On October 21, 2015, two mechanical technicians were involved in an incident while attempting to remove a blind flange from supply piping on a Magnet Cooling Water (MCW) System. The mechanical technicians were assisting a welder in obtaining final measurements to make the final connections from the MCW system to a new Cell 14 cryostat as part of the Series-Connected Hybrid magnet project. When loosening the nuts on the Victaulic flange collar, an explosion occurred. The flange, the water behind the flange, or some combination of both, struck and propelled one mechanical technician backward and into the metal support structure for the cryostat, resulting in his death. A second technician, who was approximately 10 feet away at the time of the explosion, sustained minor injuries.
Following the accident, it was discovered that a compressed air valve, which delivers the compressed air needed to seat the pneumatic MCW butterfly valve positively, was in a closed position and thus was not delivering compressed air to seat the pneumatic butterfly valve positively. Leakage of water past the valve caused high-pressure water (~325 psig) to compress the air in the supply piping. As such, a combination of pressurized water and pressurized air built up behind the Victaulic flange. The total stored energy from the compressed air and water was estimated to be approximately 1.4 million ft-lbf, equivalent to about 0.9 pounds (0.4 kg) of TNT explosive.
Recommendations / Lessons Learned
System-specific, detailed step-by-step LTV procedures must be provided that positively confirm zero energy in system prior to work being performed.
Appropriate instrumentation to indicate the presence of stored energy in the system is needed, not just in the final configuration, but during the construction phase.
Detailed Piping and Instrumentation Diagrams (P&ID) should be generated and well-understood by workers.
During construction, from inception to closeout, a person who has knowledge of every aspect of work coordination, e.g., water, pipes, valves, electrical, etc., must have a daily interaction with all workers so each person understands how their work may impact a set of workers focused on a different aspect of the project.
A comprehensive LTV program commensurate with the complexity of the system (reference OSHA 1910.147 for best practices) is needed. Written programs should not be confused with written system-specific procedures needed for de-energizing equipment. The program also needs to address high risk items such as cryogens, high pressure and high voltage de-energization. Roles and responsibilities should to be clearly stated. Steps necessary to define the process when personal lock(s)/tag(s) need to be removed by those other than the originator should also be addressed.
All isolation points must be locked in a safe position. Physical lockouts of butterfly, water, drain and vent valves should be included when appropriate.
Ensure through engineering design review that appropriate positive isolation points have been incorporated into system designs and that all credible failure scenarios have been identified and have proper controls in place to mitigate stored energy potential. Consider whether:
Block valves are needed to isolate the process side still in service from the maintenance side. Blind(s) may also be needed to further isolate the system, in combination with a bleed valve to drain/vent any fluid trapped between the block valve and the blind;
Actuators meet desired failure mode(s), when pneumatic valves are used;
Pressure and temperature gauges are installed wherever appropriate, and referenced in appropriate Standard Operating Procedure(s); and,
Compressed air supply pressures for pneumatic valve actuators are designed to be lower than the maximum values listed on the valve
nameplates.
System design should include, at a minimum (reference ASME A13.1 for best practices), standardized labeling, such as directional arrows to indicate fluid flow, indication of valve numbering, indication of whether valves are “normally open” or “normally closed”, and detailed P&IDs.
Formality of work control should consider both the complexity and consequence of the tasks to be performed. For example, complexity of a system LTV should be handled at a higher level of rigor to ensure adequate safety review by appropriate
subject matter experts.
Personnel should have complete knowledge of the system, including all the valves, indicator lights, and their meaning in terms of open or closed.
Source:
https://docs.anl.gov/main/groups/intranet/@shared/@eqo/@div/documents/lessonslearned/472201.pdf