The present invention relates generally to steam turbines, and more particularly, to operating a steam turbine reheat section of a steam turbine with an overload valve.
Steam turbines, especially those that are associated with combined-cycle power plants, can operate under various operating conditions. For example, heat recovery steam generator (HRSG) supplemental firing is often used with steam turbines in combined-cycle power plants to enable the plant to respond to fluctuations in load demand. Typically, this can be helpful in improving peak power production or enabling higher steam production to compensate for the lack of production from another unit within the plant. Generally, supplemental firing results in more steam being supplied to the steam turbine. Typically, this increase in steam flow, as a result of supplemental firing, results in an increase of steam flow to the high pressure turbine section, which will cause an increase of flow of steam supplied to the remaining lower pressure turbine sections. This increase in steam flow results in increased pressures at the steam admission inlets of these lower pressure turbine sections. An increase in pressure at the steam admission inlets of these lower pressure turbine sections has implications on the components used in these sections. For example, the components of these lower pressure turbine sections typically have to be designed to withstand substantial increases in pressures that can arise due to supplemental firing. Designing these lower pressure turbine sections to withstand substantial increases in pressures that can arise during supplemental firing can be costly and add complexity to the overall operation of the power plant.
In one aspect of the present invention, a steam turbine is provided. In this aspect of the present invention, the steam turbine comprises a high pressure turbine section and at least one lower pressure turbine section coupled to the high pressure turbine section. A main steam valve regulates flow of steam from a steam generating source to the high pressure turbine section. A reheater reheats steam exhausted from the high pressure turbine section. A reheat valve regulates reheated steam from the reheater to the at least one lower pressure turbine section at a first steam admission location. An overload valve supplies a diverted portion of the reheated steam from the reheater to the at least one pressure turbine section at a second steam admission location.
In another aspect of the present invention, a steam turbine is provided. In this aspect of the present invention, the steam turbine comprises a high pressure turbine section, a low pressure turbine section, and an intermediate pressure turbine section located between the high pressure turbine section and the low pressure turbine section. A steam generating source generates steam for use by the high pressure turbine section, the intermediate pressure turbine section and the low pressure turbine section. A main steam valve regulates flow of the steam generated from the steam generating source to the high pressure turbine section. A reheater reheats steam exhausted from the high pressure turbine section. A reheat valve regulates reheated steam from the reheater to an inlet of the intermediate pressure turbine section. An overload valve supplies a diverted portion of the reheated steam from the reheater to the intermediate pressure turbine section at a location that is downstream of the inlet that receives the reheated steam supplied by the reheat valve.
In a third aspect of the present invention, there is a method of operating a steam turbine. In this aspect of the present invention, the method comprises: supplying steam to a high pressure turbine section; reheating steam exhausted from the high pressure turbine section; supplying the reheated steam to an inlet of a lower pressure turbine section; diverting a portion of the reheated steam supplied to the inlet of the lower pressure turbine section in response to detection of a change in operating condition at the high pressure turbine section; and supplying the diverted portion of the reheated steam to the lower pressure turbine section at a location that is downstream of the inlet receiving the reheated steam.
Various embodiments of the present invention are directed to operating a reheat section of a steam turbine with inlet variable pressure flow capability to provide added capacity for the overall steam turbine during different operating conditions. In one embodiment, an overload valve is used to supply a diverted portion of reheated steam generated from a reheater (e.g., a reheater section of a steam generating source) to the steam turbine reheat section which may be an intermediate pressure turbine section. The overload valve operates in conjunction with a reheat valve that regulates the reheated steam from the reheater to an inlet of the steam turbine reheat section. In one embodiment, the diverted portion of the reheated steam from the reheater is supplied by the overload valve to the steam turbine reheat section at a steam admission location along the steam path that is downstream of the inlet that receives the reheated steam from the reheat valve.
In operation, the overload valve can throttle the supply of the diverted portion of the reheated steam to the steam turbine reheat section to accommodate changes in conditions (e.g., change in steam flow, temperature and pressure) occurring upstream in the steam turbine such as at a high pressure turbine section and the reheater. As used herein, “to throttle” or “throttling” means varying the pressure-flow passing characteristics of the overload valve by varying the effective area of valve. The operation of the overload valve can also provide the steam turbine reheat section with variable swallowing capacity under different operating conditions. Furthermore, the operation of the overload valve in conjunction with the reheat valve can provide variable pressure and flow capability at the inlet of the steam turbine reheat section. In addition, the operation of the reheat valve in conjunction with the operation of the overload valve can alleviate axial thrust in the various sections of the steam turbine (e.g., the high pressure turbine section, the intermediate pressure turbine section and the low turbine section).
Furthermore, those skilled in the art will appreciate that steam turbine 110 as illustrated in electrical power generation plant 100 is only one example of a steam turbine configuration in which the various embodiments of the present embodiment can operate and is not intended to be limiting. In addition, those skilled in the art will appreciate that electrical power generation plant 100 is only one example of a power generation plant in which the use of an overload valve can be used with a steam turbine reheat section to provide benefits such as variable inlet pressure flow capability, and is not intended to be limiting. For example, one such electrical power generation plant that the various embodiments of the present invention has applicability is with a combined-cycle power plant that uses a heat recovery steam generator (HRSG) to heat up exhaust products generated from a gas turbine to produce steam to be utilized by the steam turbine.
Referring to
As shown in
Steam exhausted from HP turbine section 125 passes through a reheater 155 that reheats the exhausted steam to an increased temperature. Although reheater 155 is illustrated in
The locations that overload valve 115 and reheat valve 160 deliver the reheated steam to IP turbine section 105 as illustrated in
As shown in
Those skilled in the art will recognize that steam turbine 110 of electrical power generation plant 100 may have other components than that shown in
The use of overload valve 115 to bypass reheat valve 160 and supply reheated steam from reheater 155 in the configuration illustrated in
Overload valve 115 and reheat valve 160 can be adjusted accordingly to withstand the substantial increases in pressures that can arise in IP turbine section 105 due to supplemental firing. In particular, the steam valves (i.e., the main valve 150 or the reheat valve 160) can change position for a particular flow and therefore throttle which will increase or decrease the pressure ahead of the valves. Using the reheat valve 160 and the overload valve 115 together will redistribute the amount of reheat flow from the boiler 120 to each steam admission location within the IP section 105. In this scenario, the controller of steam turbine 110 could detect an increase in flow of steam supplied to HP turbine section 125, and consequently open overload valve 115 to start diverting a portion of the reheated steam supplied to the inlet of IP turbine section 105. The controller would also control the reheat valve's supply of the reheated steam to the inlet of IP turbine section 105 to operate in conjunction with overload valve 115. In particular, the controller could throttle overload valve 115 and reheat valve 160 to supply an appropriate amount of flow that will facilitate swallowing the increased flow and pressure at IP turbine section 105 that arises because of changes in conditions occurring upstream at HP turbine section 125 and reheater 155. This swallowing capability can occur without resulting in an increase in pressure at IP turbine section 105. Thus, the throttling of overload valve 115 in conjunction with reheat valve 160 enables IP turbine section 105 to have variable swallowing capacity under different operating conditions.
To operate steam turbine 110 in this manner, boiler 120 provides a supply of steam to HP turbine section 125. Steam exhausted from HP turbine section 125 is reheated by reheater 155 (e.g., a reheat section of the boiler, an HRSG). Reheat valve 160 supplies the reheated steam to an inlet of IP turbine section 105. Overload valve 115 diverts a portion of the reheated steam supplied to the inlet of IP turbine section 105 in response to detection of an increase in flow of steam supplied to HP turbine section 125. In another embodiment, overload valve 115 can divert a portion of the reheated steam supplied to the inlet of IP turbine section 105 in response to detection of pressure or flow directly upstream of reheat valve 160. In another embodiment, overload valve 115 can divert a portion of the reheated steam supplied to the inlet of IP turbine section 105 in response to detection of higher temperatures at the exhaust of HP turbine section 125. For example, consider a steam turbine operating at low loads in a combined-cycle plant arrangement. Low loads and corresponding reduced steam production from the HRSG can impose increased duty on the HP section exhaust, mainly in the form of higher temperatures. These higher temperatures will either result in a limitation of plant operation or require changes to the HP section design and plant piping to accommodate higher temperatures. In this scenario, a portion of the reheated steam supplied to the inlet of the LP turbine section can be diverted in response to detection of a change in plant operating condition, especially at lower loads.
In one embodiment, the diverted portion of the reheated steam is supplied to IP turbine section 105 at a location that is downstream of the inlet receiving the reheated steam from reheat valve 160. The flow of the diverted portion of the reheated steam supplied to IP turbine section 105 can be adjusted to an amount that is maintained within a predetermined pressure range of operation. If the supplemental firing is increased and causes an increase in flow of steam supplied to HP turbine section 125, then the flow of the diverted portion of the reheated steam supplied to IP turbine section 105 can be adjusted to an amount that is maintained within the predetermined pressure range of operation. Subsequently, the diverting of a portion of the reheated steam supplied to the inlet of IP turbine section 105 can be diverted in response to detecting an end of the supplemental firing.
In addition to providing variable swallowing capability, the operation of overload valve 115 in conjunction with reheat valve 160 can be used to alleviate axial thrust in the various sections of steam turbine 110. For example, in some scenarios it may be desirable to extract high pressure steam from steam turbine 110 and use it for cogeneration purposes. As shown in
To alleviate the axial thrust that can occur by extracting steam from HP turbine section 125, the controller of steam turbine 110 can open overload valve 115 to begin diverting a portion of the reheated steam supplied to the inlet of IP turbine section 105 in response to detection of the pressure drop that arises due to extraction. In particular, the operation of the overload valve 115 and reheat valve 160 can be opened and closed in an appropriate manner to maintain the desired thrust direction and magnitude in steam turbine 110 as steam is extracted from HP turbine section 125. In this manner, the flow of the diverted portion of the reheated steam from overload valve 115 and the flow of the reheated steam from reheat valve 160 can be adjusted to an amount that is maintained within a predetermined pressure range of operation. After the steam extraction is discontinued, the diverting of a portion of the reheated steam supplied to IP turbine section 105 can be discontinued.
Technical effects of the various embodiments of the present invention include enabling the steam turbine reheat section with the capability of maximizing performance and output across a large range of steam conditions. This can include increased levels of supplemental firing and also large extractions of steam for gas turbine power augmentation. Meeting large variations in steam conditions while achieving optimum performance will provide considerable operational flexibility across a large power generation application space.
While the disclosure has been particularly shown and described in conjunction with a preferred embodiment thereof, it will be appreciated that variations and modifications will occur to those skilled in the art. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.
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