# Ratio control for multiple streams

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1. Sep 30, 2015

### princessme

I have 3 streams entering my vessel and 2 of the streams are dependent on the 3rd stream's flow rate in a certain ratio. Is there a way to control the flow rate of the 2 dependent streams using the flow rate of the 3rd stream and show it in my P&ID?

2. Sep 30, 2015

### Bystander

Yes. "... and show it in my PID?" Not clear what you're asking here.

3. Sep 30, 2015

### princessme

What I meant was how should the configuration of the control loop be in my P&ID? I'm aware of how to show the control loop for 2 streams, but I'm having issues for 3 related streams to be controlled. Any references that I could refer to for this?

4. Sep 30, 2015

### Bystander

You've a couple possibilities: the third stream flow rate can be described as a function of the flow rate of either of the other two; pick one.

5. Sep 30, 2015

### princessme

For instance, both the first and third stream are dependent on the flow rate of the first, both in different ratios. Does it mean that I need to have 2 control loops - one for streams 1 and 2, and another for streams 2 and 3?

6. Sep 30, 2015

### Bystander

Yup. Third stream as fnn of first, or of second; of second is easier since it's a fixed ratio.

7. Sep 30, 2015

### princessme

Noted. Thanks for the help.

8. Oct 3, 2015

### princessme

Can one flow transmitter be connected to two flow controllers?

What I'm thinking is that i have a flow transmitter on second stream, then one flow controller each on first and third stream connected to the FT on second stream. Does it work that way? Or do I need to have separate flow transmitters for each loop?

9. Oct 4, 2015

### Bystander

... and, when the third stream "runs away," how do you sense that upset condition?

10. Oct 4, 2015

### princessme

My plan was to put a control valve on stream 1 and 3, which are connected to a ratio station each. Each ratio station is connected to the FT on stream 2. Does that make sense?

11. Oct 4, 2015

### Bystander

Makes sense if you can be certain that which ever stream you leave as an "open loop" doesn't get plugged up, run dry, or otherwise fail.

12. Oct 4, 2015

### princessme

So I should consider doing 2 closed control loops instead?

In that case, for each control loop, I will have a FT, followed by FC, which is connected to the ratio station and a control valve on the stream. Then another FT after the control valve to send the signal back to FC? Does it sound right?

13. Oct 4, 2015

### Bystander

FT on primary stream (set point) to "ratio" for two dependent streams, and FC plus FT sensing output for each dependent stream is the way I'd do it.

14. Oct 4, 2015

### princessme

Alright, thanks for the suggestion.
Will I need two FT on my primary stream or would 1 be sufficient?

15. Oct 4, 2015

### Bystander

One does the trick --- two can have a nasty tendency to start calling each other liars.