Appendix c: Manual Controller Tuning
This tuning method is designed for small plants where the flow rate is high compared to the size of the rapid mixer, meaning a retention time of less than about 20 seconds.Procedure
It is important that the controller be tuned taking the plant's minimum flow
rate into consideration. At low flows a small change in alum dosing will have
a big effect, tuning the controller at a higher flow can make the controller
too sensitive (ie proportional band too small.) This can lead to cycling or
even instability at lower flows.
PID controller tuning is mostly a matter of determining the relationship between
the dosing pump action and the streaming current result. This can be done
with one simple test:
Quickly adjust the dosing controller output up or down by about 20% of the
current dosage (enough to have an reasonable effect, but not too much.)
You should see no change in SCM output for a certain time (maybe 20 sec) then
a relatively gradual change to another stable reading (will take 5-10 minutes)
Make a note of the time it takes for the reading to start to change (we will
call this dead time). Also record the initial and final SCM readings. (The
technical term for this is the process step response.)
Manually put the controller back to the proper dosing (you should see the
same thing happen, but the other way, double check your values on this.)
The dead time must be less than 30 seconds for this formula to apply.
Tune the controller like this:
Proportional Band
PB = 40 x ( Final reading - Initial reading ) / (final dosing - initial dosing)
(dosing is as a % of maximum, reading assumes the Accufloc's 4-20mA output
span is the default: -10.0 to 10.0)
Integral(or reset) Time
Tr = 1.2 minutes
Derivative Time
Td = 0 (this is no use)
With these settings the controller should typically take around 5-15 minutes
to mostly recover from a major disturbance, eg interruption in dosing, plant
start up, or a sudden flow rate change.
Try a few things like this to ensure it is stable, as poor tuning will probably
only show up in extreme situations. If the controller cycles then increase
the Proportional Band and / or increase the Integral Time. If it is very slow
to respond, do the opposite.
These tuning rules are quite conservative, but should be more than able to
cope with all disturbances to typical water treatment plants. Remember that
it is not beneficial to tune the controller so tightly that the chemical dosage
overshoots before the SCM has had time to respond.
If your mixer retention time is very long (more 3 minutes) or your dead-time
is more than 30 seconds then the above tuning will cause a some cycling in
the dosage response. Tuning will then depend on the individual plant. Start
by increasing the proportional band and work from there.