Key points
Many can logically figure out what to inspect on equipment. But, there seems to be great confusion around how often equipment needs to be inspected in our plants and mines. How often should a motor be inspected? A pump? A hydraulic cylinder? How does one even go about deciding the inspection frequency?
Some plants I visit inspect the equipment every shift, some once a week, some monthly, some not at all. This large variation tells me there is a lot of confusion out there.
Does Criticality Decide the Inspection Frequency?
Different equipment obviously needs different inspection frequency, but the question is; what should the inspection frequency be based on? Many say that the inspection frequency should be based on criticality of the equipment. But, if we think about that for a second, it doesn’t make a whole lot of sense.
For example, if you use a criticality scale 1 to 4, and a gearbox has the criticality “2”, what is the inspection interval? If a motor has a criticality 4 what is the inspection interval? There is no correlation between a criticality number and inspection frequency. The 2 or 4 does not provide any guidance .
You may now think that it is not a hard number, rather a comparative scale. For example, an equipment with criticality 1 should be inspected more often than equipment with criticality 3. But the foundation of a pump may have criticality 1. Let’s say it is important, but the function of a base is not likely to fail very fast.
So, intuitively we understand that it makes little sense to go out and inspect a concrete foundation every shift. So, let’s ask, why doesn’t it make sense to inspect a concrete foundation three times a day, or even every week? It doesn’t make sense because a concrete foundation will fail very slowly. The time lapse starts from deterioration until it can’t function anymore is long for a foundation.
The Failure Developing Period
The “time lapse” between start of deterioration (We call this “failure”) until breakdown is called “Failure Developing Period (FDP). I would argue that the FDP for a concrete base is many months, for simplicity, I’m going to make an educated guess of 1 year. If the FDP is one year, I’ll divide that by two (2) to get my inspection frequency.
By using logical thinking, experience and in some cases, data, we can make an estimate of the FDP. We want to divide the FDP by two to make sure we have an inspection before a breakdown and also have enough time to plan and schedule the corrective maintenance before the break down. Using the FDP, there is a logical reasoning path to decide the inspection frequency of equipment. So, how would this work in reality, let’s look at an example.
Preventive Maintenance of an Electrical Motor, an Example.
Let’s look at a typical AC Motor. This example does not include all failure modes, but it includes the most common symptoms for a failing AC motor. In my opinion, that is all you need to document your preventive maintenance (PM) 99% of the time and it will make the PM setup much faster. A bearing has around 50 failure modes, but only a few can be detected. Corrosion in a bearing is a failure mode, but can’t be detected, only a damaged bearing can be detected, so the failure mode “corroded internally” is excessive for PM purposes.
Our Example: AC Motor, 125 HP, 80% load, 24/7 operation, dusty environment.
| COMMON PROBLEM | GUESSTIMATED FDP | INSPECTION & FREQUENCY |
|---|---|---|
| Temp increase center of motor due to overload or damaged winding | weeks | Temp gun weekly |
| Vibration in bearings | 6 -12 weeks | Vibration analysis every 2-3 weeks |
| Dirt buildup on motor | 1 month | Check/clean biweekly |
| Bolts loose | 1 month | Inspect bolts biweekly |
| Frame and foundation for corrosion | 1 year | Visual detailed inspection semi-annually |
| Temperature increase inboard bearing (can’t get good temperature reading on outboard bearing) | 2 weeks | Inspect IB bearing with IR gun weekly (don’t exceed 170ºF; 77ºC) |
| Electrical junction box and cables | 1 month | Biweekly |
| Noise from bearings, winding, overload, etc. | 1 week | Other tools above will pick up source of noise earlier, recommend weekly. |
| Immediate damage such as forklift run in, something falling on motor | instant | Can’t catch problems early without an FDP. |
| Increase in load (A) | 2-4 weeks | Weekly Current (A) reading |
| etc. | etc. | etc. |
As mentioned above there are 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.
The FDP is estimated based on experience. But we must realise the FDP changes depending on:
- The type of tool we use to inspect with (Vibration analysis can catch problems earlier than listening to the bearing with a stethoscope.)
- The environment, a dirty environment could shorten the FDP, in extreme cases even the FDP for a base could be milliseconds if it is hit by a forklift truck.
- How the equipment was installed. An aligned, well-balanced motor will have a longer FDP.
- How the equipment is operated. A motor that is started multiple times an hour may have a shorter FDP that a motor that isn’t.
- The design parameters. A high load, high speed motor will have a shorter FDP than a low load, low speed motor.
Having said all that, we still have a logical approach to deciding the inspection frequency, even if it is an estimate.
Making the Inspections Practical
The inspections above have many different intervals and are executed by different personnel. It wouldn’t be practical to make a PM program with all the different intervals and inspections, so let’s consolidate a bit.
I would 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 near the equipment. They don’t take too long time to do so if they are done every week or every other is not going to make a big difference.
In this example we could group them as follows in a typical process plant environment:
| Sample Checks on an AC Motor | |
| Weekly | Temp IB Bearing Temp center Motor Vibration pen at painted spot Check Cleanliness of Motor Look at condition of junction box and cables Visually look for water on motor Check fan with stroboscope Listen for unusual noise |
| Monthly | Measure Amps Vibration analysis with Analyzer (different than pen above) |
| 6 months | Carefully check the base (steel) and foundation (Concrete) |
Other Preventive Maintenance Inspections
If it is a critical motor perhaps you want to do a full motor analysis or a test of leakage to ground.
Wait, What About Equipment Life?
The life of a component has nothing to do with inspection frequency. For example, a world-class plant has an average motor life of 18 years, some motors last 8 years some 25. However, the FDP for the most common failure modes for these motors is most likely in the 1-4 week span, so the life span has nothing to do with inspection frequency.
But there isv some equipment that may be overhauled or replaced on a fixed time basis because we know the life. There are not many components that have a known life span, but an oil, water, or air filter is often changed on a fixed time basis. There are some wear items that are changed on a fixed time basis. But motors, bearings, pumps, gearboxes, cylinders all have varying life spans.
But Criticality Must Have Some Influence
Practically, I think criticality have some influence, but it isn’t the basis for the decision on inspection frequency. I think of criticality as “additional insurance”. If the estimated FDP is 1 month and the inspection frequency is 2 weeks, I may decide to inspect a critical asset weekly. The reason is all the uncertain factors mentioned above around how it is operated, installed, designed, etc. A high criticality may inspire me to inspect it more often.
But, the foundation is still the estimated FDP.
IDCON is a reliability and maintenance management consulting and training firm
