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This paper was originally
presented at RCM-2008 Reliability Centered Maintenance Managers' Forum. Please click here for future dates and locations |
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The Seven Questions of
Reliability Centered Maintenance by Bill Keeter and Doug
Plucknette,
Allied Reliability Abstract Reliability-Centered Maintenance (RCM) is a phrase coined thirty years ago to describe a cost effective way of maintaining complex systems. The RCM method uses the answers to seven very basic questions to help determine the best maintenance tasks to implement in an Equipment Maintenance Plan (EMP). This paper focuses on those seven questions and how they help determine the EMP. Introduction On December 29th, 1978 F. Stanley Nowlan and Howard F. Heap published report number A066-579, "Reliability-Centered Maintenance". The report was the culmination of several years of work aimed at determining a new, more cost effective way of maintaining complex systems. The called it Reliability-Centered Maintenance (RCM) because programs developed through RCM "are centered on achieving the inherent safety and reliability capabilities of equipment at a minimum cost". RCM is a time consuming, resource intensive process. Many practitioners have tried to reduce the amount of time and resources required to accomplish RCM projects with varying degrees of success. The most successful ones have focused on understanding the basic goals of RCM, and on the seven basic questions that need to be asked about each asset. In this paper we will concentrate on understanding each of the seven questions and how the answers to those questions help determine a Reliability-Centered approach to asset management. The Definition of Reliability In the book Maintainability, Availability, and Operational Readiness Engineering Dimitri Kececioglu defines reliability as: "The probability that a system will perform satisfactorily for given period of time under stated conditions." Nowlan and Heap define Inherent Reliability as: "…the level of reliability achieved with an effective maintenance program. This level is established by the design of each item and the manufacturing processes that produced it. …" In The Fault Tree Analysis Guide a system is defined as: "A composite of equipment, skills, and techniques capable of performing or supporting an operational role, or both. A complete system includes all equipment, related facilities, material, software, services, and personnel required for its operation and support to the degree that it can be considered self-sufficient in its intended operational environment." When we look at these definitions in conjunction it becomes very evident that any asset management program must address system development through all phases of a systems life. There is no maintenance program that can improve the reliability of a poorly designed system. Additionally, whatever maintenance program is developed is determined by the design of the system and the goals of the organization. The Goal of Reliability-Centered Maintenance (RCM) The primary goal of Reliability-Centered Maintenance (RCM) should therefore be to insure that the right maintenance activity is performed at the right time with the right people, and that the equipment is operated in a way that maximizes its opportunity to achieve a reliability level that is consistent with the safety, environmental, operational, and profit goals of the organization. This is achieved by addressing the basic causes of system failures and ensuring that there are organizational activities designed to prevent them, predict them, or mitigate the business impact of the functional failures associated with them. The Seven Questions of RCM There are seven basic questions used to help practitioners determine the causes of system failures and develop activities targeted to prevent them. The questions are designed to focus on maintaining the required functions of the system. 1. What are the functions of the asset? 2. In what way can the asset fail to fulfill its functions? 3. What causes each functional failure? 4. What happens when each failure occurs? 5. What are the consequences of each failure? 6. What should be done to prevent or predict the failure? 7. What should be done if a suitable proactive task cannot be found? What Are The Functions of the Asset? Every facility is uniquely designed to produce some desired output. Whether it is tires, gold, gasoline, or paper the equipment is put together into systems that will produce the end product. Each facility may have some unique equipment items, but in many cases common types of equipment are just put together in different ways. Within every RCM analysis we have two types of functions. First, the Main or Primary function, this function statement will describe the reason we have acquired this asset and the performance standard we expect it to maintain. Second, are the Support Functions, which list the function of each component or maintainable item that makes up the system. The Support Functions are provided by the bottom level of equipment in most facilities such as pumps, electric motors, valves, rollers, etc. Each of those maintainable items has one or more easily identifiable functions that enable the system to produce its required output. It is the loss of these functions that lead to variation in the Main or Primary function of the system and the safety, environmental, operational, and profit output of the facility.
The key thing to remember when describing equipment functions is that we are interested in what the equipment does in relation to its operating context, not what it is capable of doing. For example, a cooling tower pump may be capable of pumping 100 gpm at 275 ft of head, but may only need to pump 75 ± 10 gpm at that same pressure. It is necessary to focus on the required and secondary functions within the system operating context in order to analyze asset functions. Our main function statement for this system would address the functionality within the operating context; Be able to pump cooling tower water at a rate of 75 ±10 gpm at 275 ± 15 ft of head while maintaining all quality, health, safety and environmental standards. The rate, the head requirement, quality, health, safety and environmental standards are all performance standards for the pump. Functions need to be well defined. Statements such as “pump water from the pond” don’t lend themselves well to understanding what functional failure would look like. A statement such as pump 1000 ± 100 gpm at 275 ± 15 ft of head from the pond make it easy to understand what a functional failure might look like. If we can only pump 800 gpm then we obviously have an unacceptable variation in output.
In What Way Can the Asset Fail to Fulfill its Functions? Nowlan and Heap said there are two types of failures. There are functional failures and potential failures. Functional failures are usually found by operators, and potential failures are usually found by maintenance personnel. In many organizations there are great debates about what constitutes a failure. In their original work Nowlan and Heap used a very good definition for failure. “A failure is an unsatisfactory condition.” Using this definition allows us to grasp the idea that equipment can continue to operate yet be considered failed. Many condition monitoring programs don’t achieve their desired output because those running the program do not recognize that a failure has occurred as soon as an unsatisfactory condition is detected. They often try to run the equipment as long as possible or until they get closer to the F of the P-F curve. At Allied Reliability we call this “managing to the F”. More mature programs manage to the P, meaning that they take action as soon as the unsatisfactory condition is recognized. Remember, the further we go along the P-F curve the higher the level of business risk we are accepting. It is equally important to recognize that there is significant value in ensuring that equipment is installed and commissioned properly.
The I-P-F curve shown above is the standard P-F curve with an I-P portion added. Point I is defined as the point of installation of the component. The I-P portion of the I-P-F curve is the failure free period. This is the time during which the operation is defect free. The I-P interval for machines that were installed improperly may be just a few seconds. The I-P interval for machines installed by well trained crafts people using well designed procedures, precision techniques, and precise measuring equipment, and commissioned by operators using well designed operating procedures may be years. The graphic above shows what the I-P-F curve for two differently installed identical machines might look like. The machine with the longer I-P interval was installed by well trained crafts personnel using a properly designed procedure and precision measuring devices, and commissioned by operators using a well designed operating procedure. The machine with the shorter I-P interval was installed by inadequately trained personnel using either no procedure or a poorly designed procedure without precision measuring devices and techniques, and commissioned by operators using either no procedure or a poorly designed procedure. The difference in lengths of the I-P portions of the curve for the two pieces of equipment may represent large sums of money. The dollars represent the additional cost of parts and labor and also the amount of additional foregone production as a result of the extra maintenance work that had to be performed. Looking at an organization’s shift in focus from F toward I is a more effective way to determine its maturity than by looking at the age of their maintenance program. Many organizations reactively maintain equipment for a long time. An organization that is constantly focused on Point F and staying clear of it, will undoubtedly be a reactive culture. Typical things heard around this organization might be “How long can we run it before it fails?” and “Just how bad is it?”. An organization’s first step toward maturity will be to shift its focus from Point F to Point P. The organization then focuses its efforts on understanding how things fail and their ability to detect these failures early. Typical things overheard in this organization may be something like: “Is this the best way to detect these defects early?” or “I appreciate you letting me know about this problem, even though it’s very early.” Further maturation results in a transition from focusing on Point P to focusing on Point I. Overheard in the hallways of this organization are things like “Take the time to do it right, it will pay big dividends for us not too far down the road” and “Let’s update the procedures for that job to reflect what we just learned”. This organization is trying to prevent failures from occurring in the first place by applying best practices with fits, tolerances, alignment standards, contamination control and well documented procedures. They will see the step change in performance and they are the ones we label “mature” not the organizations that have been doing it poorly but for a longer period of time. The functional failure statement describes the loss of the equipment’s function, not what is wrong with the equipment. A good functional failure statement will most likely not have the noun name of an equipment part in it.
What Causes Each Functional Failure? At the end of the day we will be building maintenance tasks designed to prevent functional failures from occurring. In order to do this we must understand what causes each functional failure. The cause may be the failure of some equipment part, but it can just as easily be a failure in some human activity. Improper operation and improper maintenance are likely to be the causes of failures. Remember the definition of a system. Everything and everybody in the facility has some impact on system reliability. It is very important to describe these causes or failure modes in a way that allows us to create a living program for improving asset management. Easy to use codes in the Enterprise Asset Management (EAM) system will allow us to capture data about what types of failures are occurring and to react to that data by reengineering the maintenance plan, training plan, or equipment design associated with the equipment. A well designed Failure Reporting, Analysis, and Corrective Action System (FRACAS) is a must for continuously improving system performance. For part failures we may want to use a simple three part code that consists of the part name, part defect, and defect cause.
What Happens When Each Failure Occurs? Known as Failure Effects, these statements clearly describe what happens when a failure occurs and what events are required to bring the process back to normal operating conditions. Different things can happen when a failure occurs. Not all failures are created equal. When listing failure effect statements we should fulfill the following criteria:
What Are the Consequences of Each Failure? What makes failures matter is their impact on the business. Every business has goals for profitability, safety performance, environmental performance, and operational performance. Each failure has a different impact on business performance, and it is important for the RCM team to understand the consequences of each one. Some failures are of little to no consequence, and some can result in the loss of lives, or in extreme cases total failure of the business. Most organizations use some sort of severity matrix to define the consequences of failures. The tables below represent just some of the ways this can be done.
How would your company handle creating severity rankings for failures? In most cases each failure will be ranked according to what is known as criticality. The criticality is the result of combining probability and consequence rankings together to yield a single number. The criticality will be a biased towards the business’s philosophy of safety, environmental, and operational risk. The tasks in the Equipment Maintenance Plan (EMP) generated from the RCM analysis are designed to lower the criticality of the significant failures in the system. Tasks can be rank ordered for implementation by implementing those that yield the higher reduction in criticality first. What Should be Done to Predict or Prevent the Failure? Each failure mode must be examined to determine what type of maintenance task, if any, should be used to prevent or predict it. Nowlan and Heap recognized four basic types of PM tasks.
When and how these tasks are performed depends on the failure mechanism that is present. In the original report six failure shapes were investigated. The team determined that only 11% of the failure modes present in their study of aircraft part failures would lend themselves to scheduled rework or replacement. In this instance 89% of the failure modes present would require some sort of inspection. The majority of the failure modes, 63%, could actually be made worse by time based overhaul or replacement. Clearly, some good non-invasive method of inspecting for potential failures would be very beneficial.
In some cases it is not possible to detect functional failures during normal operations. Those undetectable failures are called hidden failures. Hidden failures are usually associated with some sort of protective system that is designed to minimize the impact or prevent the high consequences associated with a failure of the protected system. Items such as pressure safety valves (relief valves), circuit breakers, high temperature interlocks, and high level interlocks are just a few examples of devices that could have hidden failures. The bad news is that the consequences of failure can be extremely high. The good news is the probability of the catastrophic event is often quite low. It requires that both the protecting and the protected item fail at the same time. In cases where functional failure is not immediately detectable during normal operations a failure finding task must be done to prevent the high consequences associated with multiple failures. Table 6, reproduced from the Nowlan and Heap report presents a comparison of the four types of tasks and their applicability. For non-critical failures the order of preference will generally be inspection, rework, and lastly discard or replacement of the item.
When Nowlan and Heap published their report in 1979 condition monitoring methods such as vibration analysis (VA), ultrasonic inspection (UE), ultra-violet inspection (UV), and other non-invasive technology based inspection methods were in their infancy and were very expensive to deploy. Now, nearly thirty years later, technology based inspection methods are relatively inexpensive and easy to deploy. These methods are really nothing more than inspection methods that can be used on a periodic basis to determine the condition of equipment. We can be almost certain that Nowlan and Heap would have recommended extensive use of these technologies had they been readily available. In any case, the task chosen must either lower safety, environmental, or operational risk to an acceptable level, or for non-critical failures be economically effective. Risk is always the top driver in the decision making process. We may have to spend more money to ensure that we meet our risk goals.
What Should be Done if a Suitable Proactive Task Cannot be Found? There may be a couple of reasons why we wouldn’t be able to find a suitable proactive task. We are either unable to find a task that will lower business risk to an acceptable level, or we are unable to find a task that is economically feasible. Each case requires a different response. In the first case, the system will have to be redesigned to that an acceptable level of risk. In the second case, we can choose a run to failure approach for the failure mode. It is important to remember that when a run to failure strategy is employed we should then put in place consequence reduction tasks to mitigate the impact of the failure. The RCM team must ensure that appropriate steps are taken to have written procedures in place to deal with the failure mode, and that proper spares levels are maintained. Conclusion Answering the seven questions of RCM properly will yield a cost effective EMP that achieves the business’ goals for safety, environmental, and operational risk. Answering the questions properly requires a cross-functional team of maintenance, operations, and engineering personnel who have a thorough understanding of how the asset works, and what the organization’s risk and profit goals are.
Bibliography 1. Reliability Centered Maintenance, Anthony M. Smith, McGraw-Hill, ISBN 0-07-059046. 2. Failure Mode, Effects, and Criticality Analysis (FMECA), Reliability Analysis Center, IIT Research Institute, 201 Mill Street, Rome, New York 13440-6916 3. DOD report number A066-579, “Reliability-Centered Maintenance”., F. Stanley Nowlan and Howard F. Heap, December 29th, 1978 |
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