D2-250 PLC PID loops control pumps in
residential water system
by
Peter Carman,
Wyoming Controls
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We used PID loops in the D2-250 CPU to control three pumps
and maintain pressure in a residential water system. The flow
demand varies from zero to 250 gallons per minute. The pumps
all have
variable speed drives with the speed reference and run commands
coming from the 250 CPU. At low flow, one pump runs alone,
as needed, to maintain the pressure. As the flow increases,
the other pumps are brought on, either in tandem with the first,
or at a constant flow rate. The flow from each pump has an
upper limit; if the total flow demand exceeds the combined
upper limits, the pressure will fall. The result is that each
pump can operate in an optimum range on its pump curve, and
a widely varying flow can be delivered and the pressure is
maintained very closely. The operator interface is a color
touch screen
that displays the pump status, flow rates, and system pressure.
All the setpoints for pump on/off and associated time delays
are easily accessible to the system operator.
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