EI 11C201
CEMP-E
1 March 1997
where:
Hs
=
total energy head in meters (feet)at the pump suction nozzle
Ps/w + V2s/2g + Zs
=
Pa
=
atmospheric pressure in KiloPascals (pounds per square foot) absolute, and
Pv
=
vapor pressure of fluid being pumped in KiloPascals (pounds per square foot)
absolute.
(3) Affinity laws. A set of relationships derived from flow, head and power coefficients for
centrifugal pumps, can be used to determine the effect of speed changes on a particular pump.
These relationships are known as affinity laws and are as follows:
Q1/Q2
= N1/N2
H1/H2 = N21/N22
= N31/N32
P1/P2
where:
N1, N2 = pump speeds in revolutions per minute (rpm).
Q, H, and P terms represent pump capacity, discharge head, and power output respectively, at
speeds N1 and N2. These relationships will be used in analyzing variable speed pump operation
in the absence of manufacturer's characteristic curves, or where characteristic curves do not
show performance at the desired speeds.
d. Pump selection. System analysis for a pumping station will be conducted to select the
most suitable pumping units which will meet service requirements, and to determine their
operating points, efficiencies, and required power.
(1) Single pump operation. A system head-capacity curve will be prepared showing all
conditions under which the pump is required to operate. The system curve will then be
superimposed onto a pump head-capacity curve, or characteristic curve, to define the pump
operating point. The point where the two curves intersect represents the head and capacity at
which the pump will operate in the given piping system.
(2) Multiple pump operation. Where two or more pumps discharge into a common header,
the head losses in individual suction and discharge lines will be omitted from the system
head-capacity curve. This is because the pumping capacity of each unit will vary depending
upon which units are in operation. In order to obtain a true picture of the output from a multiple
pump installation, the individual suction and discharge losses are deducted from the pump
characteristic curves. This provides a modified curve which represents pump performance at the
point of connection to the discharge header. Multiple pump performance will be determined by
adding the capacity for points of equal head from the modified curve. The intersections of the
modified individual and combined pump curves with the system curves give the total discharge
capacity for each of the several possible combinations. Pumps will be selected so that the total
required capacity of the pump installation can be delivered with the minimum level in the wet well
and maximum friction in the discharge line. Pump efficiency will be a maximum at average
operating conditions.
7-4
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