8/10/2019 Taco Pump Curves How to Read and Use
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PART I FundamentalsA centrifugal pump operated at constant
speeddelivers any capacity from zero to maximumdepending on the
head, design and suctionconditions. Pump performance is most
commonlyshown by means of plotted curves which are
graphical representations of a pump s
performancecharacteristics. Pump curves present the averageresults
obtained from testing several pumps of the
same design under standardized test conditions. Fora single
family residential app lication, considerationsother than flow and
head are of relatively littleeconomic or functional importance,
since the totalload is small and the equipment used is
relativelystandardized. For many smaller circulators, only theflow
and pressure produced are represented on theperforman ce curve
(Fig. 1-1 ). For larger and morecomplex buildings and systems,
economic andfunctional considerations are more critical, and
PERFORMANCE FIELD
F L O W M 3/H
6
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10
4
0 r\6 o08 -. N
=::::: s r-.8I -
r ............
4 .....006 \ o o 5 007 ~
0 r\.] 16 20 24 28 32
F L O W GPM
Fig. 1 1
performance curves must relate the hydraulicefficiency, the
power required, the shaft speed, andthe net positive suction head
required in addition tothe flow and pressure produced (Fig. 1-2).
Pumpperformance curves show this interrelation of pumphead, flow
and efficiency for a speci fic impellerdiameter and casing size.
Since impellers of morethan one diameter can usually be fitted in a
givenpump casing, pump curves show the performance ofa given pump
with impellers of various diameters.Often, a complete line of pumps
of one design isavailable and a plot called a c omposite or
quickselection curve can be used, to give a completepicture of the
available head and flow for a givenpump line (Fig. 1-3). Such
charts normal ly give flow,head and pump size only, and the
specificperformance curve must then be referred to forimpeller
diameter, efficiency, and other details. Formost application s in
our industry, pump curves arebased on clear water with a specific
gravity of 1.0.
8/10/2019 Taco Pump Curves How to Read and Use
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0
0
0
.
0
Quick Selection Curve
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1750RPM
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v , .FLOW IN GALLONS PER MINUTE
. Fig. 1 3
ow is a volume measure to establish pumppacity per unit of time,
usually as GPM.
ead is a pressure measure represented by how highe pump can lift
a column of liquid, usually in feet.convert the popular pressure
expression P S I to
et of water, multiply P S I x 2.31.
orsepower H.P.) is the term used to express themount of power
available to drive the pump.
rake Horsepower BHP) is the term used to expresse amount of
power required to drive the pump.
et Positive Suction Head Required NPSHR) is aessure measure - in
absolute units - expressed inet and indicates the pressure required
at the
ump suction to prevent cavitation. Reducing theessure at the
pump flange below the vaporessure of the liquid can cause formation
of vappr
ockets in the impeller passes. This conditionavitation) will
interfere with pump performance,d is usually accompanied by noise
as the vapor
ockets collapse. NPSHR can be thought of as themount of pressure
in excess of vapor pressurequired to prevent the formation of vapor
pockets.
et Positive Suction Head Available NPSHA) is therm used to
express the pressure available at theump suction flange. If NPSHAis
less than NPSHR.vitation problems should be expected.
mp Efficiency indicates the percentage of brakeorsepower
converted into useful work. Pumpficiency, along with flow, head,
and liquid specificavity affect the power required to drive the
pump.
he more efficient the pump, the less power requireddrive it.
ecific Gravity S.G.) is the term used to expressldtive weight of
a liquid when compared withater water= 1.0 S.G. .
2
R.P.M. is the term used to express the rotationalspeed of a
pump.
Some addi tional terms that are often used whenconsidering a
pump's relationship to the particularsystem into which it will be
installed are reviewedbelow.
Shut. )ff Head is the term used to express the headdeveloped by
a pump at zero flow.
static Head is the term used to express the pressure
at the pump discharge which the pump mustovercome before it can
produce flow. Static head isa difference in elevation and can be
computed for avariety of condit ions surrounding a pump
installation.
System Resistance is the term used to express thepressure on the
pump discharge resulting from theresistance to flow created by
friction between thefluid and the piping system. This value will
vary withflow rate.
Suction Pressure is the term used to express thepressure
observed at the pump suction connection.This may be a positive
pressure or a negativepressure.
Discharge Pressure is the term used to express thepressure at
the discharge connection. This will alwaysbe a positive
pressure.
Differential Pressure is the term used to express thealgebraic
difference between the discharge andsuction pressures. This value
represents pump head.
Service Factor is the term used to express the reserve power
availab le from an electric motor whenoperating under normal
conditions.
System Curve is a graphical representation of thehydraulic
characteristics of a piping system. When
the pump performance curve is laid over the systemcurve, the
intersection indicates the flow and headpressure of the pump when
coupled to the hydraulicsystem.
Constant Speed is the term used to express the RPMof a pump upon
which a published pump curve isbased.
8/10/2019 Taco Pump Curves How to Read and Use
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PART II T h e System CurveUnderstanding a system curve,
sometimes cal led asystem head curve, is important because
conditionsin larger, more complex p iping systems vary as aresult
of either controllable or uncontrollablechanges. A pump can operate
at any point of ratingon its performance curve, depending on the
actualtotal head of a particular system. Partially closing avalve
in the pump discharge or changing the size or
length of pipes are changes in system conditionsthat will alter
the shape of a System Curve and, inturn, affect pump flow. Each
pump model has 0definite capacity curve for a given impeller
diameterand speed. Developing a system curve provides themeans to
determine at what point on that curve apump will operate when used
in a particular pipingsystem.
Pipe, valves and fittings create resistance to flow orfriction
head. Developing the data to plot a systemcurve for a closed
hydronic system under pressurerequires calculation of the total of
these friction headlosses. Friction tables are readily available
that
provide friction loss data for pipe, valves and fittings.These
tables usually express the losses in terms of theequiva lent length
of straight pipe of the same size asthe valve or fitting. Once the
total system friction isdetermined, a plo t can be made because
thisfriction varies roughly as the square of the liquid flowin the
system. This plot represents the System Curve.By laying the system
curve over the pumpperformance curve, the pump flow can edetermined
Fig. 2-2). Care must be taken that bothpump head and friction are
expressed in feet andthat both are plo tted on the same graph. The
systemcurve will intersect the pump performance curve atthe flow
rate of the pump because this is the point at
which the pump head is equal to the requiredsystem head for the
same flow.
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