Tutorial: Coriolis mass flowmeters

03/22/2007


A relative newcomer to the instrumentation scene, Coriolis flowmeters have quickly established a unique place in the measurement toolbox. They are known for being highly accurate, capable of very wide turndown ranges and expensive. However, their growing availability and popularity proves that these capabilities are well worth the price in critical applications. Understanding them requires some basic knowledge of the technology.

They are mass flowmeters. They are affected by the mass of fluid flowing through them, not the volume. This characteristic is helpful in that they are capable of handling any type of fluid, including gas or liquid, so a Coriolis flowmeter is equally at home measuring compressed air or other gas as it is with water. (If you've ever struggled with measuring gas or air flow where pressures and volumes are variable, this is a major benefit.) On the other hand, you have to pay attention to what you're putting through it and know the fluid density to make appropriate mass to volume conversions.

Coriolis flowmeters operate by pushing the process fluid through a vibrating pipe or pair of pipes. When the pipe is empty, it vibrates in a predictable manner. When fluid is moving through it, the vibrations are disturbed in a measurable way that converts to mass which can convert to volume (if desired) based on density. They can read in pounds per minute or other mass units, or convert to volume internally to indicate volume per minute. One peculiar characteristic is that you can often feel the vibrations and hear them hum when operating. Usually the vibrating pipe is "U" shaped which gives these flowmeters their odd tennis racket or horse collar outline, but there are also designs that do the job with a straight pipe section.

Coriolis flowmeters have common characteristics:

  • High accuracy, with

  • High turndown rates, as high as 1:100, so they can cover very wide measuring ranges;

  • Unobstructed flow path internally, although they can cause pressure drops when operating near maximum flow;

  • Wide range of connection options and materials;

  • Suitable for many sanitary (dairy, food, pharma) applications;

  • Wide range of sizes, from 3 to 80,000 lb/minute (1/8 to 12 inch);

  • Wide temperature ranges, generally with sophisticated internal compensation;

  • Capability to handle odd process fluids, including slurries, viscous materials, etc.; and,

  • Insensitivity to internal flow profiling.

They do have some downsides which vary by design type:

  • Two phase flow (e.g., bubbles in liquid) can degrade accuracy drastically;

  • Sticker shock—they are expensive;

  • They can accumulate material internally and even clog;

  • Design is often bulky;

  • Internal tubes are difficult to clean; and,

  • Can be sensitive to environmental vibrations.

If you are considering a Coriolis flowmeter, keep the following points in mind. These are generalities, so discuss specific concerns with your suppliers.

  • Check specifications carefully. Suppliers rate meters in terms of mass and volume units, as well as tube diameter, so make sure you pay attention. Also check for likely pressure drop through your operating range.

  • Units can be sensitive to positioning. Check that your piping will allow the sensor to be mounted in the best orientation.

  • Watch for potential paths for bubbles to enter a liquid stream. Bubbles in liquid, or trapped gas pockets in the internal tubes, can hurt accuracy. The effect of this varies, so discuss it with your supplier.

  • Straight pipe units are easier to clean, but can be more expensive and have fewer size options.

  • If a unit is damaged (corrosion in terminal box, denting to outside case, etc.) repairs are expensive if they are possible at all. Replacement may be the only option.

  • Make sure the flowmeter characteristics you need from a Coriolis are worth the additional expense and downsides. Other technologies might be able to do the same job at a lower cost.

Coriolis flowmeters are available from a variety of suppliers:

Emerson Process Management (MicroMotion)
Endress+Hauser
Siemens
Foxboro
ABB
FCI
Krohne
Yokogawa
AW Company

Control Engineering Daily News Desk
Peter Welander , process industries editor





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