In a high pressure control valve, is there any way to keep downstream pressure constant at all the openings and different flows?
Say the upstream pressure is 140 kg/cm2 and the downstream pressure we want is 20 kg/cm2 at all the openings even at 100% opening.

 

Measure the downstream pressure and use it is as PV for the pressure control loop?

But I guess you mean something else.

 

Let's visualize what you are trying to do.

With a valve, you want to flow product into another volume (pipe, tank, etc.) and to keep the pressure within that volume constant, independent of the flow.

Two conditions will allow you to do that.

1) You MUST remove flow from that volume at the exact same rate that it is being introduced by the valve.

2) You will need a volume that is enormously large such that the pressure within it will not change much as flow into it changes.

Neither one of which is practical.

So, a question back to you. What is your objective is achieving this condition? Why do you need the downstream pressure to be constant under all flow conditions from the valve?

 

certainly possible and commonly down but only if it is compatable with the overall process design.

 

Sounds like a conventional pressure regulator to me, with the inherent proportional droop in flow rate as flow rate increases.

 

Unless you can control upstream pressure, too, it can't be done for a given flow. Flow is proportional to the square root of the differential pressure across the valve.

Now if your purpose is to regulate downstream pressure alone, and flow and upstream pressure can vary, then yes. Downstream pressure varies directly with flow, and so reducing flow reduces downstream pressure, and vice versa.

 

Let’s have a look at how a normal PID pressure control loop works. You will measure your line pressure downstream and take that to a PID controller which in turn will give an output to your control valve to open and close in order to keep the downstream pressure constant and on set point.

In order to do that the valve will have to open and close and the flow through the valve will therefore also change dependent on the downstream pressure measurement that is send to the PID controller.
Assuming that your PID controller is perfectly tuned and everything works the way it should, you will have a continuous change in flow, pressure and valve opening and closing as the control valve is trying to control the pressure downstream to a given set point. If the upstream pressure increases the flow through the valve will increase and also increase the pressure downstream. The pressure sensor downstream will see the change in pressure and the controller will compensate for it by closing the valve slightly and visa versa when the upstream pressure decreases, so yes your downstream pressure can be kept constant for all valve opening positions and all the various flows in your process.

Obviously if your upstream pressure does not change much the variance in flow, pressure and valve opening and closing will not be much but should your upstream pressure changes a lot your flow through your control valve and the valve opening and closing will be much bigger, even as much as all the way from 0 to 100% valve positions and therefore also flow, in order to compensate for these variances.
Just another note on an extreme case like above where you have to open and close your control valve from fully open to fully close in order to try and control your process you will under normal conditions seriously consider alternative means of doing the control of your process.

One such an alternative would be to move over to a split range control system where you use two valves in parallel (one small and one big valve) in the same line and connected to the same PID controller in order to control low as well as high flows and pressure variations of your process.

 

In this case diversion of flow is required.
you need a back pressure valve. it will maintain constant pressure for all load changes by diverting the flow.

 

Thanks you all, it was quite helpful.

 

if you want a minimum pressure you could use the pressure as PV at low pressures and flow as PV above that minimum.