PID cascade mode to control level?

[karivelm]

Process:
Adding defoamer to the pulp eases water removal. The pulp goes in a vacuum drum washer, which is a rotating cylinder with a screen all around. This drum lays in a pulp vat. As pulp enters the vat continuously, the drum sucks up a pulp mat, and the dirty water goes through the drum in to a tank on the lower floors. The drum speed presently adjusts to control the level in the vat, so the vat does not overflow. If the drum speeds up too much, water removal is bad and the final pulp is dirtier. This is when the operators add more defoamer to improve water removal, so the vat level can be maintained with a lower drum speed.

Problem:

I want to pump my defoamer in automatic to maintain the drum speed at a certain value. This way, if the drum starts to speed up, the defoamer will automatically increase to slow it down.

Would something like to PID llops in cascade work?
PID 1: Control Level with speed of DRUM
PID 2: Control DRUM SPEED with defoamer pump speed

How can this be implemented?

 

[bce123]

The drum speed presently adjusts to control the level in the vat

The level of water or the level of pulp?

 

[karivelm]

The level of pulp...

 

[bce123]

what determines the flow of pulp entering the vat?
and do you measure the flow of pulp entering the vat?

 

[bce123]

If the drum speeds up too much, water removal is bad and the final pulp is dirtier

could you control defoamer pump speed with the level of vat?
and control drum speed with the flow of incoming pulp?

 

[Lancie1]

Because of this condition:

..if the drum starts to speed up, the defoamer will automatically increase to slow it down.

then you need to change PID 2 to control, not Drum Speed, but Defoamer Pump Speed.

PID 1: Control Pulp Level by varying the speed of DRUM (where the Level is controled with a setpoint?)
PID 2: Control defoamer pump speed proportionly to DRUM SPEED, which makes the amount of water removal tied to the drum speed.

Really, the amount of defoamer needed should depend directly on the ratio of Water Added to Water Removed. If you can measure (or already do measure) those two quantities, then use that ratio to control how much defoarmer gets added. That would be a more direct control method.

Many times, the most direct factor cannot be measured, so a secondary, indirect factor has to be used to substiture for the real control factor. That is what you are trying to do with Drum Speed, using it as a substitute (stand-in) for the amount of water that is removed.

There may be other factors (other than Drum Speed and Defoarmer) affecting the amount of water that gets removed, such as how dirty the water is, the type of pulp, the temperature of the water and pulp, the coarse/fines in the pulp, how clogged the drum screen is, and so on. That is why using a measurure of the acutual factor to be controled (Water Removal) would be more accurate, than using the secondary indirect Drum Speed, to control the amount of Defoamer.