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Hazard Identification

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Safety Instrumented Systems

Process industry is exposed to risks from incidents like fire, explosion, equipment malfunction, etc. causing fatalities and/or monetary losses. Safety instrumented systems (SIS) have been used for many years to perform safety instrumented functions (SIF) in the process industries. If instrumentation is to be effectively used for safety instrumented functions, it is essential that this instrumentation achieves certain minimum standards and performance levels.

The Standards IEC 61508 and 61511 and ANSI/ISA-84.00.01-2004 placed many new requirements on the design, selection, installation, operation, and maintenance of instrumented safety systems. The standard is based on a SIS Safety Lifecycle addressing the associated activities from initial conception through decommissioning. These standards require that a performance target be assigned and verified for each SIF. The performance target is a quantitative target known as the Safety Integrity Level (SIL).

BakerRisk Offers Support Service for

SIL Selection/SIL Determination, using Layer of Protection Analysis (LOPA)

SIL Verification

Safety Requirements Specifications (SRS) Development
SIL Selection/Determination Services
Safety Instrumented Functions are intended to protect against specific and identifiable hazards instead of general hazards, such as fire and gas explosion. These hazards are not always clearly defined in the Process Hazards Analysis (PHA), which is often used as the primary means for defining Safety Instrumented Functions. SIF definition requires an adequate understanding of hazards associated with the chemical process, and the specific instruments that are utilized to protect against those hazards. SIL Selection/Determination requires a multidisciplinary team with the necessary understanding of the design and hazards. BakerRisk supports the SIL Selection/Determination with knowledgeable facilitators who can guide the team to define the SIF and select the SIL targets. BakerRisk also offers support services for PHAs, allowing for the possibility of combining both studies for improved efficiency.

BakerRisk supports the customer using the SIL target determination methodology as their policy required. In most cases, SIL selection/determination is accomplished using an implicit Layer of Protection Analysis (LOPA) with a pre-defined risk-matrix. The team determines the consequence of a hazard and the likelihood of the initiating event without protection layers. The number of protection layers needed are evaluated using the risk matrix. The team then identifies independent protection layers (IPL) that apply. The SIL target is the number of the protection layer as determined by the risk matrix minus the layer of protection offered by IPLs. BakerRisk utilizes the most widely used SIL selection software, based on client needs.

Layer of Protection Analysis (LOPA) is a semi-quantitative method for process hazards analysis. According to ANSI/ISA-84.00.01-2004 Part 3 (IEC61511-3 Mod), “The method starts with data developed in the Hazard and Operability analysis (HAZOP study) and accounts for each identified hazard by documenting the initiating cause and the protection layers that prevent or mitigate the hazard. The total amount of risk reduction can then be determined and the need for more risk reduction analyzed. If additional risk reduction is required and if it is to be provided in the form of a Safety Instrumented Function (SIF), the LOPA methodology allows the determination of the appropriate Safety Integrity Level (SIL) for the SIF.” BakerRisk has expertise in facilitating LOPA for PHA as well as SIL assignment.

DEFINITIONS

Safety Instrumented Function (SIF) is a function which is intended to achieve or maintain a safe state in respect to a specific hazard

Safety Instrumented System (SIS) is used to implement one or more SIFs. An SIS is composed of any combination of sensor(s), logic solver(s), and final element(s) whose successful actions will prevent a potentially hazardous outcome from being realized.

Safety Integrity Level (SIL) is a discrete level (1 to 4) for specifying safety integrity requirements of the SIFs to be allocated to the SIS.

SIL 4 is used for highest risk systems. Each function is assigned a SIL that reduces risk for a specific hazard.

SIL Verification Services

SIL Verification is the quantitative analysis to verify that the SIL target can be met with intended SIS design. SIL Verification includes determination of probability of failure on demand and spurious trip rates. SIL Verification considers the type of equipment employed, advanced voting arrangements, diagnostics, and testing frequency. BakerRisk consultants utilize fault tree analysis to perform calculations in accordance with the U.S. standard, ANSI/ISA 84.00.01-2004, technical report ISA TR84.00.02, and the international standard, IEC 61511. BakerRisk uses proven industrial fault tree analysis software for calculations.

SRS Services

The primary objective of the Safety Requirements Specification is to specify all requirements of the SIS needed for detailed engineering, system fabrication and integration, installation, commissioning and start-up. The Safety Requirements Specification shall also serve as a reference document for later stages of the Safety Instrumented System life cycle, including operation, maintenance, modification, decommissioning, etc. ANSI/ISA 84.00.01-2004 Part 1 Clause 10 [Ref.4] specifies the requirements for the SRS. Section 10.3 of the standard identifies specific items that the SRS should address.

The Safety Instrumented System needs to achieve the following key functional objectives:

The processes and technologies our consultants have worked on include: