Ultrasonic Multi-Path Flowmeters For Crude Oil Custody Transfer
Due to significant improvements in accuracy during the past decade, multi-path ultrasonic flowmeters are the fastest growing flowmeter technology, gaining wider usage in the petroleum industry for liquid hydrocarbon custody transfer measurement. With the introduction of enhanced signal processing and multiple channel design, multi-path ultrasonic flowmeters consistently and reliably meet or exceed the demanding accuracy standards required for oil custody transfer metering and fiscal accounting (less than 0.2% of measured value).
Unlike traditional technologies such as turbine meters, Coriolis meters and positive displacement (PD), the ultrasonic flowmeters contain no moving parts and do not require frequent recalibration and maintenance. They also do not need to be protected by expensive and maintenance-intensive strainers that cause pressure drops and necessitate more pumping power. Ultrasonic flowmeters also perform better in large line sizes with high flow rates and, therefore, they require less piping, fewer valves and other components, and reduced maintenance and floor space, compared to alternative technologies which require that large pipelines be split into multiple parallel measuring sections.
A factor inhibiting the growth of multi-path ultrasonic flowmeters has been the relatively high purchase cost of the meter. However, when all related costs and savings are considered, the ultrasonic technology proves to be not only the most reliable and accurate option, but also the most economical alternative for many oil custody transfer applications.
Theory of Measurement
Ultrasonic flow measurement by the differential transit-time method is now one of the most universally applied flowmetering processes. It is used for measuring cryogenic liquids, hot liquids, gas and steam up to 500°C and above. The theory of transit-time differential measurement is quite simple: one transducer transmits a signal downstream with the flow and a second transducer transmits a signal upstream against the flow along the same path, and the difference in transit times is directly proportional to the medium's mean flow velocity.
It is easy to picture the underlying physics principle. Imagine two canoes crossing a river in opposite directions on the same diagonal line, one with the flow and the other against the flow. The canoe moving with the flow needs much less time to reach the opposite bank. The time difference will be greater or smaller depending on the speed of the river current. Ultrasonic waves behave exactly the same way. A sound wave traveling in the direction of the flow of the product is propagated at a faster rate than one traveling against the flow.
Accuracy without Continual Reproving
Traditional custody transfer flowmetering systems require cumbersome and expensive "proving" recalibration procedures to maintain accuracy whenever a change of product or fluid properties occurs. For example, a ship unloading platform in the Gulf of Mexico may stop pumping operations and recalibrate three times during a single off-loading batch, because of density stratification within the holds of the tanker. Frequently recalibration is especially burdensome in maturing fields with highly variable output, and in applications where fluids come from various fields.
The cost of the proving procedure itself varies, but the use of a portable prover typically costs $2,000, and the procedure takes anywhere from 45 minutes to several hours. During this time, the pumping operation may be at a standstill and the ship sits idle, at a considerable opportunity cost; unless multiple measuring points are available.
In contrast, multi-path ultrasonic flowmeters do not require reproving when the fluid properties change. Since the medium density is the same in both directions, the differential transit time measurement is not affected by density changes to the flowing liquid. The technologically advanced ultrasonic meters with multi-channel design and advanced digital signal processing completely, instantly and automatically compensate for variations in viscosity.
For example, the KROHNE ALTOSONIC V uses ten transducers to measure flow velocity along five parallel measuring beams, thereby covering a large range of the flow profile across the pipe cross section. This approach allows for highly accurate measurements of total volumetric flow rate, even in laminar and turbulent flow and in situations with asymmetrical flow profiles and swirl. Accordingly, the ALTOSONIC V became the first ultrasonic flowmeter to meet the stringent requirements for custody transfer of high value oil products, as specified by OIML R 117 and national regulations, with an error measurement of ± 0.15% of measured value.
To maintain this high accuracy, ultrasonic meters automatically sense, and account for, temperature changes that can alter the diameter of the pipe and, thereby, distort volumetric flow calculations. In addition, the ultrasonic technology can also detect buildup of solids on the pipe interior (sound travels faster in solids), which effects pipe diameter, and can signal an alarm to indicate that cleaning is required.
Reliability and Low Maintenance
High reliability is also a critical requirement for custody transfer applications, which depend on high availability and cannot tolerate a breakdown during a transfer operation. The superior reliability of ultrasonic meters derives primarily from the fundamental fact that there are no moving parts, in contrast to the wear in the rotor seals of positive displacement meters, and in the bearings of both positive displacement and turbine meters.
The multi-path design also provides the advantage of redundancy: the multi-path ultrasonic flowmeter will continue to operate, albeit with potentially reduced accuracy, if one or more of the transducers fails. The failed transducer can be replaced while the meter continues to operate. The KROHNE ALTOSONIC V can lose up to 3 paths and still have accuracy and repeatability compliant to OIML R117.
Furthermore, the ultrasonic transducers are non-intrusive, minimizing wear and pressure drop, in contrast to traditional metering methods, which have protruding parts vulnerable to fouling and damage during normal operation. Moreover, the presence of elevated amounts of gas in the pipeline will speed up the moving parts of blades and rotors in PD and turbine meters, creating extremely high velocities and, thereby, causing the meter to break down. For ultrasonic meters, the presence of large amounts of gas or solid contents may temporarily blind a meter, but will not damage it. As soon as the liquid is free of these obstacles, the instrument will return to full functionality.
Favorable Total Cost of Ownership
Although the initial price of multi-path ultrasonic flowmeters is higher than traditional meters used for oil custody transfer, the total cost of ownership (TCO) can be considerably less.
During installation, a single multi-path ultrasonic flowmeter can substitute multiple smaller traditional meters, resulting in savings on piping, valves, and other expensive components. Moreover, multi-path ultrasonic flowmeters do not require the installation of spare meters for use while other meters are taken offline for repair or recalibration.
Most significantly, traditional meters require 3-5 times as much maintenance labor and expense compared to the ultrasonic flowmeters. Traditional meters can cause considerable pressure drop, increasing energy consumption and stress on pumps while decreasing productivity relative to ultrasonic meters. Finally, the improved accuracy, availability, and reliability of multi-path ultrasonic flowmeters can add tremendous value to oil custody transfer applications.
SOURCE: KROHNE