Factors that impact the accuracy of mass flow meters saw by thermal dispersion; the flow profile and its conditioning PART I
The selection of the point where to install the flow meter based solely on the ease of installation is an error.
Considerations on the flow profile
This note reviews the flow profile, some considerations about the end-user application. Explore the use of flow conditioners to improve the accuracy of a thermal mass flow meter from the American firm Sierra. It is especially useful in areas where it is impossible to obtain an optimum flow profile.
The velocity profile is used to determine the flow rate of a fluid that flows inside a pipeline. If the optimum profile changes, the accuracy of a thermal mass flow meter decreases. One of the factors that affect the flow profile are the previous straight stretches. If there are not enough sections, a flow conditioner can be used. This allows obtaining a more uniform speed profile, improving the accuracy of the measurement.
Basic principles of the flow profile – Speed profile
Fluids, like gas, flow at different speeds within a pipeline. As the gas enters a circular pipeline, the molecules in contact with the surface of the pipeline stop, creating a boundary layer. Gas flowing in the adjacent layers slows down due to friction, and the gas velocity in the middle section increases to maintain the mass flow through the pipe. Velocity distribution through the pipeline is the velocity profile. The flow profile becomes parabolic once the flow is “fully developed”. By knowing the velocity profile, it is possible to determine the velocity at any point within the pattern.
Understanding the speed profile, it is possible to determine the speed at any point.
The pipe and the straight section upstream of the flow sensor
Each time a flow profile becomes distorted, the accuracy of the meter decreases. The main factors affecting the flow profile are the pipe and the straight section upstream of the flow sensor, with less impact on the subsequent straight sections. In an ideal application, the optimum flow profile occurs when we have a straight, smooth pipe and sufficient straight sections that precede the mass flow meter. In the real world, however, most pipelines do not have enough straight stretches, and flow alterations occur due to; the fittings / couplings, expanders, reducers, valves, T, heads, bends, filters, heat exchangers, regulators, flanges and elbows …
Often, the straight section that precedes the meter requires greater distances to offer the proper flow profile. For example, when an elbow is introduced in the same plane of a line, it will be required that the straight section of the upstream pipeline be at least 25 diameters. Even more straight sections are needed in other types of installations. Often, it is not feasible to obtain a straight section long enough to ensure a perfect flow profile. In these cases, the flow conditioner is an excellent option to consider.
Elbow of 90 degrees in the velocity profile.
Installation of thermal mass flow meter
The location for installing a flow meter is very important and often overlooked. Select an installation site based solely on ease of installation, in most cases it turns out to be a failure. The final user must first consider the possible disturbances of the flow of the chosen location for the installation of the flowmeter, before giving it as final. For this reason, it is very important to consider the most effective location. In any case, once the location is selected, the disturbances must be taken into account so that the manufacturer can determine if a flow conditioner is required and, if so, the flow meter would be calibrated at the factory with the corresponding conditioner .
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