Production • Processing • Handling
Accuracy diagnostics of liquid ultrasonic flowmeters
Joshua W Rose looks at the specific diagnostic capabilities of liquid ultrasonic flowmeters and how diagnostic information can contribute to maintaining high accuracy custody transfer measurement.
L
iquid ultrasonic flowmeters are not new to the measurement of crude oil. Over the past decade technological improvements have enabled liquid ultrasonic meters to meet the higher
accuracy requirements needed for custody transfer measurement.
Te transit time principle of measurement has opened the window to allow the observation and measurement of aspects of the flow stream that have never been visible to traditional measurement technologies such as PD meters and turbine meters but are critical to the proper operation of a liquid ultrasonic meter. Custody transfer liquid ultrasonic flowmeters as defined in API MPMS chapter 5.8 use the well- known principle of transit time measurement to infer the volumetric flow rate through the meter. In transit time measurement a pair of ultrasonic transducers are used to send an ultrasonic sound signal through the flowing product at an angle to the flow stream. Te transit time is the time it takes for the signal to travel from one transducer to the other. Because the signal is sent across the flow stream at an angle, the transit time is greater when the signal is sent upstream against the flow stream than when it is sent downstream with the flow stream. Te difference in the transit times, or ∆T is used to calculate the linear (axial) velocity of the fluid. Te general configuration of this transducer pair is shown in Fig. 1. In this case, the time from transducer B to transducer A (tBA
) will be greater
than the time from transducer A to transducer B (tAB
).
Te meter is configured to include transducer pairs located on multiple planes spaced across the cross-section of the flow stream. Typical configurations for custody transfer meters can range from four to 18 paths located on different planes across the flow stream. Fig. 2 shows a diagram of a meter that has transducer pairs located on four planes across the flow stream. In this case, the top two planes have two paths per plane for a total of six paths located on four planes. Fig. 3 shows the six
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www.engineerlive.com path FMC Technologies Smith Meter Ultra6 Liquid
Ultrasonic Meter which employs this principle. Calculating the axial velocity of the fluid on multiple planes allows the velocity profile of the flow stream to be determined empirically.
By employing two crossing paths on a plane, not only can the axial velocity of the fluid be calculated, the transverse component of the velocity can be calculated as well. Tis allows the presence of swirl and/or crossflow to be determined empirically in addition to the axial velocity profile as shown in Fig. 4.
Liquid ultrasonic diagnostics As already mentioned, Liquid Ultrasonic Flow Meters (LUFMs) employing the transit time method enable observation and measurement of aspects of the liquid flow stream that are critical to
Fig. 1. General configuration of transducer pair to flow stream.
Fig. 2. Path configuration.
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