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Difference between a flow regulator and a pressure regulator?

by redargon
Tags: difference, flow, pressure, regulator
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redargon
#1
Nov27-08, 08:03 AM
P: 348
So far I've seen that a pressure regulator regulates the pressure from upstream to required pressure downstream and that a flow regulator regulates the flowrate, obviously.

But, theoretically, doesn't one regulate the pressure to induce a specific flowrate? what is the actual difference between these two devices (in theory or in practice)?

I've also seen that flow regualtors are more predominant in liquid applications and pressure regulators are more common in gas applications. Compressibility of a gas, I would assume, makes it more difficult to regulate the flow of a gas compared to liquid.

Any other ideas or differences that people may know of? Any help is appreciated as always, thanks.
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Averagesupernova
#2
Nov27-08, 12:06 PM
P: 2,499
Pressure regulators are used in some liquid applications. There is a pressure regulator right ahead of my water meter. I've worked on propane gas systems where the liquid is regulated down to about 30 PSI. A poor practice in my opinion, but it was common on this particular machine. An ideal pressure regulator will allow enough gas through to build up the pressure to a specified PSI no matter what the flowrate through it is. Of course in the real world pressure regulators have real world limitations. By design they are usually fairly simple consisting a diaphram, spring, and valve arranged in a feedback fashion to give good regulation.
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A flow regulator also uses feedback in some way to allow X volume-units/minute to pass through. If the input pressure is constant the flow regulator can be a simple as a ball valve or an orfice.
nucleus
#3
Nov27-08, 12:30 PM
P: 171
First you have to know what creates flow and pressure and types of pumps. A positive-displacement pump, like a gear pump, pumps a fixed volume of fluid for each revolution. They have a pressure regulator and often a pressure relief (safety device). A non positive-displacement pump, like some water pumps, slip when loaded and do not normally require a pressure regulator, although may have a safety device.

A pump produces flow only, in GPM, CFMs etc. The resistance to flow creates pressure. If you have a positive-displacement pump connect to a open line fluid would come out of the line, depending on displacement, RPM, and efficiency of pump, but there would be no resistance, hence no pressure. . However, if you plug the line there is now backpressure and it is felt by the regulator and it open to allow fluid to return to the reservoir. Flow in this case goes to zero. The higher the pressure differences the higher the flow. Pumps are rated for pressure but that is for the strength that it can with stand before breaking.

“a pressure regulator regulates the pressure from upstream to required pressure downstream”
Most of the time it is the other way round. You need backpressure for a regulator to work.
“theoretically, doesn't one regulate the pressure to induce a specific flowrate”
Yes. A flow control device can be as simple as your bath tap to one that compensates for pressure and temperature.
“I would assume, makes it more difficult to regulate the flow of a gas compared to liquid.”
Control of the flow of air is common. Welders often have a flow control to control the flow of argon.

This just scratches the surface, but a good website to learn more can be found at:

http://www.hydraulicspneumatics.com/...IndexPage.aspx

FredGarvin
#4
Nov27-08, 01:02 PM
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P: 5,095
Difference between a flow regulator and a pressure regulator?

There is no real such thing as a "flow regulator." There are pressure regulators and there are flow control valves. Term "regulator" has a very specific meaning in which it is implied that regulation of the pressure is accomplished via the balancing of various forces. You will find a lot of different types of regulators, all of which control a pressure of some kind.

A control valve which is meant to control flow is not a regulator. It is some kind of valve like a pintle or globe valve, needle valve, etc...

Like nucleus said, you haven't even scratched the surface...
redargon
#5
Nov28-08, 01:56 AM
P: 348
Thanks for the help guys.

I'm trying to scratch the surface a little more, that's why I posted the question. These things are no dark art, I'm just trying to figure out some of the terminology and the differences between different devices. A lot of people call them flow regulators instead of control valves, but hey, potato potato (that doesn't work when it's typed ) Fact is, people have different (and often wrong names for things), be that as it may, I am just looking for some experienced answers and I did get some of them here.

From what I've seen, a flow regulator (flow control valve) has a feedback pressure regulator and a controlled inlet orifice. You can obviously, adjust the orifice at the inlet to regulate the flowrate. What I don't see immediately is how it automatically adjusts itself as the upstream pressure increases or decreases. My guess is that that is what the pressure regulator downstream of the flow controlling orifice is for. I know that the flow rate across an orifice is determined by the geometry of the orifice and the pressure difference across the orifice. So if the geometry stays constant, then the downstream pressure would have to adjust itself to a change in upstream pressure to keep the flow constant.

How much more of the surface do I have to scratch? I was wondering for instance how flow regulators work on a gas that has variable density (compressibility), seeing that a change in pressure would also cause a change in flowrate as the gas is compressed or decompressed.
FredGarvin
#6
Nov28-08, 08:51 PM
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It depends on the set up that you are referring. It would help if you could point out a schematic.

What I see a lot in our industry is what is known as a delta P regulator in series with a control valve. What this does is senses the pressure on both sides of the control valve and maintains a specific delta P across the control valve. What this does is keeps the control vale insensitive to changes in supply pressure. So the only thing you have to change to control the flow is the flow area of the valve.

Again, regulators work on the idea of force balances. Usually there is a spring force providing one of the forces on a diaphragm and the other is the line pressure acting over a specified area of the regulator.

There are many different kinds of regulators so it's tough to answer when specifics aren't referenced.

Also, I would not use the notion that regulators are used in gas applications and "flow regulators" in liquid operations. I use both in both kinds. Whether you are dealing in gases or liquids, their places in the system are the same.
redargon
#7
Dec1-08, 01:55 AM
P: 348
Thanks Fred. It looks as though our application will benefit from a pressure regulator (spring and diaphragm). The gas to be used is CO2 and we need to regulate from about 60bar to about 1bar. There is massive amounts of cooling as the gas expands, so it gets a little complex and there are some strange effects depending on geometry of the system. I can estimate some of the characteristics by using a relatively simple static analysis (force balances, etc), but I haven't taken dynamic effects into account yet. I think we will just do some physical tests to get results.

It's also possible that we need to install a flow regulator after the pressure regulator, but I'm still trying to decide if that is really necessary as the flow can be controlled using specific piping and connections.

The tests should tell all (well at least show all) and I'll see how to go on from there.
FredGarvin
#8
Dec1-08, 08:29 AM
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The regulator you get should have the orifice size or Cv for the regulator depending on settings. You can always get that kind of flow curve from the manufacturer. That way you will be able to back out what flow you get at what pressure setting.

Having big delta p's like that are a pain and icing is a big concern. If possible, you may have to look at heating the upstream gas just enough to keep your lines and regulator from freezing. Depending on what you go with some simple heat tape on all of your components may be enough.

Good luck.


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