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CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Demonstrating Prior Use v4
Page 18 of 30
o
Safe detected (SD)
o
Safe undetected (SU)
•
Dangerous Failure - a failure that when it occurs does not cause the system to
perform the function which puts the system into the safe state, a failure that
occurs when the system fails to operate when the process puts a demand onto it.
Dangerous failures can further be categorised as either:
o
Dangerous detected (DD)
o
Dangerous undetected (DU)
Dangerous undetected (DU) failures can only be identified on an actual process demand
or by proof testing (providing the proof test is designed to detect the failure).
The four different failure modes are described in reliability modelling as follows:
λ
SU
, Safe undetected failure
λ
SD
, Safe detected failure
λ
DU
, Dangerous undetected failure
λ
DD
, Dangerous detected failure
Using the failure mode data as defined above allows calculation of the PFD and SFF.
For a 1oo1 component, the average failure of probability on demand (PFD) can be
calculated as follows:
Where t
ce
is the channel equivalent mean down time in hours (this is the combined down
time for all of the components in the channel of the sub-system), and can be calculated
as follows:
T1
= Proof test interval (hours)
MTTR
= Mean Time to Restore (hours)
Safe Failure Fraction (SFF) can be calculated as follows: