Coriolis technology--flowmeters for the masses

Pushing back the frontiers of Coriolis mass flow measurement technology

The advantages of the Coriolis mass flowmeter are only beginning to be realized. Operations and maintenance and repair organizations (MRO) people alike are sometimes reluctant to apply these instruments because of their perceived high initial cost as well as a belief that a Coriolis meter is a special instrument intended for special applications.

When introduced in 1977, these meters became the first commercially viable device capable of measuring the mass flow directly, as opposed to volume flow of liquids and gases. Direct mass measurement was a major advancement for the process industry, which is--or should be--more interested in metering mass than volume to ensure the tightest control.

Unlike volume, mass is unaffected by changes in pressure, temperature, viscosity, and density. Although Coriolis meters directly measure mass, they can also be employed to perform volume and density measurements.

Using volumetric flowmeters to determine mass flow requires that the volume flow measured be mathematically converted to mass flow by compensating for temperature and pressure. To maintain accuracy, meter recalibration to correct for drift can be a continuing headache. Volumetric flow instruments also wear quickly and that translates into high maintenance and replacement costs. Last, a volumetric flowmeter is actually less accurate in measuring volume than a Coriolis meter.

How a Coriolis flowmeter works
A typical dual-tube, dual-path Coriolis meter consists of two sensor loops of metal tubing through which the fluid to be measured flows equally. Two loops increase sensitivity and also serve as each other's reference points and thereby nullify the effects of external vibration.

No component of a Coriolis meter is located in the fluid stream, pressure drop is minimal, and plugging has no more chance of occurring than in any other section of pipe.

An electromagnetic coil induces the loops to oscillate like tuning forks. As liquid flows, one leg of each loop oscillates in a wave-like, twisting motion slightly out of phase with the other leg (Figure 1). Twist is generated because the mass of the flowing fluid possesses an inertia that alternately resists and reinforces the imposed oscillations. The greater the mass flow, the greater the tubes' twist.

Velocity detectors measure the amount of twist, and transmitter electronics calculate the rate of mass flow and total mass of fluid delivered. The transmitter can also determine density by comparing the resonant frequency of the fluid being measured with that known for water. Knowing density, the transmitter can then convert mass to volume.

Why Coriolis meters are easy to use, and accurate
Without moving parts in the conventional sense, Coriolis technology designs also offer very high reliability and long life. A variety of construction materials allows measurement of most corrosive fluids. The instruments require no maintenance or recalibration and are self-draining if oriented properly.

No component of a Coriolis meter is located in the fluid stream, pressure drop is minimal, and plugging has no more chance of occurring than in any other section of pipe. The clear flow path makes the meters particularly amenable to clean-in-place procedures. For these and other reasons, the life-cycle cost of a Coriolis unit is usually lower than a less expensive technology.

Coriolis flowmeters are accurate--typically to within 0.1 to 0.2 percent at their design flow rates. Reasonable variations in fluid characteristics--temperature, pressure, density, viscosity, velocity, turbulence, pulsation, and solids content--do not degrade accuracy. Coriolis meters are also unaffected by Reynolds numbers, so straight runs of pipe and other flow conditioners are not required.

The meters detect small variations in flow and provide an output down to almost zero flow. Accuracy, even at very high turndowns, is superior to most other technologies. For example, a Coriolis meter accurate to 0.1 percent at a nominal rating of 5,000 pounds per minute is still accurate to 0.6 percent at 50 pounds per minute--a 100:1 turndown!

Selecting a flowmeter
The most significant costs associated with any flowmeter occur before the instrument is purchased, during installation, and after it's in service, so don't look strictly at acquisition cost when choosing the flowmeter for your application.

First, there's upfront engineering expenses. Plant operations and instrumentation personnel can spend hours comparing and specifying from a range of alternative measurement methods and instruments.

Then, there's installation and startup. The less complicated Coriolis instrument requires no secondary transmitters, manifolds, impulse tubing, remote seals, or flow conditioning devices. The installation and startup costs of the extra devices required by volumetric flowmeters can quickly out-distance the initial cost of Coriolis technology.

Last, there are recurring future costs to consider--calibration, maintenance, performance degradation--and consequent process degradation--fugitive emissions testing, instrument replacement, among other costs. Figure 2 illustrates the present-value costs of some of these long term factors--as well as of engineering, acquisition, installation, and startup--for several types of flowmeters.

Coriolis' overall costs are low because of the meter's simplicity, long life, and little need for attention. Many users find that calibration and maintenance procedures required for other technologies simply are not necessary for coriolis.

Check utility billing
Plant utilities historically have been ignored when identifying measurement points. As consumers, we simply believe what utilities tell us and we pay our bills.

Things are changing, though, as plants face pressure to cut costs everywhere. More plants want to balance their energy consumption with their product output, as well as check utility billing accuracy. Coriolis technology provides a simple and accurate way to do this.

For example, one chemical and pharmaceutical facility recently installed a Coriolis meter to track natural gas consumption and check the utility's turbine meter. The results were surprising. The Coriolis meter indicated that the plant was receiving eight percent less gas than the utility was billing. Why? The turbine meter the utility used for billing purposes required temperature compensation... and, that circuit had never been connected!

After connection, the meters read within one percent of each other. Additional testing indicated that the Coriolis meter was actually the more accurate meter. The plant has now applied for a refund for years of over-billing by the utility.

The application of Coriolis technology in gas measurement is growing five to 10 times faster than for liquid measurement. The key advantages offered by Coriolis technology are a significant improvement in rangeability and an ability to measure gases of mixed composition accurately.

The key advantages offered by Coriolis technology are a significant improvement in rangeability and an ability to measure gases of mixed composition accurately.

In another facility, the difference in readings between precision orifice and turbine pay and check flowmeters at an ethylene production facility and at a nearby plastics plant customer was three percent--$1 million in product per year. Technicians installed three-inch Coriolis meters at both sites and agreement between meters fell to within 0.3 percent. NIST-traceable liquid-to-gas calibration tests of the Coriolis meters were then conducted to certify the meters for fiscal transfer.

Detecting pipe leaks
Consider a fatty acid and alcohol process that consumed 30 percent--up to $2,000 per month--more hydrogen than reaction chemistry predicted. Pipe leaks were suspected, generating a safety concern. The plant replaced orifice meters at the supply source and at the fatty acid reactors with one-inch Coriolis meters, and the hydrogenmass balance immediately closed to within one percent.

More plants want to balance their energy consumption with their product output, as well as check utility billing accuracy. Coriolis technology provides a simple and accurate way to do this.

Rather than leaks, an otherwise undetected process control problem that also affected product yield and quality was found to be the culprit. Understandably, the Coriolis meters are now integral to the plant's process control and safety systems.

Controlling by density?
The ability to perform accurate, on-line, full-stream, or slip-stream density measurements for concentration control and net flow in a slurry process further expands the applicability of this versatile technology.

Real-time measurement and control
Potential applications include acid, base, and net catalyst additions. An example of this occurred at a paper plant. The problem hinged on trying to control a titanium dioxide brightener's slurry flow and the slurry's titanium dioxide concentration. A former
combination of magmeters and periodic lab analysis resulted in either adding too much titanium dioxide or producing as much as 15 percent of out-of-spec paper.

A Coriolis meter now measures mass flow and titanium dioxide net solids in realtime, which provides titanium dioxide net flow information and leads to precise application of the brightener.

The savings add up to $250,000 per year, and lab work is no longer necessary. The meter has since been incorporated into a paper opacity control loop to improve titanium dioxide application response.

Maintenance costs and capacity
In a yeast separation application, control of filter presses to separate liquids from suspended yeast solids was erratic because of volumetic loading. Too often, the presses were either underused or damaged because there was no compensation for the varying solids content of the batch being loaded into the press.

Applying Coriolis technology and loading according to the slurry's solids content cut plate damage by $12,000 per year, for a one year payback. A very lucrative side benefit was a 15 percent capacity increase due to reduced downtime and increased efficiency.

In another density control application, a 50 percent caustic solution distributed by a header system to in-plant users triggered significant corrosion and freezing problems and subsequent production-halting repairs. Because most users diluted the caustic to 25 percent, the plant installed a centralized blender--controlled by Coriolis density measurements--to reduce the concentration to an accurate and repeatable 25 percent and distribute the diluted material through the header. Downtime from header damage has been completely eliminated and caustic usage better tracked.

The Coriolis instrument is now certified as an intelligent field device compatible with Fieldbus Foundation's distributed architecture.

The future
There are few tasks in fluid and gas flow measurement that a Coriolis flowmeter cannot perform. It can replace other types of flowmeters--mass or volume--in existing processes or in new construction economically. It can tighten the accuracy of flow
measurements and thereby boost process efficiency, product output, and product quality. Further, it can cut the downstream costs--and bother--of flow instrumentation in general.

What's more, by providing both mass and volume flow and totals plus density, a Coriolis meter adds control points to your process. Even an available fluid temperature output might serve as an additional control point or as a check on a transmitter.

Coriolis should be a part of your future. Developments in viscosity measurement are on the horizon, as are additional capabilities in liquid and gas measurement. The Coriolis meter is also entering the world of device-level control, which is expected to be the wave of the future in process automation. The Coriolis instrument is now certified as an intelligent field device compatible with Fieldbus Foundation's distributed architecture.

Finally
When specifying a flowmeter, consider ease of use, reduced maintenance, reliability, and accuracy that Coriolis technology offers. And, keep in mind:

• Mass is an inherent property of matter independent of gravity. Weight, on the other hand, varies with gravity. Normally mass and weight are considered equivalent at sea level, and they are usually expressed in pounds or kilograms.
• When cost-justifying new technology such as Coriolis flowmeters, present-value cost analysis brings back all costs--present and estimated future--to the present. Estimated future costs are discounted by the interest accrued by the deferred spending.