Question 5 a) A semi-batch reactor is charged with a fixed volume of substrate which is then reacted with a continuous feed of propylene oxide. The reaction rate is controlled by the feed rate of propylene oxide. Should the propylene oxide feed controller fail dangerously, leading to an excess flow of propylene oxide, then this could in turn lead to a catastrophic runaway reaction. The reactor has a high temperature Safety Instrumented Function (TZ1). This comprises a single temperature transmitter (TT1) which acts via a single relay (R1) to close one isolation valve (IV1) in the propylene oxide feed line. Based on the following data calculate the Probability of Failure on Demand Average for TZ1. [2 marks] Dangerous failure rates for items: Temperature transmitter Relay Isolation valve Trip test interval b) What is the SIL category for TZ1? 0.1 year 0.01 year 0.2 year 6 months [2 marks] c) TZ1 has a Hardware Fault Tolerance of 0. Explain what is meant by this and its significance in relation to the hardware configuration of TZ1. [4 marks] d) It is proposed to install a second identical high temperature Safety Instrumented Function (TZ2) which has a temperature sensor TT2 acting through relay (R2) to close one isolation valve (IV2) in the same propylene oxide feed line. Two options are available: i) Install as a 1 out 2 voting system. ii) Install as a 2 out of 2 voting system.