Abstract:
An evaporator system comprises an evaporator; a drum; and a pump that are in fluid communication with each other. The pump is operative to create a temporary pressure gradient during start-up of an evaporator system and transport a fluid from the evaporator to the drum prior to the fluid reaching its boiling point in the evaporator. Following the fluid reaching its boiling point in the evaporator, the fluid naturally circulates in the evaporator system.

Description:
TECHNICAL FIELD 
       [0001]    Disclosed herein is a method of controlling drum temperature transients in an evaporator system in a heat recovery steam generator. More specifically, disclosed herein is a method of using temporary forced circulation during startup to control drum temperature transients in a heat recovery steam generator. 
       BACKGROUND 
       [0002]    Heat recovery steam generators generally comprise three major components: an evaporator, a superheater and an economizer The different components are put together to meet the operating requirements of the unit. Some heat recovery steam generators may not have a superheater or may include additional components such as reheaters. 
         [0003]    The  FIG. 1  is a depiction of an exemplary prior art evaporator system  100  of a heat recovery steam generator that comprises an evaporator  102  and a steam drum  104 . The steam drum  104  is in fluid communication with the evaporator  102 . In a natural circulation heat recovery steam generator, either no flow or minimal flow is established until boiling begins in the evaporator  102 . This generally results in a very rapid rise in the steam drum  104  temperature. 
         [0004]    For example, for a cold start the water temperature inside the steam drum  104  can rise from 15° C. to 100° C. in less than 10 minutes. This produces a large thermal gradient and hence compressive stress in the steam drum  104  wall. As the pressure in the steam drum  104  increases, the temperature gradient through the drum wall is reduced and consequently the stress due to pressure becomes the dominant stress in the drum. The stress due to pressure (with increased pressure in the steam drum  104 ) is a tensile stress. The stress range for the drum is determined by the difference between the final tensile stress at full load (pressure) and the initial compressive thermal stress. Boiler Design Codes (such as ASME and EN) impose limits on the stress at design pressure. Some codes, such as for example EN12952-3, also include limits on the permissible stress range for a startup-shutdown cycle. These limits are intended to protect against fatigue damage and phenomena such as cracking of the magnetite layer that forms on the surface of the steel at operating temperature. 
         [0005]    As the pressure in the steam drum  104  increases, the wall thickness of the steam drum  104  is also increased to ensure that the tensile stress in the drum shell at design conditions does not exceed allowable stress limits specified in the design Codes. The thermal stress however, becomes greater as steam drum  104  wall thickness increases. The maximum pressure that a drum can be designed for is thus limited by the initial thermal transient. 
         [0006]    It is also desirable to have as much operational flexibility as is desirable for combined cycle power plants because these power plants are often shut down and restarted as electrical power demand varies. The addition of renewable energy sources such as solar and wind increases the need to shut down and restart combined cycle power plants due to the variation in power output from such renewable resources. Stresses in the drum during these start ups due to thermal transients can also limit the total number of times the heat recovery steam generators can be shut down and started over its operational life. 
         [0007]    It is therefore desirable to reduce the temperature transient in the drum. This will allow the use of drum type boilers at higher pressures than can be achieved with conventional natural circulation and/or allow greater numbers of start up cycles. 
       SUMMARY 
       [0008]    Disclosed herein is a method comprising creating a temporary pressure gradient during start-up of an evaporator system, where the evaporator system comprises an evaporator; a drum; and a pump; where the evaporator, the drum and the pump are in fluid communication with each other; transporting a fluid from the evaporator to the drum prior to the fluid reaching its boiling point in the evaporator; and circulating the fluid through the evaporator system via natural circulation after the fluid has reached its boiling point in the evaporator. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1  is a prior art depiction of the evaporator system; 
           [0010]      FIG. 2  is a depiction of an exemplary embodiment of the evaporator system of the present invention; and 
           [0011]      FIG. 3  is another depiction of an exemplary embodiment of the evaporator system of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which various embodiments are shown. This invention may, however, be embodied in many 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 invention to those skilled in the art. Like reference numerals refer to like elements throughout. 
         [0013]    It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0014]    It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
         [0015]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof 
         [0016]    Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower,” can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
         [0017]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0018]    Exemplary embodiments are described herein with reference to cross sectional illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims. 
         [0019]    Disclosed herein is an evaporator system that comprises a pump for circulating heated fluid from the evaporator to the steam drum. The pump provides circulation during start-up to initiate heating of the steam drum, which reduces the rate of temperature change in the drum. This reduction in the rate of temperature change in the steam drum causes reduced thermal stresses in the drum. In an exemplary embodiment, the fluid is water. 
         [0020]    The pump may be a centrifugal pump, a jet-pump pump, or the like, and its purpose is to provide a pressure gradient in the evaporator system that promotes fluid circulation from the evaporator to the steam drum before fluid (e.g., water) present in the evaporator begins to boil. In one embodiment, the pump produces a lower pressure in the steam drum in relation to the evaporator before fluid present in the evaporator begins to boil. Upon generating a lower pressure in the steam drum, fluid from the evaporator is drawn into the steam drum causing the drum to heat up gradually. The gradual heating takes place until the fluid in the evaporator reaches the boiling point, at which point the pump may be shut off or isolated. After the pump is shut off, natural circulation promotes circulation of the fluid in the evaporator system. 
         [0021]    The pump therefore operates for a short period of time, until the steam drum reaches the temperature of the boiling fluid. This allows for a pump that is smaller in size than other comparative pumps that are normally used. It also reduces stress in the wall of the steam drum. 
         [0022]    With reference to the  FIG. 2 , an evaporator system  200  of the present invention comprises an evaporator  202 , a steam drum  204  and a pump  206 . The pump  206  is in fluid communication with the steam drum  204  and the evaporator  202 . In one embodiment, the pump  206  lies downstream of the steam drum  204 . The steam drum lies downstream of the evaporator  202 . 
         [0023]    Disposed across the inlet and outlet to the pump  206  is a one-way check valve  208 . The check valve  208  permits only fluid flow from the steam drum  204  downstream to the evaporator  202  via the pump  206 . The check valve further permits only fluid flow from the evaporator  202  downstream to the steam drum  204 . The pump  206  has a first valve  210  and a second valve  212  disposed upstream and downstream of it respectively. The first valve  210  and the second valve  212  can isolate the pump  206  from the evaporator system  200  when desired. The first valve  210  and the second valve  212  can be electrically, pneumatically or manually activated. 
         [0024]    In one embodiment, in one method of operation of the evaporator system  200 , the pump  206  is used to circulate fluid from the evaporator  202  to the steam drum  204  during start up of the heat recovery steam generator to eliminate the rapid drum temperature rise that would normally occur in a natural circulation heat recovery steam generator. Once the steam drum  204  temperature reaches a predetermined value, the pump  206  is isolated and the evaporator  202  runs under natural circulation. As the pump  206  can be isolated after start-up, it does not have to be sized for full flow load, pressure and temperature. This reduces the cost of the pump  206  when compared with comparative pumps that are used for full time circulation. 
         [0025]    In another embodiment, depicted in the  FIG. 3 , the evaporator system  200  comprises a jet-pump  306  (eductor) that creates a pressure gradient in the evaporator system that promotes fluid circulation from the evaporator  202  to the steam drum  204  before fluid (e.g., water) present in the evaporator  202  begins to boil. In one embodiment, the jet-pump  306  produces a lower pressure in the steam drum in relation to the evaporator before the fluid present in the evaporator begins to boil. 
         [0026]    The jet-pump  306  creates low pressure in a downcomer  308  that is in fluid communication with the steam drum  204  as a result of which fluid is drawn into the steam drum  204  from the evaporator  202 . High velocity fluid flow in the narrow downcomer  308  induces a low pressure in the downcomer  308  relative to the steam drum  204 , which in turn causes flow in the downcomer  308 . When the low pressure is created in the downcomer  308 , the steam drum  204  is at a lower pressure than the evaporator, which causes the fluid to flow from the evaporator  202  to the steam drum  204 . In one embodiment, low pressure created in the downcomer  308  by the operation of the jet-pump  306  drives the circulation of fluid from the evaporator  202  to the steam drum  204 . 
         [0027]    The jet-pump  306  is in fluid communication with a first valve  310  and a second valve  312 . The first valve  310  is used to control the flow of feed water into the steam drum  204 , while the second valve  312  is used to isolate the jet-pump  306  from the downscomer. 
         [0028]    The jet-pump  306  of the  FIG. 3  functions in a manner similar to the pump  206  of the  FIG. 2  in that it permits a temporary fluid flow from the evaporator  202  to the steam drum  204  before the fluid present in the evaporator  202  begins to boil. 
         [0029]    As noted above, the use of a pump for temporary circulation of fluid to the steam drum has a number of advantages. These include using a pump that is smaller in size than other comparative pumps that are normally used. It also reduces stress in the wall of the steam drum and permits the use of steam drums with larger wall thickness than those that are currently used in evaporator systems that do not employ temporary circulation. This in turn allows operation of the steam drum at higher pressures or greater numbers of stop-start cycles. 
         [0030]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.