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When it comes to safety incidents, there are the huge incidents that live on forever like Bhopal, India or BP Texas City to name a few.

But then there are the small incidents that go relatively unnoticed, but prove to be great learning experiences. In an occasional series, ISSSource will edit reports about incidents that showed how a small mistake led to a pretty big safety incident.

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Failures of process safety management (PSM) systems are deadly and costly.

Major accidents have emphasized the need for process safety within the chemical and petrochemical industries. For example, the founding of the Center for Chemical Process Safety (CCPS) was a response by industry to the methyl isocyanate release at Bhopal, India, in 1984 that killed over 2,000 people and injured tens of thousands. A fire and explosion at a PEMEX LPG terminal in Mexico City, also in 1984, killed more than 600 people and injured around 7,000.

Schneider Bold

Major environmental damage also occurred because of process safety incidents. The firefighting efforts during a fire in a Sandoz warehouse in Basel, Switzerland, in 1986 caused the release of different chemicals, including pesticides, because responders failed to contain the water runoff. The release caused massive destruction to aquatic life in the Rhine River as far as 250 miles away; fishing ended up banned for six months. The environmental consequences of the Exxon Valdez spill in 1989 and the Deepwater Horizon incident in 2010 have been well-documented.

Accidents almost always have more than one cause. For quite a few years, safety experts have used the Swiss cheese model to help managers and workers in the process industries understand the events, failures, and decisions that can lead to a catastrophic incident or near miss. According to this model, each layer of protection is depicted as a slice of Swiss cheese, and the holes in the cheese represent potential failures in the protection layers, such as:
• Human errors
• Management decisions
• Single-point equipment failures or malfunctions
• Knowledge deficiencies
• Management system inadequacies, such as a failure to perform hazard analyses, failure to recognize and manage changes, or inadequate follow-up on previously experienced incident warning signs.

Incidents are typically the result of multiple failures to address hazards effectively — represented by the holes in successive slices aligning. A management system may include physical safety devices or planned activities that protect and guard against failure. An effective PSM system has the effect of reducing the number of holes and the sizes of the holes in each of the system’s layers, thereby reducing the likelihood that they will align.

The following is a case in point:
ARCO Channelview explosion: Use MoC for wastewater tank maintenance

A wastewater tank at the ARCO chemical plant in Channelview, TX, exploded during the restart of a compressor on July 5, 1990. The nitrogen purge ended up significantly reduced during maintenance, and a temporary oxygen analyzer failed to detect the buildup of a flammable atmosphere in the tank. When the compressor restarted, flammable vapors ended up sucked into the compressor and ignited. The flashback of the flame into the headspace of the tank caused an explosion that killed 17 people. Damages were around $100 million.

Event details: The 900,000-gal wastewater tank contained process wastewater from propylene oxide and styrene processes. There were thousands of feet of piping upstream of the tank where peroxides and caustic could mix. A nitrogen purge kept the vapor space inert, and an off-gas compressor drew the hydrocarbon vapors off before the waste was disposed of in a deep well.

The tank was out of service so workers could repair the nitrogen blanket compressor. During this time, the normal flow of nitrogen purge gas to the tank reduced to a minimum. Workers installed a temporary oxygen analyzer between two roof beams in the tank, and they planned to add a nitrogen purge if a high oxygen level ended up detected.

Within the tank, decomposition of the peroxides generated dangerous levels of oxygen. However, the air in the headspace of the tank was stagnant and the analyzer did not detect the oxygen buildup because it was in a dead zone. Occasional nitrogen purging was inadequate to prevent the formation of a flammable atmosphere in the headspace and in the piping to the compressor.

After maintenance wrapped up, a failed attempt to restart the compressor drew flammable vapors into the suction line of the compressor. When the compressor successfully restarted, the flammable vapors ignited and flame flashed back to the tank, causing an explosion.

Cause of incident: No one thought of the wastewater tank as being a part of the operating plant. Hence, management and workers did not understand that a chemical reaction was taking place in the tank, generating oxygen. The lack of understanding enabled a series of poor decisions, such as discontinuing the nitrogen purge, poor design and location of the temporary oxygen probe, no management of change (MoC) review of the these decisions, and no pre-startup safety review.

Key lessons from the incident: Keep in mind the chemicals that enter any wastewater tank are still prone to reaction. Ensure that proper MoC procedures end up followed before performing any maintenance work. In this incident, the workers did not know that a chemical reaction that could produce an oxygen buildup was taking place in the tank. Therefore, they did not comprehend the importance of continuing an effective nitrogen purge.

This was an excerpt from a report written by Albert Ness from the Center for Chemical Process Safety. He had 39 years of experience with Rohm and Haas, GE Plastics, the Dow Chemical Co., and ABS Consulting as a research and development engineer in plastics, agricultural chemicals, and ion exchange resins, and then, starting in 1989, as a process safety specialist.

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