Water hammer accident

A five-person crew was valving in a second stage reheater at an electric power generation station. They were slowly opening two valves to bring the reheater online. Two of the employees were stationed at one valve, and the other three were at the second valve. Two other employees were working in the immediate area, observing the reheater's drain tank levels after having adjusted an automatic valve about 30 minutes before. The drain line for the reheater ruptured downstream of the valves, releasing steam and hot water into the area. The rupture was caused by excessive pressure in the line as a result of a water hammer. The water hammer was caused by the presence of lower pressure on the downstream side of the valves than on the upstream side. This produced a back flow in the line. The operating procedure for the valves in the line during start up operations had been changed earlier in the year to eliminate water hammer that had occurred previously. However, the new changes were not incorporated into the written operating procedures being used at the time of the accident. Seven employees (all five members of the work crew plus the two other employees working in the area) received second-degree burns to multiple parts of their bodies. They were hospitalized for their injuries.