pressure drop in regulator
pressure drop in regulator
(OP)
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
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





RE: pressure drop in regulator
B.E.
You are judged not by what you know, but by what you can do.
RE: pressure drop in regulator
B.E.
You are judged not by what you know, but by what you can do.
RE: pressure drop in 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.
RE: pressure drop in regulator
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: pressure drop in regulator
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.
RE: pressure drop in regulator
RE: pressure drop in regulator
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.
RE: pressure drop in regulator
RE: pressure drop in regulator
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: pressure drop in regulator
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.
RE: pressure drop in regulator
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
RE: pressure drop in regulator
RE: pressure drop in regulator
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: pressure drop in regulator
B.E.
You are judged not by what you know, but by what you can do.