Allocation of safety functions to protection layers

NOTE Account should be taken, during the process of allocation, of other industry standards or codes. 

In order to determine the need for a SIS and its associated SIL, it is important to consider what other protection layers exist (or need to exist) and how much protection they provide. After considering the other protection layers, a determination should then be made on the need for a SIS protection layer. If a SIS protection layer is needed, a determination should then be made on the SIL for the safety instrumented function(s) of this SIS. 

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Allocation of safety functions to protection layers Allocation of safety functions to protection layers and for each safety instrumented function, the associated safety Integrity level 9 A description of the required safety instrumented function(s) and associated safety integrity requirements Description of allocation of safety requirements (see Clause 9)... 

Clause 8 - Hazard and risk assessment (Layers of Protection Analysis, Layered Risk Analysis, etc,) Clause 9 - Allocation of safety functions to protection layers L R A R R P P A... 

Allocation of safety functions to protection layers requirements - 9.2... 

The SRS is prepared before hazard and risk analysis and the allocation of safety function to protective layers in the safety lifecycle according to I EC 61511-1 (2003). It is important that the documentation covers all SIFs to be addressed during the safety lifecycle. After completing the risk analysis and SIL allocation, the SRS is created and updated according to the procedure in Figure 1. Revision... 

Used in conjunction with ISA-TR84.00.04-2005 Part 1, the example set forth in this technical report is provided to illustrate howto apply ANSI/ISA-84.00.01-2004 Parts 1-3 (lEC 61511 Mod). It is intended to demonstrate one method to meet the requirements of the standards. The reader should be aware that ANSI/ISA-84.00.01-2004 Parts 1-3 (lEC 61511 Mod) is performance based, and that many approaches can be used to achieve compliance. Some of the methods applied in this example include what-if and HAZOP techniques for hazard and risk analysis, LOPA for allocation of safety functions to protection layers, fault tree analysis for SIL verification, and ladder logic to document the application software requirements. Other techniques and tools could be utilized at each of these steps in the safety lifecycle to meet the requirements of the standards. 

The main objective is to allocate safety functions to protection layers, to determine SIF, and for each SIF determine the associated SIL. Major coverage of this clause shall mainly include ... 

Allocation of the safety functions to layers of protection taking account of potential reduction in effective protection due to common cause failure between the safety layers and between the safety layers and the Basic Process Control System (BPCS). ... 

NOTE 1 A safety instrumented function is considered to be entireiy dependent on a subsystem if a faiiure of this subsystem results in a failure of the safety instrumented function in the safety instrumented system under consideration, and the safety instrumented function has not also been allocated to another protection layer (see Clause 9). 

Alarms for which an operator or facility worker is required to evacuate an area (e.g., fire and gas alarms) and are not intended to direct the operator to take action on the process are generally not considered safety instrumented functions. These alarms should not be allocated to the BPCS but may be allocated to the SIS or to another independent protection layer. Refer to Annex F, Figure F.1, for an overview of protection layers. These alarms are generally classified as safety-related and are designed and managed in a manner that supports the allocated risk reduction. 

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