Hi Tammy,
When you have a trip, it means that something is not right. Its clear that if the machine (its components/ parts/...) fails, you will have to consider the trip and include it in your MTBF calculations. But, the interesting part of your question is, what if the machine trips on an external cause?! I would say you should look at two things before deciding to include or exclude the failure in your calculations:
1) Machine and its components are working within the designed criteria and an abnormal condition (outside design boundaries), activated the interlocks, affected the sensors or even the integrity of the equipment and mad it fail. Examples could be: flooding a pump which is not designed to work submerged, an ambient temperature of 130F for a machine which is designed to work in a max 90F ambient temp . In these cases the machine trip should not be considered a failure of the machine, but I'd agree to have it categorized as a design failure to some extent. I would not include this trip in MTBF for the machine, but for sure I'll start digging around and look for similar design caused trips and document them; at the end you may need to redesign some of the equipment (if you see a common track of failures as a result of design)
2) Machine and its components and controls system tripped due to the implemented logic or as a result of not meeting the designed criteria for the operation. This is a little different with the previous category I mentioned above. In this case, lets pick the example you had, The inlet Air temperature is higher than it should be and activates the interlock and trips the machine. If the higher temp of the inlet air is a result of the machine's operation, then you need to figure out what caused that and perhaps the machine was the source of the trip and should be included in the MTBF calculations. But if the machine is just getting air from another source and the hot air is actually coming from another source (machine/HEX/...) , then your machine and its control system did what they supposed to do, and the trip was a failure of meeting the operation design criteria at somewhere else. I would not include this trip in the machine's calculations, but will and should do more investigation to find the root cause of the high air temp (which can be a design issue in another part of the system or a failure in operation of another component of the system-which in this case you need to consider it in the calculations for that system).
There is a fine line between the examples I mentioned above, and not always that clear. But in a nutshell, if the failure is within the machine/process boundaries, you have to include it in the calculations either for the subjected machine or find the real source of the failure of that machine outside its boundaries.
To sum this up, if you have a failure/trip, that means that something is not working as it should; as a reliability professional, you'd need to find that root cause and address it, no matter how you include it in the calculations.
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Yaser Sahebi PhD
Air Liquide Large Industries
Houston TX
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Original Message:
Sent: 06-22-2021 10:07 AM
From: Tamunoteyim Karibo
Subject: Trips and Failures
Hello M&R professionals,
Hope everyone is doing well.
I have two interesting questions.
1. Are equipment trips considered as failures?
Let's say, for example, equipment trips on High inlet air temperature. Is this considered a failure?
2. Will these trips be considered when calculating the MTBF of an equipment if no action was taken to bring the equipment back online? It was just restarted after inspection.
Thank you for your response.
Tammy
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Tammy Karibo
Reliability Engineer
Enerflex
Abu Dhabi
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