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MIT16_63JS16_Tank_Report.pdf - STAMP-Based Analysis of SBS...

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1STAMP-Based Analysis of SBS Tank 731 Overflow AccidentNancy Leveson, Margaret Stringfellow, and John ThomasDescription of the Physical ProcessIn the SBS unit, tail gas is burned at 1200oF with excess air and natural gas in the tail gascombustor F-700.This converts the H2S to SO2. [A physical diagram could be inserted here, butwe did not have one that we could use.]Hot gas effluent from F-700 is cooled in the waste heat boiler E-701.Effluent gas from theboiler enters the venturi quench tower V-703 where it is quickly cooled by direct contact with a30% sulfuric acid solution to approximately 181oF.The 30% acid solution comes from thebottom of the T-704 quench separator and is pumped to V-703 via the quench circulating pumpsP-704A/B/C in a continuous loop.The acid concentration is maintained at 30% by purging asmall (< 2 gpm) slip stream to Tank-731 acid tank and replacing it with water to maintain theprocess temperature near 181oF.The acid from Tank 731 is then removed either by draining tothe process sewers (used as Lakefront pH control) or used as a product elsewhere.As the acidenters Tank 731, it is saturated with SO2.The design of Tank 731 allows for the gas that isentrained in the acid to degas off to another tower, T 707, where it can be further treated.OnThursday August 7th, when tank 731 tank overfilled, there was not sufficient residence time forthe SO2entrained in the solution to degas properly.As a result, the acid evolved sulfur dioxideto atmosphere as it overfilled to the ground.Events:The analysis starts, like any accident analysis, with identifying the proximate events includingthe physical failures and operator actions (or missing actions) related to the loss. But stoppingafter identifying these, often the end point in accident investigation, usually leads to attributingthe cause to operator error, which, as stated earlier, does not provide enough information toprevent accidents in the future. The operators may be fired or reprimanded, subjected toadditional training, or told not to make the same mistake in the future, none of which lead to longterm prevention of the same behavior if problems exist in the other parts of the safety controlsystem design. It also leads to identifying and fixing specific hardware design flaws, e.g., theredesign of a relief valve or the replacement of a flow meter, but not the flaws in the engineeringdesign and analysis process or the maintenance issues that led to that particular manifestation ofa flawed design. Examining the rest of the control structure will provide more information aboutthe flaws in the larger company safety management structure that need to be fixed.The events below are from the original accident report:08:33 - Board Operator attempts to open control valve F-47706, to begin an acid drawdown fromthe quench recirculation system.The flow meter does not indicate a flow, so the Board Operatorcalls the Outside Operator to check and see if the manual block valves at the control valve stationare closed.Note:

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Term
Summer
Professor
NoProfessor
Tags
Chemical Engineering, Control Engineering, Test, Safety engineering, Board Operator

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