control flow and pressure by moving the poppet either closer to or farther away from the orifice (B), provides a baseline (Option A) for the flow curve comparison.

Generally, a spring-loaded pressure regulator will reduce droop adequately for most basic applications. The regulator poppet will then allow additional flow by moving away from the seat as system flow demand increases. When this happens, the loading spring will relax and lower the loading force and regulator setpoint. The amount of droop this system allows depends on the loading spring rate as flow demands fluctuate. To manage the spring rate, technicians may have to frequently perform manual adjustments to maintain the desired set pressure, particularly if a high degree of accuracy is necessary.

Even though this system can be more effective than a simple spring-loaded regulator in many cases, its droop-reduction performance can be enhanced by additional components and design modifications.

Option B: Dome-loaded regulator with a pilot regulator

As the industrial fluid system becomes more sophisticated, so do the mechanisms to maintain adequate pressure. It may be necessary to employ a dome-loaded, pressure-reducing regulator as the complexity of the system increases. In this system, the loading is managed by a pressurized gas housed in a dome chamber. The gas flexes a diaphragm that moves the poppet away from the orifice and controls the downstream pressure (FIG. 3A).

Option B combines a dome-loaded, pressure-reducing regulator with a pilot regulator. This system regulates pressure changes by keeping constant pressure in the dome chamber, while the pilot regulator controls the supply of gas to the chamber. As shown in FIG. 3B, any excess dome pressure is relieved through an outlet loop.

FIG. 3. A dome-loaded regulator uses pressurized gas housed in a dome chamber to flex a diaphragm and move the poppet to control downstream pressure (A). The Option B configuration (B) features a dome-loaded regulator with a pilot regulator and a dynamic control outlet loop to control dome pressure.
FIG. 3. A dome-loaded regulator uses pressurized gas housed in a dome chamber to flex a diaphragm and move the poppet to control downstream pressure (A). The Option B configuration (B) features a dome-loaded regulator with a pilot regulator and a dynamic control outlet loop to control dome pressure.

As with the spring-loaded regulator, the poppet will allow additional flow as the system flow demands increase, though the mechanism is quite different. In this system, the diaphragm flexes downward to lower the dome pressure. This triggers the pilot regulator to allow additional gas into the dome so consistent pressure can be maintained.

When flow demands decrease downstream, the diaphragm flexes upward, causing the pressure in the dome to rise. Under these circumstances, the excess pressure vents to the downstream side through the dynamic control outlet loop.

In FIG. 1, this system is labeled Option B and shows much more robust pressure control than Option A (the spring-loaded regulator). The amount of droop is reduced, as represented by the flatter flow curve. It is clear that dome regulators equipped with pilot regulators can manage pressure more accurately over a wide range of flows. In most instances, standard dome-loaded regulators are employed with confidence that significant pressure drops will not occur. However, even this system can be improved.

Option C: External feedback line connected to a dome-loaded regulator

Additional accuracy can be achieved by adding external feedback to a dome-loaded regulator, as shown in FIG. 4. External feedback is sent to the regulator by connecting a tube from the downstream process line back to the sensing area of the dome-loaded regulator.

FIG. 4. The Option C configuration features an external feedback line connected to the dome-loaded regulator to better compensate for downstream pressure drops.
FIG. 4. The Option C configuration features an external feedback line connected to the dome-loaded regulator to better compensate for downstream pressure drops.

The external feedback line provides more precise feedback on pressure levels within the system instead of depending solely on the pressure levels in the regulator itself as is seen in standard dome-loaded regulators. In FIG. 1, this system is shown as Option C, where some droop is still observed despite a flatter flow curve than either a spring-loaded regulator or a dome-loaded regulator.

Option D: External feedback line connected to a pilot regulator

In FIG. 5, the final option connects the  external feedback line directly to the pilot regulator instead of the dome-loaded regulator. This enables the pilot regulator to make highly accurate adjustments to pressure in the dome-loaded regulator’s chamber based on actual downstream pressure. Then, the dome-loaded regulator can compensate by changing its outlet pressure.

FIG. 5. The Option D configuration features an external feedback line connected to the pilot regulator, delivering downstream pressure feedback.
FIG. 5. The Option D configuration features an external feedback line connected to the pilot regulator, delivering downstream pressure feedback.

When system flow demands rise in this configuration, pilot regulators monitor lower pressures with the aid of the external feedback line. If pressures drop, the pilot regulator increases the pressure inside the dome, which maintains the proper downstream set pressure. The feedback loop allows continuous, automatic adjustments that keep the system stable and working at optimal performance. In FIG. 1, this is represented by Option D, which clearly has the least droop and the broadest flow range.

Minimizing droop

All regulators will exhibit droop, but the right system configuration can minimize it by keeping the pressure constant as flow changes. Work with your regulator supplier to design the right system for your application. HP

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