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)

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

Pumps Classification

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 deviceFor the control of hydraulic systemsFor drainage system, removing slurries,

mud, waterFor irrigation systemsCleaning, 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

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


For Incompressible fluids (water) backward curved vanes are used (pumps)

For compressible fluids (air) forward curved vanes are used (compressors)

Backward curved Radial curved Forward curved


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

Eff

icie

ncy

and

Hea

d/P

ress

ure 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

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

Lobe Pump

Rotary PDP

Vane Pump

Rotary PDP

Screw Pump

Rotary PDP

Diaphragm Pump

Cross-section of a diaphragm pump

Rotary PDP

Piston pump

Turbines

Turbines

Turbines are devices that convert the energy of fluid into mechanical energy.

The fluid can be water, steam, flue gas etcThe 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.

Pelton Wheel

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

Francis Turbine

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.

Kaplan Turbine

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)

Gas Turbine Power Plant Cycle

Jet Engine

Turbo Jet

Jet Engine

Turboprop

Turbofan

The End

pumps & turbines

Documents

fluid pumps

pressure energy

kinetic energy

pumps applications

energy transmitters

centrifugal force

fluid turbinesa

working fluid