CHAPTER 11 - ACOUSTIC FLOW MEASUREMENT
3. Available Technology
At this time (1996), many different types of acoustic flowmeters are available. As mentioned in previous sections, transit-time and frequency difference devices, as well as devices which use chordal (multipath) or diametral (single-path) paths to measure the average velocity in the pipe are available. Some systems use clamp-on transducers mounted to the outside of the pipe wall, and some use internal or wet-mount transducers. Likewise, some transducers are mounted in a spool, and others are installed in an existing section of pipe. All these options have their advantages and disadvantages, and they will be covered later.
All acoustic flowmeters consist of the following components.
(a) Primary Device
A spool piece with the transducers installed or an existing section of the pipe to which the transducers are mounted. Transducers can also be clamped to the outside of the pipe.
(b) Measurement Section
The pipe section in which the flow rate is being measured. This section is located between the upstream and downstream transducer locations and is usually a circular cross section, but acoustic flowmeters can be used in conduits of various shapes.
The transducers transmit and receive the acoustic signals. They may be factory or field mounted by clamping, threading, or gluing them to the pipe wall. Transducers can be wetted by the fluid or can be attached to the outside of the pipe. Wetted transducers may be flush mounted, protruding, or recessed. Some wetted transducers can be replaced without taking the pipeline out of service.
(d) Acoustic Paths
Single-path or multipath measurement sections can exist where each acoustic path consists of a pair of transducers. Common path configurations are diametral and chordal (figure 11-1) or diametrically reflective.
(e) Secondary Device
A secondary device contains the electronics necessary to operate the transducers, measure the transit times, process the data, and display and store the results. Most meters have several outputs available, including analog, digital, and/or alarms either as standard equipment or options. Likewise, several outputs can be stored or sent by telemetry to another location.