What’s the right Inspection Frequency for Equipment?
If an inspection frequency that is too short, equipment will be over maintained, and resources are wasted. If it is too long, some failures are going to be missed.
How often do you need to inspect equipment? The short answer is that it will vary depending on component, operating context, environment, and load. But you must understand how to estimate the correct inspection frequency. It is not based on criticality, not the amount of resources you may have at your plant, and it is not based on life of the equipment.
Let’s start with defining what we mean by inspections. Inspections include all objective and subjective inspections.
Objective inspections (we measure something) by observation or use an instrument. Instruments can include vibration analyzer, infrared camera, voltmeter, flow meter, or ultrasonic.
Subjective inspections are those look-listen-feel-smell inspectionsIn order to set the frequency of your preventive maintenance inspections, you need to understand what Failure Developing Period (FDP) is.
Failure Developing Period (FDP) (or Pf Curve as many calls it)
The FDP is the time period from when it is possible to detect a failure until breakdown occurs. A failure is when a system or equipment is operating correctly within given parameters but has signs of problems.
For example, a centrifugal pump may be cavitating, but is still providing the required flow for the operation; this is a failure, but not a break down. The cavitations in our example will eventually develop into a breakdown. The breakdown occurs when the pump is unable to perform its intended function.
The FDP is the time difference between the failure and the break down. If the pump started to cavitate at 6 am and it broke down 6 pm 6 days later, the FDP is 156 hours.
So, what’s the Inspection Frequency?
The theoretical answer to the question is very simple. The inspection frequency should roughly be:
For example, if the estimated failure developing period is 14 days and we need some time to plan and schedule the corrective maintenance for that failure to avoid a break down. A reasonable inspection frequency is 7 days (FDP/2). If the inspection frequency is longer than 14 days, we may miss the failure and we will have a breakdown.
Inspection Tools changes the FDP
FDP changes when we have access to better tools. For example, we may be able to detect a problem with a pillow block bearing by listening to it with a stethoscope. This method may give us a warning period of a few days (on average depending on situation). However, if we use a vibration analyzer, we can probably detect the same failure at least 8 weeks in advance.
The failure is the same, but the FDP has changed! For the most part, the only reason we buy inspection tools is to extend the FDP.
In reality, the ability to detect a failure during the FDP also depends on the person’s ability to do the inspection, environment (lighting, temperature, indoor vs. outdoor, etc.), and operational parameters at the time of inspection, equipment design and accessibility, and much more.
Too many variables
Some variables that trip up many plants when calculating the FDPs are:
- Each component has many failure modes and each failure mode can have different FDPs.
- FDP may change depending on the inspection tool, technique, the skills of the person doing the inspection, and more.
- Each component is running at different speeds, different environment and different load.
- All these variables inevitably lead many plants to do the wrong thing…start a massive study to find the answers to all these variables for each component.
Why is a massive study not a good approach? I mean all you have is time, right?
This is not a good approach because in 999 times out of 1000, you will not have the data you need to do the analysis, and even if you did, the best bang for the buck is usually to get your people trained and then out there doing inspections rather than performing a big analysis.
What you will end up with when you do a complicated analysis without accurate data is a guess based on many small guesses that will take A LOT of work. So let’s not do the complicated analysis with poor data and instead do an educated guess using our experience and cut out 99.9% of the work.
Let’s look at some typical problems with an AC Motor. Note! this example does not include all failure modes, for example, if you look at bearing manufacturer manual, a bearing has over 50 failure modes. So instead we need to look at most common and most likely problems.
If it is a critical motor perhaps you want to do a full motor analysis or a test of leakage to ground.
Common Logical Error
Preventive Maintenance Inspection frequencies are based on FDP, not life of component, nor the criticality.
The life of a component has nothing to do with inspection frequency. For example, a world class plant had an average motor life of 18 years, some motors last 8 years some 25. This gives you no suggestion as to what the inspection frequency is, the failure is random, as it is for over 90% of components.
However, the FDP for the most common failure modes for these motors are most likely in the 1-4 weeks span, so life statistics has nothing to do with inspection frequency. There is no logical connection.
A common erroneous argument is “we have inspected this component for 3 years and have not found any problems”. Therefore, the inspection frequency is extended from one week to four weeks. Just because you have not found a problem has nothing to do with the FDP, it hasn’t changed just because the component is running without any indications of a failure.
Once that component fails, it may be after 15 years, the FDP may still be two weeks and you need to catch it if it is financially viable to do so. If you change the inspection period to four weeks, it is roughly 50 % + risk that you miss it.
Criticality does not affect the FDP, but it might be a factor when we assign inspection frequency.
The criticality of the motor is a deciding factor when estimating the financial aspects of a break down and the precautions taken. But, the criticality in itself will not help you decide the inspection frequency. For example, let’s say you have a simple criticality ranking of equipment of 1-10 and one equipment has the criticality 5, what is the inspection frequency? A day, a month, every shift? There is no logical connection between inspection frequency and criticality.
But, shouldn’t we inspect critical equipment more often? Perhaps, but again, you can’t find the time interval by knowing the criticality. First, figure out the FDP, then if the equipment is critical, go conservative, if it’s not critical, be liberal. Think of criticality analysis as add-on “insurance” to the FDP.
To sum up this article:
Inspection frequencies are based on FDP, not criticality or component life.
The FDP for all failure modes is quite unfeasible and impractical to predict. However, we can make a pretty good guess to what it is.
• If you don’t have very good historical data as to what the FDP is, don’t waste your time making an elaborate study, make a reasonable guess, it is what you will end up with anyway with a study without reliable data.
If you have the FDP data, ask if it is better to spend the effort in training people in how to do inspections and planning and scheduling of corrective actions instead of making an outsized study. It is much more cost effective to spend the time on making the execution of good inspections a reality.
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