PCV and BPV Operations

[SamSnead]

Hi everyone,

I am a "green" process engineer and have a couple of questions about regulator data sheets. On the data sheets that we provide for vendors to get quote, you have to put in the inlet pressure and pressure drop across the valve. I have included a diagram.

If a BPV is set at 50 psig, what will be the minimum inlet pressure? Is it 50.01 psig? My understanding of the principle is that the BPV controls the back pressure to 50 psig and opens of closes as needed to maintain the pressure at 50 psig, right?

For a PCv set at 50 psig, the upstream pressure wil be whatever the pipeline pressure is and the downstream pressure should always be 50 psig.

Are these statements correct? I appreciate all explanations.

Sam.

 

[LittleInch]

Looks correct to me.

The trick in regulator or control valve data sheets is trying to pick the two or three cases which exhibit max DP and lowest "normal" flow (i.e. not start-up or shutdown flows) or min DP and max flow and a "normal" operating case if indeed there is one as this will provide the envelop of opening percent of the valve and allow you to compare different ones depandant on your particualr case. Sometimes you want as low a DP as posisble if either the back presusre or downstream presure is less than your set point and sometime you need it to control on a low flow high DP for a long time so don't want the valve to "scream" when it's doing this so look at the dB figure...

IMHO try to avoid calling your valves BPV or PCV etc, they are all just control valves with a range of possible inputs. I've had up to four different inputs going into a control valve control loop before now so e.g. your valve may control mainly on flow rate set point, but if pressure goes above or below a certain value it then limits the downstream or has a minimum upstream pressure, all in one valve. People can get confused when a "FCV" starts acting like a "PCV". Just call them control valves... Lecture over.

My motto: Learn something new every day

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

 

[Latexman]

Fisher Technical Reference section of Regulator Handbook

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529

 

[SamSnead]

Thanks, LittleInch. What about a PV (not a PCV)? A PV usually acts like a BPV by using a sensor and a transmitter up stream to keep the pressure at a set value.

For example if you have a PV on the vapor stream out of a scrubber set at 50 psig (see attached diagram). Now, this PV acts like a BPV by keeping pressure in the vessel at 50 psig, right? If that's the case, the inlet pressure will be 50 psig. How do you figure out what the outlet pressure will be? I have a simulation that gives the pressure of that line at 48 psig. So, which will I take as the inlet pressure? the 50 psig or the 48 psig? What about the outlet pressure?

 

[LittleInch]

That's my point, it's just a control valve, stop calling it a pv or pcv or bpv.

Outlet pressure will be dependent on whatever is downstream. Only you know. Be careful that you give the control valve something to work on. Less than 2 psi is not enough and you will get fluctuations in the scrubber pressure because the valve will not have enough differential pressure to control. You probably need something in excess of 15 psi differential pressure to allow the valve enough movement to be effective. That would be one of your cases, but also look at the min pressure case.

The Fisher manual is an excellent read. Download and use it as a reference guide.

My motto: Learn something new every day

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