Single Tube Coriolis
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WWW.micromotion.com KNOWLEDGE MICRO MOTION WHITE PAPER WP-00510 Rev. A / © 2011 Micro Motion, Inc. All rights reserved. Coriolis Technology Creates Superior Meters Dual curved-tube and single straight-tube meters offer different advantages The discovery made by Gaspar Gustav de’Coriolis (1792– 1843) has made possible the development of two types of meters: the highly accurate dual curved-tube meters, and the more compact single straight-tube meters. Napoleon had asked de'Coriolis to explain why his cannon balls never went straight. History doesn't tell us whether he ever gave Napoleon a sufficient explanation—that phenomenon is caused by an entirely different effect—but de'Coriolis' research led to his discovery of the Coriolis effect. Using Coriolis technology, a whole new industry of accurate meters was spawned. What does the weather have to do with it? Most people are familiar with the Coriolis effect on our weather patterns. Because of the jet streams, the clouds tend to move east and west across the planet. However, the rotation of the planet creates the Coriolis effect, which deflects clouds as they move across the sky, causing them to spin or rotate. That rotation causes the earth's storms. Down to earth, from storms to mass measurement Applied to instrument design, Coriolis meters can measure mass flow directly as well as volume and density. In process plants, mass is a vital measurement: it is the basis of all chemical reactions, mass and energy balances, and almost all process flow operations, including most custody transfers. In process manufacturing facilities, prior to Coriolis meters, mass was measured by weighing liquids on a scale, which can be appropriate for batch operations. However, for continuous processes, meters were generally restricted to volumetric measurement. In a continuous flow situation, earlier methods of flow instrumentation, such as orifice plates and magnetic flow tubes, measured only volumetric flow. Mass flow measurements had to be corrected for changes in temperature and pressure. To illustrate, Figure 1 shows two barrels of gasoline of the same weight. When temperatures change, the volume increases, but not the mass; therefore, the barrels remain balanced. In volumetric technology, something—such as manual or automatic calculations with additional transmitters and other instruments—must compensate for temperature to convert process readings to mass. Figure 1. Fluid volume is affected by temperature and other factors A volumetric flowmeter is affected by changes in the temperature, as well as by pressure, density, viscosity and the flow profile of the process fluid. But a Coriolis flowmeter provides a direct mass measurement that is unaffected by changing process fluid characteristics. How the meter works In a dual-tube meter, a manifold that connects the two tubes splits the flow in two and directs it through each of the two tubes. In this way, 100% of the flow always goes through the sensor. The two vibrating tubes rotate around the two fixed end points, creating a Coriolis effect when mass flows through. When there's no flow, fluid on both sides of the U-shaped tube are subject to the same force operating in the same direction (opposite to the direction in which the flow tube is moving); so there is no twist (Figure 2).
Transcript of Single Tube Coriolis
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KNOWLEDGE
MICRO MOTION WHITE PAPER
WP-00510 Rev. A / © 2011 Micro Motion, Inc. All rights reserved.
Coriolis Technology Creates Superior Meters
Dual curved-tube and single straight-tube meters offer different advantages
The discovery made by Gaspar Gustav de’Coriolis (1792–1843) has made possible the development of two types of meters: the highly accurate dual curved-tube meters, and the more compact single straight-tube meters.
Napoleon had asked de'Coriolis to explain why his cannon balls never went straight. History doesn't tell us whether he ever gave Napoleon a sufficient explanation—that phenomenon is caused by an entirely different effect—but de'Coriolis' research led to his discovery of the Coriolis effect. Using Coriolis technology, a whole new industry of accurate meters was spawned.
What does the weather have to do with it?
Most people are familiar with the Coriolis effect on our weather patterns. Because of the jet streams, the clouds tend to move east and west across the planet. However, the rotation of the planet creates the Coriolis effect, which deflects clouds as they move across the sky, causing them to spin or rotate. That rotation causes the earth's storms.
Down to earth, from storms to mass measurement
Applied to instrument design, Coriolis meters can measure mass flow directly as well as volume and density. In process plants, mass is a vital measurement: it is the basis of all chemical reactions, mass and energy balances, and almost all process flow operations, including most custody transfers.
In process manufacturing facilities, prior to Coriolis meters, mass was measured by weighing liquids on a scale, which can be appropriate for batch operations. However, for continuous processes, meters were generally restricted to volumetric measurement. In a continuous flow situation, earlier methods of flow instrumentation, such as orifice plates and magnetic flow tubes, measured only volumetric flow. Mass flow measurements had to be corrected for changes in temperature and pressure.
To illustrate, Figure 1 shows two barrels of gasoline of the same weight. When temperatures change, the volume increases, but not the mass; therefore, the barrels remain balanced. In volumetric technology, something—such as manual or automatic calculations with additional transmitters and other instruments—must compensate for temperature to convert process readings to mass.
Figure 1. Fluid volume is affected by temperature and other factors
A volumetric flowmeter is affected by changes in the temperature, as well as by pressure, density, viscosity and the flow profile of the process fluid. But a Coriolis flowmeter provides a direct mass measurement that is unaffected by changing process fluid characteristics.
How the meter works
In a dual-tube meter, a manifold that connects the two tubes splits the flow in two and directs it through each of the two tubes. In this way, 100% of the flow always goes through the sensor.
The two vibrating tubes rotate around the two fixed end points, creating a Coriolis effect when mass flows through.
When there's no flow, fluid on both sides of the U-shaped tube are subject to the same force operating in the same direction (opposite to the direction in which the flow tube is moving); so there is no twist (Figure 2).
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Figure 2. Theory of operation: dual, curved-tube, no flow
When fluid is flowing, it's accelerating (changing direction) on the inlet side and decelerating on the outlet side. As a result, reactive force on the inlet side is opposed to the motion of the tube, and reactive force on the outlet side operates in the same direction as the motion of the tube. The tube is twisted very slightly by these opposing forces (Figure 3). The amount of twist is proportional to the mass flow rate.
Figure 3. Theory of operation: dual, curved-tube response to flow
To exploit the Coriolis phenomenon, a magnet is attached to one tube and a pickoff coil is attached to the other tube at both the inlet and the outlet of both tubes (Figure 4).
Because of vibration, the coil moves into the magnetic field and generates a sine wave that is proportional to that motion. Because the coils and magnets are referenced to each other (one element moves with each tube), the sine
wave represents the relative velocity and position of the tubes. When the waves cross zero, the tubes are momentarily at rest before changing direction.
Under no-flow conditions, the two sine waves on the inlet and outlet are in phase, or on top of each other. But under flow conditions, the tubes twist, and these two sine waves shift apart (Figure 4). The instrument measures the twisting, which is directly proportional to the mass flow.
Figure 4. Theory of operation, sensor pickoff elements
Get it straight
The same analogy applies to a single, straight-tube meter (Figure 5). The tube is vibrated along the center of the axis. Because the tube is fixed at the ends, the vibration creates two rotating reference frames. The rotation of the inlet side is in the opposite direction of the outlet side, creating the opposing Coriolis forces.
At the inlet side, the tube is rotating in one direction, say upward, but the fluid doesn't travel there. The fluid is restrained by the wall and the substrate and now it is vibrating up, creating a force downward (Figure 6). When the fluid passes the centerline lengthwise and is rotating in the opposite direction, the wall restricts the rotation, which creates a force in the opposite direction.
The second part of Figure 5 shows an exaggeration of the deflected shape. Again, there are two forces that are proportional to the mass flow rate. The Coriolis effect is generated as it is in the double curved tube, through the rotation of the tube about fixed ends.
The two sine waves on the inlet and the outlet are shown as Point A and Point B in Figure 6. The inlet side lags the outlet side, which is identical for either straight or curved tubes.
Software allowed the designers of the single straight tube meter to change the design to ensure the response of the Coriolis signal was not affected by changes in the density or
Coriolis Technology Creates Superior Meters Page 3 of 6
mounting options. Approximately 100,000 to 150,000 different analytical runs were performed to perfect the design.
The sensor element, which is relatively immune to environmental changes, takes into account that the Coriolis forces are changing with fluid density at the appropriate vibration levels.
Figure 6. Theory of operation, single, straight-tube (3)
Each design has its merits
Although there are similarities between dual, curved-tube and single, straight-tube meters, there also are some significant differences. The key differences are how they behave to mounting effects, fluid density changes and temperature changes.
The dual, curved-tube meter is much more immune to meter mounting variations. The two tubes are essentially identical. When they vibrate in opposition to each other, they are extremely well balanced. All the energy is contained within these two tube sets, and nothing is
transmitted out to the environment. The tubes are “unaware” of the environment, because everything is maintained within the sensing elements themselves.
The designers were able to put a relatively fixed spacer between the two tubes. Pipeline loads are transferred to the spacer and never get to the sensing element, so that when the loading on the meter flanges is changed in the field, it has no effect.
The same attributes make the dual curved-tube meter reliable despite fluid density changes. Because these two tubes are identical, when the fluid density changes within them, they respond identically.
These tubes extend from the pipeline and are relatively free to expand. Therefore, as the temperature changes, they grow in length, unrestrained, with no stresses or loads generated through expansion. A single measurement compensates for the effect of temperature on the modulus of the material.
In contrast, the single straight-tube meter's mounting presents challenges. The single-tube system was designed to be symmetric from inlet to outlet, but during vibration, the flow tube and reference tube don't vibrate to the same extent, so the single tube meter is not inherently balanced. The reference part was designed to maintain that balance, but it responds to vibration as if no vibration leaked out the ends.
As fluid density changes, it only changes in a single tube, not in the reference tube. A great deal of research was invested to understand the dynamics and make these meters stable as fluid density changes. The design of the reference tube is vital for stable vibrations throughout the entire density range.
Figure 5. Figure 5. Theory of operation, single, straight-tube (1) and (2)
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The last design challenge and perhaps the most challenging aspect of single straight-tube meters is what happens when temperature changes. Its case is fairly rigid, so it holds pipeline loads. It also holds the tube very rigidly. A fluid temperature change will change the tube, but the case will not allow it to expand.
Using Coriolis
The differences between curved tubes and straight tubes will impact user applications (Figure 7). Every plant or facility has a variety of needs for applications and measurement points, which often require significantly different specifications (Table 1).
One example would be a material balance, in which many different streams are combined. The output should be very consistent but the error tends to increase as more streams are added together. In this case, users will want very high accuracy, because the errors will multiply throughout the measurement points.
To solve this problem, the user could choose a dual curved tube meter because of its ability to deliver highly accurate and repeatable measurements. When blending or combining different materials, if operators are changing the amounts of materials that they are combining, they might run it at different flow rates. The dual curved-tube
meter's increased accuracy also gives it a higher rangeability, from 100 percent of flow to 2 or 3 percent of flow, with very little degradation of the measurement.
In some cases, when operators are just blending and filling, a mass measurement is required but it doesn't need to have the same accuracy as in the material balance situation.
Despite the dual meter's superiority, some users will want to select the single straight-tube meter. If a volumetric meter is being replaced, there might be a limited amount of space to install a new meter between closely spaced pipe flanges. The compactness of the straight tube might make it preferable.
Figure 7. Process industry applications
Dual curved-tube meter Single straight-tube meter
Technical challenges Meter mounting effectsDual tubesMechanical filteringPipe loads through spacer
Symmetric constructionLess mechanical filteringRigid caseAsymmetric vibration
Fluid density changes Inherent well-balanced systemNo changes in tube vibration
Imposed balance systemChanges tube vibrationMass of tube changes
Fluid temperature changes Flow tubes not constrainedNo induced thermal stresses
Flow tubes constrainedInduced thermal stresses
User needs Superior accuracySuperior repeatabilitySuperior immunity to field effectsHigh turndownHigh and low flows sizes (1/32-inch to 12-inch)Optional materials of construction (high nickel content)Gas applicationsCustody transfer
CompactSanitarySelf-drainingInspectableNon-pluggingLow pressure dropCleanableShear sensitive fluidsSurface finishSecondary containment
Table 1. Coriolis measurement points
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The straight tube is also easy to make highly sanitary. The surface can be polished, because there are no manifolds or bends and no changes in the flow area or reductions where particles can collect.
Micro Motion product line
Emerson Process Management’s Micro Motion division offers a broad product line that includes a complete family of Coriolis meters ranging from large to very small sensors. Specializations are available within this broad family line.
Micro Motion introduced the ELITE® meters dual curved-tube product line to be the most precise meter on the market, for times when accuracy is the most important factor.
Micro Motion T-Series single, straight-tube meter offers compact size and sanitary specifications for applications where sanitary conditions are more important, such as the pharmaceutical industry.
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Coriolis Technology Creates Superior Meters
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