INSTRU PPT.pptx

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    FLOW

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    Over the past 60 years, the importance offlow measurement has grown, not onlybecause of its widespread use for accountingpurposes, such as the custody transfer offluid from supplier to consumer, but alsobecause of its application in manufacturingprocesses.

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    Food & beverage Medical

    Mining & metallurgical

    Oil & gas transport Petrochemical

    Power generation

    Distribution

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    Velocity it is a measure of speed & directionof an object or when related to fluids, it issimply the rate of flow of fluid particles in apipe.

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    Laminar flow of a liquid occurs when itsaverage velocity is comparatively low & thefluid particles tend to move smoothly inlayers

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    Turbulent flow occurs when the flowvelocity is high & the particles no longer flowsmoothly in layers & turbulence or a rollingeffect occurs.

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    Viscosity is a property of a gas or liquid thatis a measure of its resistance to motion orflow.

    Dynamic viscosity measured in poise orcentipoise

    Kinematic viscosity measured in stokes orcentistokes

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    Early flow measurement was centred roundthe question of disputation: how much hashe got versus how much have I got. Asearly as 5000 BC flow measurement was used

    to control the distribution of water throughthe ancient aqueducts of the early Sumeriancivilisations from the Tigris and Euphratesrivers. Such systems were very crude, basedon volume per time: e.g. diverting flow in onedirection from dawn to noon, and diverting itin another direction from noon to dusk.

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    The first major milestone in the field of flowtechnology occurred in 1738 when the Swissphysicist Daniel Bernoulli published hisHydrodynamica in which he outlined theprinciples of the conservation of energy for flow.In it he produced an equation showing that anincrease in the velocity of a flowing fluidincreases its kinetic energy while decreasing itsstatic energy. In this manner a flow restriction

    causes an increase in the flowing velocity and afall in the static pressure the basis of todaysdifferential pressure flow measurement.

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    The word turbine is derived from the Latinspinning top and although the ancient Greeksground flour using horizontal turbine wheels,the idea of using a spinning rotor or turbine

    to measure flow did not come about until1790 when the German engineer, ReinhardWoltman, developed the first vane-typeturbine meter for measuring flow velocities inrivers and canals.

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    Other types of turbine meter followed. In thelate 1800sLester Pelton built the first Peltonwater wheel that turned as a result of water

    jets impinging on buckets attached around

    the outside of the wheel. And in 1916Forrest Nagler designed the first fixed- bladepropeller turbine.

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    A third milestone occurred in 1832 when MichaelFaraday attempted an experiment to use his lawsof electromagnetic induction to measure flow.With the aim of measuring the water flow of theRiver Thames, Faraday lowered two metalelectrodes, connected to a galvanometer, into theriver from Waterloo Bridge. The intent was tomeasure the induced voltage produced by theflow of water through the earthsmagnetic field.

    The failure of Faraday to obtain any meaningfulresults was probably due to electrochemicalinterference and polarization of the electrodes.

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    The last milestone occurred only three yearsafter Faraday conducted his originalexperiment when, in 1835, Gaspar Gustav deCoriolis, made the discovery of what is now

    termed the Coriolis effect, which led, nearly acentury and a half later, to the developmentof the highly accurate direct measurementmass flow Coriolis meter.

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    The cost of many liquids & gases are basedon the measured flow through a pipelinemaking it necessary to accurately measure &control the rate of flow for accounting

    purposes.

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    One of the most important primary propertiesof a fluid (liquid or gas) is its viscosity itsresistance to flow or to objects passingthrough it.

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    The viscosity of a fluid also depends onpressure but, surprisingly, pressure has lesseffect on the viscosity of gases than onliquids.

    A pressure increase from 0 to 70 bar (in air)results in only an approximate 5% increase inviscosity. However, with methanol, forexample, a 0 to 15 bar increase results in a10-fold increase in viscosity.

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    In a frictionless pipe in which there is noretardation at the pipe walls, a flat idealvelocity profile would result in which all thefluid particles move at the same velocity

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    At low flow rates the fluid particles move instraight lines in a laminar manner witheach fluid layer flowing smoothly pastadjacent layers with no mixing between the

    fluid particles in the various layers. As aresult the flow velocity increases from zero,at the pipe walls, to a maximum value at thecentre of the pipe and a velocity gradient

    exists across the pipe.

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    The shape of a fully developed velocity profilefor such a laminar flow is parabolic, with thevelocity at the centre equal to twice the meanflow velocity. Clearly, if not corrected for,

    this concentration of velocity at the centre ofthe pipe can compromise the flowcomputation.

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    As the velocity increases further theindividual paths start to intertwine and crosseach other in a disorderly manner so thatthorough mixing of the fluid takes place.

    This is termed turbulent flow.

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    Obstructions in a pipe, such as bends,elbows, reducers, expanders, strainers,control valves, and T-pieces, can all affect theflow profile in a manner that can severely

    affect measurement accuracy. Such disturbedflow, which should not be confused withturbulent flow, gives rise to a number ofeffects that include:

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    Swirl fluid rotation about the pipe axis.

    Vortices areas of swirling motion with highlocal velocity which are often caused byseparation or a sudden enlargement in pipe

    area.

    Asymmetrical profile

    Symmetrical profile with high core velocity

    caused by a sudden reduction in pipe area.

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    The onset of turbulence is often abrupt andto be able to predict the type of flow presentin a pipe, for any application, use is made ofthe Reynolds number, Re a dimensionless

    number given by:

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    Irrespective of the pipe diameter, type offluid, or velocity, Reynoldsshowed that theflow is:

    Laminar: Re < 2000

    Transitional: Re = 2000 - 5000

    Turbulent: Re > 4000

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    Volumetric flow rate The volumetric flow rate, Q,represents the total volume of fluid flowingthrough a pipe per unit of time and is usuallyexpressed in litres per second (l/s) or cubicmetres per hour (m3/h). The measurement of

    volumetric flow rate is most frequently achievedby measuring the mean velocity of a fluid as ittravels through a pipe of known cross sectionalarea A.

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    The term velocity is often used very loosely todescribe the speed at which the fluid passes apoint along the pipe. In reality, most modernflowmeters measure either the point velocity

    or the mean velocity.

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    The point velocity is the flow velocity in alocalised region or point, in the fluid and is,generally of little use in practice. It is usedmainly in research to determine, for example,

    velocity profiles or flow patterns.

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    The mean flow velocity, can be obtained bymeasuring the volumetric flowrate, Q, anddividing it by the cross-sectional area of thepipe, A:

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    If we want to measure the rate at which wateris flowing along a pipe. A very simple way ofdoing this is to catch all the water coming outof the pipe in a bucket over a fixed time

    period. Measuring the weight of the water inthe bucket and dividing this by the time takento collect this water give