FC FT Vapor 54 PT FC PC FT LT--LC Steam T Condensate Liquid ll -----X

A process instrumentation diagram for a flash drum isshown in Fig. E9.2. Steam is condensed in a steam coil to vaporize a portion of the liquid feed, and the liquid product is removed by a pump. There are five control valves for the steam flow, vapor product, liquid product, feed flow, and steamchest (which allows the steam chest to be rapidly evacuatedin emergency situations). Determine whether the five valves should be air-to-open (AO) or air-to-close (AC) for safe operation, for each of three cases:(a)The safest conditions are achieved by the lowest tempera-ture and pressure in the flash vessel.(b)Vapor flow to downstream equipment can cause ahazardous situation.(c)Liquid flow to downstream equipment can cause ahazardous situation.Discuss various scenarios of air failure (or power failure).