Bottom and fly ash result from the burning of finely ground coal in a boiler to produce electricity. These materials need to be collected and removed from power-plant equipment (Fig. 1 above).
Bottom ash comprises the mostly incombustible by-products of coal combustion that accumulate in the boiler. This coarse, granular (i.e., extremely abrasive) material is made up mostly of silica, alumina and iron with traces of calcium, magnesium and other compounds. Upon removal from the boiler, bottom ash is typically collected in a wet system, and the resulting slurry is pumped away for disposal or further treatment. (NOTE: The tendency of bottom ash to harden when exposed to water, oxygen and heat is what makes this by-product very attractive to material manufacturers.)
Fly ash comes from plant exhaust gases as they exit the boiler. A very fine, powdery material, it’s composed mostly of silica (with traces of alumina, oxides of iron, calcium, magnesium and toxic heavy metals like lead, arsenic, cobalt and copper also present). Nearly all particles are spherical in shape. Fly ash is generally collected by an arrangement of electrostatic precipitators, bag houses (that utilize fabric filters to capture the ash as flue gas stream passes through them) and, finally, a scrubber system, prior to the flue gas being discharged into the atmosphere. Vacuum pumps and centrifugal pumps are usually involved in handling fly ash.
This “Maintenance Log” article focuses on effective solutions for a problematic bottom-ash application.
A real-world case study
The bottom-ash systems (four units) at one power-gen facility operate in the 3600 RPM range. According to repair records and interviews with plant personnel, the system had experienced marginal reliability from the time it was installed during a 1998 plant expansion.
Since installation, the bottom-ash motors have experienced very short run times: 18 months to two years on average. Most recently, failures had been occurring within a matter of a few months. A majority of these events were catastrophic—due to the rotor contacting the stator. The primary cause of the failures, however, was contamination of bearing oil resulting in sleeve-bearing failure.
The 800 hp, 3570 RPM motors are direct-coupled to the centrifugal pumps by a limited-end float-gear-type coupling. The rotors are constrained with straight-sleeve oil motor bearings and lubricated with oil rings in self-contained, air-cooled housings. The system runs at ambient temperature. The centrifugal pumps use packing to seal the shaft.
After discussing operating and maintenance history with plant personnel and reviewing service-center repair records, the following observations were documented:
Discussion. . .
Bottom ash is considered an abrasive slurry and must be handled accordingly. That means paying special attention to the following items:
Recommendations. . .
Long-Term Solution: Replace existing pump and motor package with a more appropriate 1200 RPM system. This configuration would require larger pumps and motors, but wear and maintenance would be reduced dramatically. (Consider life-cycle costing, the cost of downtime and lost production.) Motors should be TEFC (Totally Enclosed Fan Cooled), specified with positive bearing housing seals and flooded lubrication to control temperature, moisture and contaminates.
Coupling installation, residual imbalance
Immediate actions. . .
ROI from corrective actions
The immediate actions detailed in this article were implemented to one unit during a scheduled outage. The unit ran trouble-free for six months, at which time the other three units were modified. Since that time, the plant has submitted a capital request to replace units with low-speed configurations. MT