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January 2009

Coriolis meters use flow vibrations

The global flowmeter market tracks at nearly $5 billion a year in revenue. One of the two fastest growing segments of this market is Coriolis (ultrasonic is the other).

Flowmeter growth is strongest in the oil and gas industry, and with crude oil trading between $50 and $150 per barrel over the last year, measurement accuracy and reliability are most important. That is where Coriolis flowmeters come in. They are very popular for custody transfer of petroleum liquids.

Technology of the gods

The first commercial meters appeared in the 1970s. They measure mass flow directly with high accuracy and rangeability.

A French engineer and mathematician, Gustave-Gaspard Coriolis, first described the Coriolis force in the early 1800s. It is an effect of motion on a rotating body and is of paramount importance to meteorology, ballistics, and oceanography.

Whereas pressure differences tend to push winds in straight paths, winds follow curved paths across the Earth. In 1835, Coriolis first gave a mathematical description of the effect, giving his name to the Coriolis force.

While air begins flowing from high to low pressure, the Earth rotates under it, thus making the wind appear to follow a curved path.

In the Northern Hemisphere, the wind turns to the right of its direction of motion. In the Southern Hemisphere, it turns to the left. The Coriolis force is zero at the equator.

This force results from acceleration acting on a mass, and anyone who walks radially outward on a moving merry-go-round experiences the force. A person must lean toward or direct the mass of his or her moving body against the force that the Coriolis acceleration produces.

If we know the force (F) acting on the body, the velocity (V) of the body, and the angular velocity () of the platform, we can calculate the person’s mass (M).

By applying this phenomenon to mass-flow measurement, we create a Coriolis mass flowmeter. Indeed, there are several types of meters leveraging the Coriolis Effect. One straight-up mass-flow device has rotors containing metal vanes that form several channels.

This gadget operates at a constant angular velocity per an external power source. Any particle of fluid traveling through the radial channel with velocity V will experience the Coriolis force, resulting in torque acting in the plane of rotation.

With the torque acting in the plane of rotation, measurement of the torque happens by placing a sensing means, such as a strain gage, in the drive shaft. To measure the mass-flow rate, we hold the angular velocity—the motor rotational speed—constant, leaving the torque as a direct measure of mass flow.

Coriolis crosses platforms

A device employing the gyroscopic principle of operation in the oscillatory mode is a tube shaped in the form of the letter “U.” An electromagnetic oscillator drives the U-shaped tuning-fork-like structure at the resonant frequency of the system, thereby producing a Coriolis acceleration and resultant force.

The force acts alternately—perpendicular to the flow path—in opposite directions, causing an oscillating moment about axis O-O of the flowmeter.

The resulting moment (m), acting about the central axis and in a plane perpendicular to the driving moment (W), produces a twist-type motion, where the deflection angle between FC1 and FC2 is directly proportional to the mass-flow rate.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Basics. A source for this article is Fundamentals of Flow Measurement, Joseph DeCarlo, an Independent Learning Module, ISA, 1984.