Dacotah Cement's Rapid City, S.D. plant recently found its cement production hindered. The source of the problem: vibrations in its 104-in.-diam., 1,250-hp, induced-draft fan, which draws hot gas off the rotary kiln before sending it to the baghouse for particulate removal.

"Since the fan is in the process air flow, some of the particulates stick to the fan and form a hard coating up to an inch thick," said Plant Maintenance Superintendent Harry Ohrtman. "Changes in temperature then cause chunks of this coating to spall off and throw the fan out of balance. It would reach a point where the fan, nearby handrails, and the fan housing were all vibrating." As a result, Dacotah had to shutdown the plant to sandblast the accumulated material from the fan. The cleaning process took nearly six hours, and the repair work almost 12.

In addition to accelerated bearing and motor wear, Dacotah faced another maintenance problem resulting from these frequent shutdowns. The steel structure of the kiln is protected from the extreme process heat by a refractory lining. But expansion and contraction in the refractory due to repeated heating and cooling from the unplanned fan shutdowns caused sections of it to fail, thus requiring further repair.

Dacotah brought in the BalaDyne Corp., which responded by directly mounting its IV automatic fan balancing system on the fan spindle in the kiln. The system head is divided into four sealed chambers that contain a high-specific gravity balancing fluid. When the balancer's controller detects an imbalance, the fluid in the chamber on the heavy side is heated, thus changing it from a liquid into a gas. This gas is then transferred through a shuttle tube to the chamber on the light side where it condenses and becomes liquid. The process continues until the controller senses that a balance has been achieved.

According to BalaDyne, finding adequate space for the balancer, while protecting it from heat radiating off the nearby 650 degrees F process air stream, posed significant installation problems. Dacotah's problems were solved by tailoring the balancing wheel to fit the available space and by designing a cooling fan and ventilation holes into the balancer's housing. Additional ventilation was provided by an externally-mounted "squirrel cage" blower.

Before installing the balancer, the fan's vibration levels would reach 20 mils. By contrast, the new balancer maintained a level of less than 1.2 mils, according to BalaDyne. As a result, the product has offset fan imbalances, thus reducing maintenance costs for Dacotah Cement.

In addition, Dacotah installed the B4Setup for Windows Software to help analyze the vibration of the fan, as well as predict future problems so that maintenance can be planned before major failures occur.