Production • Processing • Handling
‘pipeline quality oil’. In general, the free water in ‘pipeline quality oil’ should not be a problem for measurement with ultrasonic meters. Sediment, though, especially small particulates may present a problem because they can diffuse the ultrasonic signal. Gas slugs or entrained gas will not damage an ultrasonic meter but can adversely affect the measurement accuracy of the meter. Even a small number of gas bubbles can cause attenuation of the ultrasonic signal. Te degree of attenuation depends on a number of factors, such as pressure, bubble size, amount of free gas, temperature and signal frequency. Wax crystals begin to form in a petroleum product at a specific temperature known as the cloud point. If a meter is operated below the cloud point, wax can form on the measurement element, which can notably affect the meter’s accuracy. Some meters are considerably more tolerant of waxing than other meters. For example, after an initial build-up of wax on the walls of positive displacement meter, rotating blades wipe the surfaces, and the meter factor remains stable. In the case of velocity meters (eg, ultrasonic, turbine) there is a continuous meter factor shift as the wax builds up.
Principle of operation
Te operating principal of ultrasonic meters is shown in (Fig 1). Te volume throughput (Q) is equal to the fluid velocity measured (Vm) multiplied by the area (A) or Q = Vm x A, where the fluid velocity measured is proportional to the difference between an ultrasonic signal traveling with the flow (tAB
) and against the flow (tBA
these affects must be addressed. Tis is especially important with crude oil measurement as the oil is typically highly viscous with high levels of contaminants. On a qualitative level the influences of fluid
properties have been addressed by various authors. Knowledge on the quantitative affects of fluid properties on ultrasonic meter accuracy is limited. Te influence of fluid properties on the UFM performance may be classified in two main groups:
● Signal quality affects – the signal attenuation and signal-to-noise ratio (SNR) in the acoustic paths
● Flow profile affects – the robustness of the integration method used to combine the individual acoustic path measurements into a full volumetric flow rate measurement
Te signal quality of an ultrasonic meter in crude oil applications is determined by: viscosity, entrained gas, sediment and water (S&W) and wax content. Te signal strength, or more precisely, the signal- to-noise ratio (SNR), is crucial for the accuracy of the transit time measurements made in the LUFM. Reduced SNR can mean higher uncertainty of the transit time measurement, resulting in a higher uncertainty of the volumetric flow rate measurement. In the worst case, the signal can’t be discerned from the noise and the measured output is erroneous. Noise is classified as:
). Te
measurement principal is fairly simple but there are number of factors that must be addressed to achieve custody transfer measurement accuracy.
● Coherent noise (signal interference) which includes: a. Transducer ‘ringing’ effects; b. Spool-piece borne signals (acoustic cross talk); c. Liquid borne reflections (transducer ports reflections, pipe wall reflections/reverberation).
● Incoherent noise (‘signals’ with random phase relative to the measurement signal) includes: a. electromagnetic noise (RFI); b. flow noise; c. valve noise; d. structural (pipe) vibrations, etc.
Te ‘strength’ (amplitude) of the measurement signal has to ‘compete’ with noise, to give a sufficient SNR. A number of fluid-dependent factors can attenuate the measurement signal and decrease the SNR. Te factors that contribute to the sound attenuation coefficient, α, are:
Fig. 1. Operating principal of liquid ultrasonic flowmeters.
A misconception exists that ultrasonic meters are not sensitive to fluid properties. Tis is not the case. To achieve the level of precision measurement available with other metering technologies,
Where: αabs
α = αabs + αwio + αgas + αsolids + αwax , is the sound absorption coefficient of the pure fluid, and
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