Overview of Primary Product

9.
In other words, Pump 2 overpowers Pump 1. For this
reason, with Pump 2 running alone, Pump 1 should not be
started unless Pump 2 operates to the right of the point
where the curve of Pump 2 and the curve of Pumps 1 and
2 diverge (100 GPM) in Fig.3–6.
Parallel pumping is often an excellent way to obtain
optimum operating conditions and to save energy. To be
successful, however, systems and operating conditions must
be understood. This applies to both stable and unstable
pump curves.
Part IV – NPSH And Pump Cavitation
The net positive suction head (NPSH) is an expression of
the minimum suction conditions required to prevent
cavitation in a pump. NPSH can be thought of as the head
corresponding to the difference between the actual absolute
pressure at the inlet to the pump impeller and the fluid
vapor pressure. An incorrect determination of NPSH can
lead to reduced pump capacity and efficiency, severe
operating problems and cavitation damage.
It is helpful to define separately two basic NPSH consider-
ations; required NPSH (NPSHR) and available (NPSHA).
The required or minimum NPSH is dependent on the design
of a particular pump and is determined by the manufacturer’s
testing of each pump model. The pump manufacturer can plot
this required NPSH for a given pump model on performance
curve and this value, expressed as feet of the liquid handled,
is the pressure required to force a given flow through the
suction piping into the impeller eye of the pump. Required
NPSH can also be defined as the amount of pressure in
excess of the vapor pressure required by a particular pump
model to prevent the formation of vapor pockets or
cavitation. Required NPSH, then, varies from one pump
manufacturer to the next and from one manufacturer’s model
to another. The required NPSH for a particular pump model
varies with capacity and rapidly increases in high capacities.
The available NPSH, on the other hand, is dependent on the
piping system design as well as the actual location of the
pump in that system. The NPSH available as a function of
system piping design must always be greater than the NPSH
required by the pump in that system. The NPSH available
as a function of system piping design must always be greater
than the NPSH required by the pump in that system or noise
and cavitation will result. The available NPSH can be altered
to satisfy the NPSH required by the pump, if changes in the
piping liquid supply level, etc., can be made. Increasing the
available NPSH provides a safety margin against the potential
for cavitation. The available NPSH is calculated by using the
formula:
NPSHA = ha +/- hs - hvpa – hf
where:
ha = atmospheric pressure in feet absolute
hs “+” = suction head or positive pressure in a closed
system, expressed in feet gauge
hs “- ” = suction lift or negative pressure in a closed system,
expressed in feet gauge
hvpa = vapor pressure of the fluid in feet absolute
hf = pipe friction in feet between pump suction
and suction reference point.
Cavitation can be defined as the formation and subsequent
collapse of vapor pockets in a liquid. Cavitation in a centrifugal
pump begins to occur when the suction head is insufficient
to maintain pressures above the vapor pressure. As the inlet
pressure approaches the flash point, vapor pockets form bubbles
on the underside of the impeller vane which collapse as they
move into the high-pressure area along the outer edge of the
impeller. Severe cavitation can cause pitting of the impeller
surface and noise levels audible outside the pump.
The Taco pump performance curve below (Fig. 4–1) includes
a plot of the required NPSH for a Taco Model 1506. If a
pump capacity of 105 GPM is used as an example capacity
requirement, reading vertically from that GPM rate shows a
required NPSH of 4 feet. An available system NPSH greater
than 4 feet would, therefore, be necessary to ensure
satisfactory pump performance and operation.
Curve no. 2015
Min. Imp. Dia. 4.25"
Size 2 x 1.5 x 6
MS 2-18-02 PC-2015 RevB ECN10627
FEET
KPa
0
2
20
0
2
0
0
10
1
30
3
40
4
50
5
60
6
70
7
80
8
90
9
100
10
110
11
120
12
4 6 8 101 3 5 7 9 11
2
6
4
12
6
18
8
24
(121mm)
REQUIRED NPSH
6.25"
5.75"
5.25"
4.75"
4.25"
CURVES BASED ON CLEAR WATER
WITH SPECIFIC GRAVITY OF 1.0
1HP(.75K
W)
.75HP
.5HP
.33HP(.25
KW
)
1.5HP(1.1
KW
)
(159mm)
(146mm)
(133mm)
(108mm)
(.37KW)
(.56KW
)
54%
57%
42%
46%
46%
42%
54%
57%
60%
64.5%
63%
63%
60%
HEAD IN FEET
0 25 50 75 100 125 150 175 200
0
10
20
30
40
50
FLOW IN GALLONS PER MINUTE
Model 1506 1760 RPM
August 9, 2001
CI & FI Series
L/SEC
HEAD IN METERS
HEAD IN KILOPASCALS
NPSH
Fig. 4-1