Safety instrumentation systems implementation

References Guidelines for Safe and Reliable Instrumented Protective Systems, American Institute of Chemical Engineers, New York, 2007 ISA TR84.00.04, Guidelines for the Implementation of ANSI/ISA 84.00.01-2004 (IEC 61511), Instrumentation, Systems, and Automation Society, N.C., 2005 ANSI/ISA 84.00.01-2004, Functional Safety Safety Instrumented Systems for the Process Industry Sector, Instrumentation, Systems, and Automation Society, N.C., 2004 IEC 61511, Functional Safety Safety Instrumented Systems for the Process Industry Sector, International Electrotechnical Commission, Geneva, Switzerland, 2003. 

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. 

Once the severity and the probability corresponding to a scenario are estimated, that is, the risk is assessed, a decision can be made on the nature of the protection system to be implemented. If a safety instrumented system (SIS) is to be used, consisting of one or more independent protection levels (IPL), the required reliability of the protection system, constituting a so-called Safety Integrated Level (SIL) can be determined by using this risk assessment, respective of the required risk reduction. 

The skills and knowledge required to implement any of the activities of the safety life cycle relating to the safety instrumented systems should be identified and for each skill, the required competency levels should be defined. Resources should be assessed against each skill for competency and also the number of people per skill required. When differences are identified, development plans should be established to enable the required competency levels to be achieved in a timely manner. When shortages of skills arise, suitably qualified and experienced personnel may be recruited or contracted. 

Safety instrumented system audits provide beneficial information to plant management, instrument maintenance engineers and instrument design engineers. This enables management to be proactive and aware of the degree of implementation and effectiveness of their safety instrumented systems. Many types of audits, which can be carried out exist. The actual type, scope, and frequency of the audit of any specific activity should reflect the potential impact of the activity on the safety integrity. 

Management at the various levels may want to apply the relevant type of audit to gain information on the effectiveness of the implementation of their safety instrumented systems. Information from audits could be used to identify the procedures that have not been properly applied, leading to improved implementation. 

Which measurements need to be taken to detect the onset of the hazardous conditions. A simple example could be that a pressure rise above a specified vaiue needs to be detected. The value of the parameter at which action shouid be taken wiii need to be outside the normal operating range and less than the value that will result in the hazardous condition. An allowance will need to be made for the response of the system and the accuracy of measurement. In setting the limit, there will therefore need to be a discussion with those responsible for the safety instrumentation system design and implementation. 

A program to modify a programmable safety instrumented system should be managed, planned and resourced to the appropriate level to ensure the safe implementation of the change. 

A plan is created for the installation and commissioning. This step includes a comprehensive test to validate that all requirements from the original SRS have been completely and accurately implemented. Also needed is a revalidation plan that is a subset of the validation plan. When the installed system is tested and validated, the safety instrumented system is ready to provide protection when actual operation begins. 

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. 

Another difference between Basic Process Control System design and Safety Instrumented System design is that per ANSl/lSA-84.00.01-2004 (lEC 61511 Mod) these systems are designed and implemented to meet different risk reduction requirements presented by the various hazards. (Chapter 1)... 

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). 

Procedures shall be implemented to ensure prompt follow-up and satisfactory resolution of recommendations pertaining to the safety instrumented system arising from... 

Procedures shall be implemented to evaluate the performance of the safety instrumented system against its safety requirements including procedures for... 

Management of modification procedures shall be in place to initiate, document, review, implement and approve changes to the safety instrumented system other than replacement in kind (i.e. like for like). 

The following requirements may only be applied to PE logic solvers used in safety instrumented systems which implement SIL 1 or SIL 2 safety instrumented functions. 

The validation activities including validation of the safety instrumented system(s) with respect to the safety requirements specification including implementation and resolution of resulting recommendations. 

ISA-TR84.00.03, Guidance for Testing of Process Sector Safety Instrumented Functions (SIF) Implemented as or Within Safety Instrumented Systems (SIS), International Society of Automation (ISA), www.isa.orq. ISBN 978 1 55617 801-6. 

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. 

M. Lundteigen, M. Rausand, Common cause failures in safety instrumented systems on oil and gas installations Implementing defense measures through function testing, Journal of Loss Prevention in the Process Industries 20 (2007). Elsevier. 

With the introduction of safety standards lEC 61508 and 61511 (for process industries), there is a defined need for proper implementation of safety systems embedded into the main system. The safety life cycle has various phases. Phases 1 and 2 have been discussed at length in previous chapters (Chapter VI and Chapter VII and to a certain extent in Chapter IX). In this part, detailed discussions have been presented to include Phases 3—7, that is, from safety-related systems (SRSs) to modifications. This has been done purposefully so that prior to looking at the detailed implementation part of the standard, readers need to have some knowledge of the safety instrumented system (SIS), safety integrity level (SIL), and their implementation in various instrumentation components. So, this part of the discussions in conjunction with previous chapters will complete the topic of lEC 61508/61511. Safety instrumentation cannot be complete without discussions on explosion protection. With reference to lEC 60079-(0,10,14,15,17, etc.) and NEC (497,499,70, etc.), electrical area, classification of plant, explosion protection, etc. also have been included as part of this chapter to make the system complete in all respects. In view of this, these are presented in two sections. Section 1 for lEC implementation and Section 2 for explosion protection. 

Overall safety instrumentation system (SIS) implementation. ALARP, as low as reasonably practicable IPL, independent protection layer. 

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. 

Safety function Function implemented by a safety instrumented system or other safety related technological system for reduction risk of the facilities, i.e. to achieve or maintain a safe state for the process, with respect to a specific hazardous event. 

For example, the equipment tags of the Safety Instrumented Systems (SIS) should be specified in the Safety Requirement Specification (SRS), a live document made specifically for every installation (GL-070 2004 IEC 61508 2010). The PS should contain links to such relevant documentation. In addition to specific requirements for safety critical and barrier functions, the PS should have a clear description of equipment groups that are considered as part of the SCS/SBS. A properly created PS will allow the correct identification of critical equipment tags and the implementation of data into the CMMS. 

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