ABSTRACT
In Chapter 1, we saw that both failure rate ( ) and probability of failure on demand (PFD) are parameters of interest. Since unavailability is the probability of being failed at a randomly chosen moment then it is the same as the probability of failure on demand. PFD is dimensionless and is given by:
PFD UNAVAILABILITY ( MDT)/(1 MDT) ( MDT)
where is failure rate and MDT is the mean down time (in consistent units). Usually MDT 1. For revealed failures the MDT consists of the active mean time to repair (MTTR) plus any logistic delays (e.g. travel, site access, spares procurement, administration).
