module 3_lecture 10-11_ pumping
TRANSCRIPT
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JR STOKES, CHEE2003
FLUID MECHANICSLECTURE 10
Fluid Flow: Pumping
Pumping of FluidsAim: to be able to make elementary engineering estimates of the performance of
fluid machines
White: Ch 11 or Potter et al Ch 12
Turbomachinery:
Add energy to the fluid: The Pump Family Extract energy from the fluid: Turbines
Both usually connected to a rotating shaft...turbo: to spin... or ....whirl...
OLD technology....3000 years......waterwheels (1000bc), Archimedes screw pump
(250bc), paddlewheel turbines (70bc), windmills (700BC)
Liquids: Pumps
Gases: Fan (
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Pumping of FluidsPump :Add energy to the fluid i.e. adds Pressure energyto the fluid
Increases the pressure from low to high.
fluids flow from high pressure to low pressure
TurbineLosspump hhzg
V
g
Phz
g
V
g
P
2
2
22
21
2
11
1 2
2
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Classification of Pumps
2 BASIC types:
1. Positive Displacement Pumps
- Force fluid along by a volume change
2. Dynamic pumps
Also called: Rotodynamic pumps, momentum-change pumps
- adds momentum to fluid (kinetic energy) that is converted to a pressure
Pros/cons PDP Rotodynamic
Flow rate Lower High
Discharge pressure rise Very high Moderate
Discharge Pulsating / perioid flow Steady flow
Discharge & Fluid type Constant discharge
regardless of fluid viscosity
High viscosity fluid
degrade performance
Priming Self-priming Required
Control value Suction side exit
Positive Displacement Pumps (PDP)
Force fluid along by volume changes. A cavity opens, and fluid isadmitted through an inlet. The cavity closes, and the fluid is squeezedthrough an outlet (White c11)
A. Reciprocating:
1. piston / plunger;2. diaphragm
B. Rotary:1. Single:
i. sliding vane; ii. flexible tube; iii. screw; iv. peristaltic2. Multiple rotors:
i. Gear; ii. Lobe; iii. screw; iv. circumferential piston
Pulsating or periodic flow Can pump viscous fluids Applies mechanical compressiondevelops immense pressure if outlet
shut. Sturdy construction and pressure reliefvalue required
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PDP Reciprocating pump
See animations at :
http://www.ustudy.in/node/538
Highly pulsed flow (can use accumulators to smooth flow).
Very efficient, suitable for high heads at low flow.
Self priming. Reliable discharge flow, used where accuracy essential.
Not tolerant to solid particles
Leakage and poor valve operation result in delivery being less than ideal for large scaleindustry.
White fig11.1
http://www.youtube.com/watch?v=gtRdXks1lVE
Gear pumpGear pump: Good for viscous fluids and high pressures.
PDP Gear Pump
http://www.youtube.com/watc
h?v=UJYAksK3CPM&featur
e=related
Video:
http://www.ustudy.in/node/538http://www.youtube.com/watch?v=gtRdXks1lVEhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=UJYAksK3CPM&feature=relatedhttp://www.youtube.com/watch?v=gtRdXks1lVEhttp://www.ustudy.in/node/538 -
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Mono pump
A helical section of pipe rotates to move fluid along. Good for slurries and high
pressures. Can deliver fluid with less pulsation.
PDP Mono Screw Pump
http://www.youtube.com/watch?v=12Wszv1wUMkAnimation:
Peristaltic pump
Used for small deliveries - biological applications, instruments in labs (and
hospitals).
Positive Displacement Pump
http://www.youtube.com/watch?v=12Wszv1wUMkhttp://www.youtube.com/watch?v=12Wszv1wUMk -
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Positive displacement pumps Delivers high pressures on outlet (small discharge
volumes) ..control value should be on (suction or outlet ?)
PDP
Suction side
Positive Displacement Pump: Control
Positive displacement pumps Delivers high pressures on outlet (small discharge
volumes) ..control value should be suction side.
Use to Control flow
PDP
Pressure
relief value
Due to high pressure,closing a valve on outletwill cause severe damageor pipe rupture !
Suction side
Positive Displacement Pump: Control
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Dynamic Pump (Roto-dynamic pump)
Adds momentum to fluid suing fast moving blades or vanes. No closedvolume. The fluid increases momentum while moving through open passagesand then converts its high velocity to a pressure increase by exiting into a diffuser
section. (White C11)
A. Rotary:1. Centrifugal or radial exit flow2. Axial flow3. Mixed fluid (radial & axial)
B. [Special Designs
1. Jet pump or ejector; 2. Electromagnetic pumps for liquid metals 3. Fluidactuated: gas lift or hydraulic ram]
Higher flow rate than PDP, but only moderate pressure rise Steadier discharge than PDP Not for high-viscosity fluids. Requiring priming (if fill with gas/air, cannot suck up liquid below their inlet)
Dynamic Pumps: Centrifugal Pump
http://www.youtube.com/watch?v=9nL1XhKm9q8
Radial Flow: Centrifugal pumps are the most common pumps used in chemical
plants. They can pump liquids with a wide range of properties and can be
constructed from a wide range of materials
http://www.youtube.com/watch?v=iygacPUfuRA
http://www.youtube.com/watch?v=9nL1XhKm9q8http://www.youtube.com/watch?v=iygacPUfuRAhttp://www.youtube.com/watch?v=iygacPUfuRAhttp://www.youtube.com/watch?v=9nL1XhKm9q8 -
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Fig 11.2 Comparison of performance curves of typical dynamic andpositive displacement pumps at constant speed (White)
Volute-type centrifugal pump (Radial flow)
Fluid enters at the centre of a rotor or impeller, and is given kinetic energy by theblades.
In the casing, the fluid velocity is decreased, converting some of the kinetic energyinto pressure.
A lot of the energy is lost in turbulence, giving a peak efficiency of 75-80%.
Dynamic Pumps: Centrifugal Pump
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Diffuser-type centrifugal pump (Radial flow)
Similar to the Volute-type except with a stationery diffuser in the casing which is
designed to reduce turbulence and give higher efficiencies.
A number of pumps in series can be used to produce high pressure.
Dynamic Pumps: Centrifugal Pump
Centrifugal Pump: PrincipalVery simplified approach (illustrative purposes only):Section 1spinning / centrifugal action
Spinning will accelerate the fluid => Kinetic energy.
The second phase is to convert KE to Pressure (diffuser):
2
112
2
2
12
112
2
1
2
221
2
2
1
2
1122
1
2
2
2
vPP
A
AvPP
g
vv
g
PP
zg
v
g
Pz
g
v
g
vP
AvAv
This suggests that the
centrifugal force is
converted to pressure
at the outlet
Centrifugal motion
Vr=0~0 Vr=R~(wR)2
1 2
A proper analysis is in the texts..(principals important but detail in text dont need to know)this slide isto illustrate principals
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Centrifugal Pump
Bernoulli across whole system
)(
)(
0~2
2
2
121
supplied
21supplied
2
1
2
2supplied
21
2
2
supplied
1
2
PPh
g
PPh
g
vvh
g
PP
zg
v
g
Phzg
v
g
P
g
Neglect viscous work and heat transfer. hpump
includes any losses inside pump:
2
1
2
1
v1 ~ v2
supplied)..( hQgPw Power delivered to fluid, Pw
Power to drive Pump, BHP:
Efficiency:
TPbhp .w
T
hQg
P
P
p
w
w
supplied)..(
http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1
Power to fluid = Work rate = Energy / time
= specific weight x discharge x net head change
WsJsmN
P TL
T
MLLTL
T
L
L
Mw
//.
...][ 23
23
w shaft angular velocity
T shaft torque
1 hp = 746 W
Rotodynamic / Centrifugal Pump used to deliver high volumes of liquid against low to
medium pressures..control value should be at outlet !!
For control of flow, valves should be placed on the delivery side with any valve on the suction
side fully open.
If a valve on the suction side is closed to any extent, pressure gets reduced and cavitation
(formation of gas bubbles) can occur. Cavitation can lead to excessive wear and damage to
the pump (vapour pressure in lecture 1).
Use to
Control
flow
Centrifugal Pump
Suction side
Dynamic Pump: Control
delivery side
http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1 -
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Axial flow
A propeller type impeller often with adjustable pitch for the blades.
Flow is in the direction of the rotating axis of the impeller. Commonly used in
ventilation systems for movement of large quantities of air with low pressure
drop.
Dynamic Pumps: Axial Flow Pump
Positive Displacement
Centrifugal utilizes centrifugal force of rotating impeller
not fixed capacity output (like positive displacement)
pump duty point is different for each SYSTEM
Design/selection - matching pumpcharacteristics to operating system
Pumps Summary
http://upload.wikimedia.org/wikipedia/commons/f/f6/Axial_2.png -
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Pumping of fluids: Centrifugal Pump
supplied)..( hQgPw Power delivered to fluid, Pw
Power to drive Pump, BHP:
Efficiency:
TPbhp .w
T
hQg
P
P
bhp
w
w
supplied)..(
http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1
WsJsmN
PTL
T
MLLTL
T
L
L
Mw
//.
...][ 23
23
BHP = brake horse power
w shaft angular velocity
T shaft torque
1 hp = 746 W
Engineer aim: to make as high as possible over a broad range of discharge Qas possible
White pp 777
Potter pp 608
qHh supplied
For following sections, we will use the following terminology for the headbeing supplied by the pump to the fluid :
bhpPbhp
Pumping Performance curvesRotodynamic centrifugal pump characteristics
For a specific pump, a given flowrate will generate a specific head.
If the head against which the pump is delivering is high, the allowed flowrate is low
if the head against which the pump is delivering is low, higher flowrates can be achieved.
This gives us a pump characteristic curve, as shown.
When selecting a pump, the required flowrate and head should lie somewhere in the
middle of the curve to give some flexibility in operation.
Head
Generated
by pump,
Hq
Discharge Flowrate, Q
Hq = ho - Q2
White 11.3
Potter 12.2/12.4
Qmax
High head needed, Low Flow
Low head needed, High Flow
http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1http://www.youtube.com/watch?v=B8MV09HF-nY&NR=1 -
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Pumping of Fluids
Rotodynamic centrifugal pump characteristics
Performance charts are plotted for constant shaft rotation speed, n
Independent variable : Q
Dependent variable: Hq (DPincrease), bhp,
Curves typically available for commercial pumps.
Hq
Flowrate, Q
bhp
bhp
HgQ
P
P q
bhp
w.
QmaxQ*
BEP = best efficiency point,
design point
bhp =Power input to
drive pump shaft
Hq =head delivered to
fluid by pump
Pumping of FluidsRotodynamic centrifugal pump characteristics
Typical curve of Hp vs Q that can be provided by a pump
Hq
Q
increasing impeller speed,increasing impeller size
Same speed,differentsystem head
Different curves for different shaft rotation speed, n
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Pumping of Fluids
Rotodynamic centrifugal pump characteristics
They tend to be strictly applicable to a fluid of a certain density and viscosity
Constant shaft rotation speed, n3 impellor sizes
White Fig 11.7 - Measured performance curves for a centrifugal water pump:
(a) basic casing with 3 impellor sizes
Hq
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White Fig 11.7 - Measured performance curves for a centrifugal water pump:
(b) 20 % large casing with 3 larger impellors at slower speed
Hq
Pumping of Fluids
Matching pumps to System Demand
Head
Flowrate, Q
System Curve
System demand curve, hq
2
2
12q2
)(HeadSystemRequiredhgAQK
DLfzz
g
v
g
v
D
Lfz
g
vPhz
g
vP
2K
222
2
lossesminorall
L
2
2
2
22
2q1
2
11
1
1
2
Losses in system
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Pumping of Fluids
Rotodynamic centrifugal pump characteristics
different piping systems, different curves
different fluids?
different Q?
effects of valves etc
HH
Q Q
Pumping of FluidsRotodynamic centrifugal pump characteristics
For a given system, as the flowrate is increased, the head loss due to friction
will increase. This gives us a system curve, as shown.
The point at which the two curves intersect gives the flowrate for the combination of
pump and system. When choosing a pump, the system operating point is usually chosen
to coincide with pump maximum efficiency, by careful choice of Pump size & Operatingspeed
System curvePump characteristic
curve
Flowrate and Head
generatedHead
Generated
Flowrate, Q
Hqhq
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Fluid Pumping : MFEE
losseshzg
v
ghz
g
v
g
22
22
2supplied1
21
11
22
Required head from pump
g2g2D22
22
2
2
22
21
2
11
1 vvlfzg
v
ghz
g
v
gLpump
Multiply by g ~ J/kg:
m
22D22
22
1
2
11
12
2
22
2 vvlfgzvgzvghLpump
Qghmgh pumppump ...
Multiply by mass flow rate ~ J/s Power (W)
Example No. 16Cooling water (density = 1000 kgm-3) is pumped from a reservoir to a heat
exchanger and then returns to the top of a cooling tower which is 10 m above
the water level in the reservoir. Both reservoir and cooling tower are open to
the atmosphere. There is a total of 150 m of 75-mm diameter pipe, with a
friction factor () of 0.003. Note, = f/8. The head loss due to friction in the
pipe fittings and heat exchanger is equivalent to 20 velocity heads. The pumpcharacteristics are given in graph Figure Q5. Estimate the flowrate of cooling
water through the system.
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Example No. 16
Figure Q5 - Pump Characteristic Curve
10
15
20
0.003 0.0035 0.004 0.0045 0.005 0.0055 0.006
Flowrate (m 3s -1)
H
eadDeveloped
(m)
Example No. 16
Figure Q5 - Pump Characteristic Curve
10
15
20
0.003 0.0035 0.004 0.0045 0.005 0.0055 0.006
Flowrate (m 3s -1)
HeadDevelop
ed
(m)
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Cavitation in centrifugal pumps
Most likely cause of pump wear Collapsing vapour cavities Pump suction
side or pump
inlet
Net Positive Suction Head (NPSH):
Total head at the pump suction side over and above the vapour pressureof the liquid being pumped.
g
PNPSH
vapour
InletatHeadFluid
g
P
g
v
g
PNPSH
vapourinletinlet
2
2
Cavitation in centrifugal pumps
Most likely cause of pump wear Collapsing vapour cavities Pump suction
side or pump
inlet
Net Positive Suction Head (NPSH):
Total head at the pump suction side over and above the vapour pressureof the liquid being pumped.
NPSHpump is the lowest value of NPSH at which the pump can beguaranteed to operate without significant cavitation-included on pumpcurves
NPSHavailableis the absolute pressure head available at the pump suctionside
NPSHAvailable> NPSHPump for successful operation
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Pump Cavitation
Head required at the
pump inlet to keep the
liquid from cavitating
or boiling
Inlet is the low-
pressure point where
cavitation will first
occur
NPSH must be above
pump reference value
to avoid cavitation
g
P
g
v
g
PNPSH
vapourinletinlet
2
2
NPSHPump
NPSHA > NPSHPump http://www.youtube.com/watch?v=cOeNxkksruo
Pump Cavitation
Calculating NPSH: Head required at the pump inlet to keep the liquid
from cavitating or boiling
Inlet: suction creates low pressure point where cavitation could occur
Pump performance curve = NPSHPump
NPSH in actual system must be equal or greater that the value given in
pump performance curve to avoid cavitation
i.e.
NPSHA > NPSHPump
A
vapourinletinlet
NPSH
g
P
g
v
g
PNPSH
...........
2
2
Pump suction side
or pump inlet
http://www.youtube.com/watch?v=cOeNxkksruo Cavitation demo !
http://www.youtube.com/watch?v=cOeNxkksruohttp://www.youtube.com/watch?v=cOeNxkksruohttp://www.youtube.com/watch?v=cOeNxkksruohttp://www.youtube.com/watch?v=cOeNxkksruohttp://www.youtube.com/watch?v=cOeNxkksruo -
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Pump Selection Summary
Establish desired flowrate
Establish total dynamic head
Establish NPSHA(or range)
Use manufacturers performance curves
Pump should operate close to BEP
Calculate pump power requirements
Efficiency/cost/reliability
Fluid Pumping
In addition to White and Potter:
Recommended reading: Australian PumpTechnical Handbook, Produced by the Australian
Pump manufacturer's Association Ltd.
PS&E Library Call number: TJ900 .A87 1987
Main issues: MFEE and pumps
Pump selection: system curve and pump performance curve.
Pumping constraint: Net Positive Suction Head (NPSH)
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Supplementary Slides
MFEE and Pumps
Generic pumping system
(0) flow system inlet
(1) pump inlet side - suction side
(2) pump outlet side
(3) flow system outlet
0
1
2
3
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MFEE with major losses, minor losses and
shaft head for pipe flowAssumptions:
steady flow
incompressible flow
no heat term
Kinetic energy coefficients Pipe friction loss coefficient = function ReD coefficient
Another important principle = conservation of mass: Q.A = constant
qL hvvl
fzg
vz
g
v
g2g2D22
22
1
2
11
12
2
22
2
Pumping of FluidsPositive displacement pumps
Mechanical device which transfers energy by displacing volumes of fluid.
Reciprocating pump
The wheel spins around causing a
piston to move in and out of acylindrical volume. Inlet valve opens,
piston moves out, fluid is sucked into
the cylinder. Inlet valve closes, piston
moves in and fluid exits through
outlet valve. Liquid delivery is related
to the speed of the spinning wheel
and the capacity of the cylinder.
Leakage and poor valve operation
result in delivery being less than
ideal.
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Pumping of Fluids
To move fluids from one place to another, overcome resistances to flow, and raisepressure to that required at point of delivery, energy can be added to the fluid.
The energy required will depend on the height through which the fluid is to be
raised, pressure required at delivery, length and diameter of pipe, rate of flow and
physical properties of the fluid (density and viscosity).
Machines used to give a fluid energy are:
Fans or Blowers - used for gases where the pressure rise is to be small and the
gas can be treated as incompressible.
Compressors - used for gases where the pressure rise is not small and the
assumption of constant density does not hold. See Engineering Thermodynamics
Module.
Pumps - used for gases and liquids. Two types: Positive displacement
Rotodynamic (or dynamic)
Gear pump
Two gears rotate carrying liquid
between their teeth. Good for viscous
fluids and high pressures.
Pumping of Fluids
Peristaltic pump
Flexible tubing of small diameter is held stationary and slightly flattened
around a series of rotating rollers which push the fluid along. Used for small
deliveries - biological applications, instruments in labs (and hospitals).
Mono pump
A helical section of pipe rotates to
move fluid along. Good for slurries
and high pressures.
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Pumping of Fluids
Rotodynamic pumpsMechanical device which transfers energy by rotating an element (opposite to a
turbine). There are two classes of rotodynamic pumps which depend on the flow
direction - radial or axial flow.
Volute-type centrifugal pump
(Radial flow)
Fluid enters at the centre of a
rotor or impeller and is given
kinetic energy by the blades. In
the casing, the fluid velocity is
decreased, converting some of the
kinetic energy into pressure. A lot
of the energy is lost in turbulence,
giving a peak efficiency of 75-
80%.
Axial flow
A propeller type impeller often with adjustable pitch for the blades. Flow is in
the direction of the rotating axis of the impeller. Commonly used in ventilation
systems for movement of large quantities of air with low pressure drop.
Diffuser-type centrigual pump (Radial flow)
Similar to the Volute-type except with a stationery
diffuser in the casing which is designed to
reduce turbulence and give higher efficiencies. A
number of pumps in series can be used to
produce high pressure.
Centrifugal pumps are the most common pumps
used in chemical plants. They can pump liquids
with a wide range of properties and can be
constructed from a wide range of materials
Pumping of Fluids
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Pumping of Fluids
Flow Control
Positive displacement pumps - used to pump against high pressures,
usually with smaller discharge quantities. Also used for difficult or aggressive
liquids, such as those at higher temperatures or higher viscosities.
For control of flow, valves should not be placed on the delivery side. Due to the
high pressures on the delivery side, closing a valve on this side will cause severe
damage or pipe rupture. The control valve is therefore place on the suction
side with only a small relief valve on the delivery side.
Pumping of FluidsFlow Control
Rotodynamic pumps - used to deliver high volumes of liquid against low to
medium pressures.
For control of flow, valves should be placed on the delivery side with any valve
on the suction side fully open. If a valve on the suction side is closed to any
extent, pressure gets reduced and cavitiation (formation of gas bubbles) can
occur. Cavitation can lead to excessive wear and damage to the pump.