Friday, March 11, 2011

Bayer CropScience Pesticide Waste Tank Explosion

Details of the investigation carried out by US Chemical Safety Board (CSB)of the Chemical runaway reaction leading to pressure vessel explosion on 28 August 2008 that kills two and injured eight. Available include:

* Final report
* Video on CSB website and on YouTube

The incident occurred during the restart of the methomyl unit after an extended outage to upgrade the control system and replace the original residue treater vessel. The Chemical Safety Board (CSB) investigation highlighted the following issues:

* Deviation from the written start-up procedures,
* Bypassing critical safety devices intended to prevent such a condition.
* Inadequate pre-startup safety review;
* Inadequate operator training on the newly installed control system;
* Unevaluated temporary changes,
* Malfunctioning or missing equipment, misaligned valves, and bypassed critical safety devices;
* Insufficient technical expertise available in the control room during the restart.
* Poor communications during the emergency between the Bayer CropScience incident command and the local emergency response agency confused emergency response organizations and delayed public announcements on actions that should be taken to minimize exposure risk

Pre-Startup Activities

Unlike the normal methomyl restart after a routine shutdown, the August restart involved operations personnel, engineering staff, and contractors working around the clock to complete the control system upgrade and residue treater replacement. Work included finalizing the software upgrades, modifying the work station, calibrating instruments, and checking critical components. Board operators were provided time at the methomyl work station so that they could familiarize themselves with the new control functions, equipment and instrument displays, alarms, and other system features. Other personnel were completing the residue treater replacement, reinstalling piping and components, and reconnecting the control and instrument wiring. These activities progressed in parallel with the ongoing Larvin unit operation.

The methomyl control system upgrade required a revision to the SOP to incorporate the changes needed to operate the methomyl unit with the new Siemens system, and to reformat the SOP to a computerized document. However, at the time of the incident the SOP revision remained incomplete; the operators were using an unapproved SOP that did not contain the new control system operating details.

Solvent Flush and Equipment Conditioning
Many of the subsystems in the methomyl unit required a solvent flush and nitrogen gas purge to clean and dry the systems before startup. These activities were critical to safely start the residue treater system as the feed, recirculation, and vent piping had been disconnected and a new pressure vessel had been installed. The solvent-only run was also needed to verify instrument calibrations, proper equipment operating sequences, and other operating parameters in the new DCS.
The staff flushed the process equipment with solvent to remove contaminants and water that might have gotten into the system during the outage. However, contrary to the SOP 25 the staff did not perform the residue treater solvent run.26 Operators reported that solvent flow restrictions upstream impeded completion of instrument calibrations because the proper adjustments could not be made at low flow rates. Even had the staff not needed to verify the control system function and operability, the solvent run was required to pre-fill the residue treater to the minimum operating level and to heat the liquid to the minimum operating temperature before adding the methomyl containing flasher bottoms feed.

2.2 Unit Restart
Although the operations staff acknowledged that management had not prescribed a specific deadline for resuming methomyl production, onsite stockpiles of methomyl necessary to make Larvin were dwindling. Unit personnel recognized the important role of methomyl in the business performance of the facility, and a recent increase in worldwide demand for Larvin created a significant, sustained production schedule. Methomyl-Larvin operating staff told CSB investigators that they looked
forward to resuming methomyl production and a return to the normal daily work routine after the long unit shutdown.
Operator logs documented the plan to start the MSAO (a.k.a. Oxime) unit Monday morning, August 25. Methomyl synthesis needed to begin shortly thereafter. However, critical startup activities were not completed, and the staff struggled with many problems as they attempted to bring each subsystem on line. To complicate the startup problems, process computer system engineers had not verified the
functionality of all process controls and instruments in the new control system.

Control System Upgrade
The introduction of the Siemens PCS7 control system significantly changed the interactions between the board operators and the DCS interface. The Siemens control system contained features intended to minimize human error such as graphical display screens that simulated process flow and automated icons to display process variables. But the increased complexities of the new operating system challenged operators as they worked to familiarize themselves with the system and units of measurement for process variables differed from those in the previously used Honeywell system.

Human interactions with computers are physical, visual, and cognitive. New visual displays and modified command entry methods, such as changing from a keyboard to a mouse, can influence the usability of the human-computer interface and impair human performance when training is inadequate. Operators told CSB investigators they were concerned with the slower command response times in the Siemens system and they talked about the methomyl process control issues they would face during the restart, which was much more difficult to control than the Larvin process. Board operators also told CSB investigators that the detailed process equipment displays in the DCS were difficult to navigate. Routine activities like starting a reaction or troubleshooting alarms would require operators to move between multiple screens to complete a task, which degraded operator awareness and response times.

The old system display and command entry was basically a spreadsheet, or line-item display. The new system used a graphical user interface (GUI) that displayed an illustrative likeness of the process and its various components (Figure 18). The board operator selected the device that needed to be changed. This made data entry clearer, but much slower. In the old system, board operators could change multiple process variables simultaneously, but they could select and change only one variable at a time in the Siemens system.

The new control system also changed how board operators monitored multiple pieces of equipment. The methomyl board operators’ station had five display screens available to monitor the methomyl processes and one display screen dedicated to process alarms. However, operating some methomyl equipment required the operators to use at least three of the five display screens. To simplify the operation, they asked the Siemens project engineers to add equipment overview screens to display multiple pieces of equipment. The board operators believed that the overview screens would provide more effective control of the unit; however, the screens were not available for the August 2008 startup.

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