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New Coriolis Technology solves traditional flow measurement problems
by Wade Mattar

Coriolis technology offers unprecedented accuracy and reliability in measuring material flow.

It may be the most superior flow measurement technology. However, conventional Coriolis meters have had one significant limitation: They haven't performed well in measuring two-phase flow conditions, flow that involves a combination of gas and liquid mass.

Two-phase flow can cause process interruptions and measurement inaccuracies that can significantly affect production and profitability. Recent developments in digital Coriolis technology overcome the challenges of measuring two-phase flow to improve traditional pipeline flow measurement, while offering a solution for demanding applications that have been traditionally out of reach.

Partially filled flow tube
With worldwide revenues presently greater than $400 million and moving to $600 million in the near future, Coriolis meters are among the fastest-growing flow measurement technologies. These meters measure flow by analyzing changes in Coriolis force of a flowing substance.

Coriolis force arises from a mass moving within a rotating frame of reference. That rotation produces an angular, outward acceleration, together with linear velocity, to define the Coriolis force. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.

To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flow tube equipped with sensors and drivers, and an electronic transmitter that controls the oscillations, analyzes the results, and transmits the information.

Reliable Coriolis measurement depends on consistent, reliable oscillation, which four factors determine: the density of the liquid, the balance of the tubes, the dampening caused by the flow stream itself, and the physical isolation of the tubes from the environment. Compromising even one of these factors will degrade Coriolis meter performance. Yet two-phase flow compromises every one of them. Thus applications involving negligible amounts of entrained gas—even as little as 2% volume—have been poor candidates for Coriolis measurement. This has been particularly troubling in applications where reliable, highly accurate flow measurement can confer considerable bottom-line advantage, but where two-phase flow is an integral part of the process or it is necessary to begin with an empty or partially filled flow tube.

Making matters worse, entrained air may not emerge as the culprit until after a frazzled process engineer has invested many hours trying to figure out why he can't get the results he needs. Our own analysis shows up to 92% of all Coriolis measurement problems are due to entrained air or gas, yet in the vast majority of cases, users don't recognize two-phase flow as the problem.

Patented and advanced control
Coriolis technology is highly accurate in single-phase flow, with perhaps a ± 0.1% error level. Two-phase flow can boost the error rate to 20% or higher. Following are some of the profitability drains that inaccurate flow measurement causes:

Lost production: In flow-intensive operations, thousands of dollars' worth of lost production can pass undetected in minutes.

Inaccurate pricing: In custody transfer applications, where measured amount transferred defines payment price, faulty measurements raise financial havoc on either end of the transfer.

Excess downtime: When traditional Coriolis meters encounter entrained air, they render inaccurate measurements, and if the condition persists, will shut down. This cuts into valuable production time.

To determine the extent of the problem and find a cost-effective solution, Invensys commissioned a survey of process engineers. This revealed entrained air was indeed a major problem in the industry, and what customers really wanted was a cost-effective meter that provides accurate measurement despite the presence of air. Invensys collaborated with researchers at Oxford University in England to develop digital technology for accurate measurement of flow, even when the flow tube contains entrained air.

Working closely with the Oxford researchers, our engineers developed a transmitter that applied the Oxford measurement principles. The resulting patented product incorporates new signal processing techniques to provide useful measurements of both mass flow and density and the operational aspects of keeping the Coriolis meter running stably in single-phase or two-phase flow conditions.

One of the many patents it has received involves an advanced control and measurement system with high-speed digital signal processing that responds to changing flow conditions many times faster than standard Coriolis flowmeters. Another patent relates to detecting and compensating for two-phase flow conditions and generating a validated mass flow measurement.

Unloading railcars and trucks
Coriolis meters measure the mass flow of materials, which is independent of other physical parameters, as well as the ambient conditions in which the measurement takes place. Therefore, the measurement is unaffected by changes in temperature, pressure, density, viscosity, and flow profile. With the ability to handle two-phase flow and compensate for physical conditions, the advanced Coriolis flowmeters have greatly expanded fluid metering applications, including traditionally difficult situations, such as custody transfer, proving, tank truck, and tanker loading and unloading, and applications where two-phase flow is an integral part of the process.

Accurate measurement at custody transfer points is critical as competitive market conditions drive companies to develop operations that are more efficient. By minimizing wasted materials left on the bottom of transport vessels and improving transfer yields, advanced Coriolis flowmeters provide more accurate material accountability, which is a direct contribution to bottom line performance. This is a win-win situation for both entities involved in the transaction. Advanced Coriolis technology is increasingly replacing positive displacement meters for custody transfer to attain the benefits of Coriolis accuracy, while reducing total cost of ownership. With no moving parts in the fluid stream, Coriolis meters require little-to-no maintenance and are easy to install.

In pipeline flow measurement-proving applications, the frequency and duration of calibration can hinder productivity. Advanced digital Coriolis flowmeters offer a solution by providing a much faster response time, and greater accuracy and repeatable proving with small volume provers. A proving run may take as little as 20 seconds. This is particularly beneficial in multi-product pipeline applications where fluids varying from light, liquefied petroleum gases to heavy crude oils pass through a common flowmeter. For these applications, flowmeters often take place several times a day, so slashing each proving process to seconds can significantly boost productivity.

Another issue is unloading railcars and tank trucks until they are practically dry. To empty out the tank completely, invariably introduces air as the level approaches bottom. Exacerbating this is that in most cases, unloading happens at as high a flow rate as possible to speed up the process. This high flow rate tends to suck air into the flowmeter. Where a conventional Coriolis meter would shut down in this situation, advanced Coriolis meters continue to provide a useful flow measurement, enabling faster, more complete unloading of tank trucks and railcars. Even with the flow tube empty, they respond 10 times faster than traditional Coriolis transmitters, which reduce startup time while increasing production throughput and profitability.

Innovative moneymakers
In addition to improving existing flow measurement applications, advanced Coriolis technology is opening new doors for improving process efficiencies where two-phase flow is an integral part of the process.

For instance, using carbon dioxide (CO2) for enhanced oil recovery (EOR) can increase output by as much as 12%. However, accurate measurement of CO2 has been the weakness of the process. A large midstream energy company found the solution by applying advanced Coriolis metering technology as part of a three-stage EOR program. The first stage was primary oil recovery, based on natural gas driving the oil to wellheads. Secondary efforts involved water flood driven production using natural aquifers. As primary and secondary production methods declined in effectiveness, tertiary oil recovery techniques gained attention. A number of EOR options received scrutiny, and CO2 injection into the oil reservoirs was the most effective method for extracting and moving oil to the well bore. While the yields from this EOR were significant from the start, engineers felt they could do even better if they could more accurately measure the CO2 flows in each well.

The problem is when CO2 is above the critical point, it exists as a gas and is easily measurable with standard gas measuring devices such as orifice plates. However, below the critical point, it can coexist in two phases, liquid and gas.

The company transfers CO2 in pipelines to multiple injection wells throughout the field, and variations in ambient temperature and pressure outside the pipeline have a dramatic affect. On a cool morning, they could have primarily liquid CO2 in the pressurized distribution pipelines. However, in the afternoon, with elevated outside temperatures, they could have primarily gas.

Possible options considered were orifice plates with multivariable DP transmitters, Vortex meters, and conventional Coriolis flowmeters. While traditional Coriolis technology is highly accurate in single-phase flow, with a 0.1% plus or minus error level, two-phase flow can boost the error rate to 20% or higher. None of these options met the company's performance standard, so they explored new avenues of flow measurement technology.

The company tested an advanced digital Coriolis flowmeter to successfully measure two-phase CO2 and based on the results installed the flowmeters at each of the injection wells. The advanced Coriolis flowmeters improved the accuracy of CO2 measurement by 300%. This provided the immediate benefits of increasing oil output, as well as the long-term advantages of accurate flow measurement data to correlate optimum production efficiency with the volume of CO2 injected, which is critical for developing oil reservoir strategies.

The above cases are but a small sampling of the many ways in which the benefits of Coriolis accuracy can work areas that have been traditionally out of reach. Every day we are seeing new applications wherein advanced Coriolis flowmeters are working successfully to solve traditional problems.

So look at your flow measurement challenges. Advanced digital Coriolis technology may be the solution to today's problems and tomorrow's innovations.

Contact: See more details about Wade Mattar

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