Pressure Regulator Droop and 5 Ways to Minimize It

Sometimes referred to as the ‘Proportional Band’ or ‘Offset’, droop is an inherent characteristic in all self-operated and pilot-operated regulators. Droop is a decrease in outlet pressure from the set value due to an increase in media flow rate. Obviously, this is not a good thing.

The diagram below shows the droop effect in a pressure regulator at a set value of 10 psig. The green line shows how we would ideally like our regulator to perform, while the red shows actual regulator performance. The difference between these two is the incurred droop, expressed as a percentage. Increased droop means decreased accuracy, and in most cases that means waste.

What Determines Droop?

The amount of droop your pressure regulator will see is dependent on three key factors: diaphragm area, stroke length, and spring rate.

1. Diaphragm Area – This is usually restricted for economical or practical reasons. Larger diaphragm areas tend to increase overall regulator cost because they then require larger spring housing, larger bolting, etc.
   
   
2. Stroke Length – This is the distance the spring is allowed to relax proceeding from minimum to maximum flow.
   
   
3. Spring Rate – Springs with the lowest rate possible will generally be utilized by Design Engineers when designing the regulator. This allows for an adequate range of pressure adjustments while still retaining sensitivity to changes in pressure. Lowering a spring rate too much may make a regulator overly sensitive, which creates instability.
   
   

How to Minimize Droop

You may be asking yourself, how can I stop droop in my regulator? Technically there isn’t a way to stop it. However, there are ways you can minimize it:

1. Composition vs. Metal Diaphragms – Choosing a composition diaphragm will offer increased sensitivity, accuracy, and efficiency. Metal diaphragms should only be chosen when temperature or material compatibility requires them.
   
   
2. Air & Dome Loading – These loading types eliminate the need for a mechanical spring thereby increasing accuracy.
   
   
3. Altering a Key Factor – Increasing the diaphragm area, decreasing the stroke length, and decreasing the spring rate are all ways to drastically increase accuracy. Adjusting these factors may have certain negative impacts, listed above, which should be kept in mind.
   
   
4. External Feedback – Introducing an external feedback loop can assist in droop compensation.
   
   
5. Choosing the Proper Regulator – As silly as it may sound, simply choosing the proper regulator for your system can drastically reduce droop. Higher set values require stiffer springs which decrease accuracy. Your set value should be near the top end of your spring’s effective range.

There are other ways to reduce droop in regulators, but these should give you a great start at improving accuracy. Feel free to contact us with any questions; we would love to help you choose the right regulator for your fluid process system!