TM 5-814-2/AFM 88-11, Vol. 2
variable speed pump operation in the absence of
the absolute pressure at the pump inlet drops below the
manufacturer's characteristic curves, or where
vapor pressure of the fluid being pumped. To determine
characteristic curves do not show performance at the
if cavitation will be a problem, the net positive sunction
desired speeds.
head (NPSH) available will be computed, and compared
d Pump selection. System analysis for a pumping
with the NPSH required by the pump. The NPSH is not
normally a problem when discharge heads are less than
station will be conducted to select the most suitable
60 feet. However, when heads are greater than 60 feet,
pumping units which will meet service requirements,
or when the pump operates under a suction lift, or far
and to determine their operating points, efficiencies, and
out on its curve, the NPSH will be checked. The NPSH
required horsepower.
available at the eye of the impeller in feet will be
(1) Single pump operation. A system head-
calculated with the following formula:
capacity curve will be prepared showing all conditions
under which the pump is required to operate. The
NPSHA = H8 + Pa/w - Pa/w
system curve will then be superimposed onto a pump
where
head-capacity curve, or characteristic curve, to define
H8 = total energy head at pump suction
the pump operating point. The point where the two
nozzle
curves intersect represents the head and capacity at
= P8/w + V2 /2g + Z,
which the pump will operate in the given piping system.
Pa = atmospheric pressure in pounds
(2) Multiple pump operation. Where two or
per square foot absolute, and
more pumps discharge into a common header, the head
Pv = vapor pressure of fluid being
losses in individual suction and discharge lines will be
omitted from the system head-capacity curve. This is
pumped in pounds per square foot
because the pumping capacity of each unit will vary
absolute
depending upon which units are in operation. In order to
obtain a true picture of the output from a multiple pump
All head terms are in feet.
installation, the individual suction and discharge losses
(3) Affinity laws.
A set of relationships
are deducted from the pump characteristic curves. This
derived from flow, head and power coefficients for
provides a modified curve which represents pump
centrifugal pumps, can be used to determine the effect
performance at the point of connection to the discharge
of speed changes on a particular pump.
These
header. Multiple pump performance will be determined
relationships are known as affinity laws and are as
by adding the capacity for points of equal head from the
follows:
modified curve.
The intersection of the modified
individual and combined pump curves with the system
Q1/Q2 = N1/N2
curves shows total discharge capacity for each of the
2
2
H1/H2 = N 1/N 2
several possible combinations. Pumps will be selected
3
3
P1/P2 = N 1/N 2
so that the total required capacity of the pump
where
installation can be delivered with the minimum level in
N1, N2 = pump speeds in revolutions per minute
the wet well and maximum friction in the discharge line.
(rpm)
Pump efficiency will be a maximum at average
operating conditions. A typical set of system curves
Q, H and P terms represent pump capacity, discharge
with pump characteristic curves is shown in figure 5-2.
head, and power output respectively, at speeds N1 and
N2. These relationships will be used in analyzing
5-4
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