Many plants have a condition assessment program in place. But 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 your effectiveness in improving machine reliability. This approach focuses on those tests that are most cost effective when it comes to machine reliability.
Let’s consider the typical vibration program. After some research, a cost justification is approved for purchase vibration equipment and software. Then one or two technicians are trained and designated to manage the program. They’re 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, it minimizes cost per measurement, it provides a full work load, and it keeps the equipment in use. They’ve optimized the individual technology program.
But 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 to do surgery based on that? 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. Then you’ll have complete information to manage the machine. And when that PM for overhaul, which is surgery, comes up, you can make an informed decision to perform or defer it.
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 and fluid pressures. RPM and other parameters also 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 more valuable information. You’ll 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. Isn’t that what we really want?
I want to propose an approach that’s not new. Many maintenance people have been doing it for years. It’s formalized by Reliability Centered Maintenance. 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 RCM 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?
What we want to do is to maximize our effectiveness in improving machinery reliability. We need to assess machine health based on several measures. And we should only do those tests 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: First ask, what are the possible failures? Next ask, which of these failures are significant? Next ask, how can we avoid these failures? Then ask, when we can’t avoid failure, how can we get an early warning? Then, tailor a suite of tests to detect those early warning signs. Finally, collect the results of the tests at one decision point. Lets look at it in detail.
To ask what the failures 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 may be 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.
This is a partial table of the possible functions of a motor-drive system. Your particular systems may have different functions. Any machine may have both primary and secondary functions. Be sure to consider all of them.
This busy table is a partial list of the functions, functional failure and the failure modes that may cause them. It’s the starting point for looking for symptoms but because it’s so long, we need to trim it to a more manageable size. That’s the next step.
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. Use the failure data and cost data to rank the failure modes. 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 about each one: how often does this failure occur and what’s the impact on production when it does. Make it up as a questionnaire.
Possible answers are in this table. 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 and multiplying it by the score for effect 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.
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.
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 low supply voltage. A coupling may be hot because of misalignment or lack of lubrication. Make a list for each failure.
With a list of symptoms, you’re now in the position to select tests that can 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.
This table shows some of the failure modes, their causes and the symptoms they might present in a motor. It also shows some possible tests. You want to develop a similar table for your machines. After you’ve finished doing this, you have a list of the tests needed..
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 (for example the vibration spectra); they just have to understand the results 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.
So in summary, the process is: First ask, what are the possible failures? How can the machine file to meet the function it was put in the system to perform. Next ask, which of these failures are significant? Don’t ignore the small failures. They add up. Next ask, how can we avoid these failures? Design changes, part replacement, etc. Then ask, when we can’t avoid failure, how can we get an early warning? What symptoms will the failure present. Then, tailor a suite of tests to detect those early warning signs. Use both high tech and low tech methods. Finally, collect the results of the tests at one decision point.
If we do all of that, we should do this – Optimize the machines healthcare. Thank you for you attention. I’ll be happy to take any questions or to listen to any comments.
Six Steps to aHealthy Machine Jim Taylorjim.email@example.com 765-366-4285 www.machineryhealthcare.com
BackgroundTechnology Centered: Machine centered: Optimizes individual Optimizes machine technology program health spreads cost over as Provide all needed many pieces of information to assess equipment as possible machines health Minimizes cost per Decide what PMs actually measurement improve or maintain machines health Provides full workload Family physician Keeps instruments in model use
Technology CenteredSpreads cost over as manymachines as possible Minimizes cost per data point Maximizes utilization of test equipment Provides evidence of full work load to supervision Each Technology Is Optimized but Machine healthcare is sub-optimized
Is This the Best Way?Would you be happy with your doctorif on your annual physical he onlytested your pulse rate? And then sent you out to contract your own blood work and interpret the results? Then based on that limited information, he makes the decision to do surgery. A pump overhaul is surgery!
Failure after breaking system boundry 2000 1800 1600 1400 1200Failures 1000 800 600 400 200 0 < 1 wk 1 – 2 wk 2 – 3 wk 3- 4 wk 1 – 2 mo 2 – 3 mo >3 mo Time after ov erhaul Study by Corio & Costantini
Application to Machinery Healthcare Don’t hurt me!!!To get a complete picture of machinehealth, you need to run a number oftests. You’re more likely to catch something early. And when that PM for overhaul (surgery) comes up, you can make an informed decision on whether to perform or defer it.
Advantages of a Machine Centered ApproachCollecting complete data on each trip to the machine • Less machines per day • More valuable information Lets you do 2 things: • Manage failure • Defer routine overhauls Optimize Machines Healthcare
Machine Centered ProcessIt’s nothing new Reliability Centered Maintenance formalizes itBut the fact is not everyone can do RCM • You can’t afford it • You don’t have the manpower • You can’t get approvalBut you still have to maintain the machine
Machine Centered Thought Process Which of How can What are the these we avoid possible failures are these failures? significant? failures? When we cant Collect allavoid failure, how Tailor a suite of tests to get information at one can we get an decision point. early warning? early warnings.
First ask: What are the possible failuresThink about the function of the machine.How can it fail to meet that function?
System BoundariesPick yourboundaries soyou have amanageableproblem
Motor-Drive System Functions Function Downstream (Load side) Start motorFunctions Stop motorof a Motor Deliver specified torque atDrive specified RPMSystem Specified speed ramp rate up Specified speed ramp rate down accelerate load from stop to operating speed
Function Functional Failure Mode FailureFunctional Start motor Motor will not turn winding failure (stator)Failure of a Insulation Failure (stator)Motor Drive Rotor failureSystem Bearing Seized Contactor Failed Loss of Power VFD Malfunction (Start) Stop Motor will motor not stop
Next ask:Which of these failures are significant?How often it happens - frequencyWhat’s the impact when it does - consequenceRisk = frequency x consequence
Criticality SurveyScore Frequency Effect 1 1/10 yrs None 2 1/ yr A little 3 1/ month Some 4 1/ week A lot 5 1/ day Complete Safety goes to the top
Then ask:How can we avoid these failures? Adjust, lubricate, … Preventive replacement Design changes
When we cant avoid failure, ask: How can we get an early warning? Process parameters Inspections Technology
Then:Tailor a suite of tests to get early warnings? • Only do the tests needed • Don’t test just because you can Appendix A to the paper has a partial list of tests & technologies
Motor Failure Failure Failure Symptoms Measurement Mode Causeswinding Conductor vibration > ips vibration failure failure monitoring (stator) Various MCSA Various MCE excessive vibration > ips vibration vibration monitoringInsulation Breakdown Polarization index Failure (Stator) R to gnd < ohms Megger excessive temperature > °F thermometer current amperes > A ammeter voltage spike power quality monitor excessive Motor temperature > thermometer temperatu °F re Ambient temperature > thermometer °F
Finally:Collect all information at one decision point. Most important step!
Machine Centered Thought Process Which of How can What are the these we avoid possible failures are these failures? significant? failures? When we cant Collect allavoid failure, how Tailor a suite of tests to get information at one can we get an decision point. early warning? early warnings?