Venturimeter

It is a device, which is used for measuring the rate of flow of fluid through a pipe. It consists of an • Inlet section followed by • Convergent section • A cylindrical throat and• A gradually divergent cone. 

Principle

The basic principle on which it works is that by reducing the cross sectional area of the flow passage, a pressure difference is created and the measurement of the pressure difference enables the determination of the discharge through the pipe.

OBJECT

1. TO CALIBRATE THE VENTURIMETER.

2. TO OBTAIN A GRAPH BETWEEN DISCHARGE

COEFFICIENT ‘K’ AND LOG10RE.

WORKING.

AS THE CROSS SECTION OF THE THROAT IS SMALLER THAN THE CROSS SECTIONAL AREA OF THE INLET SECTION, THE VELOCITY OF FLOW AT THE THROAT WILL BECOME GREATER THAN THAT AT THE INLET SECTION, ACCORDING TO CONTINUITY EQUATION.

THE INCREASE IN THE VELOCITY OF FLOW AT THE THROAT RESULT IN DECREASE IN PRESSURE AT THIS SECTION. AS SUCH A PRESSURE IS DEVELOPED BETWEEN THE INLET SECTION AND THE THROAT OF VENTURIMETER.

THIS PRESSURE DIFFERENCE CAN BE DETERMINED EITHER BY CONNECTING DIFFERENTIAL MANOMETER BETWEEN THE PRESSURE TAPS PROVIDED AT THESE SECTIONS OR BY CONNECTING THE SEPARATE PRESSURE GAUGE AT EACH OF THE PRESSURE TAPS. THE MEASURE OF THE PRESSURE DIFFERENCE BETWEEN THESE SECTIONS ENABLES THE RATE OF FLOW OF FLUID TO BE CALCULATED.

THE CROSS SECTIONAL AREA OF THE THROAT OF VENTURIMETER SHOULD NOT BE REDUCED UNTO A CERTAIN LIMIT, OTHERWISE THE PRESSURE AT THIS SECTION DROPS BELOW THE VAPOR PRESSURE OF THE FLOWING FLUID THAN THE FLOWING FLUID MAY VAPORIZE AND THE VAPOR POCKETS MAY BE FORMED IN THE LIQUID AT THIS SECTION.

THEORY

BY APPLYING BERNOULLI THEOREM BETWEEN ENTRANCE AND THROAT SECTIONS, THE RELATION FOR DISCHARGE CAN BE DERIVED.Q = K A√2g √∆h

√((D/d)4-1)= K.C. √∆h

Here, ∆h is the difference of piezometric.

HEADS BETWEEN ENTRANCE AND THROAT SECTION AND IS KNOWN AS “VENTURI HEAD” C IS CALLED CONSTANT OF VENTURIMETER AND DEPENDS UPON GEOMETRIC PARAMETERS OF GIVEN VENTURIMETER AND K IS CALLED DISCHARGE COEFFICIENT.

IT TAKES ONTO ACCOUNT THE ENERGY LOSS BETWEEN TWO SECTIONS AND DEPENDS ON THE RE. USUALLY K VARIES FROM 0.96 TO 0.99 FOR RE>105. THE REYNOLDS NO RE IS GIVEN AS U2 D/ 4 =ט Q/ΠDט

IS THE KINEMATIC VISCOSITY OF FLUID AND DEPENDS טON THE TEMPERATURE.

EXPERIMENTAL SETUPEXPERIMENTAL SETUP

THE VENTURIMETER IS FITTED IN A PIPE LINE, TO WHICH AN INLET VALVE IS FITTED. THE PRESSURE TAPS ARE PROVIDED AT ENTRANCE SECTION 1 AND THROAT SECTION 2. A U-TUBE MERCURY MANOMETER IS FITTED BETWEEN THESE POINTS 1 AND 2. WATER TANK AND A STOP WATCH IS USED TO MEASURE THE DISCHARGE.

PROCEDURE

1. REGULATE THE INLET VALVE TO ALLOW A STEADY FLOW THROUGH VENTURIMETER

2. COLLECT THE CERTAIN VOLUME OF WATER IN THE TANK AND NOTE THE TIME T

3. NOTE THE MANOMETER READING IN THE LEFT LIMB (H1) AND RIGHT LIMB OF MANOMETER (H2). THIS DIFFERENCE IS X.

4. REPEAT THE STEPS 1 TO 3 FOR OTHER RATE OF FLOW.

5. TAKE SEVEN TO EIGHT SETS OF READINGS

CALCULATIONSCALCULATIONS

1. THE VENTURIMETER CONSTANT C IS CALCULATED AS.

C = A√2G /√((D/D)4-1)

2. THE DISCHARGE Q IS THEN CALCULATED BY VOLUMETRIC METHOD

3. THE DEFLECTION OF MERCURY MANOMETER X IS CONVERTED INTO EQUIVALENT HEAD OF FLOWING LIQUID WATER)

H = X (SM/S - 1) = X (13.6/1 – 1 ) = 12.6XH

WHERE SM IS SPECIFIC GRAVITY OF MEASURING FLUID AND S IS THE SPECIFIC GRAVITY OF WATER.

4. NOW THE SQUARE ROOT OF H IS DETERMINED.

5. THE DISCHARGE COEFFICIENT K IS CALCULATED.

K = Q/C√H

6. FINALLY THE RE IS CALCULATED CORRESPONDING TO DIFFERENT SET OF DISCHARGES.

FORMULA USED 1. Q = K.HN

2. K = CD.A1.A2√2G

√(A12-A2

2 )

WHERE ,Q= DISCHARGE (RATE OF FLOW)K= CONSTANTH= DIFFERENTIAL HEADCD= COEFFICIENT OF DISCHARGE A1= AREA OF INLET SECTION A2= AREA OF THROAT SECTIONG= GRAVITATIONAL ACCELERATIONN= 0.5(APPR.)

OBSERVATION TABLESTABLE 1

DISCHARGE MEASUREMENT TABLES.No. Area

of

collecting

tank

Depth Of Water collected Volume

of

water

collected

Time

of

collection

Discharge

Q = (A1∆h/t)

cumec

Initia

reading

Final

reading

depth

S.NO. Manometer reading Head

H cm

h½ K Re LogRe

I Limb

II Limb

Diff. ‘xcm

Table 2Dia of pipe D=Dia of throat d=

Area of collecting tank = A

Lab temperature = , kinematic viscosity = m2/sec

PRESENTATION OF RESULT

PLOT A GRAPH BETWEEN Q AND √H ON AN ORDINARY GRAPH. IT WILL BE A STRAIGHT LINE PASSING THROUGH THE ORIGIN

PLOT ANOTHER GRAPH BETWEEN K AND RE ON A SEMI LOGARTHMIC GRAPH SHEET

AVERAGE VALUE OF K =…..

PRECAUTIONS

1. DRIVE OUT ALL ENTRAPPED AIR FROM DIFFERENTIAL MERCURY MANOMETER.

2. MAINTAIN A CONSTANT DISCHARGE BEFORE TAKING ANY READING.

VIVA - VOICE

• WHAT IS THE PRINCIPLE AND USE OF VENTURIMETER? CAN IT BE USED FOR LARGE PEN STOKES. HOW WILL PRESSURE CORRECTIONS BE MADE THEN ?

• WHY IS THE LENGTH ON CONVERGING CONE SMALLER THEN THE DIVERGING CONE OF VENTURIMETER?

• WHY IS THE PRESSURE DIFFERENCE BETWEEN ENTRANCE AND THROAT SECTION INCREASED DUE TO FRICTION?

• IF THE DIRECTION OF FLOW IS REVERSED WHAT WILL BE THE EFFECT ON THE VALUE OF ‘H’ (I) IF FRICTION IS NEGLECTED. (II) FRICTION IS TAKEN INTO ACCOUNT?

• ACCURACY OF RESULT DEPENDS ON HOW GREAT IS THE VALUE OF H. IF WE REDUCE ‘D’ FURTHER, THE VALUE OF H INCREASES. CAN WE GO O REDUCING THROAT DIAMETER ‘D’ AS MUCH AS WE PLEASE?IF NOT WHAT IS THE FACTOR TO LIMIT THE VALUED/D?

• JUST BY SEEING A SEPARATE VENTURIMETER, CAN YOU ASCERTAIN THE DIRECTION OF FLOW, HOW?