Lecture Notes_HPS

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INTRODUCTION:

APPLIED HYDRAULICS AND PNEUMATICS

Practically every industrial process requires objects to be

moved, manipulated, held, or subjected to some type of force. Three most commonly employed methods for

producing the required forces/motions are

Electro mechanical- Motors, Solenoids, Levers, Cams

Pneumatics - Air

Hydraulics - Liquids

Source of Words:

Hydraulics - Hydra (Greek for Water)- aulos (Greek or Pipe)

Pneumatic - Pneumn (Greek for wind or breadth)

Syllabus Overview:

Introduction to Fluid

Power:

Fluid power technology is a means to convert, transmit, controland apply fluid energy to perform useful work. Fluid can be eithera liquid or a gas.

Fluid power in general includes hydraulics (liquid like petroleumoil, synthetic oil, water) and pneumatics (Gas like air).

Hydraulics / Oil Hydraulics:

Employs pressurized liquid

Operates @ 200 bar or even much higherUsed in high load applications where accurate speed,control/positioning is required.

Example: N.C Machine tool, Lifting machinery, Earth movingequipment, agriculture machinery

Pneumatics:

Employs compressed air

Operates at 5 to 10 bar (pressure is very limited)

System can produce only low or medium size forces but velocities obtained are usually high. (Because compressed air expands very quickly)



Difficult to control the speed because of the compressibility of air.

Example: Assembly line, Precision machiningoperations, Locators, Industrial automation.


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power was deveoped by

the pressure generated by exert ng a force ona conf ned cts und m n shed n equa magntude and n a d rectons a of the f u d conta ner

w states that thepressure nt n aconf ned qu d acts

xerted on a conf ned qud at a y n a d rectons s the same andacts at r ght ang es tothe er

IMPORTANCE of FPT: Today it would be difficult to identify a productthat has not been affected by fluid power at some point along theroute from raw material to final installation.

HISTO

RY:People have used the natural movement of both air & water.Ex. Using sails on ships. (2500BC)

Wooden Valves to control water flow through bamboo trees.(Chinese @ 4000 BC)

Built Masonry dam across Nile.(Egypt)

Roman engineers produced power (3HP) using vertical wheels.

(1st Century BC) Recent History: Three items are very essential tothe existence and comfort of human kind. 1. Transportation, 2.

Movement of water, 3. Generation & transmission of power.

Contribution of Individuals &Evolution:

1650 – Blaise Pascal; fundamental law of physics on whichthe fluid power systems are based.

1795 – Joseph Bramah’s; built first hydraulic press

1850 – Full development of Bramah’swater press

1906 – Electrical systems are replaced by hydraulicsystem for elevating &controlling guns in the battle ship of U.S.S. Virginia

1926 – Self contained package includes Reservoirs, pumps,controls & actuators. During past 50 years – FPT rose to animportant industry with the increasing emphasis onautomation, quality control, safety, and more efficientenergysystems.

BASICLAW

Basic principle of fluid lPascal .

Pascal Law states that i imass of liquid at rest a i i i i l i i ll i



i normal to the inside w ll l ii .

In simplePascalLa

generated at one poi i ili iequally in alldirections.

More Def.: Pressure e ili i rest is

transmitted equ ll i ll i i , iat any point in a liquid, i

l surfaces of thecontain .

|



Pascal’s law is valid irrespective of the shape of the vessel.

Any change in the exerted pressure is seen almost instantlythroughout the liquid.

Fluid in a system can be asrigid as steel for thetransmission of power.

APPLICATION OF THE LAW:

The force appliedto piston1ismultiplied by ten times.

Displacement is ten times

lesser .

Hydrostatics vs

Hydrokinetics: Types

of Fluid Systems:

Fluid Transport Systems – Transport fluids from one place toanother place to achieve some useful purpose.



Fluid Power Systems – Primarily designed to perform work.



Fluid power Systems: : System

Characteristics: Accuracy of

actuator movement

– Liquid can be compressed only slightly. Hydraulicsystems therefore can produce more accurate, easilycontrolled movement of cylinders and motors thanpneumatic systems

– Compressibility produces a more ‘Spongy’ operation in pneumatic systems; Not suitable where highlyaccurate movement is required.

OperatingPressure

– Hydraulics: 200 bar or more

– Pneumatics: 5 to 10 bar ; Extremely highpressure pneumatic systems normally

are not used.

ActuatorSpeed

– Pneumatic systems: When high speedmovement is required

– Rapid response cylinder operation is also possible withpneumatic systems

– Accuracy islow.

ComponentWeight

– Hydraulic systems operate at higher pressure requiresthe use of stronger materials & more massive designs to

withstand the pressure.

– Pneumatic systems operate at muchlower pressure can be manufactured



using light weight materials and designs that minimizethe amount of material.

Cost

– Pneumatic systems are more expensive to operatethan hydraulic systems.

– This cost can be directly associated withcompression, conditioning

& distribution of air.



Advantages of Fluid power Systems (FPS):

(Compared with mechanical/electrical/electromechanical power transfer system)

An easy means of multiplying and controlling force

and torque.

Infinite & stepless variable speed control for both linearand rotary motion.

Overloading the system simply stalls the actuatorwithout damage to the components. (more easilyachieved/controlled by using relief valves)

Provides an easy means of accuratelycontrolling the speed of machinesand/or machine parts.

Provides the ability to instantly stop and reverselinear and rotary actuator.

Systems easily adapt to accommodate a range of machine sizes and designs.

Systems readily adapt to external controlmethods, including mechanical, pneumatic,

electrical and electronic systems.

Constant force is possible in FPS (irrespective of workoutput moves a few millimeter or several meters perminute)

As the medium of power transmission is fluid, it is not

subjected to any breakage of parts as in mechanicaltransmission.

The parts of hydraulic system are lubricated with thehydraulic liquid itself.

Pneumatic systems provide cleanoperation with minimal fire &

explosion hazard. (So useful for painting

& mining)

The FPS are more compact & simple than a mechanicaldrive because it eliminates the need for links like cams &



gears.

Because of the simplicity & compactness the cost isrelatively low for the power transmitted.

Large volumes of compressed air may be easily stored inpneumatic systems to provide energy for intermittent,heavy system demand.



PEC / DoME / MP / III Year- Mechanical Engineering / V SEM / ME2305- Applied Hydraulics And

8 | P a g

Disadvantages of Fluid power Systems (FPS):

(Compared with mechanical/electrical/electromechanical power transfer system)

Higher safety factors associated with high pressure oil

and compressed air. (Hydraulic elements have to bemachined to a high degree of precision which increasesthe manufacturing cost of the system)

Susceptibility to dirty environments, whichcan cause extreme component

wear without careful filtration.

Fluid leakage and spills cause a slippery messy work

environment around hydraulic equipment.

Fire hazard with hydraulic systems usingcombustible oils.

Special handling and disposal procedures for hydraulic oilrequired by environmental regulations.

High cost of compressing and conditioning air for use in

pneumatic systems.Reduced accuracy in actuator speed control in pneumaticsystems caused by compressibility of air.

Noise level of pneumatic systems when air is directlyexhausted to the atmosphere from components.

Applications of Fluid Power System:

AGRICULTURE – Tractors, Chemical Sprayers, Fertilizerspreaders

AUTOMATION – Automated Transfer Machines

AUTOMOBILE – Power steering, Power Brakes, Suspension

Systems AVIATION – Landing Wheels of Aero plane &

Helicopter, aircraft trolleys BUILDING INDUSTRY – Meteringand mixing of concrete ingredients fromhopper




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CONSTRUCTION EQUIPMENT - Earth Moving Equipments likeexcavators, , Dozers, Post hole diggers

DEFENSE – Missile Launch Systems, Navigation Controls

Cont….


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Continuation…..

ENTERTAINMENT - Amusement parkentertainment rides like roller-

coasters.

FABRICATION INDUSTRY - Pneumatic Drills, Grinders,borers, RivetingMachine, Nut Runners

FOOD and BEVERAGE – Food processing equipment,

wrapping, bottling FOUNDRY – Moulding Machines, Tilting of

Furnaces, Die Casting Machines GLASS INDUSTRY – Vacuum

suction cups for handling.

JIGS and FIXTURES - Fluid power operated clamps.

MACHINE TOOLS – Automated Machine Tools, NC Machines,

MATERIAL HANDLING – Jacks, hoists, cranes, fork-lifts,

conveyor systems. MEDICAL – Breathing assistors, heart

assist devices, cardiac compressionmachine, dental drills and human patient simulator.

MOVIES – Special effect equipment using fluid poweranimations

MINING – Rock drills, excavating equipment, ore conveyorloaders.

PAPERand PACKAGING – Edge trimming,stapling, pressing, bundle wrapping

OIL INDUSTRY – Off-shore oil rigs.

PLASTIC INDUSTRY – Automatic injection mouldingmachines, raw material feeding

PRESS TOOLS – Sheet Metal Bending, Punching,

Stamping ROBOTS – Fluid power operated

Robots, Pneumatic Grippers SHIPS – Stabilizing

systems

TEXTILES – Process control, web tensioning devices. TRANSPORTATION – Hydraulic elevators, winches,

overhead trams UNDER SEA – Submarines, undersea




research vehicles.

WELDING – Full and semi-automatic welding machines

WOOD WORKING – Tree shearers, feeding, clamping and sawoperation




Functions of Fluid Power System:

FPS is made up of component groups containingparts designed to perform specific tasks.

The work may

involve simpleorcomplex

tasks, but thecomponent groupsperform specificsystem functions thatare basic to all fluidpower systems.

BASIC FUNCTIONS OF A FLUIDPOWER SYSTEM

Energy Conversion:

FPS does not generate energy. But transform it into a formthat can be used to complete a task. Process begins with a

prime mover pressurizing a fluid. End with – an actuator toperform work.

Fluid Distribution:

Various types of lines are involved. Valves and othercomponents also serve to assist in fluid distribution.

Fluid Control:

FPS requires the control & regulation of the fluid in thesystem. A number of components are used to control fluidflow rate, direction and pressure in a system.

Work Performance:

Using the energy stored in the pressurized fluid of the systemis the primary function of a FPS.

Fluid Conditioning:

FPS performance & service life require a fluid that is clean &provides lubrication to system components. Thisinvolves 1. Storing fluid 2. Removing dirt &

other contaminants 3. Maintaining proper




system operating temperature




Structure of Fluid Power System:

Physical appearance vary depends on type of fluid used, application,

and power output. Basically Structure of FPS involves five component

groups.

Powerunit

Actuators

Conducto

rs

Control valves

Fluid Conditioning

The structure of fluid power systems: : A—Hydraulicsystem. B—Pneumatic system.

Power Unit: - deals with energy conversion function of thesystem. - Consist of a prime mover, pump & reservoir. - Alsoperforms fluid maintenance.

Actuators: - group of components performs the workdone by thesystem. -Converts the energy in the system fluid to linear orrotary motion. - Basic actuatorsare cylinders for linear motion, & motors for rotary motion. -Variety of cylinders & motors.

Conductors: - Primary function; Fluid distribution. – Consist of pipes,hoses and tubes.– Intake of fluid; Distribution of fluid to and from control valvesand actuators. –

Draining of liquids.

Control valves: - Directional control valves; start/stop/changethe direction. – Pressure control valves; limits the maximum




pressure of the system. – Flow control valves; Control over fluid flowrate to control the rate of movement of an actuator

Fluid Conditioning: - Maintaining and conditioning system fluid. –Requires removal of dirt and moisture. – Assuring proper operatingtemperature. – Filters and heat exchangers are used. – Same task

can be performed by reservoir/conductors. – Filter locations; in theintake line, in high pressure working lines, in the lines that returnfluid from system actuators. – Pneumatic systems additionallyhave separator (removes water droplets), lubricator (adds a fine mistof oil to the air for lubrication)



Types of Fluid Power System

Based on Control System

Open loop system: - Does not use feed back. –Performance is based on the characteristics of individualcomponents. – Not so accurate.

Closed loop system: - Uses servovalves/digital electronics.

Based on the type of controlFluid logic control: - Controlled by Hydraulicoil/air. – Fluid logic devices provide various logicalfunctions AND/NAND, OR/NOR, etc… - Two types; Movingpart logic (MPL) and fluidics.

Electrical control: - Controlled by electrical devices. –Switches, relays, timers and solenoids (Four basicelectric devices). – help to control starting, stopping,sequencing, speed positioning, timing & reversing of actuating cylinders and fluid motors. – Electric control &Fluid power works well together where remote control isessential.

Electronic control: - Controlled bymicro electronic devices. – Electronic brain

controls, Fluid power muscles for doing work. Mostadvanced type includes PLC/Microprocessors. - Change inthe system operation will result in redoing hardwareconnections. – Its overcomed by PEC. - Program can bemodified or new program can be fed to meet the changeof operations and number of such programs can bestored in these devices; Makes more flexible system.

Based on the type of



fluid: Hydraulic fluid

power system

Pneumatic fluid

power system



BASIC SYSTEMCOMPONENTS

A basic hydraulic fluid power syst em



A basic pneumatic fluid power syst em



GENERALARRANGEMENT

A hydraulicsyst em

A pneumatic



syst em

Comparison between Hydraulic, Pneumatic and

Electromechanical Power System:

HYDRAULIC SYSTEM PNEUMATIC SYSTEM

ELECTRO-

MECHANICALSYSTEM

Pressurized Liquid is used Compressed Air is used

Energy istransmittedthroughmechanicalcomponents

Energy stored in

Accumulator

Energy stored in TankEnergy stored in

Batteries

Hydraulic Valves are used

Pneumatic Valves

areused

Variable Frequencydrives

Transmission throughHydraulic cylinders,Actuators

Transmissionthrough Pneumaticcylinders, Actuators

TransmissionthroughMechanicalcomponents likeGears, Cams

Flow rate is 2 to 6 m/s Flow rate is 20-40 m/sExcellent withminimum loss

Very smooth and precisemotion

Smooth and LessPrecise motion

More Precision butsmooth motion isnot possible

Large force can begenerated (higher controlforces and work underextreme operatingconditions.)

Limited force canbe achieved.

Large force canbe realized butpoor in efficiency.

Medium Cost High cost Low Cost

Hydraulic systems aregenerally more difficult tooperate.

simpler and easier tohandle.

Complicated indesign and difficultto handle.

Most hydraulicapplications generally

use bigger

components.

Smaller componentscompared to hydraulic.

Biggercomponents.



Dangerous andfire systems

hazardous because of leakage

Automotive brakes,control

Noisy in operations Noise depends on theapplications

Many types of tools

systems of large

aircraft, Earthmovingequipment, N.C.

Machine tool

found in anautomotive

repair shop,Industrialautomation,

Assembly line

Feeding mechanism,

semi- automaticassembly lines



PEC / DoME / MP / III Year- Mechanical Engineering / V SEM / ME2305- Applied Hydraulics AndPneumatics

Transmit fluid powerefficiently

Lubricate the moving

parts

Absorb, carry and transfer the heat generated within the system

(Dissipate heat). Be compatible with hydraulic components.

(Seals clearance between mating parts) Remain stable against a

wide range of possible physical and chemicalchanges, both during storage andwhile in use.

To remove impurities andabrasion

FLUIDS:

States of matter:

A substance exists as a solid. Asheat is added to this substance it

melts into a liquid at its melting point (see phase change), boilsinto a gas at its boiling point, and if heated high enough would enter a

plasma state in which theelectrons areso energized

that they leave their parent atoms from within the gas.

A liquid is a fluid. Unlike asolid, themolecules in a liquid have a much greater freedom to move. The forces that bind the moleculestogether in a solid are only temporary in a liquid,allowing a liquid to flow while a solid remains rigid.

A pure gas may be made up of individual atoms (e.g. a

noble gas or atomic gas like neon), elemental moleculesmade from one type of atom (e.g. oxygen), or compoundmolecules made from a variety of atoms (e.g. carbon




dioxide).

The state of matter distinguished from the solid andliquid states by relatively low density and viscosity,

relatively great expansion and contraction with changesin pressure and temperature, the ability to diffusereadily, and the spontaneous tendency to becomedistributed uniformly throughout any container.




Liquids &Gases

Relative spacing between

molecules Mass, volume &

shape Compressibility

Bulk modulus

v p

- Ve sign indicates pressure increases,volume decreases v

Physical Characteristics:

Weight versus mass: All objects are pulled towards thecenter of the earth by a force of attraction. This force iscalled the weight of the object & isproportional to the objects mass asdefined by F

w mg .

Density: Mass per unit volume. – Instrument: Hydrometer.– Hydraulic oilsused in industries: 800 – 900kg/m3.

Specific Weight: Weight per unit volume.- Calculated bymultiplying density of oil by acceleration due to gravity

Specific Gravity: Ratio of densities of oil andwater. Specific Gravity

Density of Oil

Density of Water

- Has no unit. –Important in those cases wherethe overall

system weight must be keptminimum.

Viscosity: It is a measure of the fluid’s internalresistance offered to flow/shear. – Viscosity varies withtemperature and pressure.

If the viscosity is too high (heavy weight oils), the following resultscan be e xpected:

1. Higher pressure drop due to friction 2. Excessive heatgeneration

3. Sluggish operation 4.More powerconsumption




5. Lower mechanical efficiency

6. Starvation of the pump inlet, causing cavitation.

If the viscosity is too low (light weight oils), the following results canbe expected:

1. Less film strength, and thus more wear on moving parts

2. More leakage 3.More pressure loss

3. Lower volumetric efficiencies in pumps and motors

4. Less precision control and slower responses 5.Lower overallefficiency.



Absolute/Dynamic Viscosity ( )Shear stress

; Ns

Rate of shear m

2

Kinematic viscosity ( )

Measurement of viscosity

Problems

Dynamic Viscosity;

Density of Oilm

2

Sec

Properties of hydraulic fluids:

Good Lubricity: - system contains many surfaces which are

in close contact and which move in relation to eachother. The hydraulic fluidmust separate and lubricate such surfaces. -

Protection against wear –good lubricating characteristics.

Stable Viscosity Characteristics: - very important fluidproperty. - Viscosity may be considered asthe resistance of the fluid to flow or as a measure of

internal friction. Viscosity varies withtemperature and pressure.

Fluids having large changes of viscosity with temperatureare commonly referred as low viscosity index fluids andthose having small changes of viscosity withtemperature are known as high viscosity index fluids. -Viscosity is also important with regard to the ability of fluidto lubricate.

Stable chemically and physically: Fluid characteristicsshould remain unchanged during an extended useful lifeand during storage. The fluid in a working hydraulic systemis subjected to violent usage – large pressure fluctuation,shock, turbulence, aeration, cavitation, water andparticulate contamination, high shear rates, and largetemperature variations. -

The temperatures to which the fluid will be exposed is animportant criterion in the selection of a hydraulic fluid.

System Compatibility: -Hydraulic fluid should be inert to those materials used in or near the hydraulic equipment.



If the fluid in anywayattacks, destroys, dissolves or changes parts of thehydraulic system, the system may lose its functional efficiency and may start malfunctioning. -Changes inthe hydraulic fluid itself caused by interaction with the

system material can also cause system malfunction.-Replacement of one hydraulic fluid with another involvesthe consideration of compatibility.



Good Heat Dissipation: An important requirement of thefluid is to carry heat away from the working parts.Pressure drops, mechanical friction, fluid friction,leakages, all generate heat. The fluid must

carry the generated heat away and readily dissipate it tothe atmosphere or coolers. Therefore highthermal conductivity and high specific heat valuesare desirable in the fluid chosen.

High Bulk modulus: - Oil is taken as incompressible. - inpractice all materials are compressible and so is oil. - Thebulk modulus is a measure of the degree of compressibility of the fluid and is the reciprocal of

compressibility. -The higher the bulk modulus, thelesser the material will be compressed withincreasing pressure.

Adequate Low- temperature properties: This is an important

consideration for hydraulic systems which must operatein outdoors, in low temperature environments or at high

altitudes. Low-temperature properties may be describedby the pour point or viscosity-temperature characteristicsof the fluid.

Flash point: The flash point of hydraulic oil is defined asthe temperature at which flashes will be generatedwhen the oil is brought into contact with any heatedmatter, e.g., a heated stick. The fire pint is actually theignition point of the oil.

Low Foaming tendency: The ability of a fluid to releaseair or other gases without the formation of foam is animportant characteristic of a hydraulic fluid. Excessivefoaming results in loss of fluid if the volume of thehydraulic system is exceeded. Compression of air-oilmixture by pump or actuators will increase its temperaturewhich in turn may cause fluid deterioration by thermal

breakdown or oxidation.Fire Resistant: - Optional property in a goodusable hydraulic fluid.



- Petroleum derivatives, and consequently they burnvigorously once they pass the fire point. For critical applications, artificial or synthetic hydraulic fluids areused which have high fire resistances. -Various gradesof fluids with high water content are also available now adays.



Prevent Rust Formation: - Moisture may be presentto some extent.- Moisture and oxygen cause rusting of iron partsin the hydraulic system. Rust particles can cause

abrasive wear of system components and also act ascatalyst to increase the rate of oxidation of the fluid.Fluids with rust inhibitors help to minimize rust formation.

Low in Volatility: - should have a low volatility , i.e.low vapour pressure or high boiling point characteristic.High vapour pressure may cause high back pressures orvapour-lock resulting in lack of adequate flow. The vapourpressure of a fluid varies with temperature and hence the

operating temperature range of the system is important indetermining the suitability of the fluid.

Good Demulsibility: - Moisture or water may enter ahydraulic system through contamination or condensation.

This water may either dissolve in the fluid or form twolayers. Dissolved water may produce corrosion, rusting orsludge in the fluid. Fluids with emulsifiers easily separate

the water from its main body. Generally used orcontaminated fluids are more likely to emulsify with waterthan new fluids. The resistance of a hydraulic fluid toemulsification, or how well a hydraulic fluid resistsmixing with water.

Low Coefficient of Expansion: - A low coefficient of expansion is usually desirable in a hydraulic fluid tominimize the total volume of the system required at

the operating temperature.

Low Specific Gravity: - Specific gravity of fluid is of importance only in those cases where the overall system weight must be kept to a minimum. Highspecific weight means more weight for a given volume of fluid. Heavy fluids can also cause pump cavitation andmalfunction. This aspect is important especially in the

aircraft industry.Non-toxic, Easy to Handle and Available: - Thesecharacteristics refer to the interaction of the fluid with



people who repair, handle, use or pay for the hydraulicsystem or hydraulic fluid. Obviously, it is desirable thatthe fluid be as simple to handle and as available and cheap as possible.



Various hydraulic fluids (General types of fluids):

WATER: - Should be treated with chemicals before beingused in a FPS. - This treatment removes undesirableContaminants.

Advantage: In expensive, readily available, and Fire resistant.

Disadvantage: No lubricity, corrosive, Temperature limitations.

PETROLEUM OILS: - Most commonamong the hydraulic fluids.

Characteristic of petroleum based hydraulic oils are controlledby the type of crude oil used.

- Napthenic – Low viscosity index; Unsuitable where temperature

vary.- Aromatic – High presence of benzene; more compatiblewith moderate temperature variation.

Paraffinic Oil – High viscosity index; suitable where thetemperature varies greatly.

Advantage: Excellent lubricity, Reasonable cost, Non corrosive

Disadvantage: Tendency to oxidize rapidly; not fire resistant.

WATER GLYCOLS: - Solutions of waterand glycol.

- They contain 35 to 55 % of water.

Advantage:Good fire resistance, Inexpensive, compatiblewith most pipe compounds and seals.

Disadvantage: Not good for high bearing loads, poor corrosion

resistance.

WATER OIL EMULSION: - Contain 40% water. The rest is oil,emulsifiers and other additives. The water is dispersed inmicroscopic droplets surrounded by a film of oil.

Advantage: Good fire resistance, Inexpensive, compatible withmost seals.

Disadvantage: Sometimes difficult to maintain.

PHOSPHATE ESTERS: - These are organic alcohols attachedto a phosphorous atom. They have high thermal stability. They serve as an excellent detergent and prevent building-up



of sludge.

Advantage: Excellent fire resistance, Good lubricity, Noncorrosive.

Disadvantage: Not compatible with many plastics and elastomers,

Fairly expensive.SILICONES: - Dimethyl polysiloxanes. – Have excellentthermal stability.

Advantage: Non-corrosive. Non toxic, Less volatile.




20 | P a g e

Liquids &gases

Ability of fluids to transmit forces in all

directionsFluid

characteristics

Bernoulli’s

equation

Continuity

equation Toricelli’s

theorem

LAMINAR and TURBULENT FLOW:

Normally we assume a constant velocity over the crosssection of a pipe. However when a fluid flows through a

pipe, the layer of fluid at the wall has zero velocity. Thisis due to velocity which causes fluid particles to cling tothe wall. Layers of fluid at the progressively greaterdistances from the pipe surfaces have higher velocitieswith the maximum velocity occurring at the pipe centerline.

LAMINAR

FLOW:Characterized by the fluid flowing insmooth layers.

Stream line flow because all the particles of fluid are moving

in parallel paths. No Collision of particles.

Friction is caused by the sliding of one layer or particle of fluidover another in

a smooth continuousfashion.

TURBULENT FLOW:




20 | P a g e

If the velocity of flow reaches a high enough value, the flowceases to be laminar and becomes turbulent.

Movement of a particle becomes random & fluctuates up

& down in a direction perpendicular as well as parallel to themean flow direction.

This mixing action generates turbulence due to collidingfluid particles.

This causes considerably more resistance to flow and thusgreater energy losses than that produced by laminar flow.




21 | P a g

f

R

REYNOLDS NUMBER:

Experiments by Osbern Reynolds– 1833.

If the flow in the pipe was laminar, the dye jet flowed

smoothly.

However, when turbulent flow occurred in the pipe, the dye jet would mix with the main fluid.

The nature of the flow depends on thedimensionless parameter.

VD VDe

v

Re < 2000 - Laminar flow

Re > 4000 - Turbulent flow

Between 2000 to 4000 – Critical zone between Laminar &

Turbulent. Hydraulic Systems should normally designed to

operate in the laminar flowregio

n.Head Loss due to Friction

The fluid has potential energy due to its pressure, its elevation& Kinetic energy resulting from its movement. But energylosses resulting from friction of the fluid.

The energy loss due to friction in a hydraulic systemresults in a loss of potential energy.

The head loss (HL ) in a system actually consists of twocomponents.

1. Losses in pipes

2. Losses in valves & fittings.

Head loss in pipes can be found by usingDarcy’s equation

H L

L V 2 L - Length of pipef - FrictionFactor

V – Velocity of Flowg – Acceleration due togravity




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D g HL – HeadLoss

D – Inner Diameter

The pressure drop due to friction

P Weight Density( ) x Head Loss(H L )

f 64

Friction Factor for Laminar Flow depends on Reynolod’s No. Re



For Turbulent flow friction factor depends on Reynolds No. &the roughness of the pipe.

Relative

Roughness

Absolute

Roughness( ε )

Inner

Diameter(D)

Losses in Valves & fittings:

Pressure drops are also due to valves, expansions, contractions,bends, elblows, tees & Pipe fittings.

The losses in valves & fittings in hydraulic systems arefrequently computed in terms of equivalent length of hydraulictube.

L

KDe

f

K = Factor for Valve / Fitting

Valve /Fitting K Factor

Globe Valve

Valve /Fitting K Factor

Full openHalf open

GateValve

10.012.

5

Check ValvePoppet

TypeBall type

3.00

4.00

Fullopen¾ openHalf open¼ open

0.190.904.50

24.00

Return Bend

2.20Standard Tee

1.80

Standard Elbow

0.9045

oElbow



0.42

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Lecture Notes_HPS