The energy industry is exposed to significant threats which may result in events such as fire and explosion, where such accidents can cause fatalities and asset losses. Risk factors are high, particularly for operators of large plant, with high hydrocarbon inventories.
A Safety Instrumented System (SIS) is one of the most important layers of protection against accidents and hazards in the hydrocarbon process industry. The Occupational Safety and Health Administration (OSHA) warrants that the design and implementation of a safety system meets good engineering practice. Safety performance criteria for SIS should be defined by Safety Integrity Levels (SIL). The determination of the Safety Integrity Level required for the SIS will determine the configuration of SIS to meet the required integrity level, and in turn improve the reliability of the system.
AIE has adopted a structured approach for conducting SIL studies in line with IEC 61508/61511 requirements. We have provided many SIL assessments as a part of Hazards and Operability (HAZOP) Analysis recommendations to industry operators.
In order to conduct a SIL analysis, a combination of technical documentation (Piping & Instrumentation Diagrams (P&ID), Cause and Effect charts), safety studies/documentation review (HAZOP, QRA, Firefighting and detection system data) and economical parameters (equipment cost, product and raw material costs) are required. This will lead to the identification of an initial risk level in terms of personnel safety, environmental loss, commercial impact.
The initial risk level may then be reduced by applying a range of risk reduction measures including basic process control, alarms, and mechanical devices. The total contribution from all risk reduction measures must bring the residual risk to a level below the tolerable level. Criteria to achieve adequate risk reduction minimising SIL level of safety functions will be evaluated considering the benefit in less severe maintenance requirements and lower cost for modifications.
SIL study sequence typically consists of four stages:
- Step 1 – Identification of SIFS/ EUC
- Step 2 – SIL Assessment
- Step 3 – SIL Verification
- Step 4 – Identify Recommended Actions
SIL assessment is performed in accordance with IEC 61511 and uses 2 methods based on data availability, confidence, resources, and project timelines. These methods are listed as follows,
- Risk Graph Method
- Layer of protection analysis (LOPA)
The Risk Graph method is a structured means of determining SIL by qualitatively selecting various parameters such as consequence of failure, exposure time and demand rate. A Layer of Protection Analysis (LOPA) is a semi quantitative approach to determine SIL by analysing all cause-consequence scenarios and selecting appropriate values for initiation likelihood, protection layers, independent protection layers, etc. in accordance with IEC 61511.
After SIL assessment, SIL verification analysis will be carried out to verify that the design selection of components for the Safety Instrumented Function meet the target integrity threshold (the ‘SIL.’) There are 4 SILs described in IEC-61508:
| Safety Integrity Level (SIL) | Avg. Probability of Failure on demand (PFD) | Risk Reduction Factor |
| SIL 4 | ≥ 10-5 to < 10-4 | 10,000 to 100,000 |
| SIL 3 | ≥ 10-4 to < 10-3 | 1000 to 10,000 |
| SIL 2 | ≥ 10-3 to < 10-2 | 100 to 1000 |
| SIL 1 | ≥ 10-2 to < 10-1 | 10 to 100 |
The following benefits can be achieved through application of a SIL study:
- Confirms the optimised selection is made for the Safety instrumented system based on the SIL assessment results
- Reduces the cost of instrumentation in the project stage by avoiding over-engineering.
- Provides Cost/Benefit analysis for the organisation while decide on the safety instrumented systems
- Develops the preventive maintenance plan based on the criticality of the loop
- Ensures the safety of operating personnel by ensuring the reliability of safety devices when demanded.
- Ensures the intended function of the Safety device is not compromised
References:
- Functional Safety of Electrical/Electronic/Programmable Electronic Safety Related Systems (E/E/PES): IEC-61508.
- Functional Safety – Safety Instrumented systems for the process industry sector: IEC61511-Part1 to Part3.
- Guidance on Practice for Layer of Protection Analysis (LOPA): GP 48-03.
- Safety Integirty Level (SIL) Evaluation Techniques: ISA-TR84.00.02
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