6 replies to this topic

[pmunishankar]

Dear experts,

I have a question, may be silly one...
Attached the scheme which I am intended to design, PCV and FCV in series to drop the line pressure of stream 2 to 1100 psig before it blends with stream 1 and FCV to control the flow in the range 0 - 25 MMSCFD depending up on the requirement.

My question here is " Whether PCV and FCV can be connected in series?"
PCV will be trying to close to maintain pressure at the downstream where as FCV tries to open to maintain the flow. Both will be operating complementarily and I suppose system will never be in equilibrium.

Why cant one FCV will maintain and drop excess pressure of 400 psig? can a FCV be designed with pressure drop of 400 psig??

If any one has ever designed similar design, please provide me the scheme.

Thanks a lot,
Muni Shankar

Attached Files

•  Visio-PCV-FCV in Series1.pdf   16.13KB   96 downloads

[aransari85]

Hi Munishankar,

I dont see any problem in the scheme...

PCV tries to control 1100psig pressure downstream of the PCV inlet to the FCV.

When pressure at PCV outlet decreases ( It will be by the FCV action when it needs more flow FCV will open) then PCV will open to maintain the pressure at 1100psig.

In opposite direction FCV want less flow FCV will try closing which will have effect on PCV outlet pressure which will start increasing and will result in PCV action valve closing.

Purpose of PCV is to maintain pressure at 1100 regardless of FCV flow.. which will work in this case..

I have seen these arrangement in fuel inlet to furnace, where PCV is maintaining FG header pressure and FCV in cascaded with temperature control of furnace..

Regards,
ARA

[fallah]

My question here is " Whether PCV and FCV can be connected in series?"
PCV will be trying to close to maintain pressure at the downstream where as FCV tries to open to maintain the flow. Both will be operating complementarily and I suppose system will never be in equilibrium.

Why cant one FCV will maintain and drop excess pressure of 400 psig? can a FCV be designed with pressure drop of 400 psig??

Muni Shankar,

Seems the PCV in the system be a back pressure regulator which can normally maintain the pressure of its downstream in stable manner if terminated to a tank, a header, a vessel,..., or generally a part of the system has adequate "pressure capacitance".

Hence, as per your sketch with no such capacitance due to short piping connection between PCV and FV seems it wouldn't operate with adequate stability due to interaction between two cobtrol valves.

About your second question: You can design such FCV provided that its upstream and downstream pressure to be fixed (leading to desired pressure drop) and actually in this condition you don't need to a FCV and a manual valve would do the job (adjusting fixed flowrate)!

Fallah

[deltaChe]

PCV is a self operated control valve, and it has an independent control system ( transmitter, controller, actuator altogether). Its driving froce is from its transfer medium pressure, and can't incresae pressure difference by increasing the air source pressure. So, it usually has limited size and may not be required to work with other control valve.

[S.AHMAD]

1. There is no prolem to install control valve in series. However, it is a waste of energy if the source of stream 2 is a compressor.
2. Based on the description and the drawing, in my opinion, PCV is redundant. It is not necessary. FCV alone can do the job if being sized correctly.
3. The pressure of stream 1 is the back pressure of downstream system, regardless of pressure upsttream of PCV of stream 2. It means that pressure of stream 1 is independent of source pressure of stream 2.
4. It is more make sense to control the pressure upstream of FCV. This is normally helpful to reduce sound level of control valve FCV (flowrate approaching sonic flow) if only 1 control valved being installed.
5. We can give better comments if you provide more details process description of the whole system.

[pmunishankar]

Thank you Guys...

[Dacs]

I've seen letdown (on steam system) in excess of 400 psi, to answer if a single control valve is sufficient.

Normally, this setup (PCV and FCV in series) is used to ensure smoother operation under fluctuating conditions.

One such example is fuel gas feed to a furnace. Imagine a single FCV is supplying the required flow (and throttle) while the fuel gas (usually from off-gases from refinery units) pressure is fluctuating.

This may cause flame instability that may lead to flameout of furnace burners and as such is undesirable since it will cause an upset in the operation of the plant.

So ultimately, you have to review your system if the need for this setup is really needed or if you can just do without the PCV upstream your FCV (assuming of couse a single CV is sufficient as far as letdown is concerned)

Hope this helps

Edited by Dacs, 30 January 2012 - 03:28 AM.