Ash-handling can be a maintenance nightmare if equipment isn’t specified correctly.
By Jason Kapelina, WEG Electric Corp.
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:
- The bottom-ash pumping system was misapplied for the service. Typically, a bottom-ash system will be a low-speed application (1200 to 1800 RPM). This is because of the abrasive nature of the service. Increased pump speed has numerous disadvantages beyond the increased wear factor.
- The motor’s oil-ring lubrication would be adequate under the most ideal conditions. System anomalies (vibration, low oil level, misalignment), however, can cause rapid bearing failure.
- Higher speeds generate higher temperatures, and self-contained bearing housings may not properly control/dissipate the heat.
- The motor’s labyrinth seal wasn’t adequately protecting the bearing from moisture and particulate contamination.
- The centrifugal pump’s packing (intended to seal the shaft) is designed to leak, and requires frequent maintenance (adjustment or replacement of the sleeve) to control said leakage. Given the system’s high-pressure configuration, when the packing fails, pumpage that’s sprayed from the pump shaft is forced directly onto the inboard motor bearing, leading to moisture entrainment and subsequent failure.
- By design, the limited-end float-gear type of coupling used in this system must be slightly misaligned to work properly. Furthermore, a gear coupling requires frequent maintenance (grease) every six to 12 months, depending on service. Gear couplings cannot be balanced, which can present a problem, particularly with a base speed of 3600 RPM.
Discussion. . .
Bottom ash is considered an abrasive slurry and must be handled accordingly. That means paying special attention to the following items:
- Appropriate pump design/configuration (end suction, double volute style with tangential discharge nozzles, rubber-lined)
- Proper material selection for the pump and motor (abrasion- and chemical-resistant metallurgy)
- The appropriate speed (slow, typically 800 to 1200 RPM) (Rule of Thumb: If you double the speed of a pump, you will get almost four times the shaft whip, wobble or run out and eight times the wear.)
- Sealed bearing housing (positive seal) with forced lubrication (pump and motor)
- Proper mechanical seal selection (metallurgy, elastomers, seal face combination, environmental controls)
- Coupling selection (metallic shim, pack style)
- Pump base design (groutable baseplate)
- Pump base grout (non-shrinking, free-flowing epoxy grout/Novalac resin)
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. . .
- Fit motors with positive bearing housing seals that will seal housings from moisture and contamination. There are two cost-effective ways to achieve a “positive” seal environment on the existing system:
- Employ a modified “lip” seal with sleeve (to protect the motor shaft) and pressurized housing (air).
- Install a “hybrid” flooded lube system to ensure adequate lubrication is delivered to the bearings. In addition the lube system will filter the oil and control any moisture that may enter the housing. Lube system will be fitted with heaters, pressure controls, filter, flow meter and temperature sensors. (For a diagram of this type of lube system, go to www.mt-online.com/lubesystem.)
- Replace pump packing with mechanical seals. By retrofitting the unit with mechanical seals, the primary root cause of motor bearing failure will be addressed. A split mechanical seal with a flush system will properly seal the pump shaft.
- Replace the gear-type coupling with a shim pack design; balance it to ISO G1 (4W/N) to compensate for the high speed. Ensure that the exposed keys are cut flush to the shaft profile shown in the figure at the top of this page.
- Perform precision alignment of the motor to the pump.
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
Jason Kapelina manages WEG and Electric Machinery’s large motor and generator business in the Southeast U.S. While he’s involved in a range of industrial applications, he specializes in power-gen applications. Email: jkapelina@WEG.net.