ANSI AWWA C750-19 pdf free download – Transit-Time Flowmeters in Full Closed Conduits
Sec. 1.1 Scope
This standard describes transit-time ultrasonic flowmeters for water supply service application in pipes running full. An ultrasonic flowmeter is a meter that uses acoustic energy signals to measure liquid velocity. There are currently two distinct types of ultrasonic flowmeters available: Doppler-effect and transit-time. The Doppler-effect meter is used exclusively for liquids containing solid particles or gases, and the transit-time flowmeter is used in a wide variety of applications in the water industry.
Sec. 1.2 Purpose
The purpose of this standard is to provide the minimum requirements for transit-time flowmeters, including components, performance, calibration, and verification.
Sec. 1.3 Application
Transit-time flowmeters are used in a wide variety of applications, including in raw water, in treated water, and in different stages of the treatment process, such as settled water, supernatant, backwash, and chemical lines. These meters are also used in large-diameter transmission pipelines.
Transit-time flowmeters do not create any flow restrictions in the pipeline and are, therefore, used in many cases where flow restrictions are undesirable. Transit- time flowmeters are desirable for their suitability on a wide range of pipe sizes, from 3 in. to 30 ft in diameter. Transit-time flowmeters become especially cost- effective in large-diameter pipelines. Transit-time flowmeters are easy to install in retrofit applications with minimum or no disruptions to the flow. The components are also easy to remove during maintenance. An additional advantage of transit-time flowmeters is their suitability to bidirectional flow conditions. Like any other flow measurement device, transit-time flowmeters need to be selected prudently for the conditions under which the equipment is expected to operate.
4.3.1 Measurement section. The measurement section shall either be a part of the user’s process piping or be a part of the flowmeter provided by the transit-time flowmeter manufacturer.
4.3.2 Transducers. Transducers shall either be factory-mounted on a measurement section or they shall be field-mounted. Field-mounting shall be by (1) clamping to the outside surface of the measurement section, (2) threading through a tap installed on the measurement section, or (3) mechanically fixing or bonding to the interior surface of a large-diameter measurement section. Transducers may be wetted or nonwetted. Wetted transducers are in direct contact with the liquid and inject ultrasonic pulses directly into the flowstream. The nonwetted transducers are separated from the flowstream by the wall of the measurement section or by a shield (Figure 2).
4.3.3 Electronic component. The electronic component of the transit-time flowmeter commonly includes a microprocessor that operates the transducers, makes the measurements, processes the measured data, and displays or records the results in engineering units specified by the purchaser.
4.5.1 Accuracy. Accuracy is the closeness of a measured value to its true value. Typically for rate-type flowmeters, the term is expressed as a percentage of the measured value to that of the true value. Accuracy of 99 percent, by definition, means an inaccuracy of ±1 percent. Inaccuracy expressed as accuracy is commonly used. In other words, in this case, the device would be classified as a ±1 percent flowmeter. Accuracy for this definition is percent of point, or true value, as expressed in flow rate units. For transit-time flowmeters, the relative accuracy levels are dependent on the design of choice. Table 1 presents a range of accuracy for single-path and multipath transit-time flowmeters, given the flowmeters are installed according to proper hydraulic recommendations.