What is the PC doing? Maintaining a min pressure or only preventing an excess pressure?

I assume the FC is to maintain a minimum flow.

It's unusual to have both flow and pressure to maintain min flow. Why both?

But first work out what each controller is trying to control and what happens if the flow or pressure reading is higher or lower than the set point. Doe sit want to open or close the valve?
DO the two things happen together - so in this instance high pressure probably equals low flow, so in both those instances you want the valve to open.



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What is your process application, pump design and pressures involved.

@LittleInch - yes both PC and FC are for min flow protection. In case of low flow and high prssure..the control valve should open. Valve is fail open and air to close. So when flow decrease, controller should send low signal to valve maybe ..to cause it to open? ..for pressure increase the valve should open so controller should less signal...so maybe reverse actin...having difficulty figuring it out

@hacksaw - Hydrocarbon liquid pump. normal discharge pressure is 250 psig.

The PC and FC controllers work by looking at the process signal versus a set point.

When the process signal is lower or higher than the set point, the controller outputs a position signal to the valve positioner to either open or close to try and maintain the required set point. So as the flow through the pump increases, the flow controller wants to close the valve until it is fully closed if flow is > the min flow set point. However if the pressure is at or above the pressure set point the pressure controller wants to maintain or open further the valve. SO in this case the valve wants to have sent to it the highest value.

The position signal is what goes into the high selector block which in your case take highest position to send to the valve positioner.

For your case I can't really understand why you're doing this. A pressure signal to control low flow is much less accurate than a flow signal so why is it there?

But if you really want to do that then take both position signals and stick them into a high selector block and the valve will open more based on either lower flow than your set point OR higher pressure than your set point and flow more water of liquid around the bypass, thereby increasing flow through the pump and at the same time lowering the pressure. whichever is exceeded essentially takes control of the bypass control valve.

Does that make sense?

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Most applications requiring constant process pressure with minimum flow conditions for the pump, require two valves.

Agree with hacksaw. Pressure control is usually via second valve downstream of the bypass line.

Using a bypass line to control pressure is a poor way of doing this as it wastes a lot of energy if you want a low pressure.

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Thanks little inch. What you said makes sense. I guess my only concern is that since we have a fail open and air to close valve, should we actually have a low selector instead of high selector? Since in case of blocked flow for e.g, downstream of pump, the head/pressure increases and flow decreases...using a low selector would be better? since it will send lowest signal to valve causing it to open more?

Don't think so.

The signal is the position of the valve. So you want the highest of the two positions to be sent to the valve positioner.

Sending a low position signal would close the valve.

The control action of the valve is irrelevant here. That only affects what happens if you lose air pressure. Not connected to the control logic discussion.

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@little inch - thanks, however, I am still a bit confused over selecting a high selector

From your earlier comment "But if you really want to do that then take both position signals and stick them into a high selector block and the valve will open more based on either lower flow than your set point OR higher pressure than your set point and flow more water of liquid around the bypass, thereby increasing flow through the pump and at the same time lowering the pressure. whichever is exceeded essentially takes control of the bypass control valve."

so say we talk in terms of 4-20 mA signals. Since valve is fail open and air to close...more signal to the valve means it closes more and zero signal to the valve means its full open. Essentially the control valve positioner or control valve itself will have I to P converter..that will take the signal and convert it to pneumatic signal. So now if we select a high selector we are sending a higher signal from the two controllers to valve which converts to a higher pneumatic signal and closing the valve more since the valve is air to close. Hence, my confusion whether it should be a low selector or high selector.

You can use a high or a low signal select. Just configure the controller behaviour (direct acting or reverse acting) to suit. Add a signal reversing logic block if required. There are no fixed rules to follow, as long as the control logic works under all operating scenarios. Plant control narratives should fully describe the scheme intended so the DCS controls engineer can set this up.

From the control sketch, it looks like the pressure controller is set high, while the FC is set low.

You're mixing up the signals coming from the PID controllers with the signals going to the valve positioner.

The selector block might be 4 to 20 or it might be digital within the DCS.

Your final element to the valve will be reverse acting in that a 20mA signal will equal 3psi or wha tr ever is the min output pressure into the valve.

You really need to talk to a controls engineer to explain further.

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