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Part 3 Turbomachinery - Middle East Technical University
Transcript of Part 3 Turbomachinery - Middle East Technical University
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ME 306 Fluid Mechanics II
Part 3
Turbomachinery
These presentations are prepared by
Dr. CΓΌneyt Sert
Department of Mechanical Engineering
Middle East Technical University
Ankara, Turkey
Please ask for permission before using them. You are NOT allowed to modify them.
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Fluid Machinery
β’ Fluid machinery is used to convert hydraulic energy to mechanical energy or vice versa.
Power absorbing
Work is done on the fluid
Mechanical Energy β Hydraulic Energy
Power producing
Work is done by the fluid
Hydraulic Energy βMechanical Energy
Pump Turbine
3-3
Classification of Fluid Machinery
β’ Fluid machinery can be classified based on the motion of moving parts.
1 ) Positive Displacement Machines
β’ Fluid is directed into a closed volume.
β’ Energy transfer is accomplished by movement of the boundary of the closed volume.
β’ Closed volume expands and contracts, sucking the fluid in or pushing it out.
Human heart
http://speakeasies.biz
Water well pump
http://www.bicycleaccessories.us
Tire pump
http://en.wikipedia.org
Gear pump
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Classification of Fluid Machinery (contβd)
2 ) Turbomachines
β’ Turbo means ββspinββ or ββwhirlββ in Latin.
β’ Turbomachines use rotating shafts with attached blades, vanes, buckets, etc.
β’ In ME 306 weβll study turbomachines, mostly pumps.
Axial fan
http://en.wikipedia.org
Centrifugal pump
http://www.noehill.com/nevada_county_california
/cal1012.asp
Pelton wheelKaplan type
hydraulic turbine
http://www.britannica.com
3-5
Classification of Turbomachines
β’ Pumps increase the pressure of a liquid without changing its velocity considerably.
β’ Shown centrifugal (radial) pump is the most common type.
β’ Visit www.standartpompa.com to get more information on sizes and capacities.
Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump
β’ Propellers are used to generate thrust.
β’ Marine propellers work with incompressible water and aircraft propellers work with compressible air.
β’ Pressure difference between the front and back surfaces of the blades create the thrust.
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Classification of Turbomachines (contβd)
Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller
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β’ The main difference between fans, blowers and compressors is the pressure difference they create.
β’ Fans create small pressure difference. Their main purpose is to put high amount of fluid into motion.
β’ Shown is axial fan of a wind tunnel.
Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller Fan
Classification of Turbomachines (contβd)
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Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller Fan Blower
β’ Blowers work with medium amout of flow rates and pressure ratios.
β’ They are mostly centrifugal type.
β’ Shown is an industrial type blower.
Classification of Turbomachines (contβd)
3-9
β’ Compressors work with smaller flow rates, but create very high pressure ratios.
β’ Shown is a multi-stage axial compressor.
β’ Compressors are used in gas and steam turbines, natural gas pumping stations, turbochargers, refrigeration cycles, etc.
Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller CompressorFan Blower
Classification of Turbomachines (contβd)
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Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller Compressor Pelton wheel
Fan Blower
β’ Pelton wheels have buckets attached to a rotating disk (wheel).
β’ They convert kinetic energy of a high speed liquid jet into mechanical energy.
β’ Largest ones used at hydraulic power plants have capacities up to 200 MW.
Classification of Turbomachines (contβd)
3-11
Turbomachines
Power absorbing Power producing
Incompressible Compressible Incompressible Compressible
Pump Propeller Compressor Pelton wheel
Hydraulic turbine
Fan Blower
β’ Hydraulic turbines are used at dams to generate electricity using high pressure water.
β’ Common types are Francis and Kaplan.
β’ Shown are the runner blades of the Francis turbines used at Three Gorges Dam / China.
β’ AtatΓΌrk Dam has a capacity of 8 x 300 MW.
Classification of Turbomachines (contβd)
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Turbomachines
Power absorbing Power producing
Incompressible Compressible
Steam turbine
Incompressible Compressible
Pump Propeller Compressor Pelton wheel
Hydraulic turbine
Fan Blower
β’ Steam turbines are used at power plants to generate electricity using high temperature and high pressure steam.
β’ 80 % of worldβs electricity is produced by steam turbines.
β’ AfΕin-Elbistan thermal power plant has a capacity of 4 x 344 MW.
Classification of Turbomachines (contβd)
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Turbomachines
Power absorbing Power producing
Incompressible Compressible
Steam turbine
Incompressible Compressible
Pump Propeller Compressor Pelton wheel
Hydraulic turbine
Gas turbine
Fan Blower
β’ Gas turbines are similar to steam turbines, but they use high temperature and high pressure combustion gases.
β’ A Boeing 777 is powered by 2 turbofan engines, each generting a thrust of ~500 kN.
β’ To learn how a turbofan engine operates visit http://www.youtube.com/watch?v=GpklBS3s7iU
Classification of Turbomachines (contβd)
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Turbomachines
Power absorbing Power producing
Incompressible Compressible
Steam turbine
Incompressible Compressible
Pump Propeller Compressor Pelton wheel
Hydraulic turbine
Gas turbine
Wind turbine
Fan Blower
β’ As of 2017 Turkeyβs wind energy production is 6 GW. Total available capacity is 48 GW.
β’ Worldβs total wind energy production is 490 GW, which is about 2.5 % of all electricity usage.
β’ There are wind turbines with more than 120 m rotor diameter, producing 6 MW of electricity (enough for 4500 homes) (http://www.youtube.com/watch?v=LQxp6QTjgJg)
Classification of Turbomachines (contβd)
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Turbomachines
Uncased Cased Impulse Reaction
Axial Flow Propeller
β’ Uncased turbomachines do not have a solid casing around them.
Another Classification of Turbomachines
Power absorbing Power producing
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Turbomachines
Uncased Cased Impulse Reaction
Axial Flow Propeller
Axial
β’ Fluid enters an axial flow turbomachine parallel to the axis of rotation.
β’ Fluid leaves the machine also in axial direction.
Another Classification of Turbomachines (contβd)
Power absorbing Power producing
In
Out
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Turbomachines
Uncased Cased Impulse Reaction
Axial Flow Propeller
Axial Radial
β’ In radial flow machines fluid intake is parallel to the axis of rotation.
β’ Rotating impeller blades push the fluid in radial direction.
β’ Fluid leaves the machine perpendicular to the rotation axis.
Another Classification of Turbomachines (contβd)
Power absorbing Power producing
InOut
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Turbomachines
Uncased Cased
Axial (Kaplan)
Impulse Reaction
Axial Flow Propeller
Mixed Pelton wheel
Wind turbine
Axial Radial
β’ Kaplan turbines are axial flow machines.
β’ They are preferred for low head and high flow rate configurations.
β’ Their capacities are less than Francis type, less than 200 MW.
β’ They can provide efficiencies higher than 95 %.
Another Classification of Turbomachines (contβd)
Power absorbing Power producing
InIn
OutOut
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Turbomachines
Uncased Cased
Axial (Kaplan)
Impulse Reaction
Axial Flow Propeller
Mixed Pelton wheel
Wind turbine
Radial(Banki)
Mixed (Francis)
Axial Radial
Another Classification of Turbomachines (contβd)
Power absorbing Power producing
β’ Francis turbines are the most widely used turbomachines for hydropower.
β’ They are of mixed flow type.
β’ They can provide more than 800 MW power.
β’ For more information http://www.voithhydro.com
Centrifugal Pump
β’ Centrifugal pump is the most commonly used type of turbomachine.
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Discharge(outflow)
Inflow
Eye
Casing, housing or volute
Blade
HubImpeller
Munson
3-21
Centrifugal Pump (contβd)
β’ For centrifugal pump details watch
http://www.youtube.com/watch?v=9nL1XhKm9q8 (Principles and parts)http://www.youtube.com/watch?v=Vq3hEe5jzSM (Pump Parts)http://www.youtube.com/watch?v=UrChdDwHybY (Impeller animation)http://www.youtube.com/watch?v=RvOzKhUDmJM (Computer aided blade design)
β’ Most important part is the impeller. It may have different designs such as
β’ Backward-curved, radial or forward-curved
β’ Closed (shrouded) or open
Called backward-curved if rotates in
this direction
Called forward-curved if rotates in this direction
Open
Closed (Shrouded)Open, radial
3-22
Pump Head (βπ )
β’ Consider the BE between the inlet (suction) and outlet (discharge) of a pump.
πππππ+πππ2
2π+ π§ππ =
πππ’π‘ππ+πππ’π‘2
2π+ π§ππ’π‘ β βπ
β’ Pump head is the difference between the total heads at the
pump inlet and outlet.
β’ Pump head is a positive quantity with units of length.
Pump head
πππ
πππ
π΄ππ
πππ’π‘
πππ’π‘
π΄ππ’π‘
π
π
π§
Datum
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Pump Head (contβd)
β’ Elevation difference between inlet and outlet is generally negligibly small.
β’ If suction and discharge pipe diameters are the same β πππ = πππ’π‘
β’ For this simplified case
βπ =πππ’π‘ β πππππ
=βπ
ππ
i.e. pump head is the pressure rise across the pump expressed as a head.
β’ Pump head is directly related to the power delivered to the fluid, known as water horsepower
π«π = πππ βπ
β’ Pump head can be defined as the power delivered to the fluid per weight of the fluid flowing through the pump in unit time (weight flow rate).
Weight flow rate
π for fluid
3-24
Theoretical Analysis of a Centrifugal Pump
Exercise: Consider the given schematic of a centrifugal pump. Perform a control volume analysis to derive a relation for the variation of the theoretical (ideal) pump head as a function of discharge (volumetric flow rate).
1 Inlet
2 Outlet
π1
π2
1
2
π
Blade
π½2
π½1
In
In
Out
Out
π
3-25
Theoretical Analysis of a Centrifugal Pump (contβd)
Exercise: Water is pumped at a rate of 5300 L/min through a centrifugal pump operating at 1750 rpm. The impeller has a uniform blade height, π, of 5 cm with π1 = 4 cm and π2 = 18 cm, and exit blade angle π½2 is 23o. Assume ideal flow conditions and the tangential velocity component, π1π, of the water entering the blade is zero. Determine
a) the tangential velocity component, π2π, at the blade exit.
b) the ideal head rise
c) the power transferred to the fluid.
Reference : Munson
3-26
Pump Efficiency
β’ Power necessary to run the pump (π«π), known as brake horsepower (bhp), is larger
than power delivered to the fluid due to
β’ mechanical and fluidic frictional losses
β’ flow separation on impeller blade surfaces
β’ misalignment of inlet flow velocity with impeller blade geometry
β’ internal leakage, etc.
π«π = ππ βπππ‘ π > π«π
β’ Efficiency of the pump is defined as
ππ =π«π
π«π< 1
Rotational speed of the pumpTorque supplied to
the pump shaft
π for pump
π«π (π«π β) : Pump power. Also called shaft
power. Power input to the pump.
π«π : Internal power
π«π : Fluid power. Power delivered to the fluid.
π«π : Mechanical power loss at bearings, etc.
βπ : Hydraulic friction loss (head loss)
ππΏ : Leakage loss
ππ : Mechanical efficiency
ππ = πβππ£ : Internal efficiency
(Hydraulic eff. x Volumetric eff.)
ππ = πππβππ£ : Pump (overall) efficiency
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Pump Efficiency (contβd)
ππ =π«ππ«π
ππ =π«π
π«π= πβππ£
ππ =π«π
π«π
π«π βπ ππΏ
π«ππ«ππ«π (π«π β)
Pump
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Important Parameters of a Centrifugal Pump
β’ Volumetric flow rate (discharge, capacity) π
β’ Head βπ (or simply β)
β’ Size (impeller diameter) π·
β’ Rotational speed π [rpm] or π [s-1]
β’ Power consumption π«π
β’ Efficiency ππ (or simply π)
β’ Fundamental characteristic curve of a pump is a plot of β vs. π at a given rotational speed π.
β’ It is customary to plot π«π and π on the same figure.
π
β π
π«π
3-29
Pump Characteristic Curve
β’ β β π curve of a pump is known as its characteristic curve.
β’ A pump can operate only on its characteristic curve.
β’ At a given rotational speed (π) a typical centrifugal pump characteristic curve is
Shutoff head
β’ Discharge valve of the pump is closed and π = 0.
β’ Pump is not doing any useful work.
β’ π«π = 0 & π = 0
Free delivery
β’ There is no load on the pump and β = 0.
β’ Pump is not doing any useful work
β’ π«π = 0 & π = 0
π
β
Note : This curve is for a given rotational speed.
3-30
Best Efficiency Point (BEP)
β’ The exact operation point of a pump depends on the system it is working in.
β’ Pumps are designed to work at (or close to) their maximum efficiency, but this is not always possible.
Note: All these curves are for a given rotational speed π.
Best Efficiency Point (BEP) (or design point)
π
Head (β)
Efficiency (π)
Power (π«π)
π for maximum efficiency
β for maximum efficiency
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System Characteristic
β’ A pump works at an operating point on its characteristic curve.
β’ But this operating point depends on the system that the pump is installed in.
β’ Following pump works between a suction reservoir (π π) and a discharge reservoir (ππ).
β’ BE between points π π and ππ
ππ πππ+ππ π2
2π+ π§π π =
πππππ+πππ2
2π+ π§ππ β β + βπ
β’ If ππ π = πππ = πππ‘π and ππ π = πππ = 0
β = βππ‘ + βπ
Total major and minor losses
Total geometric head (= π§ππ β π§π π)
ππ π
πππ
βππ‘
Pump
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System Characteristic (contβd)
β’ βπ consists of major and minor losses calculated as
βπ =
π
πππΏππ·π
ππ2
2π+
π
ππππ2
2π
β’ ππβs are average velocities in suction and discharge pipes.
β’ Using the continuity eqution ππ = π/π΄π
and the total loss becomes
βπ =
π
πππΏππ·π
π2
2ππ΄π2 +
π
πππ2
2ππ΄π2
or simply βπ = πΎπ2
Friction factor
Actual or equivalent pipe length
Head loss coefficient for minor losses
Pipe diameter
β’ Using this βπ in the BE we get the following system characteristic equation
β’ Minimum head the pump should provide is equal to the total geometric head.
β’ Additional pump head is necessary to overcome frictional losses. This part increaseswith the square of the flow rate.
3-33
System Characteristic (contβd)
π
β
βππ‘
βππ‘ + πΎπ2
β = βππ‘ + πΎπ2
β’ It is possible to change system characteristic in two ways.
3-34
System Characteristic (contβd)
Total geometric head (βππ‘ , the
difference between reservoir levels)
can be changed.
Friction loss can be changed by
changing πΎ. For example by opening
or closing a valve.
π
β
βππ‘ increases
βππ‘1 + πΎπ2
βππ‘2 + πΎπ2
βππ‘3+ πΎπ2
π
β
πΎ increases
βππ‘ + πΎ1π2
βππ‘ +πΎ2π2
βππ‘ +πΎ3π2
β’ A pump installed on a system will not work at an arbitrary point.
β’ It will operate at the point where pump and system characteristics intersect.
3-35
Operating Point
π
β
Pump characteristic(Supply curve)
System characteristic(Demand curve)
βππ
Operating point
πππ
β’ Normally we want the operating point to be close to the BEP (design point).
β’ However BEP is not always the most economical operating point as far as the power consumption is concerned, i.e. BEP is not necessarily the minimum π«π point.
Exercise: Water is pumped from one large open tank to a second one. The pipe diameter throughout is 15 cm and the total length of the pipe between the pipe entrance and the exit is 60 m. Minor loss coefficients for the entrance, exit and the elbow are shown and the friction factor for the pipe can be taken as 0.02. A certain centrifugal pump with the shown characteristic is suggested for this flow system. With this pump, what would be the flow rate? Do you think this pump is a good choice? (Reference: Munson)
3-36
Operating Point (contβd)
100
80
60
40
20
00 1.5 3.0 4.5 6.0 7.5 9.0
Flow rate [m3/min]
30
24
18
12
6
0
Hea
d [
m]
Effi
cien
cy [
%]
3 m
ππππππ‘ = 0.5
πππ₯ππ‘ = 1.0
ππππππ€ = 1.5
Pump
3-37
Similarity Laws for Pumps
β’ Similitude analysis is typically used
β’ to predict the performance of a pump when a different sized impeller is used in the same casing.
β’ to predict the performance of a pump when it operates at a different speed.
β’ Perform a Buckingham-Pi analysis with the following parameters
π , πβ , π· , π«π , π , π , π
to get the following non-dimensional groups
Flow coefficient : Head coefficient :
Power coefficient : Reynolds number :
ππ =π
ππ·3πβ =
πβ
π2π·2
ππ« =π«π
ππ3π·5ππ =
π
πππ·2=1
π π
3-38
Similarity Laws for Pumps (contβd)
β’ In most pump applications similarity of viscous forces are not as important as the other π groups.
β’ Two geometrically similar pumps are said to be operating under similar conditions if the remaining three π groups are equal.
ππ1 = ππ2 , πβ1 = πβ2 , ππ«1 = ππ«2
where 1 and 2 are two similar operating points (homologous points).
Exercise : Show that when affinity laws are satisfied, efficiencies of two homologous points are equal.
Exercise : How does the nondimensional performance curve (πβ vs. ππ) of two
geometrically similar pumps compare with each other? What about ππ« vs. ππ and
π vs. ππ curves?
Affinity laws
3-39
Similarity Case 1 β Different Rotational Speeds
β’ Consider a pump with a known performance. We want to determine its operation at a different speed.
β’ Pump sizes are the same (π·1 = π·2). Affinity laws are simplified as follows
π1π2=π1π2,β1β2=π1π2
2
,π«π1π«π2=π1π2
3
Delivering π1 and β1
Using π«π1
Delivering π2 and β2
Using π«π2
π·1 = π·2
π1 β π2
3-40
Similarity Case 1 (contβd)
Exercise : Head and efficiency of a centrifugal pump running at 1500 rpm are given as
β = 50 β 200 π β 24000 π2
π = 60 π β 1200 π2
It is desired to deliver 0.03 m3/s of water against a head of 36 m. Determine
a) speed of the pump
b) efficiency of the pump
c) power consumption
of the pump
0 0.01 0.02 0.03 0.04
60
β [m]
π[m3/s]
50
40
30
20
10
0
Point 1
(a point similar to point 2)
π1 = ?
β1 = ?
π1 = 1500 rpm
Point 2
(desired operating point)
π2 = 0.03 m3/s
β2 = 36 m
π2 = ?
3-41
Similarity Case 2 β Different Sizes (Impeller Diameters)
β’ Consider a pump with a known performance. We want to study the operation of a similar pump with a different impeller size rotating at the same speed.
β’ Rotational speeds are the same (π1 = π2). Affinity laws simplify as follows
π1π2=π·1π·2
3
,β1β2=π·1π·2
2
,π«π1π«π2=π·1π·2
5
Delivering π1 and β1
Using π«π1
Delivering π2 and β2
Using π«π2
π1 = π2
π·1 β π·2
3-42
Similarity Case 2 (contβd)
Exercise : Characteristic curve of a centrifugal pump is given as
β = 100 β 1000 π2
It is desired to deliver 0.1 m3/s of water against a head of 70 m. For these requirements it is thought that using a similar pump with a smaller impeller would be more efficient.
a) Determine the required percent reduction in impeller diameter.
b) Determine the percent decrease in power consumption.
Point 2
(desired operating point)
π2 = 0.1 m3/s
β2 = 70 mπ·2 = ?
Point 1
(a point similar to point 2)
π1 = ?
β1 = ?
π·1 = ?
0 0.1 0.2 0.3
120
β [m]
π[m3/s]
100
80
60
40
20
0
3-43
System Characteristic, Operating Point & Similarity
Exercise : Characteristics of a centrifugal pump at 600 rpm is given below. The pump is used to elevate water by 32 m from a lake to an open tank. Flow rate is measured as 22 lps while the delivery valve is fully open and the pump running at 600 rpm.
Determine the power consumptionfor the following operations.
a) Valve closure is increasedsuch that the frictional lossis doubled.
b) Pump speed is increasedto 720 rpm while keepingthe valve fully open.
0 10 20 30 40 50
70
β m
π [lps]
60
50
40
30
20
10
0
π [%]
3-44
Series Combination of Pumps
β’ Series combination is used if the head provided by a single pump is not enough.
β’ Same π goes through both pumps.
β’ Total head provided is the sum of individual heads.
β’ Pumps can be identical or different.
β’ More than two pumps can be combined in series.
Pump Aprovides βπ΄
Pump BProvides βπ΅
π = ππ΄ = ππ΅
β = βπ΄ + βπ΅
ππ΄ ππ΅
3-45
Series Combination of Pumps (contβd)
Exercise : Show that for two pumps combined in series overall efficiency is
π =βπ΄ + βπ΅βπ΄ππ΄+βπ΅ππ΅
β’ To get combined pump characteristic, individual pump characteristics are added vertically.
β’ If the pumps are identical
π
β
System characteristic
Operating point
Pump A or B
Pump A+B in series
3-46
Series Combination of Pumps (contβd)
β’ If the pumps are NOT identical
β’ Above a certain ππ pump B is forced to operate above its free delivery point. For such a case it just creates extra loss and should be shut off and bypassed.
π
βSystem characteristic
Operating point
Pump B
Pump A+B in series
Pump A
ππ
3-47
Series Combination of Pumps (contβd)
Exercise : Two identical pumps, with shown characteristics, are combined in series and used to transport water between two reservoirs with an elevation difference of π§ππ β π§π π = 50 m. Total length of suction and discharge pipes is 120 m. Pipe diameters are 0.12 m. Friction factor inside the pipes is 0.022. Neglecting the minor losses, determine the power required to drive both pumps.
0 0.01 0.02 0.03 0.04 0.05
90
π [m3/s]
β [m]
π [%]
70
50
30
10
β’ Parallel combination is used if the flow rate provided by a single pump is not enough.
β’ Each pump provides the same head β.
β’ Total flow rate is the sum of individual flow rates.
β’ Pumps can be identical or different.
β’ More than two pumps can be combined in parallel.
3-48
Parallel Combination of Pumps
Pump A
Pump B
ππ΄
β = βπ΄ = βπ΅
ππ΅
π = ππ΄ + ππ΅
3-49
Parallel Combination of Pumps (contβd)
Exercise : Show that for two pumps combined in parallel overall efficiency is
π =ππ΄ + ππ΅ππ΄ππ΄+ππ΅ππ΅
β’ To get combined pump characteristic, individual pump characteristics are added horizontally.
β’ If the pumps are identical
π
β System characteristic
Operating point
Pump A or B
Pump A+B in parallel
3-50
Parallel Combination of Pumps (contβd)
β’ If the pumps are NOT identical
β’ Above a certain βπ pump B is forced to operate above its shutoff head. For such a case it just creates extra loss and should be shut off and its branch should be blocked with a valve.
π
β
System characteristic
Operating point
Pump B
Pump A+B in parallel
βπ
Pump A
3-51
Cavitation
β’ In a liquid flow cavitation occurs when the local static pressure falls below the vapor pressure of the liquid.
β’ For a cavitating flow
β’ liquid locally vaporizes forming bubbles.
β’ bubbles collapse as they travel to higher pressure regions and cause erosion/surface pitting.
β’ flow becomes unsteady, causing noise and vibration.
β’ performance of the turbomachine drops.
β’ For a pump, critical low pressure region is the entrance, and for a turbine it is the exit.
β’ High speed regions like propeller blade tips are also critical.
β’ Listen to the sound of a cavitating pump : www.youtube.com/watch?v=Qw97DkOYYrg
β’ Watch propeller tip cavitation : www.youtube.com/watch?v=GpklBS3s7iU
3-52
Damage on Francis turbine blades
Damage on centrifugal pump impeller
www.pumpfundamentals.com
en.wikipedia.org
Damage on propeller blades
Cavitation Damage
en.wikipedia.org
3-53
Cavitation of a Pump and ππππ»
β’ Cavitation possibility of a pump is checked using Net Positive Suction Head (ππππ»).
β’ ππππ» is the difference between the total head at the suction side and the head corresponding to the vapor pressure.
ππππ» =ππ ππ+ππ 2
2πβππ£ππ
β’ Point π denotes the suction side (inlet) of the pump.
β’ It is used in the definition because it is the critical low pressure region.
Vapor pressureSuction pressure
Suction velocity
Total head at the suction side of the pump (Datum is
arranged so that π§π = 0)
3-54
NPSH (contβd)
β’ There are two values of ππππ» that we work with
β’ ππππ» required (ππππ»π )
β’ ππππ» available (ππππ»π΄)
β’ ππππ»π is the value that must be exceeded to prevent cavitation inside the pump.
β’ It is measured by the manufacturer of the pump and provided as an extra curve on the pump characteristic plot.
π
ππππ»π
β
ππππ»π
π
3-55
NPSH (contβd)
β’ ππππ»π΄ is the value that we need to calculate for the problem of interest.
β’ Consider the BE for the suction side of a pump
ππ πππ+ππ π2
2π+ π§π π =
ππ ππ+ππ 2
2π+ π§π + βππ
Typicallyππ π = πππ‘πππ π = 0
π§π β π§π π = π»π
βππ : Frictional losses at the suction side
of the system
ππ ππ+ππ 2
2π=πππ‘πππβ βππ βπ»π β ππππ»π΄ =
πππ‘π β ππ£ππ
β βππ β π»π
π
π π
π»π
Pump
Suction pipe
3-56
NPSH (contβd)
ππππ»π = Provided by the manufacturer , ππππ»π΄ =πππ‘π β ππ£ππ
β βππ β π»π
β’ To prevent cavitation
ππππ»π΄ β₯ ππππ»π
Exercise : What can be done to make ππππ»π΄ larger for the pump shown in the previous slide?
Exercise : How does the vapor pressure of water change with temperature? What does this information tell us about preventing cavitation?
β
ππππ»π
ππππ»π΄
CavitationNo cavitation
3-57
NPSH (contβd)
Exercise : A centrifugal pump is to be placed above a large open water tank to pump water at a flow rate of 1.4 Γ 10β2 m3/s. At this flow rate the required NPSH value is given as 4.5 m by the pump manufacturer. Water temperature is 30 oC and the atmospheric pressure is 95 kPa. Suction side pipe is short and the main head loss between the suction tank and the pump is due to a filter that has a head loss coefficient of π = 20. Other losses can be neglected. Suction pipe diameter is 10 cm.
a) Determine the maximum height that the pump can be located above the water surface of the suction tank without cavitation.
b) If you were required to place a valve in the flow path to regulate the flow rate, would you place it upstream or downstream of the pump? Why?
(Munsonβs book)
π
π π
π»π = ?
PumpFilter
3-58
NPSH (contβd)
Exercise : Centrifugal pump with the given characteristic is running at 1450 rpm to pump water at 25 β from a reservoir whose surface is 1.2 m above the centerline of the pump inlet. Reservoir is open toatmospheric pressure.
The piping system from the reservoirto the pump consists of 3.2 m of castiron pipe with a diameter of 5 cm andan average roughness of 0.05 cm.Minor losses at the suction side of thepump are; sharp edged inlet (π = 0.5),three flanged smooth 90o elbows(π = 0.3 each) and a fully open flangedglobe valve (π = 6).
Estimate the maximum flow rate thatcan be pumped without cavitation.
0 2 4 6 8 10 12 14 16 18
π [m3/h]
Nπππ»π m
βm
12
11
10
9
8
7
6
2
1
0
www.standartpompa.com
3-59
Pump Specific Speed (ππ )
β’ Specific speed is a useful non-dimensional pi-term obtained by combination ππ and
πβ to eliminate π·.
ππ =ππ1/2
πβ 3/4=π π
(πβ)3/4
β’ In the industry the following dimensional form is also commonly used
ππ π =π(rpm) π(liter/min)
β(m) 3/4
where ππ π = 2733 ππ
β’ ππ is useful to classify and compare different types of pumps at their BEP.
β’ ππ is mainly used for preliminary pump selection.
(Note that g is missing)
3-60
Pump Specific Speed (contβd)
β’ Centrifugal (radial) pumps work efficiently at low specific speeds (ππ < 1.5).
β’ Mixed pumps work efficiently at medium specific speeds (1.5 < ππ < 3.5).
β’ Axial (propeller) pumps work efficiently at high specific speeds (ππ > 3.5).
Γengelβs book
0.1 0.2 0.5 1 2 5 10
ππ
πmax
1
0.9
0.8
0.7
0.6
0.5
500 1000 2000 5000 10000 20000
ππ π
Centrifugal Mixed Axial
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Axial Pumps (Propeller Pumps)
β’ Worldβs most powerful pump: http://pressurewashr.com/the-worlds-most-powerful-water-pump/
β’ Centrifugal pumps are usually work efficiently at high β and low π.
β’ Certain applications, such as drainage and irrigation involve low β and high π.
Flow coming in
Flow going out
Fixed stator (guide) vanes
Rotating rotor blades
wikipedia.com
3-62
Pump Selection
β’ Two main inputs for pump selection are
β’ required head, β.
β’ required flow rate, π.
β’ Additional considerations for pump selection are
β’ pump speed
β’ type of fluid (highly viscous, muddy, etc.)
β’ available space, vertical placement limitations, etc. that will affect ππππ»π΄
β’ maximum allowable noise level
β’ etc.
β’ For preliminary pump selection specific speed (ππ ) is commonly used.
3-63
Pump Selection (contβd)
β’ Manufacturers provide catalogues of their pumps.
β’ Following chart is for centrifugal pumps produced by Standart Pompa.
β’ For an application with π = 100 m3/h and β = 30 m, ββ65-160ββ family seems suitable.
5 10 20 30 40 50 100 200 300 600
β m
π [m3/h]
100
80
60
40
30
20
15
10 www.standartpompa.com
2900 rpm
3-64
β’ This one is for axial pumps produced by Goulds Pumps.
β’ For an application with π = 5000 m3/h and β = 4m, ββ24-24 24ββ family seems suitable.
www.gouldspump.com
600 1000 2000 3000 5000 8000 12000 20000 40000
β m
π [m3/h]
12
10
8
6
4
2
0
Pump Selection (contβd)
3-65
Pump Selection (contβd)
β’ These are the detailed performance curves of the the pumps in the ββ65-160ββ family of Standart Pompa.
β’ There are three similar pumps with impeller diameters of 160 mm, 175 mm and 184 mm.
β’ Red curves are iso-efficinecy lines.
β’ ππππ»π and π«π curves are also
provided.
β’ One of these three pumps can be selected by considering cavitation possibility, efficiency and power consumption.
β’ The smallest pump cannot provide the required head of 30 m at the desired flow rate of 100 m3/h.
0 20 40 60 80 100 120 140
βm
π [m3/h]
20
Nπππ»π
mπ«πkW
15
10
5
0
2
6
10
20
30
40
50
25
35
45
π 184
π 175
π 160
π 160 π 184
50% 60 6570
73.5
70
6560
50
π 184
π 175
π 160
Adapted from www.standartpompa.com
2900 rpm
3-66
Turbines
0.2 0.5 1 2 10 50 100
βm
π [m3/s]
1000
300200
50
10
2
1
www.3helixpower.com
Impulse Types
Pelton wheel
Turgo
Cross-flow
Reaction Types
Francis
Kaplan
3-67
Turbines (contβd)
β’ Fundamental performance characteristic of a turbine is the power produced (π«π‘) vs. rotational speed curve at a given head.
β’ Affinity laws used for pumps are valid for turbines too.
β’ Turbine specific speed can be used for preliminary turbine selection. It is defined in a slightly different way than pumps
ππ π‘ =ππ«1/2
πβ 5/4=π π«π‘
π (πβ)5/4
π
π«π‘
At a given head β
3-68
Turbine Specific Speed (ππ π‘)
β’ Impulse turbines work efficiently at low specific speed (ππ π‘ < 0.3) (High β, low π).
β’ Francis turbines work efficiently at medium specific speeds (0.3 < ππ π‘ < 2).
β’ Kaplan turbines work efficiently at high specific speeds (ππ π‘ > 2) (Low β, high π).
Γengelβs book & www.repack-s.com
0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10
ππ π‘
πmax
1
0.9
0.8
0.7
0.6
0.5
ImpulseFrancis Kaplan
3-69
Pelton Wheel
http://www.photobucket.com
β’ Compare Pelton, Kaplan and Francis turbines: https://www.youtube.com/watch?v=k0BLOKEZ3KU
Typical bucket design
3-70
Francis and Kaplan Turbines
Rotor
Adjustableguidevanes
Draft tube
π
Adjustableguidevanes
π
π
Rotor blades
Francis turbine(radial flow)
Kaplan turbine(axial flow)
π
3-71
Francis and Kaplan Turbines (contβd)
www.geppert.at
www.tbhic.cn
Runner of a Francis turbine
Runner of a Kaplan turbine
β’ Francis turbine: https://www.youtube.com/watch?v=3BCiFeykRzohttps://www.youtube.com/watch?v=IZdiWBEzISM
β’ Kaplan turbine: https://www.youtube.com/watch?v=0p03UTgpnDUhttps://www.youtube.com/watch?v=_eLufvzh5HU
3-72
Exercise: A Francis turbine is being designed for a hydroelectric dam. Instead ofstarting from scratch, the engineers decide to geometrically scale up a previouslydesigned turbine that has an excellent performance history. The existing turbine(turbine A) has diameter π·π΄ = 2.05 m, and spins at ππ΄ = 120 rpm. At its best efficiencypoint, ππ΄ = 350 m3/s, βπ΄ =75 m π«π‘ = 242 MW. The new turbine (turbine B) is for alarger facility. Its generator will spin at the same speed (120 rpm), but its net headwill be higher (βπ΅ = 104 m).
a) Calculate the diameter (π·π΅) of the new turbine such that it operates most efficiently, and calculate ππ΅, and ππ΅.
b) Calculate the turbine specific speeds of both turbines.
Turbines (contβd)
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Turbines (contβd)
Exercise : Calculate the specific speeds of the following turbines
a) Francis type radial flow turbine at the Round Butte hydroelectric power station in Madras rotating at 180 rpm and producing 119 MW of power at a flow rate of 127 m3/s from a head of 105 m.
b) Francis type mixed flow turbine at the Smith Mountain hydroelectric power station in Roanoke, VA, rotating at 100 rpm and producing 194 MW of power at a flow rate of 375 m3/s from a head of 54.9 m.
c) Kaplan type axial flow turbine at the Warwick hydroelectric power station in Cordele, GA, rotating at 100 rpm and producing 5.37 MW of power at a flow rate of 63.7 m3/s from a head of 9.75 m.
Exercise : Learn the meaning of the following turbine related terms
Runner blade, wicket gate, stay vane, crown, penstock, draft tube, tail water
Exercise : How does hydraulic power work? http://www.youtube.com/watch?v=cEL7yc8R42k
Virtual turbines http://www.youtube.com/watch?v=HzQPNpP55xQ