Instrumented systems of safety

Introduction

When the automated systems introduce to possible hazards for the Anybody S, the Environnement or the goods, various Sécurité S is implemented. Those take part either in the prevention (by minimizing the probability of appearance of a risk), or with protection (to limit the consequences of a dysfunction). The instrumented systems of safety (LOCATED) are often used as means of prevention to fulfill these instrumented functions of safety (SIFs). To conceive these systems, two standards are used: ANSI /ISA S84.01-1996 and the IEC 61508. These two standards are based on the principle of the evaluation of the reduction of the risk necessary to reach an acceptable level of risk.

Instrumented systems of safety

An instrumented system of safety is a system aiming at putting the process in fallback position of safety (i.e. a stable condition not presenting a risk for the environment and the people), when the process takes a route involving a real risk for the personnel and the environment (explosion, fire…). LOCATED is composed of three parts:

a sensor part charged to measure the drift of a parameter (pressure, temperature…) towards a dangerous state.
a system part of logic processing (UTL) charged to collect the signal coming from the sensor, to treat this one and to order the associated actuator.
an actuator part charged to put the process in its position of safety and to maintain it.

The probability of failure on request of LOCATED is determined by calculation and the combination of the probabilities of failure of its components. These probabilities depend on the failure rates of the components, the detected dangerous failure rates and the factor which characterizes the failures of common cause.

Conformity to standards ANSI/ISA S84.01-1996 and CEI 61508

Standards ANSI/ISA S84.01-1996 and CEI 61508 lay down the regulations relating to the specification, the design, the installation, the exploitation and the maintenance of LOCATED, in order to have any confidence in its capacity to bring and/or to maintain the process in a state of safety. The necessary basic stages to ensure conformity these two security standards are:

To establish a target of safety (acceptable risk) of the process and to evaluate the existing risk.
To identify the necessary functions of safety and to assign them to the levels protection.
Déterminer if the instrumented function of safety is necessary.
To implement the instrumented function of safety in LOCATED and to determine the SIL of the LOCATED one.
Vérifier that the LOCATED one makes it possible to reach the target of safety required at the beginning.

The table gives the SIL of LOCATED according to the value of its PFD and its frequency of request.

Qualitative and quantitative methods for the determination of the SIL

The determination of the SIL of LOCATED can be obtained by various methods:

qualitative Methods: They are methods which make it possible to determine the level of SIL starting from the knowledge of the risks associated with the process.
quantitative semi Methods: The most widespread method is the matrix of risk. This matrix gives the level of SIL according to the gravity of risk and its frequency of occurrence.
quantitative Methods: They are the methods which make it possible to calculate the PFD of LOCATED starting from the probabilities of failures of their components. The most widespread methods are:

simplified equations.
trees of failures.
Markovian approaches.

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