Safety instrumentation systems

Design equipment to accommodate maximum operating envelope. Appropriate use of Safety-Related Systems (SRS) such as Safety-Instrumented Systems (SIS). 

Same sensor used for basic process control system and safety instrumented system. Failure of sensor leads to loss of control system and safety system functionality. 

Provide independent safety instrumented system (SIS) with periodic testing... 

Provide independent sensors for use in BPCS and safety instrumented system with a plausibility analysis... 

House logic-solver components of BPCS, and safety instrumented systems (SIS) in a controlled environment... 

ISA-S84.01-1996, Application of Safety Instrumented Systems for tbe Process Industries, Instrument Society of America, Research Triangle Park, NC. 

Part 2 Classification of Process Control Systems Realization, operation and testing of safety instrumented systems (December 1998)... 

Covert Fault Faults that can be classified as hidden, concealed, unannounced, undetected, unrevealed, latent, etc. In the case of safety instrumented systems covert faults impair the intended safeguarding function without being apparent to the operator. Covert faults can only be detected by testing or challenging the system. 

Safety Instrumented System (SIS) The instrumentation, controls, and interlocks provided for safe operation of the process. 

IEC (2001), IEC 61511, Functional Safety Instrumented Systems for the Process Industry Sector, Parts 1-3. (Draft in Process), Geneva International Electrotechnical Commission. 

Where process, safety, and environmental considerations permit, vacuum protection may be provided by properly sized ever-open vents. Alternatively, active protective devices and systems are required. Vacuum breaker valves designed to open and admit air at a predetermined vacuum in the vessel are commonly used on storage tanks, but may not be suitable for some applications involving flammable liquids. Inert gas blanketing systems may be used if adequate capacity and reliability can be ensured. Where the source of the vacuum can be deenergized or isolated, suitably reliable safety instrumented systems (e.g, interlocks) can be provided. 

Include provisions for detecting abnormal process conditions and bringing the process to a safe state before an emergency situation occurs (Safety Instrumented Systems). 

References Guidelines for Safe and Reliable Instrumented Protective Systems, American Institute of Chemical Engineers, New York, 2007 ISA TR84.00.04, Guidelines for the Implementation of ANSI/ISA 84.00.01-2004 (IEC 61511), Instrumentation, Systems, and Automation Society, N.C., 2005 ANSI/ISA 84.00.01-2004, Functional Safety Safety Instrumented Systems for the Process Industry Sector, Instrumentation, Systems, and Automation Society, N.C., 2004 IEC 61511, Functional Safety Safety Instrumented Systems for the Process Industry Sector, International Electrotechnical Commission, Geneva, Switzerland, 2003. 

Safety instrumented function (SIF) A safety function allocated to the safety instrumented system with a safety integrity level necessary to achieve the desired risk reduction for an identified process hazard. 

Safety instrumented system (SIS) Any combination of separate and independent devices (sensors, logic solvers, final elements, and support systems) designed and managed to achieve a specified safety integrity level. An SIS may implement one or more safety instrumented functions. 

Introduction The chemical processing industry relies on many types of instrumented systems, e.g., the basic process control systems (BPCSs) and safety instrumented system (SIS). The BPCS controls the process on a continuous basis to maintain it within prescribed control limits. Operators supervise the process and, when necessary, take action on the process through the BPCS or other independent operator interface. The SIS detects the existence of unacceptable process conditions and takes action on the process to bring it to a safe state. In the past, these systems have also been called emergency shutdown systems, safety interlock systems, and safety critical systems. 

Safe Automation and ANSI/ISA 84.01-1996 served as significant technical references for the first international standard, IEC 61511, issued by the International Electrotechnical Commission (IEC). In the United States, IEC 61511 was accepted by ISA as ISA 84.00.01-2004, replacing the 1996 standard. In 2004, the European Committee for Electrotechnical Standardization (CENELEC) and the American National Standards Institute (ANSI) recognized IEC 61511 as a consensus standard for the process industry. IEC 61511 covers the complete process safety management life cycle. With its adoption, this standard serves as the primary driving force behind the work processes followed to achieve and maintain safe operation using safety instrumented systems. 

Layer of Protection Analysis (LOPA) Scenario- based Order-of- magnitude By preidentified scenario Processes likely to require independent protection layers, such as safety instrumented systems, to meet predefined risk criteria Dependent on comprehensiveness of scenario list identified by other method(s) Higher... 

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). 

Instrumentation, Systems, and Automation Society (ISA) Standard 84, Application of Safety Instrumented Systems for the Process Industries, outlines the principles of high integrity shutdown systems. 

More and more commonly, fire alarm panel data is transferred to a safety instrumented system (SIS) for graphic annunciation though the SIS human-machine interface (HMI). 

When a gas and vapor detection system is used either for fire protection or for personnel exposure protection, the system should be managed, inspected and tested on a scheduled basis, and maintained similar to any other major element of the fire protection system or any process safety interlock or safety instrumented system. 

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

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. 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

Validation—The activity of demonstrating that the safety-instrumented system under consideration, after installation, meets in all respects the safety requirements specification for that safety-instrumented system. 

The integrity of safety instrumented systems is usually dependent on proof testing at specified intervals (often at a frequency between monthly and three-monthly). Safety management systemst2jsafety critical operating procedures such as described in (c) above are adhered to. 

Draft Standard IEC 61511, "Functional Safety Safety Instrumented Systems for the Process Industries", (Contact BSI/IEC for availability)... 

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