very large tanks control strategy quesion
very large tanks control strategy quesion
(OP)
hello,
I would like to ask if someone was involved in a level control of a *very* large tank or some advice.
We have a water treatment plant, very large, and they want to control a very large output tank level automatically with a water pump
The water demand may change and this causes problems to the level control.
As a constraint we have water turbidity, and "speeding up the pump" causes troubles, so the increments shall be moderate.
The designer made a PI control, that didn't work well, after that someone did like a step logic, working better.
But still the error in the level is too much (setpoint at 95%, you can see oscillating from 80 to 100%) and this is not a PI control, only adding or substracting steps in flow setpoint.
Also you can see turbidity going up when flow goes up.
I was thinking of adding like a correction factor, a PID correction to the controlled variable, like plus minus 10 litres/second to the steps with the same setpoint as the tank level, the same process value and the output or CV adding or substracting to the flow setpoint.
but before I would like to ask for you opinion!!
please see attached graph
kind regards,
martin.
I would like to ask if someone was involved in a level control of a *very* large tank or some advice.
We have a water treatment plant, very large, and they want to control a very large output tank level automatically with a water pump
The water demand may change and this causes problems to the level control.
As a constraint we have water turbidity, and "speeding up the pump" causes troubles, so the increments shall be moderate.
The designer made a PI control, that didn't work well, after that someone did like a step logic, working better.
But still the error in the level is too much (setpoint at 95%, you can see oscillating from 80 to 100%) and this is not a PI control, only adding or substracting steps in flow setpoint.
Also you can see turbidity going up when flow goes up.
I was thinking of adding like a correction factor, a PID correction to the controlled variable, like plus minus 10 litres/second to the steps with the same setpoint as the tank level, the same process value and the output or CV adding or substracting to the flow setpoint.
but before I would like to ask for you opinion!!
please see attached graph
kind regards,
martin.





RE: very large tanks control strategy quesion
RE: very large tanks control strategy quesion
The solution is to make your system into a low pass filter to flow where the inflow can vary but the outflow changes slowly like a low pass filter. The solution is simple, since you are NOT controlling level use a proportional only controller or a proportional band controller. This is easy to setup. Now the flow will be proportional to the measured level but the level can change quickly so the trick is to use a low pass filter. The measured level is run through the low pass filter to get a filtered level. Use the filtered level for your proportional only controller.
Adjust the time constant to get the desired results. If you chose a time constant that is too long the actual level will lag the filtered level by too much and there may be an over flow so here is the next trick.
Make the time constant for the low pass filter shorter as the tank level increases or gets past a certain point but make the low pass filter longer while the tank levels are low.
About time constants. Time "constants" are not always constant. The instantaneous time constant is something divided by the rate of change in something. In your case the time constant is the level divided by the rate of change in level or volume divided by the rate of change in volume.
This method will not control the level to any particular level but it will keep the level within the proportional bands if the in flow doesn't exceed the out flow for too long or the in flow doesn't increase too rapidly relative to the time constant but the time constant should be short using my definition of a time "constant".
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/
RE: very large tanks control strategy quesion
turbidity depends on flow rate as you can see in graph but also need to take care on flow changes because too much change "makes peaks" in turbidity.
the trick you say is very interesting, i will try make modificatons.
regards,
martin.
RE: very large tanks control strategy quesion
RE: very large tanks control strategy quesion
RE: very large tanks control strategy quesion
RE: very large tanks control strategy quesion
Also, is there some optimal outflow where the ratio of flow to turbidity is the highest or is it alway proportional? This is another place to optimize things. Do the level of the tank affect the turbidity? If the same amount of stuff is mixed in a shallow tank the concentration of stuff is greater. This may mean the keeping the water level up a little keeps the turbidity down. Just thinking. I would be looking at your data with an excel spread sheet getting answers to the questions. If the flow is constant will the turbidity change as the level changes?
Does the inflow affect the turbidity. If the in-flow affects the turbidity more than the out-flow there are other "weighting" factors for optimization.
@djs, what you suggested is what I think other people do now based on previous threads. I think your plan would work well IF the flow rate is ramped up slowly. From what I have seen the flow is ramped in steps. The key would be to ramp the set point slowly but that is what the low pass filter is doing. I suggest using a P only system because that is all that is really required and a proportional band is easy to implement. The trick is ramping the set point in an optimal way.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/
RE: very large tanks control strategy quesion
yes the lenght of the array I cannot change in this DCS...
for the flow, I guess changing the flow value is more important than how much flow is in. Operators always say "speeding up" is worse than anything.
well i have something to work on, lets see what happens.
regards,
martin.