ISSUE No. 10 : February 2008
RSA RELIABILITY NEWSLETTER
Improving reliability starts with its very basic foundation and that is through “ EDUCATION ” . . .
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AN INCONVENIENT TRUTH ABOUT PREVENTIVE MAINTENANCE
Most industries today have a form of Preventive Maintenance being performed on a routine and scheduled fashion, yet despite the very best and noble efforts performed by our maintenance craftspeople, reactive maintenance is still what most industries are experiencing today. This leads us to the question, Can Preventive Maintenance really capture failures ? or Does Preventive Maintenance caused failures ?
First let us define what Preventive Maintenance is. Preventive Maintenance is a basic maintenance performed on equipment’s and facilities. The main goal of performing task on a scheduled basis is to extend the life of the equipment and to assure its capacity in support of plant’s goal and targets. It is also a series of tasks performed at a defined frequency dictated by the passage of time, running hours or volume, that either extend the life of the asset or detect that an asset is on its critical wear curve and is on the process of failing. Preventive Maintenance activities includes regular checking and inspection, routine cleaning, lubrication, planned component replacement, minor and extensive overhauls.
PROBLEMS WITH PREVENTIVE MAINTENANCE ?
Maintenance people believe that all part’s after consistent use will reach a point of wear and tear, hence, overhauling or replacing the part before it fails on a specific fix schedule will ensure the reliability of the equipment, therefore the concept of Preventive Maintenance will help solve the problem of unexpected failure. Is this statement Right or Wrong ?
If your answer to the above statement is RIGHT, then your definitely “WRONG”. Have you in anyway experience this. Quote from the book of John Moubray on RCM page 143 : It is also borne out by the machine operator who said that every time maintenance works on it over the weekend it takes up to Wednesday to get it going again. Or let me termed it this way :
Boss : Hey, what’s the problem with your equipment ?
Tech : I don’t know boss, it’s been acting crazy since I started it this morning
Boss : Did maintenance perform some PM on it ?
Tech : I think so boss, there’s a stamp on the equipment
Boss : No wonder its running crazy, they touch it !
Isn’t it if we performed PM in our equipment, then it should be the other way around ? Are we missing something. These things simply are called Infant Mortality Failures, and a lot of factors contribute to it but most likely Infant Mortality Failures are caused by human errors committed during the process of performing scheduled overhauling or PM on a piece of equipment.
Most maintenance people believe that the more often an item is overhauled the less likely it is to fail. In reality, this is just the opposite. The truth is that scheduled overhauls / Preventive Maintenance increases failures by introducing what we call Infant Mortality Failures into otherwise stable systems. This means that if we are “NOT” equipped on how to disassemble and assemble systems in our equipment or if we lack some training or tools to perform the tasks on the equipment, the best thing to do is to leave it alone. Don’t disturb stable systems PERIOD unless we are really equipped w/ the tools & knowledge on how to do it.
Another problem with Preventive Maintenance is that most maintenance assume that similar assets will require similar activities on maintenance without considering that each equipment have their own unique operating context. Hence, Preventive Maintenance are used for similar assets without considering that different consequences apply in different operating context. This results in a large number of schedules which are wasted not because they are wrong in the technically sense, but in reality, they achieve “ NOTHING”
I used to provide this sample to explain the last paragraph in a way an ordinary layman can grasped. I used to ask a question if they have a bulb in their home they use in the evening perhaps around 30 watts, and most of them would say yes. When I ask them if they perform a sort of inspection on this bulb daily if it is working or not, then my audience simply laugh and say “Of Course Not”, when it gets busted then we simply replace it with a new one. When I told them that I have a bulb like this and we used to inspect these bulbs not once but 6x a day. Now when I asked them if I am stupid or something for inspecting this bulb 6 times a day, their reply was a resounding YES ! I tell my audience that I’ll prove to them It’s worth inspecting the bulb 6x a day and that I was not crazy or stupid in any way.
I tasted my first work in 1989, as a Preventive Maintenance helper of a shipping company and I used to be a crew for a cargo ship named MV Sea Raider. My duty was from 4am to 8am and again at 4pm to 8pm in the afternoon. Actually there were 6 shifts where we require to work 2 shifts a day totaling 8 hours. I belong to the engine room and inside the engine room were 2 generators and two 750 hp Diesel Engine which means that this cargo ship have 2 propellers. On the wall was 6 bulbs, 3 bulbs located on the port (left) and 3 bulbs at the starboard side (right), and when it is time for our duty to go down to the engine room and monitor the engines running, our first activity was to check if these bulbs were working or not, the bulbs are color coded Green, Yellow and Red. If the green bulb is on then the ship simply moved forward at a designated speed, but if the green bulb blinks continuously, then the captain or officers on deck wants us to increase the rpm and speed of the ship and we comply. If the yellow light blinks, then we simply need to stay alert as either the green or red bulb will light afterwards. When the Red bulb lights up then we need to stop the engine, if 2 red bulbs from the port and starboard lights up then both engines should be stopped. But if the Red Bulb blinks continuously then the command was to have the engines in full reverse otherwise the ship might hit something which can cause the lives of all the crew on board. Remember that ships have no breaks and in order to stop a ship you need to reverse the engines. After giving this story, I asked my audience again, if I am stupid or something for inspecting this bulb, and they answer “NO” why because the consequence of failure on the red bulb greatly differs from the bulb you have at home. Imagine failing to inspect the red bulb, and it got busted what do you think will happen to us. Well I’m still here today writing down this newsletter because we did inspect the bulb religiously at 6x a day. This same bulb where we just color coded it is the same bulb you have in your home. This simply explains what operating context is all about.
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Let’s take a sample of these three identical pumps. One is a stand alone pump while the other pump has a standby pump which is connected in parallel with the duty pump. If we are to derive the most suitable tasks for each pump I say that for Pump the most feasible task is to Predict or use Preventive Maintenance since if this pump fails, it will affect operations since this is a stand alone pump, while we can run to fail Pump B since if it fails operations won’t be affected since we have a back-up pump. For Pump C which is a standby pump, we can run it once in a while to check if it is still running. This simply explains that the conditions in which an equipment is being operated should be taken into account in determining the most feasible maintenance tasks and that similar equipment might require different tasks
Another problem encountered on Preventive Maintenance, is replacing perfectly good parts with new once since it is assumed that the part will eventually fail after all. And most of the time the assumption is wrong. This is what makes Preventive Maintenance costly. Preventive Maintenance does not guarantee that the parts to be replaced really needs to be replaced. If the equipment is in good condition, might not as well touch it and just perform the basics such as keeping the equipment clean, checking lubrication and routine inspection.
LESSONS FROM NOWLAN AND HEAP ABOUT RELIAB ILITY
The message of Stanley Nowlan and Howard Heap dates back to the late 60’s which started in the airline industry which states that first, scheduled maintenance has little or no effect on the reliability of a complex item unless the item has a dominant failure mode and second, there are many parts/items/components for which there is no effective form of scheduled maintenance. Their discovery led to the discovery of the 6 failure patterns which I believe every single maintenance should understand.
Figure 1
Figure 2
These 6 failure patterns simply tell us how a part will eventually fail after all. A study by Nowlan and Heap from the aviation industry reveals that 14% of component of an aircraft conforms to patterns A to C in which the rate of wear is directly proportional to it’s age. But around 86% of components will fail randomly with the presence of infant mortality failure. This graph simply states that if the failure pattern of the part conforms to either patterns A, B or C then this is where Preventive Maintenance or Scheduled Maintenance is feasible to use. But when the part or component failure conforms to either patterns D, E or F then this is where Preventive Maintenance will be at it’s weakest point. It is not recommended to use Preventive Maintenance on these failure patterns but rather other feasible options that can be used for this pattern of failure which includes a Run to Fail when consequences are low, Predictive Maintenance when there is evidence of a potential failure or simply redesign the system.
Hence, to answer the question, why don’t Preventive Maintenance reduce the amount of Reactive Maintenance being performed in your plant ? It is because we simply abuse the use of our Preventive Maintenance system. My experience tells me that activities on our PM checklists continues to grow by the day due to deviations from audits, customers, quality control or simply important people in your plant who visited your operations. We must understand that the lesser the amount of PM being performed on your equipment, the better off your equipment will be. Intrusive or forced maintenance will only create havoc in our operations, what is important is that each of our people understand and realize the lessons from Nowlan and Heap and understand that in the real world of maintenance there are not 1 but 6 types of failure pattern, and each part or component will fail according to the 6 failure pattern. Not all parts will wear out, most parts will fail randomly. Therefore, if we are encountering random failures PM is not the best option to use. Remember that Preventive Maintenance was designed around the theory that equipment failures are directly related to the age of the equipment. In reality only around 20% of equipment failures fit this pattern and that 80% of equipment failure are not being effectively managed by doing time-based or scheduled Preventive Maintenance Activities.
CONCLUSION ABOUT PREVENTIVE MAINTENANCE
It is important to understand that Preventive Maintenance have its own unique limitations and understanding when to use and not to use it will derive the most benefit from its application. When applied correctly PM will increase equipment’s uptime, however, if we abuse its use, then it will cause us more harm by being reactive instead of being proactive in our assets. Preventive Maintenance is feasible when the part or component will wear out directly with respect to age and its usage will definitely survive these defined age.
I think we need to resound the learning’s from Nowlan and Heap which they discovered in 1960’s since it will create a lasting impact if we really absorb and apply the learning’s, that the lessons they provided us are of great value for every person whose responsibility is to take care of their equipment and assets. There is no substitute for training since training is where we acquire these knowledge and that whatever learning we gain should be shared among our people.
48 years had past, yet most industries I’ve been through have really not absorbed the learning. What Stanley Nowlan and Howard Heap had discovered is not about rocket science or silver bullet but rather understanding that Preventive Maintenance have their own limitations and that we must not take this for granted.
It is my hope that these newsletter will serve as an eye opener so that we can reflect for a while and think that the first step in improving reliability is to acquire the correct knowledge on how to maintain our equipment’s correctly. Preventive Maintenance will always have a place in the reliability world if we understand how and when to use them correctly.
References : Reliability-Centred Maintenance by John Moubray
Reliability-Centred Maintenance Report by Stanley Nowlan
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