Mass Flowmeters Fill Key Process Needs

By Steven G. Pagano

Broadly applicable, a new generation of Coriolis mass flowmeters are quickly becoming the technology of choice for mass flow measurement and control in industrial processes. For prospective new users, this article provides highlights of mass metering capabilities, with emphasis on Coriolis technology.

Many industrial processes can benefit from flow measurement and control based on metering mass flowrates in units such as pounds per hour. A pound of any liquid or gas is a pound, regardless of changes in the measured fluid’s temperature or pressure, flow profile, or other variables, such as its density or viscosity, all of which have an effect on the accuracy of volumetric meters.

Figure 1. A multivariable transmitter combines sensors for differential pressure, pressure, and temperature, along with the computational logic to determine mass flow rates. Savings result from the need for fewer transmitters, wiring, and process penetrations.

Measuring mass flowrate offers potentials for processing improvements worth thousands of dollars in such industries as chemical, food and beverage, petrochemical, pulp and paper, petroleum refining, pharmaceuticals, and water or wastewater treatment. Typical applications include batching and blending operations, precise filling of containers, billing and custody-transfer, as well as improved process control per se.

There have been great strides in mass flowmeter designs in recent years, especially in flowmeters that employ inferential, thermal mass, and Coriolis metering technologies. Suppliers can point to more and more successful installations that are proving their value. It will pay the process or instrument engineer and plant manager to keep up-to-date on what is available and make use of this technology.

Inferential vs Direct Mass Flow Metering
Coriolis meters inherently measure mass flowrate directly (see sidebar). They can provide readings of volumetric flowrate, fluid density, and temperature as well. Thermal mass flowmeters also read directly in mass flowrate units.
Prior to the availability of such meters, the only practical answer to mass flow measurement was to use a volumetric meter and correct its readings for density of the measured fluid, using a densitometer.

In many ways, with all types of meters measuring volume and density, the instrumentation solved the basic equation:

Mass flowrate, in.-lbs/hr = Volume flowrate in gallons/hr × Density in lbs/gallon.

Mass, of course, could be in other units, such as grams or kilograms (kg), and time, in minutes or seconds.

This inferential method is still applicable today and must be used with liquid mass flowrates that exceed the capacities of Coriolis meters. A practical example of a larger-sized volumetric flowmeter with density compensation is given in Litptak[1] with a schematic showing a magnetic flowmeter equipped with a gamma radiation densitometer in a single unit. Magmeters, of course, come in sizes up to 100 inches in diameter or more.

Role of Multivarible Transmitters
Multivariable transmitters, Figure 1, represent a relatively recent technology that can readily serve to measure mass flow using inferential techniques. Here a single transmitter measures not only differential pressure (volumetric flow), but also process pressure and temperature. Since the latter two variables govern fluid density, they are just the variables needed to compute mass flow.

Taking advantage of this fact, these transmitters also contain the computational logic necessary to determine mass flow based on industry standard formulas. The flow calculation capabilities of these transmitters can include compensation for such complex variables as discharge coefficient, thermal expansion, Reynolds number, and compressibility factor.

Volumetric flow can be based on the differential-pressure measurements from such common flow elements 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 plants 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

Figure 3. Coriolis flowmeters directly measure mass flow. The converter for the remote version, A, may be up to 1,000 feet from the primary flow element. Alternatively, the converter may be mounted integral with the primary, as in B.

For certain applications, with liquids and especially with gases, a thermal mass flowmeter may offer the only viable answer. Yoder[5] gives some highlights of recent developments in thermal flowmeters. Originating with hot-wire anemometer technology, two companies introduced models for industrial applications in the 1970s. A third company used a system of flow switches.

Thermal flowmeters get their name from the fact that they use heat to implement mass flow measurement. They put heat into the flowstream and use temperature sensors to measure how fast the heat dissipates. There are several ways this dissipation is measured and related to direct measurement of mass flow. Figure 2 shows an insertion-type thermal mass sensor.

Advantages of thermal meters are that they have fast response time in measuring gases and excel in the measurement of low flowrates. They come in a form for insertion into large pipes or stacks where they can continuously monitor emissions of sulfur dioxide or nitrous oxide from power plants.
Thermal mass flowmeters generally have a slow response in measuring the mass flow of liquids. Also they are not nearly as accurate as Coriolis meters, with typical accuracies in the range of 1 percent to 3 percent.

Direct Mass Flow Measurement with Coriolis Meters
In recent years, Coriolis mass flowmeters have become the instrument of first choice in many processing plants, building up an impressive number of reference installations. Coriolis meters are well suited for industrial environments and readily tie in with complete process measurement and control systems. Today, the global market for these meters probably exceeds $480 million, shared by over a dozen suppliers Liptak[1] has tabulated by various industries scores of specific process liquids and gases being measured by Coriolis meters — some like molasses are quite viscous, others like nitric acid are quite corrosive, and unusual examples include compressed gases such as nitrogen and helium.

One interesting development is for users to standardize on them for practically all applications throughout the plant. The higher unit cost is considered justified due to 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).

Basics of Coriolis Meters

Figure 4. Vibrating dual bent tubes represent one of many tube geometries used to create the Coriolis effect for measuring mass flow.

The two basic system configurations for Coriolis mass flowmeters are shown in Figure 3 — (A) a Remote Converter and (B) an Integral Converter — mounted directly on the primary housing. The Remote Converter connects to the primary with shielded cable that can be up to 1000 feet long. The converters receive the small electric measuring signal generated by the sensing system in the primary and electronically change it into usable outputs (current, pulse frequency, or digital). These outputs can be shown on the converter display and transmitted to panel-mounted recording and control instrumentation or process control computers in a centralized control room.

The primary mounts in the flow line and houses the essential sensing system components. This system adapts Coriolis technology to obtain the electrical signal that is a direct measure of mass flowrate. It also provides a measure of fluid density and temperature.

The main feature of this sensing system is the proprietary flow tube assembly. Different manufacturers use distinctly different tube geometries for the flow path through the primary. Some use a single tube while others use a parallel pair of flow tubes. Figure 4 shows a bent tube arrangement with dual tubes. Liptak illustrates 17 tube "geometries" that help determine performance of the flowmeter.

Other main components of the sensing system are (1) detectors that precisely measure the Coriolis effect as a measure of mass flowrate and (2) a driver coil to vibrate the flow tube. (See sidebar).

Tube bore is sized to provide meter sizes from about ½" to six inches. The tube has no obstructions to the flow of fluid through it.

Advantages of Coriolis Meters

In 1835, Custave G. Coriolis, a French engineer, discovered the principle that is the basis for operation of all Coriolis flowmeters. He showed that when a body is moving longitudinally (as fluid does flowing in a pipeline) and it is also subjected to rotary motion (e.g., the pipe is rotating), these combined motions create a unique inertial force. This force, now called the Coriolis force, can be directly related to the mass flowrate through the pipe.

The Rotation of Earth is given as an example of Coriolis force that causes a slight sideways drift to a hypothetical object thrown from the North Pole toward the equator. Note that two motions are simultaneously in effect: the longitudinal flight of the object and the rotating motion of the planet.

It was not until 1972 that Coriolis force was first employed in a commercial flowmeter providing a measure of mass flowrate. This Coriolis mass flowmeter used an ingenious design concept that artificially introduced a Coriolis force on the measured fluid stream passing through a vibrating flow tube. Tube vibration simulated rotary motion of the tube.

Directly related to mass flowrate, Coriolis forces cause a change in vibration frequency and cause a twist of the tube. Different suppliers models today use refinements in design to precisely monitor these continuously varying effects and produce an electrical measurement signal. Separate electronic converters amplify this signal and provide values of mass flowrate as described with reference to Figure 3.

References 1 through 4 each provide a lengthy and complex explanation of how the Coriolis meter works, complete with equations that need definitions of the terms used. The average prospective user, however, does not need such hard-to-follow details in order to benefit from the application-proven advantages offered by Coriolis mass flowmeters.

1. Can measure mass flowrate, volumetric flowrate, fluid density, and fluid temperature — all from one instrument — and are virtually unaffected by variations in fluid properties.

2. Measure a wide variety of fluids that are often incompatible with other types of flowmeters (e.g. nonconductive fluids). Operation is independent of Reynolds Number so the meter can measure extremely viscous fluids. Compressed gases and cryogenic liquids can also be measured by some designs.

3. Provide outstanding accuracy on the order of +/- 0.1 percent to +/- 3 percent of rate, selectable with some designs. Also, extremely linear over its entire flow range, having a usable range in the order of 20-to-1 or greater. Has been successfully used at flowrates 100 times lower than given full-scale flowrate.

4. Operation is independent of flow characteristics such as turbulence and flow profile so that it does not require minimum upstream or downstream straight pipe runs or flow conditioning. May also be configured to measure flow in either direction.

5. Their flow tubes have no internal obstruction that can be damaged or plugged by slurries or solids in the flow stream. They have no process wetted moving parts that will wear out and require replacement.

6. Available in designs for sanitary applications Available in materials that will withstand corrosive fluids. In addition, certain manufacturers provide secondary containment housings as part of the standard primary sensor.

Operating Constraints
1. Coriolis flowmeters are not generally available in sizes above six inches, but flowrates up to 25,000 lb/min (11,249 kg/min) are achievable. To use this technology for larger flowrates requires that two or more meters be mounted in parallel. The increased expense would have to be justified by performance not otherwise available.
2. The cost of a Coriolis meter may be perceived as high. But the cost of a volumetric meter, densitometer, and other related equipment can be just about as high or higher when considering future maintenance and product costs relating to performance gaps.
3. Measuring mass flowrate of low-pressure gases can be difficult with a Coriolis meter. In order to generate enough mass flowrate to provide sufficient Coriolis force to be measured, the gas velocity must be extremely high. This in turn can lead to very high pressure drops across the meter.
4. Temperature of fluid being measured is limited typically to 400 F (204 C). Spitzer[3] describes one high-temperature model said to be usable up to 425 C
5. Coriolis mass meters can be sensitive to pipeline vibration.
6. Some designs are bulky, taking up valuable space resources when retrofitting existing plants (more true of earlier designs and eliminated in new generations).
7. Should not be used to measure mixtures of solids and gas, liquid and gas, and steam).

Installation Points
1. Coriolis meters must be installed so that the measuring tube is kept full; if installed where the pipeline may be only partially filled, the meter cannot make a valid measurement.

2. To ensure proper operation, the meter must be zeroed after installation, completely filled with process fluid.

3. There should be no entrained gas bubbles in the liquid being measured; they can affect meter performance. Increasing the backpressure with the installation can dissolve the bubbles. Some designs claim to handle gas or air slugs by using smart process data filtering within their processor.

4. The meter should be installed on the high-pressure side of a pump and as close to the pump as practical.

5. Preferably, the meter should be installed at the lowest point in the process. This location ensures the highest backpressure.

6. Valves, pipe elbows, or pumps installed downstream from the meter do not affect its operation.

Steven G. Pagano is senior product manager of Primary Flow Devices for ABB’s Automation Technology Products Division within the Instrumentation business unit. He can be reached at [email protected].

For More Information: www.abb.com


References
1. Liptak, Bela G., Editor-in-Chief, Process Measurement and Analysis: Instrument Engineer”s Handbook, Fourth Edition, CRC Press, 2003, www.crcpress.com.

2. McMilland, G. K., Process/Industrial Instruments and Controls Handbook, Fifth Edition, New York, McGraw-Hill, 1995.

3. Spitzer, David W., Industrial Flow Measurement, Third Edition, ISA – The Instrumentation Systems, and Automation Society, 2005.

4. Mass Flowmeters, Transactions, Volume 4, pp 58-71, Omega Press.

5. Yoder, Jesse, “Coriolis Vs Thermal — 2 Approaches to Mass Flow Measurement,” Flow Control, March 2005.