Safety instrumented system
Encyclopedia
A Safety Instrumented System (SIS) is a form of process control
usually implemented in industrial processes, such as those of a factory or an oil refinery. The SIS performs specified functions to achieve or maintain a safe state of the process when unacceptable or dangerous process conditions are detected. Safety instrumented systems are separate and independent from regular control system
s but are composed of similar elements, including sensors, logic solvers, actuators and support systems.
The specified functions, or safety instrumented functions (SIF) are implemented as part of an overall risk reduction
strategy which is intended to reduce the likelihood of identified hazardous events involving a catastrophic release. The safe state is a state of the process operation where the hazardous event cannot occur. The safe state should be achieved within the process safety time. SIFs are focused on preventing hazardous events with a health&safety or environmental consequence.
The correct operation of an SIS requires a series of equipment to function properly. It must have sensors capable of detecting abnormal operating conditions, such as high flow, low level, or incorrect valve positioning. A logic solver is required to receive the sensor input signal(s), make appropriate decisions based on the nature of the signal(s), and change its outputs according to user-defined logic. The logic solver may use electrical, electronic or programmable electronic equipment, such as relay
s, trip amplifier
s, or programmable logic controller
s. Next, the change of the logic solver output(s) results in the final element(s) taking action on the process (e.g. closing a valve) to bring it to a safe state. Support systems, such as power, instrument air, and communications, are generally required for SIS operation. The support systems should be designed to provide the required integrity and reliability
.
International standard IEC 61511
was published in 2003 to provide guidance to end-users on the application of Safety Instrumented Systems in the process industries. This standard is based on IEC 61508
, a generic standard for design, construction, and operation of electrical/electronic/programmable electronic systems. Other industry sectors may also have standards that are based on IEC 61508, such as IEC 62061 (machinery systems), IEC 62425 (for railway signaling systems), IEC 61513 (for nuclear systems), and ISO 26262
(for road vehicles, currently a draft international standard).
The safety integrity requirements may be verified by reliability analysis. For SIS that operates on demand, it is often the probability of failure on demand (PFD) that is calculated. In the design phase, the PFD may be calculated using generic reliability data, for example from OREDA. Later on, the initial PFD estimates may be updated with field experience from the specific plant in question.
It is not possible to address all factors that affect SIS reliability through reliability calculations. It is therefore also necessary to have adequate measures in place (e.g., procedures and competence) to avoid, reveal, and correct SIS related failures.
Process control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...
usually implemented in industrial processes, such as those of a factory or an oil refinery. The SIS performs specified functions to achieve or maintain a safe state of the process when unacceptable or dangerous process conditions are detected. Safety instrumented systems are separate and independent from regular control system
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
s but are composed of similar elements, including sensors, logic solvers, actuators and support systems.
The specified functions, or safety instrumented functions (SIF) are implemented as part of an overall risk reduction
Risk reduction
Risk reduction may refer to:* Absolute risk reduction or Relative risk reduction, statistical descriptors of an intervention.* Risk management* Safety Integrity Level* Hedge...
strategy which is intended to reduce the likelihood of identified hazardous events involving a catastrophic release. The safe state is a state of the process operation where the hazardous event cannot occur. The safe state should be achieved within the process safety time. SIFs are focused on preventing hazardous events with a health&safety or environmental consequence.
The correct operation of an SIS requires a series of equipment to function properly. It must have sensors capable of detecting abnormal operating conditions, such as high flow, low level, or incorrect valve positioning. A logic solver is required to receive the sensor input signal(s), make appropriate decisions based on the nature of the signal(s), and change its outputs according to user-defined logic. The logic solver may use electrical, electronic or programmable electronic equipment, such as relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
s, trip amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s, or programmable logic controller
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
s. Next, the change of the logic solver output(s) results in the final element(s) taking action on the process (e.g. closing a valve) to bring it to a safe state. Support systems, such as power, instrument air, and communications, are generally required for SIS operation. The support systems should be designed to provide the required integrity and reliability
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
.
International standard IEC 61511
IEC 61511
IEC 61511 is a technical standard which sets out practices in the engineering of systems that ensure the safety of an industrial process through the use of instrumentation. Such systems are referred to as Safety Instrumented Systems...
was published in 2003 to provide guidance to end-users on the application of Safety Instrumented Systems in the process industries. This standard is based on IEC 61508
IEC 61508
IEC 61508 is an international standard of rules applied in industry. It is titled "Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems"....
, a generic standard for design, construction, and operation of electrical/electronic/programmable electronic systems. Other industry sectors may also have standards that are based on IEC 61508, such as IEC 62061 (machinery systems), IEC 62425 (for railway signaling systems), IEC 61513 (for nuclear systems), and ISO 26262
ISO 26262
ISO 26262 is a Functional Safety standard , titled "Road vehicles -- Functional safety".This standard is an adaptation of the Functional Safety standard IEC 61508 for Automotive Electric/Electronic Systems.ISO 26262:...
(for road vehicles, currently a draft international standard).
Other names
Other terms often used in conjunction with and/or to describe safety instrumented systems include:- Critical control system
- Safety shutdown system
- Protective instrumented system
- Equipment protection system
- Emergency shutdown system
- Safety critical system
- Interlock (engineering)Interlock (engineering)Interlocking is a method of preventing undesired states in a state machine, which in a general sense can include any electrical, electronic, or mechanical device or system....
- InterlockingInterlockingIn railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...
(railway signalling)
SIS reliability
What a SIS shall do (the functional requirements) and how well it must perform (the safety integrity requirements) may be determined from Hazard and operability studies (HAZOP), layers of protection analysis (LOPA), risk graphs, and so on. All techniques are mentioned in IEC 61511 and IEC 61508. During SIS design, construction, installation, and operation, it is necessary to verify that these requirements are met. The functional requirements may be verified by design reviews, such as failure modes, effects, and criticality analysis (FMECA) and various types of testing, for example factory acceptance testing, site acceptance testing, and regular functional testing.The safety integrity requirements may be verified by reliability analysis. For SIS that operates on demand, it is often the probability of failure on demand (PFD) that is calculated. In the design phase, the PFD may be calculated using generic reliability data, for example from OREDA. Later on, the initial PFD estimates may be updated with field experience from the specific plant in question.
It is not possible to address all factors that affect SIS reliability through reliability calculations. It is therefore also necessary to have adequate measures in place (e.g., procedures and competence) to avoid, reveal, and correct SIS related failures.
SIS examples
Safety instrumented systems are most often used in process (i.e., refineries, chemical, nuclear, etc.) facilities to provide protection such as:- High fuel gas pressure initiates action to close the main fuel gas valve.
- High reactor temperature initiates action to open cooling media valve.
- High distillation column pressure initiates action to open a pressure vent valve.
See also
- Distributed control systemDistributed control systemA distributed control system refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location but are distributed throughout the system with each component sub-system controlled by one or more...
, (DCS). - Industrial control systemsIndustrial Control SystemsIndustrial control system is a general term that encompasses several types of control systems used in industrial production, including supervisory control and data acquisition systems, distributed control systems , and other smaller control system configurations such as skid-mounted programmable...
, (ICS). - Industrial safety systemsIndustrial safety systemsAn industrial safety system is a countermeasure crucial in any hazardous plants such as oil and gas plants and nuclear plants. They are used to protect human, plant, and environment in case the process goes beyond the control margins. As the name suggests, these systems are not intended for...
- PLCProgrammable logic controllerA programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
- SCADASCADASCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...
- Spurious trip levelSpurious trip levelSpurious Trip Level is defined as a discrete level for specifying the spurious trip requirements of safety functions to be allocated to safety systems. An STL of 1 means that this safety function has the highest level of spurious trips. The higher the STL level the lower the number of spurious...
- Safety integrity levelSafety Integrity LevelSafety Integrity Level is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function ....
External links
- ANSI standards Purchase IEC 61511
- ISA Standards Purchase ANSI/ISA 84.00.01-2004
- Center for Chemical Process Safety book, Guidelines for Safe and Reliable Instrumented Protective Systems