Steam turbine plant

An exemplary steam plant having a steam circuit which includes a superheater defining a boundary between a superheated steam region and an unsuperheated steam region. The steam circuit includes a branch, from a superheated steam region of the steam circuit, with a branch valve and a steam desuperheater upstream of the branch valve. The desuperheater provides cooling to the branch during flow mode operation of the branch. During a no flow mode, a first preheat line and a second preheat line provide the cooling by supplying unsuperheated steam to the branch and directing this flow through to a lower pressure region of the steam circuit.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to European Patent Application No. 10154526.7 filed in Europe on Feb. 24, 2010, the entire content of which is hereby incorporated by reference in its entirety.

FIELD

The present disclosure relates to steam turbine plants with steam circuits and branches including turbine bypasses and vent lines with relief valves, and to the temperature control of the branches during both flow and no flow modes of operation.

BACKGROUND INFORMATION

Steam circuits of steam plants can include different temperature regions wherein higher temperature regions can be made of materials with higher heat strength that can be more expensive than lower heat strength materials used in lower temperature regions. Due to a difference in cost between materials of differing heat strength, it can be advantageous to reduce the temperature in high temperature branches, such as steam turbine bypasses, of the steam circuit.

GB patent application No. 2 453 849 A discloses the use of a water injection cooler for reducing the temperature exposure in a branch. The cooler functions by spraying water in the branch so that it comes in direct contact with the steam to be cooled. While this can be effective in reducing steam temperature to the point where lower temperature alloys may be used, the disclosed system uses a steam flow to operate and therefore cannot be universally applied to branches with no flow modes, such as bypasses, bleeds or vent lines and still ensure low temperature in the branch during all operating modes.

As an alternative, IPCOM000176170D discloses injecting an inert gas in a branch that acts as a heat buffer during no flow modes of operation. When the branch is, for example, a turbine bypass, the change from no flow to flow mode may result in the inert gas entering the steam circuit. Unless removed with additional equipment, the inert gas can have a negative affect on steam turbine efficiency. In addition, this does not address the thermal shock that may occur when the branch is brought into flow mode from a no flow mode, as the method is not able ensure an adequate temperature is maintained in the branch.

An alternative is to provide external heating and cooling elements over the branch piping and associated equipment. While such arrangements are physically possible such solutions can add significant complexity and cost to the steam plant.

SUMMARY

A steam plant according to the disclosure includes a steam circuit, a superheater in the steam circuit defining a boundary between a superheated steam region and an unsuperheated steam region, and a branch from the superheated steam region of the steam circuit. The branch includes a branch valve and a steam desuperheater, upstream of the branch valve, having a cooling medium feed line. A first preheat line is connected at a first end to an unsuperheated steam region and at a second end, to a first end region of the branch. A second preheat line connected at a first end, to a second end region of the branch, upstream of the branch valve, distal and opposite the first end region and at a second end, to a point of the steam circuit that in operation has a lower pressure than the unsuperheated region to which the first preheat line is connected, so as to enable sequential steam flow through the first preheat line, the branch and the second preheat line respectively when the branch valve is closed.

A method is disclosed for controlling a steam plant, having a superheater in a steam circuit for defining a boundary between a superheated steam region and an unsuperheated steam region. A branch from the superheated steam region of the steam circuit includes a branch valve and a steam desuperheater with a cooling medium feed line. A first preheat line is connected at a first end to an unsuperheated steam region and at a second end to a first end region of the branch. A second preheat line is connected at a first end to a second end region of the branch upstream of the branch valve which is distal and opposite the first end region, and at a second end to a point of the steam circuit that in operation has a lower pressure than the unsuperheated region to which the first preheat line is connected, so as to enable sequential steam flow through the first preheat line, the branch and the second preheat line respectively when the branch valve is closed. The method includes closing the branch valve; isolating a cooling medium flow from the steam desuperheater, and opening a valve in at least one of the first preheat line and the second preheat line to enable sequential flow of unsuperheated steam through the first preheat line, the branch and the second preheat line.

A method is disclosed for controlling a steam plant having a superheater in a steam circuit for defining a boundary between a superheated steam region and an unsuperheated steam region. A branch from the superheated steam region of the steam circuit includes a branch valve and a steam desuperheater, with a cooling medium feed line. A first preheat line is connected at a first end to an unsuperheated steam region and at a second end to a first end region of the branch. A second preheat line is connected at a first end to a second end region of the branch upstream of the branch valve which is distal and opposite the first end region, and at a second end to a point of the steam circuit that in operation has a lower pressure than the unsuperheated region to which the first preheat line is connected, so as to enable sequential steam flow through the first preheat line, the branch and the second preheat line respectively when the branch valve is closed. The method includes closing the branch valve, establishing cooling medium flow to the steam desuperheater, and closing a valve in the second preheat line.

DETAILED DESCRIPTION

Exemplary embodiments of the disclosure can address the high temperature in steam circuit branches that have both flow and no flow modes of operations.

The disclosure is based, at least in part, on the general idea of using a combination of low temperature steam and water injection cooling to control the temperature in steam plant branches.

An exemplary embodiment of the disclosure provides a steam plant with a steam circuit having a superheater that defines a boundary between a superheated steam region (e.g., a higher temperature region of any known steam turbine) and an unsuperheated steam region (e.g., a region of lower temperature, by a specified margin, as compared to the higher temperature region of the same steam turbine). The steam circuit has a branch from the superheated steam region that, at a downstream region, has a branch valve and at an upstream region, a steam desuperheater. The steam desuperheater provides a means to control the temperature of the branch when it is in flow mode. During no flow mode for example, when the branch valve is shut, first and second preheat lines can control the temperature of the branch. They can achieve this, in an exemplary embodiment, by the first preheat line being connected, at a first end, to a unsuperheated steam region, and, at a second end, to an first end region of the branch. Meanwhile, the second preheat line is connected, at a first end, to a second end region of the branch, opposite and distal the first end region, and, at a second end, to a point of the steam circuit configured in operation to have a lower pressure than the unsuperheated steam region to which the first preheat line is connected. This configuration of the preheat lines can promote sequential flow of the unsuperheated steam flow through the first preheat line, the branch, and the second preheat line.

A means can thus be provided for limiting, independent of the flow mode of the branch, the temperature in the branch. In this way, a means to overcome the high temperature in the branch can be provided and thus can enable the use of less expensive lower hot strength materials of construction in the branch.

In various exemplary embodiments, the branch can be either a steam turbine bypass or a vent line with a relief valve.

In order to reduce unwanted steam leakage through the preheat lines during branch flow mode, in an exemplary embodiment, the second preheat line includes a valve, which can be an actuated block valve, check valve or manual valve.

In an exemplary embodiment, the branch can include a temperature controller for controlling the temperature in the branch during no flow mode. A control system cannot only ensure that the branch temperature can be maintained below a maximum temperature, it can enable the temperature to be maintained within a temperature range and thus avoid thermal shock concerns when changing flow modes.

In an exemplary embodiment, the controller can include a flow restricting device, in either or both the first preheat line or the second preheat line. Such a controller can provide a simple, economic means of control.

In another exemplary embodiment, the controller can include a flow modulating valve in the first preheat line, a flow modulating valve in a cooling medium feed line of the steam desuperheater, and a temperature measurement device in the branch. The temperature measurement measures the temperature of the branch. Such a controller can provide tight, predictable temperature control.

Exemplary embodiments of the present disclosure are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details and so is not limited to the exemplary embodiments disclosed herein.

In this specification, reference is made to superheated steam and unsuperheated steam. While unsuperheated steam shall be taken to mean steam that has not been superheated, unsuperheated steam cannot be taken to mean merely saturated steam as unsuperheated steam may contain a low degree of superheat. As a result, throughout this specification, superheated steam is taken to mean steam that has more than 100° C. of superheat above any superheat, unsuperheated steam may have.

In this specification reference is made to valves in terms of: location, for example, branch valve; function, for example, modulating or block; or design, for example, relief or actuated. In the absence of a particular qualifying term the valve can be taken to encompass all known suitable valves for the given purpose. The presence of a qualifying term, however, does not place a restriction on other properties of a valve. For example, a valve designated as an actuated valve may be either a modulating valve or a block valve.

FIG. 1shows a steam plant10whose purpose is to extract heat from a heat source and convert this heat into power. This can be achieved by the use of a steam circuit in which heat energy can be transferred into the steam circuit by, for example, heat exchangers and later extracted, for example, by multiple steam turbines12, such as a high pressure steam turbine12a, in combination with a low pressure steam turbine12b. The closed loop steam circuit can also include a condensate region and so a steam circuit is not taken to mean “consisting exclusively of steam.”

Some elements of a steam circuit of an exemplary steam plant10shown inFIG. 1will now be explained in more detail. The steam circuit, as a continuous loop, has a steam preheater15for vaporising/heating condensate in the steam circuit. After preheating in the preheater15, steam is superheated in a superheater16and fed to a high-pressure steam turbine12where energy is extracted. Exhaust from the high-pressure steam turbine12is once more superheated in a further superheater16and then fed into an intermediate and/or low-pressure steam turbine12for further energy extraction. The exhaust from the steam turbine12is condensed and, in completing the cycle, returned to the preheater15. While the exemplary steam plant10ofFIG. 1is shown with two steam turbines12, exemplary embodiments can be applied to steam plants10configured with one steam turbine12or alternatively more than two steam turbines12.

The steam circuit of a steam plant10can include branches20that span high and lower temperature regions of the steam circuit or else provide outlets from the steam circuit. These branches20can have both flow and no flow modes. In the context of this specification, the terms flow and no flow modes refer to the state of flow or no flow of the steam circuit steam/condensate and not to auxiliary heating and/or cooling flows, such as preheat flows, even if these flows are taken directly from the steam circuit. Exemplary branches20, as shown inFIG. 1, include vent lines13with relief valves14as well as steam turbine bypass lines whose purpose can be to either totally or partially direct steam flow around a steam turbine12.

In an exemplary embodiment, shown inFIG. 2, a branch20includes a branch valve24, which can be used to isolate the branch20from the process and thus can prevent steam flow through the branch20. The branch20further includes a steam desuperheater18, upstream of the branch valve24, to cool the branch20downstream of the steam desuperheater18. In this way, locating the branch valve24downstream of the steam desuperheater18also can ensure it is kept cool and therefore can be made of lower hot strength material.

In an exemplary embodiment where the branch20is a vent line13, the branch valve24can be a relief valve14.

In an exemplary embodiment, the steam desuperheater18can be configured to desuperheat steam from 735° C., where a suitable alloy is a nickel alloy, such as NiCr 23 Co 12 Mo, to below 600° C. and thus enable the use of a lower hot strength material such as 9-12% martensitic Cr-steel. In another exemplary embodiment, when 600° C. steam is desuperheated, a change from, for example, a 9-12% martensitic Cr-steel to a 10CrMo910 steel or equivalent can be made. In each case, the material change can be made after about 10-15 pipe diameters downstream of the steam desuperheater18. This can ensure there is adequate time for the material of the branch to cool before the material change is made.

The steam desuperheater18, shown inFIG. 2-4, can desuperheat steam by mixing or injecting a cooling medium with the superheated steam as it enters the branch20from the steam circuit. The cooling medium can be provided to the steam desuperheater18by a cooling medium feed line23in which a valve25can be located for either or both isolation or control purposes.

As shown inFIG. 2, in an exemplary embodiment, the desuperheater18acan be a mixer in which unsuperheated steam is used as the cooling medium.

As shown inFIGS. 3 and 4, in an exemplary embodiment, the desuperheater18bis a water injection cooler that can utilise water or condensate, sourced from the steam circuit, as the cooling medium.

During no flow mode, for example when the branch valve24is shut, an arrangement of preheat lines21,22can be used, as shown inFIGS. 1-4, to control the temperature in the branch20downstream of the steam desuperheater18. The control of the temperature not only enables the use of lower hot strength materials it also can prevent damage caused by thermal shock when the branch20is brought into flow mode.

In an exemplary embodiment shown inFIG. 2-4, the first preheat line21is connected, at a first end, to an unsuperheated region of the steam circuit. In separate exemplary embodiments, shown inFIG. 1, this is either at a point between the steam turbine12exhaust and steam superheater16or between the steam preheater15and the superheater16. At a second end, the first preheat line21is connected to a first end region of the branch20.

The second preheat line22is connected at a first end, to a second end region, opposite and distal from the first end region, of the branch20. At a second end, the second preheat line22is connected to a point of the steam circuit configured in operation to have a lower pressure than the unsuperheated steam region at which the first preheat line21is connected to. An example of such a location is the feed line of one of the steam turbines12, such as a low pressure steam turbine12bas shown inFIG. 1. This configuration enables sequential flow of unsuperheated steam through the first preheat line21, the branch20and then finally through the second preheat line22and back into the steam circuit.

In order to prevent reverse flow through the secondary preheat line22when the branch20is in flow mode, the secondary preheat line22, in an exemplary embodiment, can include a valve25, as shown inFIGS. 2-4, that can be shut during flow mode when reverse flow is most likely. In an exemplary embodiment, the valve25can be an actuated valve thus enabling automated operation of the valve25. In another exemplary embodiment, the valve25can be a check valve or other pipe device to prevent reverse flow.

In an exemplary embodiment, the branch20is fitted with a temperature controller for controlling the temperature in the branch20during no flow mode. In an exemplary embodiment, the controller can include a flow restriction device31, as shown inFIG. 3. The flow restriction devices31can be a flow orifice, or flow tube, fitted in either or both the first preheat line21or the second preheat line22(not shown). Thus fitted, the flow restriction device31can ensure a predetermined flow rate of unsuperheated steam can be provided through the branch20, when in no flow mode, in a cheap and technically simple way.

In another exemplary embodiment, shown inFIG. 4, the controller can include: a flow modulating valve26in both the first preheat line21and the cooling medium feed line23of the steam desuperheater18; and a temperature measurement device30in the branch20for measuring the temperature of the branch20. These control elements are elements of a logic controller such as a processor coupled to a memory that uses known control means to modulate the modulating valves26based on measurements taken from the temperature measurement device30during both no flow and flow modes.

The control of temperature in the branch20is not however limited to these two controller configurations and as such can, for example, include elements from each of these control schemes, or else incorporate other suitable control elements.

An exemplary method for configuring an exemplary steam plant10shown inFIG. 1andFIG. 2-4for no flow mode, for example, when the branch valve24is shut, can include, in no particular order, the following; isolating the desuperheater cooling flow by, for example, closing a valve25in the cooling medium feed line23to the steam desuperheater18; and opening, or ensuring open, valve25or valves25in either the first preheat line21, the second preheat line22or both the first preheat line21and the second preheat line22. The opening or ensuring open of the valve or valves25can enable sequential unsuperheated steam flow through the first preheat line21, the branch20and the second preheat line22.

An exemplary method for configuring an exemplary steam plant10, shown inFIG. 1andFIGS. 2-4for flow mode, for example, when the branch valve24is open, can include, in no particular order; establishing cooling flow to the steam desuperheater18, by, for example opening valve or valves25in the cooling medium feed line23; and closing or ensuring closed the valve25in the second preheat line22.

Although the disclosure has been herein shown and described in what are considered to be the most preferred exemplary embodiments, the present disclosure can be embodied in other specific forms. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.

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