The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #73, and was written by Greg.

In the 1980s and 1990s, Coriolis meters were expensive and were primarily relegated to critical measurements of small liquid streams. The meters were susceptible to vibration and errors from bubbles or solids. Since then the price and capability have improved. Coriolis meters can now be used on gas streams and can measure the percent bubbles and percent solids. Density measurement has always been a strong point, with a precision out to the fourth decimal place for grams per milliliter, and there is no drift and very little maintenance required.

Process gains and online metrics (Tip #61) are a function of mass flow ratios. We have gotten used to working in volumetric units because volumetric flowmeters are so prevalent. However, you do not hear of a process engineer doing a volume balance. Process analysis and modeling require a material balance for the conservation of mass. Material balances often do not close due to errors in measurements made with volumetric meters. Even when they are corrected for pressure and temperature, mass flow computations based on volumetric flow measurements assume a known and constant composition, which may not be the case. Coriolis is the only means of true mass flow measurement.

Coriolis meters have been commonly used in the front end operations of specialty chemical, food and beverage, and pharmaceutical plants. The front end operations, such as reactions, set product quality and yield. Accurate mass flow ratios enable maintaining stoichiometric ratios. Coriolis meters are now moving downstream as their precision and rangeability advantages are recognized. Coriolis meters are replacing load cells for inventory management and are increasingly being used in recovery, blending, and custody transfer.

Materials of construction have expanded to include titanium. The principal remaining reason not to use a Coriolis meter in a process stream is cost for larger pipelines. However, if the cost analysis included the process benefits from better process knowledge and turndown and the elimination of downtime and maintenance, Coriolis meters would be justified for most process streams. There is often a misunderstanding about the performance of Coriolis meters because of the different models and vintages. The newest and most capable meters, with technological advancements in sensor design and signal processing, offer an order of magnitude improvement in resolution over earlier versions.

A turbine meter may provide slightly better resolution, but only for a perfect velocity profile and for fixed composition and viscosity. In addition, a turbine meter is a mechanical device that requires maintenance and has only a 15:1 rangeability, whereas a Coriolis meter has a 200:1 rangeability.

Concept: Blending, crystallization, distillation, evaporation, and reactions benefit from material balance analysis and control from online measurement of mass flow rates and the composition of the important liquid streams. The density measurement capability of the Coriolis meter can be used as an inferential measurement of component concentrations and solids for two components or a single component, two phase mixture. The flow rate and concentration measurements can be used for intelligent feedforward signals (Tip #92), process modeling, online data analytics, online process metrics (Tip #61), and live process flow diagrams on operator screens (Tip #101).

Details: The best U-tube Coriolis meters maintain an extraordinary accuracy of 0.05% and a repeatability and resolution of 0.02% of rate for liquids, independent of concentration, viscosity, temperature, time, and upstream/downstream straight pipe runs. There is no drift. Once properly calibrated and installed in an application, there is no maintenance required. The density measurement is accurate to 0.0002 gm/cm3. The flow measurement rangeability is 200:1. Advanced meter software is capable of measuring bubble and solids concentrations. Sizes range from 2 mm to 300 mm (0.1 to 12 inch). Use the most accurate Coriolis meters for all important process streams. Straight tube Coriolis meters, used to prevent solids accumulation or erosion, have an error and repeatability about five times larger. Error and repeatability in meters used for gas flows are about 10 times larger than for liquid flow.

Watch-outs: Misalignment of flanges can put torque on the tube, creating significant error. Vibration and crosstalk from adjacent meters can cause noise. Solids can accumulate on the bottom, and erosion can occur at the turns of the U-tube when streams contain abrasive particles. Bubbles can accumulate in the meter when streams contain gases or vapors. For streams that contain solids, it is better to use straight tube meters installed vertically for self-cleaning. For streams containing gases or vapors, installation must prevent trapping bubbles or causing flashing or cavitation in the meter. For gas flows, low density due to low molecular weight and/or high temperatures will reduce accuracy, sensitivity, and repeatability. Less expensive and older U-tube models and straight tube designs offer much less accuracy and precision.

Exceptions: The cost may be prohibitive in line sizes above six inches unless the reduction in maintenance and the benefits of tighter material balance analysis and control provide an acceptable rate of return on capital investment. Particles and coatings may clog the meter in line sizes below 1/2 inch. Maximum temperature is 350°C and materials of construction are more limited than for other flowmeters. Coriolis meters are not often used in oil and gas platforms and pipelines, refineries, and petrochemical plants because process knowledge is well-established and the line sizes are large.

Insight: Coriolis meters require the least maintenance and offer the best long-term accuracy of mass flow and density measurement, independent of piping and process operating conditions for liquid and gas streams, which enables better advanced control and process analysis.

Rule of Thumb: Use Coriolis meters for measuring liquid flows in batch, fed-batch, and continuous operations where tight flow ratio and composition control, accurate process modeling, and high turndown are beneficial.  

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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About the Author
Hunter Vegas, P.E., has worked as an instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he entered the systems integration industry and is currently working for Wunderlich-Malec as an engineering project manager in Kernersville, N.C. Hunter has executed thousands of instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms. Hunter earned a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University.

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