Chemtrac SCC3500XRD, Руководство по эксплуатации, Страница: 36 / 58

Chemtrac Systems, Inc. Page 35
This procedure may need to be repeated on a monthly to seasonal basis to ensure the
setpoint remains representative of optimum process performance.
NOTE - If automatic control is used, refer to Section 3.7 on Automatic Control.
3.6 MANUAL CONTROL USING CONTROLLER
Only available on model SCC3500XRD.
Make electrical connections between Streaming Current Controller and the chemical
feed pump. The AUTO or Manual control mode is indicated on the MAIN screen. To
switch between AUTO or Manual control mode can be done by pressing both
p and q
keys from the MAIN screen. Once in Manual control mode, the pump control output
percentage can be increased or decreased by pressing the
p or q key.
The AUTO or Manual control mode can also be change from the Pump Control (1)
menu. Once Pump Control (1) menu is display, move the text cursor on the far left hand
side of the screen (
}
) to “Pump Ctrl:” then press
u
the control text will blink. The
control mode can be set by pressing the pressing p or q key, this will toggle between
AUTO and MAN mode. Set the controller to MAN. Once completed, press t key then
press
p
or
q
key to get to next parameters on that screen. Next press
q
key to “Man
Output:” then press u the control number will blink. The percent (%) number will blink,
the manual chemical feed control value output can now be increased or decreased the
by pressing
p
or
q
key to the desire manual control value. When completed, press
t
key to get to other parameters on that screen. The SAVE Function Key must be
pressed to save current setting modifications and brings up the MAIN screen.
3.7 AUTOMATIC CONTROL
Only available on model SCC3500XRD.
Automatically controlling the treatment process with the Streaming Current Controller
(SCC3500XRD) is a very simple operation, once you know how the controller works.
Most controller manuals do not explain, in simple terms, the purpose of so-called "tuning
constants" termed Proportional Band, Integral Time, and Rate. Controllers with these
three functions are sometimes called "3-mode controllers," or "PID controllers". In
order to explain the purpose of these functions, a brief review of how a simple
proportional controller works may be helpful.
Many water treatment plants have "flow controlled" coagulant feed, i.e., when raw water
flow increases, a signal is sent to the coagulant pump that immediately increases the
feed rate. In other words, there is no "lag time" between water flow increasing and
coagulant increasing. This controller works extremely well in maintaining proper dosage
as long as the water flow is the only thing changing. However, if turbidity is changing,
the proper chemical dosage cannot be maintained with a simple proportional controller.
Since the SCV is taking its sample downstream of the coagulant feed point, there will be
some "lag time" before it can sense a change in turbidity, alum feed, raw water flow, etc.
If, for instance, the SCV reading should start going more negative due to an increase in
turbidity, it will cause the controller to increase coagulant feed. This will cause the
streaming current reading to go more positive, which will cause the pump to decrease
coagulant. The result is continuous cycling above and below the setpoint, thus, the