Flyash is a waste product that is carried out of furnaces, kilns, boilers, etc. with the stack gases. Typical producers of flyash are coal-fired power plants, oil-fired power plants
, trash-to-energy facilities, steel mills
, coke ovens, foundries, and pulp and paper
The Clean Air Act (CAA - 1970 / amended 1990) requires power facilities to clean stack gases of flyash 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 Flyash is a dusty, abrasive material that is often sticky causing it to form chunks and coat heavily. Flyash has many compositions and can be insulating or slightly conductive depending on carbon content. It also tends to bridge and “rat hole”.
Flyash hoppers will run at temperatures up to 500° Fahrenheit for “Cold Side” installations and over 1000° for “Hot Side” installations Flyash is removed from the stack gas in one of two ways.
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 CAA.
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.
The dusty, abrasive nature of flyash, along with heavy coatings and high temperatures, are a difficult environment for most level measurement switches.
Flyash Level Measurement Technologies
The following is a review of the most common point level measurement devices used for Flyash Measurement.
Nuclear gauges have been used to measure fly ash in hoppers because they are a non-penetrating technology. Since they do not penetrate the hopper wall it is assumed that they eliminate the problems associated with the difficult environment inside the hopper. However, coatings on the inside of the hopper wall will absorb the emitted radiation causing false high-level indications.
Additionally,radioactive sources require periodic checks for radiation leaks and restrictions on source disposal are becoming an increasing problem. For these reasons along with the regulatory requirements and aperwork associated with nuclear gauges, many facilities are looking for switches to replace their existing installations.
Paddle wheels, a mechanical device, have often been used as a point level switch for flyash measurement. Even with the maintenance issues associated with mechanical devices, paddle wheels are one of the common technologies selected due to customer’s preference for no-calibration products.
Although relatively low priced with an initial purchase price of $200 - $300 US, paddle wheels require extensive maintenance due to mechanical wear and motor fatigue. A paddle wheel motor will fail often, sometimes in as little as 6 months from installation. Replacement motors cost approximately $100 and up. High temperatures on flyash hoppers make replacing the motor a difficult operation. This, along with the cost of installing the new motor and downtime will have a cost of ownership of many thousands of dollars over a ten-year period. High maintenance mechanical paddle wheels are a poor choice for this demanding measurement.
Tuning Forks and Vibrating Rods
Tuning forks and Vibrating Rods have been used for flyash measurement. As with paddle wheels, vibration type sensors do not require calibration. Although vibrating sensors require less maintenance then paddle wheels, they are susceptible to damage caused by abrasion or weight loading. Also, as flyash falls from the ESP or FF, it is very light. Bulk densities in the top of the flyash pile may be as little as 10 pounds per cubic foot. Tuning forks and vibrating rods will often “Rat Hole” in the light flyash.
Rat holing refers to the vibrating rod or fork “tunneling” out the granular material causing them to no longer “see” the material being measured. The sticky, coating nature of flyash, will coat the tuning fork or rod causing it to indicate a false level condition. The challenging environment of flyash measurement has proven to be too difficult for vibration technology.
RF Admittance switches
provide the best possible solution to flyash measurement. There are no regulatory requirements or source disposal problems as with nuclear gauges. RF switches can be remote mounted to allow the electronic unit to be placed in a low temperature, convenient location.
Products such as the Intellipoint
from Drexelbrook have driven shield circuitry which allows the sensor to ignore even heavy, sticky coatings left by the flyash. There are no moving parts to wear out, jam, or fail due to rat holing. Robust, industrial designed sensing elements will not bend. The Intellipoint
provides a continuous AutoVerify self-check that ensures the operational integrity of the switch at all times. The Intellipoint
is also self-calibrating. Simply install the sensor into the vessel and power it up. The microprocessor based electronic unit calibrates the switch for the optimum set point.
RF Admittance products such as the Intellipoint are clearly the most reliable and economical way to measure flyash in ESP’s and FF’s.