Safety integrity levels assessment

General References Guidelines for Hazard Evaluation Procedures, Second Edition with Worked Examples, American Institute of Chemical Engineers, New York, 1992 Layer of Protection Analysis A Simplified Risk Assessment Approach, American Institute of Chemical Engineers, New York, 2001 ISA TR84.00.02, Safety Instrumented Functions (SIF)—Safety Integrity Level (SIL) Evaluation Techniques, Instrumentation, Systems, and Automation Society, N.C., 2002. 

Once the severity and the probability corresponding to a scenario are estimated, that is, the risk is assessed, a decision can be made on the nature of the protection system to be implemented. If a safety instrumented system (SIS) is to be used, consisting of one or more independent protection levels (IPL), the required reliability of the protection system, constituting a so-called Safety Integrated Level (SIL) can be determined by using this risk assessment, respective of the required risk reduction. 

The outcome of the hazard and risk assessment and allocation process should be a clear description of the functions to be carried out by the safety systems, including potential safety instrumented systems together with safety integrity level requirements (along with mode of operation, continuous or demand) for any safety instrumented function. This forms the basis for the SIS safety requirements specification. The description of the functions should be clear as to what needs to be done to ensure that safety is maintained. 

The considerations listed below apply when an assessment is carried out on the likelihood of common cause, common mode and dependent failures. The extent, formality and depth of the assessment will depend on the safety integrity level of the intended function. The effect of common cause, common mode and dependent failures may be dominant for safety integrity levels of 3 or higher. The following should be considered ... 

During the 1990s the concept of Safety Integrity Levels (SIL) was developed [1]. It serves to assess safety-related systems and concerns aU components and subsystems required to realize safety functions from the sensor to the final element. Apart from that it applies to application software, which was developed for systems with limited variability language (no branching) or programmable logic controllers (PLC). 

Control System (BPCS), including functions of Supervisory Control and Data Acquisition (SCADA) system, the alarm system (AS) and Safety Instrumented Systems (SIS) performing defined Safety Instrumented Frmetions (SIF). Proper design of layers of protection is based on hazards analysis and risk assessment with consideration of human and organizational factors. It is essential to ensure required safety integrity level (SIL) for each of these layers. 

Dutuit, Y, Innal, R, Rauzy, A. and Signoret, J.-P. 2008. Probabilistic assessments in relationship with safety integrity levels by using Fault Trees, Reliab. Eng. Syst. Safety vol. 93 Issue 12, Dec. 2008. 

This standard would establish a consistent approach to mine hoisting risk assessments used to determinate safety integrity levels of safety related control functions at mine projects. Opinions differ as to the SIL requirements for various safety critical functions such as midshaft overspeed SILl, some people feel SIL2 end of wind overspeed at a high speed 100 persons drum hoist without shaft end arrestors, higher than SIL3. 

The preliminary risk assessment is made by a risk graph qualitative method described in lEC 61508-5 Annex E according Figure E.l (Figure 3) and Table E.l. This method has been used extensively within the machinery sector, see ISO 14121-2 and Annex A of ISO 13849-1, and enables the safety integrity level to be determined from knowledge of risk factors associated with the hoist machinery and it control system. 

Phases 1 to 5 deal with concept and analysis—risk assessments to discover the Safety Instrumented Functions (SIF) and assign an appropriate Safety Integrity Level (SIL) rating. 

The safety core-process itself starts with the Preliminary Hazard Identification (PHI) and the Preliminary Hazard Assessment (PHA). During diat phase a preliminary hazard list with severities is created via brainstorming and the use of historical data and checklists. Outputs are the preliminary hazard list, including severities and hazard target rates, and initial development process integrity level allocations as detailed in various standards, e.g. Safety Integrity Level (SIL) in lEC 61508 or CENELEC EN 50128. 

NOTE 2 The safety integrity level is defined numerically so as to provide an objective target to compare alternative designs and solutions. However, it is recognized that, given the current state of knowledge, many systematic causes of failure can only be assessed qualitatively. 

The term prior use is used in ANSI/ISA-84.00.01-2004-1. This term is applied when the owner/operator is justifying the implementation of a device in an SIF application based on prior use of the device in a similar operating profile. The standard provides specific requirements for assessing prior use for devices based on the device type, safety integrity level, and, in the case of PE logic solvers, the safe failure... 

The use of Functional Safety Assessment (FSA) is fundamental in demonstrating that a Safety Instrumented System (SIS) fulfils its requirements regarding safety instrumented function(s) and Safety Integrity Level (SIL).... The following attributes are considered good practice for Functional Safety Assessment ... 

Systematic assessment of safety integrity level requirements... 

Functional safety is the part of the overall safety arrangements that depends on a system or equipment operating correctly in response to its inputs (BS EN 61508)." Procedures for functional safety assessment and auditing should be in place. A functional safety assessment is an independent assessment and audit of the functional safety requirements and the safety integrity level achieved by the SIS. 

Copied Reoommendation 1 - Systematio assessment of safety integrity levels... 

Have all existing I C systems been assessed for their required functional safety integrity levels Are the site operators aware whether the existing (aging) I C systems meet these required safety integrity levels ... 

Safety integrity level (SIL) assessment is used during front end engineering design to develop the reliability requirements for control and protection systems. This is normally done in accordance with international standards such as EC 61508. This topic was discussed in Chapter 2. 

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