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ARC White Paper September 2004, Safety Through Layers of Protection. Risk Reduction is the Highest Priority, 2 Copyright ARC Advisory Group ARCweb com. ARC White Paper September 2004, Executive Overview. Since the publication of the IEC 61508 safety standard and more recently. the IEC 61511 standard for process safety the interest in performing rigor. ous hazard and risk analysis and applying certified Safety Instrumented. Systems SIS has increased considerably within the user community As. users become more knowledgeable about safety issues they are sharpening. their focus on overall safety with automated monitoring and testing of field. devices Users want their safety instrumented systems SISs to satisfy their. needs in a more cost effective way through integration with control sys. tems less frequent proof testing and scalable architectures They are also. looking for increased capability to modify, Increased focus on overall safety alarm limits based on process conditions. Automated monitoring testing of field device and orderly shutdown procedures in case. Closer integration with control systems, of an emergency With the release of the.
IEC 61511 safety standard for process in, Increased flexibility and scalability. dustries there is now no excuse for users, Enhanced functionality for taking care of varying. not to cost effectively put safety as the, states of a process. first priority, Use of industry standard operating systems. Major Trends in Process Safety Today the main cause of SIS failure is not. the failure of logic solvers but the failure, of field devices A protective system needs to address overall health of.
safety loops by incorporating the checking of field devices in its overall de. sign Consequently the ability to provide an integrated safety solution. from sensor to actuator should be an important criterion when selecting an. Control valves are available now that have very low probability of stem. seizure and packing failure T V certified valve controllers and actuators. are also available in the marketplace The SIS design should include lim. ited valve movement testing as an integral design feature. Suppliers are now offering similar systems for control and SIS where simi. lar configuration procedures programming languages and maintenance. procedures are used The two systems communicate with each other but. with adequate protection from corruption of one by the other Choose a. system that provides flexibility in the configuration of logic solvers along. with rich function block capabilities, Copyright ARC Advisory Group ARCweb com 3. ARC White Paper September 2004, It is important to choose a system that integrates information about the. health of the field devices into the logic Users should choose a system that. offers a transparently integrated configuration operations and maintenance. environment with the required separation between safety and control Us. ers should make sure to select a system that makes compliance with. International Safety Standards as painless as possible. Risk Reduction Is the Highest Priority, Risk is usually defined as the combination of probability and the severity of. an unplanned event That is how often can it happen and how bad is it. when it does Examples of events and their associated risks in manufactur. ing operations include loss of life or, Operating plant machinery close to their limits. limb environmental impact loss of, Transient operation states.
startup shutdown shift change work force transitions capital equipment and loss of pro. duction For many manufacturers, Use of hazardous raw materials. loss of company image can also be a, Manufacture of hazardous intermediates. significant risk factor Add to these, Presence of untrained personnel issues the realities of increased envi. Absence of safety culture ronmental awareness regulatory. Factors that Increase Risk concerns and threat of litigation and. it is easy to see why risk reduction is, becoming increasingly important to most manufacturers. The best way to reduce risk in a manufacturing plant is to design inherently. safe processes However inherent safety is rarely achievable in today s. manufacturing environments Risks prevail wherever there are hazardous. or toxic materials stored processed or handled, Higher environmental awareness.
Increased regulatory considerations Since it is impossible to eliminate all risks a manu. Emergence of safety standards, facturer must agree on a level of risk that is. considered tolerable After identifying the hazards. Maintaining company image, a hazard and risk study should be performed to. Forces Driving Lower Risk, evaluate each risk situation by considering likeli. hood and severity Site specific conditions such as. population density in plant traffic patterns and meteorological conditions. should also be taken into consideration during risk evaluation. 4 Copyright ARC Advisory Group ARCweb com, ARC White Paper September 2004. Once the hazard and risk study has ascertained the risks it can be deter. mined whether they are below acceptable levels Basic process control. systems along with process alarms and facilities for manual intervention. provide the first level of protection and reduce the risk in a manufacturing. facility Additional protection measures are needed when a basic control. system does not reduce the risk to a tolerable level They include safety. instrumented systems along with hardware interlocks relief valves and. containment dikes To be effective each protection subsystem must act in. dependently of all others, Safety Standards Guide Best Practices.
Since the publication of the IEC 61508 safety standard and more recently. the IEC 61511 standard for process safety interest in performing rigorous. hazard and risk analysis and applying certified safety instrumented sys. tems has increased considerably within the user community These. standards give guidance on best practice and offer recommendations but. do not absolve their users of responsibility for safety The standards deal. not only with technical issues but also include the planning documentation. and assessment of all activities required to manage safety throughout the. entire life of a system, The IEC 61508 Safety standard published by the International Electrotech. nical Commission IEC is applicable to a wide range of industries and. applications and is written primarily for the supplier community The IEC. 61508 standard is composed of seven parts starting with general safety re. quirements to specific system and software requirements and guidelines to. applications The standard is generic and can be used directly by industry. as a standalone standard and by international standards organizations as a. basis for the development of industry specific standards such as for the. machinery sector the process sector or for the nuclear sector It is sug. gested that in evaluating a safety system a user should select one that is. certified by an independent third party such as T V or FM. Note that the certificate from the independent body should be reviewed in. parallel with the User Safety Manual This is a document that defines the. Copyright ARC Advisory Group ARCweb com 5, ARC White Paper September 2004. restrictions on use of an SIS component The manual for a good safety sys. tem is very thin with a minimal number of restrictions Beware of a thick. safety manual it indicates that there are many complexities and limitations. associated with the use of the SIS, Safety Integrity Levels. Safety integrity is defined as the likelihood of a safety instrumented system. satisfactorily performing the required safety functions under all stated con. ditions within a given period A safety integrity level SIL is defined as a. discrete level for specifying the safety integrity requirements of safety func. tions A safety integrity level is derived from an assessment of risk it is not. a measure of risk It is a measure of the intended reliability of a system or. Safety Integrity Probability of Failure Probability of Failure. Level SIL on Demand Mode on Continuous Mode, 1 10 2 to 10 1 10 6 to 10 5. 2 10 3 to 10 2 10 7 to 10 6, 3 10 4 to 10 3 10 8 to 10 7.
4 10 5 to 10 4 10 9 to 10 8, Safety Integrity Levels. Demand Mode Where actions are taken in response to process or other. conditions no more than once per year, Continuous Mode Functions that implement continuous control to main. tain functional safety, IEC 61511 The Safety Standard for Process Industries. The IEC 61511 standard is targeted specifically at end users in the process. industry This standard provides best safety practices for all users to follow. in the implementation of a modern SIS While IEC 61508 is composed of. seven parts IEC 61511 has only three, Part 1 Framework definitions system hardware and software re. quirements, Part 2 Guidelines on the application, Part 3 Guidance for the determination of the required safety integrity.
6 Copyright ARC Advisory Group ARCweb com, ARC White Paper September 2004. IEC 61511 Part 1 is primarily normative while Parts 2 and 3 are informative. Part 1 of the IEC 61511 standard is structured to adhere to a safety lifecycle. model The hazard and risk analysis utilizes the concept of protection lay. ers and specifies the safety integrity level concept developed by the IEC. 61508 standard It also lists key issues that need to be addressed when de. veloping a safety requirement specification Issues like separation. common cause response to fault detection hardware reliability and. proven in use are also addressed in this part, IEC 61508 IEC 61511. Generic safety standard for broad Sector specific safety standard for the. range of applications process industries, Applies to all safety related systems Applies only to safety instrumented. and external risk reduction facilities systems, Primarily for manufacturers and sup Primarily for system designers inte. pliers of safety systems and devices grators and users of safety systems. and devices, Main Differences Between IEC 61508 IEC 61511 Standards.
Software safety requirement specifications are included addressing such. items as architecture relationship to hardware safety instrumented func. tions safety integrity levels software validation planning support tools. testing integration and modification In addition a section is dedicated to. Factory Acceptance Testing requirements and another section lists the in. stallation and commissioning requirements, Part 2 of the standard provides how to guidance on the specification de. sign installation operation and maintenance of safety instrumented. functions and related safety instrumented system as defined in Part 1 of the. standard It has borrowed heavily from the ISA technical report. dTR84 0 02 which provides guidance on methods to calculate the perform. ance of safety instrumented systems, Part 3 of the standard provides guidance for development of process haz. ard and risk analysis It provides information on the underlying concepts. of risk and the relationship of risk to safety integrity and the determination. of tolerable risks The ANSI ISA 84 safety standard which predates the. international safety standards will soon be updated to closely follow the. IEC 61511 standard, Copyright ARC Advisory Group ARCweb com 7. ARC White Paper September 2004, Major Trends in Safety Instrumented. As manufacturers become more knowledgeable about safety issues they. are performing more thorough hazard and risk analysis to determine their. needs more accurately They are looking for reduction of risk by increasing. their focus on overall safety They would like their SIS to satisfy their needs. in a more cost effective way by closer integration of safety with control sys. tems They are also looking for a flexible architecture along with more. scalability increased functionality for modifying alarm limits based on. process conditions and orderly shutdown procedures in case of emer. Increased Focus on Overall Safety, The main cause of an SIS failure is not the failure of logic solvers but the.
failure of field devices There has been a significant advance in the devel. opment of the architecture of logic solvers with voting circuits and. advanced diagnostics However they do not address over 90 percent of the. causes for failure which are due to the failure of sensors and actuators. Main Causes of SIS Failures, Today a protective system should address the need for checking the health. of the I O and field devices In fact it needs to incorporate monitoring of. I O components in its overall design Examples include. Sensor validation, Environment condition monitoring such as temperature and humidity. that can cause sensor degradation, Transmitter drift. 8 Copyright ARC Advisory Group ARCweb com, ARC White Paper September 2004. Common failures of electronic components are frequently due to environ. mental conditions Many electronic devices fail when exposed to elevated. humidity and temperature conditions which need to be monitored closely. Sensor calibration is also becoming an, 1 Integrate health data from field devices into.
logic solvers integral part of an SIS Use of open pro. tocols such as HART and in principle, 2 Degrade the quality of the signal from a field. input device when the input is dubious FOUNDATION Fieldbus allows for re. Users want their safety instrumented systems SISs to satisfy their needs in a more cost effective way through integration with control sys tems less frequent proof testing and scalable architectures They are also looking for increased capability to modify alarm limits based on process conditions and orderly shutdown procedures in case of an emergency With the release of the IEC 61511

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