Some maintenance issues become so prevalent, plant managers merely accept the problems as a part of the manufacturing process. However, at Plum Creek, a manufacturer of Medium Density Fiberboard (MDF) located in Columbia Falls, Mont., the reality of changing bearings every week and a half, as well as the shortening of the lifespan of their $500,000 fans, was not acceptable.
“We were experiencing numerous bearing failures as a result of high vibration levels – three times higher than the acceptable limit,” said Jack Hinman, Plum Creek MDF plant engineer. “We realized there had to be a better way.”
Building-Up A Problem
Plum Creek’s problems were occurring on their two large induced draft fans used in the production process. During fiberboard production, residuals – such as sawdust, chips and shavings – are refined, and then combined with urea-formaldehyde resin that is later pressed into boards.
The 600 hp fans, serving as fiber dryers, run in both a wet and dirty environment and operate at 1,800 rpm, removing hot exhaust air from the flash tube dryer. Any build-up on the fans can cause an imbalance - and it did. Although the build-up did not damage the shaft or the fan itself, the bearings were in a constant state of replacement. The trip level, set at 0.5 inches per second, was commonly read at 1.2 inches per second or 1.3 inches per second.
Balancing A Solution
Determined not to accept this imbalance as a permanent reality, Plum Creek set out to find a better solution. They found their answer in Lord Corp.'s RealTime balancing system. This high-speed fan balancing system continuously monitors fan vibration levels. Once levels reach a pre-set high trip point, the system switches on, commanding balance rotors on the fan shaft to adjust as needed to counteract the vibration.
Although Lord has a variety of products for the manufacturing industry, the RealTime balancing system was selected for use at Plum Creek because of its ability to make rapid balance corrections and withstand the harsh environment. The system is set up to monitor fan bearing vibration levels and the vibration phase angle, and automatically correct for unbalanced conditions. This is done while the fan is running at operating speed, eliminating costly downtime to clean and manually balance the fan.
The system's balancing ring, mounted to the fan shaft, houses two counterweighted rotors that can be repositioned to offset the unbalance detected in the fan rotor. Utilizing vibration sensors, the system monitors the fan bearing vibration. Vibration signals are received and processed by an “Adaptive Influence Coefficient” control system, which determines the balance adjustments that are required. The controller relocates the counterweighted rotors to the desired position to minimize the vibration levels. This process continues until the controller senses that balance has been restored. Typical balance cycle times range from three seconds to 10 seconds, depending on operating speed.
Lord's non-contact power supply used in the actuator coil eliminates the need for maintenance, sending power across an air gap between the stationary actuator coil and the rotating balancer ring.
Low Vibrations, Big Savings
This process improvement has added up to big savings for Plum Creek. Not only have they been able to realize an increase in production as a result of the installation of balancers on the two fans, they are also extending the life of their equipment and have minimized the wear-and-tear on the fans. Although Plum Creek still shuts-down once a month for routine maintenance, such occurrences are typically planned and not in response to fan imbalance.
Plum Creek reports that vibration levels now read as low as 0.01 ips to 0.05 ips and the decreased downtime paid for the balancer almost immediately. Hinman also recognized increased savings in labor and bearing replacement costs, both direct and indirect, and sees other opportunities at Plum Creek for the balancing technology.