Abstract:
A steam power plant is described including on a single rotor at least one high pressure turbine or turbine section having a steam exit connected in operation to a first steam reheater and at least two intermediate pressure turbines or turbine sections with a first of the at least two intermediate pressure turbines or turbine sections having a steam exit connected in operation to a second steam reheater and with a second of the at least two intermediate pressure turbines or turbine sections having a steam entry to receive steam from the second steam reheater and a steam exit connected to one or more low pressure turbines or turbine sections, whereby the at least two intermediate pressure turbines or turbine sections are each separated into a high temperature turbine or turbine section and into a low temperature turbine or turbine section.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a high-temperature steam turbine power plant with double reheat driven by a main or live steam temperature of 650 degrees C. or more in the first and second reheat and having a power output of 100 MW or more. 
       BACKGROUND OF THE INVENTION 
       [0002]    The steam power plants currently in operation have main steam temperatures of up to about 600 degrees C. When operating at this or close to this temperature, the power plant is typically referred to as “supercritical”, as the main steam pressure is above the critical pressure. Efforts to improve the efficiency of supercritical power plants through increase of the steam temperature have been hampered to a large extent by the lack of suitable materials for the boilers, turbines and supply pipes. Conventional steel even when chromium based lose their stability below 650 degrees C. 
         [0003]    The power plants designed to break the barrier as currently posed by the available materials are often referred to as ultra-supercritical or USC steam plants. To achieve a steam temperature of above 650 degrees C. and more, the parts exposed to the high temperature require Nickel based alloys which are expensive and difficult to forge into large forgings beyond the limit of 10 tons. This limit can be insufficient for the rotor of a large steam turbine plant, as described for example in the published United States Patent application no. 2008/0250790 A1. 
         [0004]    On the other hand methods described as “double reheat ” have been suggested and used for more than five decades as evidenced for example by the U.S. Pat. No. 2,955,429. Double reheat systems have an additional return flow of the main steam path back to the boiler or reheater. In a typical single reheat system; a return path directs the exit steam from a first or high pressure turbine back to the boiler for reheating before re-entering the turbine stages at a second or intermediate pressure. In a double reheat system a similar return path exists at the exit of the intermediate pressure turbine to reheat the steam before it enters at a second (lower) intermediate pressure or at a low pressure turbine stage. 
         [0005]    Double reheat systems have been suggested as a means of further increasing the efficiency of USC steam plant. A design of a double reheat USC power plant is described for example in an article by R. Blum, J. Bugged, and S. Jar, “AD700 innovations pave the way for 53 per cent efficiency” in Modern Power Systems, Nov 2008, pp. 15-19. 
         [0006]    However, the double reheat systems proposed for USC steam power plants are insufficiently optimised for the properties and costs of currently existing materials. It is therefore seen as an object of the present invention to provide USC steam power plants improved double reheat which are both, technically and economically, better suited for commercially viable installations. 
       SUMMARY OF THE INVENTION 
       [0007]    According to an aspect of the present invention, there is provided a steam power plant including on a single rotor at least one high pressure turbine or turbine section having a steam exit connected in operation to a first steam reheater and at least two intermediate pressure turbines or turbine sections with a first of the at least two intermediate pressure turbines or turbine sections having a steam exit connected in operation to a second steam reheater and with a second of the at least two intermediate pressure turbines or turbine sections having a steam entry to receive steam from the second steam reheater and a steam exit connected to one or more low pressure turbines or turbine sections, whereby the at least two intermediate pressure turbines or turbine sections are each separated into a high temperature turbine or turbine section and into a low temperature turbine or turbine section. 
         [0008]    A high temperature turbine or turbine section can receive reheated steam at temperatures above 650 degrees C., preferably above 700 degrees C. And hence the high temperature turbine or turbine section is at least partly manufactured from advanced materials such as Nickel based alloys. 
         [0009]    The four turbines or turbine sections resulting from the separation of the intermediate turbines can be preferable recombined into double-flow turbines resulting in either a double-flow high temperature turbine, a double-flow low temperature turbine, or both. 
         [0010]    These and further aspects of the invention will be apparent from the following detailed description and drawings as listed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Exemplary embodiments of the invention will now be described, with reference to the accompanying drawing, in which: 
           [0012]      FIG. 1  is a reproduction of a known ultra-supercritical steam power plant design; 
           [0013]      FIG. 2A  shows improvements as per a first variant of the present invention; and 
           [0014]      FIG. 2B  shows a second variant of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The plant design  10  of  FIG. 1  shows a known USC steam plant with double reheat. In a steam generator or boiler  11  live steam is produced from a water return cycle (not shown) and guided through the main steam feed pipe  121  and the main steam inlet valve  131  to a first turbine  14 . The turbine  14  is typically referred to as the high pressure or HP turbine. The main steam path exits the HP turbine  14  through a pipe  122 , which in turn branches into the first reheater return pipe  123 . In the first reheater  151  which is in general part of the boiler  11 , the main steam flow is reheated before entering the feed pipe  124  and passing through the inlet valve  132  into the first intermediate pressure or IP turbine  161 . 
         [0016]    At this stage the feed steam has gained through the reheating a temperature close to or even above the original live steam temperature. The first IP turbine  161  is manufactured using essentially the same high-temperature resistant materials as the HP turbine  14 . From the exit of the first IP turbine  161  leads a return pipe  125  to the second reheater  152 . 
         [0017]    In the second reheater  152  the steam is again heated to a temperature close to or even above the original live steam temperature. After passing through the second reheater the steam enters into a steam feed pipe  126  and through a valve  133  into a second intermediate pressure turbine  162 . The second IP turbine  162  is again manufactured using essentially the same high-temperature resistant materials as the HP turbine  14  and the first IP turbine  161 . 
         [0018]    The steam at the exit of the second IP turbine  162  is guided into one or more low pressure or LP turbines  163  to be finally expanded to condensing conditions. The LP turbine  163  is configured as a so-called “double-flow” turbine with two balanced branches in one inner casing. 
         [0019]    All turbines share a single rotor shaft  17  which drives an electro-magnetic generator  18 , as known in the art. 
         [0020]    The plant design  10  of  FIG. 2A  shows a USC steam plant with double reheat with a reheat system in accordance with an example of the present invention. Numerals of essentially equal components or components with similar functions are maintained with respect to  FIG. 1 . In the design of  FIG. 2A , there is again a steam generator or live steam boiler  11  to produce steam from a water return cycle (not shown) and guide it through the main steam feed pipe  121  and the main steam inlet valve  131  to a first turbine  14 . As in the example of  FIG. 1 , the turbine  14  is a high pressure or HP turbine made of materials suitable for temperatures around 700 degrees Celsius and a main steam pressure PO of about 350 bar. As before the main steam path exits the HP turbine  14  through pipe  122 , which in turn branches into the first reheater return pipe  123 . In the first reheater  151  which is typically part of the boiler  11 , the main steam flow is reheated before entering the feed pipe  124  and passes through the inlet valve  132  into the first intermediate pressure or IP turbine  161 . 
         [0021]    The first intermediate pressure or IP turbine  161  is in this example separated into a first high-temperature part  161 - 1  and a second low temperature part  161 - 2 . When referring to separation, the meaning is that the first high-temperature part  161 - 1  is housed within or surrounded by an inner casing separated from the inner casing of the second low temperature part  161 - 2 , which has its own inner casing. When referring to the step of separating the first intermediate pressure or IP turbine  161  into a first high-temperature part  161 - 1  and a second low temperature part  161 - 2 , its most significant element is hence the step of housing both parts of the turbine in different inner casings (with different supply and exit lines). 
         [0022]    At this stage the feed steam has gained through the reheating a temperature of around 720 degrees Celsius and a pressure of around 75 bar. The first high-temperature part  161 - 1  of the IP turbine  161  is manufactured using essentially the same high-temperature resistant materials as the HP turbine  14  while requiring less pressure resistance due to the reduced steam pressure from the first reheat cycle. A feed pipe  128  connects the exit of the high-temperature part  161 - 1  to the second low temperature part  161 - 2 . 
         [0023]    As the second low temperature part  161 - 2  of the turbine  161  is no longer subject to the same high temperatures as the first part  161 - 1 , it can be built using more conventional materials as applied for example in the building of steam turbines for super-critical steam. The second low temperature part  161 - 2  of the turbine  161  is shown as a double-flow turbine with two balanced branches in one inner casing. The exits of the second low temperature part  161 - 2  are combined into the return pipe  125  conveying the steam back to the second reheater  152 . 
         [0024]    The separation of the first IP turbine  161  into first high-temperature part  161 - 1  and a second low temperature part  161 - 2  has the advantage of splitting the casing and turbine parts into high temperature components and low temperature components only. Neither is it required to manufacture a full IP turbine with high-temperature components nor is it necessary to weld together casings parts or other turbine parts made from high-temperature alloys on the one side and conventional alloys on the other side. The welds between different alloys in a high-temperature steam environment are found to pose a risk for crack initiation and can be a major source of long-term defects in USC turbine plants. 
         [0025]    In the second reheater  152  the steam is again heated to a temperature around 720 or 730 degrees Celsius. After passing through the second reheater the steam enters into a steam feed pipe  126  and through a valve  133  into the second intermediate pressure turbine  162 . As with the first IP turbine  161 , the second IP turbine  162  is in this example also separated into a first high-temperature part  162 - 1  and a second low temperature part  162 - 2 . The separation means that the first high-temperature part  162 - 1  is housed within or surrounded by an inner casing separated from the inner casing of the second low temperature part  162 - 2 , which has its own inner casing. 
         [0026]    At this stage the feed steam has gained through the reheating a temperature of around 720 or 730 degrees Celsius and a pressure of around 30 bar. The first high-temperature part  162 - 1  of the IP turbine  162  is manufactured using essentially the same high-temperature resistant materials as the HP turbine  14  and the first high-temperature part  161 - 1  while requiring even less pressure resistance than the latter due to the reduced steam pressure from the second reheat cycle. The feed pipe  129  connects the exit of the high-temperature part  162 - 1  to the second low temperature part  161 - 2 . 
         [0027]    As the second low temperature part  162 - 2  of the second IP turbine  162  is also no longer subject to the same high temperatures as the first part  162 - 1 , it can be built using more conventional materials as applied for example in the building of steam turbines for super-critical steam. The second low temperature part  162 - 2  of the turbine  162  is shown as a double-flow turbine with two balanced branches in one inner casing. The exits of the second low temperature part  161 - 2  are combined into the feed pipe  127  conveying the steam to the one or more low pressure or LP turbines  163  to be finally expanded to condensing conditions. The LP turbine  163  is configured as a so-called “double-flow” turbine with two balanced branches in one inner casing. In the example the single double-flow turbine  163  represents any number of LP turbines such as one, two, three, four or five of such turbines depending on the overall mass flow rate of the steam through the plant. 
         [0028]    All the turbines of  FIG. 2A  share a single rotor shaft  17  which drives an electro-magnetic generator  18 , as known in the art. 
         [0029]    The advantages gained by the implementation of a plant as shown in  FIG. 2A  can be furthered by combining the high temperature turbine parts  161 - 1  and  162 - 1  into a double-flow casing  164  with each part having its dedicated feed and exit pipes as shown in the example of  FIG. 2B . In  FIG. 2B  numerals of essentially equal components or components with similar functions are maintained with respect to  FIG. 2A . 
         [0030]    Hence in  FIG. 2B , the high temperature turbine parts  161 - 1  and  162 - 1  are shown combined into a double-flow turbine casing  164 . The feed pipe  124  delivers the steam from the first reheater  151  to the first high temperature turbine part  161 - 1  of the double-flow turbine and the feed pipe  126  delivers the steam from the second reheater  151  to the second high temperature turbine part  162 - 1  of the double-flow turbine. 
         [0031]    The exit of the first high temperature turbine part  161 - 1  of the double-flow turbine  164  is connected to the feed pipe  128  of the low temperature part  161 - 2  of the first intermediate pressure turbine. The low temperature part  161 - 2  of the first intermediate pressure turbine is a double-flow turbine with its exits connected via the return pipe  125  to the second reheater  152 . 
         [0032]    The exit of the second high temperature turbine part  162 - 1  of the double-flow turbine  164  is connected to the feed pipe  129  of the low temperature part  162 - 2  of the second intermediate pressure turbine. The low temperature part  162 - 2  of the second intermediate pressure turbine is a double-flow turbine with its exits combined into the feed pipe  127  conveying the steam to the one or more low pressure or LP turbines  163  to be finally expanded to condensing conditions. The LP turbine  163  is configured as a so-called “double-flow” turbine with two balanced branches in one inner casing. In the example the single double-flow turbine  163  represents any number of LP turbines such as one, two, three, four or five of such turbines depending on the overall mass flow rate of the steam through the plant. 
         [0033]    As the present invention has been described above purely by way of example, the above modifications or others can be made within the scope of the invention. For example in a smaller plant design the second parts  161 - 2 ,  162 - 2  of the first and second intermediate pressure turbines can be combined into a single double-flow turbine instead of forming two separate double-flow turbines as in the examples above. For double-flow turbine casing there are several variants possible including with common outer casing and common inner casing, but also a design with one inner casing and a piston and a blade carrier on the second side is possible; and a common inner inlet casing with blade carriers on both sides. 
         [0034]    The invention may also comprise any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. Alternative features serving the same, equivalent or similar purposes may replace each feature disclosed in the specification, including the drawings, unless expressly stated otherwise. 
         [0035]    Unless explicitly stated herein, any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.