Energy-Tech Magazine

Introduction
There are nearly as many level measurement technologies available to the power industry as there are applications. The problem for most end-users is not finding a technology but determining the correct technology that will provide the best value in terms of performance, maintenance and cost of ownership.

Some level measurement methods work well with a multitude of applications, while others are more specific. The key for the end-user is to understand the limitations of each, as well as the features and benefits. This will allow them to make an informed decision that best suits their individual circumstances. Recent changes in technology might alter the way end-users address continuous and point level applications for bulk solids.

Typical applications for level instrumentation in a power facility are:

• The steam system
• Boiler drum
• Pre-heaters
• Feedwater tanks
• Condensate tanks
• Fly ash
• Lime silos
• Coal bunkers
• Ammonia tanks
• Challenges in the process

Those

References "Application Information of Radar Level Measurement for Hydrocarbon Measurement," Endress+Hauser White Paper, Alexander Roettcher & Dieter Markanic, March 2006 "In Control Part III - Liquid Level Measurement," Ask Tom! 2001, http://www.waterandwastewater.com/www_services/ask_tom_archive/in_control_part_3.htm "Capacitance Level Measurement - How it Works," Endress+Hauser White Paper, Robert A. Green. "Capacitor Theory," http://www.ece.uwaterloo.ca/~lab100/ls1notes.pdf "The Theory and Practice of Level Measurement," Endress+Hauser Inc., Wim van de Kamp. http://new.ametek.com/content-manager/files/STD//504-1200-000-LM2.pdf "Level Measurement in Fly Ash Silos - Pretty Challenging," Endress+Hauser White paper, Rob Vermeulen.

companies that collect, store or use fly ash in their production processes know it is not easy to measure the level in fly ash silos. Why is it so difficult and what makes fly ash so inconsistent? Why should instrumentation engineers be alert and particular about their choice of instrumentation for this difficult level application?

Fly ash properties
First, fly ash is not always the same. Fly ash collected at a coal-fired power plant is different then what is collected from the stag in a cement plant. Both processes might burn the same coal, but the temperatures the coal was burned at and also the process of burning itself has an impact on the properties of the fly ash collected. Naturally, fly ash collected from heavy oil or waste burning installations also will have different properties as well.

So what are these properties? The main properties are weight of the fly ash, size of the particles, dielectric constant, temperature and last but not least: How sticky is it?

Measurement principles used to measure fly ash
In the past, mechanical systems were used. Here the specific density of the fly ash determines the accuracy and reliability of the measurement. A weight was lowered on a cable or metal tape into the silo. When the weight touched the level in the silo, the electronics detected the slack in the cable or band, reversed the motor and started counting the length of the tape that indicated the amount of free space in the silo.

This is a simple and robust way of measuring in fly ash silos, but it also can be costly. It requires relatively high maintenance as the moving, mechanical parts wear out and need replacement, especially in a dusty environment. With the current focus of modernization and building plants that will reduce maintenance and ownership costs, this technology is being abandoned.

Ultrasonic level measurement devices
Ultrasonic measurement devices also were used. Some worked well and some had intermittent performance during filling. This technology depends on the density of the fly ash. It makes an emitted sound wave that reflects to a surface and back to the sensor, and then the distance to the level is calculated. A big advantage of ultrasonic measurement devices is that they have no moving parts and are, in principle, maintenance free. The problem is getting the sound waves through a thick dust of blown-in fly ash to the level. Sound is being deflected by the dust particles. So success depends primarily on the amount of fly ash pneumatically conveyed, the density of the ash and the shape of the particles. The temperature of the ash also could cause a problem since the speed of sound changes with temperature. Materials used to build ultrasonic sensors limit the use of this technology in high temperature applications as well.

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