TABLE OF CONTENTS Flow measurement is a key parameter used by plants for reading values (including accounting needs) and for controlling processes. Of the typical process measurements flow, level, temperature, and pressure flow tends to be the most difficult and, therefore, the one for which incorrect devices are most likely to be selected.
The technology of flow measurement has evolved to the point where highly accurate and reliable devices are now on the market. Moreover, new measurement principles are being introduced, and existing principles are continuously improved upon. As a starting point, it should be mentioned that no single flowmeter can cover all flow measurement applications. For that reason, flowmeter selection has been referred to as both a science and an art. This chapter provides some of the basic knowledge plant personnel need in order to select the correct flowmeter. It is essential that the person selecting the instrument considers the actual users' experiences.
Flowmeters operate according to many different principles of measurement. They can be broadly classified into four categories:
Flowmeters that have wetted moving parts (such as positive displacement, turbine, and variable area). These meters utilize high-tolerance machined moving parts, which determine the meter's performance. These parts are subject to mechanical wear and thus are practical for clean fluids only.
Flowmeters that have wetted non-moving parts (such as vortex, differential pressure, target, and thermal). The lack of moving parts gives these meters an advantage. However, excessive wear, plugged impulse tubing, and excessively dirty fluids may cause problems for these meters.
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