A process instrumentation diagram for a flash drum is shown 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 control valves for the steam flow, vapor product, liquid product, feed flow, and steam chest (which allows the steam chest to be rapidly evacuated in emergency situations). Determine whether the five valves should be fail-close (F/C) or fail-open (F/0) for safe operation, for each of three cases: (a) The safest conditions are achieved by the lowest temperature and pressure in the flash vessel. (b) Vapor flow to downstream equipment can cause a hazardous situation. (c) Liquid flow to downstream equipment can cause a hazardous situation. Discuss various scenarios of air failure (or power failure).
Problem 9.2A process instrumentation diagram for a flash drum is shown in Fig. E9.2. Steam iscondensed in a steam coil to vaporize a portion of the liquid feed, and the liquid product isremoved by a pump. There are control valves for the steam flow, vapor product, liquidproduct, feed flow, and steam chest (which allows the steam chest to be rapidly evacuatedin emergency situations). Determine whether the five valves should be fail-close (F/C) orfail-open (F/0) for safe operation, for each of three cases:(a) The safest conditions are achieved by the lowest temperature and pressure in the flashvessel.(b) Vapor flow to downstream equipment can cause a hazardous situation.(c) Liquid flow to downstream equipment can cause a hazardous situation. Discuss variousscenarios of air failure (or power failure). Step-by-step solution Step 1 of 3 ^There are control valves for the steam flow vapor product, liquid product, feed flow, and steamchest in a process instrumentation diagram for a flash drum (Refer to Figure E9.2 in the Textbook).(a)The safest conditions are achieved by keeping temperatures and pressures lowest in the flashvessel. To achieve this condition, the following set-up has been required:VALVE 1:- Fail closeVALVE 2:- Fail openVALVE 3:- Fail openVALVE 4:- Fail openVALVE 5:- Fail closeSet valve 1 as fail-close puts off more heat from entering into flash drum and set valve 3 asfail-open permits the steam chest to drain. Set valve 3 as fail-open puts off pressure build-up inthe vessel. Valve 4 should be fail-open to vacate the system and help in maintaining thepressure low. Valve 5 should be fail-close to stop any further pressure build-up.
