Safety-instrumented systems SIS

NOTE - ANSI/ISA-84.01-1996 may discuss SIL of an SIS. The reader should determine what the specific intent is by analyzing the application of the term. 

As stated previously, SIF is assigned a target SIL. An SIF is implemented using an SIS per the safety lifecycle. SIS is a process-sector specific term. 

No reproduction or networidng permitted without license from IHS 

Copyright International Society of Automation Provided by IMS under license with ISA 

No reproduction or networking permitted without license from IMS 

---

Design equipment to accommodate maximum operating envelope. Appropriate use of Safety-Related Systems (SRS) such as Safety-Instrumented Systems (SIS). 

Provide independent safety instrumented system (SIS) with periodic testing... 

House logic-solver components of BPCS, and safety instrumented systems (SIS) in a controlled environment... 

Safety Instrumented System (SIS) The instrumentation, controls, and interlocks provided for safe operation of the process. 

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. 

Introduction The chemical processing industry relies on many types of instrumented systems, e.g., the basic process control systems (BPCSs) and safety instrumented system (SIS). The BPCS controls the process on a continuous basis to maintain it within prescribed control limits. Operators supervise the process and, when necessary, take action on the process through the BPCS or other independent operator interface. The SIS detects the existence of unacceptable process conditions and takes action on the process to bring it to a safe state. In the past, these systems have also been called emergency shutdown systems, safety interlock systems, and safety critical systems. 

More and more commonly, fire alarm panel data is transferred to a safety instrumented system (SIS) for graphic annunciation though the SIS human-machine interface (HMI). 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

The international standard IEC 61511 [2] gives advice on the design of safety instrumented systems (SIS) and presents a layer concept to achieve reliability of protection systems. These principles can be applied to the protection of chemical reactors [3]. Figure 10.3 represents this layer of protection principles. The first layer is the process itself, meaning that it should be designed in such a way that it cannot give rise to a runaway reaction. Some concepts for achieving this objective are reviewed in Section 10.3. 

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 Safety Instrumented Systems (SIS) and critical alarms assigned a Class 1 include those that have been mandated as such by state or federal agencies an in-house technical safety review committee HAZOP studies and specific alarms deemed critical by operations supervisors. All of these Safety Instrumented Systems and alarms are on a regular prooftesting schedule. 

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. 

Application of Safety Instrumented Systems for the Process Industries, ISA-S84.01, Instrument Society of America, ISBN 1-55617-590-6, Research Triangle Park, NC, 1996. The objective of this document is to define the requirements for Safety Instrumented Systems (SIS) for the process industries. The SIS addressed here includes electrical, electronic, and programmable electronic technology. 

Also, the design practice includes P ID documentation, database specification and verification of purchased equipment, control design and performance analysis, software configuration, real-time simulation for DCS system checkout and operator training, reliability studies, interlock classification and risk assessment of safety instrumented systems (SIS), and hazard and operability (HAZOP) studies. 

Safety instrumented functions are derived from the safety function, have an associated safety integrity ievei (SiL) and are carried out by a specific safety instrumented system (SiS). For exampie, ciose vaive XY123 within 5 s when pressure in vessei ABC456 reaches 100 bar . Note that components of a safety instrumented system may be used by more than one safety instrumented function. 

Addition of layer of protection analysis (LOPA) in PHA to determine the safety integrity level (SIL) gap for safety instrumented systems (SIS)... 

A working definition of the Safely Lifea/cle is that it is an engineering process utilizing specific steps to ensure that Safety Instrumented Systems (SIS) are effective in their key mission of risk reduction as well as being cost effective over the life of the system. Activities associated with the Safety Lifecycle start when the conceptual design of facilities is complete and stop when the facilities are entirely decommissioned. Key activities associated with a Safety Lifecycle are outlined below. 

Functional safety is thus the primary objective in designing a safety instrumented system (SIS). To achieve an acceptable level of functional safety, several issues must be considered that may not be part of the normal design process for automation systems. These issues are provided as requirements in international standards. 

TR84.00.02-2002, Technical Report, Safety Instrumented System (SIS) - Safety Integrity Level (SIL) Evaluation Techniques, NC Research Triangle Park, ISA, 2002. 

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. 

Control System (BPCS), including functions of Supervisory Control and Data Acquisition (SCADA) system, the alarm system (AS) and Safety Instrumented Systems (SIS) performing defined Safety Instrumented Frmetions (SIF). Proper design of layers of protection is based on hazards analysis and risk assessment with consideration of human and organizational factors. It is essential to ensure required safety integrity level (SIL) for each of these layers. 

Safety instrumented systems (SIS) play a major part in industrial risk management as risk reduction measures. The main European standard for functional safety of SIS, denoted electrical / electronic / programmable electronic (E/E/PE) safety-related systems, is the EC 61508 (lEC, 2005a). The second edition will soon be adopted in 2009 (EC, 2009). Objectives are to enable the design of SIS, and the development of apphca-tion sector standards. Such examples are EC 61511 (lEC, 2004) for process industry, and EC 62061 (EC 2005b) for machinery. One of the main contributions of EC 61508 is to consider the overall system and software safety life cycle. The standard fi amework, with the corresponding normative parts and subclauses, is ... 

ABSTRACT This paper gives a brief review of the methods for calculating the Probabilily of Failure on Demand (PFD) of Safety Instrumented Systems (SIS) indicated in related lEC and ISA standards. The goal of the paper is to provide advice for SIL practitioners in industry and consulting on which of the possible approaches is most suitable for a given situation. The paper concludes that the sunphfied ecpiations provided in the referred standards are suitable for most SIS found in practical applications. For more complex apphcations where the equations do not directly apply, either customized formulas can be developed or simulation studies can be performed. 

The lEC 61508 standard (lEC 1998) is currently under revision and discussion by the community. Among other issues the methods for calculating the probability of failure on demand (PFD) of safety instrumented systems (SIS) are being revised. This paper reviews the existing methods and provides some indications for practitioners on which method can/should be used under which conditions. The authors would like to point to two papers with a similar theme, namely (Rouvroye 1999) and (Rouvroye 2002). The focus of these papers is on comparing different analysis techniques. 

ABSTRACT Common cause failures (CCFs) are an important part of reliability analysis when working with safety instrumented systems (SIS), and engineers have been aware of these types of failures since the midseventies (Fleming, 1974). The purpose of this paper is to develop a strategy for analyzing CCFs by smdying an example of an oil-pressure system. This paper presents an example which the standard j8-factor model is unable to describe properly. 

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. 

Safety instrumented systems (SIS) are frequently used in the oil and gas industry to detect the onsets of hazardous events (e.g., gas leakages and high pressures) and to mitigate their consequences to humans, the environment, and material assets. Failure to do so can lead to major accidents and it is therefore of vital importance to monitor the performance of the SIS in the operational phase. 

Incidents created by malicious acts (hacking, sabotage, virus infections). E.g. someone hacking into the Safety Instrumented Systems (SIS), taking control of the production and doing harm. 

---