On March 4, 2005, Employee #1 (leadman) performed a hydrostatic pressure test on a large stainless steel pressure vessel at a plant manufacturing pressure vessels. The tank was cylindrical, about 14 in. diameter and 24 in. long. The tank was pressurized to 150 number for the test. Upon completing the test, he was draining the water from the tank. He soon discovered that the water would not drain very quickly, as the internal configuration of the tank was such that a vacuum was being created, and there were inadequate air openings to displace the draining water. After consultation with the plant supervisor and plant manager, it was decided that compressed air would be pumped into the tank to force the water out for a short time, then more openings would be exposed, and the water could drain by itself. The 110-number air was pumped into the tank by Employee #1, and the supervisor and the water began draining. The supervisor turned the job back to Employee #1. The air hose was disconnected sometime later, and the compressed air was also allowed to bleed off.