Just like the goose and gander, what's good for liquid is good for gas. Long used to accurately measure liquid flow, Coriolis meters now bring similar benefits to gas applications. As a result, Coriolis gas measurement is growing four to five times as fast as liquid applications, according to Fisher-Rosemount's Micro Motion Inc. (Boulder, Colo.).

No longer a niche technology for special applications, Micro Motion reports that Coriolis gas meters are proving they can deliver accurate measurements, especially under changing conditions and wide turndowns, with no flow conditioning; better reliability due to no moving parts and less needed devices; and lower costs also because of fewer devices, reduced maintenance, and quicker payback from improved accuracy. Coriolis gas meters also require fewer connections and intrusions into process lines, which further cuts costs by preventing leaks and fugitive emissions.

Coriolis gas meters' direct mass measurement ability eliminates the need for former densimeter or pressure-, temperature-, and composition-based measurements. Direct measurement also reduces the chance of process upsets and off-spec product, according to Micro Motion. The company adds that later corrections to standard conditions are also aided because direct mass flow is more accurate than traditional volumetric flow.

One large, Pacific Northwest-based pulp mill uses Micro Motion's Elite CMF100 Coriolis meter to measure oxygen used to delignify its pulp stock. Chlorine is usually used to bleach pulp stock, but it's dangerous to handle and generates environmentally hazardous dioxins. This makes oxygen delignification more attractive because it removes lignins from pulp fibers without attacking the fibers themselves and greatly reduces chlorine use. "It's a great benefit to have an in-line meter capable of measuring mass flow of oxygen directly and accurately without the need to compensate for pressure and temperature," report the mill's operators.

Micro Motion's Elite Coriolis meters can be used in many other process applications and installed in numerous orientations, including vertical pipe runs. Because the meters are insensitive to orientation and gas swirl effects, straight pipe runs aren't needed. Recalibration for different fluids isn't required, and the factory water calibration transfers to gas measurement. Depending on the application, usable turndown of 100:1 can be achieved, which Micro Motion claims is unprecedented for any gas flow measurement technology.

Micro Motion says its Elite Coriolis meters achieve gas flow accuracies of ±0.5% of actual mass flow rate and gas density accuracies of ±0.002 g/cc. The meters are available in 1/4- to 3-in. (6- to 80-mm) widths and can meter gas flows greater than 30,000 scfm (57 x 10⁶ NM³/hr.) or 2,500 lb/min. (68,000 kg/hr.) depending on inlet pressure.

In one case, Micro Motion reports several Asian refiners spent years struggling to accurately transfer ethylene gas to chemical users, and had monthly disputes of 2-3% on amounts transferred becasue of volumetric devices. Replacing them with Elite flow meters reduced transfer variance to 0.3%, which settled the dispute and also cut maintenance costs.

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