Basic Fundamentals Of Safety Inst. Sys

Basic Fundamentals Of Safety Instrumented Systems
• • • • • • • Overview Definitions of basic terms Basics of safety and layers of protection Basics of Safety Instrumented Systems Safety Integrity Level (SIL) Probability of failure upon Demand (PFD) Quiz

Overview
The operation of many industrial processes, especially those in the chemical or oil & gas industries, involve inherent risk due tothe presence of dangerous chemicals or gases. Safety Instrumented Systems (SIS) are specifically designed to protect personnel, equipment, and the environment by reducing the likelihood or the impact severity of an identified emergency event. Explosions and fires account for millions of dollars of losses in the chemical or oil & gas industries each year. Since a great potential for loss exists,it is common for industry to employ Safety Instrumented Systems (SIS) to provide safe isolation of flammable or potentially toxic material in the event of a fire or accidental release of fluids. This course will explain the basic concepts, definitions and commonly used terms in Safety Instrumented Systems and provide a basic understanding of SIS related concepts.

Definitions
Following arecommon terms related to Safety Instrumented Systems. Covert Fault: Faults that can be classified as hidden, concealed, or undetected.

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Dangerous Failure: Failure which has the potential to put the safety instrumented system in a hazardous or fail-to-function state. Demand: A condition or event that requires the safety instrumented system to take appropriate action to prevent ahazardous event from occurring, or to mitigate the consequence of a hazardous event. Mitigation Layer: A protection layer that reduces the consequences of a hazardous event. Examples include emergency depressurization on detection of confirmed fire or gas leak. Overt Faults: Faults that are classified as announced, detected, or revealed. Prevention Layer: A protection layer that reduces the frequencyof occurrence of a hazardous event Probability of Failure on Demand (PFD): A value that indicates the probability of a system failing to respond to a demand. The average probability of a system failing to respond to a demand in a specified time interval is referred as PFDavg. PFD equals 1 minus Safety Availability. Process Hazard Analysis: It requires identifications of hazards, causes ofaccidents, possible outcome of accidents, safeguard to prevent and recommendation to implement measures to reduce process risk. Protection Layer: Any independent mechanism that reduces risk by control, prevention or mitigation. This could be a process engineering mechanism such as the size of vessels containing hazardous chemicals etc., a mechanical engineering mechanism such as a relief valve, a safetyinstrumented system or an administrative procedure such as an emergency plan against an imminent hazard. These responses may be automated or initiated by human actions. Proven-In-Use: A component may be considered as proven-in-use when a documented assessment has shown that there is appropriate evidence, based on the previous use of the component, that the component is suitable for use in a safetyinstrumented system. Redundancy: Use of multiple elements or systems to perform the same function. Redundancy can be implemented by identical elements (identical redundancy) or by diverse elements (diverse redundancy). Reliability: Probability that a system can perform a defined function under stated conditions for a given period of time. Safe Failure Fraction: Safe failure fraction (SFF) is arelatively new term resulting from the IEC 61508 and IEC 61511 committees’ work to quantify fault

tolerance and establish the minimum level of redundancy required in a safety instrumented function. Per IEC, “Safe failure fraction is the ratio of the (total safe failure rate of a subsystem plus the dangerous detected failure rate of the subsystem) to the total failure rate of the subsystem.” (In...