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Steam Turbine Thermal Expansions

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    Steam Turbine Thermal Expansions Steam Turbine Thermal Expansions Document Transcript

    • Steam Turbine Thermal ExpansionsThe G2 ST represented below, the HP/IP in a single casing with the thrust bearing inthe 2nd standard. The 2nd standard is fixed to pedestal, where the 1st is free to slide withHP/IP casing thermal expansion. The SXD measures the HP/IP casing expansion, theDXD measures the difference between the casing and the HP/IP rotor expansion, andthe RXD measures the expansion of the two LP rotorsThe expansion measurements can be look at as if they were thermometers, indicatingthe temperature of the turbine components. For example if the HP/IP casing was 200”and pre-warmed to 200F above ambient, the SXD should indicate an expansion of0.240”.The DXD indicates the difference temperature between the HP/IP casing and its rotor.This indication is used to indicate the severity of a thermal transient. When the unit hasbeen operated at steady state for a long period of time, the casing and rotor will be at asteady temperature and the DXD will indicate normal (or in GE terminology “greenmark”) The GE scaling for DXD is confusing, given that the range is 0 to 1” with greenbeing mid scale. The rotor, being completely immersed in this steam and being lessmassive than the casing expansions will occur before the casing. The casing expansionwill lag the rotor during any transient.During startup, the temperature of the steam admitted into the HP/IP will be increasing,the rotor heats faster than the casing and the rotor goes long. If the startup rate is toosevere and the rotor long alarm occurs, tripping the unit may not be the best optionsince the Poisson ratio effect will make the rotor go more long as it slows down andwind age from bottled steam will add more heat.
    • Controlling the steam temperature and the rate of increase is the desired solution.Allowing for the lag of the casing, once steady state operation at load, the rotor andshell will reach temperature and DXD will return to near the green mark. The SXDshows the casing expansion and can be used to determine its average temperature .
    • The G3 ST represented below, the HP and IP are separate casings with the thrustbearing in the 2nd standard. The 3nd standard is fixed to pedestal, where the 1st and 2ndare free to slide with HP and IP casing thermal expansion. There are two SXD’s thatmeasure the HP and IP casing expansion, with the HP SXD measuring the sum of bothcasings There are two DXD’s that measures the difference between the casing andtheir rotor expansions. The RXD measures the expansion of the LP rotors and the IPDXD (IP rotor minus IP casing)