July 1, 2009

Designing an Ultrasupercritical Steam Turbine

Heinrich Klotz,Ken Davis, and Eric Pickering

Compact and Reliable Configuration

The resulting USC steam turbine configuration for TPP is a condensing tandem compound single-reheat, 3,600-rpm steam turbine generator set. The proven Alstom STF60 design includes a four-casing steam turbine with a single-flow HP turbine, a double-flow IP turbine, and two double-flow downward exhaust low-pressure (LP) turbines. The IP turbine is connected to the LP turbines through crossover pipes. The complete turbine configuration is illustrated in Figure 2.

2.    Compact and reliable arrangement. The TPP steam turbine is a condensing tandem compound single-reheat design. It includes a four-casing steam turbine with a single-flow high-pressure turbine, a double-flow intermediate-pressure turbine, and two double-flow downward exhaust low-pressure turbines. The turbine operates at 3,600 rpm. Courtesy: Alstom

HP and IP turbine casings are separate in Alstom’s standard design. This compact design is not prone to self-excited vibration, which may occur in some combined HP/IP turbines. Unbalanced axial thrust forces experienced in some combined HP/IP designs cannot occur because the axial thrust of each cylinder is balanced under all operating conditions, including load rejection and bypass operation. Furthermore, the long-term efficiency of the separate HP and IP casing design is superior because steam cannot leak through the seal between the HP and IP sections.

The compact casings, together with the single-bearing design, lead to a short overall shaft length and thus a shorter turbine. All turbine and generator bearings are independent of the casing structure and are directly supported on the foundation. This principle makes shaft alignment easy, shortens overall unit erection time, and ensures long-term, stable running behavior of the steam turbine generator set. Inlet valves are directly flanged to the HP and IP casings in the design used at TPP. This eliminates turbine inlet loop or connection piping and yields higher efficiency, better accessibility, and ease of maintenance.