CALIBRATION OF VENTURIMETER AND ORIFICMETER Aim: To determine the discharge coefficient of given orifice meter and venture meter and its variation with Reynolds number. Also prepare a calibration chart for the given. Apparatus: An orifice meter and a venturi meter, Suitab Suitable le manome manometer ter (with (with mercur mercury y as the manome manometri tric c fluid) fluid) for measuring pressured drop across the meters, Water circulation system consisting of water reservoir, a centrifugal pump pump having having delivery delivery line line with with by pass. pass. Flow Flow contro controll valves valves are provided on delivery line as well as by pass line. Calibrated tank for measuring the flow rate of water through orifice meter. Stop watch •
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Theory: This experiment introduces us to two devices that are used to measure flow. These instruments are based on the same principle as following: "The press pressure ure differen difference ce betwee between n any two points points on a taperi tapering ng pipe pipe through which the fluid is flowing depends on the change of levels and on the change in velocities and therefore on the volume rate of flow" One, the venturi meter, meter, is a device used for measuring measuring the rate of flow of a fluid through a pipe. Water is allowed to flow through the meter at different rates ranging from zero to the maximum and the corresponding pres pressu sure re diff differ eren ence ces s show shown n in the the mano manome mete terr are are note noted. d. The The actu actual al discharge is determined using the measuring tank and the stop watch.
Figure 1Venturi Meter A Venturi Venturi Meter consists of a short short converging converging conical conical tube leading to a cylindrical portion, called the throat, of smaller diameter of that of the pipeline, which is followed by a diverging section in which the diameter increases again to that of the main pipeline.
The function of the converging portion is to increase the velocity of the fluid and temporarily lower its static pressure. The pressure difference between inlet and throat is developed. This pressure difference is correlated to the rate of flow. The second, the orifice meter, is again a device used for measuring the rate of flow of a fluid through a pipe. It works on the same principle as a venturi meter. It consists of a flat circular plate which has a circular sharp edged hole called orifice, which is concentric with the pipe. It has an opening in it smaller than the internal diameter of the pipeline, placed in a flange connecting the two portions of the pipeline. The orifice diameter is about 0.5 times the diameter of the pipe. A differential manometer is connected at section 1 which is at a distance of about 1.5 to 2 times the pipe diameter upstream from the orifice plate, and at section 2, which is at a distance of about half the diameter of the orifice on the downstream side from the orifice plate.
Figure 2 Orifice Meter The orifice meter comes under the category of "constant area variable head flow meter." The principle of these meters is conversion of pressure head into velocity head, which increases due to constriction placed inside the closed conduit carrying stream of fluid. Construction of orifice meter is simplest amongst all the flow meters in that it consists of a plate with a hole drilled in it. In principle, it is essentially similar to a venturi since it obstructs the flow of fluid, however, due to the absence of guiding passage on the downstream passage of the orifice meter fluid comes out in the form of a free jet. This difference in the flow physics of the two flow meters leads to difference in the value of discharge coefficient and irrecoverable pressure loss even when the area ratios for two are identical. By assuming steady-state, incompressible (constant fluid density), inviscid, laminar flow in a horizontal pipe (no change in elevation) with negligible frictional losses, Bernoulli'sequation reduces to an equation relating the conservation of energy between two points on the same streamline:
or:
By continuity equation: or V 1 = Q / A1 and V 2 = Q / A2 :
Solving for Q:
and:
The above expression for Q gives the theoretical volume flow rate. However, in actual practice there are considerablefrictional lossesin orifice meter dueto boundary layer separation and eventual wake formation on its downstream side and actualvolumetricflow rate through orifice meter is less than the theoreticaldischarge.Introducing the beta factor β = d 2 / d 1 as well as the coefficient of discharge Cd :
The value of Cd is usually less than 1, for the Venturi Meter it is close to one, and for the Orifice it is about 0.65, due to the larger losses in the case of Orifice meter. Observation: Orifice Meter
Sr. no.
Height Time in Take tank n
Mercu ry level
Discharg e
H
T
Rm
Q=H*Atan k/t
uni t
Cm
T
cm
cm3/s
1
21
30
15
700.0000
2
19.5
30
13
650.0000
3
11.8
30
4.4
393.3333
4
13.7
30
6.8
456.6667
5
15.1
30
8
503.3333
6
16.7
30
10
556.6667
7
18.1
30
11.5
603.3333
Velocit y
V=Q/S0 cm/s 454.72 26 422.24 24 255.51 08 296.65 24 326.96 72 361.61 27 391.92 76
C0
Qactual
C0=Qac/k*Rm ^.5)
Qact=Qt/C0 cm3/s
0.7249
965.6192
699573
0.7231
898.9425
649604
0.7521
522.9818
393094
0.7024
650.1517
456388
0.7138
705.1885
503026
0.7060
788.4248
556327
0.7136
845.4914
602966
We see that C0 = approx 0.72 Graphs
Figure 3Qactualvs R
Figure 4 Discharge Coeff.vslog(Reynold Number) Venturi Meter
Reynold s number r=ρ*v*d /μ
Height Time in tank Take n
Mercu ry level
Discharge
H Cm
T T
Rm cm
Q=H*Atank/ t cm3/s
9.5
30
1.8
316.6667
15
30
4.9
500.0000
17.5
30
7.8
583.3333
15.5
30
5.5
516.6667
17.2 21.7
30 30
7.4 12.5
573.3333 723.3333
16.8
30
6.5
560.0000
22
30
13
733.3333
Velocit y
V=Q/S0 cm/s 205.70 79 324.80 19 378.93 55 335.62 86 372.43 95 469.88 363.77 81 476.37 61
Discharge Coefficient C0 C0=Qact/k*Rm^ .5)
Qactual
Qact=Qt/ C0 cm3/s
Reynold s number r=ρ*v*d /μ
0.9467
334.5003
316474
0.9060
551.8973
499695
0.8377
696.3179
582978
0.8836
584.7114
516352
0.8453 0.8206
678.2286 881.4857
572984 722892
0.8810
635.6484
559659
0.8158
898.9425
732886
Here the C0 value vary from 0.8 to 0.95 Graphs: Figure 5 Q vs R
Figure 6 Discharge Coeff.vslog(Reynold Number)
Discussion: Orifice meter is a device used to measure the discharge through pipe. The principal of this meter is conversion of pressure head into velocity head. Inside the orifices meter due to constriction placed a pressure difference between two sections is developed, which determines the discharge through the pipe. The discharge through the orifice meter is calculated by the formula Qact = Co * Qt
where Qt = theoretical discharge, which is measured sing manometer reading Co = discharge coefficient of orifice meter which is the ratio of actual flow rate and theoretical flow rate The discharge coefficient is also dependent upon the diameter ratio of orifice and pipe, shape of orifice, location of pressure taps and the Reynolds number. For a given set up the first 3 parameters are fixed so in the experiment we observe the behavior of discharge coefficient with change in Reynolds number. According to theoretical explication discharge coefficient remains constant for higher Reynolds number.
Result: •
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The discharge coefficient for orifices meter varies in-between 0.70 to 0.77 This all data are calculated at a higher value of Reynolds number. The graph plotted for discharge coefficient verses Reynolds number is very much straight line. This describes that due to high pressure loss in orifices meter the velocity of flow also get reduced. The graphs of manometer reading verses flow rate were come out to be parabola which was expected theoretically.
Venturimeter is a device used to measure the discharge through pipe. The principal of this meter is by reducing the cross section area of flow passes, pressure difference is created and measurement of pressure difference enables the determination of the discharge through pipe. The discharge through the venture meter is calculated by the formula in the same way as in orifice meter. Result: •
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The discharge coefficient for venture meter varies in-between 0.80 to 0.95 The graph plotted for discharge coefficient verses Reynolds number is very much straight line. In comparison of orifice meter, venture meter has less pressure loss. The explanation is that the pressure losses in the convergent section but it pressure regain in divergent section.
Explanation for Error: •
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One of the sources of error is the place where the pressure taps are installed are right or not. There can be error in the measurement of manometer reading.
Precautions: •
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Ensure that there is no air in manometer if there are some air bubbles are present than using air release valve slowly release the air in manometer. After release the air close the air valves again. Never close the delivery line and by pass line simultaneously.
Conclusion:
For both meters it was noted from plots that the difference in pressures are directly related to the rate of discharge. • Coefficient of discharge have a varying value depending on the rate of discharge, where in the venturi meter the value of Cd increase gradually at first with increasing the rate of discharge till a point approaching one, then started to descend with further increment in the rate of discharge. For the Orifice meter the same scheme was noticed but with the ultimate value of 0.7 • The Coefficient of discharge " Cd" for the venturi Meter was greater than that for the Orifice Meter, which indicates that the head losses occurred in the Orifice are larger than that in the venturi, this result was predicted theoretically. • A practical error in taking the reading was noticed when the Cd of that reading was calculated, which showed increasing value of Cd of the previous results. • The arrangement of Orifice Meter is much economical than the use of the venturi meter, but as seen in this experiment the venturi meter provide more accurate results that the Orifice meter, So the choice of which to use in a specified laboratory depends on the financial ability of the organization occupying the device and on the accuracy needed.
