Psd Ceu 195 jan13 Pumps

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CEU1

95

Pumps

Continuing Education from theAmerican Society of Plumbing Engineers

January 2013

ASPE.ORG/ReadLearnEarn


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The most common type of pump used in plumbing systems

is the centrifugal pump, although some applications requireother types. For plumbing, the centrifugal pump stands out

because of its simple design and suitable head (pressure).Further, its rotational speed matches that of commonly avail-

able electric motors; drive belts or gears are rarely employed.With small sizes, the motor shaft is typically coupled directly

to the pump impeller, resulting in a compact design and asimple installation, even for fire pumps.

This chapter focuses on centrifugal pumps, but pumps in

general are explored, including differences in pump types,performance characteristics, applications, installation, and

environmental issues.

APPLICATIONSPump applications in plumbing include specialty pumps forliquid supplies, pressure boosters for domestic water supply,similar supply pumps for fire suppression, water circulation

for temperature maintenance, and elevation increases fordrainage systems. Except for the circulation application,

pump systems theoretically are open systems, meaning thatthe liquid is transferred from one reservoir to another of a

higher elevation. The applications vary in the nature of theliquid, the dutywhether for daily use or for rare firefighting

useand the magnitude of elevation changes.

PUMP BASICS

Machines that move water, or any liquid, are called turbo-machines. Commonly referred to as pumps, these machinesadd energy to the liquid, resulting in a higher pressure

downstream. This added energy is called head, which refersback to the days of dams and water wheels. The descent ofwater was expressed as a level of energy per pound of water.

The water descended adjacent to the dam through the waterwheel, and the vertical distance between the water levels on

either side of the dam was measured. In contrast to waterwheels, all pumps add energy, but the amount is expressed

in the same terminology.In theory, if a sufficiently tall, open-top vertical pipe is

mounted on a pipe both downstream and upstream of a

pump, the liquid level in both can be observed. The leveldownstream will be higher than the level upstream. This

difference in elevation between the two levels is called thetotal head for the pump. Another element of pump head is

the difference in elevation between the upstream pipe andthe pump; a distinction is made if the upstream elevation is

above or below the elevation of the pump inlet.

Pump Types and Components

For all pumps, the basic parts consist of a passage andmoving surface. The passage is simply referred to as tpump casing. A prime mover, such as an electric motor b

sometimes an engine, adds torque to the moving surfaOther parts include shaft bearings and various seals, su

as the shaft seal.Pumps may be categorized as positive displacement, ce

trifugal, axial, or mixed flow. Positive-displacement pum

deliver energy in successive isolated quantities whether a moving plunger, piston, diaphragm, or rotary eleme

Clearances are minimized between the moving and unmoviparts, resulting in only insignificant leaks past the movi

parts. Common rotary elements include vanes, lobes, a

gears.When a pump with a rotating surface has significant cle

ance between itself and the stationary passage, the pump donot have positive displacement. If the direction of dischar

from the rotating surface, called the impeller, radiates inplane perpendicular to the shaft, the pump is a centrifug

pump. If the direction is inline with the shaft, the pumpaxial. If the direction is partly radial and partly axial, tpump is mixed flow. Examples of a centrifugal pump,

axial pump, and a positive-displacement pump, respectiveinclude an automobile water pump, a boat propeller, and t

human heart.Compared to positive-displacement pumps, centrifu

and axial pumps are simple and compact and do not haflow pulsations. Centrifugal pumps provide greater total he

than similarly sized axial pumps, but they provide lower floThe operation of a centrifugal pump includes the outwaradial projection of the liquid from the impeller as it rotat

In addition, if a gradual expanding passage is provided afthe impeller, the high velocity is converted to a high sta

pressure. This idea follows the law of conservation of enerand is quantified in Bernoullis equation. If the expandi

passage wraps around the impeller, it is called a volute.The quantity and angle of the blades on the impeller a

the shape of the blades vary. They may be two straight blad

positioned radially, many curved blades angled forward,more commonly, many blades angled backward to the dir

tion of rotation. While forward blades theoretically impgreater velocity, the conversion to pressure is unstable exc

within a narrow speed range.Pipes generally connect to pumps with standa

flanges, but they may also connect by pipe threads or s

der joints. The centerline of the inlet pipe may be alignwith the pump shaft. Figure 4-1 shows this type; it is

Reprinted fromPlumbing Engineering Design Handbook, Volume 4. 2012, American Society of Plumbing Engineers.

Note: In determining your answers to the CE questions, use only the material presented in the corresponding continuing educatio

article. Using information from other materials may result in a wrong answer.

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ferred to as an end-suction design. The outlet generally

falls within the plane of the impeller. If the inlet and outletconnections align as if in a continuation of the pipe run, as

shown in Figure 4-2, the pump is referred to as inline.Casing materials are generally cast iron and, for domestic

water supply, cast bronze. Other materials include stainless

steel and various polymers. Impeller materials also includecast iron, bronze, and various polymers. Pump bearings and

motor bearings vary between traditional sleeves and rollerelements such as steel ball bearings. Bearings on each side

of the impeller minimize shaft stresses compared to a pair ofbearings on one side. At the other extreme, the pump itselfhas no bearings, and all hydraulic forces are applied to the

motor bearings. The combination of these materials, design

features, and array of pump sizes results in pumps being t

most varied of the worlds manufactured products.The greatest pressure in any pumped system is with

the pump casing, which includes the shaft seal. Anoth

concern with this seal occurs when the pump is not opering, when a stored supply of pressure applies continuo

static head against the seal. This seal traditionally has bedesigned with a flexible composite material stuffed arou

a clearance between the shaft and the hub portion of t

pump casing, referred to as a stuffing box. A mechanical rangement applies pressure to the flexible material throu

routine adjustments. Some leakage is deliberately requirso provisions for the trickle flow must be included, such

with the installation of a floor drain.Another seal design consists of a simple O-ring. M

advanced seals include the mechanical seal and the wet rodesign. In a mechanical seal, the interface of two polishsurfaces lies perpendicular to the shaft. One is keyed a

sealed to the shaft, and the other is keyed and sealed to tpump casing. Both are held together by a spring and a fl

ible boot. Some pumps include two sets of these seals, athe space between them is monitored for leakage. Often

special flow diversion continuously flushes the seal area.In the wet rotor design, the rotor winding of the mo

and the motor bearings are immersed in the water fland are separated from the dry stator by a thin, stationastainless steel shield called a canister. The shield impart

compromise in the magnetic flux from the stator to the rotso these pumps are limited to small sizes.

DETERMINING PUMP EFFICIENCYHigh efficiency is not the only characteristic to examine

selecting a pump. It is explored here, nonetheless, to demstrate the impact of alternatives when various compromiare considered.

An ideal pump transfers all of the energy from a shaftthe liquid; therefore, the product of torque and rotation

speed equals the product of mass flow and total head. Howevhydraulic and mechanical losses result in performance deg

dation. Hydraulic losses result from friction within the liquthrough the pump, impeller exit losses, eddies from suddchanges in diameter, leaks, turns in direction, or short-circ

paths from high-pressure sections to low-pressure sectioMechanical losses include friction in bearings and seals. T

amount of hydraulic and mechanical losses is from 15 perceto 80 percent in centrifugal pumps and lesser amounts

positive-displacement pumps.Design features in centrifugal pumps that minimize h

draulic losses include a generous passage diameter to redu

friction, an optimal impeller design, a gradual diametchange and direction change, placement of barriers agai

short-circuits, and optimal matches of impeller diameterspump casings. The design of a barrier against short-circu

includes multiple impeller vanes, seals at the impeller inland minimal space between the impeller and the pumcasing. The seals at the impeller inlet are commonly in t

form of wear rings. Enclosed impellers, as shown in Figu4-3, achieve higher heads because of the isolation of t

inlet pressure from the liquid passing through the impell

JANUARY 2013 Read, Learn, Earn

Figure 4-1 Portion of a Close-Coupled Centrifugal PumpWith an End-Suction Design

Figure 4-2 Inline Centrifugal Pump with a Vertical ShaftPhoto courtesy of Peerless Pump Co.


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Axial flow directed into the impeller of a centrifugal pu

may come from one side only (single-suction pump, reback to Figure 4-1) or both sides (double-suction pump, Figure 4-4). The single-suction design creates axial forces

the pump shaft. The double-suction design balances thforces. In addition, double-suction pumps have a slower in

velocity, which helps prevent cavitation.Since most pumps are driven by electric motors, a co

plete review of pump efficiency should include considerat

of motor efficiency, which varies with torque, type of motspeed, type of bearings, and quality of electricity. Ma

fractional-horsepower, single-phase motors experiencdramatic loss of efficiency at light loads. A three-phase m

tor achieves peak efficiency at slightly less than full loHigh-speed motors and large motors offer greater efficienc

than slower or smaller motors. Polyphase, permanent spcapacitor, and capacitor-start/capacitor-run motors are mefficient than split-phase, capacitor-start/induction-run, a

shaded pole motors.A centrifugal pumps first cost can be minimized by

signing for the best efficiency points (BEP) of the operatflow and head. A lower total head also results in less bear

and shaft stresses, leading to a longer expected pump lifAn appreciation of the benefits of investing in efficien

in a plumbing system can be realized by identifying the mnitude of power in various parts of a building. For exampa domestic water heaters energy input may be 1,000,0

British thermal units per hour (Btuh) (293 kW), while circulation pump may be 700 Btuh (205 W). Hence, in t

situation an inefficient pump is of little consequence. Excsive circulation increases standby losses, but a more effici

heat exchanger in the water heater will provide the mtangible benefit. While the importance of a fire pump for fisuppression is paramount, efficiency invested there is l

important than a reliable pump design.

CENTRIFUGAL PUMPCHARACTERISTICSThe characteristics of centrifugal pumps can be reduc

to two coefficients and one value referred to as the specspeed. The coefficients and a set of relationships, called

finity laws, allow similarly shaped centrifugal pumps tocompared. In general, the coefficients also apply to axial a

mixed-flow pumps, as well as turbines and fans.Deriving the coefficients starts with the law of cons

vation of momentum. That is, the summation of forces

the surface of any fixed volume equals the aggregateangular-momentum vectors multiplied by the flows at ea

of those vectors. Since the applied energy into the liquidthe fixed volume around the impeller is only the tangent

movement of the impeller, only the tangential velocity vtors are considered. For constant density and for radial a

tangential velocities at the inlet and outlet of an impellthe momentum equation becomes:

Equation 4-2

T=d2r2vt2Q2 d1r1vt1Q1

where T =Torque, foot-pounds (N-m)

thus, the original efficiencies are maintained over the pumps

useful life.Equation 4-1 illustrates the relationship between flow,

total head, efficiency, and input power for pumps with coldwater. For other liquids, the equation is appropriately ad-justed.

Equation 4-1

P = Q h [Q h 9.81]3,960 e ewhere P =Power through the pump shaft, horsepower (W) Q =Flow, gallons per minute (gpm) (L/s) h =Total head, feet (meters) e =Efficiency, dimensionless

Impellers with diameters significantly smaller than anideal design generally compromise efficiency. The efficiency of

centrifugal pumps varies greatly with head and flow. Hence,a pump with 85 percent efficiency at one flow may be only50 percent at one-third of that flow.

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READ, LEARN, EARN: Pum

Figure 4-3 Enclosed Impeller

Figure 4-4 Centrifugal Pump with aDouble-Suction Inlet Design


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d2 = Density at the outlet, pounds per cubic foot (kg/m3)

r2= Radius at the outlet, inches (mm) vt2 = Tangential velocity at the outlet, feet per second

(fps) (m/s) Q2 = Flow at the outlet, gpm (L/s) d1=Densityat the inlet, pounds per cubic foot (kg/m

3) r1 = Radius at the inlet, inches (mm) vt1 = Tangential velocity at the inlet, fps (m/s) Q1 = Flow at the inlet, gpm (L/s)

From Bernoullis equation of an ideal flow through any

type of pump, total head is a measure of power per flow andper specific weight. Since power is the product of torque and

rotational speed, the above equation can be related to theBernoulli equation. For steady-state conditions, the inlet flowequals the outlet flow. The relation becomes:

Equation 4-3

h =P

=(r2 vt2 r1 vt1) n

d g Q g where h =Total head created by the pump, feet (m) P =Power, horsepower (W) n =Rotational speed, revolutions per minute (rpm)

(radians per second) g =Gravity constant

With the velocity of the tip of a rotating surface at its

outside radius designated as U, the equation is:

Equation 4-4

h

=

U2vt2U1vt1g

For centrifugal pumps, flow is proportional to the outlet

radial velocity. In addition, vt1=0 since inlet flow generallyis moving in an axial direction and not in a tangential direc-tion. Thus:

Equation 4-5

h

=

U2

vt2g

Figure 4-5 shows the velocity vectors of the flow leaving

the impeller. Vector vr2represents the velocity of the waterin a radial direction, Vector X represents the velocity of the

water relative to the impeller blade, and Vector Y representsthe sum of X and U. Thus, it is possible to resolve these vec-tors into tangential components and derive the following:

Equation 4-6a

vt2=U2 vr2cot B=U2[1 (vr2/U2) cot B]

Equation 4-6b

h

=

U2

U2[1 (vr2/U2) cot B]g

Equation 4-6c

h= U22[1 (CQ) cot B]

gFor a given flow, the vr2/U2ratiois constant and is defined

as a capacity coefficient, CQ.For a given impeller design, CQ

and Angle B are constant. Hence, [1 (vr2/U2) cot B] is con-stant and is defined as a head coefficient, CH. Equation 4-7shows the relationship between this coefficient, the head,

and the impellers tip velocity.

Equation 4-7

CH=hgU2

2

With the various constants identified in Equation 4-6c, ttotal head is directly proportional to the square of the imp

lers tip velocity, U2. Recall that the tip velocity is a produof the impellers rotational speed and the impellers radiThus, the total head is proportional to the square of t

impellers radius or of its diameter, and it is proportionalthe square of the impellers rotational speed, in rpm (radia

per second). This is the second pump affinity law.Additionally, since flow is directly proportional to ar

and velocity at any section through a pump, at a particusection the flow is proportional to the velocity of the implers tip. Hence, flow is proportional to the rotational spe

of the impeller and to the diameter of the impeller. Thisthe first pump affinity law.

Since power is the product of flow and head, powerdirectly proportional to the cube of the velocity. This is t

third pump affinity law.Table 4-1 summaries the three pump affinity laws. Ea

function is directly proportional to the corresponding vain the other columns.In addition, it is customary to combine flow and head w

the rotational speed and set exponentials, so this speed apears to the first power. The result, nQ0.5/h0.75, is called t

specific speed of the pump.When the flow rate, head, and a given pump speed a

known, the specific speed can be derived, and the design

an economical pump can be identified, whether centrifugaxial, or mixed flow. Specific speed also allows a quick cl

sification of a pumps efficient operating range with a meobservation of the shape of the impeller.

The affinity laws allow easy identification of pum

performance when the speed changes or the impeller diaeter changes. For example, doubling the speed or impel

diameter doubles the flow, increases the head by four, aincreases the required motor power by eight.

PERFORMANCE CURVESSince centrifugal pumps do not supply a nearly constant fl

rate like positive-displacement pumps, characteristic pumcurves are provided by manufacturers to aid in selectingpump. Under controlled conditions, such as with water a

certain temperature, these curves are created from measuments of impeller speed, impeller diameter, electric pow

flow, and total head. The standard conditions are created

such groups as the Hydraulic Institute. As can be observthe shape of the curve in Figure 4-6 agrees with Equati4-6c. This pump curve represents a particular impeldiameter measured at a constant speed, with its total he

varied and its resulting flow recorded. Efficiency is plotton many of these curves, and the BEP is sometimes mark

Additional curves usually include shaft input power, msured in horsepower (W), efficiency, and net positive suct

head (NPSH).While a curve is plotted for a given pump and with a giv

diameter impeller, a pump in operation under a constant he



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and speed has one particular flow. The point on the pump

curve of this flow and head is referred to as the duty pointor system balance point. The pump will provide that flow if

that head applies.In plumbing, a particular flow may be required for a sump

pump or hot water circulation pump. In domestic water and

fire suppression supply systems, the head varies with thequantity of open faucets, outlets, hose streams, or sprinkler

heads. Further, the quantity of such open outlets varieswith time. Thus, the duty point rides left and right along

the curve with time.Another curve that represents the buildings distribution

piping at peak demand can be plotted on a pump curve. This

second curve, called the system head curve or building systemcurve, is shown in Figure 4-6. Equation 4-8 represents this

familiar curve, where p1represents a pressure gauge read-ing at the pump inlet and p2and h2represent pressure and

elevation head respectively at a particular system locationsuch as at a remote fixture. The last term represents theentire friction head in the piping between the two points

including control valves, if any, at the pump. The curves

shape is parabolic. This curve is applicable to any liquid thathas a constant absolute viscosity over a wide flow range (aNewtonian fluid).

Equation 4-8

hp=(p2 p1)/d+h2+f(L/D)(v2/2g)

At no flow, the friction term becomes zero since velocityis zero, and the point where this curve crosses the vertical

axis is the sum of the remaining terms.To select a pump, determine the peak flow and use Equation

4-8 to calculate the required pump head. The flow and head

identify the duty point. Most catalogues from pump man

facturers offer a family of centrifugal pumps in one diagraSeparate graphs, one for each pump housing and shaft spe

show the pump performance for each of several impelleFigure 4-7 illustrates such a graph for a pump measured1,750 rpm (183 radians per second). Pick a pump impel

that at least includes the duty point. An optimal pump is owhose pump curve crosses this point. However, with m

pump selections, the pump curve crosses slightly above tpoint.

For example, if the duty point is 100 gpm at 30 feethead (6.31 L/s at 9.14 m of head), the impeller number 6in Figure 4-7 is a suitable choice because its pump curve (t

solid line matched to 694) crosses above the duty point. Powrequirements are marked in dashed lines in Figure 4-7. T

pumps motor size, in horsepower or kilowatts, is identifiby the dashed line above and to the right of the duty point

more precise motor required can be estimated at 1.6 hp (kW), but engineers typically pick the 2-hp (1.5-kW) mosize. Select the motor with a nominal 1,800-rpm (188 ra

ans per second) rotational speed. The pumps efficiency c

be estimated if efficiency curves are included on the chaComparing the efficiencies of several pumps can lead to ideal choice. Alternatively, the flow and head of the du

point can determine the ideal power requirement. A pumefficiency is found by dividing the ideal power, from Equati4-1, by the graphically shown power. With this example, t

efficiency is 0.758/1.6 = 47 percent.The shape of a pump curve varies with the impeller desig

A rapidly dropping head due to increasing flow is charactized by a steep curve. Flat curves represent a slight variat

from no flow to BEP, often defined as 20 percent. The latis preferred in most plumbing applications that employ o

pump because of the nearly uniform head. Figure 4-8 shosteep and flat curves and the corresponding blade design

A pump with a steep curve is advantageous when a h

head is required in an economical pump design and the flis of less consequence. For example, a sump pump, whi

has a sump to collect peak flows into its basin, may havehigh static head. With a generous volume in the sump, t

total time to evacuate the sump is secondary; therefore, t

Figure 4-5 Net Fluid Movement From an ImpellerRepresented by Vector Y

Table 4-1 Centrifugal Pump Affinity Laws

FunctionTip

VelocityRotational Speed,rpm (radians/sec)

Impeller Radius (orDiameter), in. (mm)

Flow U n R

Head U2 n2 R2

Power U3 n3 R3

Figure 4-6 Typical Pump Curve Crossing a System Curve

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pumps flow is of less concern than its head. Further, as theinlet flow increases and the water level rises, the head reduces

and the pump flow increases.A pump design with some slope in its curve is desired for

parallel pump configurations. The sum of the flows at each

head results in a more flat curve. For control, the drop inhead as the demand increases may serve as an indicator to

stage the next pump.

A pump with nearly vertical steepness is desired for drain-age pumps that are part of a system of pumps that dischargeinto a force main. This performance characteristic allowsa nearly uniform flow for a wide variation of heads. Some

centrifugal and all positive-displacement pumps exhibit thischaracteristic.

STAGINGTo obtain greater total head, two pumps can be connectedseries; that is, the discharge of one pump becomes the inof the other. As a convenience, pump manufacturers ha

created multistage pumps in which two or more centrifupumps are joined in a series by combining all of the imp

lers on a common shaft and arranging the casing to dirthe flow of a volute into the eye of the next impeller (s

Figure 4-9).Another way to obtain greater head is by using a rege

erative turbine pump. Unlike other centrifugal pumps, t

outer edge of the impeller and its volute are intentionaemployed with higher velocities by using recirculation o

portion of the flow from the volute to pass just inside the of the impeller. The close dimensions of these pumps lim

their use to clean liquids.Applications of high-head pumps inclu

water supplies in high-rise buildings, deep w

ter wells, and fire pumps for certain automastandpipe systems.

SPECIALTY PUMPSTo select a specialty pump, the following mu

be considered: pressure increase, range of flonature of the energy source (electricity, amanual, etc.), whether the liquid contains p

ticulates, whether pulses are tolerable, accurain dispensing, self-priming requirement, wheth

the pump is submerged, and if the pump requian adaptation to its supply container.

Domestic Booster PumpsA domestic booster pump system typically u

multiple parallel centrifugal pumps to increa

municipal water pressure for the buildindomestic water distribution. Particular dsign issues such as sizing, pump redundan

pressure-reducing valves, other pump controadjustable-frequency drives, high-rise buildinand break tanks are described inPlumbing Egineering Design Handbook,Volume 2, Chap5: Cold Water Systems. The same issues ap

for private water systems that require a wpump.

Fire PumpsThe water supply for fire suppression requir

a pump that is simple and robust. In additiothe slope of the performance curve is limited

fire pump standards. NFPA 20: Standard for Installation of Stationary Fire Pumps for F

Protection limits the curve to not less than

percent of the rated total head for 150 perceof the rated flow. A variety of listing agenc

monitor pump manufacturing to certify compance with one or more standards. The design

a single-stage or multistage centrifugal pumgenerally qualifies. A double-suction centrifu

Figure 4-7 Typical Pump Curves and Power Requirements

Figure 4-8 Blade Shape and Quantity Versus Performance Curve

Source: Figures 4-7 and 4-8 courtesy of Weil Pump Company Inc.



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pump with enclosed impeller, horizontal shaft, wear rings,stuffing-box shaft seals, and bearings at both ends histori-

cally has been used. The pump inlet connection generally isin line with the outlet connection.

A recent variation, for small fire pumps, includes a verticalshaft and a single-suction design with the impeller fastened

directly to the motor shaft. Pump bearings, shaft couplings,and motor mounts are eliminated in this compact design.

In applications for tank-mounted fire pumps, the impeller

is suspended near the bottom of the tank, and the motor orother prime mover is located above the cover. Between the

two is a vertical shaft placed within a discharge pipe. NFPAcalls these pumps vertical lineshaft turbine pumps. Flexibility

in their design includes multistaging, a wide range of tankdepths, and several types of prime movers.

Water Circulation PumpsMaintaining adequate water temperature in plumbing is

achieved through circulation pumps. Applicable generally forhot water, but equally effective for chilled water to drinking

fountains served by a remote chiller, the circulation pumpmaintains a limited temperature change. Heat transfer from

hot water distribution piping to the surrounding space isquantified for each part of the distribution network. Fora selected temperature drop from the hot water source to

the remote ends of the distribution, an adequate flow in thecirculation can be determined from Equation 4-9. Since the

nature of circulation is as if it were a closed system, pumphead is simply the friction losses associated with the circula-tion flow.

Equation 4-9

Q

=

q [q

]500T 4,187T

where Q =

Flow, gpm (L/s) q =Heat transfer rate, Btuh (W) T =Temperature difference, F (C)

For example, if it is determined that 1,000 Btuh transfers

from a length of hot water piping and no more than 8F isacceptable for a loss in the hot water temperature, the flow

is determined to be 1,000/(5008)=0.250 gpm. In SI, ifit is determined that 293 W transfers from a length of hot

water piping and no more than 4.4C is acceptable for a lossin the hot water temperature, the flow is determined to be293/(4,187

4.4)

=

0.0159 L/s.

Drainage PumpsWhere the elevation of the municipal sewer is insufficientor if another elevation shortfall occurs, pumps are added

to a drainage system. The issue may apply only to onefixture, one floor, or the entire building. Elevation issuesusually apply to subsoil drainage, so this water is also

pumped. Lastly, if backflow is intolerable from floor drainsin a high-value occupancy, pumps are provided for the floor

drains.The terminology varies to describe these pumps, but typi-

cal names include sewage pump, sump pump, sewage ejector,lift station pump, effluent pump, bilge pump, non-clog pump,

drain water pump, solids-handling sewage pump, grindpump, dewatering pump, and wastewater pump.

Drainage pumps generally have vertical shafts, cylindcal basins, and indoor or outdoor locations. Some pum

are designed to be submerged in the inlet basin, othersa dry pit adjacent to the basin, and in others the motor

mounted above with only the pump casing and impelsubmerged. In any design, provision is required for airenter or leave the basin as the water level varies.

The nature of solids and other contaminants in the wathrough these pumps necessitates several types of pum

designs. For minimal contaminants, the design may be wan enclosed impeller, wear rings, and clearance dimensio

that allow -inch (19-mm) diameter spheres to pass througSuch a pump may be suitable for subsoil drainage or graywater pumping.

For drainage flows from water closets and similar fitures, manufacturers provide pumps of two designs. O

design uses an open recessed impeller, no wear rings, aclearance dimensions that allow 2-inch (50-mm) diame

spheres to pass through.The other, referred toas a grinder pump (see

Figure 4-10), places aset of rotating cutting

blades upstream of theimpeller inlet, which

slice solid contaminantsas they pass througha ring that has acute

edges. Efficiency is com-promised in both types

for the sake of effectivewaste transport, in the

latter more so than inthe former, but withthe benefit of a reduced

pipe diameter in the dis-charge piping. Grinder

pumps are available incentrifugal and positive-

displacement types.The installation of a

pump in a sanitary drain

system includes a sealedbasin and some vent

piping to the exterior or

to a vent stack. In somecases, the pump can beabove the water level,

but only if a reliableprovision is includedin the design to prime

the pump prior to eachpumping event.

Figure 4-9 Multistage or VerticaLineshaft Turbine Pump

Photo courtesy of Peerless Pump Co.

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PUMP MAINTENANCEThe selection of a pump includes factors such as the needto monitor, repair, or replace the pump. Pumps in acces-

sible locations can readily be monitored. Sensors on remotepumps, such as seal leak probes and bearing vibration sen-

sors, assist in pump monitoring to prevent a catastrophicpump failure.

Pump maintenance can be facilitated when disassemblyrequires minimal disturbance of piping or wiring. Disas-

sembly may be with the casing split horizontally along ahorizontal shaft or with the casing split perpendicularly tothe shaft. The latter allows impeller replacement without

disturbing the pipe connection to the pump body.Complete pump replacement can be facilitated with

adequate access, shutoff valves, nearby motor disconnects,minimal mounting fasteners, direct mounting of the motor onthe pump housing (close-coupled pump), and pipe joints with

bolted fasteners. A simpler arrangement, commonly used forsubmersible drainage pumps, allows removal of the pump

from the basin by merely lifting a chain to extract it. Thelift or return is facilitated by special guide rails, a discharge

connection joint held tight by the weight of the pump, and

a flexible power cable.

ENVIRONMENTAL CONCERNSIn addition to any concerns about how a pump may affect the en-vironment, the environment may affect the design requirement

for a pump. An example of the former is a provision in an oil-

filled submersible pump to detect an oil leak, such as a pro

in the space between the shaft seals that signals a breachthe lower seal. Another example is vibration isolation fopump located near sensitive equipment.

The external environment can affect a pump in maways. For instance, a sewage ejector may be subjected

methane gas, causing a potential explosion hazard. Lof power is a common concern, as are abrasive or corros

conditions. The former can be prevented with the inclusi

of a parallel pump powered by a separate battery, and corrmaterial selection can help prevent the latter. Other examp

include the temperature of the water through the pump, ttemperature of the air around the pump, and the nature

any contaminants in the water. Sand and metal shavings arconcern with grinder pumps as they can erode the blade

PUMP CONTROLSPump controls vary with the application. A small simpsump pump may have a self-contained motor overlo

control, one external float switch, an electric plug, and control panel. A larger pump may have a control panel w

a motor controller, run indicator light, hand-off auto swit

run timer, audio/visual alarm for system faults, and buildiautomation system interface.

A control panel should be certified as complying with oor more safety standards, and the panel housing should

classified to match its installation environment. Motor cotrol generally includes an electric power disconnect and t

related control wiring, such as power-interrupting contragainst motor overload, under-voltage, or over-current.

The largest pumps often include reduced-voltage starteDuplex and triplex pump arrangements include these cont

features for each pump as well as an alternator device thalternates which pump first operates on rising demandmicroprocessor may be economically chosen for applicatio

involving at least a dozen sensor inputs.A booster pump has additional controls such as l

flow, low suction pressure, high discharge pressure, a ticlock for an occupancy schedule and possibly a speed cont

such as a variable-frequency drive.A circulation pump may include a temperature sensor th

shuts down the pump if it senses high temperature in t

return flow, which presumably indicates adequate hot wain each distribution branch. A time clock for an occupan

schedule shuts down the pump during off hours.The controls for a fire pump may include an automa

transfer between two power sources, engine control if plicable, and pressure maintenance through a secondapump, which is called a jockey pump. The control of a dra

age pump includes one or more float switches and possiblhigh water alarm.

INSTALLATIONPumping effectiveness and efficiency require uniform veloc

distribution across the pipe diameter or basin dimensionsthe pump inlet. An elbow, increaser with a sudden diamechange, check valve, and any other flow disturbance at t

pump inlet create an irregular velocity profile that reduthe flow and possibly the discharge head. To avoid air entra

Figure 4-10 Cross-Section of a Grinder Pump withCutting Blades at the Inlet

Photo courtesy of Ebara.



10/12

ment, eccentric reducers with the straight side up are used

on inlet piping rather than concentric reducers.In addition to shutoff valves, pump installations may

include drain ports, pressure gauges, automatic or manual

air release vents, and vibration isolation couplings. Pressuregauges upstream and downstream of the pump allow easy

indication of the rated pump performance. Check valves areprovided for each pump of duplex and similar multiple-pump

arrangements, fire pumps, and circulation pumps.

A fire pump includes provisions for periodic flow testing.Fire pumps also may include a pressure relief valve if low

flows create high heads that exceed pipe material ratings.A pump requires a minimum pressure at its inlet to avoid

cavitation. Destructive effects occur when a low absolutepressure at the entry to the impeller causes the water to

vaporize and then collapse further into the impeller. The re-sulting shock wave erodes the impeller, housing, and seals andoverloads the bearings and the shaft. The pockets of water

vapor also block water flow, which reduces the pumps capac-ity. Cavitation can be avoided by verifying Equation 4-10.

Equation 4-10

hrha hv+hs hf

where hr=Net positive suction head required (obtained from

the pump manufacturer), feet (m) ha=Local ambient atmospheric pressure converted to

feet (m) of water hv=Vapor pressure of water at applicable temperature,

feet (m) hs=Suction head (negative value for suction lift), feet

(m) hf=Friction head of piping between pump and where hs

is measured, feet (m)

Increasing hsresolves most issues regarding cavitation,

generally by mounting the pump impeller as low as pos-sible. Note that hrvaries with flow and impeller diameter:

ha=33.96 feet (10.3 m) for an ambient of 14.7 pounds persquare inch (psi) (101 kPa) and hv=0.592 feet (0.180 m) for

water at 60F (15.5C). Suction head, hs, may be the inletpressure converted to head, but it also may be the vertical dis-

tance from the impeller centerline to the surface of the waterat the inlet. The ambient head, ha, also may need adjustingfor sewage pumps, with the basin connected to an excessively

long vent pipe. Reciprocating positive-displacement pumpshave an additional acceleration head associated with keeping

the liquid filled behind the receding piston.Submergence is a consideration for pumps joined near or

in a reservoir or basin. A shallow distance from the pumpinlet to the surface of the water may create a vortex forma-

tion that introduces air into the pump unless the reservoirexit is protected by a wide plate directly above. In additionto lost flow capacity, a vortex may cause flow imbalance and

other harm to the pump. To prevent these problems, thebasin can be made deeper to mount the pump lower, and the

elevation of the water surface can be unchanged to keep thesame total head.

Redundancy can be considered for any pump ap-plication. The aggregate capacity of a set of pumpsmay exceed the peak demand by any amount;

however, the summation for centrifugal pumps

volves adding the flow at each head to create a composperformance curve. Discretion is further made to the amouof redundancy, whether for each duplex pump at 100 perce

of demand or each triplex pump at 40 percent, 50 percent67 percent. For efficiencys sake, a mix may be considered

a triplex, such as 40 percent for two pumps and 20 percefor the third pump.

GLOSSARY

Available net positive suction head The inherent ener

in a liquid at the suction connection of a pump.

Axial flowWhen most of the pressure is developed by t

propelling or lifting action of the vanes on the liquid. Tflow enters axially and discharges nearly axially.

Bernoullis theorem When the sum of three types of ergy (heads) at any point in a system is the same in a

other point in the system, assuming no friction lossesthe performance of extra work.

Brake horsepower (BHP)The total power required bypump to do a specified amount of work.

Capacity coefficient The ratio of the radial velocity oliquid at the impeller to the velocity of the impellers t

Churn The maximum static head of a pumptypically thead when all flow is blocked.

Design working headThe head that must be availablethe system at a specified location to satisfy design requiments.

DiffuserA point just before the tongue of a pump casi

where all the liquid has been discharged from the impelIt is the final outlet of the pump.

Flat head curveWhen the head rises slightly as the flis reduced. As with steepness, the magnitude of flatneis a relative term.

Friction headThe rubbing of water particles against eaother and against the walls of a pipe, which causes a pr

sure loss in the flow line.

Head The energy of a fluid at any particular point of a fl

stream per weight of the fluid, generally measured in f(meters).

Head coefficient Pump head divided by the square of tvelocity of the impeller tip.

HorsepowerThe power delivered while doing work at t

rate of 500 foot-pounds per second or 33,000 foot-pounper minute.

Independent headHead that does not change with flo

such as static head and minimum pressure at the enda system.

Mechanical efficiencyThe ratio of power output to powinput.

Mixed flowWhen pressure is developed partly by centrifuforce and partly by the lift of the vanes on the liquid. T

10 Read, Learn, Earn

JANUARY 2013



11/12

flow enters axially and discharges in an axial and radial

direction.

Multistage pumpsWhen two or more impellers and casings

are assembled on one shaft as a single unit. The dischargefrom the first stage enters the suction of the second and soon. The capacity is the rating of one stage, and the pressure

rating is the sum of the pressure ratings of the individualstages, minus a small head loss.

Net positive suction head (NPSH) Static head, velocityhead, and equivalent atmospheric head at a pump inlet

minus the absolute vapor pressure of the liquid beingpumped.

PackingA soft semi-plastic material cut in rings and snuglyfit around the shaft or shaft sleeve.

Potential headAn energy position measured by the workpossible in a decreasing vertical distance.

Pumps in parallelAn arrangement in which the headfor each pump equals the system head and the sum of the

individual pump capacities equals the system flow rate atthe system head.

Pumps in seriesAn arrangement in which the total head/capacity characteristic curve for two pumps in series can

be obtained by adding the total heads of the individualpumps for various capacities.

Pump performance curve A graphical illustration ofhead horsepower, efficiency, and net positive suction head

required for proper pump operation.

Radial flowWhen pressure is developed principally by cen-

trifugal force action. Liquid normally enters the impellerat the hub and flows radially to the periphery.

Required NPSHThe energy in a liquid that a pump must

have to operate satisfactorily.Shutoff BHPOne-half of the full load brake horsepower.

SlipA loss in delivery due to the escape of liquid inside a

pump from discharge to suction.

Specific speedAn index relating pump speed, flow, and head

used to select an optimal pump impeller.

StandpipeA theoretical vertical pipe placed at any point in

a piping system so that the static head can be identified byobserving the elevation of the free surface of the liquid in

the vertical pipe. The connection of the standpipe to thepiping system for a static head reading is perpendicular tothe general flow stream.

Static headThe elevation of water in a standpipe relativeto the centerline of a piping system. Any pressure gauge

reading can be converted to static head if the density ofthe liquid is known.

Static pressure headThe energy per pound due to pressure.The height a liquid can be raised by a given pressure.

Static suction head The vertical distance from the freesurface of a liquid to the pump datum when the supply

source is above the pump.

Static suction lift The vertical distance from the f

surface of a liquid to the pump datum when the supsource is below the pump.

Steep head curveWhen the head rises steeply and continously as the flow is reduced.

Suction head The static head near the inlet of a pump abothe pump centerline.

Suction lift In contrast to suction head, this vertical dim

sion is between the pump centerline and a liquids surfathat is below the pump.

System head curveA plot of system head versus syst

flow. System head varies with flow since friction and velity head are both a function of flow.

Total discharge head The sum of static head and velochead at a pump discharge.

Utility horsepower (UHP)Brake horsepower divided drive efficiency.

Total head The total head at the pump discharge minsuction head or plus suction lift.

Variable-speed pressure booster pumpsA pump usedreduce power consumption to maintain a constant build

supply pressure by varying pump speeds through couplior mechanical devices.

Velocity headThe velocity portion of head with its unconverted to an equivalent static head.

Water horsepowerThe power required by a pump motfor pumping only.



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READ, LEARN, EARN: Pumps

CE Questions Pumps (CEU 195)

The energy a pump adds to a liquid is called what?1.horsepowera.headb.churnc.liftd.

In a _______ pump, the direction of discharge from the impeller2.radiates in a plane perpendicular to the shaft.

positive-displacementa.axialb.

centrifugalc. mixed-flowd.

Hydraulic losses are caused by _______.3.leaksa.turns in directionb.friction within the liquid through the pumpc.all of the aboved.

Which of the following helps minimize hydraulic losses?4.gradual diameter and direction changesa.placing barriers against short circuitsb.matching the impeller diameter to the pump casingc.all of the aboved.

The _______ of double-suction pumps helps prevent cavitation.5.slow inlet velocitya.small impellerb.

dry sealc.multiple vanesd.

Power is directly proportional to the cube of the velocity is the6._______ affinity law.

firsta.secondb.thirdc.fourthd.

The ________ is where the system head curve and the pump7.curve meet.

system head curvea.best efficiency pointb.system balance pointc.required NPSHd.

A pump with a _______ is advantageous when a high head is8.required in an economical pump design.

flat curvea.

steep curveb. vertical steepnessc.none of the aboved.

High-head pumps can be used for _______.9.high-rise water suppliesa.deep water wellsb.certain automatic standpipe systemsc.all of the aboved.

The curve for a fire pump must be10. not less than _______percent of the rated total head for _______ percent of the ratedflow.

80/150a.65/150b.35/150c.75/150d.

A _______ can reduce flow and possibly head.11.check valvea.elbowb.increaser with sudden diameter changec.all of the aboved.

_______ is the12. static head near the inlet of a pump above thepump centerline.

discharge heada.potential headb.suction headc.friction headd.

ASPE Read, Learn, Earn Continuing EducationYou may submit your answers to the following questions online at aspe.org/readlearnearn. If you score 90 percent or higher on the test,

you will be notified that you have earned 0.1 CEU, which can be applied toward CPD renewal or numerous regulatory-agency CE pro-

grams. (Please note that it is your responsibility to determine the acceptance policy of a particular agency.) CEU information will be kept

on file at the ASPE office for three years.

Notice for North Carolina Professional Engineers: State regulations for registered PEs in North Carolina now require you to complete ASPEs

online CEU validation form to be eligible for continuing education credits. After successfully completing this quiz, just visit ASPEs CEU Valida

tion Center at aspe.org/CEUValidationCenter.

Expiration date: Continuing education credit will be given for this examination throughJanuary 31, 2014.

Psd Ceu 195 jan13 Pumps

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