Safety instrumentation systems final elements

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 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. 

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 safety instrumented system, like a basic process control system, is also composed of sensors, controUer(s), and final elements. Although much of the hardware appears to be similar, safety instrumented systems and basic process control systems differ very much in function. The primary function of a control loop is generally to maintain a process variable within prescribed limits. A safety instrumented system monitors a process variable and initiates action when required. 

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). 

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). 

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. 

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. 

A. Coleman, Safety Instrumented Systems Control Valves as Final Elements, Emerson Process Management Chemical Engineering, January 2011. 

FINAL CONTROL ELEMENTS FOR SAFETY INSTRUMENTED SYSTEMS... 

Final Control Elements for Safety Instrumented Systems... 

Predicting health of final control element of safety instrumented system hy digital valve controller, in Presented at ISA Automation West, R. Ali — Fisher Controls Int l., LLC, 2004- www.isa.org. 

From lEC 61508 Part 6, a definition of commonly used architectures in safety instrumented systems is available. The elements used in a single or multiple configuration can be either sensors or final elements—mainly for input sensors, and only a few for the final element on account of cost [4]. Typical interfaces of these with an intelligent control (DCS/PLC) system are shown in Fig. Xl/1.3.2-1. The configuration may be lool, which is quite vulnerable because single instrument failure wUl make the loop unavailable. 

Final element This represents that part of a safety instrumented system which is responsible for implementation of the physical action necessary to achieve a safe state. 

These valves are often designed to be operated remotely either by operator action at a panel in a safe location or by an automatic trip system. Similar valves are used on offshore platforms where they are known as ESDVs. Such valves are effectively final elements in a functional safety system and all fall within the same guidelines applicable to safety instrumented systems. 

This final chapter aims to summarise a number of the themes which regularly crop up within the book. A second aim of this chapter is to offer some speculations regarding the likely directions the field of Patient Safety Culture (PSC) will take in the coming years, as well as potential areas for future exploration. A key argument is that the field of PSC needs to move beyond ciurent concerns and expand its theoretical and methodological horizons. Part of this will involve improving the way in which we develop and test PSC surveys, tools and instruments. A second element is the need to take a closer look at some of the key lessons from other industries and work towards an improved application of a wider systems approach to future PSC research and practice (Wilson 2014). 

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). 

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. 

The European standard EN 12186 (formerly the DIN G491) and more specific the EN 14382 (formerly DIN 3381) has been used for the past decades in (mechanically) instrumented overpressure protection systems. These standards prescribe the requirements for the overpressure protection systems, and their components, in gas plants. Not only the response time and accuracy of the loop but also safety factors for oversizing of the actuator of the final element are dictated by these standards. Independent design verification and testing to prove compliance to the EN 14382 standard is mandatory. Therefore, the users often refer to this standard for HIPPS design. 

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