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Guided Wave Radar Instrumentation in Steam Drum and Feedwater Heater Applications

With the addition of non-conventional, renewable generation to the energy mix, many operators have questions and concerns about level technology and how it can improve the efficiency of their power plants. Magnetrol® expert Donald Hite recently answered questions about guided wave radar level control instrumentation as part of a power industry-focused webinar, “Optimizing Level Control to Meet New Generation Demands.” This week’s blog post shares some of that Q&A.

Question: What is the standard mounting for a guided wave radar instrument in a feedwater heater and in a steam drum? Should the instruments be mounted externally or internally?

Answer: The standard mounting for both a feedwater heater and a steam drum is an external chamber. Level instrumentation on feedwater heaters are never directly inserted – the internal construction of a heater prevents top entry of level sensing devices; thus, limiting them to external mount only. Historically, guided wave radar is also mounted externally on a steam drum. However, recent research and testing has explored the possibility of internal installation. MAGNETROL has proven that direct insertion of guided wave radar into a steam drum is not only a viable, but also a more effective and less complicated method of measuring the level. As of right now, all of our direct insertion GWR installations in steam drums are being done internationally due to the fact all of the R&D and testing was accomplished in that market space. On the domestic front, direct insertion is a new concept and will catch on as plants are made aware of the cost and performance benefits, which I should say are significant.

Question: One of the claims made about guided wave radar is that it requires no external input to achieve and maintain accuracy. What does this mean?

Answer: The fundamental principle of Guided Wave Radar technology does not rely on specific gravity or inference as the basis for the measurement. Consequently, parameters which normally affect such devices have relatively little impact on the measurement. It doesn’t require any external input to compensate for process conditions that affect the specific gravity of the material being measured. This is in contrast to how many other level instruments function. Take a displacer, for instance. A displacer is designed to be most accurate at the specific gravity of the material being measured at operational conditions. Any fluctuations from the design criteria relative to specific gravity have a negative impact on the accuracy. A similar statement can be made about differential pressure technology. These vulnerabilities, if uncorrected, reduce accuracy during critical startup and cycling conditions where swings in specific gravity are commonplace. Guided wave radar gets its return signal from the surface of the material being measured. As a consequence, its accuracy is not contingent on correcting for fluctuations in specific gravity. The key is ensuring the material being measured is of sufficient dielectric value during all application conditions to return a signal that can be tracked.   Although the saturated steam in the vapor space does affect the propagation speed of microwave energy, the correction is self contained and accomplished internally in the unit, completely transparent to the end user. When you use guided wave radar in any given steam application, you no longer need to introduce other measurements, which could be sources of additional error, to determine the true level.

Question: How does guided wave radar work with foaming applications?

Answer: There are essentially three scenarios when using guided wave radar on foaming applications. In the worst case scenario, the foam absorbs all of the microwave energy eliminating GWR as a level solution. Although rare, it does occur from time to time. Another possibility is that it has no impact at all. The microwave energy passes through the foam unchallenged and the level is easily acquired. In a third scenario, the foam is of sufficient dielectric value to create a reflection which can be tracked as level while low enough to allow the energy to pass through creating a second reflection from the actual liquid surface. In this scenario the instrument can be configured to track either the foam or liquid level.

Question: Is guided wave radar now the industry’s preferred option for drum level control?

Answer: At the moment, it is not. The most common technologies you see used on drum level applications are differential pressure and conductivity. Up until recently, the process isolation seal material used in GWR probes for saturated steam applications could not tolerate the temperature common on high-pressure drums. GWR has always been a viable level solution on the LP and IP drums. Most customers prefer to standardize on a single technology for all drums. Since GWR could not handle the HP drum it did not see much service for this reason.

To learn more about guided wave radar technology and optimized level control in steam drum and feedwater heater applications, view the webcast “Optimizing Level Control to Meet New Generation Demands.