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Fly Ash Hoppers Under An Electrostatic Precipitator

Application Problem:

There is a need in the power industry to measure the bin level inside fly ash hoppers under an electrostatic precipitator. These hoppers are usually ganged together per generating unit, in arrays from 4 to 32 depending on unit capacity. It is important that a properly maintained level monitoring system be implemented to prevent material from backing up in the hopper and damaging the precipitator, reducing precip efficiency or both. Several properties of fly ash present difficulties in applying process instrumentation correctly: repose angle, temperature, dielectric constant, material buildup and space limitations are the most prominent.


Typically the power industry has focused on using radiometric (gamma) or capacitance type devices. The radiometric device provides a non-contact method of measurement, the radiation passes through the sidewall of the hopper to a detector on the opposite side. This eliminates material properties from affecting the level measurement and gives a relatively simple installation. The drawback to radiometric devices centers around the cost of ownership – documentation, periodic testing, training and maintaining a site Radiation Safety Officer (RSO) is normally required, along with disposal costs which can exceed original purchase price.

Traditional capacitance devices provided a lower cost alternative with limited success. The low dielectric constant of Fly Ash and the temperature extremes make reliable setting of a traditional capacitance probe difficult. The sensitivity needed for low dielectric product detection often leaves the probes in a state where false triggers can be caused with changes of temperature.

Hawk uses the Gladiator Admittance switch product line to provide a reliable and robust solution to the problem of detecting fly ash levels, without the need for the regulatory, safety and administrative concerns which are raised when using radiometric (gamma/nuclear) devices

The Gladiator Admittance series extends the performance of capacitance and RF probes greatly through use of an extremely stable oscillator core which exhibits almost no drift with process temperature changes. High stability allows higher sensitivity to be used when setting switch points, and so greatly improves the ability to reliably detect products having lower dielectric constants. High temperature ceramic insulation is used in the construction of the Gladiator heavy duty probe for fly ash applications, and the rugged 36mm stainless steel sensing element will withstand heavy impact loads without bending or damage. Lighter duty versions and Teflon insulated versions are also available for less demanding process conditions.

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