1. Eternal Discussion
Much has been said, is discussed, and will be discussed about the different "methods" of executing Maintenance actions.
Although it is true that a Maintenance Program is based on a group of Maintenance tasks and actions to be carried out, taking into account a group of resources (Materials, Human and Financial), it must also be admitted that the only thing that differentiates these actions and / or tasks, it is under what "name" they are executed and not under what method or form it is done. Analyzing the types of Maintenance that are known until today (and it can be said that they are accepted by the majority) we can conclude that they all have the same objective (maintain the Objective function), that they are all executed with resources (different according to the effective actions to be carried out) and that all are executed after some “time” has passed, during which the equipment / system consumes a resource (meter1) and this identifies the loss of performance levels,depending on the operation to be performed, the same action and / or task is repeated. We can include among the similar criteria the study being carried out, the Root Cause analysis, and the measurement of certain values, which will make up the performance control indicators, both for the equipment and for the Maintenance function in general. But why do they differ? And most importantly, how do I know which one to use?
Before answering these questions, it is necessary to take into account an interesting element. We will agree that the types of Maintenance that are recognized (the most general) are classified between Preventive Maintenance, Predictive Maintenance and Corrective Maintenance.
Within these three (3) the remaining classifications are incorporated. In addition, it is these three systems that in some way or another recognize the RCM methodology, where one of its fundamental results, after analyzing the failures and their possible consequences, is to define the actions that prevent it and under what type of Maintenance is it carried out.
But only the RCM is able to classify the Maintenance actions in Preventive, Predictive and Corrective? Do you believe. It is necessary to wait to execute an RCM analysis to know under what type of Maintenance I attend to each team according to the actions that I must carry out? Of course not. This last question is part of another Eternal Discussion, in which I prefer not to comment. Now, in some way, it is necessary to know what Type of Maintenance I am executing, because from here, I will know what and how I should act.
For this, we will start from the Maintenance approaches, in a simplified way, with the aim of understanding the criteria.
But only the RCM is able to classify the Maintenance actions in Preventive, Predictive and Corrective? Do you believe. It is necessary to wait to execute an RCM analysis to know under what type of Maintenance I attend each team according to the actions that I must carry out? Of course not. This last question is part of another Eternal Discussion, in which I prefer not to comment. Now, in some way, it is necessary to know what Type of Maintenance I am executing, because from here, I will know what and how I should act. For this, we will start from the Maintenance approaches, in a simplified way, with the aim of understanding the criteria.
2. Maintenance Approaches.
Everything in life exhibits two forms of execution, to prevent the occurrence of an event, or after the event occurred. This is why the focus of the Maintenance works is reduced to the PROACTIVE (before) and REACTIVE (after) focus, where the reference to the occurrence or not of NON-CONFORMITY is evident, that is, the appearance of a fault. From here, it is necessary to identify Maintenance actions, carried out before a NON-Compliance, or after its occurrence. If a fault occurs, for example, a boiler pressure relief valve, which is clogged, in the closed position, and the boiler, due to over-pressure (dangerous, isn't it?), Stops working, the action necessary to take it would be reactive, since the "main" equipment stopped working. Now if this same valve is stuck,it is discovered before the overpressure occurs, so the actions are clearly Proactive, since the nonconformity did not occur.
This simple example demonstrates the need to differentiate Preactive actions - fundamentally based on Preventive Tasks (Frequency of Intervention), Predictive Tasks (Based on Condition) and Evaluation (Inspection, Search for Failures and Errors) - from Reactive actions, characterized fundamentally for Corrective Maintenance. It is good to incorporate these elements in the analysis, since it depends on understanding the difference, similarities and coincidences that exist between these “Types of Maintenance”, except perhaps for Corrective Maintenance. We will then go on to analyze each Type of Maintenance, under this approach, but in order to establish under which criteria I use each, within a Maintenance Program2.
3. Types of Maintenance Actions.
According to the Maintenance approaches already explained, criteria of Preventive, Predictive and Corrective Maintenance will be treated. For this, it is necessary to establish some elements as a starting point, especially for the importance they deserve.
PREVENTIVE MAINTENANCE.
In an article I found the following statement: "The purpose of preventive maintenance is to: Find and correct minor problems before they cause failure." It is incredible that they refer to "Minor Problems" when most companies in the world base their Maintenance on Preventive. Don't be fooled, almost everything you do. does in your company, it is Preventive Maintenance, unless corrective measures are the order of the day. Likewise, it is defined in this article that “preventive maintenance was designed with the idea of anticipating and anticipating machine and equipment failures”. In this case, I agree. The raison d'être of Preventive Maintenance is to PREVENT the occurrence of failures, but it is a pity that it does not say how.
Conceptually, it is always established that it is based on the execution of Maintenance tasks and / or actions, according to a frequency of intervention, in which the equipment / system is supposed to work free of failure, and which incredibly, is related to the Indicator Average Time Between Failures (TMEF), and how many problems it gives us, if it is not calculated and analyzed correctly. It is also possible that the intervention frequency will be established by the equipment / system manufacturer, but it is never thought that this failure-free working time was foreseen for specific conditions, some of them almost ideal in laboratories and test benches. For this reason, the fundamental success of Preventive Maintenance will be in defining this frequency of intervention correctly, so that no failure will surprise us.only because the probability that it did not fail was not met.
Another critical point is the definition of the METER that best characterizes the fall in performance, for which an intervention frequency is defined. These meters are the ones that govern the behavior of the equipment. If it is a Calendar Meter (Day, Month, Year, Weeks, etc.), it must be taken care that the actions are not being executed outside of real time, since a team that does not work twenty-four (24) hours a day does not will coincide with a frequency of intervention, established with a calendar meter. In this case, a Technological Meter is envisaged, which does correspond to the intervention of the team / system, depending on the Fall in Performance. If Maintenance actions are carried out preventively for a Turbo-Group, every 10 months of work, and this Turbo-Group works 24 hours a day, no problem,but if he works only 5 hours a day, then it is essential that the Maintenance actions are carried out, based on a Technological Meter, such as Moto-Hours. Therefore, Preventive Maintenance is performed according to the Frequency of Intervention, of a meter that characterizes the Decrease in the Performance of the Equipment / system.
PREDICTIVE MAINTENANCE.
Predictive Maintenance is misinterpreted by many maintainers. First, there is no talk of Predictive Maintenance when Inspection actions are performed. Nor is it predictive the fact of measuring the variables of Vibrations with the best team in the World. This is why Predictive Maintenance is very complex. Above all, Predictive Maintenance is defined as the execution of Maintenance actions based on the actual technical state of the equipment, based on the measurement and monitoring of some symptom parameter and intervention according to desired, permissible or alarm levels. From here, Predictive Maintenance is: Measurement of Variables that identify a Symptom Parameter, which is known as State Monitoring. This status monitoring is executed by Planning Inspections,that are executed according to a frequency, predicted based on the characteristics of the Variable in question and the Symptom Parameter identified.
From the study and analysis of the Variable, a Maintenance action to be executed is established, which can be Planned, if its execution is foreseen, depending on the characteristics of the equipment, or Corrective, if it is urgently necessary to execute it. As you can see, Predictive Maintenance is much more than measuring and taking readings, it is all a “DYNAMIC” study that has as its main criterion, the Tracking of a Variable that identifies a Symptom Parameter. In summary, Predictive Maintenance is performed based on the Measurements, Analysis and Diagnostics carried out on the equipment, which in this case characterizes the decrease in performance.
CORRECTIVE MAINTENANCE.
Corrective Maintenance is understood as those actions (planned or NOT) that aim to reestablish the performance level of a Equipment / System, after the occurrence of a failure, which may or may not be expected. Based on this concept, we can identify two factors. First, that the actions and their elements can be planned or NOT, which implies that the occurrence of the failure is expected, with all the resources available for its solution, but it is not scheduled (execution date), which makes a difference. The second factor is that the occurrence of the failure can be expected or NOT, since the equipment / system is worked in this case until it enters the failure state, under certain specific conditions that identify it, fundamentally the criteria cost;where the costs of avoiding the failure (Plan a Preventive or Predictive action) are much greater than the cost of unavailability that is incurred if the nonconformity occurs. It is clear that this includes an absence of operational risks and risks to human life, as well as risks to the environment, criteria that are not unique to the RCM.
Summarizing: Preventive Maintenance actions are all those “Tasks” that are executed based on a frequency of intervention, identifying a Calendar or Technological meter that identifies the failure that is to be avoided. Similarly, Predictive Maintenance is characterized by the execution of inspections (preventive) to measure or not a variable, which identifies a Symptom Parameter, and which in turn establishes alarm criteria to intervene the equipment / system, before failure occurs, either by preventive actions, or urgently (corrective). Finally, Corrective Maintenance includes those actions planned or not, which involve working the team to its fault state, to then execute the actions already planned.These differences establish a correlation between what is Maintenance Planning and Scheduling. However, it is inevitable to keep in mind those failures not foreseen (in any way) and that will surely occur within the daily operation of the equipment.
An example of this can be found in a water supply pump, in which, according to its Objective Function, these groups of actions can be established:
- A. Preventive Maintenance: Filter cleaning, fixing adjustments, exterior paint. Inspections B. Predictive Maintenance: Check condition of fasteners, check cavitation, Check condition of Bearings. Corrective Maintenance: Replacement of Bearings, breakage of the impeller, replacement of broken shafts.
All these actions, in their different forms, are treated under different types of Maintenance. Now which one to use? For this, a Decision Diagram is presented.
4. Decision diagram. Examples.
With the aim of being able to decide in a coherent way what type of Maintenance action will be executed, the following Decision diagram is established, in order to facilitate the approaches to be executed, and without forgetting that we are preparing the way for the RCM, since will use this Diagram as long as a Reliability Based Maintenance Methodology is not implemented.
To explain this decision diagram, we will use three (3) examples:
EXAMPLE # 1: Let's take a hydrocarbon pumping station, in which the Pump, Gearbox and Motor equipment are considered Critical. In this case, you can have critical equipment for Operational Safety or Environmental Impact, otherwise, we would have to ask ourselves if the failure causes High Shutdown Times. We start in the Diagram from this point, to carry out a Technical-Economic analysis, to check the justification of Predictive Maintenance. Upon obtaining a positive response, we will analyze whether it allows any type of follow-up. In this case, the equipment allows, among other factors, to measure vibration variables, analyze oil consumption, check noise levels, among other factors. Following the diagram,We got to wonder if there is capital for the incorporation of a Monitoring Program, in order to guarantee the execution of the actions. Well, in this case the answer is NO (something very common). However, there is capital to track other actions. Therefore, we have as Predictive Maintenance actions: Alignment of the pump-gearbox coupling. For oil study, seal changes and Maintenance to gearbox. And for Compression Testing, we have Maintenance intake / exhaust valves and Cylinder change. On the other side of the diagram, we have to ask ourselves, if it is possible to Identify Failure Modes, and from here, establish the Control Meters, for the following Preventive Maintenance actions: Oil Change, Verification of Water Losses,Radiator Cleaning, Anchor Adjustments, Inspections. As you can see, we have two groups of Maintenance tasks, which are aimed at preventing the objective function of the equipment from failing. But, what happens when Predictive Maintenance is not justifiable, even when we work with Critical teams?
EXAMPLE # 2: In another Hydrocarbon pumping station, we have a Fluid Control Valve, as equipment to Plan, but it is necessary to select under which Optics to work. Taking into account (in theory) that if the valve failed, it would only affect the supply of hydrocarbons to customers, but it does not have implications for the environment. For this reason, the decision is made to continue along the NO line; and we got to the question of whether it causes high downtime. In this case, the failure of this valve (automatically controlled), which would occur in the middle of “nothing”, that is, in the field, far from the repair units, it is clear that its replacement, or Maintenance, would be delayed. So the answer would inevitably be YES. From now on,the question is marked in a justification of Predictive Maintenance. This valve has failures in the electric drive controlled by sensors, all with electronic technology, which complicates the possibility of applying diagnostic techniques.
Applying Preventive Maintenance and taking a frequency established by a Meter, which will initially be set by the manufacturer, and which will later be varied, analyzing equipment failures, and adjusting interventions. Note how critical equipment, under whatever glass you look at, is not treated with Predictive Maintenance, just because it is not possible to track it, identify a Symptom Parameter, and select a Technique for this. It would not be unusual, that even identifying a Symptom Parameter, it would be unnecessary to apply Predictive techniques, since economically it is not justified, especially since the cost of avoiding the failure is much greater than the losses that its occurrence causes. This point, which is analyzed in the question: Justify Predictive? it is much more than a simple economic analysis,it is also an assessment and study to find the most optimal variant, to avoid the occurrence of this failure.
EXAMPLE # 3: To have a broader view of the usefulness of this graph, let's look at a final example of an Electric Power Transmission Line. This system, which is understood as its Critical System classification, presents a group of Maintenance Tasks, classified between Preventive and Predictive Maintenance. It is known that its objective function (transmitting electricity at 230 KV and power of 130 MW) may fail due to conductor breakage, short circuit, conductor fall due to vegetation, among other potential failures. In this case, to prevent conductor breakage, Preventive Inspections are carried out, not only on the vegetation, but also on the state of the structure. In addition, it includes application of Thermography techniques, to check the state of the joints, and analysis of the corona effect. As you can appreciate,here two Maintenance Tasks are combined, under two different optics, but aimed at avoiding the same failure. No matter what it costs, we are referring to a system that has to maintain (by state regulations) 98% availability, which implies that it CANNOT FAIL. Regardless of this regulation, there are failures that cannot be avoided with Predictive techniques, so it is necessary to plan and schedule strict compliance preventive actions. An example of this are the Periodic Inspections to avoid the short circuit, mixed with a Corona Effect Analysis (Predictive technique) to avoid decreasing the required Availability. As you can see, the possible variants are infinite, but there is only one rule:guarantee the objectives of operation (operational safety) or the Technical-Economic feasibility of the Task.
5. At the End…
The analysis allows us to obtain a profile for the presentation of the Maintenance actions, under the Preventive, Predictive or Corrective Maintenance variants, without having to wait for the incorporation of the RCM Methodology. However, if you can use it, it would be the best decision you could make. This decision graph tries to reveal a path, but the path, and its deductions, are your responsibility. Dear reader, I invite you to try, so that you notice, that it is not so difficult to know how to maintain the equipment and / or systems of your company.
