Probability Failure on Demand (PFD) – SIL Verification

The term Probability of Failure on Demand (PFD) is commonly used in the context of Safety Instrumented System (SIS). It is a measure of the reliability of a Safety Instrumented Function (SIF) and represents the likelihood that the system will fail to perform its intended safety function when demanded.

The PFD is expressed as a probability ranging from 0 to 1, where a lower value indicates higher reliability. A PFD of 0 means the system is perfectly reliable, while a PFD of 1 signifies complete failure.

To calculate the PFD, it is mandatory to consider the failure rates of individual components within the Safety Instrumented System. This includes sensors, logic solvers, and final elements (e.g. valves or actuators). The PFD is determined based on the architecture of the safety system and the redundancy implemented to achieve the required level of reliability.

Anyhow, the PFD is just one metric in the overall assessment of safety instrumented systems. Other factors, such as architecture constraints, and systematic capability.

The calculation of the Probability of Failure on Demand (PFD) is guided by industry standards and codes related to function safety such as IEC 61508, IEC 61511, and ANSI/ISA-TR84.00.02

SIL requirements according to ISA-TR84.00.02 and IEC/EN 61508

SIL according to ISA-TR84.00.02

SIL	Demand mode of operation (probability of failure on demand average)
3	< 10^-3 to > 10^-4
2	< 10^-2 to > 10^-3
1	< 10^-1 to > 10^-2

SIL according to IEC/EN 61508

SIL	Low Demand (Average probability of failure on demand)	High Demand or Continuous Mode of operation (Probability of dangerous failure per hour)
4	<10^-4 to >10^-5	< 10^-8 to > 10^-9
3	<10^-3 to >10^-4	< 10^-7 to > 10^-8
2	<10^-2 to >10^-3	< 10^-6 to > 10^-7
1	<10^-1 to >10^-2	< 10^-5 to > 10^-6