Logic solver safety integrated systems

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. 

This example illustrates how one particular SIS logic solver integrator develops safety application software for its customers. This software is typically developed as a part of an overall system integration process that is discussed in the section below. 

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

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). 

SIS Logic solver Software requirements Integrated systems Function protected against changes and override See user and/or safety manual requirements See user and/or manual requirements... 

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 ... 

An integrated control/safety system with same operating system may have systematic design and common cause errors. Another important point here is that, third-party certificate of compliance of the equipment is not really sufficient, especially for end-user who has the ultimate responsibility with the plant operating company s management, not the vendor. International standards call for compliance of SIS logic solvers to lEC 61508. Such third party certification does not cover ... 

However, common HMI for safety system operator is permitted. These are elaborated to indicate that FGS is treated as another safety system — SIS. Also, Clause 10.4.7 of NORSOK 3001 2008 demands that ESD system shall operate as an independent system. ESD functions shall be functionally and physically segregated from other systems/functions but the ESD logic solver can be an integrated node in the overall SAS ... 

Comparative attributes of various types of logic solver for high-integrity protection (safety) systems (author s judgments). 

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. 

The standard tells us that the method to be used is to decompose the SIS into major sub systems (e.g. sensors, logic solvers, actuators, also I/O sub systems) and apply the design rules for fault tolerance to ensure that each sub system meets the SIL value needed for the application. Then when we put the whole loop together we can work out if the overall SIS will meet the safety integrity requirements we have been asked to achieve. The SIL value we get will be limited to the lowest SIL value of the series connected sub systems as shown in the diagram below based on Figure 5 in lEC 61508 part 2. 

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