In a process plant, flammable gas that is accidentally released can be detected by an automatic gas detection system or by a process operator working in the area. The process operator can only detect gas if she is present in the area where the gas is released. There are several operators working in the plant, but only one operator is on duty at any time. If she is not present, she cannot detect the gas, but if she is present, there is a possibility that this may happen. An operator is present 30% of the time. The probability that the operator will not detect the gas is 0.3. If the gas is released, there is a possibility that the release may ignite. The probability of ignition is 0.1 given that gas is released. The frequency of gas release is 0.5 yr−1. If the gas is detected (automatically or by the operator), the operator will try to escape and there is a 50% probability that she escapes in time if she is present when a gas release takes place. Given that someone is in the area when ignition occurs, the probability of being killed is 0.2.
(a) Prepare an event tree with hazardous event (top event) “Gas released” and end events “Operator killed” and “Operator not killed” (b) What is the frequency of an operator being killed? (c) What is LSIR due to gas releases? Assume that each operator is present in the area 500 h/yr (one year is 8760 hours). (d) What is the average individual risk (AIR) for an operator due to gas releases? (e) What is the potential loss of life (PLL) per year?