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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
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.
2
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
5
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.
6
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.
7
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.
8
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.
9
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.
10
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
12
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.
14
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
15
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
12
2
1
2
212
16
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
17
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
20
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).
22
Performance of Pump
Head and Capacity
BHP (Brake Horsepower) and Capacity
Efficiency and Capacity
NPSH (Net Positive Suction Head) and Capacity
25
Pumps in Series
31
When two (or more) pumps are arranged in serial their
resulting pump performance curve is obtained by adding
their heads at the same flow rate as indicated in the figure below.
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.
32
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
46
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.
47
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
48
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