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PFD / Availability (MTBF)Free online tool · works on PC and mobile

PFD and Availability: Reliability Metrics Driven by MTBF/MTTR

How Availability Is Calculated

Availability A = MTBF / (MTBF + MTTR), the proportion of time a system is in a usable state. MTBF is the mean time between failures and MTTR the mean time to repair. For safety instrumented systems, availability affects the system's readiness to respond to a safety demand and is a fundamental input for the PFDavg calculation in SIL verification. It can also evaluate how maintenance strategies affect availability, balancing maintenance cost against downtime loss.

How PFDavg Is Assessed

In low-demand mode, the probability of failure on demand of a safety instrumented function (PFDavg) is determined by the failure rate (lambda-DU, dangerous undetected failures), the proof-test interval T and repair time, with the simplified form PFDavg = lambda-DU x T / 2. The tool computes PFDavg from the failure rate and test interval and compares it with the SIL requirement. Distinguish safe from dangerous failures in the calculation, because the two contribute completely differently to PFDavg.

Applicable Scenarios

SIL verification of safety instrumented functions (SIF); conversion among reliability metrics MTBF/MTTR/availability; spare-parts and maintenance-strategy evaluation; and checking reliability figures in equipment reports and tender documents. It can also evaluate how maintenance strategies affect availability, balancing maintenance cost against downtime loss. Remember to distinguish safe from dangerous failures, since the two contribute completely differently to PFDavg.

Outputs and Precautions

The tool outputs availability, failure rate, PFDavg and a conclusion against the SIL target. Note that lambda-DU must distinguish detectable from undetectable failures; a shorter proof-test interval T lowers PFDavg but raises maintenance cost; and common-cause failures (the beta factor) significantly affect the actual PFD of redundant architectures and should be included. Optimize the test interval and maintenance plan around the PFDavg target, balancing safety and cost.

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Frequently Asked Questions

Does a larger MTBF always mean higher availability?

Yes, when MTTR is unchanged, but availability also depends on MTTR: with a shorter repair time, even a moderate MTBF can give high availability.

How do PFDavg and failure rate relate?

In low-demand mode PFDavg = lambda-DU x T / 2: the higher the failure rate and the longer the test interval, the larger the PFDavg and the lower the SIL compliance.

How do I distinguish dangerous from detected failures?

Dangerous failures (DU/DD) are those that prevent the safety function from operating; they are divided by self-diagnosis capability into detected (DD) and undetected (DU). SIL calculations mainly use lambda-DU.

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