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

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

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