Many plants have a condition assessment program in place; usually those programs operate in relative isolation, concentrating on only one or two technologies. The people responsible for them work to maximize the efficiency of the application of the technology. Therefore the application of the technology is optimized, rather than the results.
A machine-centered, as opposed to a technology-centered, approach to condition assessment will maximize the effectiveness of technology in improving machine reliability. This approach focuses on those tests and tasks that are cost effective when it comes to machine reliability.
Background: A plants condition assessment program might include vibration monitoring, oil analysis and thermography. And they may also have preventive maintenance tasks calling for routine overhaul of some machines based on running time or calendar time. This has the potential to make major contributions to the reliability of the machinery. But usually, these various sets of data never meet. And the routine overhaul - it's not effected by collected data. So the payback is never fully realized.
Let's consider the typical vibration program. After some research, a cost justification is made and approved to purchase vibration equipment and software. Then one or two technicians are trained and designated to manage the program. They are told to make the vibration program run. In the absence of any measure of cost-benefit, they make the decision to apply the vibration to as many pieces of equipment as possible. From their perspective, it's a smart move: it spreads the cost of equipment and training over as many pieces of equipment as possible, minimizes cost per measurement, provides a full work load, and keeps the equipment in use. It optimizes the individual technology program.
Is this the best strategy to improve machine reliability? Would you be happy with your doctor if on your annual physical he only tested your pulse rate? And then maybe he makes a decision on whether to do surgery based on that? (That pump overhaul is surgery!) Probably not! You'd like to see him make a number of tests - blood work, EKG, chest x-ray, etc. Then he'll get a complete picture of your health. And have a lot better basis to make a decision on surgery.
The same principle applies to machinery. To get a complete picture of machine health, you need to run a number of tests. And when that PM for overhaul (surgery) comes up, you can make an informed decision on whether to perform or defer it.
Condition assessment involves a lot more parameters than just vibration, oil condition and IR. Process parameters such as temperatures, pressures, flow rates and operating speed all have things to tell us about health of our machinery. For example, suction and discharge pressure of a pump, along with motor amps, RPM and flow rate will give a good indicator of impellor condition. Based on those, you may decide to defer that pump overhaul for another year or two. That's a big savings.
If you have a technician going to a machine to collect data for one technology, why not collect all the data you need? Instead of just vibration, how about trending bearing temperatures, fluid pressures, RPM and other parameters that contribute to a complete picture of the machines health. It means that more time will be spent at each machine, and fewer machines will be assessed in a day. But you have much more valuable information. You will also save transit time, prep time, and administrative time associated with multiple trips to the machine. And you'll save time by just applying a technology to those machines where it's cost effective. You haven't optimized the technology, but you have optimized the machine's healthcare. And isn't that what we really want?
Proposal: I want to propose an approach that's not new or unique. Many savy maintenance managers have done it for years. Reliability Centered Maintenance formalizes it. I call it Machine Centered Healthcare.
I believe that Reliability Centered Maintenance is the best approach for critical machines. But not every plant can afford, can get approval, or has the manpower for a Reliability Centered Maintenance program. It's expensive in the short run. I'm proposing a thought process that will help you decide how to maintain your machines in a less formal manner with less paperwork than reliability centered maintenance.
A machinery-centered approach looks at the machine first, and by asking a series of questions, helps you decide how to maintain the machine's health. What tests should be done? What routine PM should be done? How can we make the overhaul/no-overhaul decision?
First Ask, What Are The Possible Failures?: To ask what the failure are, first we need to know what the machine is supposed to do. What is its primary function? At first glance, you might say that a pump's primary function is to pump a liquid. In reality, its primary function is to keep a supply tank full. As the process draws liquid from the tank, the pump replaces it. If the pump can't pump at a sufficient rate, the supply tank will go empty. That minimum rate will vary from process to process. Look beyond the obvious to the real function of the machine.
Once you've decided what the machine's function is, ask what can happen to prevent it from meeting that function. In the case of the pump, the answer might be the impellor wearing out reducing available head, bearing failure causing low RPM, a crack in the casing or worn-out seal causing liquid to be lost reducing flow, or a number of other possible failures. At this point, you're just brainstorming. Don't consider whether the failure is likely or has much impact. We'll do that in the next step. For now, just get a complete list.
Next Ask, Which Of These Failures Are Significant?: Now that you have a list of possible failures, you want to decide which ones you should worry about. Some failures are so unlikely that you won't worry about them; others have such a low consequence that their impact and cost is minor.
Machinery history is the best way to determine how often a failure occurs and what its impact is. You do have one, don't you?
However, we can do it without the history. I've had success in the past using a subjective evaluation. Make a list of the failures and ask two questions: how often does this occur and what's the impact on production when it does. Make it up as a questionnaire. Possible answers are in Table I below. This may sound simplistic but it works.
Now send the questionnaires to a cross section of maintenance, production and management personnel. When you get them back, average the scores for each item.
The significance of a failure is the combination of two factors: frequency and effect. By taking the score for frequency score (1 to 5) and multiplying it by the score for effect (1 to 5) you'll get a composite score for each failure in the range of 1 to 25. Rank the list by the composite score. The higher the composite score, the greater the significance of the failure.
Now you have to make a judgment call - which failures should you worry about? Often, only a few will have a high rank and you can concentrate on them. Other times most will have a high rank. This is where your knowledge of the machine and professional judgment come into play.
Next Ask, How Can We Avoid These Failures?: Starting at the top of the list, ask "how can we avoid this failure?" Is there some action we can take that will keep the failure from occurring? Can we change the design? Can we replace a part that has a predictable wear-out period? Can we adjust or lubricate to avoid failure? The list you make here should be the start of your preventive maintenance list for that machine.
Then Ask, When We Can't Avoid Failure, How Can We Get An Early Warning?: There will be some failures that we can't avoid. For those, we ask "How can we detect the failure before it occurs?" What are the symptoms of the failure? Most failures show symptoms before they happen. A pump may have to be run faster because of a worn impellor. A motor may draw more amps because of misalignment or a seal that is too tight. A coupling may be hot because of misalignment or lack of lubrication. Make a list for each failure.
Then, Tailor A Suite Of Tests To Detect Those Early Warning Signs: With a list of symptoms, you're now in the position to select tests that measure or detect that symptom. For each symptom, try to get as many independent tests as possible. The more information you have, the more confident you'll be in your call. You should have at least two tests for each failure that can confirm each other and avoid false positives (or negatives).
As you're considering tests, don't limit yourself to high tech methods. Process parameters are also valuable. And one of the most valuable tests is the operator and maintainer. An experienced person, familiar with the machine, making a conscious effort to sense a particular effect, can be very effective at assessing the health of a machine.
Finally, Collect The Results Of The Tests At One Decision Point: Doing the tests without putting all the information together is not effective. I recommend that each machine have one or two individuals assigned to monitor its health. They should be trained in assessing all the information provided by the tests. Notice I didn't say, "trained to evaluate the data". They don't have to analyze the data (vibration spectra); they just have to understand the results (information) of that analysis.
They should receive the results of the tests along with any other pertinent information on a regular basis. Then they can use that information to manage the machine. They can use it to adjust lubrication intervals, decide when adjustments are needed or part replacement is indicated. And that overhaul? They may decide it's not needed after all.
Summary: Condition assessment programs are often structured to optimize the application of the technology. This spreads the cost of equipment and training over as many pieces of equipment as possible, minimizes cost per measurement, provides a full work load, and keeps the equipment in use. But it means that we may be spending time and resources taking data that is not particularly valuable in improving reliability and reducing costs. And the information derived does not get integrated into overall machine healthcare decisions.
What we want to do is to maximize the effectiveness of the technology in improving machinery reliability. We need to assess machine health based on several measures. And we should only do those tests and tasks that are cost effective from the point of view of the machine. The question is, how do we decide what to do? I propose we follow a systematic process to identify that.
In summary, the process is:
1. First ask, what are the possible failures?
2. Next ask, which of these failures are significant?
3. Next ask, how can we avoid these failures?
4. Then ask, when we can't avoid failure, how can we get an early warning?
5. Then, tailor a suite of tests to detect those early warning signs.
6. Finally, collect the results of the tests at one decision point.
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Editors note: Our thanks goes to James for this article - as the first a what we hope is a long series of common sense "how to" articles. Stay tuned. - Terry O