ACHRNEWS

Don't Forget About The Critical Fan

July 20, 2004
Sometimes it seems that 99.99 percent of the engineers who specify a fan for an application do so with the assumption that the fan they specify will run forever and never experience a performance problem. For a gambler, those are pretty good odds, and for the majority of fan applications the assumption is a safe one.

But what about your customer's fan application? Is their fan just another piece of hardware, or is it an important part of the process system? Can they afford to ignore it, entrusting its future care and operation to the operators, the maintenance department, or to people who make production decisions without ever going near the equipment itself? Almost always, they can. Almost.

However, in some applications, the fan is not only important to the process, it is critical to the process. Let's examine a few case studies that demonstrate how a fan can be a critical component of an air system.

Case No. 1

A brick company used a high-pressure centrifugal fan to supply combustion air for a kiln. This company was widely known for producing bricks of excellent quality. Not surprisingly, they placed a great deal of emphasis on maximizing output. Their production facilities were not about to win an award for cleanliness, but after all, it was a brick plant and the main goals were to make bricks and profit.

Maintenance was on the list of important considerations, but the maintenance men were often drafted to help out here and there with production work. When the maintenance men returned to their maintenance work, it would occasionally happen that they did not resume work at the exact point or on the same equipment where they left off. These are the casualties of an imperfect world, or in this case, a slightly imperfect brick plant.

It happened that the high-pressure fan was one of these casualties. The fan was an arrangement 8 (see AMCA 99-2404 for information on fan arrangements), and the fan bearings received no grease. During the next lubrication cycle, there was a great hurry to prepare for a vacation shutdown, and the bearings missed another scheduled lubrication.

When the plant resumed operation after vacation, the operators noticed a vibration that seemed to be coming from the fan, but since it wasn't severe, they ignored it. Several hours later, in the middle of the night, the fan failed catastrophically. The loss of the fan's combustion air required the operators to shut down the kiln.

The result of this failure was several weeks of lost production time. There was a vibration that warned of a brewing problem with the fan, but no one with enough fan experience was there to interpret the warning properly. After the failure, there was no replacement on hand, and the fan vendor was several states away.

Case No. 2

In a large city in the Midwest, a multistory office building was served by an air handler that employed a single vaneaxial fan. Because the air handler was in a hard-to-access location in the top story of the building, maintenance department personnel never went near it unless they absolutely had to do so. Because of this lack of attention, the fan was never physically examined or maintained in any way.

One hot summer day, cool air stopped flowing from the air handler. Occupants of the floor below the equipment room reported hearing banging noises from the floor above just before they noticed that there was no air flowing from the ceiling diffusers. Shortly thereafter, the office temperature spiked, and there was a frantic call to the fan manufacturer.

Examination of the air handler showed that the fan had become unbalanced and eventually dislodged another part that jammed the impeller, causing the circuit breaker to blow.

The result of this fan failure was that the building's tenant was forced to send employees home for a few days of unplanned vacation until the building owner was able to obtain and install a re-placement fan. By a stroke of extremely good fortune, the fan manufacturer was able to build the replacement from stock components and deliver the fan by private carrier overnight.

The noise heard by some of the occupants was an indication of trouble, but they were in no way able to know how serious the trouble was, let alone do anything about it. Failure to inspect the fan, plus relying upon a single fan instead of having parallel fans in the air handler, placed the entire system at risk.

Case No. 3

A bag house installation at a metals plant employed a single fan. After five years of uninterrupted service, the weld between one of the impeller blades and the back plate failed, causing a high imbalance and setting off a series of subsequent weld failures that resulted in a catastrophic failure of the impeller.

The already-destroyed impeller continued to turn in the housing for several minutes before employees realized what the problem was and turned off the power to the fan.

Those additional minutes of turning destroyed the fan housing and the steel base the fan motor was mounted on; it also did severe damage to the concrete under the fan motor base. The loss of this fan caused the plant to be idle for several days.

When the fan was originally quoted, plant management was advised to order an upgraded version with welding of a higher quality level. This the company declined, saying that the installation was regarded as "light duty," as the fan would operate only a few hours a day. The higher quality level of welding was not regarded as either necessary or important.

Unfortunately for all concerned, not long after installation, "light duty" operation soon turned to "continuous duty," and the fan was operated without a break for five years. While a fan might run forever without a structural problem, the ordinary vibration associated with fan operation was aggravated by dust and dirt buildup on the impeller blades. For a certain weld on one blade, the increased vibration proved to be more than the weld could handle, starting the domino-like sequence of failures.

Once again, a fan critical to a process, operating far beyond its intended duty, was never shut down for inspection. No one was monitoring it, and the company was without a spare or a standby fan waiting in parallel.

Cautionary Tales

Given these case histories, the need for prudent prevention measures is obvious. Prevention can take the form of a handy spare impeller and shaft, or, better yet, a standby fan in parallel ductwork.

Even the expense of backup hardware can be avoided if a fan is operated at the duty it is designed for, taken out of service periodically for inspection (and any deficiencies remedied), and has its bearings lubricated properly at the required intervals.

Last but not least, fans should be equipped with a vibration monitoring device that will alert the operator of impending problems and shut down the power if vibration rises to a pre-catastrophic failure level.

This article was prepared by Air Movement and Control Association International Inc. (AMCA), a not-for-profit international association of the world's manufacturers of related air system equipment. For more information, visit www.amca.org.

Publication date: 07/26/2004