TDC3000 Ratio Control


Thread Starter


Have you ever seen a ratio controller? One that controls, lets say, the flow into, two different vessels, in this case two reactors. We set the ratio flow on two different control loops controlling two different control valves. This way you can choose to increase or decrease flow to the reactors as long as the total percentage equals 100.

The problem is that the feed to these two valves come from another vessel that has it's own level control valve. What happens in an upset situation is that if the ratio controllers have satisfied their ratio at one flow rate and the flow rate through the level control valve swings, you get out of control and wind out your level control loop and never catch up, because the ratio control is still at the same ratio, i.e.,same output. You end up having to put the ratio controllers on manual and sling them open until the level can find set point.

I thought about running the level with the ratio loops and just put the level loop in manual at 100%, but I am not sure how to do the ratio logic if I do so.

Anyone have any suggestions...

Bruce Durdle

As I read your description, you are taking an outlet stream from one vessel through your level control valve, themn splitting the level control
discharge stream into two separate streams: you want to keep the ratio of these two streams as set.

It sounds as if you are trying to control 3 valves - the LCV and the 2 FCVs - to meet 2 conditions - level and ratio. This never works and one of the systems will take itself out of action by winding up as you have described.

If this is the case, you need to get rid of the level control valve (or run it at 100%) and adjust the two flow control valves in proportion from the output of the level controller. EG - if the required ratio is 60%:40%, use the controller output to generate the setpoints for each flow controller, but have one FC setpoint = LC O/P x 0.6; the other SP = O/P x 0.4.

Your strategy looks incorrect:
Level is one animal. Flow an other one.
Have two independant flow loops, then ratio one to the other. Any DCS should be able doing the job. Analog controllers were capable of same strategy.

Hello What type of level control do you use. Are you saying it can't reset itself because the ratio controllers are drawing too fast? Or is the unit not capable of resetting itself?

Thanks Bob Hogg [email protected]

Johan Bengtsson

As I interpreted the first post he tried to say that he tries to use three controllers to control the same thing, the flow out from a tank (and thereby the level)

This mean, either will the controller aimed at controlling the level or the controllers aimed at controlling the flows succed, not both.

The solution is as Bruce explained, to have the level controller adjust the setpoints for the other two (standard cascade control) except that
there is not one secondary controller but two and they should divide the effort by the pre set ratio.

/Johan Bengtsson

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Satish Salankimatt

You can achieve this in many ways:

case 1.
You can control level of a tank in two ways either by controlling the feed or by controlling the discharge. If you are controlling the discharge it means that it has constant feed and discharge may be any amounts.

case 2:
But if you discharge has got limits then you can have control at discharge. You can let the discharge at its own rate and control the feed to make up the level.

You can achieve ration control in many ways:

1. as bruce has suggested keep the lcv at 100% and give 60% and 40% of level controller o/p to respective fcv's.

This is like contolling the discharge. Summary of this is that if the feed is constant then discharge also should be constant so there is no actual controlling involved as you are just diverting the discharge.

In this case total discharge tro both fcv's will be equal to total feed. As inlet is always constant discharge is also always constant.

2. you have a level control loop then the o/p of the controller you have the ratio and give to fcv's. The difference here is that the lcv is not always at 100% but it is almost like as expained in 1.

3. you have a level control loop then two flow control loops with flow transmitters.

To explain further I need whether the flow requirement of the reactors is constant or is varying.