Safety integrity level risk reduction

Safety instrumented function (SIF) A safety function allocated to the safety instrumented system with a safety integrity level necessary to achieve the desired risk reduction for an identified process hazard. 

Once the severity and the probability corresponding to a scenario are estimated, that is, the risk is assessed, a decision can be made on the nature of the protection system to be implemented. If a safety instrumented system (SIS) is to be used, consisting of one or more independent protection levels (IPL), the required reliability of the protection system, constituting a so-called Safety Integrated Level (SIL) can be determined by using this risk assessment, respective of the required risk reduction. 

Safety integrity levels (SILs) provide targets for risk reduction (Security EAL-Evaluation Assurance Level Correlation to SIL )... 

The defining of the term Safety Integrity Level (SIL), a number from 1 to 4 representing the risk reduction required for the safe operation of a particular functional component. SILs are essentially an analogue for the extent of rigour required during the development process. 

The required safety integrity level of the instrumented function shall be derived by taking into account the required risk reduction that is to be provided by that function. For those SILs, the target PFDj,yg on demand and the target frequencies of dangerous failure are hsted in Table 3.8 [ANSI/ISA-84.00.01(2004) Part 3] for each SIF. Several risk analysis methods ranging from qualitative to fully quantitative can be deployed based on the severity and complexity of the scope, as listed in Table 3.9. 

Table 3.8 Safety integrity levels probability of failure on demand, target risk reduction factor and target frequency of dangerous failure to perform the SIF.

The consequence severity and the likelihood frequencies determine risk. In some cases, the risk of a hazard is within tolerable levels, and no risk reduction is needed. For these cases, no SIS is required. In other cases, risk reduction is required, and the quantity of risk reduction is specified by an order-of-magnitude level called the safety integrity level (SIL) as indicated in Figure 1-7. 

Safety Integrity Level Probability of Failure on Demand (PFDavg.) Low Demand Mode Risk Reduction Factor (RRF)... 

In ANSI/ISA-84.00.01-2004 (lEC 61511 Mod), 3.2.71, a safety instrumented function is defined as a "safety function with a specified safety integrity level which is necessary to achieve functional safety." This standard, 3.2.68, defines a safety function as a "function to be implemented by a SIS, other technology safety-related system or external risk reduction facilities, which is intended to achieve or maintain a safe state for the process, with respect to a specific hazardous event."... 

The risk graph method, proposed in lEC 61508 and lEC 61511, is useful in the process of determination of required risk reduction level associated with safety integrity level (SIL) of safety instrumented function (SIS). However, this normative document presents only an illustrative example which shows how the risk graph method can be used. 

The purpose of the study is to provide a minimum safety integrity level of HIPPS for determination of required risk reduction performance. A systematic approach and an example to assign the acceptable safety level of a pilot HIPPS model are proposed based on the LOPA. 

The risk reduction strategy is outlined in ISO 12100 and IS014121 and a number of different methods for determining safety integrity level requirements are shown in these standards and in the already mentioned ISO 13849, lEC 62061 and lEC 61508 standards, they are all shown as informative annex in these standards. 

In [lEC 62061] the result of the risk analysis is that the risk reduction requirements on the E/E/PE-based safety functions end up in a certain SIL (Safety Integrity Level). Three levels exist from SIL 1 (lowest risk reduction) up to SIL 3 (highest risk reduction). 

The required safety integrity level of a safety instrumented function shall be derived by taking into account the required risk reduction that is to be provided by that function. 

Safety integrity level (SIL) Target average probability of failure on demand Target risk reduction... 

Refer to lEC 61508-1, table 2 (for low demand mode operation) or table 3 (for continuous or high demand mode operation) to determine the safety integrity level (SIL). The SIL then guides the selection of the techniques used for the avoidance of systematic faults in both hardware and software, so that as the risk reduction increases, or the hazard rate decreases, there is a reduction in the likelihood that systematic failures (including those resulting from incorrect specification) will result in a hazard. 

Layer of protection analysis (LOPA) LOPA is a systematic and structured way of quantification of risk reduction and safety integrity level (SIL) determination. Usually, it starts its work on the data developed in HAZOP analysis. For each documented undesired event with an initiating cause, it provides an independent protection layer (IPL), which will mitigate or prevent the hazard. Then, the total amount of risk can be determined. If safety instrumented function is necessary, LOPA methodology can be used to determine SIL also. From ISA 84 transaction it is found that LOPA is a simplified risk assignment tool used to evaluate the effectiveness of IPLs that are designed to reduce the likelihood or severity of an undesirable event. Quantitative PHA LOPA deals with single cause consequence pairs. Detailed documentation is possible and can be applied for continuous process. 

SIS consists of a number of SIFs. Each SIF will have some risk reduction factor and this indicated by safety integrity level (SIL). 

Table 10.2 Safety Integrity Levels (ISA-84.01 andlEC61511) SIL RRF (Risk Reduction PFDavg (Probability Factor) of Failure on...

The safety integrity level (SIL 3 in this case) is allocated based on a process hazard and risk assessment. It forms the basis for the risk reduction target for the safety instrumented system/SIL (HIPS in this case). For on-demand systems such as a HIPS, the SIL defines the probability of... 

Allocating the target Safety Integrated Level (SIL) (or Risk Reduction Factor RRF) for each SF,... 

Safety requirements allocation To allocate a safety function to SIS, non-SIS and external risk reduction measures. To allocate safety integrity, level to each safety function. EUC, the EUC control system and human factors. Specification from stage 4. Allocation decisions for SIS, Non SIS and external measures. Expansion of the SRS for the SIS. 

With an understanding of the role of safety instrumented systems in risk reduction we are able to introduce the concept of safety integrity and safety integrity level , (SIL). 

In order to get a scale of performance safety practitioners have adopted the concept of safety integrity levels or (SILs). The SILs are derived from earlier concepts of grading or classification of safety systems. The principle is illustrated in the next figure where the layer of protection provided by an SIS is seen to be quantified as a risk reduction factor from which it can be converted to a PFDavg and referenced to an SIL classification table. 

The required risk reduction defines the quality or safety integrity level of the protection system... 

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