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Understanding the Types of Maintenance and Choosing the Right Strategy


This article provides an overview of the nine main types of maintenance, ranging from preventive to corrective maintenance and everything in between. Although specific terminology may vary, it's important to ensure that everyone is talking about the same thing and that the underlying principles are clear. Whether you refer to it as condition-based maintenance or predictive maintenance, what matters most is that we can have a meaningful conversation about when and how to use it. As such, this article also explores the appropriate applications of condition-based maintenance. Many people have questions about the different types of maintenance, so this quick overview provides a helpful starting point:

There are nine distinct types of maintenance, which can be classified into two main categories: Preventive Maintenance and Corrective Maintenance. 

The Types of Maintenance

Preventive Maintenance is performed proactively before any equipment failure occurs. This category encompasses:

  1. Time-Based Maintenance (TBM), 
  2. Failure Finding Maintenance (FFM), 
  3. Risk-Based Maintenance (RBM), 
  4. Condition Based Maintenance (CBM), 
  5. Predictive Maintenance (PdM). 

On the other hand, Corrective Maintenance is reactive and occurs after equipment failure. It includes: 

  1. Deferred Corrective Maintenance (DCM)
  2. Emergency Maintenance (EM). 

The remainder of this article provides an in-depth discussion of each of these maintenance types.

Before delving into specific definitions, it's important to first understand the fundamental distinction between Preventive maintenance and Corrective maintenance.

To start, it's helpful to distinguish between preventive maintenance and corrective maintenance at a high level: 

• Preventive maintenance involves performing tasks before a failure occurs. These tasks can be aimed at preventing the failure, minimizing its consequences, or assessing the risk of it happening. 

• Corrective maintenance, in contrast, involves restoring equipment functionality after a failure has already occurred. It's worth noting that corrective maintenance can also result from a deliberate "run-to-failure" strategy.

The types of maintenance listed in this article do not encompass Autonomous Maintenance or Autonomous Care, which is also known as Front Line Maintenance in some organizations. Autonomous Care involves CLAIR activities (Clean, Lubricate, Adjust, Inspect, and Repair), which combine various maintenance strategies and are performed more frequently by frontline employees.

1-PREVENTIVE MAINTENANCE

Preventive maintenance is a type of maintenance that involves replacing or restoring an asset at a fixed interval, regardless of its current condition. This is achieved through scheduled restoration and replacement tasks. Essentially, the goal of preventive maintenance is to prevent failure or reduce the likelihood of it occurring by performing regular maintenance while the equipment is still functioning.

According to this strategy, maintenance is performed based on time, usage, or other predetermined factors. The idea is to perform maintenance before a problem arises, instead of waiting for something to break down or fail completely.

Preventive maintenance is generally considered less expensive and less disruptive than corrective maintenance. It allows maintenance to be scheduled in advance and minimizes unexpected downtime. However, it also requires a significant investment of time and resources to keep track of the maintenance schedule and perform the necessary tasks.

Preventive maintenance can be scheduled either based on time or usage. Time-based maintenance involves performing maintenance tasks at regular intervals, such as weekly, monthly, or every three months. Usage-based maintenance, on the other hand, involves performing maintenance tasks based on the number of cycles, hours of operation, or other usage-related factors. For instance, your car may require maintenance every 10,000 kilometers to keep it running smoothly. Similarly, equipment may require maintenance after a certain number of cycles or hours of operation to ensure it stays in good condition.

Besides the conventional regular interval approach, there are additional methods that fall under the umbrella of preventive maintenance, such as 

Time-Based Maintenance (TBM), 

Failure Finding Maintenance (FFM), 

Risk-Based Maintenance (RBM), 

Condition-Based Maintenance (CBM), 

Predictive Maintenance (PDM). 

In the subsequent sections, I will delve into each of these categories of maintenance, explaining when they should be employed.

Planned Maintenance and Preventive Maintenance are not the same: 

Planning refers to the maintenance planning and scheduling process, so Planned Maintenance involves preparing the maintenance work for execution with all necessary job steps, labor, parts, and tools identified and organized. 

Preventive Maintenance, on the other hand, is maintenance that has been identified to prevent or mitigate a failure mode. All Preventive Maintenance should be Planned Maintenance, as it should go through the normal maintenance planning and scheduling process. 

The opposite of Planned Maintenance is Unplanned Maintenance, which is inefficient and should be avoided. Unplanned Maintenance is not properly prepared and is planned on the fly, as the job is done. The only time you should perform Unplanned Maintenance is when a high-priority work request comes in, which is so urgent that it requires Emergency Maintenance, where you break into the Frozen Weekly Schedule to complete the work without going through the normal planning and scheduling process.

2-TIME-BASED MAINTENANCE 

Time-Based Maintenance (TBM) involves replacing or renewing an item at a fixed time, interval, or usage, regardless of its condition, to restore its reliability. This approach is referred to as Scheduled Restoration or Scheduled Discard tasks in John Moubray's book, RCMII. It is important to note, however, that all maintenance should be scheduled through a Weekly Schedule, with the exception of Emergency Maintenance, which requires immediate attention and cannot be planned.

Time-Based Maintenance aims to prevent failure of known wearing parts that have a predictable Mean Time Between Failure (MTBF). This strategy assumes that the failure is age-related and has a clear service life, or that it is not worth the effort to assess the condition and a time-based replacement is more economical and still effective. 

However, it is important to note that Time-Based Maintenance can only manage age-related failure modes and should only form a small part of your overall maintenance program since more than 70% of the failure modes in your plant are not age-related.

In various industries, companies have to perform specific tasks to meet regulatory compliance requirements, which are often done on a fixed time interval basis using Time-Based Maintenance. However, even with compliance-related maintenance, there may be opportunities to engage with regulators and consider transitioning to risk-based approaches. For instance, using Risk-Based Inspection (RBI) for vessel inspections instead of conducting internal vessel inspections every four years is a good example of this.

3-FAILURE FINDING MAINTENANCE (FFM) 

Failure Finding Maintenance tasks aim to detect hidden failures, usually associated with protective functions such as pressure safety valves and trip transmitters. Since this type of equipment is not required to function until something else has failed, its failure modes are hidden, making it crucial to detect these failures before relying on the equipment to protect you. 

It is important to note that failure-finding maintenance does not prevent failure but merely detects it. Once detected, the failure must be repaired. These tasks are carried out at fixed time intervals, typically derived from legislation or risk-based approaches.

4-RISK-BASED MAINTENANCE (RBM)

In Risk Based Maintenance (RBM), maintenance resources are allocated to assets based on their associated risk in the event of a failure. This is achieved by using a risk assessment methodology that considers the likelihood and consequence of a failure.  Assets with a higher risk and significant consequence of failure would require more frequent maintenance and inspection, whereas low-risk equipment may only need occasional or limited maintenance. The RBM approach ensures that maintenance efforts are targeted where they are most needed, optimizing the use of limited resources.

By implementing an effective Risk Based Maintenance process, you can reduce the total risk of failure in your plant in the most economical way. This method involves continuously optimizing the frequency and scope of maintenance activities based on the results of testing and inspection, as well as a comprehensive risk assessment. Risk-Based Maintenance can include Risk-Based Inspection for static equipment such as vessels, piping, and pressure relief valves. Essentially, it is a form of preventive maintenance.

5-CONDITION-BASED MAINTENANCE (CBM)

Condition Based Maintenance (CBM) is a maintenance strategy that aims to prevent failures by detecting physical evidence of a failure in its early stages. While many failure modes are not related to age, they often give some sort of warning before they occur. By identifying these warning signs, it is possible to take action to prevent the failure from happening or minimize its consequences. CBM has a wide range of applications beyond traditional condition monitoring techniques, such as those typically used for rotating equipment.

The P-F curve is a key concept in Condition Based Maintenance, as shown in the figure below. 

It illustrates that as a failure begins to develop, the equipment gradually degrades until it can potentially be detected (point "P"). If the failure is not detected and addressed, it will continue to progress until a functional failure occurs (point "F"). The P-F interval, which is the time between points P and F, is a critical window during which inspections can potentially identify impending failure and provide an opportunity to take corrective action.

It's crucial to understand that the purpose of CBM is not to prevent failures by renewing equipment throughout its life but to intervene before a failure happens, as this approach is typically more cost-effective and minimizes the impact on availability. In short, condition monitoring does not repair machines or prevent failures, but rather allows you to identify issues before they lead to a breakdown.

A general guideline is to schedule CBM tasks at intervals that are one-half or one-third of the P-F interval. The effectiveness of CBM compared to breakdown maintenance is highly dependent on the length of the P-F interval. When there is ample warning of an impending failure, repairs can be planned, materials and resources can be gathered, and breakdowns can be avoided (though production may still be interrupted during the maintenance period). However, if the P-F interval is only a few days, the necessary organizational and workplace actions will be similar to those required for a breakdown, and the benefits of CBM will be greatly reduced.

Effective implementation of CBM as a maintenance strategy depends on early intervention, which requires an efficient and effective process for data gathering, analysis, decision-making, and intervention. However, for failure modes with large variability in the P-F interval, condition monitoring may not be an effective strategy.

6-PREDICTIVE MAINTENANCE (PDM)

In the past, Predictive Maintenance (PDM) was often used interchangeably with Condition Based Maintenance (CBM). However, with the recent advancements in Artificial Intelligence (AI), the decreasing costs of equipment sensors (IIoT), and the use of machine learning, there is now a clear distinction between Predictive Maintenance and Condition Based Maintenance.

Predictive Maintenance (PDM) is a more sophisticated version of Condition Based Maintenance (CBM), leveraging the potential of numerous online sensors to detect any signs of equipment performance deterioration and predict impending failures. The field is rapidly evolving and attracts the interest of many large organizations, making it an exciting and dynamic area for Maintenance & Reliability professionals. However, even the most advanced PDM approaches must be grounded in reliable principles and comprehension of the underlying factors.

Predictive Maintenance is a type of Preventive Maintenance as both aim to conduct maintenance before failure occurs. The selection of the maintenance type depends on the failure mode and its characteristics. For random failure modes, condition-based or predictive maintenance is suitable, while for age-related or uneconomical failure modes, time-based maintenance is appropriate. For example, a turbine with thousands of liters of oil is best maintained on condition, whereas dealing with 50 liters of oil, time-based maintenance is more suitable.

The main differences between preventive maintenance and predictive maintenance:  

  • Preventive maintenance encompasses various maintenance types that are carried out before a failure occurs. Predictive maintenance is a form of preventive maintenance that utilizes technology to monitor the equipment's condition and identify potential issues before they turn into failures. 
  • Typically, when people mention preventive maintenance, they're usually referring to Time Based Maintenance, which involves scheduled repairs or replacements on a fixed interval, regardless of the equipment's actual condition. The interval can be based on time (e.g., days, weeks, months) or usage (e.g., operating hours, cycles, kilometers).

7-CORRECTIVE MAINTENANCE (CM)

Corrective Maintenance (CM) involves restoring the function of an item after it has failed. This strategy is typically employed when the impact of failure on safety or the environment is deemed insignificant, and preventing failure is either not possible or not cost-effective. 

Corrective Maintenance may be the deliberate result of a Run to Failure strategy, or it may result from unforeseen failures that could not be prevented through preventive maintenance.

A run-to-failure strategy can be a viable option for certain equipment, such as general area lighting or smart process instrumentation without trip functionality, where the impact of the failure is limited and does not require immediate repair. However, it is important to carefully consider the failure modes under consideration and ensure that they do not have the potential to become emergency maintenance. 

Adopting a run-to-failure strategy for critical equipment that requires immediate restoration upon failure can trap an organization in a reactive maintenance environment, which is expensive, inefficient, and unsafe. 

Therefore, while a run-to-failure strategy can be beneficial in certain circumstances, it is crucial to make an informed decision.

Preventive maintenance is generally considered better than breakdown maintenance because preventing a failure is cheaper and safer than letting equipment fail. Preventive maintenance also has less impact on production compared to running equipment until failure. However, there are cases where running equipment to failure is the best strategy, such as with general area lighting where trying to replace lightbulbs before they fail would be wasteful and the consequence of failure is low.

8-DEFERRED CORRECTIVE MAINTENANCE

We divided corrective maintenance into two sub-types in the maintenance types chart:  

  • Deferred corrective maintenance 
  • Emergency maintenance (EM).

It is crucial to minimize the amount of Emergency Maintenance in organizations, as it is expensive, causes longer equipment outages, and poses a safety risk. To address this, it is important to prioritize corrective maintenance work requests and allocate enough time for proper planning and scheduling. Deferring the work request where possible can be an effective strategy to reduce the amount of Emergency Maintenance, leading to more cost-effective, efficient, and safe maintenance practices.

9-EMERGENCY MAINTENANCE (EM)

Emergency Maintenance (EM) is a type of corrective maintenance that demands immediate attention and disrupts your Frozen Weekly Schedule, assuming you have one. It can create chaos and throw your plans off-track, although some people thrive in such an environment and may even be hailed as heroes for working long hours to restore production. 

However, in terms of achieving Reliability, it is a dead end. Therefore, Emergency Maintenance is the one type of maintenance that we should strive to minimize as much as possible. In fact, World Class organizations aim to limit their Emergency Maintenance to less than 2% of their total maintenance. 

Emergency Maintenance is the most costly type of maintenance as it demands immediate attention and often lacks proper planning, resulting in inefficiencies and higher expenses. According to various sources, Emergency Maintenance can be up to 3-5 times more expensive than well-planned preventive maintenance.

Frequently, people ask "What is breakdown maintenance?" While it's not a separate type of maintenance in this explanation?. Breakdown maintenance is essentially corrective maintenance triggered by equipment failure. The urgency of the repair will depend on the risk associated with the failure. However, for many, breakdown maintenance is seen as synonymous with emergency maintenance - repairs that must be done right away.


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