How effective is preventive maintenance (PM) in reducing downtime? As with many questions related to maintenance, the answer is "it depends."

I'll illustrate with an example. In a large, multi-line pulp-and-paper mill, a 20-inch knife-gate valve on the main effluent header failed, shutting down the entire operation for three days. As manufactured, the stainless-steel valve yoke had been attached to the valve body with stitch welds. Those welds failed, likely from fatigue. The pressure forced the spade out of the valve, and the escaping 180-degree effluent submerged the effluent lift pumps that feed the treatment system.

Could this failure have been avoided if the preventive maintenance system had been better designed? It could not, at least not economically in a process plant like a paper mill. The reasoning was that if the scope of the PM inspection program included regular checks on items such as manufacturers' shop welds on all plant equipment as well as other possible checks at that level of detail, then it would be unrealistically expensive. PM inspections for that plant should be limited to probable failure modes on most equipment. This event was not the result of a probable failure mode.

When the downtime in the plant was analyzed over a number of years, the total losses from "probable" and "unlikely" failure modes were nearly equal. The unlikely failures were less frequent, but each one resulted in a large production loss.

Of course, the depth to which it is reasonable (and economical) to search for potential failures depends on the criticality of the equipment, defined as the consequence of failure. For instance, during the design and maintenance of commercial aircraft, every possible failure mode for each component is examined, and actions are taken to either prevent failure or to prevent a failure from affecting the safety of the aircraft (e.g., by providing back-up systems). Such analysis in a large manufacturing operation is usually not economical or necessary. However, for very critical systems, such as those containing hazardous chemicals, a much more rigorous design process and a correspondingly higher level of inspection detail is justified.

So what action should be taken when an unlikely failure occurs? One possibility is to extend the PM program to include that specific mode of failure on all similar equipment. This is only practical if the failure development period for each failure mode is long enough to allow regular inspections to detect a potential failure with enough warning for repairs to be made before a breakdown occurs.

The best way to deal with unlikely failures is to conduct a root cause analysis and take whatever action is required to prevent similar failures in the future. This often involves some redesign. In the case of the valve example, the attachment of the valve yoke to the valve body was redesigned to prevent further failures.

PM Program Design

If the PM program is well-designed, most of the "probable" failure modes can be avoided with a reasonable failure development time.

Good downtime records are of great value when setting up a PM program. After one plant recorded all of its production losses against the equipment that caused them, a Pareto analysis revealed that 80 percent of all unscheduled maintenance downtime could be attributed to 87 items, or less than 1 percent of more than 12,000 equipment items. A new PM program that focused on these 87 items and similar equipment reduced unscheduled maintenance downtime by more than 50 percent within 18 months.

In a process plant, such as a pulp-and-paper mill, PM inspections should consume no more than 15-20 percent of available maintenance manpower. It is also a good "system check" to occasionally compare the total hours of work generated by your PM program each year with your total available manpower.