CHAPTER 11 - ACOUSTIC FLOW MEASUREMENT
5. Installation Considerations
Many of the errors considered above can be eliminated or decreased by following the manufacturer's installation guidelines. Errors and their sources should be addressed prior to flowmeter installation. The following paragraphs cover areas which should be examined. More detailed descriptions are available in American Society for Testing and Materials (1984), American National Standards Institute (1985), and Laenen (1985).
(a) Acoustic Path Length and Angle
Changes in acoustic path length and angle can be caused by significant changes in pressure or temperature, along with external loading on the meter section. The measurement section should be examined to determine if any of these conditions exist. In addition, path lengths and angles must be known with a high degree of accuracy.
(b) Signal Detection
Entrained air, suspended solids, and transducer fouling may lower accuracy or prevent operation by weakening the acoustic signal. Electrical and acoustical noise caused by mechanical vibration, other electrical devices, or cavitation can also disrupt the flowmeter's operation.
(c) Secondary Flows
Secondary flow or crossflow affects a meter's performance and should be evaluated in the system design. This design evaluation should include selecting a diametral path or chordal path meter and deciding whether a second crosspath is necessary. Path orientation should also be considered. Typically, the measurement section should be located as far downstream as practical from upstream bends, transitions, valves, and pumps.
(d) Velocity Profile Integration
Chordal-path meters use numerical integration techniques to compute the flow rate. It is important that the chordal paths are positioned in accordance with the particular locations specified by the manufacturer.
In general, acoustic flowmeters do not require a field calibration when manufacturer's suggested installation criteria have been met or exceeded. However, in some cases, unusual installation conditions or the need for a high degree of accuracy may require a calibration. Three methods exist for flowmeter calibration: (1) laboratory calibration, (2) field calibration, and (3) analytical techniques. For more information on calibration techniques, a good reference is the ANSI/ASME standard MFC-5M-1985 (American National Standards Institute, 1985).