The Hazards of Dead Legs

Oct 19th, 2010 | By | Category: Health and Safety

Dead legs are sections of process piping that have been isolated and no longer maintain a flow of liquid or gas. They are often formed in industrial process piping as processes are changed or modified. Dead legs have been the cause of several industrial incidents, especially during the winter months.

Dead Leg Danger

In February 2007 an oil and gas refinery witnessed the destruction in its de-asphalting unit caused by a forgotten dead leg in the process piping for liquid propane. Valves were used to isolate a section of process piping when the process sequence was changed; however, a foreign object prohibited one of the valves from closing completely. As a result, moisture from the processing of the liquid propane leaked through the valve into the dead leg. Weather conditions during February were below freezing for four days, which was long enough for the moisture to freeze and rupture the piping. Once the temperature rose above freezing, the ice melted, allowing liquid propane to flow from the ruptured piping into the dead leg. This quickly formed a vapor cloud that was carried by the wind to an ignition source nearby. The Chemical Safety Board, which investigated the accident, said, “The accident resulted in multiple injuries, damage exceeding US$50 million and reduced operation for nearly a year.” 

Recommended Procedures for Dead Legs

As a result of this accident and others, the U.S. Chemical Safety Board recommended several changes to the American Petroleum Institute (API)standards that address the hazards of dead legs. The Chemical Safety Board recommends that any industrial or chemical facility consider the following for process piping containing hazardous materials:

  • Develop a written freeze protection program to ensure that process piping and any dead legs are properly protected. This would include installation of a heat trace or insulation around piping exposed to weather conditions, as well as routine determinations of mechanical integrity.
  • Conduct periodic inspections to identify and maintain dead legs. This should include inspection and maintenance of associated valve systems or isolating devices and mechanical integrity testing.
  • Implement a management of change program to ensure hazard evaluations are conducted when a change in process equipment or technology is proposed. A management of change program is a documented approach to requesting, evaluating risk, planning and implementing a change to a process, equipment, control technology or system. It requires the review and approval of a multidisciplinary team (including operations; engineering; and environmental, health and safety management) to ensure that potential hazards are not missed, all required documents are maintained, and communication and training are provided to all personnel affected by the change.

The refinery accident demonstrated the importance of reviewing the potential for dead legs when process piping is changed. It also shows the damage that could be experienced if dead legs are not properly maintained or removed. Process piping should be continually evaluated, and Piping and Instrumentation Diagrams (P&IDs) should be properly maintained with necessary updates or revisions. These simple tasks will go a long way toward preventing a major incident. 

About the Author

Allison K. Lloyd is in the Pittsburgh, U.S.A. office of Environmental Resources Management. She has more than fourteen years of diverse industrial and consulting experience in environmental, health and safety management, including process safety compliance, incident command, behavior-based safety, ISO 14001, OHSAS 18001 and U.S. OSHA’s Voluntary Protection Program.

Photograph:Pipe Flower by Mark Puplava, Chicago, Illinois, U.S.A.

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