Safety instrumentation, definition

The detailed functional safety requirements specification should include all necessary functions during all modes of operation of the process being protected. Additionally, the periodic testing of all the safety instrumented functions should be provided. This typically requires the definition of maintenance override capabilities so the sensors and final elements can be tested without shutting down the process. The same methodology described in the paragraph above can be used to document these requirements. 

A working definition of the Safely Lifea/cle is that it is an engineering process utilizing specific steps to ensure that Safety Instrumented Systems (SIS) are effective in their key mission of risk reduction as well as being cost effective over the life of the system. Activities associated with the Safety Lifecycle start when the conceptual design of facilities is complete and stop when the facilities are entirely decommissioned. Key activities associated with a Safety Lifecycle are outlined below. 

Emphasis should be placed on the last phrase of the SIF definition, "specific hazardous event." This phrase helps one clearly identify what equipment is included in the safety instrumented function versus auxiliary equipment not actually needed to provide protection against the hazard. 

Many never consider a person to be part of a safety instrumented function. When reviewing the definitions used in functional safety standards, there is little indication that this was ever intended. A process operator takes action when normal process operating conditions are violated. The operator normally responds to these violations from alarms. The operator action is, therefore, normally considered as being part of the alarm layer of protection, not the SIS. 

A failure occurs when a device at some level (a system, a unit, a module, or a component) fails to perform its intended function. To many, the definition is clear. Disagreement may occur, but when this happens it is usually a matter of properly defining "intended function." For safety instrumented systems, the definition of intended function is usually clear and should be properly recorded in the safety requirements specification. 

Most practitioners define "Fail-Safe" for an instrument as a failure that causes a "false or spurious" trip of a safety instrumented function unless that trip is prevented by the architecture of the safety instrumented function. Many formal definitions have been attempted that include "a failure which causes the system to go to a safe state or increases the probability of going to a safe state." This definition is useful at the system level and includes many cases where redundant architectures are used. 

Many practitioners define "Fail-Danger" as a failure that prevents a safety instrumented function from performing its automatic protection function. Variations of this definition exist in standards. lEC 61508 provides a definition similar to the one used herein, which reads "failure which has the potential to put the safety-related system in a hazardous or fail-to-function state." The definition from lEC 61508 goes on to add a... 

The safety and availability of a set of equipment used for a safety instrumented function may benefit from testing. However, that depends on redundancy and how often the demand occurs. Three modes of operation have been defined in lEC 61508 for equipment providing a safety instrumented function continuous demand mode, high demand mode and low demand mode. This book will use the lEC 61508 definitions to designate those three different situations. 

These two causes will result in the definition of two different safety instrumented functions because the consequences and likelihood associated with the two scenarios are different. These differences are summarized in the SIF list for well 1 (Table 13-1). 

Functional safety—safety instrumented systems for the process industry sector—Part 1 Framework, definitions, system, hardware and software requirements (lEC 61511-1 2003 + Corrigendum 2004) German version EN 61511-1 2004... 

Functional Safety Safety Instrumented Systems for the Process Industry Sector - Part 1 Framework, Definitions, System, Hardware and Software Requirements... 

Throughout the prooess industry, multiple terms have been used to refer to what are now called safety instrumented systems. This annex is intended to clarify the differences between interlocks, permissives, inhibits, safety functions, SIS, and SIF. Sinoe many of these terms have historical references, the terminology Is discussed in relation to ANSI/ISA-84.01-1996 and lEC 61508. This facilitates understanding of how these terms evolved into the current definitions of ANSI/ISA-84.00.01-2004-1. 

ANSI/ISA-84.00.01-2004-1 utilizes "safety instrumented function" (clause 5.0) in lieu of "safety interlock." NOTE - ANSI/ISA-84.00.01-2004-1 does utilize "safety function" (clause 4.0) identical to its lEC 61508 definition (see clause 1.1.3). H.3 Permissives H.3.1 Definition... 

As discussed in 1.1.3, "safety function" has different meanings if you are comparing ANSI/ISA-84.01-1996 with ANSI/ISA-84.00.01-2004 (or lEC 61508). It can be seen from the above definitions that ANSI/ISA-84.00.01-2004 and lEC 61508 definitions are essentially identical. However, ANSI/ISA-84.01-1996 has not defined "safety function" but has used it as a synonym for safety instrumented function (see 1.4.3). The result is the definition of "safety function" changes when transitioning from ANSI/ISA-84.01-1996 to ANSI/ISA-84.00.01-2004. 

Clauses 8 and 9). To address the ANSI/ISA-84.01-1996 "safety function" definition, ANSi/iSA-84.00.01-2004-1 added the term "safety instrumented function."... 

Figure H.1 — Definition of Safety Instrumented System and Safety Instrumented Function ( SIS user interface may be part of the SIF. Refer to ISA-TR84.00.04-1 Annex B)...

Instrumented system used to implement one or more safety instrumented functions. An SIS is composed of any combination of sensor (s), logic solver (s), and final elements(s) (ANSI/ISA-84.00.01-2004-1 Clause 3.2.72 - portion of definition only). 

SIF stands for safety instrumented function. SIF consists of sensors, logic solver, and final control element combination. SIF takes the system or process in to safe zone in the event of hazardous situation/event, which is determined by predefined conditions for the process (see Clause 8.1 also, for definition as per various standards). 

When discussions are to be developed for safety instrumentation functions, associated controls (logic solver) cannot be kept aside. Now, for discussions on control systems, special attention is necessary for batch process and controls. Plant processes and productions can be categorized as discrete process, continuous process, and batch process. Out of these three types, batch process is more complex in the sense that the problems are logisticaUy more challenging. In order to meet this challenge, complex issues are divided into simpler steps, then solving them, and all the solutions are integrated suitably to arrive at a solution for the complex issue. In this way, batch controls follow a special pattern and established standards. Definitions of various processes, with examples, are discussed as follows ... 

It is interesting to note the heading of the chapter It is named like this to indicate that safety instrumented functions (SIFs—in plural) constitute a safety instrumented system (SIS). From the discussions in previous chapters, readers have come across varieties of definitions of accidents, hazards, and risks from different perspectives. Also there were a few things common such as the accidents are not always negative, and there are always aim to avoid accidents so that there is not loss to the system, personnel, and environment. At the back of the mind, we always try to develop and incorporate some things to reduce risks and to maintain work safety. Therefore, safety work involves some activities, measures, and techniques, which can contribute or help to reductions in losses in different forms, and human injury or fatality. There are quite a good number of elements involved in work safety these include, but are not limited to the following ... 

From lEC 61508 Part 6, a definition of commonly used architectures in safety instrumented systems is available. The elements used in a single or multiple configuration can be either sensors or final elements—mainly for input sensors, and only a few for the final element on account of cost [4]. Typical interfaces of these with an intelligent control (DCS/PLC) system are shown in Fig. Xl/1.3.2-1. The configuration may be lool, which is quite vulnerable because single instrument failure wUl make the loop unavailable. 

Definition of the safe state of the process for each identified safety instrumented function and any combined concurrent safe states that could cause a hazard. ... 

Two of the key standards to do with safety instrumentation are EC 61511 and lEC 61508. They are performance-based, nonprescriptive standards that provide a detailed framework and a lifecycle approach for the design, implementation, and management of safety systems applicable to a variety of sectors with different levels of risk definition. 

Title Functional safety instrumented systems for the process industry sector Description This standard is an adaptation of lEC 61508 for the process industry and provides details on a general framework, definitions and system software and hardware requirements. 

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