Level contol dyamic stability

[Peckett]

Hi,

We are designing a wate tank with level contol on the outlet valves. My question is how do we size the tank to provide a stable system that can handle flow changes allowing the control valves to maintain control.

Obviously a small tank will be harder to maintain control.

Is there a rule of thumb for sizing the tank liquid surface area and control range or is a full dynamic analysis with nyquist plots etc required?

Any assistance appreciated.


Ben

 

[BigInch]

I've never seen anyone do a Nyquist plot for a tank level control. In pipeline work tanks are more typically sized for minimum and maximum flowrates in order to average flow over time thus allowing a pump to operate closer to BEP for longer periods. Better than being designed to operate over an extreme range of flowrates at large inefficiencies. Should you be looking at process high and low flowrates with an on/off valve attached to HL and LL switches, rather than a PID w/control valves? I don't know, but if your requirement is to hold a tank level to a very very close tolerance, maybe you need to make Nyquist plots to be absolutely sure you can do it. On my side of things, surface area is usually controlled by available plot plan area and economic build height of a tank wall.


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[FeX32]

Make sure the differential equation you use to model your system is stable in closed loop form at least.
If you are running open loop then be sure that the D.E. has all its poles in the left half plane.
What kind of control are you using? Just an on off switch?

Fe

 

[Peckett]

Thanks for the responses. We normally just use common sense and judgement to determine if the system looks right. However, with large flows and a relatively small tank, I was interested to see if there are any rules of thumb.

The control valve will be on a PID loop on the tank outlet. Inflow is from another process over which we have no control.

Cheers,

Ben