Safety instrumentation systems scope

Where loss of control could lead to severe consequences, the integrity of the basic process control system and the protective safeguards must be designed, operated and maintained to a high standard. Industry standards such as ANSI/ISA-S84.01 (1996) and IEC 61508 (2000) address the issues of how to design, operate and maintain safety instrumented systems such as high temperature interlocks to achieve the necessary level of functional safety. The scope of these standards includes hardware, software, human factors and management (HSE 2000). 

Safety instrumented system audits provide beneficial information to plant management, instrument maintenance engineers and instrument design engineers. This enables management to be proactive and aware of the degree of implementation and effectiveness of their safety instrumented systems. Many types of audits, which can be carried out exist. The actual type, scope, and frequency of the audit of any specific activity should reflect the potential impact of the activity on the safety integrity. 

Although some consider fire and gas systems to be outside the scope of a safety instrumented system, many others classify these functions as safety instrumented functions. The criteria is based on needed risk reduction. When consequence or likelihood reduction is achieved by these functions and the risk reduction needed is greater than 10, ANSl/lSA-84.00.01 (lEC 61511 Mod) (Ref. 9) requires the hmction be classified as a SIR... 

The purpose of this annex is to provide guidance for establishing instrument setpoints for safety instrumented functions in the process industries. The scope of ANSI/ISA-84.00.01 is requirements for... a safety instrumented system, so that it can be confidently entrusted to place and/or maintain the process in a safe state." This annex provides guidance on instrument uncertainty calculations and setpoint determination for instruments used in an SIS to ensure that each SIF responds to achieve or maintain the safe state of the process within one-half of the process safety time with respect to a specific hazardous event. If measurement uncertainty is not considered in the determination of an SIS setpoint, the SIF may not detect the presence of a valid process demand. 

The general scope of safety analysis for HWRs, covering the accident categories considered, safety barriers challenged, and the technical disciplines involved are summarized in Table 4.2. Failures in safety support systems (such as instrument air) are addressed in the PSA. 

This standard sets out practices in the engineering of systems that ensure an industrial process s safety through the application of instrumentation. The scope of lEC 61511 includes the following items [19,20] ... 

A common cause failure could affect all the pumps of the safety systems because of an insufficient protection in their instrumentation and control system. Besides, limitation in the scope of testing has a direct impact on the reliability of the ECCS and containment spray systems. 

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