Fluid Engineering Mechanics · Analysis of Fluid Machines: Pumps and turbines, types, principles of...

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Analysis of Fluid Machines: Pumps and turbines, types, principles of operation, performance characteristics, Performance curve, similarity laws, pumps in series and parallel, selection of pumps and turbines Dr. Muhammad Ashraf Javid Assistant Professor 1 Fluid Engineering Mechanics Chapter 7

Transcript of Fluid Engineering Mechanics · Analysis of Fluid Machines: Pumps and turbines, types, principles of...

Analysis of Fluid Machines: Pumps and turbines, types, principles of operation, performance

characteristics, Performance curve, similarity laws, pumps in series

and parallel, selection of pumps and turbines

Dr. Muhammad Ashraf Javid

Assistant Professor

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Fluid Engineering Mechanics

Chapter 7

Objectives

Understand the role of pumps and turbines as energy-conversion

devices and use, appropriately, the terms head, power and efficiency.

Be aware of the main types of pumps and turbines and the distinction

between impulse and reaction turbines and between radial, axial and

mixed-flow devices.

Match pump characteristics and system characteristics to determine

the duty point (demand point)

Calculate characteristics for pumps in series and parallel

Select the type of pump or turbine on the basis of specific speed.

Understand the mechanics of a centrifugal pump and an impulse turbine.

Recognize the problem of cavitation and how it can be avoided.

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Energy Conversion

Energy Transfer in Pumps and Turbines

Power

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Machine Efficiency

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Pumps

Pumps convert mechanical energy to fluid energy

A pump usually refers to a machine used for incompressible fluids

(water, oil); fans, blowers,

Types of pumps

Positive displacement

Centrifugal pump

Axial flow pumps

Mixed flow pumps

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Pumps: Types

Positive Displacement Pump

These types of pumps displace fixed

volumes of fluid during each cycle or

revolution of the pump.

No longer used for distribution system

pumping in most water systems, but

portable units may be used for

dewatering excavations during

construction.

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Pumps: Types

Centrifugal Pump

Frequently used in water distribution systems.

Water enters the pump through the eye of the spinning impeller and outward from the vanes to discharge pipe.

A centrifugal pump consists of: A rotating element (impeller) and housing which encloses the impeller and seals the pressurized liquid.

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Pumps: Types

Axial Flow pumps

In axial-flow pumps, the flow

enters and leaves the pump

chamber along the axis of the

impeller, as shown in Figure

In mixed flow pumps,

outflows have both radial and

axial components.

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Pumps: Types

The pumps illustrated in Figure are both single-stage pumps, which means

that they have only one impeller.

(a) Typical centrifugal pump installation. (b) Typical axial-flow pump installation.

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Pumps: Types

In multistage pumps, two or more impellers are arranged in series in such a

way that the discharge from one impeller enters the eye of the next

impeller. These types of pumps are typically used when large pumping heads

are required.

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Specific Speed

For pumps, the commonly used definition of specific speed (also called shape number), ns , is given by

where any consistent set of units can be used. In SI units, ω is in rad/s, Q in m3/s, g in m/s2, and hp in meters.

It is common practice in the United States to define the specific speed, Ns,

as

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Problem:

It is desired to deliver 100L/s at a head of 270m with a single stage pump.

(a). What would be the minimum rotative speed that could be used.

Assuming that the minimum practical specific speed, Ns, is 10

(b). For the conditions of (a) how many stages must the pump (Ns=10)

have if a rotative speed of 600 rpm is to be used.

rpm

Q

hN

h

QN

ps

p

s

21061000/100

270102/1

4/3

2/1

4/3

4/3

2/1

stagepermh

N

Qh

p

s

p

6.50

1910

1.06002/12/1

4/3

Total Reqd. Stages=270/50.6=5.34

6 stage are required

a.

b.

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Problem:

Determine the specific speed of a pump that is to deliver 125L/s against a

head of 45m with a rotative speed of 600rpm.

2.1245

1000/1256004/3

2/1

4/3

2/1

s

p

s Nh

QN

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Head Added by Pump (Total Dynamic Head)

If a pump has been selected, Bernoulli’s equation can be rearranged to solve

for the head added by a pump

Where,

ha=head added by pump (TDH)

hf= head loss in attached pipe and fittings

P=Atmospheric pressure

V=velocity

Z=elevation

fa hZZVVPP

h

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2

1

2

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Total Dynamic Head

To determine the size of the pump, one must know the

total dynamic head that the pump is expected to provide.

Total dynamic head (TDH) consists of

The difference between the center line of the pump and the

height to which water must be raised.

The difference between the suction pool elevation and

centerline of the pump

Frictional losses in the pump and fitting

Velocity head

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Calculation of TDH from Pump Test Data

TDH=Hs + HL + Hv

Where

Hs= Total static head (difference between elevations of pumping source and

point of delivery

HL = Friction losses in pipes and fittings

Hv= Velocity head due to pumping

Substituting from Bernoulli’s Equation

TDH=Hs + HL + V2/2g

TDH=Hs + HL + Q2/2gA2

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Cavitation

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Net Positive Suction Head and Capacity

NPSH is the difference between suction pressure and vapor

pressure.

NPSH Available (NPSHA): The absolute pressure at the suction port

of the pump. AND

NPSH Required (NPSHR): The minimum pressure required at the

suction port of the pump to keep the pump from cavitating.

Capacity of Pump

The higher the specific gravity of the fluid, the more power (amps)

required. The amount of fluid the pump will move is determined mainly

by the width of the impeller and the shaft speed. Capacity is normally

measured in gallons per minute (gpm.) or cubic meters per hour (m3/hr).

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Calculation of the theoretical required

power of a pump

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Calculation for Pump Efficiency

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Performance of Pump

Head and Capacity

BHP (Brake Horsepower) and Capacity

Efficiency and Capacity

NPSH (Net Positive Suction Head) and Capacity

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Pump Curve vs System Curve

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Pumps in Parallel

When two or more pumps are arranged in parallel their

resulting performance curve is obtained by adding their

flowrates at the same head as indicated in the figure below.

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Turbines

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Turbines

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Turbines

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Turbines

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Summary

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Dimensionless Parameters for Turbines

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Similarity Laws for Turbines

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Power Specific Speed

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Power Specific Speed

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Use of Specific Speed to Select

Turbine Type

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Example

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Paper Guidelines

CO-1: Introduction to fluid mechanics, properties of fluid and

thermodynamics

3-sub-questions

CO-2: Fluid Static, Forces on immersed bodies, buoyancy and

floatation

3-sub-questions

CO-3: Continuity equation, Bernoulli's equation, and

Discharge Measurement, Momentum and Forces in Fluid Flow

3-sub-questions

CO-4: Analysis of flow through pipes

3-sub-questions

CO-5: Analysis of machines

3-sub-questions

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Paper Guidelines

You have to answer only four questions

Read all the questions first and select the easiest one for

answer. i.e. take most difficult question at the end. Never

start with difficult question.

Time management is key during exam. Do not spend a lot

of time on a single question.

Kindly read the statement of each question carefully and

try to understand it before proceeding towards solution.

i.e. what is given and what is unknown?

Draw figures to improve your understanding and to make

solution simple.

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Theory Sample questions

1. Explain the following fluid properties: specific weight, compressibility, viscosity, and surface tensions

2. Derive Newton’ s equation of viscosity.

3. Differentiate between entropy and enthalpy

4. Define the following pressure terms: gauge pressure, vacuum pressure, absolute pressure and atmospheric pressure.

5. Write gauge pressure equation for differential when two vessels at the same level or at different level or inverted manometer.

6. Differentiate between steady and unsteady flow, uniform and non-uniform flow with examples.

7. Derive continuity equation with stating its assumptions.

8. State Bernoulli's theorem and derive its equation with stating its assumptions.

9. Differentiate between forced and free vortex flow with examples

10. Derive Darcy Weisbach equation

11. Differentiate between positive displacement and centrifugal pump.

12. Differentiate between impulse and reaction turbine.

13. Explain any two or three types of minor losses in pipes with sketches.

14. Using concept of impulse momentum principle show that

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