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# My supplier has given me PFD values only (λD missing)

## Background

PFDavg (or unavailability, Un) is defined as:

Un ≅ λMDT

In the context of Safety Instrumented Functions (SIFs), Mean Down Time (MDT) is typically taken as the midpoint of the proof test interval, hence:

PFDavg ≅ Un ≅ λ * (Tp / 2)

A PFDavg value cannot therefore be assigned to an element without the proof test interval (Tp) and the appropriate failure rate, λ.

## Alternative Approach #1

If you have a PFDavg value for an element that you wish to use in SIL Comp©, the following relationship provides an approximate and conservative λD value, which can then be assigned to an element:

λD ≅ PFDavg / (Tp / 2)

Where:

λD = λDD + λDU

## Alternative Approach #2

If you have a PFH value for an element that you wish to use in SIL Comp©, the following relationship provides an approximate and conservative λD value, which can then be assigned to an element:

λD ≅ PFH

Where:

λD = λDD + λDU