Page 450 - The Mechatronics Handbook

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For example, an ideal flowmeter would be able to directly measure the volumetric flowrate of a liquid
whether it be all liquid or contain a second phase of gas. Unfortunately most of the flowmeters described
above can usually only be used to meter two-phase flows when the second component is small. A review
of the performance of conventional flowmeters in two-phase flows can be found in [10].
The alternative to direct flow measurement is to use an inferential method. An inferential method for
liquid/gas flow would require the velocity of the gas and the liquid phases (v g and v l ) and cross-sectional
fraction of the gas phase (α) to be independently measured in order to calculate the volumetric flowrate
of the mixture Q m :

(
Q m = v l A 1 a) + v g Aa (19.69)
–
The overall uncertainty of the flowrate measurement would depend on the accuracy with which the
independent measurements can be made. The velocity of the liquid and gas phases cannot be assumed
to be the same, and the way the gas is distributed in the liquid (the flow regime) will change depending
on factors such as gas content, individual phase velocity, and pressure. Even in a simple case such as this,
it is clear that multiphase flow measurement is by no means straightforward. For this reason commercial
multiphase flowmeters are generally expensive and targeted at very specific applications.
The most common two-phase flows are liquid/gas (e.g., gas in water), liquid/liquid (e.g., water in oil),
gas/solid (pneumatically conveyed solids) and liquid/solids or slurries (e.g., coal in oil). Each presents
its own measurement problems and it is not feasible to discuss all possible metering combinations for
these types of flow in a overview article such as this. For further details of two-phase flow measurement
techniques see [11].

Flowmeter Installation
No matter how good a flowmeter is, correct installation is essential if it is to measure with the uncertainty
specified by the manufacturer. The calibration of most flowmeters is sensitive to changes in flow velocity
profile and in such cases the flowmeter should be installed to ensure that a fully developed and stable
velocity profile is present at the measurement point. Manufacturers’ data sheets will contain recommen-
dations for the minimum straight length of pipe required before and after a flowmeter to ensure that
this is the case. Flow conditioners (or flow straighteners) can be used to correct a distorted velocity profile
and remove swirl in applications where long straight lengths are not possible. However, the limitation
of all flow conditioners is that they restrict the flow and so produce an unrecoverable pressure drop in
the pipeline.
Installation should always ensure that the pipe is completely full at the metering point and that no
unwanted second phase is present. In liquid flows entrained gas can be produced from a number of
sources, including cavitation and leaking seals. While in a gas flow, an unwanted liquid phase can be
produced by processes such as condensation. In most cases flowmeters will produce metering errors if a
second phase is present in the flow. While it is possible to remove unwanted phases from the flow upstream
of the metering point, it is better to take care with the process, pipework design, and flowmeter installation
to ensure that this situation does not occur in the first place.

Flowmeter Selection
There is no such thing as a flowmeter which is equally good for all applications and given the large
number of commercial flowmeters and the variety of data sheets available, the choice can sometimes
appear bewildering. While at first sight more than one flowmeter may meet a particular application,
selecting the most appropriate can be more difficult. In general the best flowmeter will be the one that
can meet the performance specification at the lowest total cost (this is a combination of purchase price
and cost of maintenance).

©2002 CRC Press LLC