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Reliability Questions

Reliability Questions

Reliability Questions

Hi Everybody,

I have recently become a reliability expert in training at my company (old reliable, they're gonna call me). Although I have a fairly decent grasp on the majority of the basics, there's a few minor things regarding the basics that I could use some clarification with:

1. I know that one of the key parameters in reliability is the mean time before failure:

MTBF = Timespan/Number of failures

I feel really dumb asking this, but how far exactly do you go to classify a "failure" for reliability purpose when calculating MTBF. I know that a failure is when the equipment is unable to perform its intended function, which can range from a component needing to be replaced to resume operation (bearing failure, seal failure, etc.), up to the equipment not performing to its intended design conditions/output (not enough pressure, not enough flow, etc.). I guess electric breaker trips and start-up issues are considered failures as well since they render the equipment unusable unexpectedly, but are you interested in these as well from a reliability view (assuming they're part of the system your analyzing). Even if something like a valve needs to be replaced or a suction screen for a pump needs to be cleaned, are they classified as failures if they are a part of your system and if the equipment needs to be isolated/pulled out of service to fix or replace? In a nutshell, are you just interested in the physical failures as the ones previously mentioned, or is any unplanned shutdown/downtime of an equipment (including breaker trips, inability to start-up, valve replacements, screen cleaning, etc.) considered when calculating MTBF? I would like some clarification simply because, obviously, the MTBF heavily depends on it.

2. I know quite well that there is a difference between availability and reliability, but I know lots of people can get the two confused. I know reliability at it's most basic form is defined as:

R(t) = exp(-t/MTBF)

where t is the mission time, and MTBF is the mean time before failure from before, and F is the failure rate. However, I've seen a few websites, a colleague, and I believe somewhere in this forum that reliability is defined by:

R = Uptime/(Uptime + Unplanned downtime due to failure)

However, this second definition seems more like availability instead of reliability. Is the second definition closer to availability, or can it describe reliability if you just look at the downtime due to failure? My gut says this is availability and not reliability.

I appreciate any help/clarification you all can provide me with. Thanks

RE: Reliability Questions

1. MTBF is whatever the customer, user, or make defines it to be. Most people would consider anything that stops normal operation as a threshold definition of a failure.

2. Your first equation is the failure density function. The integral of that is a reliability.

Your second equation is the availability, which is often treated as reliability, i.e., 90% reliability to mean 90% availability.

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RE: Reliability Questions

There's quite a difference between availability and reliability. Availability is, shall we say, the "ability to provide service when needed" vs. "the capability to keep on going". This difference could be great, for example, if a number of components had to function for a successful start up of the unit in question, as a motor starter would affect the availability of a motor. Both sub-units would initially have to function in order to achieve a successfull motor run, thus the availability of the unit in question would depend on the probability of a successful start of all subcomponents.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Reliability Questions

Predicted Availability == MTTF/(MTTF + MTTR)
Where MTTF is mean time to failure for the system and MTTR is the mean time to repair for the system.

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