Safety integrity issues

Thus far the components that make up a control system have been described without considering in detail how they are integrated to ensure that the systems have sufficient robustness, or safety integrity, for the safety application in which they will be used. Safety integrity refers to the reliability with which the system will perform the safety-related tasks it has been designed to perform, as well as the extent to which it will continue to perform its safety functions in the event of faults occurring. We are therefore interested in system performance in terms of reliability and fault tolerance. 

It would seem, intuitively, that the greater the contribution that a control system makes to safety the more reliable it should be and the more resistant it should be to faults that would adversely affect the safety function. These are important concepts that are covered in two standards that are relevant to machinery safety applications and which are the subject of this section of the 

BS EN 954 1997 Safety of machinery - safety-related parts of control systems. Part 1. General principles for design 

BS EN 954-1 1997 is the British version of EN 954-1, which was written by CEN Technical Committee 114, a European standards-making committee. It is a Type B1 standard which is meant to be used by technical committees preparing Type B2 and Type C European standards. It is interesting to observe that the UK voted against this standard at the formal vote stage of its development, for reasons that will become apparent. 

The aim is to ensure that those parts of a control system that perform safety functions have adequate safety performance for the application. Safety performance, as previously noted for safety integrity, relates to the reliability and fault tolerance of the safety-related parts, and their ability to provide the required level of risk reduction. It is therefore axiomatic that the process of satisfying the requirements of the standard for any particular machine starts with a risk assessment of that machine. 

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Systematic safety integrity issues including avoidance of failures, control of systematic faults, system behavior on fault detection, and proven equipment issue. 

Agent Hydrolysis and Treatment of Metal Parts (Steps 8, 10, 13, and 16), 115 Treatment of Hydrolysates and Dunnage by Supercritical Water Oxidation (Steps 11, 12, and 14), 116 Assessment of Integration Issues, 118 Component Integration, 118 Process Operability, 119 Monitoring and Control Strategy, 119 Maintenance Issues, 119 Process Safety, 120 Worker Health and Safety, 121 Public Safety, 121... 

Besides the main components, particular emphasis was placed on the essential subsystems/peripherals/balance-of-plant, including utility and auxiliary subsystems (instrument/operating air supplies, nitrogen supply, demineralised water/KOH systems, ventilation, etc.), process and safety control subsystems, and extensive test data acquisition subsystems. Also power conditioning (converters and inverters) as a way of improving the operability and efficiency of the overall system was considered. Some of the integration issues investigated in detail were ... 

In addition to establishing joint working groups to address integrated safety management issue, we should begin to benchmark the safety authorization basis at selected Russian facilities. 

Dutuit, Y, Innal, R, Rauzy, A. and Signoret, J.-P. 2008. Probabilistic assessments in relationship with safety integrity levels by using Fault Trees, Reliab. Eng. Syst. Safety vol. 93 Issue 12, Dec. 2008. 

U.S. Department of Transportation (2002), Federal Railroad Administration Issues Final Rule Regarding Safety Integration Planning for Future Railroad Consolidations, http //www.dot.gov/affairs/fra0202.htm. Washington, D.C. (accessed 3/25/02). 

Cooling within a reactor is typically provided by a reflux condenser. Since polymerization is a highly exothermic process, temperature control is a safety concern as well as a product integrity issue. Temperatiue control is primarily... 

SIL stands for safety integrity level. It is a measure of performance of an SIS. It is determined by PFD for SIF (SIS). There are four SIL levels represented by number, viz. SIL 1, 2, 3, 4. Higher the SIL number, the better will be the performance and lower will be PFD value. However, with an increase in SIL number, the cost and complexity of the system increases, but risk level reduces. It is worth noting that there could be individual component PFD but not SIL. SIL is only given to a system (SIS). SIL certification can be issued by the company (self-certification allowed), or other competent authority to indicate that appropriate procedure, analysis, and calculation... 

Systematic failure normally occurs on account of design failure, including incorrect specifications, using a component not fit for the operation, and or due to error in software. Safety life cycle is adapted for systematic faults. So safety standards meant for E/E/PEs take care of both. SISs (Ref. Chapter VII) are developed to prevent or mitigate hazardous events to protect people or the environment, or prevent damage to process equipment. In this connection another important issue is SIL (Chapter VIII), which is a discrete level for specifying the safety integrity requirements of safety functions, but is not a measure of risk. SIL provides means for risk reduction to a tolerable level. The fundamental question, in case of functionally safe instrumentation, is how frequently failures of function will lead to accidents. The answers can be ... 

The safety specification (Clause 7.2) shall include details on both the safety function and the safety integrity level. E/E/PEs safety requirements need to be expressed in clear, precise, unambiguous, verifiable, testable, maintainable, and feasible way so that these are comprehensible. Major issues shall include ... 

Often SIL is considered as the overall measure for SIS reliability (safety integrity). From the standard, it is clear that there are three important issues in system integrity, and these are hardware, software, and systematic integrity. SIS reliability quantification is influenced by many factors, for example, selection of RBD, etc. On account of dependence on environmental conditions and human interaction, it is not always easy to keep independence between SIS and rest of the system. Naturally, it is important to be aware of some side effects introduced by... 

Fire and Gas System and Safety Integrity Level Issues With Short Planning Discussions... 

For any safety loop comprising several components, the safety integrity level (SIL) achievement is a joint responsibility of end-user and supplier, as will be clear from Table IX/1.0-1. Why discussing this here These are discussed here to show that equipment manufacturer/system integrator or end-user is not only responsible for the same in isolation. In a safety life cycle, there are several phases involving several activities. So, at various stages there will be involvement of either end-user or sup-plier/manufacturers. The same issue has been elaborated in Clause 1.0.1 refer to Fig. IX/1.0-1 also. 

A piece of equipment, or software, eannot be exeluded from this safety-related category merely by identifying that there are alternative means of protection. This would be to prejudge the issue and a formal safety integrity assessment would still be required to determine whether the overall degree of protection is adequate. 

Usability issues were detected for example when the users worked with the safety integrity measures suggested by the standard. The tabular form arrangement provided by lEC 61508 was cumbersome for many people especially when there is a need to... 

Policy that places employee safety and health on the same level of importance as customer, patient, and client safety. The responsible implementation of this policy requires management to integrate issues of employee safety and security to ensure that this protection is part of the daily functioning of the workplace. 

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