pressure drop in regulator 1

[santu398]

Hello,

I have the source (air receiver) pressure more than 7 bar. I have connected a 1 inch regulator in between receiver and machine. I have observed 7 bar pressure at the regulator and set that to 6.5 bar. Now I started the machine (which consumes 40 cfm) and I observed the pressure at regulator dropped to 6 bar. I have checked the receiver pressure is more than 7 bar. I have changed the pipe line from 1 inch to 2 inch assuming the pipeline is restricting flow. The flow capacity of regulator is 220cfm. But the same problem repeats. I want the pressure 6.5 at machine. Please explain me what could be the reason

 

[berkshire]

Have you checked how much air can actually get through the regulator?
B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

Not the flow capacity, the actual footage.
B.E.

You are judged not by what you know, but by what you can do.

 

[iainuts]

Regulators can not maintain their set pressure exactly. The decrease in downstream pressure which occurs as flow rate increases is due to how the valve functions. All regulators function basically the same way. Some are spring loaded and some are gas loaded (called "dome loaded" regulators). Regardless of which type you have, the valves function the same, but for the sake of explanation, I'll explain it using the more common spring loaded regulator.

The spring acts against a diaphragm, pushing down on the diaphragm. This assembly also pushes down on the poppet that controls flow. The further down the spring pushes, the farther the diaphragm moves and the further open the poppet goes. The further the poppet opens, the more gas that can get through and the higher the flow rate through the valve. The gas pressure downstream of the valve is being 'measured' by the diaphragm with that pressure on the inner side and atmospheric pressure on the outer side. So the force on the assembly is balanced by the spring pushing down and the pressure pushing up on the diaphragm.

When you set the regulator at a certain pressure under a no flow condition, the pressure downstream is acting on this diaphragm and the spring is balancing that pressure by applying an equal force downward while the poppet is on the seat, stopping flow. As downstream pressure drops, force up on the diaphragm also drops and the spring extends and the valve opens a little bit. But a spring has a spring constant (k) so the valve will come to equilibrium with a slightly lower pressure downstream, balanced by the slightly lower spring force and with the valve slightly open. As pressure downstream decays even further, the valve has to open more and the spring has to extend more and the forces balance at this lower pressure and lower spring force.

So the spring constant (k) is what is affecting the balance on the diaphragm. If you have an infinitely small spring constant, there would be no decay in downstream pressure with an increase in flow rate. But there is no such thing as an infinite spring constant and real regulators experience a decay in the downstream pressure set point as flow rate increases. This phenomenon is sometimes called "droop" although I've heard manufacturers call it by different names. It's a function of the spring constant and diaphragm area.

If you need a regulator with minimal droop, try a dome loaded regulator. The effective spring constant of a large volume of gas is generally lower than a spring.

 

[zdas04]

as iainuts says, the set pressure is largely a function of flow rate. When I've had similar problems I found that if I set the regulator while flowing to the maximum flow rate load, then all of the other scenarios were closer to the setpoint. If I set it with minimal flow the droop sets in and the pressure is lower than the setpoint for most loads.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[MintJulep]

The flow capacity of regulator is 220cfm.

Pressure regulators are not constant flow devices and their performance cannot be characterised by a single number.

There should be a family of curves that relate inlet pressure, outlet pressure (or pressure drop) and flow.

The full statement that you might have seen in your cut sheet is probably something along the lines of "220 cfm at 10 bar inlet and 1 bar pressure drop."

It seems likely that you are attempting to operate this regulator outside of its happy place.

 

[3DDave]

Are there pressure regulators with down-stream/remote pressure sensors?

 

[btrueblood]

3DDave: Yes, but I don't know if they are a commonly available product. Look for a "differential pressure regulator". Be careful, as poor choice in line sizes can lead to regulator chatter and surging flow.

And to the OP question and iainuts good writeup, regulators can be designed to compensate for droop over at least a large part of their rated operating range, look for terms like "precision", "low droop", "high turndown" and "instrumentation" regulators. They will typically cost quite a bit more than regulators you can buy at a hardware store.

 

[Compositepro]

It can be shocking to learn how poorly most pressure regulators actually do regulate pressure. You can also obtain regulators that are extremely precise if that is your requirement, but these are not "standard" and they are expensive. Most precision regulators have limited flow capacity so if you need higher flow you can use a "volume booster",which is similar to a "dome loaded" regulator.

 

[Lou Scannon]

Bear in mind there are two causes of droop: increasing flow rate, and decreasing supply pressure. Some regulators have been designed to negate one or both of these mechanisms of droop, but these too, go off the proverbial cliff when their design capacity has been exceeded.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Tmoose]

better oxy acetylene welding rigs use two stage regulators.

Adding a second regulator in line to first lower the pressure to the "control" regulator might be an interesting experiment

I'd set the regulator while flowing and measuring pressure at the device.
Some painters install pressure gauges at the spray gun and set the pressure at the gun while spraying to compensate for line loss andprobaly regulator pressure drop as well.

 

[LittleInch]

santu,

Iainuts has given you an excellent response. If you really need 6.5 bar at any time, flowing at different rates, not flowing etc, then you either need a better regulator or you need to use a control valve with input from a pressure controller.

You say the inlet pressure is "more than 7 bar". how much more? What is our regulator sized for? For small differences in pressure drop the poppet has to move along way to get a small change in pressure, If your inlet pressure was higher, you might find the outlet pressure is more in line with your requirements.

However if you need 6.5 bar running, why not just adjust it? Is it vital that the pressure doesn't go to 7 bar on no flow?? This all seems a bit too precise for me.

Your responses to the comments would be appreciated by all those who make them...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[chicopee]

Set the pressure regulator to 7 psig instead of 6.5 psig and note what results.

 

[LittleInch]

A lot less than 6.5 barg !

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[berkshire]

One bar, 14.2 lbs plus or minus
B.E.

You are judged not by what you know, but by what you can do.