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Non-Contact Radar Level Measurement (Part 2 of 3)by David W Spitzer and Walt Boyes
Still another proprietary technique can be used for liquids with dielectric constants as low as 1.05. The radar beam passes through the vapor space and liquid before being reflected back to the sensor by bottom of the vessel. Information from the reflected signal is then processed to infer the liquid level in the vessel.
Noting that the radar signal travels to and from the material, degradation of the signal strength between the sensors and the material can cause radar level measurement systems to fail to operate. Degradation can occur at the antenna, in transit to/from the material, and at the surface of the material.
Dirt or other coatings on the radar antenna can cause the received signal to be weak. When accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating. In many applications, the sensor may be shielded from the process and/or continuously purged with gas to keep it in operation.
Careful analysis of the sensor design should be performed. Some designs inherently tend to keep the sensor from accumulating material while others tend to inherently accumulate material.
Excerpted from The Consumer Guide to Non-Contact Level Gauges.
Small Measurement Errors Add Up: How a 1% Error Can Produce a $1,000,000 Lossby David W Spitzer
In our hectic workaday lives, we sometimes lose
track of the importance of our work. We
often view our work as providing technical expertise, however the economic
impact of our work can be mind-boggling.
I recently had the opportunity to audit a number of steam flowmeters for
a cogeneration company that produces electricity and sells steam to a local
host that uses the steam for its processes.
The substance of an excerpt
from the report reads (in part):
Relatively small
measurement errors can have a relatively large economic effect in custody
transfer installations for high value products, large pipelines, or both. For example, assuming that the average annual
steam flow rate is 100,000 lb/hr of steam valued at $10.00 per 1000 lb, the
flowmeter would pass approximately $8,760,000 of steam per year (100,000 lb/hr
* 8760 hrs/yr * $10.00 / 1000 lb).
The large
economic value of the fluid over time means that even small measurement errors
can be significant. In the above
example, a 0.01 percent measurement error would result in a billing error of
$876 per year. Therefore, even small
errors such as rounding or unit conversion can result in significant billing
errors. Conversely, measurement errors
of a few percent can result in billing errors that can approach $1,000,000 per
year.
In many such installations, total
steam consumption is more than (the stipulated) 100,000 lb/hr, so the economic
implications can be considerably larger.
It takes only a one percent error on another client's billing of $100
million per year for the error to exceed $1,000,000 per year!
There is much that you can do
after you are armed with this information.
Initially, you can evaluate heat and material balances to determine the
magnitude of suspected flow measurement errors.
This can be followed by an examination the entire flow measurement
system with a "fine-toothed comb". Be
sure to examine ALL aspects of the installation including the upstream and
downstream piping, process conditions, flow element, taps, impulse tubing (or
other process connections), instrument installation, instrument specifications,
calculations, configuration, wiring and communications, indicator, totalizer
and other aspects of the measurement as may be required.
Remember that the idea behind this approach is to
find, identify and quantify flow measurement problems. While cursory investigation may uncover some
issues, detailed investigation is advised because errors associated with
virtually any of the above aspects of the flow measurement system can result in
errors that can exceed $1,000,000 per year in many processes.
This article originally appeared in Flow Control magazine.
Which Flowmeters Are Volumetric? by David W Spitzer
The subject matter for flow
measurement related to testing for a teaching position at a technical school
was recently circulated. It included mass flowmeters (thermal and Coriolis) and
volumetric flowmeters (Annubar, nozzle, positive displacement, magnetic
turbine, vortex shedding, ultrasonic, Pitot, orifice plate, rotameter, Venturi,
and V-cone). Which of the following groupings are correct for volumetric
flowmeters?
C. Positive displacement, magnetic, turbine, vortex
shedding, ultrasonic, rotameter
D. Annubar, nozzle, positive displacement, magnetic
turbine, vortex shedding, ultrasonic, Pitot, orifice plate, rotameter, Venturi,
and V-cone
E.None of the above
The equations for fluid flow in a
pipe are:
Q = A * v
W = rho * Q
where Q is the flowing volume, A is the cross-sectional
area of the pipe, v is the velocity of the fluid in the pipe, W is the mass
flow rate, and rho is the fluid density.
Some flowmeters (volumetric)
measure the flowing volume of the fluid while others measure the mass of the
fluid. Other flowmeters measure the velocity of the fluid and infer the flow
rate using the cross-sectional area of the pipe (A). Still other flowmeters
infer the flow by measuring the velocity head (0.5 x density x velocity2) across the
flowmeter.
The flowmeters listed in
Answer A are differential pressure flowmeters. They present an obstruction to
the flow that creates a differential pressure that is measured and used to
infer the flow in the pipe. These flowmeters are not volumetric flowmeters so
Answer A is not correct.
The flowmeters listed in
Answer B measure the velocity of the flowing fluid. They are not volumetric
flowmeters. Answer B is not correct.
With regard to Answer C, positive displacement
flowmeters measure the volume of the fluid. However, the remaining flowmeters
measure velocity. Therefore, Answer C is not correct. Similarly, Answer D is
not correct. This leaves Answer E as the correct answer.
Additional Complicating Factors
Testing will apparently be administered based on
this subject matter.
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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