Droop is an inherent characteristic of all self-operated and pilot-operated regulators. The term is often used when discussing the accuracy of pressure reducing valves, or PRVs. Also known as Proportional Band or Offset, droop is defined as the deviation from setpoint as flow increases through a regulator. Droop is expressed as a percentage. The inverse of droop is accuracy. As an example, if a regulator has 20% droop, that regulator has 80% accuracy. Accuracy is one of many important selection criteria one must consider when choosing a regulator.
Three variables determine droop in pressure regulators:
- Stroke Length – the shorter, the better.
- Diaphragm Area – the larger, the more accurate.
- Spring Rate – the lighter the spring, the more sensitive the regulator.
Different regulator designs can provide more accurate regulation (less droop):
- Self-operated: 10% – 30% droop
- Pilot operated regulators: 5% – 10% droop
- Dome loaded valves: 2% – 5% droop. Accuracy is very high due to the elimination of the spring.
If flow demands are relatively constant, or 10-30% deviation from set point is tolerable, a self-contained regulator should be used.
If the fluctuations are great, or accuracy is essential, it may be necessary to go to pilot-operated valves or instruments.
If you are considering using a control valve to control pressure, determine if a regulator would be accurate enough for pressure reduction applications.
• A piloted regulator has less droop than a non-piloted regulator.
• Air loaded regulators are much more accurate because the spring is eliminated. In some models, droop is virtually eliminated.
• High-flow regulators are less accurate than standard regulators.
• Typically, the shorter the overall stroke, the less the amount of Droop.
• Larger diaphragms will increase overall accuracy.
• Regulators supplying a medium to multiple users/vessels will be less accurate than regulators supplying a medium to a single unit/vessel.
• The set point should be toward the high end of the selected spring range to give increased accuracy of regulation.