October 2001
Gamma radiation, fiber optics, and dopants meet at the mill
by Stephen Prout , Jack Rodgers
After a brief dance with radar technology, company reverts to nuclear ways.
The Alabama River Cos.’ complex, located in southwestern Alabama, includes both an older southern hardwood bleached Kraft pulp facility and a newer southern softwood (pine) bleached Kraft pulp facility. New environmental regulations, market price fluctuations, increased production, and process reliability have challenged the older facility.
A recent challenge involved its aging and deteriorating 20-year-old nuclear system level detection system. The mill decided to seek out a new measurement technology that leverages radar.
The system cost approximately one-third as much as an ion chamber nuclear level detection system and doesn’t require any regulatory licensing paperwork. The initial results were so good that the mill installed the low-cost radar in the pine mill’s chip bin, too.
Within three weeks the unit failed, and the mill revisited nuclear technology. It discovered a new and even better nuclear level detection weapon. The company hasn’t looked back.
Source holder shields
One of the important requirements in maintaining final pulp quality is a reduced Kappa variability. A key process parameter in achieving this goal is chip bin level control. Level control is important for several reasons:
- Stable chip feed to the digester
- Uniform presteaming of chips
- Complete condensation of flash steam
- Uniform deaeration of chips to prevent chip floating in the digester
- Reduced Kappa variability
- Reduced Knotter rejects
- Increased production rates
For about 20 years, a nuclear ion chamber had been successfully operating in this application and had proved to be a rugged and reliable detector. Indeed, the technology came into service in the late 1950s.
The conventional nuclear gauge system consists of three main components: the nuclear sealed source capsule’s source holder, the nuclear detector, and the electronics that provide the 4–20 mA DC output signal used by the programmable logic controller that controls the chip feed process.
The source holder and the detector are typically mounted 180° apart on the vessel. It is a nonintrusive technology, so the system components are mounted on the outside of the vessel and are not susceptible to process fouling. This reduces maintenance issues over intrusive technologies.
The electronics can lodge with the detector or at a remote location. Gamma radiation, in the form of electromagnetic energy, is constantly emanating from the nuclear sealed source capsule in the source holder.
The State of Alabama Department of Public Health governs the use of radioactive sealed source capsules in source holders within the mill. Cesium-137 or cobalt-60 are the radioactive isotopes typically utilized in the source capsules.
Radar new on process vessels
The source holder design is such that radiation is directed via a collimator (provides a beam of gamma radiation rays) through the vessel wall, across the diameter of the vessel, and through the other vessel wall, striking the detector housing and the ion chamber inside.
The source holder shields radiation from the area around the source holder outside the vessel and through the use of a collimator directs a slice of radiation to the detector. As the wood chips build up level in the vessel, the wood chips attenuate the electromagnetic energy that reaches the detector.
As the process material in the vessel blocks more and more radiation, the detector sees less radiation, which translates as high level. The nuclear gauge detector sees the maximum amount of radiation with the vessel empty and the least amount of radiation with the level at 100%, or full, condition.
But the mill’s nuclear system had reached the point where the source holders were deteriorating, and the electrical and instrumentation maintenance were an ever-increasing job.
The mill decided to replace the ion chamber nuclear level detection system with a radar level detection system. Using radar gauges on process vessels is an emerging technology. Using through-air radar gauges has overcome many problems of previous intrusive technologies, such as dusty and steamy environments.
From a cost standpoint, the radar gauge was significantly less expensive than the nuclear gauge and does not involve regulatory issues. Mounting on top of a vessel flange on the top of the bins and not coming into direct contact with the process offered advantages over other nonnuclear technologies.
Calculate time of flight
The through-air pulse radar gauges operate by transmitting a microwave pulse from the gauge toward the product surface. The product reflects the pulse back to the gauge, which uses the time of flight to calculate the distance and thus the product level in the vessel. These gauges are useful in dealing with liquids and solids.
The strength of the pulse reflection is what determines the success of the gauge. One reason the radar unit failed was the buildup of tacky fines inside the radar cone.
Also, at the bottom of the hardwood chip bin a vibrating system assists in chip flow to the metering twin-screw feeder. In the middle of this vibrating cone is a cone-shaped shield whose purpose is to allow presteaming without chips plugging the steam lines. The noise-to-signal ratio increased at low level and made it difficult to determine the level. Engineers attributed this noise to chips spreading out as they fall, and the chip bed shaking from the vibrator.
Technicians tried positioning the radar at an angle to minimize the effect of interference of chip fall and wall effects. The radar level measurement would also not read true high levels due to a baffle on the feed chute.
The mill tried different characterization calibrations to determine high levels, but none proved effective. Varying chip moisture and particle size distribution may have caused a different angle of repose and change of the reflectance of the radar and characterization of the level.
Alabama River revisited the nuclear technology because it relies on the concept of absorbance rather than reflectance. After exploring various nuclear technologies, a detection system that was lightweight, easy to install, and less susceptible to vibration emerged. The mill had this system up and running in three days.
Meet scintillating fiber
During the ordering cycle for new equipment, the supplier introduced a new scintillation flexible fiber-optic continuous level detector (SFFOD), available in lengths up to 23 feet.
Scintillation detectors have been in use for some time as a standard for nuclear density measurements. The scintillation technology offers increased sensitivity over the ion chambers when exposed to the same level of gamma radiation. This characteristic provides for the use of smaller sources and reduces the radiation fields associated with nuclear gauging systems.
The nonintrusive mounting arrangement for the scintillation detector system is the same as for the ion chamber. A source provides gamma radiation, which travels from one side of the vessel to the other. The gamma radiation strikes scintillation crystal housed in the detector.
The crystal is made of a plastic material with special dopants. The dopants provide a characteristic to the plastic such that when gamma radiation strikes the crystal, it excites the crystal and creates light photons in the crystal structure. A photo multiplier tube (PMT) detects these photons of light.
The PMT senses the photons of light and converts the light sensed into an electrical signal. Increasing the radiation to the detector increases the quantity of photons and translates as a lower chip level. The detector output is either a frequency output to a remote mounted electronics package to develop the 4–20 mA DC output or the 4–20 mA DC itself.
The SFFOD, however, leverages fiber optics and offers further advantages over the previously installed ion chamber gauge system, not the least being that it has greater sensitivity. The principle of operation of the SFFOD is the same as previously described for a scintillation detector.
Instead of a rigid plastic crystal, the SFFOD utilizes flexible fiber-optic strands of the special plastic material. Many strands bundle together to form the detector. By using the fiber-optic strands, the photons of light created within the strands move efficiently through the individual strands to the PMT.
The SFFOD is flexible and lightweight. Compared with two 10-foot ion chamber detectors weighing 350 pounds, the 23-foot SFFOD weighs less than 40 pounds. Being lightweight and flexible significantly reduces the installation and shipping cost, compared with the ion chamber detector design.
Even with the installation being 200 feet in the air, plant personnel installed the detector without the need of a crane and expensive rigging this time. With the conduit for the old detectors still existing, the gauge was commissioned and in service within a few hours.
Finally, the longer SFFOD eliminated the need for the two sets of electronics that had to be electrically connected on the side of the vessel for the ion chamber. One was enough. Alabama River is so pleased with this new nuclear and fiber-optic apparatus that it’s moving it into other areas of operations. IT
Figures and Graphics
- Gamma radiation, fiber optics, and dopants meet at the mill (pdf version)
- Ion chamber as energy cell
- Nuclear ion chamber gauge
- Scintillation detector with crystal
- Scintillation detector with fiber optics . . . even better!
Sidebars
Behind the Byline
Stephen Prout is a P.E. who works at Alabama River Pulp as an instrument and process engineer. Jack Rodgers is vice president of nuclear business at Ohmart/VEGA in Cincinnati.
Read questions answered by our experts or join the email list.
