Xylem Wastewater (Technical Manual) - Manual (Page 18)

PAGE 18
Wastewater
Goulds Water Technology, Bell & Gossett,
Red Jacket Water Products, CentriPro
It is acceptable and increasingly more common to operate three-
phase wastewater pumps using VFD’s or variable frequency
(speed) drives. We have successfully tested and operated all our
premium cast iron construction, three-phase pumps between 30
and 60 hertz operation. The pumps should never be operated
below 30 hertz (the VFD must be programmed for a minimum
speed of 30 hertz to prevent continuous operation) or above 60
hertz due to increased motor HP loading, higher amperage and
the resultant heat rise (see HP in 70 hertz Performance Multipli-
ers).
The “Affinity Laws” state that for a given pump, the capacity will
vary directly with a change in speed, the head will vary as the
square of the speed change and the required power will vary as
the cube of the speed change. (The Affinity Law formulas can be
found in the Water Products Technical Manual, TTECHWP). The
Performance Multiplier Chart provides shortcut multipliers that
eliminate having to solve the Affinity Law equations.
To calculate a pump’s total performance range when using a
VFD, use the 30 hertz data to create a minimum speed curve, the
VFD controlled pump should always be operated between 30
hertz and the published 60 hertz curve. Where it operates at any
given moment is irrelevant.
Q
1
, H
1
and BHP
1
are determined at the pump’s rated speed
N
1
(rpm).
Q
2
, H
2
and BHP
2
are determined at speed N
2
(rpm).
Use the multipliers with a minimum of 3 data points taken from
any standard, 60 Hz curve to determine the performance of that
pump at a new speed.
WASTEWATER PUMPS AND VARIABLE SPEED DRIVES
Hertz Performance Multipliers
70 – Q
2
= Q
1
x 1.17 H
2
= H
1
x 1.37 BHP
2
= BHP
1
x 1.6
60 – Use the standard published curve data
50 – Q
2
= Q
1
x .83 H
2
= H
1
x .69 BHP
2
= BHP
1
x .57
40 – Q
2
= Q
1
x .67 H
2
= H
1
x .45 BHP
2
= BHP
1
x .3
30 – Q
2
= Q
1
x .5 H
2
= H
1
x .25 BHP
2
= BHP
1
x .125
An example would be, solve for Q
2
, H
2
and BHP
2
for a 60 Hz
pump that produces 100 gpm (Q
1
) @ 100’ tdh (H
1
) using 5 hp
(BHP
1
) when it is operated at 30 Hz :
Answers: 100 gpm x .5 = 50 gpm, 100’ TDH x .25 = 25’ TDH
and 5 hp x .125 = .63 hp.
VFD’s save energy while reducing the thrust on the motor bear-
ings and the starting torque on the shaft and impeller.
Contact Customer Service for details, pricing and availability of
our full line of VFD products.
Variable Speed Drives