Logic solver defined

Manual means that are independent of both the SIS logic solver and the BPCS control system may be provided to allow the operator to initiate a shutdown in an emergency. The requirements for manual shutdown are normally defined in the SRS. 

Table 6 of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Modi defines the basic level of fault tolerance for sensors, final elements, and non-PE logic solvers having the required SIL claim limit in the first column. The requirements in Table 6 are based on the requirements in lEC 61508-2 for PE devices with a SFF between 60 and 90 %. The requirements are based on the assumption that the dominant failure mode is to the safe state or that dangerous failures are detected. 

The application software should also take into consideration the diagnostics provided by the PES and be developed to take the appropriate actions defined in the logic solver safety manual. 

SIF is defined as a collection of sensors, logic solver and final elements that protect against a particular hazard. There may be multiple sensors and final elements for each safety function as shown in Figure 3.8. The fundamental work process for SIL evaluation includes the following. 

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. 

Practitioners often prefer a more functional definition of SIS such as "A SIS is defined as a system composed of sensors, logic solvers and final elements (Figure 2-1) designed for the purpose of ... 

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 objective of a factory acceptance test (FAT) is to test the logic solver and associated software together to ensure it satisfies the requirements defined in the safety requirement specification. By testing the logic solver and associated software prior to installing in a plant, errors can be readily identified and corrected. 

FAT should take place on a defined version of the logic solver. 

Defined software safety life cycle - required activities defined to develop application software for each programmed SIS subsystem (sensor, logic solver, and final elements) -12.1.1.1... 

The FAT should be conducted on defined version of logic solver -13.2.3... 

Have suitable internees between the sensors and final elemertts and the logic solver been specified, and has the scaling between input and output signals and plant parameters been defined ... 

The device selection process in ANSI/ISA-84.00.01-2004 Clause 11.5 is illustrated in Figure L.1. This process defines when prior-use information is considered sufficient to Justify use of a device in an SIF application and no claim limit per lEC 61508 is required. The prior use evaluation fulfills specific requirements in ANSI/ISA-84.00.01-2004-1, Clauses 11.4 and 11.5.3 through 11.5.6. The methodologies and approaches for prior-use Justification may vary, depending on the device type (e.g. programmable versus non-programmable, field device versus PE logic solver), the device s installed base, and the owners/operators experience with the device. 

Analyze the logic solver components to define the potential dangerous, hardware failure modes. 

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. 

Interface The interface between sensor and logic solver and the interface between logic solver and final elements must be properly defined especially when... 

Corrective action procedure for test failure Competence of test persormel Defined version of logic solver Each SIF and performance characteristics Each test Test procedure and description Each test s chronological description of activities Test tool, environment, and interface details for each test... 

Figl. 1 defines the SIS as bounded by sensors, logic solver and actuators with associated interfaces to users and the basic process control system. We are talking about automatic control systems or devices that will protect persons, plant equipment or the environment against harm that may arise from specified hazardous conditions. 

Solid-state logic solvers are available to provide high integrity solutions that avoid dependency on software. They are considered particularly suitable for well-defined applications where software flexibility is not required. The need for small and simplistic solutions, however, remains and the option to use simple relay based systems is always available. 

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