Good Fluid Flow – Venturi Meter And Orifice Plate Report Example

Abstract

Flow measurement has remarkable application in the everyday world. Even for advance scientific requirement, this method of measurement, is very useful. The different type of equipment used for this measurement have been set up. Different type of orifice plate meters and the Venturi meter are discussed. The accuracy related recommendations are also given. The experiment is set up and 6 different readings based on the different fluid flow rate have been made. The theoretical measurements are estimated and the corresponding measurements are taken from the experiment. It is observed that the difference is acceptable and this difference comes from the experiment uncertainties.

Introduction

The importance and application of the analysis and measurement of fluid flow lies in the fact that it has several merits. It is very useful for determining blood-flow rates in arteries and it has very important application where fluid flow is required, for instance, the fluid flow of liquid oxygen in rockets. Since there are several different type of applications of fluid flow analysis, ranging from rocket propulsion to biological application, like blood flow analysis, so at time different meters, equipments and apparatus is required to suit the type of application at hand. The fluid flow meters and equipments must have the property to be able to measure precisely the temperature and the pressure to report accurate findings. Thus, some experiments would require fairly high degree of accuracy and this can be achieve by a costlier equipment. Other instruments can report mildly accurate reading and measurements and so can be used where very high precision is not a great concern.
An important metering principle is usually employed in fluid flow analysis. This principle is related with the placement of some kind of restriction within the pipe within which the fluid’s flow has to be conducted and measure. This restriction causes a pressure drop which is then measured with the desired accuracy. The difference in the measure drop, caused by this restriction has to be measured by the suitable device. There are different kinds of devices available in the market offering this property of accurately measuring the pressure drop. One of such devices is the Venturi meter.

Venturi meter

Venturi meter can use fluid acceleration with the help of a convergence cone having an angle ranging from 15 degrees to 20 degrees. As we are aware that the cone has an upstream side and its throat, so a fluid flowing through it will surely have a pressure difference. The pressure at the upstream of the cone and the pressure of the fluid at the throat is different, and this difference is measurable due to the difference in the height of the capillaries in-built in the venture meter as shown in the below figure:
The difference in the height is h as the kinetic energy at the throat is higher here, while the kinetic energy at the upstream is lower. The difference in the kinetic energy causes the increase in potential energy or the pressure energy. There are various other shapes and sizes of venture meters available, but, it was researched that most venturis are constructed in the similar geometric pattern. For instance , from the documentation of Lambda Square Inc., another shape of venture meter is found as shown in the below figure:

Assuming that liquid will not compress, the Bernoulli’s principle could be applied to determine the pressure drop as:

Cd is the discharge coefficient and for no friction devices, this coefficient = 1. Otherwise, for small frictions, as obviously for the Venturi meters, the coefficient is close to 1. As a benchmark, the coefficient value of 0.975 is considered as standard, but the values can vary remarkable, depending obviously on the friction and the Bernoulli’s principle.

Orifice meter

Orifice meter comprises of a plate where a circular shaped hole is drilled at the center of it. There are 2 pressure taps, one upstream and other one at downstream. There are a few methodologies of placing taps and these methods are crucial in determining the coefficient of the meter. Moreover, there are 4 different type of orifice meters as shown in the below figure:
AS shown clearly in the above diagram, the 4 orifices are round shaped, sharp edged, short tubed and Borda type. Having mentioned the 4 types of orifice, there are other characteristics of orifice meters which affect the coefficients. The locations are enumerated as:
a. flange location, where the tap location is 1” upstream and 1” downstream.
b. Tap location is called vena contracts where tap is located 1 pipe dia. Upstream & o.3 – o.8 pipe dia. Downstream.
c. pipe location means that the tap is 2.5 time pipe dia. Upstream & 8 times pipe dia. Downstream.
Based on this property the coefficient also changes remarkably and for the sake of benchmark, 0.6 is considered as standard.

Procedure

1. Use the Perspex Venturi meter which has the characteristic of having long & gradual convergent part, then throat, then the divergence part as shown in the below figure:
2. Orifice plate meter is also used and the type recommended is the one with the brass plate having hole of small dia. The fluids need to flow through the orifice.
3. As also shown in the above figure that there is a right-hand elbow, wherein the bend loss coefficient can be ascertained.
4. A control valve is also provided which helps return the water to hydraulic bend.
5. Pressure is measured at the entry point A as seen above, at B which is the venturi throat & its exit (Ventrui meter exit = C), Orifice meter flanges ( E & F ) and before that, orifice meter entery which is D as shown above. Finally at the right angle pressure tap labeled G & H.
6. The set-up if this experiment is then connected to a water supply ( hydraulic bench )
7. Ensure that there is no air trapped inside the apparatus and it better to use flexing to remove, if any.
8. The main power supply at the bench is then switched on and the inlet water valve is opened
9. The time required to fill the water tank is noted and the reading are also recorded at all the pressure points as noted above.
10. The valve settings are then changed and all the above readings are noted again.
11. The readings are recorded and the coefficients are evaluated for Venturi and the Orifice meters.

Results and Discussion

The results are recorded in the below table:
The coefficient of discharge are evaluated for orifice plate and Venturi Meter. 6 set of readings are taken as shown in the above figure. The time taken and the flow are recorded. These are plotted n the below graph.

The graphs are also plotted based on this data:

Conclusion
In this task the details of the Venturi and the orifice meters were given. Different types of orifice meters and Venturi meters were discussed along with their applications and accuracy. An experiment set up was made to determine the discharge coefficient using some of the meters. At least 6 different flows were simulated and the corresponding readings were taken as shown in the results charts. The estimated coefficient of discharge and the theoretical values were compared. The difference between the current and the prescribed one are attributed to the experiment uncertainty. This uncertainty arises due to lack of steadiness of the pressure meareuments.

REFERENCES

1. Flowmeters: Introduction, efunda (engineering fundamentals), http://www.efunda.com/DesignStandards/sensors/flowmeters/flowmeter_intro.cfm
2. Simon & Schuster, 2000, New Millennium Encyc. & Reference Library
3. Prandtl, L., and Tietjens, O.G., 1934, , Applied Hydro- and Aeromechanics, Dover Pubs., 1957. [Based on Prandtl’s Lectures. Composed by Prandtl’s student, Tietjens, who turned the lecture notes into a text. Translated by J.P. Den Hartog. First published by United Engineering Trustees
4. Bird, R.B., Stewart, W.E., & Lightfoot, 1960. E.N., Transport Phenomena, John-Wiley & Sons,
5. Ross, S.M., 1998, A First Course in Probability, 5th ed., Prentice-Hall