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Difference between a flow regulator and a pressure regulator?

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.
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.
- 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.
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...
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.


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