Method and apparatus for improved reduced load operation of steam turbines

The present application relates generally to steam turbines and more particularly to the improved performance of steam turbines at reduced-load, part-load, or off-design operating conditions.

Steam turbines are mechanical devices that convert the thermal energy of pressurized steam into rotational mechanical work. In order to improve thermal efficiency, large steam turbines generally include several sections, such as a high pressure section, an intermediate pressure section, and a low pressure section. Within each section, multiple stages may be used in the expansion of the steam.

The sections of large steam turbines possess various design characteristics so as to permit the extraction of the largest possible amount of energy from the expansion of steam through the turbine sections. It is common practice to introduce initially high pressure steam into a high pressure section of the steam turbine. Steam exiting the high pressure section of the turbine is then directed to a reheater before being introduced into a low pressure section of the turbine, where it continues its expansion. In many turbines the steam passes through an intermediate pressure section before being introduced into a low pressure section.

Because of the increase in volume as the steam expands through the stages of each section, the annulus area of each stage must increase in order to convert efficiently the thermal energy of the steam to rotational mechanical work. This is accomplished by lengthening the turbine blades or buckets from stage to stage, by increasing the diameter of the rotor upon which the blades are mounted, by adding two or more flow paths in parallel, or combinations thereof.

When two flow paths of a section of a turbine are in parallel, this is known as a double-flow section. It is a common practice to have one or more of the sections configured in a double flow arrangement in which steam entering a middle portion of the section encounters a diverging flow path. Following entry into this middle portion of one of the turbine sections, the steam exits in opposite directions with both flows directed to rotate the turbine shaft in the same direction. Thus, for example, steam entering from the top or bottom exits toward the left and right.

The choice of the single-flow or the double-flow low pressure section design may depend on the volume of steam designed to flow though the low pressure section at normal, full-load conditions. The best performance of low pressure steam path design may be achieved when steam velocity exiting the last stage buckets of the low pressure section is about 600 feet per second. Lower or higher velocities may degrade the performance of the steam turbine. In order to adjust the exit velocity of the low pressure section, the annulus area of the last stage buckets may be increased or reduced. If steam flow volumes are large enough, however, increasing the length of the turbine blades or the diameter of the rotor upon which the blades are mounted may not be feasible or desirable. Instead, a double-flow section may be used in order to increase the exit annulus area of the low pressure section. Similarly, if a single double-flow low pressure section does not provide sufficient annulus area to achieve adequate steam exit velocities, two or more double-flow low pressure sections may be used in parallel. In turbines having a number of double-flow low pressure sections, approximately equal portions of the steam flow may be introduced to each of the double-flow low pressure sections.

Steam turbines are designed to operate efficiently under normal, full-load operating conditions. In some circumstances such as a reduced demand for electricity, however, the steam flow to the turbine is reduced for economic reasons. When the flow is reduced below design conditions, the performance of the low pressure section of the steam turbine may drop significantly due to large separation at the hub of the last stage buckets. Exhaust hood performance also may decline. The low steam path efficiency and poor exhaust hood performance may cause poor turbine efficiency at reduced-load, low-load, or off-design operating conditions.

There is a desire, therefore, to provide a method and apparatus for improving the operation of steam turbines at reduced-load. Such a method and apparatus may improve the efficiency of steam turbines at such operating conditions.

In one embodiment, the present application provides a method of operating a steam turbine at reduced load, including providing a number of low pressure sections, providing a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections, and operating the flow control system to limit the flow of steam to the at least one of the number of low pressure sections when the load is below a predetermined value.

Another embodiment of the present application provides a method of operating a steam turbine at reduced load, including providing one double-flow low pressure section having two steam flow paths, providing a flow control system operable to limit the flow of steam to at least one of the two flow paths, and operating the flow control system to limit the flow of steam to the at least one of the two flow paths

A further embodiment of the present application provides a steam turbine. The steam turbine may include a number of low pressure sections and a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections.

These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.