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leakage, or other small demands. A PM pump
is provided to address this issue. Most
stations, but not all, include a PM pump. Door
switches are set as follows (from end of line fill
mode):
System control switch: AUTO
All Individual Pumps switches: ON
Override/Active switch: ACTIVE (Low
Discharge
Pressure
and
differential
pressure faults enabled)
VFD Bypass switch: VFD
NOTE:
As downstream pressure drops to
more than 5 PSI below setpoint, the PM pump
starts up. It runs until the system pressure
builds up to 5 PSI above setpoint, and then
shuts off.
Two issues can come into play here. One is
cycle time. This refers to the number of times
an hour that the PM pump starts and stops
(one cycle). If the amount of cycles is
excessive, either the system leaks have to be
repaired, or the start/stop parameters need to
be tuned (normally by dropping the pressure
below setpoint to start, or increasing the
pressure above setpoint to stop). Must-run
times can cause the opposite problem. The
PLC determines the must-run time for each
pump. If this value is set too high, the PM
pump reaches its "pressure above setpoint"
and stops before the must-run time setting
indicates. This can lead to an over-
pressurization situation. Address this problem
by lowering the must-run time.
WARNING: Excessive Run Time Hazard
Excessive must-run times, stop times, or stop
pressure setting can cause system over-
pressurization, pipe damage, and potentially
cause personal injury. Consult your service
technician or the factory if you are unsure about
any settings.
Failure to follow these instructions could result in
serious personal injury, death, or property
damage.
Normal Operation
Normal Operation occurs when heads are
turned on, or a demand for water exists. Door
switches are set as follows (from end of line fill
mode):
System control switch: AUTO
Pump 1 switch: ON
Pump 2 switch: ON
Pump 3 switch: ON
PM Pump switch: ON
Override/Active switch: ACTIVE (Low
Discharge
Pressure
and
differential
pressure faults enabled)
VFD Bypass switch: VFD
When the pressure drops 5 PSI below
setpoint, the PM pump starts and continues to
run until the pressure is 5 PSI above the
setpoint. When this pressure is reached, the
PM pump turns OFF. If the pressure continues
to drop to 10 PSI below setpoint, the VSP
starts. Five seconds later, the PM pump turns
OFF. This is the operation sequence for
Combo 1.
When the VSP starts, the startup ramp in the
program lowers the setpoint to 2 PSI above
the actual line pressure. The VFD adjusts the
speed of the VSP to maintain the discharge or
downstream pressure at the setpoint. The
setpoint ramps up at the rate of 1 PSI every 4
seconds until the normal setpoint is reached.
As the setpoint rises, so does the speed of the
VSP. This feature prevents the VSP from
coming up to full speed too quickly and
prevents the lag pump(s) from starting
prematurely. This ramp-up feature allows the
system to develop a controlled and smooth
startup from a completely de-pressurized
condition, minimizing pipe breakage from
water hammer. This is the operation sequence
for Combo 2.
If the VFD reaches full speed driving the main
VSP and the discharge pressure is more than
10 PSI below the setpoint, the demand for
water is greater than the capacity of the VSP.
An XL (across-the-line) pump starts as the lag
pump. This operation sequence occurs during
Combo 3.
When an XL pump starts, and the VFD is
driving another pump, the inverter (VFD)
speed immediately ramps down to the inverter
“speed when lag pump starts” setting. This
permits the XL pump to start with very little
pressure surge, or related water hammer.
Once the lag pump is at full speed, the VSP
ramps back up to try and maintain setpoint
pressure.
