Production • Processing • Handling
A select few multi-path ultrasonic meters use velocity from four chordal paths (Fig. 3) with the VPC to accurately determine the average velocity over the complete flow and viscosity range.
Specifying an ultrasonic flowmeter Key characteristics to look for in an ultrasonic meter for crude oil include:
● A multipath meter with an integration method, like VPC, to improve performance on high- viscosity low-Reynolds Numbers applications.
● Robustness in correcting asymmetric axial flow velocity profiles.
● Compensation transverse (non-axial) flow components (swirl, crossflow, etc).
Fig. 3. Four-path arrangements for velocity profile correction.
Metering systems can also have valves, strainers, elbows, tees, and headers upstream of the meter. Tese elements can distort the flow profile and introduce swirl and crossflow upstream of the meter. Since we are measuring velocity, any change created by these elements will affect the measurement accuracy. Flow conditions are used to minimize these affects but a robust integration method with crossflow compensation is also important to optimize performance At least one liquid ultrasonic meter utilises two additional paths with the VPC to correct for swirl and cross flow (Fig. 4).
Te key benefits of Ultrasonic Technology, low pressure and low maintenance are highly attractive for crude oil measurement. Tey are driving the technology, but as with any meter, there are limitations. Like turbine meters, they are best operated at the higher flow ranges for optimum accuracy. However, with techniques such as VPC, accurate measurement at the lower flow ranges can also be achieved. No pressure loss results in reduced operating costs. No moving parts results in increased service life and may reduce the frequency of proving since usage wear is a key reason why meters must be recalibrated. Te measurement technique is susceptible to installation effects and fluid properties. As with all meters, liquid ultrasonic flowmeters need to be proven. In-situ proving, though difficult, is indisputably the best method to reduce the total measurement uncertainty. Proving the meters in a laboratory offers an alternative, but at substantially higher risk of measurement error. Even though a specific ultrasonic meter may compensate for installation effects, such as swirl or cross flow, there isn’t any means of verifying this without field proving. Te measurement accuracy is further compromised by the fluid property effects that were discussed. Tis is especially true for crude oils, because their properties are difficult to simulate even in a laboratory that tests with hydrocarbon fluids. Until more data is available quantifying how liquid ultrasonic flowmeters react to different installation conditions, arguments advocating methods other than in-situ proving under actual operating conditions are questionable. Ultrasonic meters can provide accurate
measurement over a wide range of crude oil applications if they are properly applied, proven and operated. ●
Enter 50 or ✔ at
www.engineerlive.com/iog Fig. 4 .Two added paths for swirl and crossflow correction. 50
www.engineerlive.com
Raymond J. Kalivoda is petroleum measurement development manager for FMC Technologies, Erie, Pennsylvania, USA.
www.fmctechnologies.com
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