When a main steam turbine valve fails to shut properly during an emergency turbine trip or normal shutdown, a turbine overspeed event can take place, resulting in catastrophic failure of the turbine and generator components; as well as endangering the lives of plant personnel. An example of a steam turbine overspeed event is described in a NIOSH Report Investigation #01MI011. In this event, fragments projected from a steam turbine that broke apart during an overspeed trip test resulted in the death of a plant worker in Michigan in 2001. These kinds of events can be avoided by careful periodic testing, inspection and maintenance of valves and proper valve testing. The situation of steam turbine valves sticking or not seating properly is potentially worse in situations where the plant operates in valves wide open mode, sliding pressure or even in partial arc admission to the turbine, where multiple valves are used. Valves that are inspected and maintained properly will stroke and control steam flow easily.
Although steam turbine overspeed events seldom happen, when they do,
Construction materials for steam turbine valves have improved through the years to those that are more suitable for high pressure and high temperature applications.
Steam oxidation and magnetite exfoliation
Around the world for both drum type and once-through supercritical boilers, steam temperatures have increased to as high as 1,100°F or greater for cycle efficiency or heat rate improvement in both final superheater and reheater sections. Austenitic steels such as SA 213 347H, 320 or type 321 stainless steels have high chromium and nickel contents and are suitable for high pressure and high temperature applications due to their high creep resistance properties and tensile strength.
Unfortunately they are not oxidation resistant due to steam, and with time they form oxides of chromium (spinel) and that of iron (magnetite) that exfoliate, carrying solid particles that deposit on steam turbine valve internals and the turbine components. During plant start-up, the exfoliated scales are carried in the steam flow, causing solid particle erosion of the valve seats and blades of the steam turbine. The oxide material, apart from causing steam blockage-related failures on boiler tubes, can eventually impact long-term SPE damage on start up.
A simplified chemical reaction of steam with austenic steels at very high temperatures can be summarized as follows:
4Fe + 2Cr + 8H2O → Fe3O4 + FeCr2O4 + 8H2 (1)
The austenic stainless steel has a different coefficient of thermal expansion than the magnetite oxide layer (Fe3O4) and when the tubes cool down during the boiler shutdown, different expansion rates between the austenitic tube and the oxide layer cause the magnetite layer to exfoliate from the tube bore. However the spinel inner layer, or iron chromate (FeCr2O4), next to the tube is much harder and does not exfoliate. It is the exfoliated magnetite that is transported with steam during start up to the valves and turbine.
In the last few years the number of power plants reporting oxide exfoliation in their boilers, and subsequent tube failures, has increased. Although there has not been any reported valve failures from these power plants, the problem of oxide deposits on valve parts and the steam turbine is likely to have been experienced.