What is Safety Integrity Level?
Industrial safety used to be a mechanical concept before the widespread adoption of silicon based digital technology. Safe work practices, hazardous material control and protective armoring were just some of the few ways to ensure safety in industries. The 21st century, however, brought rapid changes within existing digital technology and interconnected concepts of reliability, productivity and profitability under one umbrella of safety.
Process control industries top the list of industries that require multi-dimensional safety controls. High temperature, flammability, presence of toxic materials, etc. make it imperative for checks to be in place to make sure human and machines can work without any harm. The issue of reliability in modern industries was becoming more serious, yet lacking standardization. Therefore, after years of work, the ISA SP84 committee and IEC 61508 joined their resources to come up with an achievable safety threshold.
Safety Integrity Level (SIL) was one of the key values that allowed customers to form logical and objective comparisons of equipment reliability from various industries. In summary, SIL can be seen as a quantitative measurement of performance.
The entire safety mechanism starts up with a Safety Instrumented System (SIS), which plays a pivotal role in providing an encapsulation around industrial processes. Sometimes called an emergency/safety shutdown system, SIS’s purpose is to monitor predetermined set points and take processes to a safe state. The major components of an SIS includes:
- Input signal interfacing
- Data processing
- Output signal interfacing
- Logic solver
- Actuators for control
Within a Safety Instrumented System resides:
- Safety Instrumented Functions
- Safety Integrated Level
Safety Instrumented Functions
This is a function that has a specified Safety Integrated Level held within an SIS, so that a state of safety can be maintained within a control system. For instance, a process vessel sustains pressure buildup until a threshold is reached. As soon as the sensors sense this, a safety valve is opened to release the pressure and bring it to normal set points. This as a whole represents a Safety Instrumented Function.
Safety Integrated Level
Who decides the upper & lower bounds of a process? What is meant by failure within a control system? All these queries lie under the domain of SIL.
SILs are measure of the safety risk of a specific process, and has paramount importance within a safety instrumented system. Historically seen, safety has always been regarded as a binary problem; either a process is safe or its unsafe. The new standards however categorize safety into several subclasses, making it a discrete problem. Each level is associated with a specific order of magnitude of risk. As a rule of thumb, the higher the SIL level, the greater the impact of a failure.
The safety standards require an SIL level to be assigned to an SIF within an instrumented system. The decision regarding assignment is carried out through Hazard Analysis, based on the amount of risk that must be eliminated to keep the process running within an acceptable level. After an SIL has been set, thorough tests must be conducted in a controllable environment to make sure the SIS can handle all associated process risks.
The Four SIL Levels:
Hazard Analysis is carried out to determine the SIL level of a process. The effectiveness of an SIS is dependent upon its ability to perform its function completely when called upon. For assignment of a target SIL, complex processes are carried out by experts which include:
- Simple calculations
- Fault Tree Analysis
- Layer of Protection Analysis
- Markov Analysis
For field instruments, SIL levels are devised through the following:
- Failures Modes, Effects & Diagnostic Analysis (FMEDA), which is most robust when certified by a 3rd party through a systematic analysis technique.
- Proven in Use is used by a customer when sufficient data regarding the instrument is available to carry out a historic analysis.
SIL ratings may be topped either by using equipment of higher rating or through redundancy, depending on the system constraints. Redundancy may be more suitable in some environments, but it does have requirements other than hardware like software and re-structuring of the system.
In conclusion, SIL isn’t an isolated entity or term; it is one of the pillars of a Safety Instrumented System, and highly dependent upon the conditions in which the process requires automatic control.
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