11

TurbomachineryTurbomachineryTurbo : latin prefix, means spinTurbo : latin prefix, means spin

Energy is extracted or supplied by a rotating Energy is extracted or supplied by a rotating shaftshaft

Not all pumps use rotating shaft so we can call Not all pumps use rotating shaft so we can call them fluid machine ( nowadays turbomachine them fluid machine ( nowadays turbomachine is used for all )is used for all )

22

Pump: adds energy to a Pump: adds energy to a fluid, resulting in an fluid, resulting in an increase in pressureincrease in pressure

( not necessarily increase ( not necessarily increase in velocity) across the in velocity) across the pump. pump.

Turbine: extracts energy Turbine: extracts energy from the fluid, resulting in from the fluid, resulting in a a decrease in pressure decrease in pressure

(not necessarily decrease (not necessarily decrease in velocity) across the in velocity) across the turbine.turbine.

33

General comments on the pipeline General comments on the pipeline designdesign

ease and cost of pumpingease and cost of pumping=f(=f(Design of pipeline Design of pipeline ))

-Design piping to minimize energy requirement -Design piping to minimize energy requirement of pumping ( especially energy losses due to of pumping ( especially energy losses due to friction and shock losses)friction and shock losses)

-Keep the total length of pipeline as short as -Keep the total length of pipeline as short as possiblepossible

44

maintain same pipe diameter, if changes are maintain same pipe diameter, if changes are unavoidable use reducing or expanding unions unavoidable use reducing or expanding unions rather than abrupt changes in diameter rather than abrupt changes in diameter

Use the correct type and minimum number of Use the correct type and minimum number of fittings. avoid using gate valve ( on-off valve) fittings. avoid using gate valve ( on-off valve) to regulate flow rateto regulate flow rate

Then select a suitable pumpThen select a suitable pump

55

Factors influencing the choice of the pumpFactors influencing the choice of the pump

1- 1- The quantity of liquid to be handledThe quantity of liquid to be handled: affects the size: affects the size and typeand type of the pump of the pump, , determines determines if parallel if parallel pumps pumps neededneeded. . 2- 2- The head against which the liquid is to be pumpedThe head against which the liquid is to be pumped .. determined by the difference in pressuredetermined by the difference in pressure and velocity and velocity, , the vertical height of the downstream and upstream the vertical height of the downstream and upstream reservoirs and by the frictional losses which occur in reservoirs and by the frictional losses which occur in the delivery line. the delivery line.

The suitability of a centrifugal pump and the The suitability of a centrifugal pump and the number of stages required will largely be determined number of stages required will largely be determined by this factorby this factor

( as D decreases, v increases, energy loss ( as D decreases, v increases, energy loss increases in turn expensive pumping….so optimum increases in turn expensive pumping….so optimum pipe diameter must be selected)pipe diameter must be selected)

66

3- 3- The nature of the liquid to be pumpedThe nature of the liquid to be pumped..

a)a) consistency ( viscosity)consistency ( viscosity) the viscosity largely determines the the viscosity largely determines the frictional losses and hence the power requiredfrictional losses and hence the power required

b) densityb) density c) temperaturec) temperature ( effects viscosity, pump material, cavitation ) ( effects viscosity, pump material, cavitation ) d) corrosive and/or erosive nature of fluidd) corrosive and/or erosive nature of fluid ( (The The corrosive corrosive

nature will determine the nature will determine the material of constructionmaterial of construction .With .With suspensions, the suspensions, the clearance clearance in the pump must be large in the pump must be large compared with the size of the particles.compared with the size of the particles.))

e)shear deformatione)shear deformation( fluid flow characteristic and crystal ( fluid flow characteristic and crystal habbit may change under high shear)habbit may change under high shear)

e) lubricating properties of fluide) lubricating properties of fluid ( for all metal rotary pumps ( for all metal rotary pumps requires fluid to be lubricant)requires fluid to be lubricant)

77

4- 4- The nature of power supplyThe nature of power supply.. ( though most ( though most pumps are coupled to electric motors either pumps are coupled to electric motors either directly or via pulleys and v-belts’ directly or via pulleys and v-belts’ reciprocating reciprocating pumpspumps can be also actuated by can be also actuated by steam or steam or compressed air)compressed air)

5-5- hygenic requirements.hygenic requirements. In the food industry In the food industry contamination must be quarded against all contamination must be quarded against all timestimes

88

CategoriesCategoriesFans, Blowers, and CompressorsFans, Blowers, and Compressors

– FansFans: : Low Low pressure gradientpressure gradient, High , High volume flow ratevolume flow rate.. Examples include ceiling fans and propellers.Examples include ceiling fans and propellers.

– BlowerBlower: : Medium Medium pressure gradientpressure gradient, Medium , Medium volume flow volume flow raterate.. (ie centrifugal blowers found in furnaces, leaf blowers, (ie centrifugal blowers found in furnaces, leaf blowers, and hair dryers.)and hair dryers.)

– CompressorCompressor: : High High pressure gradientpressure gradient, Low , Low volume flow volume flow raterate.. (ie air compressors for air tools, refrigerant (ie air compressors for air tools, refrigerant compressors for refrigerators)compressors for refrigerators)

99

Fans:Fans:

ΔΔP =P = from from 2 or 32 or 3”” of water up to about 0.5 psi. of water up to about 0.5 psi.

classified into three types: classified into three types:

The propeller typeThe propeller type :: electric fan electric fan

Plate fanPlate fan : 8 to 12 : 8 to 12 plate steel plate steel blades blades inside a casing. inside a casing. ((ΔΔP P from 0 from 0 to 5to 5”” water water))If If blades curved blades curved :: ΔΔP P up to 27 up to 27”” water. water.

The multi-blade fansThe multi-blade fans ΔΔP P from 0 to 5from 0 to 5”” water. water. (( much higher efficiencies much higher efficiencies and and deliver much larger volumes for deliver much larger volumes for a given size of drum than steel-plate fans.a given size of drum than steel-plate fans.))

1010

Blowers:Blowers:

Any pump of the rotary type Any pump of the rotary type

can be used as a blowercan be used as a blower

( ( two or three lobestwo or three lobes) )

ΔΔP P from from 0.5 to 10 psi.0.5 to 10 psi.

The appearance of centrifugal blower The appearance of centrifugal blower resembles a centrifugal pump, except that the resembles a centrifugal pump, except that the casing is narrowercasing is narrower and and larger impeller diameterlarger impeller diameter..

The operating speed The operating speed ≥≥ 3000 rpm3000 rpm..

1111

Compressors:Compressors:

process gas - up to process gas - up to 100 m100 m33/sec/sec at the inlet - to an outlet at the inlet - to an outlet pressure of pressure of 20 atm.20 atm. ( smaller capacity up to ( smaller capacity up to several hundred several hundred atmatmss. . )) cooling is needed on the high pressure units. cooling is needed on the high pressure units.

Axial flow machinesAxial flow machines : :up to 300 mup to 300 m33/sec, press of 2 to 10 atm./sec, press of 2 to 10 atm.

Rotary positive displacement compressorsRotary positive displacement compressors: : press to 6 atm. press to 6 atm.

Most compressorsMost compressors operating at discharge press above operating at discharge press above 3 atm3 atm are reciprocating positive displacement machines.are reciprocating positive displacement machines.

IfIf required compression ratio is greater than that can be required compression ratio is greater than that can be achieved in one cylinder, achieved in one cylinder, multistage compressorsmultistage compressors are used. are used.

The maximum pressure ratioThe maximum pressure ratio normally obtained normally obtained in a single in a single cylinder is 10cylinder is 10 atm but values above 6 are unusual atm but values above 6 are unusual

1212

Airflow through a centrifugal air compressor

1313

PUMPSPUMPS

-piston-diaphram

Centrifugal, dynamic, kinetic

-Helical screw-gear-lobe-peristaltic

RotaryReciprocating

Positive displacement

1414

CategoriesCategories

Positive-displacement machines Positive-displacement machines – Closed volume is used to squeeze or suck Closed volume is used to squeeze or suck

fluid. fluid. – Pump: human heartPump: human heart

Turbine: home water meterTurbine: home water meter

1515

All PD pumps not be allowed to pump against a closed delivery line since over pressure can develop.

A pressure relief valve should be incorporated in the discharge line to allow controlled release of any excessive pressure through mal operation.

1616

Reciprocating PD pumpsReciprocating PD pumpsPISTONPISTON pumps pumps-single cylinder( pulsating flow)-single cylinder( pulsating flow)-multi cylinder( smoother flow)-multi cylinder( smoother flow)-can handle high -can handle high µµ-develop high -develop high ΔΔPP-don’t use with abrasive fluids-don’t use with abrasive fluids-delivers accurately -delivers accurately

known volumes soknown volumes soused as used as metering pumpmetering pump

1717

1818

Diaphragm pumpDiaphragm pump

SimpleSimple

Limited Limited ΔΔP developmentP development

Check valves had to be usedCheck valves had to be used

InexpensiveInexpensive

Able to handle corrosive and abrasive fluidsAble to handle corrosive and abrasive fluids

Fluid is not in contactFluid is not in contact

with most of thewith most of the

moving partsmoving parts

1919

Rotary PD pumpsRotary PD pumps

2020

HELICAL SCREWHELICAL SCREW pump pump

Can handle very high pressureCan handle very high pressure

Abrasive liquids can be runAbrasive liquids can be run

Should not be run dryShould not be run dry

Ie. Extruders in Ie. Extruders in

food productionfood production

2121

Gear pumpsGear pumpsExternal gear pumpExternal gear pump

As # of teeth inreases flow gets smootherAs # of teeth inreases flow gets smoother

develop high develop high ΔΔP (up to 200 bar )P (up to 200 bar )

don’t use with abrasive fluidsdon’t use with abrasive fluids

High shearHigh shear

2222

Internal gear PD pumpsInternal gear PD pumps

-develop high ΔP (up to 200 bar )-Gentler than external gear PD-More expensive and complicated

2323

lobe pumplobe pump- can transfer fluid at flowrates up to 500 m3/hr- can deliver total heads of 20 bar.- easy to clean- can handle shear sensitive products- Most widely used PD pump for food aplications

2424

Peristaltic PumpPeristaltic Pump

-most hygenic pump available)

-can generate heads of up to 5m at flows of up to 10 m3/hr.--most gentle : Fragile blood cells are not damaged by this pump. Orange segments, wallnut pieces can be transferred.

  (another application: The EnTire Self-Inflating Tire system)

2525

Jet pumpsJet pumps: :

Bernoulli’s principle Bernoulli’s principle

no moving partsno moving parts

hin + vin2/2 = hout + vout

2/2

High velocity creates vacuum

2626

Air lift pumpAir lift pump Air is injected to the Air is injected to the

bottom of the vertical pipe.bottom of the vertical pipe.

The specific gravity of the The specific gravity of the water inside the pipe water inside the pipe becomes less than that becomes less than that outside the pipe.outside the pipe.

This difference transfers This difference transfers the liquid upward in the the liquid upward in the pipe. pipe.

simplesimple used for used for petroleum,petroleum,

handling of handling of hazardous hazardous fluidsfluids, the design of , the design of bioreactorsbioreactors, recycle , recycle aeration in sludgeaeration in sludge digestersdigesters..

2727

Geyser pumpGeyser pump

Geyser Pump was Geyser Pump was invented in 1999invented in 1999 Geyser Pump was Geyser Pump was developed to overcome developed to overcome weak points of airlift pumpweak points of airlift pump. .

2828

Following weekneses of airlift pump is preventedFollowing weekneses of airlift pump is prevented

2929

3030

Dynamic machines (centrifugal, kineticDynamic machines (centrifugal, kinetic) ) – No closed volumeNo closed volume. Instead, rotating blades supply . Instead, rotating blades supply

or extract energy.or extract energy.– Enclosed/Ducted PumpsEnclosed/Ducted Pumps: torpedo propulsor: torpedo propulsor

– Open PumpsOpen Pumps: propeller: propeller

or helicopter rotoror helicopter rotor

3131

– Enclosed TurbinesEnclosed Turbines: hydroturbine: hydroturbine

– Open TurbinesOpen Turbines: wind turbine: wind turbine

3232

The faster you spin,The faster you spin, the more water comes out the more water comes out the small hole,the small hole, the water the water is pressurized insideis pressurized inside t the cup using centrifugal he cup using centrifugal force in a similar force in a similar f fashion to ashion to a centrifugal pump.a centrifugal pump.

3333

How to select a centrifugal pumpHow to select a centrifugal pumpexpectexpecteded to deliver exactly the flow rate you require. to deliver exactly the flow rate you require.

The flow rateThe flow rate =f( =f( physical characteristics of your physical characteristics of your system suchsystem such::lengthlength,,size of the pipessize of the pipes, , elevation elevation difference difference ))

The pump manufacturer has no means of knowing The pump manufacturer has no means of knowing what thesewhat these constraints will be.constraints will be.

This is why This is why buying a centrifugal pumpbuying a centrifugal pump is is more complicatedmore complicated than than buying a buying a positive displacement pumppositive displacement pump which will which will provide its rated flow no matter whatprovide its rated flow no matter what system you install it in.system you install it in.

3434

Dynamic Pumps includeDynamic Pumps include – centrifugal pumpscentrifugal pumps: fluid enters : fluid enters

axially, and is discharged axially, and is discharged radially.radially.

– mixed--flow pumpsmixed--flow pumps:: fluid enters fluid enters axially, and leaves at an angle axially, and leaves at an angle between radially and axially.between radially and axially.

– axial pumpsaxial pumps:: fluid enters and fluid enters and leaves axially.leaves axially.

3535

Centrifugal PumpsCentrifugal Pumps

Snail--shaped scroll Snail--shaped scroll

Most common type of Most common type of pumppump: homes, autos, : homes, autos, industry. industry.

3636

Centrifugal PumpsCentrifugal Pumps

3737

Centrifugal Pumps: Blade Centrifugal Pumps: Blade DesignDesign

3838

Pump HeadPump Head

Net HeadNet Head

Water horsepowerWater horsepower( ( useful energy useful energy actually delivered to fluid)actually delivered to fluid)

Brake horsepowerBrake horsepower ( ( energy energy supplied to pump)supplied to pump)

Pump efficiencyPump efficiency

3939

Matching a Pump to a Piping SystemMatching a Pump to a Piping SystemPump-performance curvesPump-performance curves for a for a centrifugal pump (centrifugal pump (H=f(V), H=f(V), bhp=f(V), bhp=f(V), ηη=f(V))=f(V))

BEPBEP: best efficiency point: best efficiency pointH*, bhp*, V*H*, bhp*, V* correspond to BEP correspond to BEP

Shutoff headShutoff head: achieved by : achieved by closing outlet (closing outlet (VV=0),=0),

At this point At this point ηη=0( no flow)=0( no flow)

Free deliveryFree delivery: no load on : no load on system (system (HHrequiredrequired = 0) = 0)

At this point At this point ηη=0( pump is not =0( pump is not doing any work)doing any work)

4040

Suction pressureSuction pressure : : the pressure at the pump’s the pressure at the pump’s suction nozzle. suction nozzle. ((mostmost impimp. . press inside the pump.press inside the pump. ) )

If the If the suction pressure is inadequatesuction pressure is inadequate, it leads, it leads to to cavitation.cavitation.

pumps need a gauge at the suctionpumps need a gauge at the suction nozzle to nozzle to measure the pressure entering the pump.measure the pressure entering the pump.

Discharge pressureDischarge pressure =( suction press+press =( suction press+press developed by the pump)developed by the pump)

PressPress.. at the pump discharge nozzle as measured by at the pump discharge nozzle as measured by aa gauge.gauge.

4141

Flow rate depends on elevation difference or static headFlow rate depends on elevation difference or static head

his his velocity will be moderatevelocity will be moderate and correspond to the amount of and correspond to the amount ofeenergy he can supply to overcome the friction of the wheels on the road and nergy he can supply to overcome the friction of the wheels on the road and

the changethe change in elevation in elevation

4242

Negative static head increase flow rate.Negative static head increase flow rate.

4343

High static head decreases flow rate.High static head decreases flow rate.

4444

4545

Flow rate is limited by friction in the system Flow rate is limited by friction in the system when the static head is zero.when the static head is zero.

4646

Suction headSuction head

Suction liftSuction lift

Discharge headDischarge head

Total headTotal head

4747

4848

4949

5050

Matching a Pump to a Piping SystemMatching a Pump to a Piping System

Steady operating Steady operating point:point:

Energy equation:Energy equation:

5151

If we are lucky If we are lucky operating point is at or near BEPoperating point is at or near BEP of pump. In most cases pumps do not run at of pump. In most cases pumps do not run at their optimum efficiencytheir optimum efficiency

If If eff is a major concerneff is a major concern, , Select a new pumpSelect a new pump such that operating point as close as to BEP such that operating point as close as to BEP pointpoint

We can We can reach thisreach this sometimes sometimes by changing by changing rotational speed rotational speed

The most common situation is that an engineer The most common situation is that an engineer selects a pump heftier( more powerfulselects a pump heftier( more powerful) than ) than actually required. So actually required. So volumetric flow rate will be volumetric flow rate will be largerlarger than needed, than needed, a valve or a dampera valve or a damper is is used to decrease the flow rate to desired level used to decrease the flow rate to desired level

5252

What is the pump rating?What is the pump rating?

The manufacturer will The manufacturer will rate the pumprate the pump at its at its optimum optimum totaltotal head and flowhead and flow, this point is also known as the , this point is also known as the best efficiency point or best efficiency point or B.E.P.B.E.P.

At that flowAt that flow raterate: : most efficientmost efficient, , minimal vibration andminimal vibration and noise. noise.

the life of the pump will suffer if you operate too far the life of the pump will suffer if you operate too far away from its normalaway from its normal r rating. ating.

As a As a guidelineguideline, , aim for a variationaim for a variation of plus or minus of plus or minus 15% on total head15% on total head..

5353

Operating Point of a Fan in a Ventilation SystemOperating Point of a Fan in a Ventilation SystemA A local ventilation system local ventilation system ((hood and exhaust ducthood and exhaust duct) ) is used to remove air andis used to remove air and contaminants produced contaminants produced by a dry-cleaning operation .by a dry-cleaning operation . The ductThe duct is round and is constructed of galvanized is round and is constructed of galvanized steel with longitudinal seams andsteel with longitudinal seams and withwith joints every 30 in (0.76 m).joints every 30 in (0.76 m).

ID of the duct is ID of the duct is D D 9.06 in9.06 in (0.230 m), (0.230 m),itsits total length is total length is L L 44.0 ft (13.4 m 44.0 ft (13.4 m). There are). There are five five CD3-9 CD3-9 elbowelbows along the duct. The s along the duct. The equivalent roughnessequivalent roughness of thisof this duct is duct is 0.15 mm0.15 mm, and , and each elbow has a minor (local)each elbow has a minor (local) loss coefficient of loss coefficient of KLKL CC00 0.21. 0.21. the minimum required volume flow rate through the ductthe minimum required volume flow rate through the duct is is VV 600 600 cfmcfm, or 0.283 m, or 0.283 m33/s at 25C. /s at 25C. hood entry loss coefficient as 1.3hood entry loss coefficient as 1.3 based on duct based on duct velocity.velocity.When the When the damperdamper is fully open, its loss coefficient is is fully open, its loss coefficient is 1.81.8. . A A centrifugalcentrifugal fanfan with with 9.0-in inlet and outlet diameters9.0-in inlet and outlet diameters is is available. Its performanceavailable. Its performance data are shown in Table 14–1data are shown in Table 14–1,, as listed by the manufacturer.as listed by the manufacturer. Predict Predict the operating point the operating point of this local ventilation system, and of this local ventilation system, and draw a plot ofdraw a plot of required required and available fan pressure rise as functions of volumeand available fan pressure rise as functions of volume flow rateflow rate Is Is the the chosen fan adequate?chosen fan adequate?

5454PP11=P=P22=P=Patmatm , v , v11=0 , z=0 , z22-z-z11=0=0

5555

5656

5757

Selection of Pump Impeller SizeSelection of Pump Impeller SizeA washing operation at a power plant requires A washing operation at a power plant requires 370 370 gallons per minute (gallons per minute (gpm)gpm) of water.of water.

The required The required net head is about 24 ft at this flow rate.net head is about 24 ft at this flow rate.

A newlyA newly hired engineer looks through some catalogs hired engineer looks through some catalogs and decides to purchase theand decides to purchase the 8.25-in impeller8.25-in impeller option of option of the Taco Model 4013 FI Series centrifugal pumpthe Taco Model 4013 FI Series centrifugal pump of of Fig. 14–15.Fig. 14–15.If the pump operates If the pump operates at 1160 rpm,at 1160 rpm, as specified in the as specified in the performanceperformance plot, she reasons, plot, she reasons, its performance curve its performance curve intersects 370 gpm at intersects 370 gpm at HH 24 ft. 24 ft. The chief engineer, who is very concerned about The chief engineer, who is very concerned about efficiency, glancesefficiency, glances at the performance curves and at the performance curves and notes that the notes that the efficiency of this pumpefficiency of this pump at this at this operating operating point is only point is only 70 percent70 percent..

5858

He sees that the He sees that the 12.75-in impeller12.75-in impeller option achieves a option achieves a higher efficiencyhigher efficiency (about (about 76.5 percent76.5 percent) at the same flow) at the same flow rate. rate. He notes that a He notes that a throttle valvethrottle valve can be installed can be installed downstream of the pumpdownstream of the pump to to increase the required net increase the required net headhead so that the pump operates at this higher so that the pump operates at this higher efficiency.efficiency.He asks the junior engineer He asks the junior engineer to justify her choice of to justify her choice of impeller diameterimpeller diameter..Namely, he asks her to calculate Namely, he asks her to calculate which impellerwhich impeller option option ((8.25-in or8.25-in or 12.75-in12.75-in) would ) would need the least amount of need the least amount of electricityelectricity to operate (Fig. to operate (Fig. 14–16). 14–16).

Perform the comparison and discuss.Perform the comparison and discuss.

5959

6060

Manufacturer Performance PlotManufacturer Performance Plot

1160 rpm, 370 gpm, 8.25” impeller diameter corresponds, 3.20 bhp, head 24 ft, 1160 rpm, 370 gpm, 8.25” impeller diameter corresponds, 3.20 bhp, head 24 ft, ηη==70%70%1160 rpm,370 gpm,12.75 “impeller diameter corresponds, 8.78 bhp, head 72 ft, 1160 rpm,370 gpm,12.75 “impeller diameter corresponds, 8.78 bhp, head 72 ft, ηη=76.5%=76.5%

6161

6262

Cavitation can be termed as “the Cavitation can be termed as “the heart attack of the pump”.heart attack of the pump”.

6363

Pump Cavitation and NPSHPump Cavitation and NPSH

Cavitation should be avoidedCavitation should be avoided due due to to erosion damage and noiseerosion damage and noise..

Cavitation occurs when P < PCavitation occurs when P < Pvv

Net positive suction headNet positive suction head

NPSHNPSHrequiredrequired curves are created curves are created

through systematic testing over a through systematic testing over a range of flow rates range of flow rates VV..

6464

Definition Definition NPSHa NPSHa (available)(available)

This is the This is the energy in the fluid at the suction energy in the fluid at the suction connection of the pumpconnection of the pump over and above the over and above the liquid’s vapor pressure. liquid’s vapor pressure.

It is a characteristic of theIt is a characteristic of the system and wesystem and we say that the NPSHa should be greater than the say that the NPSHa should be greater than the NPSHrNPSHr

(NPSHa > NPSHr(NPSHa > NPSHr +3 ft +3 ft). ).

6565

Definition of NPSHr (required)Definition of NPSHr (required)

It is a It is a characteristic of the pumpcharacteristic of the pump and is indicated on the and is indicated on the pump's curve. pump's curve. It varies by It varies by design, size, and the operating conditionsdesign, size, and the operating conditions

An easy way to understand NPSHr is to call it the minimum An easy way to understand NPSHr is to call it the minimum suction pressure necessary to keep the pumped fluid in a liquid suction pressure necessary to keep the pumped fluid in a liquid state.state.

According to the Standards of the Hydraulic Institute, According to the Standards of the Hydraulic Institute, a suction lift testa suction lift test is performed on the pump and the pressure in the suction vessel isis performed on the pump and the pressure in the suction vessel is lowered to the point where the lowered to the point where the pump suffers a pump suffers a 3% 3% loss in total head.loss in total head.

This point is called the NPSHr of the pumpThis point is called the NPSHr of the pump. .

6666

To increase NPSHaTo increase NPSHa

1- raise the level in the suction vessel1- raise the level in the suction vessel

2- elevate the suction vessel2- elevate the suction vessel

3- lower the pump3- lower the pump

4-reduce the friction in the suction piping4-reduce the friction in the suction piping

5-lower the temp of the fluid in the suction 5-lower the temp of the fluid in the suction vesselvessel

6-pressurise the suction vessel6-pressurise the suction vessel

6767

(EXAMPLE) Maximum Flow Rate to Avoid Pump Cavitation

The 11.25-in impeller option of the Taco Model 4013 FI Series centrifugal pump of Fig. 14–15 is used to pump water at 25C from a reservoir whose surface is 4.0 ft above the centerline of the pump inlet.

The piping system from the reservoir to the pump consists of 10.5 ft of cast iron pipe with an ID of 4.0 in and an average inner roughness height of 0.02 in.

6868

There are several minor losses:

a sharp-edged inlet (KL 0.5),

three flanged smooth 90 regular elbows (KL 0.3 each),

and a fully open flanged globe valve (KL 6.0).

Estimate the maximum volume flow rate (in units of gpm) that can be pumped without cavitation.

If the water were warmer, would this maximum flow rate increase or decrease? Why? Discuss how you might increase the maximum flow rate while still avoiding cavitation.

6969

At 400gpm NPSHr=4 ftAt 400gpm NPSHr=4 ft

7070

Assumptions 1 The flow is steady. 2 The liquid is incompressible.

Properties For water at T = 25 C, ρ =997.0 kg/m3, µ=8.91x10-4 kg/m·s, and Pv= 3.169 kPa. Standard atmospheric pressure is Patm 101.3 kPa.

We apply the steady energy equation in head form along a streamline from point 1 at the reservoir surface to point 2 at the pump inlet,

7171

Insert eq 2

Since we know Patm, Pv, and the elevation difference, all that remains is to estimate the total irreversible head loss through the piping system, which depends on volume flow rate. Since the pipe diameter is constant,

For a given volume flow rate, we calculate speed V and Reynolds number Re. From Re and the known pipe roughness, we use the Moody chart (or the Colebrook equation)to obtain friction factor f. The sum of all the minor loss coefficients is

7272

Since the actual NPSH is much higher than this, we need not worry about cavitation atthis flow rate. We use a spreadsheet to calculate NPSH as a function of volume flow rate, and

the results are plotted in Fig. 14–21. It is clear from this plot that at 25C, cavitation occurs at flow rates above approximately 602 gpm—close to the free delivery.

7373

If the water were warmer than 25C, the vapor pressure would increase, the viscosity would decrease, and the density would decrease slightly.

where we see that the maximum volume flow rate without cavitation decreases with temperature (to about 555 gpm at 60C). This decrease agrees with our intuition, since warmer water is already closer to its boiling point from the start.

7474

General Symptoms of Cavitation General Symptoms of Cavitation

and its Affects on Pump Performance and its Affects on Pump Performance

and Pump Partsand Pump Parts

           

  Reduction in capacity of the pumpReduction in capacity of the pump::

The The formation of bubbles causes a volume formation of bubbles causes a volume increase increase decreasing the sdecreasing the space available for pace available for the liquidthe liquid and thus diminish pumping capacity. and thus diminish pumping capacity. ((when water changes state from liquid to gas when water changes state from liquid to gas its volume increases by approximately its volume increases by approximately 1,700 1,700 timestimes.    .    

7575

Decrease in the head developedDecrease in the head developed::

BubblesBubbles are are compressiblecompressible. The head developed . The head developed diminishes drastically because diminishes drastically because energy has to energy has to be expended to increase the velocity of the be expended to increase the velocity of the liquidliquid used to fill up the cavities, as the bubbles used to fill up the cavities, as the bubbles collapse. collapse.

The Hydraulic Standards Institute defines The Hydraulic Standards Institute defines cavitation cavitation as as condition of 3 % drop in head condition of 3 % drop in head developeddeveloped across the pump across the pump

7676

Abnormal sound and vibrationsAbnormal sound and vibrations::

It is movement of bubbles with very high It is movement of bubbles with very high velocities from low-pressure area to a high-velocities from low-pressure area to a high-pressure area and subsequent collapse that pressure area and subsequent collapse that creates shockwavescreates shockwaves producing producing abnormal abnormal sounds and vibrationssounds and vibrations. .

It has been estimated that It has been estimated that during collapse of during collapse of bubblesbubbles the pressures of the order of the pressures of the order of 101044 atm atm developsdevelops..

The sound of cavitation can be described as The sound of cavitation can be described as similar to small hard particles or similar to small hard particles or gravel rapidly gravel rapidly striking or bouncing off the interior partsstriking or bouncing off the interior parts of a of a pump or valve. pump or valve.

7777

To distinguish between the noise due to a To distinguish between the noise due to a bad bearing or cavitation, operate the pump bad bearing or cavitation, operate the pump with no flow. The disappearance of noise with no flow. The disappearance of noise will be an indication of cavitationwill be an indication of cavitation..

The excessive vibration caused by cavitation The excessive vibration caused by cavitation often subsequently causes a failure of the often subsequently causes a failure of the pump’s pump’s seal and/or bearings.seal and/or bearings. This is the most This is the most likely failure mode of a cavitating pumplikely failure mode of a cavitating pump

7878

Damage to pump partsDamage to pump parts::Cavitation erosion or Cavitation erosion or pitting pitting

During cavitation, the collapse of the bubbles During cavitation, the collapse of the bubbles occurs at occurs at sonic speedsonic speed ejecting destructive ejecting destructive micro micro jetsjets of extremely high velocity (up to of extremely high velocity (up to 1000 m/s)1000 m/s) liquid strong enough to cause extreme erosion of liquid strong enough to cause extreme erosion of the pump parts, particularly impellers. the pump parts, particularly impellers.

High head pumps are more likely to suffer from High head pumps are more likely to suffer from cavitationcavitation erosion erosion..

The The most sensitive areasmost sensitive areas where cavitation where cavitation erosion has been observed are the erosion has been observed are the low-pressure low-pressure sides of the impeller vanessides of the impeller vanes near the inlet edge. near the inlet edge.

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Mechanical deformations Mechanical deformations

  longer duration of cavitationlonger duration of cavitation condition can result in condition can result in unbalancingunbalancing (due to un-equal distribution in bubble (due to un-equal distribution in bubble formation and collapse) of radial and axial thrusts on formation and collapse) of radial and axial thrusts on the impeller. leads to following mechanical problems:the impeller. leads to following mechanical problems:

·       ·       

    bending and deflection of shafts,bending and deflection of shafts,

· bearing damage and rubs from radial vibration,· bearing damage and rubs from radial vibration,

·  thrust bearing damage from axial movement, ·  thrust bearing damage from axial movement,

·   breaking of impeller check-nuts, ·   breaking of impeller check-nuts,

·   seal faces damage etc.·   seal faces damage etc.

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8181

We say that the We say that the motor generates horsepower motor generates horsepower (HP),(HP), and that the and that the pumppump consumes brake consumes brake horsepower (BHp).horsepower (BHp).

The The differencedifference between HP between HP (output) and BHp (output) and BHp (input) is what is (input) is what is lost in the power transmissionlost in the power transmission; ; thethe bearings, shaft, and coupling between the bearings, shaft, and coupling between the motor and the pumpmotor and the pump

8282

PUMP EFFICIENCYPUMP EFFICIENCY

8383

8484

the practical applications the practical applications centrifugal pumpscentrifugal pumps

The pump curve, the H-Q curve, is in a descending profileThe pump curve, the H-Q curve, is in a descending profile..

Q and H changes irreversibly Q and H changes irreversibly

At times, At times, in normal industrialin normal industrial production, the flow must rise and production, the flow must rise and fall, but the pressure or head mustfall, but the pressure or head must remain a constant.remain a constant.

Many industrial processes experience seasonal rises and falls. Many industrial processes experience seasonal rises and falls. This meansThis means that flow varies. that flow varies.

8585

The The pasteurization process for milkpasteurization process for milk and and ice ice creamcream requires requires heatingheating the the milk to a specified milk to a specified temperature and pressuretemperature and pressure for a specified for a specified time time to killto kill all germs and bacteria in the milk. all germs and bacteria in the milk.

This This pressure is constantpressure is constant although although the the production of milk and ice cream production of milk and ice cream goes goes up and up and down withdown with consumption and the seasonal consumption and the seasonal changes. changes.

8686

sterile watersterile water production to production to preparpreparee medications for medications for injectioninjection..

require boiling therequire boiling the water at 35 psi, and pumping the water at 35 psi, and pumping the water at 40 gpm to 70 gpm,water at 40 gpm to 70 gpm, according to consumption. according to consumption.

The 35 psi is a constant The 35 psi is a constant pressure needed pressure needed for the water for the water toto pass through the heat exchanger, and a bank of pass through the heat exchanger, and a bank of filters. filters.

byby varying the diameter of the impeller, varying the diameter of the impeller, so so that the that the pump can pump 40pump can pump 40 gpm at 35 psi, gpm at 35 psi, or or 50 gpm at 35 psi, 50 gpm at 35 psi, or 70 gpm at 35 psi. or 70 gpm at 35 psi.

This allows theThis allows the operator to operator to use use the same pump and the same pump and motor, and only change themotor, and only change the impeller diameter impeller diameter depending on the needs of production.depending on the needs of production.

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This preciseThis precise manipulation of pumping parameters manipulation of pumping parameters could not be obtained by openingcould not be obtained by opening and closing valvesand closing valves

A A pump consuming 10 BHP with a 10 inch impeller,pump consuming 10 BHP with a 10 inch impeller, would only consume 7.3 horses with a would only consume 7.3 horses with a 9 9 inch impeller.inch impeller.

This means a 10% reduction in the impeller diameter, This means a 10% reduction in the impeller diameter, would bringwould bring about almost 30% reduction in energy. about almost 30% reduction in energy.

These energy savings will easilyThese energy savings will easily cover the cost of cover the cost of multiple impellers and the manpower to change themmultiple impellers and the manpower to change them frequently.frequently.

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PARALLEL PUMPSPARALLEL PUMPS

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PUMPS IN SERIESPUMPS IN SERIES

9090

The third pump will not be able to open the check valve with two pumps keeping it closed. So in practice, you can operate any one pump, or any two pumps, or four pumps, but not three pumps. (12 gate valves and 2 check valves exist)

9191

Pump PipingPump Piping

9292

Level indicator.

Inadequate design , bubbles entrained

9393

9494

Prefer Y branch rather than TPrefer Y branch rather than Tto reduce turbulentto reduce turbulent

9595

Use an eccentric pipe reducerUse an eccentric pipe reducer