Figure 1 7
Figure 2 7
Figure 3 8
Table 1 10
Table 2 10
Table 3 11
Graph 1 11
Graph 2 12
5. INTRODUCTION
The aim of this experiment is to find the relationship between the flowrate and the pressure drop for a venturi and an orifice measurement device. Also, we need to find the correction coefficients for a venturi and an orifice measurement device, and lastly to compare the values obtained with the literature.
In order to perform this experiment, we will turn on the tap and take readings of the pressure difference for the venturimeter between the points A and B respectively and also take the readings of the orifice plate between the points E and F. All of these points are linked to the manometer.
The measurement of a fluid is a key requirement on any processing plant. Although modern technology has facilitated the development of the non-intrusive measurement devices, the economics of many industries need simple intrusive devices, which rely on the simple relationships such as that between pressure drop and flowrate, still be used.
6. PRINCIPLES
Venturi Meter
This is an instrument used for measuring flowrate of compressible and incompressible fluids within a pipeline. It consists of a venturi tube which has a constricted throat that increases the velocity and decreases the pressure. A few disadvantages of venturi meters are that they cannot be changed for measuring a pressure that is beyond a maximum velocity, they occupy a considerable amount of space and are very expensive.
Figure 1: A diagram of a venturi meter [1]
Orifice Plate
An orifice plate is a device used for measuring the rate of fluid flow. It uses the same principles as the venturi tube; they both apply the concept of the Bernoulli’s principle. The Bernoulli’s principle is a law that states that the pressure of a fluid and the velocity of a fluid are related, and hence if the velocity of the fluid was to increase, then the pressure would decrease and the same applies vice versa.
Figure 2: An Orifice Plate Flange Assembly [2]
The orifice plate is simply a thin plate which has a hole in the middle. It is often placed in a pipe so that fluids can flow. When the fluid is flowing within the pipe, it has a certain velocity and a certain pressure. When this fluid reaches the orifice plate, where the hole in the middle is, the fluid is forced to go through the small hole and at this point a maximum convergence occurs shortly downstream of the physical orifice at a point called the vena contracta (Figure 3).
Figure 3: A flat-plate, sharp-edge orifice [3]
7. Methodology
- We will observe the apparatus and sketch a diagram which will show all the relevant components.
- We will then inspect the manometers and make sure that they can be read clearly.
- We then make sure that the recirculation tank for the hydraulic benches is fully charged.
- We will split the work load among us so that one person would be in charge of recording the change in the height between tubes A and B and between E and F. The other person would be in charge of recording the time taken for the water to increase to 9 kg (first weight to be added), then 18 kg(second weight to be added) and then 27 kg(last weight to be added).
- We will vary the flow rate and measure the resulting drop across the venturi and orifice devices using the manometers.
- We will start at a low velocity and then gradually increase the velocity.
- When we have obtained sufficient amount of data, we will calculate the change in pressure and mass flow rate and then plot a graph of the average flow rate against the change in pressure. This will help us find the relationship between the orifice and the venturi meter.
8. Results
The table below is the results from the experiment that was conducted on the venturimeter.
Table 1
The table below shows the pressure differences worked out for the Venturi and Orifice plate. This was worked out using the following formula:
P = ρg(h1-h2)
ρ = 1000 kg/m3, g = 9.81 m/s2, h = ? m
Table 2
The graph below shows the results obtained for flow rate against pressure difference. Both the Venturi and Orifice plate are on the same graph so their results can be compared easily. The flow rate is on the Y-axis and the pressure difference is on the X-axis. The graph shows that as the flow rate increased, so did the pressure difference on both the Venturi and Orifice plate. There is no time in the results where the pressure difference is constant. There is one anomaly in the results, which is the last result of the orifice plate. This is due to the pump in the tank stalling and having to be restarted. After the restart, the height of all the tubes that measure the pressure had changed and therefore the last result differs from the rest.
GRAPH 1
The relationship between the two flow rates measured, are also compared. First the flow rate taken by using the weights has to be converted into litres/min so a direct comparison can be made. This is done below:
((27/Q)*1000)/(60*9.81)= Q litres/min
Table 3
The graph below shows the comparison between Q (the flow rate that was measured using weights and the bottom tank) and Q’ (the flow rate given by the rotameter). Q is on the Y-axis and Q’ is on the X-axis. As you can see from the graph, they both have the same increase every time a new flow rate is measured. This gives a straight line graph. We tried to increase Q’ (the X-axis) by 2 litres per minute every measurement taken but the graph shows that the increases aren’t regular, but still the correlation between both flow rates still stay very similar.
GRAPH 2
The correction coefficients are worked out using the fomulas below:
Venturi
= 8.84
Orifice Plate
= 0.84
9. Discussion
The main aim of the experiment was to determine what happened to the pressure difference as the flow rate was increased. As predicted before the experiment had begun, the pressure difference did increase as the flow rate was increased. This wasn’t a surprise, but the way the pressure difference increased was. It is not linear as I thought it would be, it’s a slight curve, showing that as the pressure increased more and more, it had more of an effect on the difference between the two measured points. The pressure of the water through the orifice plate drops; this is due to the water not being able to get to points behind the plate so easily, areas represented by the symbol . As the water goes through the ring, the sharp edges on the plate make the water go through the orifice a certain way, like over a weir. The areas make the water cause a pressure drop, and as the flow rate increases, the water has even less chance to get to these areas and the area increases, causing a bigger drop.
The measurements of hA, hB, hE and hF are all quite accurate, but to a degree of error of plus or minus 1mm. This is due to the water levels going up and down and having to estimate the middle of the oscillation. The water does this most likely due to the pump not having a perfect rhythm and not producing a constant flow rate, no head tank is used in this experiment. Bubbles can also cause errors in the data collected but we were very careful to try and get all the bubbles out of the pipes before we took our measurements. As mentioned previously in the results section, the last set of results for the pressure measurements were at a different accuracy. There is a certain amount of water that circulates around the bottom tank, through the water pump, through the Venturimeter and back into the bottom tank. After the water has been pouring into the middle tank, to measure the flow rate, with the 27 kg of weights, there is no more water to be taken out the bottom tank. Because the flow rate keeps increasing, there is more of a chance the water pump will stall because it is trying to suck up air. So on the last result, we encountered this problem and had to turn off all the equipment and restart it. The last measurement was out because the levels in the tubes were changed by the demonstrator by accident. This was the only reading to be affected so the overall curve in the graph can still be recognized.
The data shows clearly what is happening to the pressure and to the flow rate. The graphs in the results section show all the correlations more clearly.
Our results compared to the published literature are quite good. Our results aren’t exactly the same because of differences with equipment, but the correlation in graphs are the same (the way the curve bends). The theory shows that all our results make sense. The pressure does drop through a Venturi and through an Orifice plate, as it’s supposed to. The pressure difference does increase as the flow rate is increased too.
Our findings fulfill all our aims and objectives. We have found the correlation between our flow rates and pressure drops. We have found the correction coefficients for both the Venturi and Orifice plate.
10. Conclusion
The conclusions for this experiment are that the pressure difference increases as the flow rate increases due to the shape of the venturimeter in a whole.
The results show that the measurement of the flow rate taken by us using weights was as accurate as the rotameter. The graph shows this clearly by being a straight line.
Always check to make sure that the bottom tank is full of water to stop the water pump stalling and so producing different results and making you have to start the whole experiment again.
All our results are very similar to the findings of the published literature, but do have bigger errors due to experimental error.
11. References
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[1]
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[2]
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[3]
- FLUID FLOW MEASUREMENT (VENTURI)-(handout)
