The experiment of flow meter measurement is conducted to introduce the students to the three different types of commonly used flow meter in the industry. The three flow meters involved in the experiment are Orifice flow meter, Magnetic flow meter, and Coriolis flow meter. It is aimed to help the students to understand the typical methods of flow measurements of an incompressible liquid, in which in this case is water.
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In the industries, many different flow meters are used to measure the different types of fluids. The choice of the flow meter might depend on many factors such as the temperature, pressure conditions, density, viscosity, conductivity or even the conditions of the fluids such as slurries or normal fluid. The selection of the type of flow meters might also be accounted to the accuracy of the flow meters itself.
Therefore, the experiment serves to expose the students to the types of flow meters available in the market and to introduce them to the basic working mechanism of the named flow meters. They may be asked to select a suitable flow meter for the plant or industry when they are in the work environment. Therefore, the basic knowledge of the flow meters as well as their mechanisms and working principles is important in order to be technically sound in the work environment.
2.0 INTRODUCTION & THEORY
There are many types of flow meters available in the market. In order to select the correct flow meter for a use, it is important to study the characteristics of each and every of the flow meters as well as their working mechanism.
For an orifice flow meter or also known as differential pressure flow meters, it uses the concept of pressure drop to measure the flow rate of a fluid. When the fluid flows into a tube of a different size, its velocity changes so is its pressure. For instance, when it moves into a tube of a smaller size, the fluid’s velocity increases and it accelerates. In effect of this, the pressure of the fluid decreases according to Bernoulli’s law. As a result, a differential pressure is created or more specifically, a pressure drop. The magnitude in the pressure drop determines the velocity of the fluid that increases when it enters the smaller tube. The following formulae can be used to calculate the parameters for the experiment:
V = k (h/D)0.5
Q = kA(h/D)0.5
W = kA(hD)0.5
Where h: pressure differential
V: flow element velocity and the
Q: the volumetric flow
W: mass flow
K: discharge coefficient of the element
As for a magnetic flow meter, it utilizes the concept of conductivity to measure the flow rate of the fluid. When a fluid with certain conductivity passes through the magnetic field created by the magnetic flow mater, a voltage is induced. It happens as a result of the fluid crossing the magnetic field or flux of the system. As a result, a voltage is induced and the magnitude of the voltage produced is proportional to the velocity of the fluid that passes through the flow meter. Therefore, a measure of the flow rate is possible. This is known as Faraday’s law of electromagnetic induction. The induced voltage can be calculated using the formula:
Where : E = Induced voltage [proportional to velocity]
: B = Magnetic flux density
: v = mean vcity of the media
: d = Distance between the sensing electrodes
Figure 1: Magnetic flow meter Figure 2: Cross- section view of a magnetic flow meter
For a coriolis meter, it is also known as inertial flow meter or a mass flow meter. It measures directly the mass or quantity of a fluid that passes through the flow meter. The flow rate is then obtained by dividing it by time in which mass per unit time is obtained. For a coriolis flow meter, if the density is constant, then the measurement of the flow rate is simple whereby it is just the mass over time. However in some cases, density changes as the computation might be more complex. A coriolis meter has the ability to measure different types of fluids such as liquids, slurries and gases. In the tubes due to Coriolis forces, the shape of the tube will be deformed and this will cause a vibration. A formula can be used to computate the mass flow.
Where Ku : temperature dependent stiffness of the tube
K: a shape-dependent factor
D: the width
Î¤: the time lag
Î©: the vibration frequency
Iu: the inertia of the tube
Figure 3: Coriolis Flow meter
From the data obtained from the experiment, we can compare and contrast the different types of flow meters that are available on the market. There are specifically three types of flow meters that we study in this experiment, namely, orifice flow meter, magnetic flow meter and coriolis flow meter. The main observation to be done through this experiment is to determine how the flow rate an incompressible fluid is measure, in which in this case is water.
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For the orifice flow meter or also called as differential pressure flow meter, we can observe a trend in the readings. At the beginning of the experiment, a flow rate of 10L/min is set, followed by 15L/min and lastly 20L/min. The accuracy of the reading compared to the calculated value is relatively high. However, a trend is observed. The accuracy is higher at flow rate of 10L/min, reduces for the 15L/min and is the highest for the 20L/min flow rate. This accuracy can be caused by the errors present in conducting the experiment. For a orifice flow meter, a pressure drop is produced when the fluid passes through a barrier or restriction. If the fluid passes through a tube of smaller diameter, it accelerates with a higher velocity. According to Bernoulli’s Law, when a fluid accelerates, its pressure decreases and therefore creates a pressure drop. An orifice flow meter’s accuracy is typically ±0.2% of the calibrated span. So, at the low end of a 10:1 flow range (10% flow), which corresponds to a differential pressure range of 100:1, the flow meter would have an error of ±20% of the actual reading. As a result, differential producing flow meters is limited to use within a smaller range or a low flow.
As for the magnetic flow meter, a same trend is observed whereby the accuracy of the reading starts high for flow rate of 10L/min, and then drops for flow rate of 15L/min and it is highest for the 20L/min flow rate. As for a magnetic flow meter, it can be considered as an ideal flow meter because it measures a wide range of fluid measurements with low conductivities. It does not use the principle of pressure drop and therefore, pressure drop in this case is negligible. The concept behind the flow measurement for this device is Faraday’s Law of Electromagnetic induction. When a conductor passes through the magnetic field, a voltage will be induced and the intensity of the voltage is proportional to the velocity of the moving fluid. In simple terms, the faster the fluid moves, the higher the voltage induced and this signifies or informs the experimenter on the flow rate of the fluid. This measuring device is independent of the viscosity, density, pressure & temperature of fluid. Its accuracy is about ±0.5% of the flow rate.
Again for the coriolis flow meter, a same trend is observed. The accuracy is highest for flow rate of 20L/min, followed by 10L/min and lastly 15L/min. A coriolis flow meter measures the amount or quantity or mass of the fluid that flows through it. The flow rate is then obtained as the mass per unit time, for instance kg/s. Similar to a magnetic flow meter, a coriolis flow meter is not affected by the changes in pressure, temperature, viscosity and density When a fluid enters the tube, a force is exerted to cause the tube to vibrate, the tube will twist or rotate as a result of the acceleration acting in the opposite directions. The advantage of a coriolis flow meter is that it measures directly the flow rate of a fluid without considering the factors of temperature, pressure or even the viscosity of the fluid. The accuracy of the coriolis flow meter is approximately ±0.1%.
4.0 ERROR ANALYSIS
Base on the experiment, there are several errors affecting the outcome of the experiment:
- Fluctuating flow rate results in the difficulty to read the water level accurately especially during initial part of the experiment because the water flow is not stable and the time keeper might not start the time accurately when the water level reaches 0L/min.
- The fluctuating of the flow rate also makes it difficult to set up the flow rate of the liquid in constant, especially when we need a constant value at 10L/min, 15L/min and 20L/min.
- The start of time-keeping is not synchronized and might not be accurately started when the water level of tank reaches 0L/min due to the different response time of the experimenter.
- Parallax error occurs when reading the water tank level, due to restrictions like height of the reader in which some may be too tall and otherwise.
There are several methods that can overcome or minimize the errors:
- The problem arising from the fluctuating flow of water can be solved by repeating the experiment and obtaining an average of the measurements.
- To reduce the parallax error, make sure that the water level is parallel with your eyes.
- To increase the accuracy of time, use digital stopwatch rather than analog stop watch or other methods.
In conclusion, coriolis flow meter has the highest accuracy, followed by the magnetic flow meter, and last in the order is orifice flow meter. The objective of the experiment which is to compare the accuracy of each flow meter as well as to study the mechanism of the flow meters is achieved. The usages of these flow meters are different in the industry. The selection of flow meters is based on the specification that can fulfill the requirement of the industry.
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