I am having a problem obtaining a proper proportional band for a level controller. We have a pneumatic valve before a pump on the outlet of our liquid tank. In trying to tune the controller, I would like to find a proportional band that yield an over damped response. However, I cannot achieve this even at the maximum of the controller (865% !). Could this be a problem with my controller, or more likely an issue with the valve (which seems to be functioning fine despite its age) or pump (which is certainly oversized for the flow rate we need)? Any thoughts would be appreciated.

 

There are a number of checks you need to do to solve your problem, namely:

1. Are the controller's air nozzles and bellows still to specification, or are they worn out and widened?

2. Is your controller properly set, i.e. does the change in level give the designated shift of output signal at 100% pb? You should find this in the valve's documentation.

3. Is your valve travel and valve spring pre-loading still to specification, given the valve age?

4. Are the valve internals still to specification, or is the valve worn out?

5. Does the valve travel smoothly or is it jerking, indicating worn guide bushings?

6. The controller most probably makes use of a float to 'measure' the level in the tank. Is the float still intact, and its guides OK?

7. Is the small pipework within the controller still OK and not damaged/worn?

You indicated that the setup has been working for quite some time, so given that the system is a mechanical system with moving parts, I expect that wear and tear has taken its toll.

 

The larger the tank, the lower the PB. You do not say what size. Try starting at 20% going down. Virtually no integral. Say .8 to 1.2 RPM.
By the way, should you not have the valve after the pump?

 

Hi,

I would be happy to help, while I have the time at the moment. When trying to tune a system, I believe in estimating what the system can do and should do based on the physical equipment design. This will define physical constraints that can put limitations on the control loop. Then I would jump to the control and tuning. Can you supply the following information?

a.) P&ID of the control loop with the source of the media being pumped and the destination

b.) What is the media and the properties of the media being pumped

c.) Dimensions, elevations and levels of the source and destination

d.) Pump curve

e.) Length of run between source and destination, line size(s), number of elbows and equipment in-line that would restrict flow

f.) Anticipated/desired flow rates (min, normal, design/max)

g.) Valve Cv and characteristics

h.) Make and model of controller

Regards
James Fountas
james [at] fountas.net