Major accidents across the world that have resulted in a fire, explosion, chemical spill, toxic gas release or other catastrophic failures have raised awareness on designing safety systems, engineering and operating practices in such a way as to prevent major accidents and catastrophic failures or to control them when they arise.
Industry experts started addressing functional safety and established an approach for reducing risk in the process plant through the development of standards IEC 61508, IEC 61511, and ANSI/ISA 84.
Safety integrity level (SIL) looks at failures of protective layers/systems and the consequences of those failures. SIL gives attention on preventing a failure much more than reducing the consequences of that failure.
The global importance of Safety integrity level (SIL) has grown significantly in the oil and gas, petrochemical and other process industries over the last 10 years.
In order to fully understand SIL and its significance, it is important to know the overall concept known as Functional Safety, and how it relates to Safety Instrumented Systems (SIS) within the process industries. Functional Safety is part of the overall safety program which depends on the correct functioning of a safety-related system.
Functional safety is a term used to describe the safety system that includes the correct functioning of the logic solver, sensors, and final elements to achieve a desired risk reduction level and brings the process to a safe state. Functional Safety is achieved when each safety function is implemented, operates correctly and the process risk is reduced to the desired level.
A Safety Instrumented System is designed to prevent or mitigate hazardous events by taking a process to a safe state when predefined conditions are violated and reduce the risk. Other common terms for SISs are safety interlock systems, emergency shutdown systems (ESD), and safety shutdown systems (SSD). SIS performs one or more safety functions
Safety Instrumented Systems and Functions (SIS &SIF)
The purpose of a SIS is to take a process to a “safe state” when predefined limits have been exceeded or when safe operating conditions have exceeded beyond the limits. The role of the SIS is to reduce risk by implementing Safety Instrumented Functions (SIF).
The safe operation in a SIS works by sensors, logic programmers, processors and final elements designed in such a way causing a stop whenever safe limits are exceeded (for example, process variables such as pressure, temperature and reaction rate exceeds at very high alarm rates) or event preventing the operation under adverse conditions.
Examples of safety systems:
- Emergency Shutdown System
- Safety Shutdown System
- Safety Interlock System
- Fire and Gas System
Concept of SIS/SIF/SIL
The concept of SIS, SIF, and SIL can be understood by considering the installation of a pressure vessel containing flammable liquid. It is designed to operate at a certain operating pressure by the Basic Process Control Systems (BPCS).
If the process control system fails, the pressure vessel will be subjected to an over-pressure condition that could result in a vessel failure leading to the release of the flammable contents breaking into fire or explosion. The risk in this scenario is considered intolerable; implementation of SIS can further reduce this risk situation to a tolerable risk level.
Irrespective of the Basic process control systems, the SIS system prevents or mitigates the hazardous condition resulting from over pressure of the pressure vessel bringing the pressure vessel to a safe state.
The SIS will have a Safety Instrumented Function (SIF) which includes a pressure transmitter that can sense over pressure/ an intolerable level of pressure, a logic solver to control the system logic, and a control valve which might open the contents of the vessel into a safe location (flare pit, storage tank, etc.), thus bringing the pressure vessel to a safe state.
The accuracy of safety functions implemented through an SIS is determined by the magnitude of risk reduction and is expressed in terms of safety integrity level (SIL). Safety Integrity is the probability that the SIS performs the required Safety Functions satisfactorily to an acceptable level.
The determination of the SIL is the process of determining the magnitude of risk reduction to the safety instrumented function (SIF). Many different techniques are used to demonstrate the SIL to the SIF which are qualitative, quantitative or a combination of both, based on the application.
LOPA: LAYER OF PROTECTION ANALYSIS
A layer of protection analysis (LOPA) is one of the predominant methods to determine the SIL. LOPA is a detailed method for evaluating plant hazards and assessing the risk. Every industrial facility has multiple layers of safety protection. Each layer of protection has its own level of risk reduction. Each layer of protection must be independent of the others, which means that if one safety layer fails, other layers are not affected and can still perform.
The LOPA method must be conducted by a diverse team with data developed in the hazard and operability analysis (HAZOP) study, and describes each identified hazard by documenting the initiating cause and the layers of protection that prevent or mitigate the hazard. The total amount of risk reduction can then be determined by risk assessment and the need for further risk reduction is analyzed. If additional risk reduction measures are required, it is to be provided in the form of an SIF, and then the LOPA methodology determines the appropriate SIL for the SIF.
LOPA (Layer of Protection Analysis) is a semi-quantitative tool which determines the combination of probability of occurrence and severity of consequences and the likelihood of failure of independent protection layers (IPLs) to estimate risks.
LOPA addresses the key questions like:
- “How safe is safe enough?”
- “How many independent layers of protection are needed?”; and
- “How much risk reduction each layer should provide?”
- It is generally used as an extension of process hazard analysis (PHA)
Oil gas, petrochemical and other process industries typically use various protection layers to reduce the likelihood of a hazardous consequence. These layers of protection are Independent series of elements related to the process design and maintenance that includes process design, Basic Process Control Systems (BPCS), Safety Instrumented Systems (SIS), passive devices e.g. dikes and blast walls, active devices such as relief valves and human intervention such as Emergency Response and Community Emergency Response.
LOPA is basically used for high-risk scenarios where the quality of the decision must be higher and allows a multidisciplinary team to assess the risk with greater clarity and decide on more clearly defined approach and reduce the risk to tolerable level.
Article by Dr.Yashoda Tammineni,
MSc, Ph.D.
HSE,HOD at NIFS
