Split Range

I would not use a fail open and a fail closed valve together

I would use the 2 valves you have and split range one 4 to 12 mA 0 to 100% open

and the other 12 to 20 mA 0 to 100% open

better still split range them at the DCS

this is will not work her coz the 1st valve is inlet and the other is drain both valve have the same signal(4-20)mA "individual loop control" i can change the operating range for valve (4-12)mA, (12-20)mA

Then how do you expect to "Split Range" them? If one is on the input and one is on the output then I am at a loss to understand what you intend to do.

It seems senseless to me with only the information you have given. Perhaps with a better explanation of the desired outcome and the configuration of the installation someone can make a suggestion.

"Need INPUT!"
TT3

Typically split range valves work opposite to each other. I.e. one in direct action and the other in reverse action. Hence you need one FO valve and the other FC valve.

Typically split range means one valve works frist then if this is not doing the job to control the second valve will kick in nothing to do with the valve being FO or FC at all

as the Fail action is want you want the valve to do if there is a fault(like loss of air supply)

Fail open and Fail Close is different to if a valve is setup to be direct action or reverse action

I have to assume as to exactly what is existing in the field.I am assuming the controller is local and not in DCS.

Assume the fail open valve is used to feed liquid and let us call it valve valve I(inlet)

Let us call the fail close valve as valve "O"(outlet)

Callibrate the valve "I" postioner such that at 4 mA gives 3.0 psig(valve open)and at 12 mA.gives 15.0 psig(valve closed)

Now calibrate the positioner for valve "O".such that 12.0 mA gives 3 psig (valve closed)and 20 mA gives 15 psig.(fully open)

Assume the level controller is direct acting ,that is the output decreases as the level decreases and vice a versa.

When the level is exactly at the set point the output is 12 mA,and both valves are closed.

If the level goes down (less than 12 mA)the output goes down opening valve "I".

If the level goes up output goes up (>12 ma) opening valve "O"

As said before all this split range can be done easily in the DCS and positioners don't need special calibration.

The 3 and 15 psig I have mentioned is just for explaing the scheme and it could be any other pressure required to operate the valve(like 6 to 30 psig)

I don't know why it says guest,but the answer above I sent is from SRINI

Hi there,

Firstly the control action you are referring to here is not a split range control. It is a push-pull type control action.

Hence your need for the feed valve to fail close and your outlet valve to fail open. Depending on the valves actions (direct/indirect) and the failure position requirements (open/close) you have many different combinations to calibrate the valve. You might also end up with an overlap where both valves might actually be open at the same time - results in better control.

For more information - try a google search on push pull systems. There is a wealth of information out there, and it will give you a more detailed explanation.

Kind regards,

Craig

Sorry Ali Zude,

I did not realize that both valves are fail closed.

In that case the inlet valve "I" needs to have a reverse acting positioner,so 4 mA gives 15 psig output(valve full open) and 12 mA gives 3 psig output and valve full closed.

This would work

split range are used with only one control application ie with one controller or one pid block with two split control outputs . as per the application we can check for f/o or f/c valve .

if you want to change to 2 f/c valves than carry out hazop study bcoz safety first as you are changing the process control application .

secondly you can change the action of the actuator as required .

you can change the action of the valve direct or reverse in postioner itself.