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Ultrasonic Flowmeter Applications (Part 2 of 2)by David W Spitzer and Walt Boyes
Straight run requirements for ultrasonic flowmeters are not excessive, so this technology can be applied where limited straight run is available. Applications where the velocity profile changes with the flow rate can cause erratic operation of some ultrasonic flowmeters. Some ultrasonic flowmeters have Reynolds number constraints, so they are generally not applied in applications where the liquid exhibits a relatively high viscosity. Other designs correct for Reynolds number, however their performance is usually degraded in the transitional flow regime. Ultrasonic flowmeters generally exhibit their best performance in the laminar and turbulent flow regimes.
Clamp-on or insertion ultrasonic flowmeter sensors are often installed to measure the flow through large pipes to avoid the expense of a (large) full-bore spool piece. The ultrasonic sensors for most pipe sizes are similar, so flowmeter cost is almost independent of pipe size. These attributes of ultrasonic flowmeters often make this technology less expensive and more convenient to install.
Ultrasonic flowmeters measure liquid velocity, from which the volumetric flow rate is inferred. The measurement is linear with liquid velocity and exhibits a relatively large turndown. In addition, the range of flow measurement is relatively large.
Ultrasonic flowmeters can be applied to gas flows, particularly stack gas, flare gas, and natural gas. Some ultrasonic flowmeters can measure the flow of liquids in partially filled pipes.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.
What Do We 'Know'? Revelations in Flowmeter Straight Run Effectsby David W Spitzer
Sometimes we 'know' things that are not
true. For example, we used to 'know' that the earth was flat. Columbus 'knew' that he could sail eastward to reach the Far East but did not 'know' that a few
continents would block his quest. During
the last few centuries, we 'knew' that the Earth was round but could only see
it courtesy of some wonderful Apollo Mission pictures from NASA only forty
years ago. Such is the case of what we 'know' about straight run requirements for flowmeters in relatively large pipes.
We used to 'know' that flowmeters
require 10 pipe diameters of upstream straight run and 5 pipe diameters of
downstream straight run. Based upon
experimental test results, we 'know' that the flowmeter will measure accurately
if a minimum straight run is installed upstream of the flowmeter. We now 'know' that the minimum upstream
requirement is often much more than the 10 pipe diameters that we used to 'know' that we needed.
By inference, we 'know' that the
number of pipe diameters required is independent of pipe size. For example, 2-inch and 48-inch flowmeters
requiring 6 pipe diameters of upstream straight run would require 1 and 24 feet
of upstream straight run respectively.
Stated differently, in this example, 1 and 24 feet of upstream straight
run dissipate velocity profile distortions to the extent that they are small
enough to not affect the flow measurement.
However, recent flowmeter data
taken on four continents shows that this dissipation does not occur as quickly
in larger pipes as it does in smaller pipes.
This appears to be because the fluid in larger pipes does not have as
extensive contact with the pipe wall as do fluids in smaller pipes and due to
the large amount of momentum present in the fluid flow. As a result, more pipe diameters of straight
run are required to dissipate flow profile distortions in larger pipes.
One can conceptualize this
phenomenon by imagining a large pipe with swirl in its center that has little
or no contact with the pipe wall. It is
not likely that this swirl involving of tons of fluid per second will be
sufficiently dissipated in (say) 6 pipe diameters. In other words, flowmeter performance can be
adversely affected in this application and the effect will likely be large due
to the large pipe size and large flow rate involved.
This is something that I 'know' (for now).
This article originally appeared in Flow Control magazine.
Know Flow with No Flowmeterby David W Spitzer
There are times when there is no flowmeter
installed but it is necessary to determine the flow through a pipe. What techniques can be used to determine the
Control valve calculations
None of the above
Both of the above
Process calculations can be used to calculate
flow rate in many applications. In a
simple application where two flows streams are combined, the combined flow rate
can be calculated by summing the flow measurements from each individual flow
stream. In one application, the
unmeasured gas flow to certain nozzles was calculated and controlled in real
time by mathematically adding the measurements from the two incoming gas
flowmeters and subtracting the measurement from another gas stream flowing to
other nozzles. This type of calculation
is essentially a mass balance of part of the process.
More complex process calculations are also
possible by performing an energy balance on a part of the process. For example, steam flow to a heat exchanger
can be calculated using the specific heat, temperature rise, and flow rate of
the liquid being heated in conjunction with the heat value of the steam. In one application, the heat flow into a
boiler was calculated and controlled in real time by mathematically adding the
product of the fuel flows and their heat contents.
Another method to calculate the flow rate is to
use a control valve as a flowmeter. The
flow rate, differential pressure, and fluid properties are typically used to
calculate Cv to size control valves.
This same relationship can be used in reverse to calculate the flow rate
when the valve position, Cv at that valve position, differential
pressure, and fluid properties are known.
Knowledge of these parameters is often limited so the accuracy with
which the flow rate can be calculated is also often limited. Nonetheless, this can be a useful tool when
other measurements are lacking.
Additional Complicating Factors
The answer to the question depends upon a number
of factors including the process, availability of process measurements, and
whether the control valve has a usable position indicator.
This article originally appeared in Flow Control magazine.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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