FURROW PUMP
8525 SW St. Helens
Drive
P.O. Box 1849
Wilsonville, OR 97070
(503) 682-4411 / (503) 682-4411 |
Dear Earl:
I have followed this bid specification closely as far as
intent, in taking two independent 4-20 mA signals (pH and flow) and
summing them to control a single pump. To do that, I’ve selected a PLC with a Touchscreen display for the system controller. This will allow us, for example, to assign
a percent of influence to flow and pH – i.e., telling the controller to pay 80% of
its attention to the flow signal and 20% to the pH signal, in coming up with the summed
signal going to the pump motor controller. Using a PLC lets us take 4-20 mA
signals in, and give out a single proportional signal to the motor controller
to maintain a chemical feed in
proportion to both flow and pH. Because of this, we can put an LMI Digi-Pulse Flow Monitor
on the pump discharge to confirm that each time the pump is told to stroke it really does,
and that chemical flow is really occurring. That means we can get an alarm if there is a
pump failure or loss of prime (or even if you’ve run out of chemical, if there is no
low level control). As an interesting but pertinent aside, if you
can count the number of actual pump strokes, you can add a calibration cylinder to the
package and determine the volume per stroke, based on a given stroke length setting. A
simple multiplication in the PLC gives us both flow rate and flow total, so we can measure
actual chemical flow. This bid specification asks for a computation of the amount of acid
needed to neutralize the backwash water to be treated. An SCR motor tachometer can’t
do this, but with an actual flow measurement we can.
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