Objective: To understand how to take safety into account in control valve select
ID: 702361 • Letter: O
Question
Objective: To understand how to take safety into account in control valve selection A process instrumentation diagram of a flash drum is shown below. 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 the steam chest (which allows the steam chest to be rapidly evacuated in emergency situations). Determine which of the five valves should be fail close (F/C) or fail open (F/O) for safe operation the following case: FC) (FT Vapor PT PC FT Steam Liqul Condensate a) The safest conditions are achieved by the lowest temperature and pressure in the flash vessel.Explanation / Answer
SINCE SAFEST CONDITIONS ARE ACHIEVED BY LOWEST TEMPERATURE & PRESSURE:
1. THEREFORE, VALVE ON STEAM LINE(VALVE 1) SHALL BE FAIL CLOSE.IF VAVE IS FAIL OUEN, THEN IN CASE OF INSTRUMENT AIR FAILURE, STEAM FLOW RATE CAN NOT BE CONTROLLED & HENCE EXCESS STEAM SHALL CAUSE HIGH TEMPERATURE OF COLUMN. THEREFORE, VALVE ON STEAM LINE MUST BE FAIL CLOSE.
2. VALVE ON CONDENSATE LINE (VALVE 2) SHALL BE FAIL OPEN. REASON FOR THIS IS CONDENSATE RETURN LINE NEEDS TO BE OPENED. OTHERWISE, STEAM CONDNENSING IN COIL WILL BE TRAPPED IN COIL & HENCE THIS SHALL CREATE BACK PRESSURE ON STEAM LINE & STEAM FLOW SHALL BE DISTURBED. HENCE FOR STEADY OPERATION OF CLOUMN, VALVE ON CONDENSATE LINE MUST BE FAIL OPEN.
3. PRESSURE CONTROL VALVE (VALVE 3) SHALL BE FAIL OPEN. IF VALVE IS FAIL CLOSE, THEN IN THE EVENT OF INSTRUMENT AIR FAIL, VALVE SHALL CLOSE & VAPOUR GENERATED IN COLUMN SHALL NOT BE ABLE TO PASS FROM COLUMN WHICH SHALL RESULT IN INCREASED PRESSURE OF COLUMN WHICH IS NOT DESIRED. HENCE PRESSURE CONTROL VALVE SHALL BE FAIL OPEN TYPE.
4. LEVEL CONTROL VALVE (VALVE 4) SHALL BE FAIL OPEN. REASON FOR THIS IS IF VALVE IS FAIL CLOSE, THEN IN THE EVENT OF INSTRUMENT AIR FAILURE, VALVE SHALL GET CLOSE & LIQUID LEVEL SHALL RISE IN COLUMN WHICH SHALL RESULT IN FLOODING OF COLUMN. ADDITIONALLY, SINCE THIS VALVE IS INSTALLED ON DISHARGE LINE OF PUMP, IN THE EVENT OF VALVE CLOSURE PRESSURE IN DISCHARGE LINE SHALL KEEP ON INCREASING DUE TO BLOCKED OUTLET (PUMP SHUT OFF PRESSURE SCENARIO) UPTO PUMP SHUT OFF HEAD (OR HIGHER IF PUMP IS RECIPROCATINNG). HENCE, FOR SAFE OPERATION OF PUMP ALSO THIS VALVE NEED TO BE OPENED IN CASE OF INSTRUMENT AIR FAILURE. HENCE LEVEL CONTROL VALVE SHALL BE FAIL OPEN.
5. FLOW CONTROL VALVE ON FEED LINE SHALL BE FAIL OPEN. REASON FOR SAME IS DUE TO SAFE OPERATION OF COLUMN AS IF DUE TO INSTRUMENT AIR FAILURE IF VALVE GETS CLOSED (FAIL CLOSE) THEN ALSO STEAM COIL MAY RUN AT DRY CONDITION (DRY CONSITION IS DEFINED AS WHEN THERE IS NO LIQUID OUTSIDE THE COIL SO ALL HEAT IS TRANSFERRED TO COIL. HENCE COIL SAFETY MAY BE COMPROMISED. ADDITIONALLY, HOLD UP LIQUID MAY ALSO GET EXCESS HEAT FOR SOME TIME WHICH MAY RESULT IN EXCESS PRESSURE & TEMPERATURE OF COLUMN. HENCE, TO AVOID ALL THIS FLOW CONTROL VALVE ON FEED LINE MUST BE FAIL OPEN.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.