A Question and my Response on MTTF
So this corrosion engineers walks into NoMTBF and send me a message.
Hi, I am corrosion engineer. May be you know for risk assessment of heat ex-changer tube bundle in API-581 , mean time to failure (MTTF) term is defined and used for risk assessment.
Would you please give me more information about MTTF and what history data required to calculate MTTF?
Thank u so much
First off MTTF and similar metrics are used for situations with a constant failure rate. Meaning that every hour a piece of equipment has the same chance to failure as any other hour, anytime.
This is generally not true and certainly not true for corrosion failures. When the right conditions exist, corrosion starts, grows and eventually over time leads to failures. The older the equipment the more likely it will fail due to corrosion, thus not a constant failure rate.
My advice is to avoid using MTTF or MTBF.
I would take a look at the models and data you have and use Weibull or other life data distribution to model the time to failure. From there you can convert to MTTF although it will not be meaningful during the first half of the lifetime generally by a wide margin.
I would ask the risk analysis folks what time frames they need failure rate information and provide estimates suitable for each time frame. An overall MTTF is pretty misleading and may alter the risk assessment results.
If you’d like to talk about better ways to work between reliability and risk assessments, let me know.
Just dawned on me I didn’t answer your question.
The data you need for the calculation is the total hours of operation of the equipment divided by the number of failures – pretty simple. So if you have 100 pumps, and all but one runs for 100 hours. The one fails at say 50 hours. Then the calculation is (( 99 x 100 ) + (1 x 50)) / 1 failure = for mttf of 9950 hours.
If there are no failure, still tally operating hours and divide by one (rather than zero when bad things mathematically occur).
- Be sure what you are measuring and reporting using MTTF actually has a constant failure rate or close enough to constant that it doesn’t matter.
- Send over your questions and maybe become the next NoMTBF blog post.
Third and most important, if you have a NoMTBF button or mug or whatever, please take a picture of it in the wild and send over. Also send along a short note on how it has helped start conversations around MTBF.
I’ll create a page on the site showcasing the ways these devices are starting conversations.