PID Flow Control

PLC75

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Join Date
Jul 2015
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US
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7
Hello All,

I have a question regarding Flow Control. Below is the scenario:

I have a level transmitter and flow meter to open/close control valve at the outlet. When level reaches certain value (high) I want to open the control valve and maintain XX barrels per day of flow. When level reaches certain value (high high) I want to open the control valve and maintain higher XX BPD of flow. When level reaches certain value (low), I want to close the valve.

For this scenario, I do not see a need for cascade loop control since I am not trying to maintain any level. I will only need flow based PID control loop.

I am getting flow using a flow meter that outputs certain frequency to give out flow rate. So when I get that flow rate that is current value, is this value in seconds meaning barrels per second currently flowing? What set-point should I set to flow control PID since my required set-point is in barrels per day and the flow-rate I get is in seconds?

As far as logic implementation, do I need to look at the totalizer and keep track of accumulated flow so I will be looking at the accumulated flow and required barrels per day flow and adjust my output so that at the end of the day I get XX barrels per day.

Thanks.
 
Multiply by 86400 to give you barrels per day.

What is the normal range of flow?

Can you reconfigure your flow transmitter to give you BPD?
 
So if reconfigure the flow meter to BPD then I can use PV against the SP to output the signal to control valve using PID?

Then do I need to keep track of accumulated flow or if I reconfigure the flow meter to BPD then I don't have to worry about in PLC.
 
So if reconfigure the flow meter to BPD then I can use PV against the SP to output the signal to control valve using PID?

Yes

Then do I need to keep track of accumulated flow or if I reconfigure the flow meter to BPD then I don't have to worry about in PLC.

I don't understand the question.

Keeping track of accumulated flow is up to you, but it does not have any thing to do with controlling flow as you described above.
 
Can you use a cascaded PID?

One flow controller that maintains a particular flow by adjusting the valve.
One level controller that maintains a level(boundary) by adjusting the set point for the flow controller.
 
Mickey, the reason I am talking about flow accumulation is to determine control valve output.

For example, when the level high is reached PID controller will start opening the control valve then I will get frequency reading from flow meter which depending upon the k-factor I will convert to BPD. I will use this converted value as PV to the PID controller against SP to determine the control valve output value to maintain this x BPD flow rate. So I was thinking to use the accumulated value and see how much flow I have left to meet x BPD and use that value to control the control valve.

With my little experience with PID, I don't understand how will the flow rate in BPD will control the valve as compare to pressure loop where you read current pressure and maintain that pressure. There is no per day, it is controlled by what is currently reading.
 
I am not sure if cascade PID is required. Do you see any advantage of using cascade here?
I am planning to do simple compare with level reading and set the flow set-point to flow controller. Since there is no requirement to maintain any level.
 
Not sure, but the way you described the problem, you want to increase your flow with high level and decrease it with a low level. To me, that could be solved with a PID using a deadband on the input. There are other ways for sure, but this would be one possible solution. I did not read your original comment regarding not needing cascade control, sorry about that ;).

Reading through it one more time, I have a question. At the end of the day, assuming you have the amount of barrels required, you shut of the valve? Will the tank level keep rising?

Just thinking out loud here, but you could use:

(required barrels - achieved barrels) / time to end of day to calculate your set point.

Lets hope from a reply from Peter, he might have the ideal solution for you.
 
I am a little confused exaclty what you are getting from your flow transmitter.

If you are getting an instantaneous rate rate, then that can be used as your process variable for the PID.
 
There is no need for cascaded loops.

You have a simple flow control loop. The derivative function is probably not needed, and so the rate setting would be zero, making it technically PI.

The Process Variable is the flow. The output, the manipulated variable, is valve position.

There will be two setpoints, which you will move into the PI loop setpoint as needed. One setpoint will be used at the "H" level, the other at the "HH" level.

You will convert the frequency to a flow rate, just as you would convert a 4-20 mA signal to a flow rate. The input is just Hz, not current.
 
So when I get that flow rate that is current value, is this value in seconds meaning barrels per second currently flowing?

There's probably 10,000 flowmeters in the world market and most all are configurable for different flow ranges and flow rated units.

There's no way in the world that anyone except you and your work colleagues can know which units your flow meter is using.

It might be lpm or it might be bpd? Only the meter knows. Go check its display to see what the display says the flow rate unit is. If it's a blind flow meter (no indicator) check its configuration to see what the flow units are.

The relative magnitude of the flow rate value depends on its units.

All of these values are the same flow rate, but the value itself is radically different:

1 BPS = 60 BPM = 3600 BPH = 86200 BPD
0.067 BPS = 4 BPM = 240 BPH = 5,760 BPD

the flow-rate I get is in seconds?

What volume in seconds? gallons/sec? liters/sec? barrels/sec?
 
A simple PI control on the flow meter is all that is required. Nothing has been said about if more fluid is being added to the tank during this process. As the tank level changes the flow will change due to a lower pressure drop unless the valve is opened more as the tank level decreases but the integrator should take care of that. The tank level isn't require except to start the process UNLESS you want to use it to estimate the flow for a give pressure drop and valve opening like a feed forward but that is a lot of work and probably not worth it.
 
Thanks everybody for your input.

Here is what I understand from flow meter and previous posts for logic implementation:

- The flow meter has scalable frequency output so I can set 1KHz as XX BPD.
- I will use the raw value Hz from the flow meter and scale it just how you scale 4 to 20ma AIs.
- Then use scaled value as PV to my PID controller which will maintain the BPD depending upon the level (H or HH).
- As far as tank getting full, there is an interlock to the inlet valve where it closes on high high level switch.
- Depending upon the level, I may or may not get X BPD of flow everyday because it will only open when LAH or LAHH occurs.
- I will have a totalizer that will keep track of BPD but it will not affect the flow PID controller in anyway.

Just to clarify my understanding on flow meter inputs: The frequency it outputs looks at the current flow and outputs the frequency in BPD to state that if the flow remains the same then you will this many barrels at the end of the day. Please let me know if this is correct statement.

Please let me know if I am moving in right direction or any suggestion for improvement.
 
Your understanding of the flow meter operation is correct. It is indeed measuring instantaneous flow, and the value reported would be the volume accumulated over the prescribed time.
 

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