Flow Control Valve (FCV) to Pressure Control Valve (PCV)

You cant make a FCV into a PCV, but you can add a PCV(just by teeing a PCV unit and a pressure transmitter/transducer) upstream. The PCV can just act as a pressure limiting device that will bypass any extra flow(without tripping/overloading the prime mover. But remember that any bypassed flow on the PCV(can we term it Pressure relief valve?!) becomes a power loss(high pressure drop at certain extra flow). If your pump flow can be varied, then a pressure transducer(enough) may dictate any pump control to limit the flow through the FCV thereby reducing pressure drop across the FCV. And if your pump is a fixed displacement type, you must have a PCV just in case a high pressure occurs otherwise the prime mover trips.

Why is there a need to control the flow in the original design? For water pumps IMO, the pumps are sized enough to deliver the required flow(less restricted without FCV) against any friction and head, and eliminating more problems/components.

You seem to have checked the important things - Cv, code etc. I am not an instrument or control specialist but I do not see a problem, the valve does not know whether it is controlling pressure or flow, it merely opens or closes in response to a control signal which could come from either a flow meter or a pressure transducer.

Hello Jose,

Using water medium you can control flow with a valve up or down stream of the flow sensor (upstream is usually easier-less turbulent), but pressure control of the fluid coming from the renamed (from FCV to PCV) valve will require that the pressure transmitter sense the downstream pressure. The time constant of the measured variables may be similar or slower depending upon the volume of your well and the pipe connection to the well volume. It needs to be understood where the pressure sensor will be placed in the injection piping or the well itself. A remote transmitter would place the sensor in the underground cavity.

Generally speaking, if the upstream process is sensitive to flow changes, you would be better off with the old FCV, but if ultimately you need to control the well pressure, it should have its own pressure sensor that will read true well pressure independent of the delivery flow. You might want to consider keeping the FCV, but remotely set the flow set point with the well pressure. You may want to provide alarms for high and low flows. If you don't need to control the flow and the well has a large enough capacity you might save some energy by changing the valve to on-off operation and shut the pump down when the valve is closed! If the pump can not be stopped, maybe a variable frequency drive will save energy and the pump. The valve may evolve to a double check valve with a leak alarm.

Regards,

Luther

hello every one

firstly check for the shut off pressure if the existing pressure exceeds shut off pressure of the control valve than what I feel is the size of the actuator and the spring range should change also what class the leak is acceptable

preferable get a new valve

Hi Jose,

You should also take into account the characteristic of the valve. Usually a FCV has a linear action. PCV would be equal percentage.

If your characteristic is correct, the CV and all your calculated data are acceptable - then there is no reason why you should not be able to use your existing valve.

Regards,

Craig

hi, craigza

I think here only looking into the data sheet is not sufficent, as the application changes ,design of the valve comes into picture to suit the application .

operating pressure has changed consideradly around 10 times as well as parameter has changed this will affect the rangeability of the valve this is what is should not be ignored . which emphasis to either change the actuator or go for new valve .

please comment to explore

regards

How can you say pressure has increased ten times. What are you assuming?

Luther M

Dear Luther,

I am not assuming ,as per the data given I am presenting the solution ,it is clear that for fcv application operating pressure is 20 psi and to change to pcv application it is 240 psi so around 10 times got it ! so it is required to check the data sheet for shut off pressure .check my other discussion

Hi all,

Of course it is not sufficient to just look at the data sheet.

Step 1 - Determine the valve characteristic needed.

Step 2 - Look at process data.

Step 3 - Now look at costs.

Step 4 - Calculate required CV.

Step 5 - Now source valve.

Cost will determine the valve type you will use. If you need a very tight shutoff for example you will not be using a butterfly valve. On the other hand you could well use a butterfly valve for level, pressure, flow or temperature control.

The actual process data and cost will determine the type of valve you need. Physically - there is no difference between a FCV or a PCV - besides the valve characteristic. And depending on the valve type you use this can be changed in the positioner via a cam or if the positioner is smart via switches/software.

As Jose said - he has calculated the required CV for the new loop and the existing valve fits in with this as well as the extended pressure. If this is the case - the only thing now that he needs to check is the valve characteristic. If it is correct for the new process, or he can change it in the positioner - then there is no need for a new valve. He can use his existing valve. Flow - linear, pressure - =% (rule of thumb).

Regards,

Craig

hi all

three factors are essential to look into this valve

1 . shut off pressure .,2. maximum diffrential pressure across the valve .,3 . spring range and actuator size of the valve .

1 if shut off pressure of the valve is less than the existing pressure chance that the valve may pass and willnot close .

2. dp max of the valve also determines opening and closing of the valve it relates to same shutoff pressure

3. increase in pressure will increase pressure over the plug there by counter act by the actuator hence size to be check for increase in air pressure rangeability of the valve ie the new calibration set the start to 10% and the flow control is between 5% to 95% than the rangeability appears to be 90% as the start has overseat the valve for existing spring range the start may be say 10 % and opens fully at 80% ( excess spring adjust )than rangeability goes to 70 % that means at this stage even thou the valve is eq% the controls will be like on off valve so this will affect the performance of the valve as well as the whole loop.

I think this will clear the problem

regards

There is some data missing here.

What are you supposed to measure? That's an important question to answer first.

I once encountered a process where the primary control was flow. However, when the line pressure dropped below a certain point, the control changed to pressure (using the same control valve).

The reason for that was that the upstream process needed to be maintained at a high pressure for reasons which have to remain secret. I'm sure the more experienced will figure out why. I've only seen particular type of control (over-ride control) just this once.

When you had flow control, the pressure was only 20psi and will become 240psi when you start using pressure control. That's a big difference. Your flow will probably drop drastically so I don't know why you're changing control types.

regards,

Vulcan