A Case for Specialty Level Measurement Technologies
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20 Level Measurement There are at least seven technologies commonly avail- able for point or continuous level measurement, whether a liquid, slurry or solid material. They each have their own spe- cific niche or fit in the maze of level applica- tions. 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 men- tioned above is invasive to the vessel contain- ing 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 tem- peratures? The answer may very well be devic- es which are rarely used (by comparison), more expensive and sometimes misunderstood, i.e. radiation detectors and microwave beam- breakers. 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, mul- tiple units can typically be combined to mea- sure greater distances and levels in larger ves- sels. Measurement error for continuous level is ±1 percent. This technology is also used for measuring material density and weight in cer- tain embodiments, but these applications are outside the scope of this column. The radiation-based instrument system con- sists of three primary components, a gamma source holder, a detector and associated elec- tronics. The radioactive gamma source is typi- cally 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- 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 mea- surement, 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 avail- able and are claimed to reduce the amount of radioactive source material needed by so much that periodic testing and documenta- tion 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 measure- ment 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 tech- nologies 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. Joe Lewis Managing Director, BlueLevel Technologies A Case for Specialty Level Measurement Technologies March 2011 • www.ProcessingMagazine.com Write In 231
Transcript of A Case for Specialty Level Measurement Technologies
20 Level Measurement
There are at least seven technologies commonly avail-able for point or continuous
level measurement, whether a liquid, slurry or solid material. They each have their own spe-cific niche or fit in the maze of level applica-tions. 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 men-tioned above is invasive to the vessel contain-ing 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 tem-peratures? The answer may very well be devic-es which are rarely used (by comparison), more expensive and sometimes misunderstood, i.e. radiation detectors and microwave beam-breakers. Their advantages stem from the fact that they are non-invasive or do not come in contact with the material at all.
Radiation-Based DevicesRadiation-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, mul-tiple units can typically be combined to mea-sure greater distances and levels in larger ves-sels. Measurement error for continuous level is ±1 percent. This technology is also used for measuring material density and weight in cer-tain embodiments, but these applications are outside the scope of this column.
The radiation-based instrument system con-sists of three primary components, a gamma source holder, a detector and associated elec-tronics. The radioactive gamma source is typi-cally 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-
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 mea-surement, 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 avail-able and are claimed to reduce the amount of radioactive source material needed by so much that periodic testing and documenta-tion 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 measure-ment 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 tech-nologies 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.
Joe LewisManaging Director, BlueLevel Technologies
A Case for Specialty Level Measurement Technologies
March 2011 • www.ProcessingMagazine.com
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www.ProcessingMagazine.com • March 2011
Microwave Beam-BreakersBeam-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 accu-rate. In the absence of material a strong signal exists at the receiver. When mate-rial 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 advan-tages 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.
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 micro-wave beam-breakers is over $1,000. Of course prices and overall operating costs vary by brand and specific tech-nology. 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.blueleveltech-nologies.com. You can follow BlueLevel on Twitter@BlueLevelTech and check out their Facebook page at http://www.facebook.com/pages/BlueLevel-
Technologies/97182916337. Their Expert’s Blog is at www.blueleveltechnologies.com/blog. Mr. Lewis can be e-mailed at [email protected].
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