New insight into the average probability of failure on demand and the probability of dangerous failure per hour of safety instrumented systems

Abstract

The aim of this paper is to give a new insight into some fundamental concepts of the IEC 61508 standard. First, low and high or continuous demand modes of operation of safety instrumented systems are examined by analysing their official definitions given in the IEC 61508 and IEC 61511 standards. In this context, the paper proposes a new criterion for distinguishing these two modes of operation. A study allowing the determination of accident frequency is also presented, where the system under study consists of one element under control and its associated safety instrumented system. Second, the relationship between the average probabilities of failure on demand and the risk reduction factor is studied. It is shown that the commonly used approach (the standard approach) may lead to an optimistic value for the risk reduction factor. Finally, the paper clarifies the nature of the probability of failure per hour of a safety instrumented system and proposes different ways to compute this in the general case, based on fault tree, Markov model, and Petri nets approaches.

Proof-testing strategies induced by dangerous detected failures of safety-instrumented systems

“…Petri nets have been adapted to SIS reliability analysis [5,7,16] especially for testing strategies of SISs [10,12], and is also a recommended modeling approach in IEC 61508 [2] and ISO 12489 [8]. The international standard IEC 62551 [4] defines the terminology of Petri nets in dependability analysis.…”

“…for measuring the effects of system architectures [6,14,20], effects of testing strategies [11,19], different demand modes and associated measures [1,7,9,13], and varying modeling methods [2,5]. More information about research achievements and directions of SISs can be found in [15,16].…”

Reliability of safety instrumented systems: Where to direct future research?

“…Very few scientific articles have been written about reliability assessment of high‐demand systems [18–20], and they all indicate that this field is immature. Currently, Markov modeling seems to be the preferred and most flexible approach for this purpose, but still, all features are not studied.…”

New PFH-formulas for k-out-of-n:F-systems

“…The PFH may be calculated based on the simplified formulas [8,12], fault tree analysis [12,13], Markov models [12], and Petri nets [12][13][14]. Some researchers prefer not to distinguish the SIS operational modes and use a common reliability measure with basis in Markov modeling [3,[13][14][15]17,19,23].…”

“…Some researchers prefer not to distinguish the SIS operational modes and use a common reliability measure with basis in Markov modeling [3,[13][14][15]17,19,23]. Despite the limitations in modeling capacity, the simplified formulas are still preferred by most practitioners, due to their simplicity.…”