MTBF – Mean Time Between Failures

What is detectable – controllable?

Statistical Distribution, Maintainability,
response to Condition Based Maintenance, inspections and monitoring

What is acceptable

This Standard is defining which defects to tolerate and which ones you should stop from developing.

It lists “Defect Symptoms“, describing practically all defects occurring at the equipment of any production plant.

When certain attributes had been added (following list column (b) in parentheses), you can also standardize capacities regarding where they are tolerable and how they can be controlled.

This standard represents the result of analyzing several hundred thousands of defects occurring in industrial plants of nearly any kind.

Legend:

“Defect Symptom” describes what you see when looking at a defective element.

Column (a): Digits in front of the dot summarize similar appearance; digits following the dot identify differences inside one appearance, resulting from differences of the following columns.

Column (b): Verbal description of Defect Symptom.

Column (c): Factor in the Weibull Equation indicating the form of the statistical distribution: “1,0” indicates random distribution; “1,5” indicates logarithmic (non-symmetric) normal distribution; “2” and higher indicate symmetric (Gauss) normal distribution (the larger alpha, the narrower the distribution).

Column (d): Statistical value. Resulting from dividing the total number of components in a plant, that actually develop this Defect Symptom, by the number of defects at the same entity actually occurring per year.

Column (e): Statistical value: Period of time, until which 10% of all components, developing this Defect Symptom, actually fail.

Column (f): Resulting from column (d): “M” (multiple), when MTBD is 9 years and less (you have to tolerate); “S” (single) when MTBD  is from 9 to 30 years (within your own experience you should not meet them more frequently than just once); “N” (non) when MTBD is larger than 30 years (you should stop them from re-occurring at all).

Column (g): “yes” indicates that with more than 50% of all elements developing this Defect Symptom, this defect would develop more frequently, when the element will not be cleaned, lubricated, impregnated or conserved.

Column (h): “yes”  indicates that with more than 50% of all elements developing this Defect Symptom, this defect could be by periodic inspection economically detected, before the defect would trigger a break down of the peace of equipment affected (i. e. you should control and not let occur any loss event).

Is this defect standard universally applicable?

Some initially doubt that such a standard would apply equally to all plants and processes.

The only exception, however, are plants that are designed from the outset for a useful life of considerably less than ten years and whose elements are subject to correspondingly higher specific stresses.