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Stan: When are nuclear devices used?
Ram: Nuclear devices are used if the level device needs to be completely external to the process due to extremely high pressures and temperatures or exceptionally hazardous materials. Nuclear is also used for difficult applications where radar is not possible because the dielectric constant is too low and other devices, such differential pressure (DP) transmitters, are not suitable.
Greg: What are the application considerations?
Ram: The use of nuclear point sources instead of strip sources creates a nonlinearity that requires compensation by signal characterizations. The measurement is highly repeatable with an accuracy of about 1% if the effect of density changes on characterization is negligible, and the location of the source relative to the detector is correct. The beam path must avoid agitators, eductors, spargers and dip tubes. Vessels jackets complicate the installation in order to avoid the effect of varying jacket fluid. The measurement may not be possible when coolant coils are present.
Stan: Why are plants reluctant to use nuclear?
Ram: Besides the burden of paper work, a radiation officer must do periodic wipe and shutter tests. A record of background radiation survey should be maintained for NRC audit. Also, the plant cannot do maintenance on the vessel until the radiation officer shuts off the source and authorizes the work.
Greg: Bubblers and diaphragm seals offer a way of isolating the DP transmitter from the process. What are some of the considerations in their application?
Ram: For bubblers, an accurate purge flow regulator and unlimited purge source is needed to insure the purge rate and hence, the purge pressure drop is constant. The end of the tube may be cut at a 60º angle or raised off the bottom when stratification of the fluid causes buildup. For diaphragm seals, larger seals diameters and better evacuation of any bubbles from the fill have reduced the 95% response time to less than 16 seconds. The use of fill with a low-volume temperature effect has reduced the effect of unequal capillary lengths and uneven temperatures (e.g., shade versus sun) on accuracy. The level measurement accuracy for a well-designed bubbler or capillary installation with no density changes, shrink or swell is about ½%.
Stan: Larry Stichweh at Monsanto documented how pressure transients may cause a temporary rise or fall in the DP transmitter signal due to the compressibility of the purge gas. Larry advocated the consideration of a liquid purge. In some applications, the drying effect of a purge gas has caused caking at the tip of the bubbler. A small amount of water or solvent introduced into the purge gas solved the problem.
Greg: The use of Coriolis mass flowmeters on a vessel discharge stream can provide the density measurement needed to correct level measurements affected by density. The totalization of mass flow from these meters can also provide an on-line check of the inventory of process mass important for material balances and process modeling. Accurate level and flow measurements greatly enhance process knowledge.
Stan: There are some heavy reasons to use floats instead of radar, such as heavy vapors, heavy foam and heavy emulsions. Greg has his own lighter "Top Ten Reasons"
10. There is something to be said for tradition.
9. It works in my toilet.
8. I want a signal I can see and feel.
7. It is kind of like a tape measure.
6. Mechanical engineering rules!
5. These systems are my legacy.
4. I am into float trips.
3. It has its ups and downs, but always ends on top.
2. Cheap, cheap!
1. I don't want any newfangled software.