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It is very important to have a good look at how and where the unit is mounted to avoid the build-up of this sticky fly ash.
The benefit of guided wave radar compared to free space is that it can measure in the storage silo and close to the collectors. The benefit of free space radar is that it is non-contact and is usually a better solution for sticky fly ash when mounted at the right place and hooked up to a purge to keep it clean.
A very tough application you might face is level control of your boiler feedwater, or in any water measurement that there is high pressure, high temperature or both present.
We would like to get away from mechanical devices with moving parts such as displacers. Is there something on the market that will do this measurement?
The answer is yes. The solution is TDR, a guided wave radar, (time domain reflectometry) device. It uses time of flight technology with microwave energy to give you a reliable and accurate measurement.
How does this work? It's actually very simple, but first
This vapor, which is now compressed steam, will cause a slower time of flight for the microwave energy and also causes an error in the calculation of the level. The energy will now slow down in the vapor space, and as shown in the graph below, there is potential for a large error.
Vapor space, which could be ambient air, nitrogen, or any other gas, has a dielectric constant of 1. On the other end of the scale is tap water, for example, which has a dielectric of around 80. Water is an excellent reflector of microwave energy. (Note: Water in boiler systems is generally more pure than tap water, which does reduce the dielectric constant.)
Now you must address the problem of vapor space in these applications. Earlier, we discussed that vapor space is usually a dielectric constant of 1. Unfortunately, the vapor space in a high temperature or high pressure water measurement has a higher dielectric constant. This is caused from the compression of the steam.
Example for steam:
Medium: Water at 592°F/725 psig
Bypass: MR=39?; x=3.9?
Operating point: 50%MR
> Error gas-phase: 7.1% of 23.6? = 1.7?
Now that we understand the potential for error, how do we correct it? It can be done manually, which requires additional components, but some manufacturers also do this with a dynamic compensation.
So what is dynamic compensation and how does it work? Gas phase compensation is real time, dynamic compensation. It consists of a reference point on the probe that is located in the vapor space, above the 100 percent point. This reference point is an area of the probe that has a larger outer diameter than the rest of the probe.
This point causes a small reflection of energy back to the transmitter. The distance is predetermined during manufacturing and programmed in the electronics of the transmitter. The reference point is continuously monitored and when the pressure and temperature rise, as well as the dielectric constant of the vapor space, there is a shift of this reference point. The shift will be to a farther distance, making it seem as though the reference point has gotten farther away. Of course, this would not be possible and the patented algorithms in the transmitter will automatically compensate for this error and give an accurate measurement. The illustration below shows this shift.
As you can see, there is a good solution for these applications. If you want to move away from mechanical devices this might be an excellent option.
Editor's note: This paper, PWR2009-81179, was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org.
Keith Riley has worked for Endress+Hauser as a Level Product manager since April 2008. Prior to that, he was a product manager and regional sales manager for L.J. Star Incorporated, as well as a product manager for TycoValves [Penberthy]. He has more than 15 years of sales and marketing experience in the process industry. Riley graduated from Iowa State University in 1984 with a B.A. in Marketing.
Kris Worfe has been marketing manager for Level Products with Endress+Hauser since January 2007. He joined Endress+Hauser in January 2002 and has done direct outside sales and inside sales in that time. He is a graduate of Texas A&M University with a B.S. in Marketing.
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