Safety integrity level risk graph

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 preliminary risk assessment is made by a risk graph qualitative method described in lEC 61508-5 Annex E according Figure E.l (Figure 3) and Table E.l. This method has been used extensively within the machinery sector, see ISO 14121-2 and Annex A of ISO 13849-1, and enables the safety integrity level to be determined from knowledge of risk factors associated with the hoist machinery and it control system. 

In Chapter I, risk matrix has been discussed. Risk graph is similarly used to categorize risks additionally it can be used to determine SIL in process industries (especially when there many hazards to analyze). The discussions are presented naturally in a generalized way, however for each application one needs to calibrate the same. In this approach, there are four sets of parameters, viz. C, F, P, and W (see Fig. VII/1.3.4 1), each arranged as a column. All of these parameters describe the nature of the hazardous situation when safety instrumented systems fail or are not available. One parameter is chosen from each of four sets and the selected parameters are then combined to decide the safety integrity level allocated to the safety instrumented functions (Annex Dlof IS/IEC 61511 3 2003). Let an example with exposure to radioactive radiation case be analyzed. 

Calibrated risk graph for safety integrity level (SIL) assignment. 

Integrity of SRP/CS and safety function is expressed in terms of performance levels (PLs). Control risk assessment is used to determine the required PL (PLr) using a risk graph see Figure 9.5. 

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