Measurement of Flow 14

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Fluid Flow Measurement Flow measurement is the quantification of bulk

fluid movement.

Flow can be measured in a variety of ways.

Positive-displacement flow meters accumulatea fixed volume of fluid and then count the

number of times the volume is filled to measureflow.

Other flow measurement methods rely onforces produced by the flowing stream as itovercomes a known constriction, to indirectlycalculate flow. Flow may be measured bymeasuring the velocity of fluid over a known

area.


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Fluid Flow Measurement

Classification of flow rate measurements Volume flow

Industrial process control Heat transfer and cooling

Mass flow Industrial process control

Fluid velocity Experimental study of flow phenomena

Flow visualization Image of overall flow field

L=length m=masst=time p=mass density of fluid


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Bernoulli's Equation

The Bernoulli Equation can be considered to bea statement of the conservation of energyprinciple appropriate for flowing fluids.

BE is named after the Swiss scientist DanielBernoulli who published his principle in his bookHydrodynamica in 1738.


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Bernoulli's EquationThe Bernoulli equation states that,

where

points 1 and 2 lie on a streamline,

the fluid has constant density,

the flow is steady, and

there is no friction.

Bernoulli equation can give great insight into thebalance between pressure, velocity andelevation.


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Where:

Dynamic/velocity pressure

The piezometric head or hydraulic head (thesum of the elevation z and the pressure head)

The total pressure (the sum of the staticpressure p and dynamic pressure q)


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The constant in the Bernoulli equation can benormalised. A common approach is in terms oftotal head or energy head H:


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Bernoulli effect`:It is the decrease in pressure as the velocity of a

fluid increases.


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FLOW METERS

There are many hundreds of types of flow metersdepending on the make and application. Theymay be classified roughly as follows.

1. POSITIVE DISPLACEMENT TYPES2.INFERENTIAL TYPES

3. VARIABLE AREA TYPES

4.DIFFERENTIAL PRESSURE TYPES

OS S C S


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1.POSITIVE DISPLACEMENT TYPESThese types have a mechanical element that makes

the shaft of the meter rotate once for an exact

known quantity of fluid. The quantity of fluid hencedepends on the number of revolutions of the metershaft and the flow rate depends upon the speed ofrotation. Both the revolutions and speed may be

measured with mechanical or electronic devices.Some of the most common listed below.

Rotary piston type.

Vane type.

Lobe type or meshing rotor.

Reciprocating piston type

Fluted spiral gear.


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2.INFERENTIAL TYPE METERS

The flow of the fluid is inferred from some effectproduced by the flow. Usually this is a rotorwhich is made to spin and the speed of the rotor

is sensed mechanically or electronically. Themain types are :

Turbine rotor types

Rotary shunt types

Rotating vane types

Helical turbine types


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3.VARIABLE AREA TYPES

There are two main types of this meter

Float type (Rotameter)

Tapered plug type.


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4.DIFFERENTIAL PRESSURE FLOWMETERS

These are a range of meters that convert flow rateinto a differential pressure. The important typesconform to BS 1042 and are

ORIFICE METERS VENTURI METERS

NOZZLE METERS

PITOT TUBES

4 DIFFERENTIAL PRESSURE FLOW


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4.DIFFERENTIAL PRESSURE FLOWMETERS


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Venturi Meters


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Venturi Meters

where

C = is a dimensionless coefficient from approximately0.935 to 0988 (small to large throat velocity and

diameter)

B = is the ratio of D2/D1


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Venturi Meters

D1 = are the inside diameters at the upstream

D2 = are the inside diameters at the throatsections

A2 = the ares of the throat section

change in h = the head differential

sg = specific gravity of the manometer liquid


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Venturi MetersNote that:

-The discharge coefficient, C, is a constant value forgiven venturi dimensions

-If D2=D1, then B=1, and Q is undefined; if D0>D1, youget the square root of a negative number(neither does

not apply to venturi)-The coefficient, C, must be adjusted to accomodate

variations in water temp.

-The value of B is usually between 0.25 and 0.50, but

may be high as 0.75

-Venturi meters have benn made out of steel, iron,concrete, wood, plastic, brass, bronze, and othermaterials


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Venturi Meters


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ORIFICE METERS


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ORIFICE METERS


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ORIFICE METERS


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ORIFICE METERS


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ORIFICE METERS


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References en.wikipedia.org/wiki/Volumetric_flow_rate

www.engr.sjsu.edu/bjfurman/courses/ME120/me120pdf/FlowMeasurement.pdf

www.freestudy.co.uk/instrumentation/tutorial2.pdf

ocw.usu.edu/biological_and_irrigation_engineering/Irrigation___Conveyance_Control_Systems/6300__L13_FlowMeasurementinPipes.pdf

hyperphysics.phy-astr.gsu.edu/hbase/pber.html

www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

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Measurement of Flow 14