Safety instrumentation systems logic solver

The safety instrumented system logic solvers addressed include Electrical (E)/Electronic (E)/ and Programmable Electronic (PE) technology. Where other technologies are used for logic solvers, the basic principles of this standard may also be applied. This standard also addresses the safety instrumented system sensors and final elements regardless of the technology used. This International Standard is process industry specific within the framework of the lEC 61508 series. 

Programmable logic controllers (PLC) often seem a logical choice for safety instrumented system logic solvers. After all, the PLC was designed... 

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

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. 

At PPG, Class 1 Prooftesting also covers 250 Safety Instrumented System loops in the PSM Safety Systems. A Safety Instrumented System (SIS) is composed of sensors, logic solvers, and final control elements for the purpose of taking the process to a safe state when predetermined conditions are violated. SISs are normally controlled by a PLC with the sole function of monitoring a process to insure operation is maintained within the safe operating envelope. 

This International Standard addresses the application of safety instrumented systems for the Process Industries. It also deals with the interface between safety instrumented systems and other safety systems in requiring that a process hazard and risk assessment be carried out. The safety instrumented system includes sensors, logic solvers and final elements. 

The major safety instrumented system integration services provided with the SIS logic solver consisted of a number of activities including. 

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. 

The analog signal comparison can be performed between two or more transmitters that are part of the SIF or between a SIF transmitter and a DCS transmitter (measuring the same process variable). In order to take credit for the comparison it needs to be performed in the SIS logic solver. The software in the Safety Instrumented System is safety critical rated. This is not the case for the DCS software. 

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). 

The present paper considers the safety of different systems or components. A safety instrumented system (SIS) is a system which consists of sensors, logic solvers and actuating items. The sensors may for instance be gas detectors, the logic solver could be a computer and the actuating items may be shut down valves. A fire and gas detection system with an alarm or a sprinkler system is an example of a SIS. A SIS is constructed to take the process into a safe state if a dangerous event occurs. 

NOTE 2 Diagnostic coverage is applied to components or subsystems of a safety instrumented system. For example, the diagnostic coverage is typically determined for a sensor, final element or a logic solver. 

NOTE Instrumented systems in the process sector are typically composed of sensors (for example, pressure, flow, temperature transmitters), logic solvers or control systems (for example, programmable controllers, distributed control systems), and final elements (for example, control valves). In special cases, instrumented systems can be safety instrumented systems (see 3.2.72). 

The following requirements may only be applied to PE logic solvers used in safety instrumented systems which implement SIL 1 or SIL 2 safety instrumented functions. 

ANSI/ISA-84.00.01-2004-1 introduced the concept that safety functions are identified during the hazard and risk analysis and allocated to protection layers. When the safety function is allocated to the safety instrumented system, the function becomes a safety instrumented function. The SIF is designed to mitigate a specified safety-related process risk using sensor(s), logic solver(s), and final element(s). At this time, SIF is a process industry sector specific term. 

Prior to the release of lEC 61508, PE logic solvers used in safety instrumented system applications were often certified for compliance with the German standard DIN N E 0801. These PE logic solvers were rated based on AK Classes 1 through 6. These certifications can be considered in the evaluation of the acceptability of the use of the PE logic solver in an SIS application, however owners/operators should note that the current recognized standard for PE devices (sensors, logic solvers, and control elements) is lEC 61508. 

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. 

Safety instrumented system (SIS) SIS is meant to prevent, control, or mitigate hazardous events and take the process to a safe state when predetermined conditions are violated. An SIS can be one or more SIFs, which is composed of a combination of sensors, logic solvers, and final elements. Other common terms for SISs are safety interlock systems, emergency shutdown (ESD) systems, and safety shutdown systems (SSDs). So, SIS is used as a protection layer between the hazards of the process and the public. SIS or SIF is extremely important when there is no other non-instrumented way of adequately eliminating or mitigating process risks. As per recommendations of standards lEC 61511 2003 (or ANSI/ ISA-84.00.01-2004), a multi-disciplinary team approach following the safety life cycle, conducts hazard analysis, develops layers of protections, and implements an SIS when hazardous events cannot be controlled, prevented, or mitigated adequately by non-instrumented means. 

Safety Instrumented Systems, The Logic of Single Loop Logic Solvers Moor Industries Worldwide, Brochure, Moore IndustrieS International, Inc., January 2014-... 

We begin by revisiting the basic configuration of a safety instrumented system to help us to recognize the role of the logic solver. 

The details in the standard are applicable to all parts of the SIS and apply directly to persons building up a safety system from major parts such as instruments and logic solvers. When there is any doubt about what is the correct practice to use in a given design this section of lEC 61508 should be consulted. 

The next figure is tree structure diagram showing typical components of life cycle cost for a safety instrumented system. A more comprehensive list would have to show details of all project related costs. For annual costs the model has to includes items such as the service agreement and software licensing costs for the logic solver. A model with suitable... 

Can a relay logic system be used to satisfy the requirements for a SIL 3 Safety Instrumented Function What are some of the issues that need to be addressed in using relays as a logic solver ... 

LIC-01 controls the level in the vessel as part of the BPCS system. SIF-15 is the safety instrumented function. The input signal to the SIF is LT-2, and the output signals from the logic solver operate XV-1, XV-2, and P-01. 

An operator-initiated SIF is often associated with a never exoeed never deviate alarm, where the operator is expected to mitigate risk in much the same manner as an automated SIF. Operator-Initiated SIFs are generally used when it is not possible to completely automate the function. The manually initiated action is typically comprised of the sensor detecting the hazardous condition, the logic solver that determines that the safety condition exists, alarm presentation, human response, and the equipment used by the operator to bring the process to a safe state. When risk reduction is taken for an operator-initiated SIF, the PFDavg should be determined for the instrumented system. This is discussed further in B.6. 

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 ... 

As per lEC 61511-3 2003 Clause 9.4.3, operator action as part of safety instrument functions (SIFs) can be credited with a level of risk reduction greater than 10 when the system from the sensor to the final element can be designed and evaluated as an SIS per the requirements of lEC 61511. A typical automated SIS, popularly known as an industrial automation and control system (LACS), from the sensor to the final element can be conceived, as shown in Fig. VIII/1.4-1 or Fig. VII/1.3-1 where the main constituents are sensor, logic solver, and final element. When an operator action such as through the display/alarm is necessary this needs to be as shown in Fig. XI/2.4.3-1. 

ISA Standard, Safety Instrumented Functions (SIF)— Safety Integrity Level (SIL) Evaluation Techniques Part 5 Determining the PFD of SIS Logic Solvers via Markov Analysis, TR84.00.02-2002, Part 5, 2002. ISA-The Instrumentation, Systems, and Automation Society. Research Triangle Park, NC. 

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