ME2512: Thermofluid mechanicsPractical 1 Calibration of Venturi and orifice meters

Name:Aaron RichesStudent Number:12845817

Introduction There are different methods to calculate flow rate Q [m/s], three of these methods, Bulk Measurement, venturi and orifice will be used in this experiment, where the bulk measurement is used to develop calibration curves for the two other methods.In this experiment we determine the coefficients of calibration for the venturi meter and orifice meter by means of calculation the flow rate via bulk measurement and with TheoryBulk measurement can be used where a linear flow is present, can be used to calculate the flow rate. When linear flow is established, for a set period of time, the volume of liquid is then collected and the volume of fluid is then measured.

In general, it is usually easier to weigh the fluid and using the specific weight, convert the weight to volume, than it is to measure the volume.Venturi meter and an orifice plate meter are two devices that are used to measure flow rate of fluid in pipes. These two devices work on a technique based on the Bernoulli equation. (1)P [Pa] = pressure, = Specific weightu = velocity of fluidg = acceleration due to gravityz = vertical distance from a datum*Subscript indicates a particular position along the stream line.Flow is often distinguished by an average velocity in a pipe. If flow rate Q and the cross sectional area of the pipe is A, then the average velocity of the cross section can be calculated as:(2)

Substituting equation (2) into (1), solving for Q, we get:(3)

Venturi

A venturi meter (Figure 1) is made up of a chamber of same size as attached pipe, a smooth entrance cone, a neck or throat, and a gradual gradient exit cone, which again connects to the pipe. Taps are located in the entrance chamber and throat to enable pressure measurement.If the cross sectional areas and pressures of both the entrance chamber and the throat are known, the equation (3) can be used to calculate the flow rate Q. However due to some energy being dissipated due to turbulence between the pressure taps, equation (3) will over predict the flow rate. Equation (3) is altered and a coefficient of calibration is added to account for the over prediction, giving:

(4)

Orifice

An Orifice Meter (Figure 2) Consists pf a plate with a sharp edged concentric circular hole about the centerline of the pipe, which creates a partial blockage of the pipe. Characteristics of the orifice is different than that of the venture in that the minimum cross section of the streamline occurs downstream from the orifice instead of at the orifice, is called vena contracta (the narrowest point of the flow). Its exact position depends on the fluid velocity, but the pressure tap position is fixed therefore a coefficient of contraction, and the area of the orifice is used to specify the cross sectional area of the vena contracta.(5)Substituting (5) into (4),(6)Which is often written as:(7)Coefficients for both the orifice and venturi will vary with fluid flow velocity. Common variations of this type are expressed as functions of Reynolds number R:(8)Where d is the diameter of the throat, and is the kinematic viscosity of the fluid.AimsThis experiment aims to determine the calibration coefficients for a venturi meter, and an orifice meter. Methodology Apparatus Venturi Meter Orifice meter Drum and Scales Manometers Water Supply, and valve StopwatchThe venturi meter and the orifice meter is connected in series to a pipeline of laboratory water supply. As seen in (figure 3), A valve is connected at the beginning of the pipe to control the flow, and the downstream end flows out into the atmosphere, and into the drum for collection.

Figure 3. Apparatus DiagramProcedures1. We first determined the mass of the drum, using the scales, then added 30kg to the scales2. The valve was then turned to an arbitrary position, the manometers at the orifice was read, and the same with the orifice manometers. At the same time the drum was put under the end stream, and was timed to when 30kg of water had filled it, we promptly emptied it and repeated two more times.3. Step 2 was repeated 10 times, with all measurements recorded.

Results & Discussion Results & AnalysisRaw data

VenturiOrificeBulk

Measurementleftrightleft right

13034605914.62

14.5

14.41

z, average time6411914.51

22428504716.44

16.13

16.4

z, average time529716.32333333

31918292721.97

21.85

21.85

z, average time375621.89

4813151430.32

30.36

30.61

z, average time212930.43

51519323020.53

20.44

20.69

z, average time346220.55333333

62327474716.74

16.66

16.7

z, average time509416.7

72630555215.26

15.46

15.38

z, average time5610715.36666667

82933625914.5

14.63

14.38

z, average time6212114.50333333

93036636014.3

14.26

14.35

z, average time6612314.30333333

102124413917.7

17.68

17.75

z, average time458017.71

Table 1.1 Raw Data Obtained, and average time, and change in manometer height z.flow [Q]ReCvC

0.002068125355.871.7329418451.380553

0.001838111430.281.7089559441.35925

0.0013783093.361.5107575191.333995

0.00098659773.701.442546391.3335

0.0014688497.261.67849351.350255

0.001796108916.981.703491281.349627

0.001952118367.481.7493137261.374746

0.002068125413.491.7614799921.369726

0.002097127167.121.7311398771.37754

0.001694102705.461.6932323991.379528

Table 1.2 Calculated flow, and orifice & Venturi coefficients

Graph1.1

To find the coefficients, the following Equations used:

Faculty of Science and EngineeringSchool of Engineering & Physical Sciences

Page 10

Example Calculations Flow rate 1

Graph 1.1 was produced, from the calculated Reynolds Number and the coefficients for the venturi and the orifice meters.Error AnalysisQuantitate and Non-Quantitative errors were evident in this experiment.Non-Quantitative errorsNon-Quantitative errors, are that which effect the results of the experiment through issues at the time of the experiment.1. Flow rate measurement via the bulk measurement method may not have been accurate due to the inadequately slow speed in which the bucket was put under the stream and removed to collect 30kg. Timing of this procedure may also be inaccurate as it is up to the reaction time of the person timing to start/stop the timer.2. Manometer readings could be miss read due to parallax error, and the mercury oxide deposits within the manometer at the instance where water meets the mercury.3. We took assumption in the fact that the water supplied was at a steady constant flow. Without a flow gauge we were unable to confirm this. Therefore there is a potential inaccuracy if the flow was not steady.4. The scale used to weigh the bucket and the water, is of an outdated system, which with the pace of the experiment would have been highly inaccurate.Quantitate errorsQuantitative errors are that which arise from calculations, and measurements gathered.Weight scale error: minimum measure 0.2kg, therefore error: 0.1kgTime error: minimum measure 1s, therefore error: .05sError in manometer: H= 64mm 0.5mmDiscussionAfter completing the equations for the coefficients of calibration of the orifice and venturi, and plotting them on the graph 1.1, both devices seem to have converged to similar constant values when graphed against Reynolds Number. The max Cv = 1.76, The max C = 1.38Whereas the supplied graph is approximately: Max Cv = 0.91 Max C = 0.69Showing a considerable margin of error: 48.29% for Cv 50% for CThe coefficients of calibrations experimental values were decisively inaccurate, although the orifice meter coefficient being significantly further out than that of the venturi meter. Hypothetically this may have been a resultant from turbulence caused by the venturi meter being in close proximity upstream of the orifice, creating a pulsating effect on the manometer gauge, making it even more difficult to read.I do believe, if the two meters were run separately on their own pipeline instead if in series, would determine a better outcome, as the flow rate would be of the single meter not being effected on by the other.

Conclusions The values of the coefficient of calibration for the venturi meter was found to be 1.76, while the coefficient of calibration for the orifice was found to be 1.38. These values are unrealistic as they need to be less than 1 to have affected the flow rate and lowered this value. Accuracy in this experiment was particularly poor reaching an error of 50% for the orifice meter. In saying that, this experiment was successful to its aim, as we were able to replicate the process of determining the coefficient of calibration, and have a similar graph as supplied.There are some improvements that could be made, where the resultant error would be significantly less:1. Digital scales for the weighing of the drum and fluid,2. Separate the venturi, and orifice, or increase the distance between them,3. Flow meter to ensure the flow is steady,4. Instead of measuring to a particular weight I water, measure water to a fixed time that way the flow rate could be measured much more accurately.Practical Questions1. What are the units for Bernoullis equation? The units are in meters.2. Can you hypothesise a limit for the value Cv? The value for Cv cannot exceed that of Q and also cannot be less than 0, as if more than Q then there is a blockage, and less than zero cannot happen as it would then seem to reverse the flow rate. Ideally the value should be between 0 and 1 in an idealistic situation.

References

ME2512 Practical 1 Handout, JCU Douglas campus Semester 1 2015