7117897 pumps-turbines

Pumps & Turbines
(Turbo-machines)
(ME 268)

Turbo Machines
Turbo machine is defined as a device that
extracts energy of imparts energy to a
continuously flowing stream of fluid by the
dynamic action of one or more rotating
blade rows. (Latin Turbo means to spin or
whirl)

Classification
According to energy consideration
 Machines that supply energy to fluid (Pumps)
An increase in pressure takes place in pumps, fans,
compressors and propellers.
 Machines that extracts energy from fluid
(Turbines)
A decrease in pressure takes place in turbines, wind
mills.
 Machines that are a combination of both
(Energy transmitters and torque converters)

More classifications
Shrouded or un shrouded
 Depending on whether the rotating member is
enclosed in a casing or shrouded in such a way
that the working fluid cannot be diverted to flow
around the edges of the impeller.
Turbines/pumps are shrouded
Aircraft propellers/wind mills are unshrouded.

Classifications contd…
Based on direction of flow
 Axial flow
 Radial flow
 Mixed flow
Based on the manner of transmission of
energy
 Kinetic displacement (Centrifugal pumps and
turbines)
 Positive displacement (Reciprocating pumps)

Pumps
A pump is a device used to move gases,
liquids or slurries. A pump moves liquids
or gases from lower pressure to higher
pressure, and overcomes this difference in
pressure by adding energy to the system.
Mechanical Energy Hydraulic energy

Pumps Classification (contd)…
 Pumps are divided into two fundamental types based on
the manner in which they transmit energy to the pumped
media: kinetic or positive displacement.
 In kinetic displacement, a centrifugal force of the
rotating element, called an impeller, “impels” kinetic
energy to the fluid, moving the fluid from pump suction to
the discharge.
 Positive displacement uses the reciprocating action of
one or several pistons, or a squeezing action of meshing
gears, lobes, or other moving bodies, to displace the
fluid from one area into another (i.e., moving the material
from suction to discharge).
 Sometimes the terms ‘inlet’ (for suction) and ‘exit’ or
‘outlet’ (for discharge) are used.

Pumps Applications
To deliver fluid at a higher elevation or at a
long distance.
To deliver fluid at a pressurized device
For the control of hydraulic systems
For drainage system, removing slurries,
mud, water
For irrigation systems
Cleaning, car wash

Centrifugal Pumps
The hydraulic machines that converts the
mechanical energy into pressure energy
by means of centrifugal force acting on the
fluid are called centrifugal pumps.
3 important parts are
 Impeller
 Volute casing
 Suction and delivery pipes.

Centrifugal Pumps (Contd…)
The rotating part of the centrifugal pump is
called impeller. It is a rotating solid disk
with curved blades. Impellers could be
open, semi-open or closed.
Open Semi - Open Closed

Backward curved Radial curved Forward curved
For Incompressible fluids (water) backward
curved vanes are used (pumps)
For compressible fluids (air) forward curved
vanes are used (compressors)

Casing is an airtight passage surrounding the
impeller which converts the kinetic energy of the
fluid leaving the impeller into pressure energy.
Suction pipe is connected to the inlet of the
pump and other side is dipped into the fluid in a
sump. Delivery pipe is connected to the outlet
of the pump and other end delivers the fluid at
required height.

Working principle
 The impeller is keyed onto a shaft which is mounted on
bearings and is coupled to a motor which rotates the
impeller.
 The kinetic energy of the impeller is transmitted to the
fluid and its velocity increases.
 The volute casing converts the kinetic energy of the fluid
to pressure energy. The pressure at the center of the
impeller (eye) decreases as the fluid flows outward. The
decrease in pressure causes the fluid of the sump to
continuously flow through the suction pipes.
 The high pressure fluid is delivered through the delivery
pipe.

Priming
 The pump casing must be filled with liquid
before the pump is started, or the pump will not
be able to function.
 To ensure that a centrifugal pump remains
primed most centrifugal pumps have foot valves
installed or are located below the level of the
source from which the pump is to take its
suction.

Cavitations
 If the suction pressure at the eye of the impeller falls
below the vapor pressure of the fluid being pumped, the
fluid will start to boil.
 Any vapor bubbles formed by the pressure drop at the
eye of the impeller are swept along the impeller vanes
by the flow of the fluid. When the bubbles enter a region
where local pressure is greater than saturation pressure
farther out the impeller vane, the vapor bubbles abruptly
collapse.
 This phenomenon is called cavitation.

There are several effects of cavitations
 It creates noise, vibration, and damage to many
of the components.
We experience a loss in capacity.
 The pump can no longer build the same head
(pressure)
 The output pressure fluctuates.
 The pump's efficiency drops.

Effect of cavitation

Prevention of cavitation
 Raise the liquid level in the tank
 Lower the pumping fluid temperature
 Reduce the N.P.S.H. Required
 Use a pump with a larger, impeller eye opening.
 Pump should be airtight
 Friction losses should be decreased

NPSH (Net positive suction head)
 To avoid cavitation in centrifugal pumps, the
pressure of the fluid at all points within the
pump must remain above saturation pressure.
The quantity used to determine if the pressure
of the liquid being pumped is adequate to avoid
cavitation is the net positive suction head
(NPSH).

 The net positive suction head available (NPSHA) is
the difference between the pressure at the suction of the
pump and the saturation pressure for the liquid being
pumped.
 The net positive suction head required (NPSHR) is the
minimum net positive suction head necessary to avoid
cavitation.
 NPSHA must be greater than NPSHR to avoid
cavitation.
NPSHA > NPSHR
NPSHA = Psuction – Psaturation = Pa + Pst – Pst - hf

Configuration of pumps
 Pumps in parallel
For high flow rate requirement
Head or pressure developed is same as the
individual pump
Flow rate is the summation of the individual pumps
 Pumps in series
For high head or pressure requirement
Flow rate remains same as the individual pump
Head or pressure is the summation of two pumps.

High velocity vs. High pressure
Water can be raised from one level to a higher
level in two ways – High pressure and High
velocity
 High velocity method is very inefficient since the
friction increases with proportional to the square
of the velocity
 High pressure method is efficient because of
low friction.

Characteristics curve
Discharge, Q
Efficiency and
Head/Pressure
Head (Pump Curve)
Efficiency
Fig: Characteristics curve of a centrifugal pump
System curve
Operating point

Specific Speed (NS)
 It is the speed of a pump with a discharging
capacity of 1 m3/sec and a head of 1 m.
 NS = n √Q / H3/4
n = speed of the pump
Q = discharge of the pump
H = head of the pump
 Pump selection is done based on the specific
speed.

Positive Displacement Pumps
A positive displacement pump causes a liquid or
gas to move by trapping a fixed amount of fluid
and then forcing (displacing) that trapped
volume into the discharge pipe.
 Periodic energy addition
 Added energy forces displacement of fluid in an
enclosed volume
 Fluid displacement results in direct increase in pressure
Two types of PDPs
 Reciprocating PDP (Tube well, diaphragm pump)
 Rotary PDP (Gear pump, Vane pump)

Reciprocating PDP
 In a reciprocating pump, a volume of liquid is drawn into
the cylinder through the suction valve on the intake
stroke and is discharged under positive pressure through
the outlet valves on the discharge stroke.
 The discharge from a reciprocating pump is pulsating.
 This is because the intake is always a constant volume.
 Often an air chamber is connected on the discharge side
of the pump to provide a more even flow by evening out
the pressure surges.
 Reciprocating pumps are often used for sludge and
slurry.

Reciprocating PDP
Cross-section of a diaphragm pump

Rotary PDP
A rotary pump traps fluid in its closed casing and
discharges a smooth flow.
They can handle almost any liquid that does not
contain hard and abrasive solids, including
viscous liquids.
They are also simple in design and efficient in
handling flow conditions that are usually
considered to low for economic application of
centrifuges.
Types of rotary pumps include cam-and-piston,
gear, lobular, screw, and vane pumps

Rotary PDP
External Gear Pump

Rotary PDP
Internal Gear Pump

Rotary PDP
Diaphragm Pump
Cross-section of a diaphragm pump

Turbines
Turbines are devices that convert the
energy of fluid into mechanical energy.
The fluid can be water, steam, flue gas etc
The energy of the water can be in the form
of potential or kinetic energy.
Steam turbine and gas turbine uses the
thermal energy of steam and flue gas
respectively.

Turbines Classification
 According to the energy used
 Impulse turbine
 Reaction turbine
 Direction of water flow
 Axial flow - Radial in axial out
 Inward flow - Outward flow
 According to the head available to the inlet of turbine
 High Head Turbine (250-1800m), Pelton Wheel
 Medium Head Turbine (50-250m), Francis Turbine
 Low Head Turbine ( <50m), Kaplan Turbine
 According to the specific speed
 Low specific speed ( <50) Pelton wheel
 Medium specific speed (50 < Ns < 250) Francis
 High Specific speed ( >250) Kaplan
 According to the fluid used
 Water Turbine (Pelton Wheel, Francis Turbine, Kaplan Turbine)
 Gas Turbine
 Steam Turbine

Turbines Classification (Contd…)
Impulse Turbine
 All available head of water is converted into kinetic
energy or velocity head in a nozzle. The water shoots
out of the nozzle and hits a bucket which rotates a shaft.
 Water is in contact with atmosphere all the time and
water discharged from bucket fall freely
 The flow is similar to open channel flow and works
under atmospheric pressure.
 The kinetic energy of water is converted to mechanical
energy.
 The water entering the turbine exerts a force in the
direction of the flow.
 Pelton wheel is an example.

Turbines Classification (Contd…)
Reaction Turbine
 The entire water flow takes place in closed conduit and
under pressure.
 At the entrance to turbine/runner only part of the energy
is converted to kinetic energy, remaining into pressure
energy
 The flow is similar to the closed conduit flow.
 The water exerts a reaction opposite to the direction of
its flow while leaving the turbine.
 Reaction turbines may be inward or outward or radial
flow.
 Francis turbine, Kaplan Turbines are some example

Application of Turbines
Almost all electrical power on Earth is
produced with a turbine of some type.
Very high efficiency turbines harness
about 40% of the thermal energy, with the
rest exhausted as waste heat.
Most jet engines rely on turbines to supply
mechanical work from their working fluid
and fuel as do all nuclear ships and power
plants.

Impulse Turbine
Pelton Wheel
 It consists of a wheel mounted on a shaft.
 Buckets are mounted on the periphery of the wheel
 Water is impinged on the buckets and energy is
transferred
 The water has only kinetic energy
 Each bucket is shaped like a double hemispherical cup
with a sharp edge at the center.
 Pelton wheel is used for high head of water (150-
2000m)
 The flow is tangential.

Reaction Turbine
 Francis Turbine
 The Francis turbine is a reaction turbine, which means that the
working fluid changes pressure as it moves through the turbine,
giving up its energy. A casement is needed to contain the water
flow. The turbine is located between the high pressure water
source and the low pressure water exit, usually at the base of a
dam.
 The inlet is spiral shaped. Guide vanes direct the water
tangentially to the runner. This radial flow acts on the runner
vanes, causing the runner to spin. The guide vanes (or wicket
gate) may be adjustable to allow efficient turbine operation for a
range of water flow conditions.
 As the water moves through the runner its spinning radius
decreases, further acting on the runner. Imagine swinging a ball
on a string around in a circle. If the string is pulled short, the ball
spins faster. This property, in addition to the water's pressure,
helps inward flow turbines harness water energy

Kaplan Turbine
The Kaplan turbine is a propeller-type water
turbine that has adjustable blades.
It is an inward flow reaction turbine
Because of the adjustable blades it is possible to
run at maximum efficiency at any load
Water flows through the guide vanes, and then
flows axially through the runners.
The runner blade angles can be changed by a
lever.
It can work on very low head but requires high
flow rate.

Gas Turbine
 Gas turbine works due to the flow of flue gas through the
stator and runner blades.
 Gas turbines have 3 major components
 Compressor
 Combustion chamber
 Turbine
 Compressor compresses air and supplies it to the
combustion chamber.
 In the combustion chamber the fuel is burnt with the help
of the compressed air and the product of combustion
also called flue gas is flowed through the turbine
 The flue gas moves the turbine blades.

Gas Turbine Application
 Gas turbine has two major applications
 In power generation
 For propulsion (Jet Engine)
 In power generation the main target is to rotate the
generator shaft with the help of the turbine.
 In the propulsion engines, the main target of the turbine
is only to run the compressor. The Flue gas while getting
out of the turbine gives a reaction force which gives the
propulsion. (Jet engine)
 In modern aircraft engine, the turbine also acts as a
propeller. In this type of engine only 25% of the
propulsion comes from the reaction of the flue gas and
the remaining 75% propulsion comes from the propelling
action. (Turboprop, Turbofan)

7117897 pumps-turbines

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