Last week we talked about how regulators work and we discussed how the spring plays a very important role. Remember that we used the set screw to compress the spring as a way of showing the regulator our desired downstream pressure. Let’s assume that we set our regulator for 40 psi while the flow was at 25 gallons per minute. If the pressure started to fall, the upward force generated by the diaphragm would be reduced, and the spring would then push the valve plug down and farther open to pass more flow to keep the pressure from falling any further. So now we will assume that our flow has now increased to 40 gallons per minute, and the force balance has been re-established.
Question: Is our downstream pressure back to 40 psi?
Answer: No, it is somewhat lower than 40 psi!
Why not? This is a regulator, why is the pressure lower? Two very important things happened when the downstream pressure started to fall because of changing conditions: First, there had to be an error before there could be a correction, and second, the spring was not as compressed as it was before it pushed the plug down and further open. Let’s examine these one at a time:
“…there had to be an error before there could be a correction”. When you are driving down a highway, you do not turn the steering wheel to keep going straight. If you did you would go off the road, i.e. you would have a loss of control. You wait until road curves before you turn the wheel to stay on the road. A regulator will not and cannot act until something changes, otherwise it would lose control altogether.
…the spring was not as compressed as it was before it pushed the plug down and further open”. Springs follow Hooke’s Law, which says the amount of force a spring exerts depends on how far it is compressed. Since our spring is not as compressed as it was when we first set the regulator at 40 psi, its downward force is lower, which means that in order for our force balance to be regained, the upward force exerted by the diaphragm must also be lower. In order for the diaphragm to exert a lower upward force, the pressure must be lower. How much lower depends on how much the flow had to increase from the original 25 gallons per minute from the original setting.
Typically, you can expect the pressure to fall anywhere from 10% to 30% from its original setting depending on the individual application. This fall in pressure is commonly referred to as droop. The droop effect is unavoidable with any spring operated regulator, however it can be managed.