Tutorial: Coriolis flowmeters for gas service

More than once in these pages we have discussed the tricky process of measuring compressed air or other gas flows. (See earlier stories on rotameters and thermal mass flowmeters .) The compressible nature of gas makes it more difficult to measure than liquids, particularly when your goal is to convert the reading to a standardized temperature and pressure or mass measurement. In critical situations, a Coriolis flowmeter may be the best solution, although it will not be the least expensive.

In the larger instrumentation picture, Coriolis flowmeters are known for being highly accurate, capable of very wide turndown ranges, and costly. However, their growing availability and popularity proves that these capabilities are well worth the price in critical applications, especially for gas service. They work in this application because they are mass flowmeters. Their natural output is pounds or kilograms per unit of time, not volume. The mass value has to be converted to volume based on the density of the product at given conditions. For compressed air use, volume units will normally be standard cubic feet per minute (SCFM) or normal cubic meters per hour (Nm3/hr).

Coriolis flowmeters operate by pushing the process fluid (either gas or liquid) 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. 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 in general have common characteristics:

• High accuracy, with

• High turndown rates, up to 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;

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

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

• Some designs are sensitive to mounting position, but this is usually less important in gas service;

• Wide temperature ranges, generally with sophisticated internal compensation; and,

• Insensitivity to internal flow profiling.

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

• Check specifications carefully. Suppliers rate designs in terms of mass and volume units, (but normally based on liquids) as well as tube diameter, so make sure you pay attention. A pound’s a pound whether it’s gas or liquid, but don’t be surprised if you have to hunt for specifications expressed in SCFM. They aren’t always listed in the literature.

• Check for likely pressure drop through your operating range. In gas service, this can be problematic if you expect to run at the upper limit.

• Most designs can be used interchangeably between gas and liquid applications with appropriate transmitter programming changes, but a few are not suitable for gas at all. Most offer standard conversion factors for typical gas volume measurements.

• 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. How much accuracy do you really need on a compressed air measurement? Other technologies might be able to do the same job at a lower cost.

Coriolis flowmeters are available from a variety of suppliers:

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

You can also search online at Control Engineering Supplier Search .

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Daily News desk

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