Selecting where to install the flowmeter based solely on ease of installation is a mistake.
Flow profile considerations
This note reviews the flow profile, some end-user application considerations. Explores the use of flow conditioners to improve the accuracy of a thermal mass flowmeter from the American firm Sierra. It is especially useful in areas where it is impossible to obtain an optimal flow profile.
The velocity profile is used to determine the flow rate of a fluid flowing in a pipe. If the optimum profile changes, the accuracy of a thermal mass flowmeter decreases. One of the factors affecting the flow profile is the upstream straight sections. If there are not enough sections, a flow conditioner can be used. This allows a more uniform velocity profile to be obtained, improving measurement accuracy.
Basic principles of flow profiling – Velocity profiling
Fluids, such as gas, flow at different velocities within a pipeline. As gas enters a circular pipe, the molecules in contact with the pipe surface are stopped, creating a boundary layer. The 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 rate through the pipe. The velocity distribution through the pipe 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.
By understanding the velocity profile, it is possible to determine the velocity at any point.
The pipeline and the straight run upstream of the flow sensor
Each time a flow profile is distorted, the accuracy of the meter decreases. The main factors affecting the flow profile are the pipe and the straight run upstream of the flow sensor, with less impact on the downstream straight runs. In an ideal application, the optimum flow profile occurs when we have a straight, smooth pipe and sufficient straight runs preceding the mass flow meter. In the real world, however, most pipelines do not have enough straight runs, and flow disturbances occur due to; fittings/couplings, expanders, reducers, valves, tees, headers, bends, filters, heat exchangers, regulators, flanges and elbows….
Often, the straight run preceding the meter requires greater distances to provide the proper flow profile. For example, when an elbow is introduced in the same plane as a line, the straight run of pipe upstream will be required to be at least 25 diameters. Even more straight sections are needed in other types of installations. It is often impractical to obtain a straight run long enough to ensure a perfect flow profile. In these cases, the flow conditioner is an excellent option to consider.
90 degree bend in the speed profile.
Installation of thermal mass flowmeter
The location for the installation of a flowmeter is very important and often overlooked. Selecting an installation site based solely on ease of installation more often than not proves to be a failure. The end user must first consider the possible flow disturbances of the location chosen for the installation of the flowmeter before making it final. For this reason, it is very important to consider the most effective location. In either case, once the location is selected, disturbances should be taken into account so that the manufacturer can determine if a flow conditioner is required and, if so, the flowmeter would be factory calibrated with the appropriate conditioner.
For more information, please contact MATELCO, S.A . from the contact section.
