Difference between a flow regulator and a pressure regulator

[C26M13]

Dear Engineers,

What are the difference between Flow control valve (FCV) and Pressure control valve (PCV) with respect to their working, design and application in controlling the fluid?

I have attached a sketch to show the control system (PCV and FCV in series) to control the fluid before it goes to the separator.
The PCV is to control the pressure before it goes to the separator. The FCV has a pressure controller input based on the pressure downstream of the separator.

Example:
Downstream of PCV: 60 bar
Pressure in the separator: 30 bar

I am not quite understand on the combination of PCV anf FCV in that scenario. Is the FCV regulate also the pressure to the separator(i.e. changed in pressure from 60 bar to 30 bar)?

Can somebody help me with these questions?


Thanks in advance.

 

[zdas04]

Both valves work on the stream the same way. The big difference is input. A pressure regulator looks at pressure (either upstream or downstream), and matches that pressure against a spring tension to either throttle the valve towards open or shut. A flow control valve must have a logic controller somewhere that takes input from a flow measurement device and directly positions the valve towards open or shut. In either case, the valve can only make the flow area larger or smaller.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

I agree with zdas04.

This is a common misunderstanding.

A control valve simply opens and closes and creates a variable pressure difference which varies due to flow rate through it and position open.

What controls the position of the valve internals is up to you. You can have multiple inputs into the controller and one of them will take precedence over the other or it can switch depending on the ranges you set.

As you've drawn it you simply have two control valves in series both controlling on pressure.

This two step approach to controlling pressure isn't unusual, but that's all you're doing. there is no direct flow input into this control loop.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mk3223]

As both valves are working under the pressure control, the first valve, "PCV", can becalmed as a self-controlled pressure regulator, and the second valve, "FCV", is a pressure control valve controlled by "PC".

 

[C26M13]

Thank you all.

For this case, the feed flow controller (FCV) has a pressure controller input based on the pressure (PC) downstream of the separator. And the PCV regulates the pressure to provide steady inlet pressure to FCV which will set the flow to the separator.

So, from the attached drawing and as per your explanation, that means that both control valves (PCV and FCV) are responsible for the pressure drop occur within the line to the separator until it gets 30bar in the separator(they are regulating the pressure and set the flow to the unit). Correct me if I am wrong.

Thanks,
D'costa

 

[georgeverghese]

From your sketch and your narrative, it looks like the the feed line PCV operates from a PIC sensing pressure between the PCV and the FCV, while the FCV operates on a primary FIC with reset on cascade from the exit line PIC.

From your narrative, it doesnt seem clear why you need the feedline PIC / PCV. The FIC loop would work just as well with variable pressure at its inlet and the sensing FE / FT, provided there is a pressure compensation algorithm enabled in the DCS for flow correction. Suspect this is a high set PIC / PCV set up on the feedline to enable mechanical design pressure reduction downstream of the PCV. If this were to be true, this PCV kicks in only at high pressure, and is "dormant - full open " when pressure is lower than its high setpoint.

 

[MortenA]

Personally i would _never_ put up a control loop like the one on the sketch - unless it had something to do with the valve e.g. cavitation in the trim or noise or something! A valve is a valve is a valve! The control loop is what is the difference. You may even experience poor control in this example since the two control loops may start to act against each other!

Furthermore, it seems a lot better to have the gas outlet control the pressure in the vessel and then have a LCV on the liquid outlet. Thats the "traditional way" and it makes a lot of sense.

See attached

https://files.engineering.com/getfi...5-bd2b-42e6-808a-b0a2e80e93b9&file=vessel.JPG

 

[C26M13]

Thanks @georgeverghese and @MortenA.

The PCV is to regulate the pressure to provide a steady inlet pressure to the FCV, which sets the flow into the unit (plant).

 

[MortenA]

maybe that sound logically to you, but to me it just wont work very well! Your flow regulating valve should be designed to be able to control the flow disregarding the dP (maybe its not).

 

[ChEMatt]

The first sketch won't work. You can't control pressure on the vessel from that point. The pressure will be controlled by the resistance to flow downstream of the vessel. It will not work and you will not be able to set the pressure easily on the vessel.

If you want a steady flow, put in a flow controller on the inlet, and put a pressure controller on the vapor stream (as was drawn by MortenA). They won't fight each other as they're looking at two different variables. This is how you would set up a distillation column, for example. Having a consistent flow is very desirable, if not required.

In the case of a separator, you may not need to put a flow controller on the inlet. A bit of variability is not a big issue. If you're coming off of an oil well, however, surges might need to be accounted for so an inlet regulator may be helpful to control instantaneous surges in flow.

That being said, what MortenA drew is good. Pressure controller on the vapor outlet, measuring the pressure UPSTREAM of the controller. Then a level controller ensuring you don't drain the vessel out.

-m

 

[C26M13]

Thank you everyone for contributing your valuable knowledge.

Yes, I do agree with the suggestion and got the points on the control system. I think my drawing was not correct.

Actually, to PC on the drawing purely to show the operating pressure of the vessel where the pressure drop has occurred upstream of the pressure letdown station. This PC to maintain the vessel pressure within the operating pressure within the safe operation limit.