6. Flowmeter Selection Guidelines

(a) Single-Path Versus Multi-Path Flowmeters

Single-path meters are generally a lower cost alternative. They are also less complex which allows easier installation. Multi-path meters perform better under variable and/or nonideal velocity profile distribution situations caused by upstream and downstream flow disturbances. Acoustic path orientation varies among meters; paths can be either crossed or parallel, or either chordal or diametral. The appropriate path configuration depends on site-specific constraints, economics, and the application.

(b) External Mount Versus Through-Wall Transducers

External mount transducers are the easiest to install and require minimal surface preparation. As a result, installation is inexpensive when compared to through-wall transducers. External mounting transducers are nonintrusive, so they do not disturb the flow. They are also easily removed and replaced without taking the pipe out of service. It should be noted that errors in flow measurement caused by variable or changing wall and/or liner thickness can be significant.

Through-wall transducers are usually wetted or covered with a protective material. This type of transducer mount may provide increased acoustic signal strength because no signal attenuation occurs through the pipe wall. Through-wall transducers can be flush mounted, protruding, or recessed. Protruding or recessed transducer mounts can cause a local flow disturbance which may affect the flowmeter accuracy. Errors caused by a protruding or recessed transducer or the protective covering require a detailed analysis of the installation configuration. The potential for transducer fouling from various waterborne contaminants (algae, minerals, etc.) also exists.

Theoretical equations used for acoustic flow measurement are based on the assumption that the transducers are in direct contact with the fluid. A protective covering or an external mount transducer will change the transit times and path angles. These changes are usually mathematically modeled by the manufacturer and corrected for by the secondary device.