Safety instrumentation systems sensor

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. 

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

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 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. 

Sensors in a safety instrumented system measure process variable conditions in order to recognize a potential hazard. Usually these are the same process variables that are used for control. So the first and perhaps most important consideration when selecting sensors for safety applications is that they accurately and reliably measure the process variable. Another key parameter is that any process wetted materials must be compatible with the chemicals of the process. These are two of the key principles required in a "well designed system."... 

Sensors designed for safety instrumented system applications t)rpically have excellent built-in automatic diagnostics. That attribute is one of the main advantages of sensors designed per lEC 61508. However, it must be remembered that automatic diagnostics must be annunciated so that a repair team can quickly restore correct operation. Without annunciation and effective repair, the diagnostics do little good. 

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. 

This internationai standard addresses the appiication of safety instrumented systems for the Process industries, it aiso requires a process hazard and risk assessment to be carried out to enabie the specification for safety instrumented systems to be derived. Other safety systems are oniy considered so that their contribution can be taken into account when considering the performance requirements for the safety instrumented systems. The safety instrumented system inciudes aii components and subsystems necessary to carry out the safety instrumented function from sensor(s) to finai eiement(s). 

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. 

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. 

D. Perry, Selecting Sensors for Safety Instrumented Systems as per lEC 61511 (ISA 84.00.01-2004), Emerson Process ManagementdRosemount Division (internet document) http //www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Doc... 

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. 

In modeling for safety instrumented systems the basic events will typically be the failures of a sensor and/or the occurrence of an initiating event in the process operations. Figure 3.19 provides an example of an FTA model for a batch process with potential instrument and process equipment failures. 

Failure rates for instrumentation used in safety instrumented systems should be calculated as a sum of the possible failures of the process connection, the sensor or transmitter itself and its associated cabling and power supplies. Similarly failure rates for actuators must take into account air supplies, solenoids, drive cylinders and valves. 

The Safety Protection Subsystem of the Plant Protection and Instrumentation System (PPIS) is that portion of the PPIS which performs lOCFRlOO-related functions. It includes the reactor trip Instrumentation hardware and associated system sensors which are used to detect abnormalities in the plant... 

Typical safety-related systems employ pressure and level sensors which use small bore instrumentation lines. Most operating plants contain safety-related equipment and systems, parts of which are exposed to the ambient environment. These lines generally contain liquid (e.g., borated water) which is susceptible to freezing. Where systems or components and their associated instrumentation are exposed to sub-freezing temperatures, heat tracing and /or insulation is used to minimize the effects of cold temperatures. 

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. 

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