Production • Processing • Handling
αwio
+ αgas
+ αsolids
and αwax
account for contaminants
in the crude oil that cause excess attenuation due to entrained water, gas, solid particles, and (wax) at the transducers.
Sound absorption (αabs ) is the attenuation due to viscosity and the influence of absorption on the
SNR. Table 2. shows the absorption coefficient αabs in terms of attention/distance between transducers. Te absorption increases with viscosity and the distance between transducers, therefore the larger the meter diameter the higher the absorption.
Table 2. Sound absorption coefficient for water and oil samples* Sound
Specific Gravity
Sample
Water (distilled) Light oil Medium oil Brad Penn Heavy oil Extra heavy oil * Data at 70°F
1.00 0.81 0.85 0.86 0.87 0.88
Velocity [ft/s]
4,856 4,420 4,598 4,666 4,729 4,856
Water droplets (αwio Viscosity [cSt]
- 4
14 20 55
337 αabs
@ 1 MHz [dB/in]
0.11 0.18 0.10 0.23 1.14
) in the oil cause excess sound
attenuation due to scattering of the sound waves by the droplets. Te effect on the SNR is accounted for in the attenuation coefficient αwio
. Te coefficient is
determined by the water droplet size and distribution, the amount of water in the oil, the pressure and temperature, the oil type and the ultrasonic flowmeter signal frequency. Because of the complexity of this relationship, it is difficult to determine the attenuation coefficient with a high degree of certainty. In the development phase of one recently introduced liquid ultrasonic meter, extensive testing was done on water-in-oil affects on signal attenuation. Te following is a summary of these tests:
● Te ultrasonic meter may operate with up to 5 per cent water-in-oil depending on the size of the water droplets.
● Te influence of pressure on water-in-oil absorption is minimal as expected because of the low compressibility of the fluids.
● Temperate can heighten or lower the attenuation depending on a number of factors.
Free gas-in-oil (αgas ) in the form of gas bubbles,
causes excess sound attenuation due to scattering of the sound waves by the bubbles and bubble resonances. Te parameters that affect this coefficient are: bubble size and distribution, the
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amount of free gas present in the oil, the pressure and temperature, the oil type and the LUFM operating frequency. Gas-in oil is a highly complex condition that can have a profound affect on performance. As found in the development of the liquid ultrasonic meter cited above, even small amounts of entrained gas can adversely affect ultrasonic meter performance. A summary of the
gas-in-oil affects on signal attenuation are: ● For small bubbles (less than 0.5 mm diameter) a concentration as low as 1,000 ppm (0. 1 per cent) can momentarily or completely interrupt the measurement signal.
● For bubbles over 0.5 mm diameter a concentration up to 10,000 ppm (1%) may be tolerable before the SNR is reduced to a critical level.
● Te attenuation is based on path length so the larger diameter meters are proportionately more affected by entrained gas than smaller diameter meters.
● Low pressure can significantly increase the signal attention.
● Temperature has minimal affect. Solid particles-in-oil (αsolids
) like free gas-in-oil,
can cause excess sound attenuation due to scattering of the sound waves by the particles. Te same parameters that affect free gas-in-oil also affect the αsolids
coefficient. Te effect of solid particles-in- oil has not been studied in detail. Cursory testing shows results similar to gas-in-oil.
Wax can affect the SNR and the meter’s K-factor. If the temperature is below the cloud point, wax contamination may build up at different surfaces of the ultrasonic flowmeter. Possible influences which may be important for the ultrasonic flowmeter’s performance include the following:
Wax layer build-up: ● At the transducer, fronts may shift the transit
times and cause a continuous meter factor shift as the wax builds up. Cause attenuation (αwax
)
can reduce the SNR. Due to the relatively small difference in the acoustic impedance between oil and wax, a thin wax layer may not affect the SNR significantly, unless the layer becomes thick, and not homogeneous.
● In the transducer, cavities may reduce the acoustic isolation of the transducer from the spool piece, and increased acoustic “cross-talk” through the spool piece. Since cross-talk acts as coherent noise, this results in reduced SNR, and thus can reduce accuracy of the transit time measurements.
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