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By Walt Boyes, Editor in Chief
Let's say you have a reactor vessel. It is 6 ft. (1.8 m) in diameter, glass-lined, has a big agitator in it, and has both a jacket made of 1-in. (25-mm) copper cooling coils and a 4-in. (100-mm) layer of insulation covered with thin steel lagging. Worse yet, there are no accessible entrances into the top of the vessel that aren't already being used for something. For the process to work, you must measure the level in the vessel with significant precision. You've even tried weigh cells, but there isn't enough precision to just weigh the contents of the reactor, with all that tare weight. Oh yeah, and you can't stop the reactor to modify it, and since it is a glass-lined and code-stamped vessel, you can't drill any holes in it either. What do you do?
Or, suppose you're making glass for a variety of products. The glass is produced by melting silica sand, glass frit from recycled bottles and some trace minerals in a very hot furnace with firebrick walls that are over 1 ft (300 mm) thick. The glass is too hot to pump, so it must flow by gravity down a firebrick-lined channel to where it is cast or molded or extruded. Your requirement is that you have to measure the level of the molten glass and control it to ±0.0005 in. (±0.013 mm), or the process doesn't work. Glass castings have holes called holidays in them, and extruded glass, whether tube or sheet, has flaws and holes. What do you do?
You are responsible for the air pollution control system for a very large coal-fired power plant. You have electrostatic precipitators that remove the fly ash from the stack gas before it gets released into the atmosphere, causing international pollution incidents and costing your utility millions in air-pollution-control violation fines. But the hoppers that hold the precipitated fly ash keep plugging up, and fly ash is very hot and also acts like concrete and sticks to everything. You need some way to tell when the hoppers are full, so you can empty and clean them, but anything you stick into the hopper just gums up and fails so fast that you have given up.
Sound familiar? Nearly every plant, from mining to wastewater and every process vertical in between, has a level application that is both critical and difficult, if not impossible, to measure.
Since the 1950s, the answer to all of these applications has been the proper application of a gamma level gauge. Gamma gauges work based on both the inverse-square law—radiated energy decreases with the square of distance—and the fact that dense materials absorb gamma energy—1 in. (25 mm) of steel, for example, cuts the energy from a gamma beam by 50%
Very early on, engineers came up with the idea that rising material or liquid would change the amount of energy reaching a detector on the other side of the vessel from an emitting source. In the case of a point level switch measurement (Figure 1), rising material would simply trigger a relay if the energy beam were interrupted. In the case of a continuous level measurement (Figure 2), the rising material would cause a decrease in the intensity of the energy beam reaching the detector that could be calibrated to be proportional to the rise in level, and when the level fell, then the energy would likewise increase.
In order to figure out how much energy will reach the detector, essentially all you have to do is to add up the densities and thicknesses of all the materials between the energy source and the detector, and make the energy beam intense enough to pass through all that material and reach the detector. Safety requires that the intensity of the energy beam be designed to be as small as possible and still make the measurement.
"Modern detector designs have made it possible to use significantly lower activity sources than in previous years," says Mick Schwartz, business unit manager of Berthold Technologies USA LLC (www.berthold.com), a manufacturer of gamma level gauging products. "This means that the risk of exposure to gamma energy for personnel is minimized and amenable to proper safety precautions. Gamma energy does not cause any of the measured product or the vessel to become radioactive."
All manufacturers of gamma level gauges have software that makes the calculation of energy source size straightforward. You or the vendor plug in the numbers for the thicknesses and densities of the material, not forgetting the air gap between the walls of the vessels—air has density, and energy decreases with the square of distance—and the software spits out an optimized energy source size and, in most cases, the appropriate housing design and detector selection.
So let's look at how to do the level application in the jacketed vessel we talked about earlier. This is not quite as easy as putting a source and a detector across from each other because there are vessel internals, including an agitator, that have to be missed. The way to do this application is to "shoot a chord" of the vessel's diameter—that is, put the source and detector off to one side of the diameter. Because the thicknesses that the energy beam will shoot through will be greater, the source activity that will be required will be greater by some amount than shooting the diameter would be. The blades of the agitator need to be considered, and, if possible, eliminated by shooting the chord between the blades and the vessel wall. If that isn't possible, many gamma level gauges can be programmed to ignore the repetitive density fluctuation caused by blades swinging into and out of the beam. It just makes the signal noisy.
Now let's look at the glass level gauge. There's a lot of firebrick on either side of the glass channel, so it may be necessary to drill holes in the firebrick to reduce its thickness. This will cause the temperature on the outside to rise, so the detector must be water-cooled to bring the internal temperature of the electronics down to the normal range.