Abstract: Employees #1 and #2 were told to weld a catwalk on a 28 foot by 5 foot diameter tank that contained 1,500 gallons of water. The tank catches fumes of ammonia sulfate and acid. The tank was new and had just been put into use the previous Saturday. Employees #1 and #2 were seen at 9:30 a.m. hoisting equipment to the top of the tank. At 10:00 a.m., two other employees talked with them. Then at 10:05 a.m., an explosion occurred. Employee #1 was thrown approximately 10 feet up and landed on the ground 15 feet from the tank. Employee #2 was thrown approximately 100 feet into a river. His body was not recovered. The top of the tank has not been found. The tank had not been checked for possible explosive fumes. Neither employee was wearing a safety belt or tie off. Welding of the catwalk could have been done at any time during the previous week before startup. A blueprint draftsman stated that the welding was not something that was needed to start up production. According to a sales rep and parts order person, there had never been a policy of testing tanks before welding.