Types of Maintenance Strategies

The importance of proper maintenance for all types of systems cannot be overstated. No matter what type of equipment is selected, inadequate maintenance can lead to unreliable operation/production that can, in turn, result in excessive downtime and lost savings and productivity that cannot be made up.

A comprehensive maintenance program employing the practice of repair before failure generally represents a form of inexpensive insurance. There are three basic approaches to ongoing maintenance:

• Unscheduled maintenance, which is performed only when an incident occurs.

• Scheduled, or preventative, maintenance, which is performed on a schedule of periodic tasks designed to preclude failure; costs are a fairly predictable variable that can be included in the investment analysis.

• Predictive, or on-condition, maintenance, which is based on the equipment's condition; measurement of machine conditions are trended over time, allowing for prediction and avoidance of impending failure. Generally, design and cost of a long-term maintenance program represents a trade-off between more frequent, less costly routine procedures and more costly, unscheduled procedures and long-term OM&R costs. Unscheduled maintenance strategies are often less costly when equipment is new. Over the long term, however, unscheduled maintenance strategies can result in longer downtime, and if system failures do occur, they are more likely to be serious. Often, more rigorous routine preven-tative maintenance programs produce lower life-cycle maintenance costs.

Preventive maintenance with long-term schedules allows maintenance costs to be more predictable. Components are serviced or replaced before failure occurs or performance degrades in order to increase system availability and efficiency. The downside is that certain procedures may be initiated before they are actually required. Over the life of the system, this may mean that the unit is taken out of service more often and cost outlays are greater than they would be with a predictive maintenance strategy.

In establishing preventative maintenance schedules, site-specific conditions of fuel, air, water, and other environmental factors should be considered. If, for example, the fuel is dirtier than is assumed in manufacturer specifications, schedules for inspecting, cleaning, and replacement might be shortened. Operational factors, such as loading and start/stop frequencies, should also be considered. In addition, schedules should be modified over time based on empirical observation.

Predictive maintenance results in a more finely tuned approach. Components are serviced or replaced most efficiently with respect to their actual condition. If a component demonstrates a field life 5,000 hours beyond the expected life, replacement based on scheduled maintenance wastes

5,000 hours worth of component life. If the component fails 5,000 hours earlier than its expected life, an unscheduled, and potentially inopportune, outage may occur. Processes such as oil analysis can be used to predict actual component wear and, therefore, actual replacement need rather than timed replacement need. The downside of predictive maintenance is the additional cost of monitoring equipment and procedures. To be effective, more data must be gathered and analyzed on a continual basis.

The OM&R plan should include detailed descriptions of systems, the specific requirements for system testing, and a listing of measurements to be taken with methods and means indicated. The testing and measurement process should continue throughout the life of the system to ensure that quality maintenance is performed and that preventative maintenance is effective. This process should also provide necessary data for documentation that equipment performance is in accordance with manufacturers' warrantees.

The OM&R plan should be extended beyond major system components to include auxiliary components such as fans, pumps, and coolers, as well as peripheral elements such as facility water quality control. This includes potable water, wash, rinse, and other process water, chilled water, condenser water, and various equipment cooling circuit water. Cooling towers are commonly the source of system coolant water. Careful water quality control is necessary not only to maintain system performance and limit deposits, but also to ensure health and safety, which can be at risk if proper biological controls are not in place. Of course, minimizing health and safety risk should be considered as the paramount OM&R objective, followed then by objectives such as maintaining system reliability, performance, and service life.

Microprocessor technology allows for close monitoring of equipment performance via remote data link. This technology provides a way to anticipate impending problems and correct them before an untimely outage occurs, and it extends the interval between scheduled maintenance procedures. In cases where metering and monitoring is used for system optimization or for ongoing savings verification processes, the same systems can often be used for predictive maintenance purposes and for other alarm functions. The computerized monitoring system can be linked with the OM&R resource library to provide immediate access to information required to make necessary corrections.

Often, effective strategies combine all three approaches.

Unscheduled maintenance can be used for inexpensive, easily replaceable, or non-critical components. Scheduled maintenance can be used for components that are more costly or critical, or for those whose end of service life is predictable. Finally, predictive maintenance can be reserved for the most costly or critical components.

The benefits of well-conceived long-term maintenance programs include more efficient use of labor, greater productivity, and fewer unplanned interruptions. It also allows materials and services to be priced out, bid, and contracted for in advance, instead of under the pressure of an emergency situation. Finally, costs can be predictably accrued, resulting in more effective budgeting and accounting.

Regardless of the strategy, project savings should be substantial enough to cover all maintenance and overhaul costs. If not, the project should not have been judged economical during the life-cycle analysis performed as part of the Detailed Feasibility Study and should only have been implemented on the basis of other benefits.

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