How to Set Frequencies for Preventive Maintenance Inspections

Tor Idhammar, IDCON INC
Tags: preventive maintenance, maintenance and reliability

How to Set Frequencies for Preventive Maintenance Inspections

For those of you who want to indulge in the details of setting preventive maintenance (PM) inspections, let’s begin with defining what I mean by inspections. Inspections include all objective inspections (we measure something) using an instrument – for example, a vibration analyzer, an infrared camera, a volt meter, a flow meter or ultrasonic equipment. Inspections also include all subjective inspections (look-listen-feel-smell). In order to set inspection frequencies, we need to understand what a failure-developing period is.

The failure-developing period (FDP), also called the Pf Curve by some, is the time period from when it is possible to detect a failure until we have a breakdown. A failure is when a system or piece of equipment is operating correctly within given parameters, but it shows signs of problems.

For example, a centrifugal pump may be cavitating, but it is still providing the required flow for the operation. We have a failure but not a breakdown. The cavitation 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 breakdown. If the pump started to cavitate at 6 a.m. and it broke down at 6 p.m. four days later, the FDP is 108 hours.

Inspection Frequency

The inspection frequency should roughly be FDP divided by two. For example, if the failure-developing period is 14 days and we need some time to plan and schedule the corrective maintenance for that failure, I think a reasonable inspection frequency is seven days (FDP/2). If the inspection frequency is longer than 14 days, we may miss the failure and will have a breakdown on our hands. So our rule of thumb is:

 Inspection frequency = FDP/2.

However, the real problem is that we don’t know what the FDP is. There is no standard, no documentation. Most plants usually have no history on FDP. So what do you do? Let’s dig deeper with some additional information.

Inspection Tools Change the FDP

We also need to understand that the 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 by putting a screwdriver to our ear (and the bearing). This method may give us a warning period of a few days (on average, depending on situation).

However, if we buy a vibration analyzer, we can probably detect the same failure at least six 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 with accuracy.

In reality, the ability to detect a failure and the FDP also depends on:

Many Variables

Each component has many failure modes, and each failure mode has different FDPs. We also know that each FDP may change depending on the inspection tool, technique, the person doing the inspection and more. On top of all this, each component is running at different speeds, different loads and in a different environment. Everything is different. Now we are in trouble.

At this point of reasoning, many plants go the wrong way, in my opinion. Some plants make the conclusion that a massive study needs to be done in order to find the answers to all of these questions. Why is this not a good approach? It is not because in 999 times out of 1,000 you will not have the data you need to do the analysis.

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 data is a wild, somewhat educated guess with a lot of work. So let’s not do the complicated analysis and instead do a wild, somewhat educated guess using our experience and cut out 99.9 percent of the work? Does that sound good to you?

A Wild, Somewhat Educated Guess

Let’s look at some typical problems with an AC motor. This is far from all failure modes. For example, if you look at the SKF manual, a bearing has more than 50 failure modes. Therefore, we need to look at the common problems.

Example: AC motor, 125 horsepower, 80 percent load, 24/7 operation, dusty environment

As mentioned above, there are many more failure modes. I have picked some common problems to illustrate my point.

If we look at the right column, there are many different inspection frequencies, even when we do a simplified analysis. Our estimates are just guesswork and will vary as a result of who is doing the inspection, the type of tool and the environment, so we should not take the numbers too seriously; they are estimates.

I would, therefore, look at some of the shorter inspection intervals and then add some of the longer-interval inspections to those since we may as well do the longer ones when we are there. They don’t take too long to do, and we are just guessing on the intervals.

When I look at this AC motor, I would group them as follows in a typical process plant environment:

Note: Preventive Maintenance and Condition Monitoring Standards are sold by Noria.

Other Inspections

If it is a critical motor, perhaps you want to do a full motor analysis or a simple leakage to ground. I have yet to have a vendor be able to explain what the estimated FDP is using their tools for the above inspections.

Common Logical Error

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 may have an average motor life of 18 years. Some motors last eight years and some 25. However, the FDP for these motors are most likely in the one- to four-week span, so life statistics have nothing to do with inspection frequency. A common erroneous argument is “we have inspected this component for three years and have not found any problems.

Therefore, we extend the inspection frequency from one week to two weeks.” The fact that you have not found a problem has nothing to do with the FDP. It hasn’t changed just because the component is running well. Once that component fails, it may be after 15 years, the FDP may still be two weeks and you need to catch it. If you change the inspection period to two weeks, there is roughly a 50 percent risk that you will miss it.

Criticality does not affect the FDP but is practically a factor when we assign inspection frequency.

Our AC motor bearings are equally critical to our foundation for the operation of the motor. If either fails, the motor stops. However, the FDP and the inspection frequency are different because we base the inspection frequency on the FDP.

The criticality of the motor may change the selection of the inspection frequency because we are uncertain of the FDP. The FDP is a guess. So a very critical component may be checked more frequently because we don’t really know the FDP. It is an insurance policy.

In summary …