Coriolis technology offers unprecedented accuracy and reliability in measuring material flow in pipelines and is often hailed as among the most superior flow measurement technologies. But conventional Coriolis meters have had one significant limitation: they have not performed well in measuring two-phase flow conditionsflow 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 measurementwhile offering a solution for demanding applications that have been traditionally out of reach.
With worldwide revenues presently greater than US$400million and expected exceed US$600million in the near futureCoriolis meters are among the fastest-growing flow measurement technologies. These meters measure flow by analysing changes in Coriolis force of a flowing substance.
Coriolis force is generated in a mass which is moving within a rotating frame of reference. That rotation produces an angularoutward accelerationwhich is factored with linear velocity to define the Coriolis force. With a fluid massthe Coriolis force is proportional to the mass flowrate of that fluid.
To use Coriolis force for measurementa Coriolis meter has two main components: an oscillating flowtube equipped with sensors and driversand an electronic transmitter that controls the oscillationsanalyses the resultsand transmits the information.
Reliable Coriolis measurement depends on consistentreliable oscillationwhich is determined by the following four factors: the density of the liquidthe balance of the tubesthe dampening caused by the flow stream itselfand 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. So applications involving negligible amounts of entrained gas – even as little as two per cent volume – have been poor candidates for Coriolis measurement.
This has been particularly troubling in applications where reliablehighly accurate flow measurement can confer considerable bottom-line advantagebut where two-phase flow is an integral part of the process or it is necessary to begin with an empty or partially filled flowtube.
Making matters worseentrained air may not emerge as the culprit until after a frazzled process engineer has invested many hours trying to figure out why he cannot get the results he needs. Our own analysis shows that up to 92percent of all Coriolis measurement problems are due to entrained air or gasyet in the vast majority of cases two-phase flow is not even recognised as the problem.
Coriolis technology is highly accurate in single-phase flowwith perhaps a ± 0.1percent error level.
Two-phase flow can boost the error rate to 20percent or higher. Following are some of the profitability drains that inaccurate flow measurement causes:
- Post production: in flow-intensive operations thousands of dollars’ worth of lost production can pass undetected in minutes.
- Inaccurate pricing: in custody transfer applicationswhere measured amount transferred defines payment pricefaulty measurements raise financial havoc on either end of the transfer.
- Excess downtime: when traditional Coriolis meters encounter entrained airthey render inaccurate measurements - and if the condition persistswill shut down cutting into valuable production time.
To determine the extent of the problem and find a cost-effective solutionInvensys commissioned a survey of process engineers. This revealed that entrained air was indeed a major problem in the industryand that what customers really wanted was a cost-effective meter that could provide accurate measurement despite the presence of air. Invensys partnered with researchers at Oxford UniversityEngland to develop digital technology for accurate measurement of floweven when air was entrained in the flowtube.
Working closely with the Oxford researchersengineers at Invensys Process SystemsFoxboro Measurements and Instruments Division developed a transmitter that applied the Oxford measurement principles. In the resulting patented productthe Foxboro CFT50 digital Coriolis flow transmitterincorporates new signal processing techniques to provide useful measurements of both mass flow and densityand the operational aspects of keeping the Coriolis meter running stably in single-phase or two-phase flow conditions (Fig.1).
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.
Coriolis meters measure the mass flow of materialswhich is independent of other physical parametersas well as the ambient conditions in which the measurement is made. Thereforethe measurement is unaffected by changes in temperaturepressuredensityviscosity and flow profile.
With the ability to handle two-phase flow and compensate for physical conditionsthe advanced Coriolis flowmeters have greatly expanded fluid metering applicationsincluding traditionally difficult situations such as custody transferprovingtank truck and tanker loading and unloadingand 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 more efficient operations. By minimising wasted materials left on the bottom of transport vessels and improving transfer yieldsadvanced Coriolis flowmeters provide more accurate material accountabilitywhich 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 accuracywhile reducing total cost of ownership. With no moving parts in the fluid streamCoriolis meters require little-to-no maintenance and are easily installed.
In pipeline flow measurement proving applicationsthe frequency and duration of calibration can hinder productivity. Advanced digital Coriolis flowmeters offer a solution by providing a much faster response timeand greater accuracy and repeatable proving with small volume provers. A proving run may be accomplished in as little as 20seconds or less. This is particularly beneficial in multi-product pipeline applications where fluids varying from lightliquefied petroleum gases to heavy crude oils pass through a common flowmeter. For these applicationsflowmeters are often proven several times a dayso 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 completelyinvariably introduces air as the level approaches bottom. This is exacerbated by the fact that in most cases unloading is done at as high a flowrate as possible to speed up the process. This high flowrate tends to suck air into the flowmeter.
Where a conventional Coriolis meter would shut down in this situationadvanced Coriolis meters continue to provide a useful flow measurementenabling fastermore complete unloading of tank trucks and railcars.
Even with the flowtube emptythey respond ten times faster than traditional Coriolis transmitterswhich reduce startup time while increasing production throughput and profitability.
In addition to improving existing flow measurement applicationsadvanced Coriolis technology is opening new doors for improving process efficiencies where two-phase flow is an integral part of the process.
For instanceusing carbon dioxide (CO2) for enhanced oil recovery (EOR) can increase output by as much as 12percent.
Howeveraccurate measurement of CO2 has been the Achilles' heel of the process. A large midstream energy company found the solution by applying advanced Coriolis metering technology as part of a three-stage EOR programme.
The first stage was primary oil recoverybased on natural gas driving the oil to wellheads. Secondary efforts involved waterflood driven production using natural aquifers.
As primary and secondary production methods declined in effectivenesstertiary oil recovery techniques were examined. A number of EOR options were studied and CO2 injection into the oil reservoirs was determined to be the most effective method for extracting and moving oil to the wellbore.
While the yields from this EOR were significant from the startengineers felt that they could do even better if they could more accurately measure the CO2 flows in each well.
The problem is that when CO2 is above the critical point it exists as a gas and is easily measured with standard gas measuring devices such as orifice plates. Howeverbelow the critical point it can coexist in two phasesliquid 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 morningthey could have primarily liquid CO2 in the pressurised distribution pipelines. But in the afternoonwith elevated outside temperaturesthey could have primarily gas.
Possible options considered were orifice plates with multivariable DP transmittersVortex metersand conventional Coriolis flow meters.
While traditional Coriolis technology is highly accurate in single-phase flowwith a 0.1percent plus or minus error leveltwo-phase flow can boost the error rate to 20percent or higher.
None of these options met the company’s performance standardso they explored new avenues of flow measurement technology.
The company tested an advanced digital Coriolis flowmeter to successfully measure two-phase CO2 andbased on the results installed the flowmeters at each of the injection wells. The advanced Coriolis flowmeters improved the accuracy of CO2 measurement by 300percent.
This provided the immediate benefits of increasing oil outputas well as the long-term advantages of accurate flow measurement data to correlate optimum production efficiency with the volume of CO2 injectedwhich 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 be attained in areas that have been traditionally out of reach. Every day we are seeing new applications wherein advanced Coriolis flowmeters are being successfully used to solve traditional problems (Fig.2).
So take a look at your flow measurement challenges. Are you simply writing off lost materials without knowing exactly what is causing them? Are you experiencing downtime that could be better spent producing profit? Are you investing in outdated technology?
If you answer yes to any of these questionsor just feel that better flow measurement would improve your process in any wayadvanced digital Coriolis technology may be the solution to today’s problems and tomorrow’s innovations.
Wade M Mattar is Flow Specialist with Invensys Process SystemsFoxboro Measurements and Instruments DivisionFoxboroMAUSA. For more informationvisit www.foxboro.com/instrumentation"