Introduction:
Purpose of this system is to maintain the constant pressure in water
supplying pipe in spite of discharge from pipe. This system consist of three pumps, out
of which one is running continuously on VFD & try to maintain the pressure in
pipe at required level & other two pumps are switched on one by one if first
pump failed to maintain the required pressure. Also there are alternations of
pumps after 24 hrs. It means if in first cycle, first pump is operating on VFD,
second is auxiliary & third is standby, after 24 hrs.change over will takes
place & second pump will come on VFD, third will be auxiliary & first will
be stand by and so on.
Set-up for the system:
VLT 2800 drive for automatic pump speed adjustment w.r.t. Pressure feedback
from pipe, PLC 16 inputs & 12 outputs along with 1 PLC expander 16 inputs &
12 outputs for switching on & off pumps on VFD & star-delta & auto-change over
of pumps. Operator Interface unit of Prizm for status display, parameter entry
& system alarms indications with alphanumeric display.
Operating Sequence:
Auto operations:
1. After giving cycle start signal o/p contactor of drive & delta
contactor of star delta starter for first pump gets on & first
pump will starts on VFD with maximum RPM, which will develops the
pressure in pipe. After developing the pressure at required level
(value set in drive parameter) motor will rotate at minimum defined
RPM. If requirement of water demands, pressure goes on decreasing,
so RPM of pump goes on increasing with internal PID action, to
achieve the required pressure. If pressure drop down with increasing
water discharge from pipe such that one pump failed to develop it
within desired period (time will derived in PLC logic), PLC will
detects this & generates the o/p to start the second pump on star-delta.
Now flow of water in pipe increases suddenly which causes friction in
pipe, to minimize the friction loss in pipe, pressure set point in drive
automatically increases. Pressure in pipe increases with two pumps, when
pressure in pipe reached at predefined level set in PLC, PLC removes the
o/p of star-delta of second pump.
2. In above case after starting second pump if pressure level failed to
develop within desired period (time specified in PLC logic) PLC generates
the o/p to start the third stand by pump. Now as 3 pumps are running at a
time, friction in pipe increases with increasing flow. To minimize the
friction loss in pipe pressure, set point in drive again increases automatically
with starting the third pump.
3. As with 3 pumps, pressure goes on increasing, which detects by PLC
continuously. When pressure will reach to the required level (set point
define in PLC logic), PLC wait for some time (time specified in PLC logic)
to get the pressure stable & after getting the feedback of pressure PLC
will stop the third pump. After stopping the third pump again PLC will check
whether pressure level is within the range, if it is in range then PLC
will wait for desired period (time defined in PLC logic) & will shut down
the second pump.
24 hour Auto-change over of pumps:
To reduce the load on one pump & to maintain the duty cycle of pump
auto-change over facility is provided. Auto-change over action will take
place after each 24 hour (duty cycle can be decide by user). This
action performs during the period when water demand is less. During
this action second pump will run on VFD, which was auxiliary pump in
previous cycle, the third pump will become auxiliary, which was stand
by in previous cycle & first pump will become stand by which was
running on VFD.All this change over action will performed by using PLC logic.
Auto-change over timer bypass:
In case if one of the pump failed to start due to any reason this facility is
provided. For this facility one toggle switch will be provided. By activating
the bypass switch externally by operator, which will detect by PLC at it’s
input. After detecting the bypass action, PLC will itself bypass the running
pump on VFD & will start the pump on VFD that is next to the pump, which was
running on VFD earlier & next pump will become auxiliary. During auto-change
over, bypassed pump will not start. This condition will remain as it is until
failed pump doesn’t get repaired & bypass switch will not get deactivated. In
this situation as only two pumps is healthy, 24-hour auto-change over will
takes place within remaining two healthy pumps. This facility is also applicable
to the pump which is running on star-delta & if star-delta will trip.
