Flow path for steam turbine outer casing and flow barrier apparatus

A steam turbine may include a turbine section including a rotor. An inner casing is provided about the turbine, the inner casing including an upstream end, a downstream end and an inner casing exhaust port positioned at the downstream end allowing exhaust steam to exit the inner casing. An outer casing is provided about the inner casing, the outer casing including an upstream end, a downstream end and an outer casing exhaust port positioned at the upstream end of the outer casing. A flow path extends between the inner casing and the outer casing through which the exhaust steam passes in an upstream direction from the inner casing exhaust port to the outer casing exhaust port. A flow barrier may be provided in the flow path between the inner casing and the outer casing.

BACKGROUND OF THE INVENTION

The disclosure relates generally to steam turbines, and more particularly, to a flow path for an outer casing of a steam turbine.

Steam turbines often are very large in size and consequently have large material mass. Steam turbines also operate at high temperatures that create a number of challenges. One challenge is to ensure proper thermal response of parts, such as an outer casing. Typically, outer casings of steam turbines are not provided with any special thermal response system other than to provide some steam leakage and specific stage steam conditions. These thermal response techniques, however, use higher temperature steam. One approach to provide better thermal response has been to position the outer casing exhaust port at the middle of the lower half of the outer casing. Unfortunately, this configuration does not impact the region of the outer casing that drives clearances.

Another challenge is to provide an appropriate amount of clearance between outer and inner casings to avoid contact therebetween caused by the differential thermal expansion in parts thereof as they increase to the high operating temperatures. Most steam turbines address the differential thermal expansion by providing sufficient clearance between casing parts to handle any worst-case situation. This latter approach, however, increases machine size and may increase machine material mass. Another approach to the clearance issue has been to use heating blankets to bring the outer casing up to temperature before startup.

BRIEF DESCRIPTION OF THE INVENTION

A first aspect of the disclosure provides a steam turbine comprising: a turbine section including a rotor; an inner casing about the turbine, the inner casing including an upstream end, a downstream end and an inner casing exhaust port positioned at the downstream end allowing exhaust steam to exit the inner casing; an outer casing about the inner casing, the outer casing including an outer casing exhaust port positioned adjacent to the upstream end of the inner casing; and a flow path between the inner casing and the outer casing through which the exhaust steam passes from the inner casing exhaust port to the outer casing exhaust port.

A second aspect of the disclosure provides a steam turbine comprising: a turbine section including a rotor; an inner casing enclosing the turbine, the inner casing including an upstream end, a downstream end and an inner casing exhaust port positioned at the downstream end allowing exhaust steam to exit the inner casing; an outer casing about the inner casing, the outer casing including an outer casing exhaust port positioned adjacent to the upstream end of the inner casing; a flow path between the inner casing and the outer casing through which the exhaust steam passes from the inner casing exhaust port to the outer casing exhaust port; and a flow barrier in the flow path between the inner casing and the outer casing, wherein an end of the outer casing adjacent to the inner casing exhaust port has a shape configured to direct the exhaust steam from the inner casing exhaust port to the flow path.

A third aspect of the disclosure provides an apparatus comprising: an arcuate flow barrier having an outer extent configured for coupling to an inner portion of an outer casing of a steam turbine and an inner extent configured for coupling to an outer portion of an inner casing of the steam turbine, the arcuate flow barrier directing flow of steam in a particular direction between the inner casing and the outer casing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings,FIG. 1shows a side cross-sectional view of one embodiment of a steam turbine100. Steam turbine100includes a turbine section101including a rotor102that includes a rotating shaft104and a plurality of axially spaced rotor wheels106. As understood, a plurality of rotating blades (not shown) are mechanically coupled to each rotor wheel106within a inner casing122. More specifically, the blades are arranged in rows that extend circumferentially around each rotor wheel106. As also understood, a plurality of stationary vanes (not shown) extend circumferentially around shaft104within inner casing122, and the vanes are axially positioned between adjacent rows of blades. The stationary vanes cooperate with the blades to form a stage and to define a portion of an operative steam flow path through turbine section101. In operation, steam enters a steam inlet110of turbine section101and is channeled through the stationary vanes. As illustrated, steam inlet110is positioned intermediate an upstream end130and a downstream end132of inner casing122(and also outer casing120) for delivering operative steam to inner casing122. The vanes direct steam downstream against the blades. Steam passes through the remaining stages imparting a force on blades causing rotating shaft104to rotate. At least one end of steam turbine100may extend axially away from rotor102and may be attached to a load or machinery (not shown) such as, but not limited to, a dynamoelectric machine such as a generator or a motor, and/or another turbine.

Steam turbine100also includes an outer casing120that extends about inner casing122. As noted above, inner casing122extends about turbine section101. As understood, each casing120,122may be formed in semi-circular sections joined along a horizontal mid-line, the upper halves of the outer and inner casings being illustrated. Inner casing122may include forward and aft shell sections mounted for radial contraction and expansion relative to outer casing120. As partly noted above, inner casing122includes an upstream end130, a downstream end132and an inner casing exhaust port134. Inner casing exhaust port134may be any opening at downstream end132of inner casing122allowing exhaust steam to exit inner casing122. As used herein, “upstream” and “downstream” indicate positions relative to an operative steam flow through turbine section101, which is left-to-right inFIGS. 1 and 2.

In contrast to conventional steam turbines, outer casing120includes an outer casing exhaust port140that is positioned adjacent to upstream end130of inner casing122. Conventionally, outer casing exhaust ports are positioned adjacent to, i.e., immediately downstream or radially outward from, inner casing exhaust port134. Positioning of outer casing exhaust port140adjacent to upstream end130provides a flow path144between inner casing122and outer casing120through which the exhaust steam passes in a direction from inner casing exhaust port134to outer casing exhaust port140. As used herein, “adjacent” means near or close to upstream end130, e.g., either upstream or slightly downstream from upstream end130. Outer casing exhaust port140may be radially outward relative to at least part of upstream end130of inner casing122. In one embodiment, an end142of outer casing120adjacent to inner casing exhaust port134has a shape configured to direct the exhaust steam from inner casing exhaust port134to flow path144, e.g., curved, curved with vanes or otherwise structured to direct steam towards flow path144.

The direction of steam flow in flow path144is upstream compared to the operative steam flow in turbine section101, i.e., generally right-to-left inFIGS. 1and2—opposite to the operative steam flow in turbine section101. Consequently, exhaust steam in flow path144passes over an inner surface150of outer casing120and an outer surface152of inner casing122, cooling each casing. In particular, flow path144allows a temperature of outer casing120and a temperature of inner casing122to each follow a temperature of rotor102. As used herein, “follow” means that if rotor temperature increases, outer casing and inner casing temperatures also increase such that the relative movement between rotor and casings is minimized. Similarly, if rotor temperature decreases, outer and inner casing temperatures decrease. From a technical perspective, the lower casing temperature that results permits a wider range of applicable materials for outer casing120. Embodiments of the invention are also very simple to implement and do not require additional parts and their inherent risk of failure. Further, the ability to use lower grade materials results in lower product cost. Reduction in clearances improves the overall performance of steam turbine100.

Referring toFIGS. 2-4, in an optional embodiment, a flow barrier160,260is positioned in flow path144between inner casing122and outer casing120. Flow barrier160,260may have any shape sufficient to direct flow of steam in a particular direction between inner casing122and outer casing120, but is generally arcuate as illustrated inFIGS. 3 and 4, which are cross-sectional views along line A-A inFIG. 2. Flow barrier160,260may be made of any now known or later developed material capable of withstanding the environmental conditions of steam turbine100, e.g., steel. As observed best inFIG. 2, flow barrier160,260directs exhaust steam towards a lower part164of flow path144between inner casing122and outer casing120. The active cooling of outer casing120reduces the axial clearances needed between stationary and rotating parts, which improves performance. As illustrated, in one embodiment, flow barrier160,260is immediately downstream, i.e., using the direction of operative fluid flow in turbine section101, of outer casing exhaust port140. However, this position may not be necessary in all cases. In one embodiment, flow barrier160,260includes an arcuate partition extending between approximately 160° to approximately 220° circumferentially between inner casing122and outer casing120, and in one particular embodiment, partition160,260extends approximately 200° circumferentially between the casings (shown via dashed lines inFIGS. 3 and 4).

As shown inFIGS. 3 and 4, arcuate flow barrier160,260includes an outer extent170configured for coupling to an inner portion172(e.g., surface150(FIG. 2) or other internal structure) of outer casing120and an inner extent174configured for coupling to an outer portion176(e.g., surface152(FIG. 2) or other external structure) of inner casing122. Consequently, arcuate flow barrier160,260has a radial length L (FIG. 3only) that approximately matches a space between inner portion172of outer casing120and outer portion176of inner casing122. Any now known or later developed techniques for coupling parts within a steam turbine100and allowing appropriate thermal expansion may be employed, e.g., mechanical couplings, welding, slide joints, etc. InFIG. 3, flow barrier160is coupled to inner casing122using the aforementioned techniques. In an alternative embodiment, shown inFIG. 4, flow barrier260is integral with inner casing122, i.e., it is formed as part of inner casing122.