Flyash is a particulate by-product produced by coal-fired power plants, trash-to-steam / co-generation plants, oil-fired power plants, steel mills, coke ovens, and foundries. To avoid pollution, this particulate which travels with the exhaust gas, must be removed prior to releasing it up the stack and into the atmosphere. The particulate that is removed from the gas is collected in hoppers. A level switch is used to detect when the hopper needs to be emptied. The collected flyash is a hot (> 300 ºF), dusty, abrasive material that is often sticky causing it to coagulate and coat everything it contacts, and depending on carbon content, it can be electrically insulating or slightly conductive. This makes it difficult to measure. The US EPA Clean Air Act requires power facilities to clean stack gases of
fly ash particulates prior to releasing the gases to the atmosphere. The EPA can levy strict penalties for non-compliance including fines and facility shut downs until corrective action has taken place It also tends to bridge and “rat hole”. Fly ash hoppers will run at temperatures up to 500° Fahrenheit for “Cold Side” installations and over 1000° for “Hot Side” installations.
Fly ash is removed from the stack gas in one of two ways. |
Fly Ash "Cold Side" Null-Kote™ level switches are an excellent solution to
fly ash measurement. Null-Kote™ circuitry provides a driven guard which allows the sensor to ignore even heavy, sticky coatings left by the
fly ash. The level control has no moving parts to wear out, jam, or fail due to rat holing. There are no regulatory requirements or source disposal problems as with nuclear gauges. Robust, industrial designed sensing elements will not bend. |
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Fly Ash
"Hot Side" Microwave Penetrating "Through the Wall" level switch
is the best technology for
detecting the level of "Hot Side" fly ash.
Consisting of a transmitter and a receiver installed face to face.
The transmitter emits a continuous low power microwave beam toward the
receiver and an output signal changes state when the beam is obstructed.
They are non-contact, non-invasive, insensitive to the application area, and possess solid-state reliability. |
Electrostatic Precipitator (ESP) | Electrostatic Precipitator (ESP) - The gas is run through an electrostatic high voltage grid. As the gas passes through the grid, the flyash particulates in the gas become charged and adhere to collecting electrodes. The flyash then is shaken off the collecting electrodes and falls into a collection hopper. If the flyash level reaches the grid, it will cause them to short out requiring repair or replacement. If the grids fail, flyash will be emitted into the atmosphere resulting in non-compliance with the Clean Air Act. |
Baghouses - Also called FF’s for Fabric Filters, use “Bags” as filters to remove the flyash. The stack gas is run through the filters and the flyash is collected in the bags. Occasionally the bags are shaken and the flyash falls into a collection hopper. If the flyash level reaches the bags, they become plugged requiring a manual clean out. Also, the weight of the flyash can damage the bags, tearing them and allowing large amounts of flyash into the atmosphere again resulting in non-compliance. Both ESP’s and FF’s require point level measurement devices to indicate ash level. Typically, each hopper will have a high level switch to indicate when the hopper should be emptied. Some hoppers will have a baffle down the middle requiring two high level switches, one for each side of the baffle. In some cases, low-level switches are also used to indicate an empty condition. | 
Baghouse (FF) |
The dusty, abrasive nature of flyash, along with heavy coatings and high temperatures, are a difficult environment for most level measurement switches. Princo’s RF Null-Kote™ level switches
and/or
Microwave level
switches
are clearly the most reliable and economical way to detect flyash level in ESP’s and FF’s. |
