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© Engineering Safety Consultants Limited
Page 21
PFD Calculations
PFD is the numerical value that describes the probability
that the will fail to operate when required.
The PFD of a single channel element is:
•
λ
DU
is the dangerous undetected failure rate per hour;
•
T
p
is the proof test frequency
If
λ
DU
Tp
is small (<0.1) then:
PFD
λ
DU
Tp
© Engineering Safety Consultants Limited
Page 22
Probability of failure with time
•
λ
DU
is the dangerous undetected failure rate per hour;
•
T
p
is the proof test frequency
PFD
Time
PFD
λ
DU
Tp
Tp
0
1