Control Valves in Parallel!


Thread Starter


Hi all

We shall receive Steam from a steam header by one of our contractor companies. They have engineered 2 x parallel control valves (A and B) regulated by an upstream pressure transducer (PT).

Their control philosophy is basically the A valve has a higher setpoint than the B valve. The A valve is normally in service @30 barg and the B valve regulates if the downstream pressure controlled by A valve drops below the setpoint to the B valve setpoint(lets say 29barg).

Now, I think these valves will fight each other if a process disruption goes below the setpoint of the B valve SP of 29barg! As both valves will be open and trying to regulate the pressure with I wrong?

After a HAZOP we are encouraging our Contractor to install PSV's downstream of the PT to maintain system pressure below the piping spec of 34 barg as I am not entirely convinced tha their "parallel pressure control" will work

Yes, everybody who tries to control one measurement with 2 actuators has problems. It is not that this cannot work (it can actually work reasonably well), but often it doesn't work because it isn't tuned properly or it is designed by someone who doesn't understand the likely dynamics of the process. Add to this 10 years of valve cycling and you get worn actuators contributing to the problems as well. In practice it usually works "well enough" for the supplier to complete the project and leave.

This first point I would advise is that you should not tune for overshoot - This just introduces instability into the system.

The second point is that you need to minimise the interactions between the 2 loops. Using a different setpoint for each loop is one way of doing this, but it is crude and assumes that the system is basically stable apart from the occasional load change which means you need some boost from the second actuator.

The real problem you have is that you are breaking the basic rule of ONE MEASUREMENT = ONE CONTROL LOOP.

You may get better results by using a single PID controller with a split range output. ie. Once Valve A gets above 100%, use the same PID to start opening valve B. In this case there is only one measurement and one PID but with an increased actuator range.

Of course, at this stage of the project you are unfortunately in the hands of your equipment supplier, which is a whole different problem.

You did not say anything about the relative sizes of the 2 valves, or why 2 valves are required. I am guessing here, that valve A cannot handle the maximum flow condition and so valve B was added to meet this flow requirement. Perhaps the high flow condition does not occur very often and would require special (expensive) trim to meet the full turn-down. If so, the parallel control scheme you described can work if tuned properly.

However, if you have a 40 barg steam source and the piping downstream of the pressure reducing valve has a maximum rating of 34 barg, you MUST have a relief valve on the downstream piping no matter how many control valves you have in parallel! Codes require it.
Hi there,

I am not sure what your contractor have done but it seems they are trying to do a split range configuration. That is the right thing to do in a application like this since a split range setup will enable the valves to cope with any kind of upsets or varying pressure demands your plant might have. It will also be able to cope with emergency situations during a trip or any other very rapid change in steam pressure demand lower or higher.

Since the valves are fail close they will close if something goes wrong with their signal or the valve itself. They cannot open and should be spring to close (not spring less double acting - you should check this) and most probably are. So they are completely safe and over pressurizing on the downstream side cannot happen unless someone put the controller on manual and open the valves fully and leave them like that or if the downstream PT fails. To be safe you can install a extra PT as a backup, so the controllers will read the pressure from either this PT or that PT with a warning in the CCR of one of the PT’s have failed. You can also make the valves reverse acting (Normal: PT fails = 4mA to PIC = 20mA to valves = valve opens 100%, reverse = valve close with a 20mA signal and opens with 4mA) so even of the PT fail the valves will still close.

Bottom line, if anything goes wrong with the PT, valves, their signal, the controller or the cables the valves should close.
If you still feel uncomfortable install a trip transmitter next to the control transmitters to cut the signal to the valves when the pressure goes to high down stream.

You also need to check if they have done a proper split range configuration on the valves so I will explain how it should work.

In this case we calibrate the “A” valve to open from fully close to fully open with a positioner signal of 4 to 12 mA. We then calibrate the second “B” valve to open from fully close to full open with a positioner signal of 12 to 20 mA.

Both valves will receive a full 4 to 20mA signal from it's own PID controller, but will only react based on the 4 to 12 or 12 to 20mA calibration that was done on each valve's positioner. (a point of confusion sometimes so some technicians)

Both PID controllers will receive a identical signal from either it’s own PT or one common PT downstream.

In a situation like yours, where you use identical size valves you can also take the signal from one controller and send it to both valves as well, but it seems they already used two controllers, but with the same input from one or two pressure transmitters down stream. That is fine if it is like that, either way, as long as both controllers receive identical signals all the time.

Each valve will then be controlled individually from its own controller and on it's own PID tuning set which should also be set identical on both controllers.

During normal operations the “A” valve alone might be big enough to control the down stream steam pressure at the 30Bar setpoint without any problems, based on the input from the pressure transmitter, and the “B” valve will stay close all the time. If the steam pressure demand increases to something higher than what the “A” valve can handle by himself the “B” valve needs to help, and will start opening up, once the “A” valve is fully open.

Once the demand falls away the “B” valve will start to close and then the ‘A” valve until the process is back to setpoint again.

Since I don’t know the size of the valves it could well be that in order to control the 30Bar the “A” valve will be fully open all the time and the “B” valve open a certain percentage so the “B” valve could be the main controlling valve in that case. It doesn’t matter as long as the “B” valve is only open a small percentage during normal operations that’s fine.

I am sure you can see by now why it is therefore important to have the setpoints on both controllers set to the same as well, and not different as it is now.

I had to make a lot of assumptions so if there is something I missed let me know.
I used 2 valves in parallel on a gas pressure application. I can't remember what the pressure was, < 20# I think. One was a large butterfly about 16" the other about 4.

The fast pressure control loop operated the small valve while the larger valve's controller operating slower looked at the small valve's position and tried to maintain it at mid travel. Something like that would work very well in a flow control loop/

Thanks Guys

Much appreciated. By some of your comments I am convinced that the vendor will not be able to control these loops without correct tuning.

The valves are sized identical with the philosophy of A valve failing (not maintaing pressure), then the B valve shall take over the control. Not as easy as it sounds.