Page 131 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 109

. What is a self-priming pump? maximum diameter (d 2 ) in designing a good
& A self-priming pump is a pump that draws liquid up impeller.
from below pump inlet (suction lift), as opposed to a & N s is 500–5000, d 1 /d 2 > 1.5 for radial flow pump;
pump requiring flooded suction. 5000–10,000, d 1 /d 2 < 1.5 for mixed flow pump; and
. “Centrifugal pumps are usually self-primed pumps.” 10,000–15,000, d 1 /d 2 ¼ 1 for axial flow pump.
True/False? & Specific speed is also used in designing a new pump
& False. by size-factoring a smaller pump of the same specific
speed. The performance and construction of the
. Define specific speed for a pump.
smaller pump are used to predict the performance
& Specific speed, N s , is a dimensionless design index
and model the construction of the new pump.
that identifies the geometric similarity of pumps. It is
& Pump may be damaged if certain limits of N s are
used to classify pump impellers as to their type and
exceeded and efficiency is best in some ranges.
proportions. Pumps of the same N s but of different
size are considered to be geometrically similar, one . How does specific speed indicate pump type to be
pump being a size factor of the other. selected?
& Specific speed is defined as the speed in revolutions & If N S > 500, centrifugal pump is the choice.
per minute at which a geometrically similar impeller & If N S < 500, positive displacement pump is a likely
would operate if it were of such a size as to deliver candidate.
1 gpm flow against 1 ft head. & Some kinetic pump types can generatevery high head
& Specific speed, (such as regenerative turbine pumps), and the specific
speed criterion, as suggested above would not hold
N S ¼ NQ 1=2 =H 3=4 ð5:2Þ true for them.
. What is suction specific speed?
whereNisthenumberofrotations perminuteandQis
& Suction specific speed is expressed as
the flow rate at or near BEP measured in gpm. (BEP is
defined and discussed later). H is the head developed 1=2 3=4
per stage at BEP at maximum impeller diameter in ft. N SS ¼ NQ =ðNPSHÞ : ð5:3Þ
& In general, pumps with a low specific speed have a
& The formula for N SS is same as for N S , by substituting
low capacity and high specific speed, high capacity.
NPSH R for H.
& The understanding of this definition is of design
& Suction specific speed, N ss , is a dimensionless num-
engineering significance only, however, and specific
ber or index that defines the suction characteristics of
speed should be thought of only as an index used to
a pump. It is calculated from the same formula as N s
predict certain pump characteristics.
by substituting H by NPSH R .
& Specific speed determines the general shape or class
& N ss is commonly used as a basis for estimating the
of the impellers. As the specific speed increases, the
safe operating range of capacity for a pump. The
ratio of the impeller outlet diameter, d 2 , to the inlet or
higher the N ss is, the narrower is its safe operating
eye diameter, d 1 , decreases. This ratio becomes 1.0
range from its BEP. The numbers range between
for a true axial flow impeller.
3000 and 20,000. Most users prefer that their pumps
& Radial flow impellers develop head principally
have N ss in the range of 8000–11,000 for optimum
through centrifugal force. Radial impellers are gen- and trouble-free operation.
erally low-flow high head designs. & In multistage pump the NPSH R is based on the first-
& Pumps of higher specific speeds develop head partly
stage impeller NPSH R .
by centrifugal force and partly by axial force. A
. What are the desirable suction-specific speeds for a
higher specific speed indicates a pump design with
centrifugal pump?
head generation more by axial forces and less by
& For single-suction pumps, N SS > 11,000: excellent;
centrifugal forces.
N SS ¼ 7000–9000: average; and N SS < 7000: poor.
& An axial flow or propeller pump with a specific speed
& For double-suction pumps, N SS > 14,000: excellent;
of 10,000 or greater generates its head exclusively
N SS ¼ 9000–11,000: average; N SS < 7000: poor.
through axial forces. Axial flow impellers are high
flow low head designs. . What are the ranges of specific speeds for centrifugal
pumps? Are these ranges more or less for reciprocating
& Specific speed identifies the approximate acceptable
pumps?
ratio of the impeller eye diameter (d 1 ) to the impeller