CHAPTER 11 - ACOUSTIC FLOW MEASUREMENT

1. Transit-Time Acoustic Flowmeters

Transit-time acoustic (ultrasonic) flowmeters are based on the principle that transit time of an acoustic signal along a known path is altered by the fluid velocity. A high frequency acoustic signal sent upstream travels slower than a signal sent downstream. By accurately measuring the transit times of signals sent in both directions along a diagonal path, the average path velocity can be calculated. Then, knowing the path angle with respect to the direction of flow, the average axial velocity can be computed (figure 11-1).

An acoustic flowmeter is a nonmechanical, nonintrusive device which is capable of measuring discharge in open channels or pipes. These flowmeters can provide continuous and reliable records of flow rates over a wide range of conditions including flow in both directions. Some typical applications include:

• Acceptance testing of hydraulic machinery (turbines and pumps).
• Flow measurement in conduits of large (360 inch [in]) and small (1/2 in) diameter.
• Hydroelectric powerplant management.
• Volumetric metering.
• Wastewater or water treatment plants.
• Laboratory and field calibration of other flow measurement devices.

Two common methods are used to calculate discharge using acoustic flowmeters:

(a) Diametral-Path Flowmeter

One or more pairs of acoustic transducers are mounted diametrically opposed in the measurement section (figure 11-1a). An average axial velocity is measured along each acoustic path, and volumetric flow rate is computed based on the known cross-sectional area of the conduit. This technique is based on an assumption that the velocity profile shape in the measurement section is very similar to a fully developed, turbulent velocity profile. As a result, flowmeter accuracy depends on how well the actual velocity profile compares to the assumed profile. A poor velocity profile will result in flow measurement errors.

A modification of the diametrically opposed, single-path flowmeter uses the opposite inside wall of the pipeline to reflect the acoustic signal to the receiving transducer. The transducers are tightly secured with straps on the same side of the pipeline. Transducer spacing depends on pipe diameter and wall thickness. Accuracy can be within +/-2 percent of actual under good conditions.

(b) Chordal-Path Flowmeter

This type of flowmeter uses four or more acoustic paths which are mounted on chordal paths across the measurement section (figure 11-1b). The average axial velocity component for each acoustic path is used to establish the velocity profile. The velocity profile is then numerically integrated over the conduit's cross-sectional area to determine the volumetric flow rate. As a result, flowmeter accuracy is relatively independent of the velocity profile. Furthermore, this type of flowmeter does not require calibration to reach the manufacturer's specified accuracy (usually between 0.5 to 2 percent of the flow rate).

The following advantages and disadvantages identified for acoustic flow measurement are discussed below.

(c) Advantages

• High accuracy, which can be achieved independent of velocity profile, flow rate, and liquid temperature.
• Bidirectional flow measurement capability.
• Nonintrusive, incurring no head loss.
• Field calibrations are generally not required.
• System cost is almost independent of pipe size.
• No moving parts and easily serviceable.

(d) Disadvantages

• Relatively high initial cost.
• Requires electronic technician to troubleshoot and service.
• Must be programmed for each pipeline material, diameter, and wall thickness.
• Entrained gases and/or suspended sediment affect the acoustic signal strength.