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When you’re in the business of manufacturing thermal mass flow meters you need to provide accurate flow measurement. Flow measurement is impacted by flow profile. In this post, I explain “What is flow conditioning?” and review “flow profile.”
Flow Profile
The accuracy of a flow meter is dependent on the flow profile of the substance in the pipe. The key factors affecting flow profile are the piping and straight run upstream of the sensor while there is some minor impact from the downstream straight run. For this reason, the flow profile is a significant factor we need to understand, to choose the right location for any thermal flow meter application.
The desired flow profile can usually be achieved when having a distance of 15 to 25 pipe diameters of straight run upstream of the meter, following a single elbow. Greater distances are required for other pipe geometries. In the real world, however, we are not always able to provide sufficient straight run to secure a “fully developed flow profile.”
What is Flow Conditioning?
Anytime there is a change in the size, the direction of a pipe in which gas flows, or a disturbance, the flow profile has been impacted and accurate flow measurement is hindered. Whenever there is a limited straight run, and a developed profile cannot be achieved, flow conditioning becomes necessary to create a known and repeatable velocity profile and to enhance the flow meter’s accuracy.
In-Line Flow Body Style Meter
In the Sage Metering In-Line flow body meter, the flow conditioning is built into the body of the thermal mass flow meter. In these cases, the recommended upstream distances ahead of the flow body can be significantly reduced due to the built-in flow conditioning.
Insertion Style Meter
For insertion style meters, it is recommended to use a Captive Flow Conditioner assembly which consists of two conditioning plates positioned between two flanges. (Pictured)
The chart on the right reveals the amount of straight run that is required for different pipe configurations for installations with and without flow conditioning ahead of the flow meter location.
Sage flow conditioners are sold along with insertion-style Sage meters. Because the flow profile following the flow conditioner is different from the normal fully developed turbulent flow profile that occurs with sufficient run, it is necessary to calibrate an insertion flow meter with the flow conditioner. Without sufficient straight run, repeatability of results is only assured when calibrating the meter with the conditioner, and installing the units properly.
