Brochure

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 func
-
tion 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 subse
-
quent 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(.75KW)
.75HP
.5HP
.33HP(.25KW)
1.5HP(1.1KW)
(159mm)
(146mm)
(133mm)
(108mm)
(.37KW)
(.56KW)
54%
57%
42%
46%
46%
42%
54%
57%
60%
64.5%
63%
63%
60%
HEAD IN FEET
0255075 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