Safety instrumentation systems defined

NFPA 85, Boiler and Combustion Systems Hazards Code, provides guidance for steam boilers and similar high reliability automatic combustion systems. In general, NFPA 85 combustion system control and safety instrumentation systems requirements exceed those defined in NFPA 86 and in API RP 556. 

Application of Safety Instrumented Systems for the Process Industries, ISA-S84.01, Instrument Society of America, ISBN 1-55617-590-6, Research Triangle Park, NC, 1996. The objective of this document is to define the requirements for Safety Instrumented Systems (SIS) for the process industries. The SIS addressed here includes electrical, electronic, and programmable electronic technology. 

The organizational structure associated with safety instrumented systems within a Company/Site/Plant/Project should be defined and the roles and responsibilities of each element clearly understood and communicated. Within the structure, individual roles, including their description and purpose should be identified. For each role, unambiguous accountabilities should be identified and specific responsibilities should be recognised. In addition, whom the individual reports to and who makes the appointment should be identified. The intent is to ensure that everyone in an organization understands their role and responsibilities for safety instrumented systems. 

The skills and knowledge required to implement any of the activities of the safety life cycle relating to the safety instrumented systems should be identified and for each skill, the required competency levels should be defined. Resources should be assessed against each skill for competency and also the number of people per skill required. When differences are identified, development plans should be established to enable the required competency levels to be achieved in a timely manner. When shortages of skills arise, suitably qualified and experienced personnel may be recruited or contracted. 

The installation and commissioning of the change should follow the procedures defined in lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (lEC 61511-1 Modi for installation and commissioning of safety instrumented systems. 

Alarms with defined operator response Critical alarms Safety instrumented systems Pressure relief devices Blast walls and dikes Deluge systems Flare systems... 

It must be recognized that the Safety Lifecycle and associated Safety Instrumented Systems need to be part of an overall Process Safety Management System (PSM) for the entire plant. PSM can be defined as a program or set of activities involving the application of management principles and analyses to ensure the overall process safety of process plants. PSM, therefore, covers aU aspects of process safety, not just functional safety. 

The ANSI/ISA-84.00.01-2004 (lEC 61511) standard (Ref. 1) defines a safety instrumented system (SIS) as an "instrumented system used to implement one or more safety instrumented functions. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s)." lEC 61508 (Ref. 2) does not use the term SIS but instead uses the term "safety-related system." That term defines the same concept but uses language that can be broadly applied to many industries. 

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. 

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. 

The RPS is a sort of SIS (Safety Instrumented System) (Torrres et al., 2009). A SIS is defined as an instrumented system used to implement one or more safety instrumented control functions. A SIS is composed of any combination ofsensors, logic solver and final elements" (lEC 61511, 2003). The standard lEC 61508 requires every safety function to achieve a determined Safety Integrity Level (SIL). For low demand operating systems the SIL levels are defined in terms of average probability of failure on demand (PFDavg, see Table 2). 

A procedure shall be defined and executed for a functional safety assessment in such a way that a judgement can be made as to the functional safety and safety integrity achieved by the safety instrumented system. The procedure shall require that an assessment team is appointed which includes the technical, application and operations expertise needed for the particular installation. 

SIS stands for safety instrumented system. SIS is designed to prevent or mitigate from happening of a hazardous event, by taking the process to a safe state whenever a predefined or predetermined conditions occur to the system. It is a combination of sensors, logic solvers, and final conttol elements. In PEs, it consists of both hardware and software. In fact, emergency shutdown system (though shown separately in Fig. 1/ 7.0-2) will be a part of the same. There could be a number of SIF (defined next) in SIS. 

With the introduction of safety standards lEC 61508 and 61511 (for process industries), there is a defined need for proper implementation of safety systems embedded into the main system. The safety life cycle has various phases. Phases 1 and 2 have been discussed at length in previous chapters (Chapter VI and Chapter VII and to a certain extent in Chapter IX). In this part, detailed discussions have been presented to include Phases 3—7, that is, from safety-related systems (SRSs) to modifications. This has been done purposefully so that prior to looking at the detailed implementation part of the standard, readers need to have some knowledge of the safety instrumented system (SIS), safety integrity level (SIL), and their implementation in various instrumentation components. So, this part of the discussions in conjunction with previous chapters will complete the topic of lEC 61508/61511. Safety instrumentation cannot be complete without discussions on explosion protection. With reference to lEC 60079-(0,10,14,15,17, etc.) and NEC (497,499,70, etc.), electrical area, classification of plant, explosion protection, etc. also have been included as part of this chapter to make the system complete in all respects. In view of this, these are presented in two sections. Section 1 for lEC implementation and Section 2 for explosion protection. 

The safety integrity level (SIL 3 in this case) is allocated based on a process hazard and risk assessment. It forms the basis for the risk reduction target for the safety instrumented system/SIL (HIPS in this case). For on-demand systems such as a HIPS, the SIL defines the probability of... 

Note that this diagram defines safety integrity as applicable to all risk reduction facilities. It is a general term but when it is applied to the safety instrumented system it becomes a measure of the system s performance. 

The SIS safety requirements specification should be developed in association with the non-SIS protection layers. The lEC 61508 standard calls for the overall safety requirement to be defined first in phase 4 followed by an allocation phase 5, which defines the sharing of protection duties across the layers of protection. The final SRS for the safety-instrumented system is then part of the detail design activities for the SIS known in lEC terms as the realization phase (phase 9). This procedure can be rather confusing at first but it appears to be designed to ensure that the basics of the SRS are in place and verified before the design team goes too far with the technical specifications for the SIS. 

It should be noted here that the lEC 61508 standard places much importance on the role of project management in the delivery of a safety instrumented system. In part 1 of the standard clause 6 provides two pages of items defining the management and technical activities that are necessary for the achievement of the requiredfunctional safety. ... 

An example of a risk situation is used in this exercise. We are asked to use layers of protection analysis to arrive at a risk reduction model for the situation. The quantitative analysis method is then used to define the safety integrity level (SIL) required for the safety instrumented system. This model can also be used to check the practical application of qualitative methods for determining SILs. 

To define the layers of protection proposed for a polymer autoclave to reduce the risk of exposing site persoimel to toxic vapors. To draw a risk reduction model incorporating the layers of protection and use the model to decide the required SIL for the safety instrumented system. ... 

Requirements for a safety instrumented system as defined from the hazard study are ... 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

A Safe Operating Procedure developed to create a uniform method to ensure that appropriate steps are taken prior to bypassing or removing an alarm, instrument, or shutdown system IWim service is described in the section that follows. This procedure can provide an effective way of communicating the status of an impaired instrument. The procedure has been in use for over five years. It assumes that all instrumentation has been classified into three safety critical systems. [7] (These classes have been defined in Chapter 9, but are repeated here.)... 

To achieve this, the sum of the diagnostic test interval and the reaction time to achieve a safe state should be less than the process safety time . The process safety time is defined as the time period between a failure occurring in the process or the basic process control system (with the potential to give rise to a hazardous event) and the occurrence of the hazardous event if the safety instrumented function is not performed. 

In ANSI/ISA-84.00.01-2004 (lEC 61511 Mod), 3.2.71, a safety instrumented function is defined as a "safety function with a specified safety integrity level which is necessary to achieve functional safety." This standard, 3.2.68, defines a safety function as a "function to be implemented by a SIS, other technology safety-related system or external risk reduction facilities, which is intended to achieve or maintain a safe state for the process, with respect to a specific hazardous event."... 

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... 

In case of incorrect diagnosis or no reaction on time against abnormal event occurred due to fast dynamic of the process, the SIS/ESD (emergency shutdown) system will operate without operator intervention to stop technological process executing defined safety instrumented functions (Fig. 3) to mitigate consequences. 

Based on Part 1 the standard provides guidelines for the specification, design, installation, operation and maintenance of Safety Instrumented Functions and related safety instmmented system as defined in lEC 61511-1. This standard has been organized so that each clause and sub clause number therein addresses the same clause number in lEC 61511-1. ... 

Systems with especially highly important safety functions are defined in "Guide for Priority Groups of Safety Function of Light Water Nuclear Power Reactor Facilities" and instrumentation air source facilities come under this definition. 

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