124247093 Fluid Mechanic in Pipeline

7/27/2019 124247093 Fluid Mechanic in Pipeline

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FLUID MECHANICSFLUID MECHANICS

FORFOR

CHEMICAL ENGINEERSCHEMICAL ENGINEERS


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IntroductionIntroduction

Fluid mechanics, a special branch of general mechanics, describesFluid mechanics, a special branch of general mechanics, describesthe laws of liquid and gas motion. Flows of liquids and gases play anthe laws of liquid and gas motion. Flows of liquids and gases play animportant role in nature and in technical applications, as, forimportant role in nature and in technical applications, as, for

example, flows in living organisms, atmospheric circulation, oceanicexample, flows in living organisms, atmospheric circulation, oceaniccurrents, flows in rivers, wind- and water loads on buildings andcurrents, flows in rivers, wind- and water loads on buildings andstructures, gas motion in flames and explosions, aero- andstructures, gas motion in flames and explosions, aero- andhydrodynamic forces acting on airplanes and ships, flows in waterhydrodynamic forces acting on airplanes and ships, flows in waterand gas turbines, pumps, engines, pipes, valves, bearings, hydraulicand gas turbines, pumps, engines, pipes, valves, bearings, hydraulicsystems, and others. The Fluid Mechanics is essential insystems, and others. The Fluid Mechanics is essential in ChemicalChemicalEngineeringEn

gineering because the majority of chemicalbecause the majority of chemical processingprocessing

operations are conducted either partly or totally in the fluid phase.operations are conducted either partly or totally in the fluid phase.Examples of such operations abound in the Biochemical, chemical,Examples of such operations abound in the Biochemical, chemical,energy, fermentation, materials, petroleum, pharmaceutical,energy, fermentation, materials, petroleum, pharmaceutical,polymers and waste-processing industries.polymers and waste-processing industries.

So what is a Fluid?So what is a Fluid?

A fluid is defined as a substance that deforms continuously whilstA fluid is defined as a substance that deforms continuously whilstacted upon by any force tangential to the area on which it acts.acted upon by any force tangential to the area on which it acts.Such a force is termed aSuch a force is termed a shear forceshear force, and the ratio of the shear, and the ratio of the shearforce to the area on which it acts is known as theforce to the area on which it acts is known as the shear stressshear stress..

The rate at which the fluid deforms continuously depends not onlyThe rate at which the fluid deforms continuously depends not onlyon the magnitude of the applied force but also on a property of theon the magnitude of the applied force but also on a property of the

fluid called itsfluid called its viscosityviscosit

y or resistance to deformation and flow.or resistance to deformation and flow.


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PressurePressure

Force per unit area is called pressure, andForce per unit area is called pressure, and

its unit is the Pascal, N/m2 in the SIits unit is the Pascal, N/m2 in the SIsystem and psia, lbf/in2 absolute, in thesystem and psia, lbf/in2 absolute, in the

English system.English system.

PForce

Area

F

A= =

1 10

1 10 10

3

2

6

2

3

kPa

N

m

MPaN

mkPa

=

= =


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The pressure used inThe pressure used in allall calculations of state iscalculations of state isthethe absolute pressureabsolute pressure measured relative tomeasured relative toabsolute zero pressure. However, pressuresabsolute zero pressure. However, pressures

are often measured relative toare often measured relative to atmosphericatmosphericpressurepressure calledcalled gagegage oror vacuumvacuum pressures.pressures.In the English system the absolute pressureIn the English system the absolute pressureand gage pressures are distinguished by theirand gage pressures are distinguished by their

units, psia (pounds force per square inchunits, psia (pounds force per square inchabsolute) and psig (pounds force per squareabsolute) and psig (pounds force per squareinch gage), respectively; however, the SIinch gage), respectively; however, the SIsystem makes no distinction between absolutesystem makes no distinction between absolute

and gage pressures.and gage pressures.


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The relation among atmospheric, gage, and vacuum pressures is shownThe relation among atmospheric, gage, and vacuum pressures is shown

below. Small to moderate pressure differences are measured by abelow. Small to moderate pressure differences are measured by amanometer and a differential fluid column of height h corresponds to amanometer and a differential fluid column of height h corresponds to apressure difference between the system and the surroundings of thepressure difference between the system and the surroundings of themanometer. This pressure difference is determined from the manometermanometer. This pressure difference is determined from the manometerfluid displaced height asfluid displaced height as

P P Pgage abs atm=

P P Pvac atm abs=

P P Pabs atm gage=


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VelocityVelocity If the fluid passes through a plane of areaIf the fluid passes through a plane of area

AA normal to the direction of the velocity,,normal to the direction of the velocity,,the correspondingthe corresponding volumetricvolumetric flow rate offlow rate offluid through the plane is Q =fluid through the plane is Q =u A.u A.

The correspondingThe corresponding massmass flow rate is m=flow rate is m=ppQQ ==p u A,p u A, wherewherepp is the (constant) fluidis the (constant) fluiddensity.density.

When velocity is multiplied by mass itWhen velocity is multiplied by mass itgivesgives momentum,momentum, a quantity of primea quantity of primeimportance in fluid mechanics. Theimportance in fluid mechanics. Thecorrespondingcorresponding momentummomentum flow rateflow ratepassing through the areapassing through the areaAA isis MM == mumu ==pp

uu22A.A.


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Basic lawsBasic laws.. When applying these laws, the procedureWhen applying these laws, the procedure

is first to identify a system, its boundary,is first to identify a system, its boundary,and its surroundings; and second, toand its surroundings; and second, to

identify how the system interacts with itsidentify how the system interacts with its

surroundings. Refer to Fig. and let thesurroundings. Refer to Fig. and let the

quantity X represent either mass, energy,quantity X represent either mass, energy,or momentum.or momentum.


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DensityDensity

S= p/ pS= p/ pww Degrees A.P.I. (American PetroleumDegrees A.P.I. (American Petroleum

Institute) are related to specific gravityInstitute) are related to specific gravity

ss by the formulaby the formula

A.P.I= 141.5/S -131.5A.P.I= 141.5/S -131.5

Thus, for the crude oil listed in Table 1.1,Thus, for the crude oil listed in Table 1.1,

indeed gives 141.5/0.851indeed gives 141.5/0.851 131.5 =131.5 =

35A.P.I.35A.P.I.


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Densities of gases.Densities of gases. ForFor idealideal gases,gases,pVpVnRT,nRT, wherewherepp is theis the absoluteabsolute pressure,pressure,

VVis the volume of the gas,is the volume of the gas, nn is theis thenumber of moles,number of moles, RR is the gasis the gas

constant, andconstant, and TTis theis the absoluteabsolute

temperature. Iftemperature. IfMM is the molecularis the molecular

weight of the gas, it follows that:weight of the gas, it follows that:

p= PM/RTp= PM/RT

For NONFor NON

idealideal

gasesgases

p= PM/ZRTp= PM/ZRT


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VISCOSITY

All fluids offer resistance to any force

tending to cause one layer to move overanother. Viscosityis the fluid propertyresponsible for this resistance. Sincerelative motion between layers requires

the application of shearing forces, that is,forces parallel to the surfaces over whichthey act, the resisting forces must be inexactly the opposite direction to theapplied shear forces and so they too areparallel to the surfaces.


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SURFACE TENSION

Surface tension arises from the forcesbetween the molecules of a liquid

and the forces (generally of adifferent magnitude) between theliquid molecules and those of any

adjacent substance.

U itU it


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UnitsUnits

An important component to the solution toAn important component to the solution to

any engineering thermodynamic problemany engineering thermodynamic problemrequires the proper use of units.requires the proper use of units.


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124247093 Fluid Mechanic in Pipeline