Incident Synopsis
One man was killed and another severely injured while working with a portable battery power supply.
At a test facility, a water-submersible portable battery power supply was used to power lighting. The battery power supply contained two 12-volt lead-acid automotive batteries, a wiring harness, and switching relays mounted in an air-tight case suitable for submersion in water. The case possessed ½-inch aluminum walls and a 13.8-pound lid. The box had been used periodically over two years.
After charging all night, the battery power supply was moved into place and connected to the lighting. Two technicians started to test the unit. One technician rested his hand on the case lid while the second leaned over the lid and threw the switch to activate the system. Immediately upon throwing the switch, the box exploded, hurling the lid upward 35 feet with deadly force where it lodged, embedded 4 inches into a low-density concrete ceiling. The technician leaning over the power supply was killed instantly and the other lost the ends of his middle and index fingers of his left hand, was sprayed on the chest and face with battery acid, and suffered trauma.
Causes
The immediate cause of the accident was procedural error. The power supply box incorporated a pipe penetration with a hand valve in the lid and a pressure gauge plumbed into the aluminum box. The protocol for charging used this equipment to vent the box and check pressurization. It was determined that the hand valve had been closed during charging, preventing venting.
The charging characteristics of lead-acid storage batteries vary with the condition of the battery and are proportional to the ampere-hour capacity of the battery. For a 100 ampere-hour 12-volt lead-acid battery, hydrogen can be evolved at a rate of several hundred standard cubic centimeters per hour both during operational and inactive periods. During charging, when a battery is close to being fully charged, hydrogen and oxygen both off gas in a flammable mixture ratio at a rate of several thousand standard cubic centimeters per hour. Therefore, without venting, the box likely possessed a near stoichiometric mixture of hydrogen and oxygen which was pressurized to greater than atmospheric pressure. The box incorporated polarized power sockets and magnetic reed switches which were operable outside the battery box but were sealed from the interior of the box. Throwing the switch would introduce a spark within the interior of the box, readily igniting the mix. Hydrogen-air and hydrogen-oxygen mixtures which are flammable over a broad range [4 to 94% by volume in pure oxygen] are readily ignited with a spark produced by these means.
Contributing Factors
There were several factors contributing to the accident:
- Few of the engineers and technicians were aware of the hazards of lead-acid storage battery operation. Many were aware of the off gassing characteristic of batteries, but thought the problem was minor based on their experience with batteries in marine applications. Even a previous incident of over-pressurization that occurred with this battery box, reported and reviewed by both operations and safety personnel, did not change this outlook. Personnel lacked training.
- The design and fabrication of the power supply was not inherently safe. Automatic ventilation or inert gas purging was not incorporated into the box. Switches with arcing contacts were built into the box. It was learned afterwards through testing that it was only by chance explosions had not occurred before. Under normal conditions of use, the geometry of the box, location of the switch, and behavior of the gas mixture around the switch led to a mixture concentration above the upper flammability limit [> 94 % hydrogen by volume]. With the error in procedure and formation of a flammable mixture, the arcing of the switch initiated the explosion. Finally, the aluminum box was too strong and without pressure relief. Without an overpressure failsafe incorporated into the design, the lid became a deadly projectile.
- Safety review procedures used at this facility for hazardous operations relied on the experience of the workers using the equipment and didn’t seek outside viewpoints.
- Battery power supplies require adequate ventilation for all operations, or the battery box should be designed to:
- Eliminate all sparking devices
- Ventilate during charging operations
- Provide inert gas purge and pressurization methods, and
- Provide adequate pressure relief
- Give consideration to other power supply methods.
- Organizational operating procedures and staffing should as necessary ensure that all equipment designs and work procedures are reviewed by an independent engineer. When hydrogen equipment and procedures are modified, the changes should be reviewed.
- Safety manuals should be revised to incorporate information on storage battery handling and operations safety practices. Safety expertise should be made convenient. Provide adequate training.