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Level Measurement
A Case for Specialty Level Measurement Technologies
Joe Lewis
Managing Director, BlueLevel
Technologies
There are at least seven
technologies commonly available for point or continuous
level measurement, whether a liquid, slurry or
solid material. They each have their own specific niche or fit in the maze of level applications. We have spoken about several before,
and white papers exist that characterize each
fit. These technologies include hydrostatic,
capacitance, radar, ultrasonic, acoustic, rotary
paddle, vibrating element, RF capacitance, tilt,
pressure sensitive diaphragm and many others.
However, some applications are so difficult,
or present so tough a challenge that none of
the common technologies will work. What are
these challenges and what is the solution?
Every level measurement technology mentioned above is invasive to the vessel containing the material to be detected or measured.
But what technology do you use for high level
control of toxic carcinogens, highly abrasive
or corrosive materials, or in vessels under
extremely high pressure and/or extreme temperatures? The answer may very well be devices which are rarely used (by comparison), more
expensive and sometimes misunderstood,
i.e. radiation detectors and microwave beambreakers. Their advantages stem from the fact
that they are non-invasive or do not come in
contact with the material at all.
Radiation-Based Devices
Radiation-based level instruments have
been in use for more than 30 years. They are
available as point level control devices and as
continuous level transmitters, typically limited
in distances of about 15-25 feet, however, multiple units can typically be combined to measure greater distances and levels in larger vessels. Measurement error for continuous level
is ±1 percent. This technology is also used for
measuring material density and weight in certain embodiments, but these applications are
outside the scope of this column.
The radiation-based instrument system consists of three primary components, a gamma
source holder, a detector and associated electronics. The radioactive gamma source is typically mounted on the outside of the vessel on
one side. The installation is such so the gamma
energy is emitted towards the detector, which
is mounted outside on the opposite side of the
vessel. Some source holders may be capable
of providing source energy for multiple vessels
located directly next to each other.
Here is how they typically work. As the thin
band of gamma energy is emitted from the
source holder it passes through the wall of the
vessel that the source holder is mounted on. In
the absence of material (liquid, slurry or solid)
in the vessel the energy will then pass through
the opposite vessel wall and be detected at
the detector element. The walls of the vessel
reduce the energy detected. The introduc-
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March 2011 • www.ProcessingMagazine.com
tion of material in the vessel between the
source holder and the detector will result in
a further reduction in energy at the detector,
and this change is compared to the change
when empty. This results either in a point level
output (material presence) or a varying analog
output based on the amount of change of
energy detected (the level of the material).
Like most all technologies for level measurement, improvements have been made
over the years and today far less radiation
source is required, using only a fraction of the
amount required years ago. In addition, an
average working life of one to two decades
is achieved. Advances have also led to the
flexible detector. Two methods of detecting
gamma energy in the flexible detector are the
use of a liquid scintillating fill fluid or the use
of special scintillating fiber bundles. Both offer
dramatic improvements in sensitivity leading
to a reduction in source energy requirements,
as well as a dramatic reduction in detector
weight from as much as 15lbs/ft to 1lb/ft. In
addition, “ultra-low source” systems are available and are claimed to reduce the amount
of radioactive source material needed by so
much that periodic testing and documentation normally required by nuclear regulating
bodies might be eliminated. These systems
are also said to be able to be installed and
removed without the presence of a licensed
person in attendance.
While it is a major advantage to be able
to measure point or continuous level
in difficult applications without being
invasive to the vessel or ever being in
contact with the material, the tradeoff is
a high installed cost including the cost
of the equipment, the source material
and installation, including licensing and
material disposal when the level gauge
has reached its life. But in these difficult
applications, a better choice may not
exist. Radiation-based level measurement systems have their place.
One cited example is the use of a point
level radiation-based system for high level
control in power plant flyash hoppers.
Flyash can be very abrasive, reducing
the life expectancy of invasive probes. In
power plant applications it is common to
see internal hopper temperatures of 1,000
degrees F or more, eliminating other technologies or driving their special product
costs up considerably. In this application a
source holder capable of providing shared
source energy to multiple hoppers can be
used, reducing installed costs. The use of
externally-mounted detectors eliminates
maintenance issues associated with the
abrasive nature of flyash and extends the
life of the sensor to its maximum.
21
Microwave Beam-Breakers
Beam-breaker technology has also been around for decades and it has
a good niche of applications as well. The beam-breaker uses microwave
or radar technology and consists of a transmitter, receiver and associated
electronics. The electronics may be mounted integral to the transmitter/
receiver component or separately. They typically are based on either
K-band or X-band radar and have a wide angle so the mounting of the
receiver in relation to the transmitter does not have to be highly accurate. In the absence of material a strong
signal exists at the receiver. When material exists between the transmitter and
receiver, the signal at the receiver is
diminished greatly and indicated by a
change in the unit’s output.
Microwave beam-breakers are point
level sensing devices and used for level
control. The primary advantage is that
they are not invasive into the vessel or
process. The elimination of an invasive
probe element can be advantageous in
certain applications where very abrasive
or corrosive material exists. In addition,
using non-conductive “windows” in
the vessel wall can allow for installation
where the transmitter and receiver are
also not in contact with the material in
the vessel offering additional advantages and immunity to abrasion.
The primary disadvantage of the
microwave beam-breaker is that it is
higher in cost than other common level
control technologies and installation
can be more expensive as well due to
multiple components.
Technologies/97182916337. Their Expert’s Blog is at
www.blueleveltechnologies.com/blog. Mr. Lewis can be e-mailed at
[email protected].
Conclusion
While somewhat less known and
less used, both radiation-based level
sensors and microwave beam-breaker
level controls have a place and fit in the
broad scheme of level measurement
and monitoring applications. Wherever
a true non-contact or non-invasive
sensing device is needed or preferred,
these two technologies offer the best
solution. The only question is, do you
need this attribute or not? Market
research reports have suggested the
average cost of radiation-based level
sensors is over $10,000, and microwave beam-breakers is over $1,000.
Of course prices and overall operating
costs vary by brand and specific technology. The “need” is usually answered
when you decide whether or not you
can afford to pay for it.
For more information about point and
continuous level sensors and their
applications go to www.blueleveltechnologies.com. You can follow BlueLevel
on Twitter@BlueLevelTech and check
out their Facebook page at http://
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