My supplier has given me PFD values only (λD missing)
PFD (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:
PFD ≅ Un ≅ λ * (Tp / 2)
A PFD value cannot therefore be assigned to an element without the proof test interval (Tp) and the appropriate failure rate, λ.
If you have a PFD 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 ≅ PFD / (Tp / 2)