Steam turbines are the heart of power generation facilities, therefor steam turbine manufacturers and owners want to protect their steam turbines from various issues. One of the dangers they must protect against is that of overspeed – the rotation of the turbine shaft at speeds approaching or exceeding the maximum allowable limits of the steam turbine.
Overspeed can occur with a turbine trip and/or a load rejection when the steam admission valves are closed to prevent further steam flow from entering into the turbine. Stopping the flow of new steam will result in the decay of the steam pressure already within the turbine, thus allowing the relatively high-pressure steam in the extraction system to flow back into the turbine. This flow of steam back into the turbine will result in a transfer of its energy to the turbine blades, increasing the shaft’s speed of rotation. If the speed of shaft rotation exceeds the maximum allowable limits, it could result in a catastrophic failure of the turbine.
To protect against overspeed, check valves should be used in extraction steam systems anywhere there are large volumes of steam and water. The location, type and number of check valves that should be used are typically dependent on the volumes of steam and water and the rate at which it can flow back into the turbine. Weir check valves are largely used for steam turbine protection.
