Patent Publication Number: US-2017370252-A1

Title: Apparatus for reducing windage loss of steam turbines

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2016-0080323, filed on Jun. 27, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Exemplary embodiments of the present disclosure relate to an apparatus for reducing or preventing windage loss of steam turbines for reducing or minimizing damage to a blade caused by a rise in temperature at an outlet stage of a high-pressure turbine due to operation of the steam turbine. 
     Generally, steam is supplied to a heat supply and storage tank after a high-pressure turbine. When a medium-pressure turbine is started-up for cogeneration in a state in which a condenser is not operated, the high-pressure turbine is rotated and a temperature at an outlet stage of the high-pressure turbine rapidly rises occurs. The rapid rise in temperature at the outlet stage may be referred to as a windage loss phenomenon. 
     In a case in which the windage phenomenon is continued, a turbine positioned at the last stage of the high-pressure turbine that is heated as described above causes damage to a bucket. 
     The bucket positioned at the stage corresponds to, for example, a sixth-stage turbine or a seventh-stage turbine among multiple turbines in the high-pressure turbine. When the turbine positioned at the stage is damaged, operation thereof needs to be stopped. 
     In particular, since when the cogeneration is simultaneously performed, district heating may also be stopped at the same time, a method capable of reducing or minimizing the windage loss for stable operation of the high-pressure turbine and stable cogeneration, is needed. 
     BRIEF SUMMARY 
     An object of the present disclosure is to provide an apparatus for reducing or preventing windage loss of steam turbines capable of reducing or minimizing damage to a last-stage bucket caused by the windage loss in a high-pressure turbine under a condition that the high-pressure turbine, a medium-pressure turbine, and a heat supply and storage tank are started-up. 
     In accordance with one aspect of the present disclosure, there is provided an apparatus for reducing or preventing windage loss of steam turbines, including: a turbine unit including a high-pressure turbine, a medium-pressure turbine, and a low-pressure turbine; a condenser in which steam passing through the low-pressure turbine is supplied and heat-exchanged; a tank unit in which steam generated at the time of operation of the turbine unit is converted into condensed water after being condensed, and stored; a steam supplier supplying additional steam to an inlet stage of the high-pressure turbine to reduce or minimize a rapid rise in temperature at an outlet stage of the high-pressure turbine due to speed-up of the medium-pressure turbine under a condition that the condenser is not operated; a sensor sensing the temperature at the outlet stage of the high-pressure turbine and internal pressure; and a controller controlling an amount of steam supplied by the steam supplier by receiving data on the temperature and the pressure sensed by the sensor. 
     The steam supplier may include an extension pipe extending toward between a second-stage turbine and a third-stage turbine among turbines configuring the high-pressure turbine, a steam generator supplying steam to the extension pipe, and a first control valve positioned at any extended position of the extension pipe and of which an opening degree is controlled by the controller. 
     The extension pipe may supply steam at a side of a turbine blade provided in the high-pressure turbine in a state in which the extension pipe is orthogonal to a shaft of the high-pressure turbine. 
     The extension pipe may have an end formed in a nozzle form. 
     The extension pipe may supply steam in a rotation direction of the high-pressure turbine. 
     The extension pipe may include a first extension pipe extending toward a turbine blade in order to supply steam in a rotation direction of the high-pressure turbine, and a second extension pipe positioned opposite to the first extension pipe and extending to supply steam in the rotation direction of the turbine blade. 
     The apparatus may further include a return pipe of which one end is connected to the outlet stage of the high-pressure turbine and the other end is connected to the tank unit, and through which the steam supplied to the inlet stage of the high-pressure turbine is drained. 
     The return pipe may include a second control valve controlling an amount of drained steam. 
     The controller may control pressure of the steam supplied by the steam supplier based on pressure data according to pressure drop between the second-stage turbine and the third-stage turbine. 
     The steam supplier may further include a pressure controller installed at the extension pipe in order to supply the additional steam supplied to the high-pressure turbine at different pressure by the controller. 
     Steam drained from the medium-pressure turbine may be stored in a heat supply and storage tank in which heating water for heating is stored. 
     In accordance with another aspect of the present disclosure, there is provided an apparatus for reducing or preventing windage loss of steam turbines, including: a turbine unit including a high-pressure turbine, a medium-pressure turbine, and a low-pressure turbine; a condenser in which steam passing through the low-pressure turbine is supplied and heat-exchanged; a tank unit in which steam generated at the time of operation of the turbine unit is converted into condensed water after being condensed, and stored; a steam supplier supplying additional steam to an inlet stage and an outlet stage of the high-pressure turbine to reduce or minimize a rapid rise in temperature at an outlet stage of the high-pressure turbine due to speed-up of the medium-pressure turbine under a condition that the condenser is not operated; a sensor sensing the temperature at the outlet stage of the high-pressure turbine and internal pressure; and a controller controlling an amount of steam supplied by the steam supplier so as not to generate damage at the outlet stage by receiving data on the temperature and the pressure sensed by the sensor. 
     The steam supplier may include a first extension pipe extending toward between a second-stage turbine and a third-stage turbine among turbines configuring the high-pressure turbine, a third extension pipe extending toward the last stage of the high-pressure turbine, and a steam generator supplying steam to the first and third extension pipes. 
     The steam supplier may further include a first control valve positioned at any extended position of the first and third extension pipes and of which an opening degree is controlled by the controller. 
     The controller may perform a control to simultaneously supply steam to the first and third extension pipes or to supply the steam to any one of the first extension pipe and the third extension pipe. 
     When the steam is simultaneously supplied to the first and third extension pipes, the controller may control pressure of the steam supplied to the first extension pipe and pressure of the steam supplied to the third extension pipe to be different from each other. 
     The apparatus may further include a return pipe of which one end is connected to the outlet stage of the high-pressure turbine and the other end is connected to the tank unit, and through which the steam supplied to the inlet stage of the high-pressure turbine is drained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating an apparatus for reducing or preventing windage loss of steam turbines according to a first embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating an example of an extension pipe according to the first embodiment of the present disclosure. 
         FIG. 3  is a diagram illustrating another example of an extension pipe according to the first embodiment of the present disclosure. 
         FIG. 4  is a block diagram illustrating a controller and a peripheral structure associated with the controller according to the first embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating operation of the apparatus for reducing or preventing windage loss of steam turbines according to the first embodiment of the present disclosure. 
         FIG. 6  is a diagram illustrating an apparatus for reducing or preventing windage loss of steam turbines according to a second embodiment of the present disclosure. 
         FIG. 7  is a diagram illustrating operation of the apparatus for reducing or preventing windage loss of steam turbines according to the second embodiment of the present disclosure. 
         FIG. 8  is a diagram illustrating operation of the apparatus for reducing or preventing windage loss of steam turbines according to the second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure will be described below in more detail with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. 
     An apparatus fore reducing or preventing windage loss of steam turbines according to a first embodiment of the present disclosure will be described with reference to the drawings.  FIG. 1  is a diagram illustrating an apparatus for reducing or preventing windage loss of steam turbines according to a first embodiment of the present disclosure,  FIGS. 2 and 3  are diagrams illustrating various examples of an extension pipe according to the first embodiment of the present disclosure, and  FIG. 4  is a block diagram illustrating a controller and a peripheral structure associated with the controller according to the first embodiment of the present disclosure. 
     Referring to  FIGS. 1 to 4 , an apparatus for reducing or preventing windage loss of steam turbines according to the present embodiment is to reduce or minimize damage to a last-stage bucket of a high-pressure turbine  110  caused by the windage loss by supplying additional steam such that a temperature at a specific position does not rapidly rises in a state in which there is no steam in the high-pressure turbine  110  using the steam. 
     Further, according to the present disclosure, high-temperature steam of which a temperature is decreased by passing through a turbine unit  100  is reused as a heat source for cogeneration, thereby improving energy reuse efficiency. 
     The apparatus for reducing or preventing windage loss of steam turbines according to the present disclosure includes the turbine unit  100 , a condenser  200 , a tank unit  300 , a steam supplier  400 , a sensor  500 , and a controller  600 . 
     The turbine unit  100  includes the high-pressure turbine  110 , a medium-pressure turbine  120 , and a low-pressure turbine  130 , and may transfer rotational force to a generator  2  since the generator  2  is connected to a shaft of the high-pressure turbine  110 . 
     The high-pressure turbine  110  includes, for example, a first-stage turbine to n+1-th-stage turbine, for example, a first-stage turbine to a seventh-stage turbine. A clutch  3  controlling connection and disconnection of the rotational force to the low-pressure turbine  130  is installed in the shaft connecting between the medium-pressure turbine  120  and the low-pressure turbine  130 . 
     The condenser  200  is positioned below the low-pressure turbine  130  and has a heat exchange pipe extending and bent at a predetermined length while passing through the condenser  200  multiple times. In the condenser  200 , when the steam of the low-pressure turbine  130  is supplied, the steam is changed into medium-temperature steam through heat exchange. 
     The tank unit  300  which is a tank storing steam discharged from an outlet stage  112  of the high-pressure turbine  110 , may have a predetermined size and the predetermined number. 
     In the tank unit  300 , the steam generated at the time of operation of the high-pressure turbine  110  of the turbine unit  100  is converted into condensed water after being condensed, and stored. 
     The steam supplier  400  supplies additional steam to an inlet stage  111  of the high-pressure turbine  110  to reduce or minimize a rapid rise in temperature at the outlet stage  112  of the high-pressure turbine  110  due to speed-up of the medium-pressure turbine under a condition that the condenser  200  is not operated. 
     The steam supplier  400  includes an extension pipe  410  extending toward between a second-stage turbine and a third-stage turbine among the turbines of the high-pressure turbine  110 , a steam generator  420  supplying steam to the extension pipe  410 , and a first control valve  430  positioned at any extended position of the extension pipe  410  and of which an opening degree is controlled by the controller  600 . 
     The extension pipe  410  extends between the second-stage turbine and the third-stage turbine while passing through an inspection hole pre-formed to inspect between the second-stage turbine and the third-stage turbine in an outer casing forming an appearance of the high-pressure turbine  110 . 
     The extension pipe  410  is a pipe or a tube having a general circular cross-section and includes a heat insulation pad for heat insulation at an outer circumferential surface to reduce or minimize heat loss, thereby reducing or minimizing heat loss to the outside even when the steam is supplied. 
     As the steam generator  420 , for example, a boiler unit is used to supply steam to the extension pipe  410 , and the boiler unit includes a main boiler and an auxiliary boiler. The main boiler supplies steam to the extension pipe  410 , and when the main boiler is broken down or malfunctions, the auxiliary boiler is operated, thereby stably supplying the steam to the extension pipe  410 . 
     It is to be noted that the steam generator  420  may also be configured to include a component generating steam other than the above-described boiler unit. 
     The additional steam is supplied through the extension pipe  410 , and thus is moved to the outlet stage  112  at which stagnant high temperature is maintained and then causes flow of fluid to move to a return pipe  50  to be described below together with gas having high-temperature heat energy, thereby reducing or preventing a phenomenon that the temperature at the outlet stage  112  is overheated, and maintaining a circulation state of the steam. 
     Therefore, a phenomenon that the temperature rapidly rises at the outlet stage  112  of the high-pressure turbine  110  may be reduced or minimized. 
     The first control valve  430  is provided to control an amount of steam supplied to the high-pressure turbine  110  through the extension pipe  410 , and an opening degree thereof is selectively controlled by the controller  600  to be described below. 
     Referring to  FIG. 2 , the extension pipe  410  according to the present embodiment is disposed to supply steam at a side of a turbine blade  114  provided in the high-pressure turbine  110  in a state in which the extension pipe  410  is orthogonal to the shaft of the high-pressure turbine  110 . 
     The turbine blade  114  rotates in a specific direction. When the high-pressure turbine  110  is rotated in a state in which there is no steam, windage loss phenomenon that the temperature at the outlet stage  112  of the high-pressure turbine  110  is overheated may occur. 
     According to the present embodiment, the extension pipe  410  extends toward the above-mentioned stage of the high-pressure turbine  110  and an end thereof extends toward a direction in which the turbine blade  114  rotates so that the steam is supplied in the rotation direction of the turbine blade  114  to reduce or minimize the windage loss phenomenon. 
     The end of the extension pipe  410  extending between the second-stage turbine and the third-stage turbine of the high-pressure turbine  110  may have a nozzle form, and in this case, the nozzle may include a single nozzle or a plurality of nozzles. 
     In the case in which the extension pipe  410  is configured in the nozzle form, a discharge speed of the steam may be increased, thereby allowing the steam to be supplied toward the second-stage turbine and the third-stage turbine at a high speed. 
     Referring to  FIG. 3 or 5 , the extension pipe  400  according to the present embodiment may include a first extension pipe  412  and a second extension pipe  414  unlike the above-described embodiment. The first and second extension pipes  412  and  414  are illustrated in order to assist in understanding of the description by way of example, thus are not necessarily limited to the form illustrated in the drawings. 
     However, disposition thereof may be similar to that illustrated in the drawings, and the number thereof may also be changed. 
     The state in which there is no steam in the high-pressure turbine  110  may not be maintained by supplying the steam to the turbine blade  114  through the first and second extension pipes  412  and  414 . 
     The first extension pipe  412  extends toward the turbine blade  114  in order to supply the steam in the rotation direction of the high-pressure turbine  110 , and the second extension pipe  414  is positioned opposite to the first extension pipe  412  and extends to supply the steam in the rotation direction of the turbine blade  114 . 
     The disposition of the first and second extension pipes  412  and  414  may be variously changed without being limited to the layout illustrated in the drawings. For example, the first extension pipe  412  may extend toward the 12 o&#39;clock direction from the left side and the second extension pipe  414  may extend toward the 6 o&#39;clock direction from the lower right side, based on a front surface of the high-pressure turbine  110 . 
     In this case, the steam sprayed toward the turbine blade  114  from the first extension pipe  412  and the steam sprayed toward the turbine blade  114  from the second extension pipe  414  are supplied respectively in the directions shown by arrows and the turbine blade  114  rotates in a dotted line arrow direction. 
     Here, in the turbine blade  114 , a position where the additional steam is supplied through the first extension pipe  412  and a position where the addition steam is supplied through the second extension pipe  414  are different from each other. However, the additional steam may be easily moved to the outlet stage  112  along an axis direction in the high-pressure turbine  110  and the additional steam moved to the outlet stage of the high-pressure turbine  110  may forcibly move gas of which high temperature is maintained to the outlet stage  112 , thereby reducing or preventing a rise in temperature at the outlet stage  112  of the high-pressure turbine  110 . 
     Accordingly, the last-stage bucket may be stably used without being damaged due to windage loss at the outlet stage  112  of the high-pressure turbine  110 . 
     According to the present embodiment, the apparatus for reducing or preventing windage loss of steam turbines includes the return pipe  50  of which one end is connected to the outlet stage  112  of the high-pressure turbine  110  and the other end is connected to the tank unit  300 , and through which steam supplied to the inlet stage  111  of the high-pressure turbine  110  is drained. 
     The return pipe  500  includes a second control valve  52  controlling an amount of drained steam. One return pipe  50  or a plurality of return pipes  50  may be provided, and the number of the return pipe  50  is not particularly limited. Further, the additional steam is mixed and drained together with the high temperature gas moved to the outlet stage  112 . 
     Referring to  FIGS. 4 and 5 , the sensor  500  according to the present embodiment includes a temperature sensor for sensing the temperature at the outlet stage  112  of the high-pressure turbine  110 . 
     The temperature sensor is not damaged and stably operated under high temperature condition, and a pressure sensor sensing internal pressure may be provided together with the temperature sensor. 
     Data on temperature sensed by the sensor  500  is transmitted to the controller  600 , and the controller  600  controls the steam supplier  400 . 
     For example, the controller  600  controls pressure of steam supplied by the steam supplier  400  based on pressure data according to pressure drop between the second-stage turbine and the third-stage turbine. 
     In the high-pressure turbine  110 , as the stage number increases from the first-stage turbine to the n+1-th-stage turbine, the pressure drop occurs, and the controller  600  may perform a control for pressure compensation for additional steam that is additionally supplied according to the pressure drop, such that it is possible to provide steam while maintaining optimal pressure. The steam pressure may be easily controlled as described above by inputting, to the controller  600 , the pressure drop according to the stage number from the first-stage turbine to the n+1-th-stage turbine using pressure data values provided to a manufacturer of the turbine in advance. 
     For example, as the stage number is changed between the second-stage turbine and the third-stage turbine, pressure difference due to the pressure drop is generated between the second-stage turbine and the third-stage turbine. Pressure of the additional steam supplied through the extension pipe  410  may be controlled according to the corresponding stage number of the high-pressure turbine  110  in consideration of variables according to the pressure drop, and the additional steam may be supplied at the controlled pressure, thus the additional steam may easily move to the outlet stage  112 . 
     The steam supplier  400  includes a pressure controller  700  installed at the extension pipe  410  in order to supply the additional steam supplied to the high-pressure turbine at different pressure by the controller  600 . The pressure controller  700  may be configured as a separate valve, and the number thereof is not particularly limited. Further, the supply pressure of the additional steam may be easily controlled by the pressure controller  700 , thereby minimizing windage loss of the high-pressure turbine  110 . 
     Steam drained from the medium-pressure turbine  120  is stored in a heat supply and storage tank  800  in which heating water for district heating is stored. The heat supply and storage tank  800 , which is provided for cogeneration, stores high-temperature steam, and is connected with a separate pipe to supply the high-temperature steam to a district requiring heating. 
     An apparatus for reducing or preventing windage loss of steam turbines according to a second embodiment of the present disclosure will be described with reference to the drawings. 
     Referring to  FIGS. 6 to 8 , an apparatus for reducing or preventing windage loss of steam turbines according to a second embodiment of the present disclosure includes a turbine unit  100  including a high-pressure turbine  110 , a medium-pressure turbine  120 , and a low-pressure turbine  130 , a condenser  200  in which steam passing through the low-pressure turbine  130  is supplied and heat-exchanged, a tank unit  300  in which steam generated at the time of operation of the turbine unit  100  is converted into condensed water after being condensed, and stored, a steam supplier  4000  supplying additional steam to an inlet stage  111  and an outlet stage  112  of the high-pressure turbine  110  to minimize a rapid rise in temperature at the outlet stage  112  of the high-pressure turbine  110  due to speed-up of the medium-pressure turbine  120  under a condition that the condenser  200  is not operated, a sensor  500  sensing the temperature at the outlet stage  112  of the high-pressure turbine and internal pressure, and a controller  600  controlling an amount of steam supplied by the steam supplier  4000  so as not to generate damage at the outlet stage  112  by receiving data on the temperature and the pressure sensed by the sensor  500 . 
     The apparatus for reducing or preventing windage loss of steam turbines according to the present embodiment is to reduce or minimize damage to a last-stage bucket of the high-pressure turbine  110  caused by the windage loss by discharging stagnant high temperature gas together with additional steam by supplying the additional steam such that a temperature does not rapidly rises in a state in which there is no steam at the outlet stage of the high-pressure turbine  110 , when the turbine unit  100  is operated using high-temperature steam generated in the steam turbine using the steam. 
     In particular, according to the present embodiment, the steam supplier  4000  extends toward the inlet stage  111  and the outlet stage  112  of the high-pressure turbine  110 , respectively, and the additional steam may be supplied to the inlet stage  111  and the outlet stage  112  at the same time by the controller  600  to be described below, or may be selectively supplied to any one of inlet stage  111  and the outlet stage  112 . 
     In this case, the problem caused by the windage loss may be reduced or minimized by selecting supply position of the additional steam based on a temperature state of the outlet stage  112  of the high-pressure turbine  110 . 
     Since main configurations of the present embodiment are similar to those of the aforementioned first embodiment, the steam supplier  4000  having different configuration will be mainly described. 
     The steam supplier  4000  supplies additional steam to the inlet stage  111  of the high-pressure turbine  110  to minimize a rapid rise in temperature at the outlet stage  112  of the high-pressure turbine  110  due to speed-up of the medium-pressure turbine  120  under a condition that the condenser  200  is not operated. 
     The steam supplier  400  includes a first extension pipe  4100  extending toward between a second-stage turbine and a third-stage turbine among the turbines configuring the high-pressure turbine  110 , a third extension pipe  4200  extending toward the last stage of the high-pressure turbine  110 , and a steam generator  4300  supplying steam to the first and third extension pipes  4100  and  4200 . 
     The first and third extension pipes  4100  and  4200  are pipes or tubes having a general circular cross-section and include a heat insulation pad (not illustrated) for heat insulation at an outer circumferential surface to minimize heat loss, thereby reducing or minimizing heat loss to the outside even when the steam is supplied. 
     The heat insulation pad may reduce or minimize heat loss of the additional steam generated from the steam generator  4300  by blocking the heat loss to the outside when a temperature of the outside air is maintained to be low. 
     As the steam generator  4300 , a boiler unit for supplying steam to the first and third extension pipes  4100  and  4200  is used, and the boiler unit includes a main boiler and an auxiliary boiler. 
     The first extension pipe  4100  extends toward between the second-stage turbine and the third-stage turbine of the high-pressure turbine  110 , and the third extension pipe  4200  extends toward the last-stage turbine of the high-pressure turbine  110  or a turbine of a stage adjacent to the last stage. 
     Referring to  FIG. 7 , the controller  600  according to the present embodiment may perform a control to simultaneously supply steam to the first and third extension pipes  4100  and  4200  or to supply the steam to any one of the first extension pipe  4100  and the third extension pipe  4200 . 
     When the controller  600  performs a control to simultaneously supply the steam to the first and third extension pipes  4100  and  4200  extending toward the high-pressure turbine  110 , a high temperature state around the last-stage bucket of the high-pressure turbine  110  may be changed to a low temperature state in the least amount of time. In this case, a phenomenon that a bucket of the outlet stage  112  of the high-pressure turbine  110  may be reduced or minimized, thereby improving safety and efficiency at the same time. 
     Referring to  FIG. 8 , when the steam is simultaneously supplied to the first and third extension pipes  4100  and  4200 , the controller  600  according to the present embodiment may control pressure of the steam supplied to the first extension pipe  4100  and pressure of the steam supplied to the third extension pipe  4200  to be different from each other. For example, the pressure of the additional steam supplied to the first extension pipe  4100  may be higher than that of the additional steam supplied to the third extension pipe  4200 , thereby reducing or minimizing overheating phenomenon due to windage loss. 
     The reason why the pressure of the additional steam supplied to the first extension pipe  4100  is high is to maintain sufficient pressure energy to be supplied to the last-stage turbine of the high-pressure turbine  110 . 
     The apparatus for reducing or preventing windage loss of steam turbines includes a return pipe  50  of which one end is connected to the outlet stage  112  of the high-pressure turbine  110  and the other end is connected to the tank unit  300 , and through which steam supplied to the inlet stage  111  of the high-pressure turbine  110  is drained. 
     The steam supplier  4000  includes a first control valve  4400  positioned at any extended position of the first and third extension pipes  4100  and  4200  and of which an opening degree is controlled by the controller  600 . 
     The first control valve  4400  is operated so that the opening degree thereof is increased or decreased depending on an amount of supplied additional steam, and thus is operated so that the additional steam is supplied under optimal condition according to the internal temperature and pressure of the high-pressure turbine  110 . 
     According to the embodiments of the present disclosure, it is possible to stably reduce or prevent the windage loss phenomenon that may be generated when the steam turbine including the heat supply and storage tank is started-up, thereby reducing or minimizing damage to the high-pressure turbine. 
     According to the embodiments of the present disclosure, since the optimal amount of additional steam supplied to the high-pressure turbine may be calculated and supplied, it is possible to economically operate the turbine unit without operating the condenser. 
     According to the embodiments of the present disclosure, since additional steam may be supplied to the inlet stage and the outlet stage of the high-pressure turbine, respectively, it is possible to reduce or minimize a rise in temperature of the high-pressure turbine. 
     The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Moreover, the above advantages and features are provided in described embodiments, but shall not limit the application of the claims to processes and structures accomplishing any or all of the above advantages.