|Home||Search||Emissions||Pollutants||About the Database|
|State Police #||11-02073|
|Pollutant||Duration||Point Source||Greenhouse Gas||Criteria Pollutant||Ozone forming chemical||Amount of Release|
|Sulfur Dioxide||1 h 13 m||flare #2||NO||YES||NO||3,585.5 pounds|
|Hydrogen Sulfide||1 h 13 m||flare #2||NO||NO||NO||7.0 pounds|
Accident Classified As: Reportable Quantity
Incident occurred due to an upset of the refinery's fluid catalytic cracking unit (FCCU). The FCCU Wet Gas Compressor has three speed probes wired to the Triconex system, which performs the governor controls for the compressor's desired speed. The Triconex system processes the three speed readings and sends an electronic output signal (basically an average speed value of the three probe readings) to the CCC control system (which includes a 1st and 2nd stage antisurge controller). The CCC controller uses the speed signal to determine if the the compressor is "running" of "stopped". The upset began when the 1st Stage Wet Gas Compressor speed signal malfunctions sending a false reading to the compressor speed controller to decrease speed. The speed controller interpreted the low speed signal to mean the compressor was shutting down. To protect he compressor it fully opened the anti surge spillback creating a recycle from the compressor discharge to the suction. Since the low speed signal was false and the machine was not shutting down the recycle overloaded and the capacity of the Wet Gas Compressor and it could not keep up with the gas coming from the Main Fractionator. With the Wet Gas Compressor maxed out, the gas from the Main Fractionator Overhead could not get out and the pressure controller opened to the flare. The flaring ceased when the spillback valve was closed and the pressure reduced on the Main Fractionator. The flaring was a result of over pressuring of the Fractionator. The over pressure condition occurred due to the Wet Gas Compressor antisurge recycle valve going fully open due to a false low speed reading reading on the 1st stage anti surge controller.
Investigation into the cause of the false low speed reading did not lead to any definitive cause. All testing ad inspections done after the event did not reveal any issues with the controller and/or associated circuitry. The condition could not be recreated by technical experts while troubleshooting.
Operations took immediate action to begin the process of stabilizing the FCCU by cutting back unit charge rate. Both the 1st and 2nd Stage Wet Gas Compressor speed controllers were set to from automatic to manual while E&I began troubleshooting the speed signals. Actions were taken to provide the operator an alarm in the event the condition reoccurs with instructions on how to mitigate until technical support arrives. All wiring associated with the CCC Control system was inspected on 4/11/11. Voltage checks were performed on all circuits and the speed signal was tested on 4/11/11. The CCC controllers will be replaced and the data recording software will be upgraded during the 2011 FCCU turnaround for a target completion date of 8/31/11. Flare # 4 exceeded maximum opacity limits during this incident.