By Teresa Hansen, Associate Editor
Coal-fired power plants can have up to 25 coal pulverizer drives, depending on their size. Any failure in the pulverizers, therefore, can cause big problems. When a large coal-fired plant in the Midwest experienced coupling failures in its coal pulverizer drives, it didn’t take the management and maintenance staffs long to recognize that something had to be done to mitigate the failures and maintain the plant’s reliability.
Large electric motors, through a worm gear arrangement, drive coal pulverizers as well as exhaust fans that blow the pulverized coal into the furnaces. The shafts from the motor, worm gear and exhaust fan must be connected mechanically. It is here that a relatively small, yet vital component, a coupling, comes in. If this coupling fails, the pulverizer quits operating. This particular power plant was designed to use “gear” type couplings. Gear couplings allow axial movement of the shafts, increasing the wear on gears and other components, such as the sleeve bearings in the motors. Eventually, these components can fail prematurely, which is exactly what happened in this case.
Production loss due to downtime became an issue. Replacement costs amounted to thousands of dollars per bearing. Preventative maintenance costs associated with gear couplings also became a burden due to time spent inspecting and lubricating the couplings to keep them operative. The power plant’s rigorous preventative maintenance program required regular gear coupling disassembly for gear teeth inspection, cleaning and relubrication. Unless extensive, time-consuming measurements were performed, visual coupling inspection was a judgment call. Additionally, coupling replacement required heavy machinery to move the motors to perform the inspections and maintenance.
Lovejoy sales engineer, Scotty Pucheu, analyzed the situation and determined that the current gear coupling design was typical of the technology available when the plant was built in the early 1970s. The gear couplings did not limit the motor shaft’s axial movement. “Sleeve” type bearings were used in the motors, meaning thermal growth of the shaft and the shaft’s position at magnetic center increased wear and shortened life spans not only of the motor sleeve bearings but of the gear couplings as well.
The Lovejoy SX disc coupling has no moving parts and requires no lubrication, translating into long coupling life.
“Increased sliding velocity of the gear teeth in the gear coupling creates a lot of wear,” says Pucheu. He says it’s akin to rubbing two sticks together. “The motor moves a bit, the gear on the coupling moves a bit, and this motion could be repeated up to a million times over the course of the day, contributing to premature failure.”
A possible solution was to install gear couplings with limited end float. But that meant the maintenance issues associated with gear couplings would continue. Because gear couplings are difficult to assess for wear, inspection and replacement often requires removing the motors. Furthermore, gear couplings would have to be disassembled, inspected, cleaned and regreased regularly.
Unwilling to compromise on maintenance costs, the plant manager decided to invest in Pucheu’s recommendation to install Lovejoy SX Disc couplings.
Unlike gear couplings, the Lovejoy SX disc coupling is balanced. This means it experiences less vibration than a gear coupling as it wears and it performs limited end float function without special design considerations for the sleeve bearings. The disc coupling has no moving parts and requires no lubrication, which translates into long coupling life.
Preventative maintenance/inspection of the coupling’s flexible elements is now possible using a strobe light, even while the pulverizer is running. Shutdown and disassembly is no longer required. Disc pack replacement can be predicted and performed without moving machinery or disturbing the original alignment.
Another bonus was Lovejoy’s ability to provide, in the same space allowed by the original design, a heavier-duty coupling than the original gear type. Nothing had to be moved or modified for installation and the component cost was comparable to that of a gear coupling.
“Until our disc coupling came along, it wasn’t practical to put a disc coupling in this system,” says Pucheu, “but, Lovejoy’s design allows for more capacity and misalignment capability than most other disc type couplings available.”
In addition, Pucheu points out that the disc coupling signals the maintenance staff, letting them know when it needs replacing. “With a gear coupling, it’s a judgment call,” said Pucheu, “unless you take it completely off the machine and measure it extensively, no one can tell visually if the gear teeth are acceptable.”
With the Lovejoy disc coupling, the disc’s outer leaf will crack before the entire disc coupling fails, signaling to the maintenance staff it needs replacing.
“Coal pulverizers are critical equipment,” says Pucheu. “If you’re not blowing coal, you’re not generating power and you’re losing revenue.”
The plant has been successfully using the disc couplings for almost two years. “Plant management approved a proposal to convert all gear couplings on 24 coal pulverizers to Lovejoy disc couplings,” says Pucheu, “based upon the benefits and cost savings.”