Further Reading
Section 9.3.3 Section 9.3.3 Part 1
10.1 Background
10.1.1 Maintenance Practices
Maintenance protocols may be based on reactionary, preventive, condition or reliability‐cen- tered principals. Reactionary maintenance is the repair of equipment following a failure – fix- ing a flat tire for example. For critical equipment, especially for a large steam‐turbine generator, repairing only after a failure has occurred, while at times necessary, is the least efficient and most costly maintenance protocol. Reactionary maintenance practices can result in uncertain production, loss of warranty provisions, and extensive collateral damage following a rotating or stationary component failure. Most large steam turbines are maintained on a preventive basis.
Preventive or time‐based maintenance is a maintenance program that performs repair on a set schedule – often proposed by the manufacturer. The schedule may be based on the manu- facturer’s research and field experience that yields a schedule based on the fleet average and not the specific machine. The manufacturer’s schedule would also be developed with its inter- ests as a priority, and those may be quite different from the owner’s. As an example, the manufacturer may propose a conservative repair cycle so that equipment failures are rare and their equipment performs better than their competition in terms of reliability. The owner might be willing to accept a less conservative repair period in order to increase production and reduce maintenance, especially in the case where experience shows a longer repair cycle is possible.
Condition‐based maintenance (CBM) is a flexible maintenance regime based on measure- ments of the condition of a component, equipment item, or system. Measurements such as vibration, temperature, lubricating oil condition, or process parameters are monitored and inter- preted so that maintenance is only performed when required. Condition‐based maintenance has the potential to reach the minimum amount of maintenance without failure but there are draw- backs. Condition‐based maintenance instrumentation systems can be expensive. The results require accurate interpretation and judgment. Interpretation cannot be perfect, so setting of the maintenance period requires a measure of judgment as much as the science behind the method.
Reliability‐centered maintenance (RCM) is a maintenance practice developed by executives of the airline industry and currently defined by the technical standard SAE JA1011. It was originally established to institute the minimum required level of maintenance to preserve
system function. It was intended to be a complete maintenance protocol including failure analysis, and analysis of equipment requirements, results of a failure, programmatic processes that prevent failures, and so forth.
When changing from a preventive, time‐based maintenance program recommended by the equipment manufacturer or the maintenance services provider to the CBM or RCM, it is important to consider any additional risks the owner would have to accept. Those risks may include the loss of warranty coverage if the time between overhauls exceeds contract limits.
Risks can also include loss of collateral damage coverage, reliability guarantees, and so forth.
Therefore, careful consideration of contact details, where appropriate, must be an integral part of an evaluation of maintenance practices.
Condition‐based maintenance can be impractical when there is a long planning period required for an overhaul. For example, the time it takes to prepare for a refinery or chemical plant turnaround can exceed several months. Waiting until a critical equipment item requires immediate repair would cause unnecessarily high labor and parts costs, and could result in uncertain product delivery. In addition, seasonal pricing patterns often dictate that major maintenance occurs during a certain time of the year. Therefore, a corporation often prefers time‐based maintenance for large complex systems, those with seasonal pricing pressure, or where there are other risks imposed by a condition‐based system.
Figure 10.1 illustrates the four types of maintenance protocols mentioned above on a prob- ability chart. Reactive maintenance would occur the least often of the three methods and would maximize the operating period between repairs. Preventive maintenance would be the most conservative, occurring on the shortest cycle, which would be predetermined based on experience or a set of criteria. Condition‐based maintenance and RCM would likely occur somewhere between preventive and reactive maintenance. With CBM, some cycles would be shorter than others depending on how the equipment or system behaved during the cycle.
10.1.2 Economic Model for Maintenance
The three maintenance practices discussed in section 10.1.1 have in common the assumption that they are optimized for the type of equipment or company performing the maintenance.
0 0.2 0.4 0.6 0.8 1 1.2
Failure probability (–)
Operating time Preventive
Reactive CBM
RCM
Figure 10.1 Maintenance practices.
All three may be part of a maintenance system and applied depending on the equipment size, cost of repair, or impact of a failure on the economic performance of the company. Figure 10.2 is an example of an optimization that accounts for the probability of a failure, the cost of the failure, including the lost production during maintenance, and the cost of normal repair prior to the failure. If properly optimized, the company would operate with a maintenance interval that minimizes the total lifecycle cost of maintenance.
The model utilizes the normal probability distribution, with the cost of repair, and an assumed standard deviation. Programs such as CBM have an underlying objective to increase the certainty when a part may fail so that maintenance intervals can be extended without undue risk effectively decreasing the standard deviation. Due to the interpretive nature of CBM programs, however, uncertainty of the moment of failure cannot be completely eliminated; and a measure of conservatism must be built into the decision process to avoid unexpected events.
Comparatively, the mean time to failure and standard deviation are known with greater certainty with CBM than with preventive maintenance but both programs have similar objectives, and can follow a similar economic model.
10.1.3 Operating Costs other than Maintenance
Much of the literature and research devoted to preventive maintenance and CBM focuses on extending run time and repairing equipment prior to a failure. However, preventing the added cost that results from a failure, which may include increased outage durations, additional parts replacement, expediting fees, unstable operations, and so forth, does not capture increases in variable operating costs due to performance degradation.
For cases in which lost performance is significant when compared to the cost of maintenance, economic optimization should account for changes in output and the cost of production.
Systems that can be impacted by performance degradation include those with compressors and heat exchangers that can become fouled, long‐distance piping systems prone to scale or sludge buildup, turbines exposed to impurities, and so forth. Whenever a process requires periodic cleaning, or the repair of worn parts, performance impacts on operating costs could be an important consideration in addition to the avoidance of a failure in a maintenance program.
Life cycle costs
Repair cycle period Cost due to failures
Planned maintenance
Sum of planned and failure maintenance
Figure 10.2 Life cycle maintenance costs.