Probability of Failure upon Demand PFD

Probability of Failure upon Demand PFD

By understanding how components of an Safety Instrumented System SIS can fail, it is possible to calculate a Probability of Failure on Demand PFD. There are two basic ways for SIS to fail. The first way is commonly called a spurious trip which usually results in an unplanned but safe process shutdown.

While there is no danger associated with this type of SIS failure, the operational costs can be very high. The second type of failure does not cause a process shutdown or nuisance trip. Instead, the failure remains undetected, permitting continued process operation in an unsafe or dangerous manner.

If an emergency demand occurred, the SIS would be unable to respond properly. These failures are known as covert or hidden failures and contribute to the probability PFD of the system failing in a dangerous manner on demand.

The PFD for the Safety Instrumented System SIS is the sum of PFD’s for each element of the system. In order to determine the PFD of each element, the analyst needs documented, historic failure rate data for each element.

This failure rate (dangerous) is used in conjunction with the Test Interval TI term to calculate the PFD. It is the test interval TI that accounts for the length of time before a covert fault is discovered through testing.

Increases in the test interval directly impact the PFD value in a linear manner; e.g. if you double the interval between tests, you will double the Probability of Failure on Demand, and make it twice as difficult to meet the target Safety Integrity Level SIL.

The governing standards for Safety Instrumented Systems SIS state that plant operators must determine and document that equipment is designed, maintained, inspected, tested and operated in a safe manner.

Thus, it is imperative that these components of Safety Instrumented Systems be tested frequently enough to reduce the PFD and meet the target SIL.

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