For a system dependent process risk, risk should be quantified thus the system can apply to risk reduction, there are various possibility for quantifying risk, for example, LOPA (Layer Of Protection Analysis) and Risk graph. All the case risk is accessed without protective measures. For risk analysis initially, the risk causing variable is identified and used to plot risk analysis.
Here we are using the Risk graph from IEC 61511.
Risk graph for IEC 61511:
Risk graphs under IEC 61511 plots risk level into SIL level by a simplified analysis based on the knowledge of the risk factors associated with the process and its control system.
The method consists of a tree-like graph where each stage represents a risk factor and the ramifications of the different values that each factor can take. A risk graph is intended to perform a graded assessment of a dangerous scenario based on a series of parameters that represent those risk factors, considering that there is no SIF in place. The SIL is calculated by selecting each parameter from a predetermined set of values.
Consequence factor C - This parameter accounts for serious injuries when the area is occupied. It must consider the area affected by the danger and vulnerability of the personal.
Exposure time factor F - Probability (based on the fraction of time) that the exposed area is occupied at the time of the hazardous event. Typical values are a) less than 10% of the time, and b) 10% of the time or more.
Probability of avoiding the hazard P - his is assuming that all protection systems fail to respond. The parameter’s value is actually the probability of failing to avoid the hazard.
Demand rate W - This is the likelihood of the hazardous condition occurring in the absence of
a SIF, usually in frequency per year.
The value in the box under the W parameters indicates the target SIL. If the box shows an “a” there is no specific requirement for a SIL value. If the box shows a “b”, this indicates that a single SIF is not sufficient to provide the required risk reduction.
The parameters of the graph can include numerical factors or simply be qualitative. In any case, the values of the parameters must be derived by calibrating the Risk Chart with the corporate risk criteria. Calibration refers to the assignment of numerical values to each of the parameters of the Risk Graph. Therefore, the corporate risk criteria that indicate the levels of risk tolerability would be incorporated into the parameters of the calibrated chart.
The calibration of the qualitative parameters is quite subjective and requires considerable judgment. When numerical values are assigned to the parameters of the Risk Chart, it becomes a semiquantitative method. Refer to the original source for the full table.
To learn about SIL: