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Recent advances
in the development and performances of Coriolis meters have meant that the
measurement of the mass flow rate of gases, such as natural gas for custody
transfer applications, is now a reality.
International
standards
This has been
reflected by the large acceptance of this technology within the natural gas
industry. As an example, Micormotion has supplied 5,000 Coriolis meters for
natural gas applications in the last 3 years. This industrial acceptance
motivated ISO to develop a standard through the ISO Technical Committee—ISO Standard
TC30/SC12. In addition to this ISO standard, there is also an
engineering technical report prepared by AGA entitled Coriolis Flow
Measurement for Natural Gas Applications.
For additional
information on Coriolis meters and their use in liquid service, see Inference
liquid meters.
Although there
is no ISO standard for natural gas measurement using Coriolis measurement, some
countries have issued type-approval certificates for natural gas measurement
using Coriolis meters. These countries include: The Netherlands (Netherlands
Inst. for Metrology and Technology), Germany (Physickalisch-Technische
Burdessarstalt), Canada (Measurement Canada), and Russia (Gosstandard).
Coriolis meter
overview
A Coriolis
meter comprises two main parts:
§ A sensor (primary element)
§ A transmitter (secondary element)
See Fig.
1.
Fig. 1—Coriolis flowmeter (Courtesy of
Daniel Industries).
With this
design, the gas flows through a U-shaped tube. The tube is made to vibrate in a
perpendicular direction to the flow. Gas flow through the tube generates a
Coriolis force, which interacts with the vibration, causing the tube to twist.
The greater the angle is twisted, the more the flow increases. The sensing
coils, located on the inlet and outlet, oscillate in proportion to the
sinusoidal vibration. During the flow, the vibrating tubes and gas mass flow
couple together because of the Coriolis force, causing a phase shift between
the vibrating sensing coils. The phase shift, which is measured by the Coriolis
meter transmitter, is directly proportional to the mass flow rate. The
vibration frequency is proportional to the flowing density of the flow. However,
the density measurement from the Coriolis meter is not normally used as part of
the gas measurement station. Like other meters, the Coriolis is usually mounted
in a meter tube. Because the device is insensitive to flow disturbances, there
is no requirement for any form of flow conditioning, straight lengths, or meter
tube.
Theory of
operation
Coriolis meters
operate on the principle that, if a particle inside a rotating body moves in a
direction toward or away from the center of rotation, the particle generates
inertial forces that act on the body. Coriolis meters create a rotating motion
by vibrating a tube or tubes carrying the flow, and the inertial force
(Coriolis force) that results is proportional to the mass flow rate. By
measuring the amount of inertial force or deflection, it is possible to infer
the mass flow rate. It is this phenomenon that is harnessed within the Coriolis
flowmeter.
It is also
important to consider any additional uncertainty associated with the
through-life stability of the Coriolis meter. There are two main influencing
factors: the change in flow-tube structural characteristics caused by erosion
of the tube wall by abrasive particles and the coating of the flow tube by
debris. Abrasion of the flow tubes by abrasive particles can directly affect
the flow calibration of the meter. Coating of the flow tubes by debris is only
a concern at low fluid flow velocities when the meter is not self-cleaning.
This influence does not affect the meter’s calibration and only affects the meter’s
zero. It can be corrected by regular zero checks for drift and zeroing, if
required. Both of these influences can be identified as occurring under flowing
conditions by monitoring the drift in flowing density over time.
Advantages and
disadvantages
The advantages
and disadvantages for Coriolis meters are shown in Table 1.
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Table 1
Sizing
Gas Coriolis
meters, like all Coriolis meters, are mass devices. The sensitivity of the
meter to measure small amounts of mass flow determines the low end of the
metering range. The upper end of the measurement range is most often determined
by the largest acceptable pressure loss. The pressure loss across the meter
increases with flow rate and the corresponding velocity through the meter.
Velocities through the meter can be a substantial fraction of the speed of
sound but clearly should not exceed about 0.5 Mach.
References
1. ISO Standard TC30/SC12, Measurement of
Fluid Flow in Closed Conduits—Mass Methods. 2005. Geneva, Switzerland: ISO
Technical Committee.
2. Coriolis Flow Measurement for Natural
Gas Applications, technical report. 2001. Washington, DC: AGA.
