Measuring mass flow rate offers potential for improvements worth thousands of dollars in industries such as chemical, food and beverage, pulp and paper, petroleum, pharmaceuticals, and in water/wastewater treatment facilities. Although great strides have been made in the design of inferential and thermal mass flowmeters in recent years, refinements in Coriolis metering technology have made these instruments particularly effective in the process industries.
Increased accuracies of advanced instruments for measuring mass flow rate can offer significant savings in petroleum processing industries. Mass flow measurements account for such variables as temperature, pressure, density, and viscosity, which can otherwise degrade the accuracies of volumetric flowmeters.
Complementing Measurement and Control
Coriolis flowmeters are well-suited for industrial environments and readily tie in with complete process measurement and control systems. Today, the global market for these devices probably exceeds $480 milion—shared by more than a dozen suppliers.
One interesting development is that users have standardized these meters for practically all applications throughout a facility. Plant engineers can justify the higher unit cost of Coriolis mass flowmeters because of the improved accountability (accuracy), integration of process flow measurements into one unit (less hardware), and elimination of the need to correct flow profiles before the fluid enters the meter (pipe runs).
Coriolis mass flowmeters have two basic system configurations—a remote converter and an integral converter that is mounted directly on the primary housing (see Figure 1). The remote converter connects to the primary with shielded cable that can be up to 1000 ft. long. The converters receive the small electric measuring signal generated by the sensing system in the primary and electronically change it to usable outputs (current, pulse frequency, or digital). The converter can show the outputs on its display and transmit them to panel-mounted recording and control instrumentation or process-control computers in a centralized control room.
Figure 1. Coriolis flowmeters directly measure mass flow. The converter for the remote version (A), can be up to 1000 ft. from the primary flow element. Alternatively, the converter can be mounted integral with the primary, as in (B).
The primary mounts in the flow line and houses the essential sensing system components. The system adapts Coriolis technology to obtain the electrical signal that is a direct measure of mass flow rate. It also provides a measure of fluid density and temperature.
The main feature of the sensing system is the proprietary flow-tube assembly. Different manufacturers use different tube geometries for the flow path through the primary. Some use a single tube, and others use a parallel pair of flow tubes. Figure 2 shows a bent tube arrangement with dual tubes. There are, in fact 17 tube geometries that help determine performance of the flowmeter. Tube bore is sized to provide meter sizes from ~½–6 in. The tube has no obstructions to fluid flow.
Figure 2. Vibrating dual bent tubes represent one of many tube geometries used to create the Coriolis effect for measuring mass flow.
Other main components of the sensing system include detectors that precisely measure the Coriolis effect to detemine the mass flow rate, as well as a driver coil that vibrates the flow tube (see sidebar).
Coriolis meters must be installed in such a way that the measuring tube is kept full— valid measurements are not possible on partially filled pipelines. And the meter must be zeroed after installation.
Entrained gas bubbles can affect meter performance, but increasing the backpressure with the installation can solve this problem. Some designs claim to handle gas or air slugs by using smart data filtering within their processor.
The meter should be installed on the high-pressure side of a pump and as close to the pump as practical, preferably at the lowest point in the process to ensure the highest backpressure. Valves, pipe elbows, or pumps installed downstream from the meter do not affect its operation.
Coriolis meters can measure mass flow rate, volumetric flow rate, fluid density, and fluid temperature and are virtually unaffected by variations in fluid properties. These instruments can measure a wide variety of fluids, such as nonconductives, that are often incompatible with other types of flowmeters. Because Coriolis devices are independent of Reynolds number, they can measure extremely viscous fluids. Some designs can also handle compressed gases and cryogenic fluids.
Coriolis meters provide accuracy on the order of ±0.1% to ±3% of rate, selectable with some designs. They are extremely linear over their entire flow range, with a usable range 20:1. They have successfully measured flow rates 100 X less than F.S.
Coriolis meters are not influenced by turbulence, flow profile, and other flow characteristics, so do not require minimum upstream or downstream straight pipe runs or flow conditioning. They can also be configured to measure flow in either direction.
The flow tubes of Coriolis meters have no internal obstruction that can be damaged or plugged by slurries or solids in the flow stream. And they have no process-wetted moving parts that wear out and require replacement.
The meters are available in designs for sanitary applications and are constructed of materials that can withstand corrosive fluids. Additionally, certain manufacturers provide secondary containment housings as part of the standard primary sensor.
Coriolis flowmeters are not generally available in sizes >6 in., but they can handle flow rates as high as 25,000 lb./min. (11,249 kg/min.). To use the technology for higher flow rates, the system must have two or more meters mounted in parallel.
Measuring the mass flow rate of low-pressure gases can be difficult with a Coriolis meter. To generate a rate high enough for the Coriolis force to be measured, the gas velocity must be extremely high. This in turn can lead to high-pressure drops across the meter.
Measurable fluid temperatures are typically limited to 400° F (204° C), although one model is said to be capable of handling temperatures as high as 425° C.
Coriolis mass meters can also be sensitive to pipeline vibration. And some designs are bulky, taking up valuable space when retrofitting existing plants (truer of earlier designs and eliminated in new generations).
Finally, Coriolis meters shouldn't be used to measure mixtures of solids and gas, liquid and gas, or steam.
The cost of these devices is on the order of 37%–43% higher than that of conventional flowmeters, but when you consider their superior accuracy and take into account the price of other related equipment you would need to add to do the same job, (e.g., a volumetric meter and densitometer), along with their maintenance requirements, it is not difficult to justify the price differential. Before Coriolis meters were available, the only practical answer to mass flow measurement was to use a volumetric meter and desi-tometer to correct its readings.
Inferential vs. Direct Mass Flow Metering
Coriolis meters inherently measure mass flow rate directly. They can provide readings of volumetric flow rate, fluid density, and temperature. Thermal mass flowmeters also read directly in mass flow rate units.
In many ways, with all types of meters measuring volume and density, the instrumentation solves the basic equation:
mass flow rate, in lb./hr. = volume flow rate in gal./hr. X density in lb./gal.
Of course, mass can be measured in other units, such as grams or kilograms, and time can be shown in minutes or seconds.
The inferential method is still applicable, and must be used with liquid mass flow rates that exceed the capacities of Coriolis meters. A practical example of a large-size volumetric flowmeter with density compensation is given in Litptak (see For Further Reading), with a schematic showing a magnetic flowmeter equipped with a gamma radiation densitometer in a single unit. Magmeters come in sizes ≥100 in. in dia.
Multivariable transmitters (see Figure 3) represent a relatively recent technology that can readily measure mass flow using inferential techniques. A single transmitter measures not only differential pressure (volumetric flow), but also process pressure and temperature. Because the latter two variables govern fluid density, they are the only ones needed to compute mass flow.
Figure 3. A multivariable transmitter combines sensors for differential pressure, static pressure, and temperature, along with the computational logic to determine mass flow rates. Savings result from the need for fewer transmitters and process penetrations, as well as less wiring.
Taking advantage of this, the transmitters also contain the computational logic necessary to determine mass flow based on industry standard formulas. The flow calculations of these transmitters can include compensation for complex variables including the discharge coefficient, thermal expansion, Reynolds number, and compressibility factor.
Volumetric flow readings can be based on the differential pressure measurements from common flow elements such as an orifice, averaging Pitot tube, venturi, nozzle, or wedgemeter.
Typical industrial mass flow applications for multivariable transmitters include combustion control to balance air and fuel flows; boiler steam flow for efficiencies and load management; and ammonia plant processes, where mass flow of the natural gas feed is balanced with steam flow to control the steam/carbon ratio in the primary reformer.
Thermal Mass Flowmeters
Another alternative to Coriolis meters is the thermal mass flowmeter. For certain applications with liquids and especially with gases, this device may offer the only viable solution.
Thermal flowmeters get their name from the fact that they use heat to measure mass flow. To do this, they put heat into the flowstream and use temperature sensors to meas- ure how quickly the heat dissipates. There are several ways this dissipation is measured and related to direct measurement of mass flow. Figure 4 shows an insertion-type thermal mass sensor.
Thermal meters have fast response time in measuring gases and excel in the meas-urement of low flow rates. They are designed for insertion into large pipes or stacks, where they can continuously monitor emissions of sulfur dioxide or nitrous oxide from power plants.
Figure 4. Thermal mass flowmeters offer direct measurement of mass flow, and they are well-suited for determining low flow rates of gases. They inject heat into the flow stream and measure its dissipation via temperature sensors. The illustration shows an insertion-type thermal mass flowmeter.
Thermal mass flowmeters generally have a slow response in measuring the mass flow of liquids. Also, they aren't nearly as accurate as Coriolis meters, with typical accuracies in the range of 1–3%.
How Coriolis Mass Flowmeters Work
For Further Reading
Liptak, Bela G. 2003. Process Measurement and Analysis: Instrument Engineer's Handbook, 4th ed., Boca Raton, CRC Press, www.crcpress.com.
"Mass Flowmeters," Transactions, Vol. 4, Omega Press, pp. 58–71.
McMillan, G. K. 1995. Process/Industrial Instruments and Controls Handbook, 5th ed., New York, McGraw-Hill.
Spitzer, David W. 2005. Industrial Flow Measurement, 3rd ed., The Instrumentation Systems and Automation Society.
Yoder, Jesse, "Coriolis vs. Thermal—Two Approaches to Mass Flow Measurement," Flow Control, March 2005.