Inspection Detail

Inspection: 1025967.015 - Maac Machinery Co., Inc.

Inspection Information - Office: Naperville Area Office

Investigation Summary

At approximately 9:30 p.m., on February 2, 2015, an employee, a welder, who had been employed by the company for approximately 10 work days, was welding and simultaneously pressurizing approximately a 500 gallon mild-steel vacuum tank for air leak testing. Arc welding process was utilized, Gas Metal Arc Welding GMAW i.e. Metal Inert Gas MIG with the Miller #24 MIG welder (Millermatic Model No. 252 and Argon inert gas tank). The vacuum tank was placed horizontally on the movable platform that allowed for its transport and also rolling it around its axis for worker's convenience during welding and other tasks such as air leak testing. The facility's air compressor regulated to approximately 134 psi was used for pressurizing the tank through its connections from one of the shop's main air lines and a quick disconnect hose attached to the air regulating fixture on the tank. A spray bottle with soapy water solution was prepared for air leak testing of the welds' sealing and penetration. Prior to performing the tank's air leak testing, Employee #1 was welding caps on the tank's ends and its middle seam where two rolled cylinders were connected to form this tank. Once the current welding task was completed, Employee #1 capped and/or plugged all other openings in the tank and connected the air regulating fixture equipped with a psi gauge and two handle-type ball valves to the tank caps 0.25-inch relief valve opening. Then, Employee #1 connected a quick disconnect air hose from the piped shop compressed air supply to one of the handle-type ball valves on the air regulating fixture that allowed for pressurizing of the tank. Next, Employee #1 began to weld the tank's middle seam while air pressure was developing. The heat from the welding process imposed onto the tank further increased the internal pressure being built-up with supplied compressed air. This condition was observed by the Employee #1's direct supervisor during a walkthrough of the welding area. According to the direct supervisor's recollection, at that time he advised Employee #1 to stop the operation. Employee #1 acknowledged the command, stopped welding, and began to walk in the direction away from his supervisor, along the long side of the tank, towards Employee #1's tools and Welder Unit Number 24. When the employee walked around the end of the tank, and next to the tank's welded cap's edge, the cap ruptured (mechanical explosion due to rapid expansion of gas), striking Employee #1. The ruptured cap was forcefully projected across the aisle from where the work was being performed. The cap's welds broke at the rolled stock's edge connection in the over-pressurized environment. Employee #1 was killed at the scene as a result of multiple bodily trauma to head, upper, and lower body parts resulting in death.

Keywords: Flying Object, Pressure Release, Rupture, Struck By, Tank, Valve, Welder, Welding