Module 3_Lecture 10-11_ Pumping

7/29/2019 Module 3_Lecture 10-11_ Pumping

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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 (


2/26

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


3/26

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


4/26

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


5/26

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


7/26

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


8/26

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%.



9/26

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.


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


13/26

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


14/26

Pumping of Fluids


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


15/26

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


16/26

Pumping of Fluids


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


17/26

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.


18/26

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)


19/26

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


20/26

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


21/26

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)


22/26

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


23/26

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.


24/26

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.


25/26

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


26/26

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.

Documents

Module 3_Lecture 10-11_ Pumping