Page 449 - The Mechatronics Handbook

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The flow velocity v, which is calculated using Eq. (19.68), is the average velocity along the transmission
path between the two transducers, and so the flowmeter’s calibration will be very dependent on the flow
velocity profile. The problem of flow regime dependency can be significantly reduced by using a config-
uration of several parallel ultrasonic beams and averaging the measured mean velocity along each beam
to calculate the overall fluid velocity. This is analogous to numerical integration, and a wide range of
multibeam configurations have been proposed, each with their own advantages.
Single path ultrasonic transit-time flowmeters have a typical accuracy of ±2% of reading over a range
of at least 100:1. Although this type of flowmeter can be used with liquids or gases, clamp-on designs
can only be used with liquids. Multibeam flowmeters have an improved accuracy, but are more expensive.
However, they are finding increased use in high value applications like the custody transfer of natural
gas. Unlike most other flowmeters, the cost of transit-time flowmeters does not increase significantly
with pipe diameter. Transit-time flowmeters are intended for use with clean fluids, although most can
still operate if there are a small amount of impurities present in the flow.

The Coriolis Flowmeter
The Coriolis flowmeter can be used to measure the mass flowrate of a fluid directly. As the name suggests,
the principle of operation makes use of the Coriolis effect discovered by Gustave Coriolis in 1835. The
fluid being metered passes through a vibrating tube, and as a result of the Coriolis force acting on it, the
fluid will accelerate as it moves towards the point of maximum vibration and decelerate as it moves away.
This will result in flexure of the tube, the amount of flexure being directly proportional to the mass
flowrate of the fluid.
The first commercial Coriolis flowmeter used a U-shaped tube, but now many different configurations
exist, including dual loops and straight through designs. Each design has its own advantages, with factors
such as accuracy, repeatability, and pressure drop varying from design to design.
Whichever design is used, the Coriolis flowmeter is a high accuracy instrument, which may be used
to meter gas or liquid with an accuracy of typically ±0.25% of reading. Measurement range varies with
design, but 100:1 is possible for U-tube designs and 50:1 for straight tube designs. Since the flowmeter
measures mass directly, changes in density, viscosity, pressure, and temperature do not effect the calibra-
tion of the flowmeter. The flowmeter is also not affected by changes in flow velocity profile or other flow
disturbances such as swirl. The flowmeter does not obstruct the flow at all, and can be used to meter
flow in both directions. However, the pressure drop across U-tube designs can be a limitation with viscous
fluids.
The major disadvantage of the Coriolis flowmeter is its cost, which is high in comparison to most
other flowmeters. This cost may be justified in applications where the product cost is high, or where
mass flowrate of the fluid is required. The cost and weight of the Coriolis flowmeter increase significantly
with increasing pipe diameter, and as a result are usually limited to pipe diameters with diameters less
than 100 mm.
Unlike most of the flowmeters discussed so far, the Coriolis flowmeter can meter some difficult two-
phase flows. For example, reliable measurements of the mass flowrate of liquid/gas mixtures are possible
if the gas component is small and well distributed, and therefore the mixture is acting like a pseudo-
homogeneous fluid. The percentage of gas that can be tolerated by the flowmeter will depend on the
viscosity of the liquid component. The less viscous the liquid the more likely the gas is to separate out
and cause problems. Liquid/solids flows (slurries) may also be metered, although the user has to com-
promise between avoiding particle dropout and avoiding excessive fluid velocities which would result in
accelerated wear of the flow tube.

Two-Phase Flow
There is a growing demand in areas such as the petroleum and food industries to be able to measure
two-phase flows such as liquid with entrained gas, or liquid with solids. Yet the measurement of such
flows nearly always presents difficulties to the process engineer.

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