13
AUTOMATIC OPERATION
Overview
The primary benefit of Variable Speed
Pumping (VSP) Systems is to ensure surge-
free starts and stops while maintaining a
constant down-stream line pressure with no
mechanical pressure-regulating valve. This
minimizes pipeline failures due to surges and,
not incidentally, reduces the utility bill for the
station.
Automatic operation is selected by the system
control switch. In automatic operation, all
pump activity is determined by the PLC. The
position of the individual pump switch lets the
PLC know whether or not it is available to run.
In most cases, all pump switches are in the
ON position. When a pump or a motor is
removed from the system, turn the pump
switch OFF, and the PLC will not try to run
that pump.
The PLC brings one pump on at a time in
order to satisfy the start criteria (usually
setpoint
pressure).
Some
stations
are
designed to start and stop the pumps based
on remote start signals, level probe signals, or
other criteria. The standard start and stop
sequence is as follows:
Pressure below setpoint to start
Combo 1
5 PSI
Delay time to start Combo 1
0 SEC
Pressure above setpoint to stop
Combo 1
5 PSI
Delay time to stop Combo 1
5 SEC
Pressure below setpoint to start
Combo 2
10 PSI
Delay time to start Combo 2
0 SEC
Pressure above setpoint to stop
Combo 2+
30 PSI
Delay time to stop Combo 2+
90 SEC
Pressure below setpoint to start
Combo 3+
5 PSI
Delay time to start Combo 3+
10 SEC
NOTE:
The criteria for Combo 4 and above
are the same as Combo 3.
The "pressure above setpoint to stop" and
"delay time to stop" parameters only apply to
Combo 1. The values of 30 PSI and 90
seconds were intentionally selected since they
are out of range and will never be used.
There are three (3) main types of auto
operation – line fill, system charging, and
normal operation.
Line Fill Mode
Line Fill Mode is used when downstream
pressure is significantly below setpoint, such
as during the initial startup, or when leaks in
the lines are repaired. Operation is as follows
(performed in order):
System control switch: OFF
All Individual Pump switches: ON
Override/Active switch: LDP Override
(Low Discharge Pressure fault disabled)
VFD Bypass switch: VFD
System control switch: AUTO
As soon as the system control switch is turned
to the AUTO position, the VSP (Variable
Speed Pump) with the lowest run time starts
at a reduced speed. When the VSP starts, the
startup ramp in the program lowers the
setpoint
to
2
PSI
above
the
actual
downstream pressure. The VFD adjusts the
speed of the VSP to reach the desired
setpoint. The setpoint ramps up at the rate of
1 PSI every 4 seconds until the normal
setpoint is reached.
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 start up from a
completely
de-pressurized
condition,
minimizing pipe breakage from water hammer.
Once the pressure above setpoint value is
reached, the VSP shuts down. The pump
switches can be left in ON and the system
control
switch
left
in
AUTO.
The
Override/Active switch should be returned to
the ACTIVE position. The station is now in
system "charging" mode.
System Charging Mode
If there are no heads open, and the pressure
is within the defined range of operation,
pressure may slowly drop due to system
