Abstract:
A method of controlling stress in a boiler pressure vessel comprises limiting the diameter of a drum ( 10 ) of the boiler pressure vessel and preheating at least a portion of the wall ( 12 ) of the drum ( 10 ). Limiting the diameter of the drum ( 10 ) allows pressure in the drum ( 10 ) to be increased for a given mechanical stress. Furthermore, preheating the wall ( 12 ) of the drum ( 10 ) reduces peak thermally induced stresses in a material from which the drum ( 10 ) is fabricated.

Description:
TECHNICAL FIELD  
       [0001]    The present application is generally directed to systems and methods for controlling variables in boiler pressure vessels. More particularly, the present application is directed to systems and methods for reducing stresses in the walls of boiler pressure vessels. 
       BACKGROUND  
       [0002]    A boiler pressure vessel (hereinafter “boiler”) is a closed vessel comprising a shell and containing a liquid that can be heated under controlled conditions using a fuel or hot gases. The shell is a drum (hereinafter “drum” or “boiler drum”) that is defined by one or more walls. Chemical energy contained in the fuel is converted into thermal energy, which heats the liquid in the boiler and causes it to vaporize. The mixture of liquid and vapor enters the drum. The walls of the drum are designed to withstand pressures exerted by the vaporized liquid. The vaporized liquid can be taken from the drum and used to provide work or as a source of heat. 
         [0003]    Starting a boiler that is initially at ambient conditions often causes rapid temperature changes to be experienced across the walls of the drum. These temperature changes can generate thermal stresses within the walls. Such stresses can cause crack initiation and growth in the material of the wall. In some cases, such stresses can also cause crack initiation and growth in a magnetite layer that forms on the inside of the walls of the drums that contain water. 
         [0004]    In both natural circulation boilers and assisted circulation boilers in which water is heated and vaporized into steam, the drum is a steam drum utilized to separate steam from the water. In boilers that operate at high pressures and/or have large drum diameters, the wall thickness is greater (as compared to boilers that operate at lower pressure and/or have small drum diameters) to maintain acceptable pressure stress levels. Increased wall thickness results in increased thermal stresses within the walls. High stresses within the walls of the drums also occur at various sites or penetrations that extend through the wall. Typical penetrations include nozzles and the like. Since the penetrations are points of weakness in the drum walls, the maximum operating pressure of the boiler is effectively restricted due to limitations imposed by the European Norm (EN) code on maximum stress ranges in boilers (and more particularly in boiler drums). The range of stress also limits the number of rapid startups that the boiler may be subjected to as well as the total number of startups over the life of the boiler. 
         [0005]    Thick-walled boiler drums are generally heated only on their inside surfaces, which results in temporary and uneven temperatures in the wall, particularly during the startup period. As the wall thickness increases, so does the temperature gradient through the wall. The induced thermal stress increases for a given rate of internal temperature change as the drum wall thickness increases. Over time, the wall heats up to a uniform temperature, thereby eliminating this type of thermal stress. The pressure stress then dominates. Such stresses due to thermal gradients and internal pressure, when applied and removed repeatedly, can cause crack initiation and growth in the component material. The need to limit stresses to prevent such cracks can effectively limit the rate of temperature change in the drum. By limiting the rate of temperature change, the operational flexibility (e.g., maximum pressures attainable) of the boiler is decreased. Such flexibility is desirable to provide for rapid start-ups to respond to changes in power demand. 
         [0006]    An additional constraint on boiler drums in compliance with EN code requirements is the limitation on stress range to avoid magnetite cracking. To avoid magnetite cracking, the difference between the highest compressive stress and the highest tensile stress should not exceed 600 mega pascals (MPa). This stress range is illustrated in  FIG. 1 , which illustrates a typical stress history for a steam drum during a boiler startup. Thermal stress that occurs early in the startup process is shown as diminishing as the temperature of the drum wall becomes more uniform as steady state operating conditions are approached. As steady state conditions are approached, the stress due to internal pressure dominates the thermal stress. For a given drum diameter, the positive hoop stress (tensile) can be reduced by increasing the drum wall thickness, but this increases the negative stress due to through wall temperatures at start up and limits the rate or number of starts. 
       SUMMARY  
       [0007]    According to aspects illustrated herein, there is provided a method of controlling stress in a boiler pressure vessel. This method comprises limiting the diameter of a drum of the boiler pressure vessel and preheating at least a portion of the wall of the drum. Limiting the diameter of the drum allows pressure in the drum to be increased for a given mechanical stress. Furthermore, preheating the wall of the drum reduces peak thermally induced stresses in a material from which the drum is fabricated. 
         [0008]    According to other aspects illustrated herein, there is provided a method of operating a boiler pressure vessel. This method comprises applying local heating to a portion of the boiler pressure vessel prior to a startup operation of the boiler pressure vessel, during an operation of the boiler pressure vessel, and/or during a shutdown operation of the boiler pressure vessel. In applying local heating to the boiler pressure vessel, thermally induced stresses in the boiler pressure vessel are reduced. 
         [0009]    According to other aspects illustrated herein, there is provided a method of controlling variables in a boiler pressure vessel. This method comprises providing a steam drum of a boiler; controlling mechanical stress in a wall of the steam drum by limiting the diameter of the steam drum; and controlling thermal stress in the wall of the steam drum by heating at least a portion of the steam drum. The heating of the portion of the steam drum is effected by preheating penetrations in the steam drum and/or an area surrounding a penetration in the steam drum during at least one of a startup period and a shutdown period of the boiler pressure vessel. 
         [0010]    The above described and other features are exemplified by the following Figures and Detailed Description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    Referring now to the Figures, which are exemplary embodiments, and wherein like elements are numbered alike: 
           [0012]      FIG. 1  is a graphical representation of a typical stress history for a steam drum; 
           [0013]      FIG. 2  is a schematic representation of a vertical section of a steam drum of a boiler; and 
           [0014]      FIG. 3  is a perspective view of a vertical section of a steam drum of a boiler. 
       
    
    
     DETAILED DESCRIPTION  
       [0015]    Referring now to  FIG. 2 , one exemplary embodiment of a steam drum of a boiler is shown generally at  10  and is hereinafter referred to as “drum  10 ” or “steam drum  10 .” The drum  10  can be from a natural circulation boiler, an assisted circulation boiler, or any other type of boiler. The drum  10  is of an elongated cylindrical shape and has a wall  12  that is penetrated by nozzles  14  that receive a high temperature steam/liquid mixture and discharge this mixture into an annular space  16  between a drum liner or baffle  18  and an inner surface  15  of the wall  12 . The wall  12  also has an exterior surface  17 . The nozzles  14  may extend beyond the inner surface  15  of the wall ( FIG. 2 ) or they may terminate at the inner surface  15  ( FIG. 3 ). A liquid  26  such as, for example, water pools in the bottom of the drum  10 . One or more steam separating units  24  are located outside the volume enclosed by the baffle  18 . Steam from the steam/liquid mixture  34  and from the vaporization of the water  26  passes through a drying assembly  32  and is removed through an outlet  30 . The configuration of  FIG. 2  is not limited to that as shown, as other configurations are possible. 
         [0016]    Upon operation of the boiler, particularly at startup from ambient conditions, the nozzles  14  and the areas  15   a  of the inner surface  15  of the wall  12  surrounding the nozzles  14  are affected by the steam/liquid mixture  34 . Temperature transients (e.g., the movement of heat from one area to another) through the materials of the nozzles  14  and the wall  12  produce thermal stresses. Accordingly, the nozzles  14  and the areas  15   a  surrounding the nozzles, namely, the drum wall  12  and particularly at the inner surface  15 , are subjected to stress from the high temperature steam/liquid mixture  34 . Mechanical stresses such as hoop stress in the wall  12  of the drum  10  are also encountered as the result of pressure. 
         [0017]    Mechanical stress in the wall  12  is a function of various process variables, namely, the radius of the drum  10 , the thickness of the wall  12 , and the internal pressure of the drum  10 . This can be described by the equation: 
         [0000]      σ m   =f ( PR/t )
 
         [0000]    where: 
         [0018]    σ m  is the hoop stress of the drum; 
         [0019]    P is the internal pressure; 
         [0020]    R is the drum radius; and 
         [0021]    t is the drum wall thickness. 
         [0000]    For a given internal pressure and stress, reducing the drum radius or diameter results in the thickness of the wall  12  of the drum  10  being reduced. 
         [0022]    One approach to accommodating mechanical stress that is applicable to both natural circulation boilers and assisted circulation boilers with steam production greater than 50 kilogram per second (kg/s) to enable operation at higher pressures, which is desirable due to the resulting higher cycle efficiency, is to limit the thickness of the wall  12  of the drum  10 . The thickness of the wall  12  is limited by using a relatively small diameter steam drum, for example, a steam drum having an inside diameter of between about 1,000 millimeters (mm) and about 1,775 mm. When the diameter of the drum  10  is reduced and the thickness of the wall  12  is limited to a value that is consistent with drums having inside diameters of greater than about 1,775 mm, the value for P can be increased for a given hoop stress. Typical wall thicknesses could range from about 70 mm to about 150 mm. 
         [0023]    Thermal stresses within the wall  12  of the drum  10  also occur at the nozzles  14  or other penetrations through the wall  12  to the inner surface  15  as well as at the inner surfaces  15   a  proximate the nozzles  14 . Referring to  FIG. 3 , a localized high stress range area is shown at  20 . This localized high stress range area  20  is located on the inner surface  15  proximate the area at which the nozzle  14  penetrates the wall. The stress in this localized high stress range area  20  is at least twice the stress in any other area in the rest of the drum. 
         [0024]    It has been discovered that applying local heating to at least portions of the drum  10  in a controlled manner can reduce the temperature transients and thermal stresses within the drum  10 . 
         [0025]    One approach to applying local heating to accommodate thermal stress is to preheat the nozzles  14  and the area  15   a  adjacent thereto (e.g., the inner surface area  15   a  of the wall  12  in the area of the nozzle  14 ) prior to boiler startup when the drum  10  is at ambient pressure conditions. In one embodiment, the local heating may be applied on the exterior surface  17  of the drum  10  proximate the area at which the nozzle  14  enters the drum  10  (e.g., area  17   a ). This would reduce the peak thermally induced stresses in a material from which the wall  12  of the drum  10  is fabricated that would otherwise limit the number of startups from ambient conditions or even prevent use of drum-type boilers above certain pressure ranges due to the EN code limits of stress ranges. Locally preheating of the nozzles  14  and/or the wall  12  may be used as an alternative to or in conjunction with limiting the diameter of the drum  10 . 
         [0026]    It should also be appreciated that the approach is not limited to being undertaken at startup of the boiler, as the nozzles  14  and the wall  12  could be heated during a shutdown operation. In doing so, the rate at which heat is dissipated from the nozzles  14  and the wall  12  would be reduced, thereby reducing the thermally induced stresses in the material of the nozzles  14  and the wall  12 . 
         [0027]    In addition to reducing thermally induced stresses by using local heating, it is contemplated that local heat uses much less energy than would be required to heat the entire drum  10  (e.g., the entire inner surface  15 ) and the fluid  26  that it contains, thereby reducing operational costs. Without any sort of preheating feature in place, the number of cold starts could potentially be limited to an absolute maximum in the specification (e.g., 300) as compared to an essentially unlimited number of cold starts with preheating. 
         [0028]    The maximum possible thermal stress for a given ramp up in temperature (temperature transient) is also a function of various process variables and varies approximately as the square of the thickness of the wall. Reduced thickness would result in reduced thermal stress for the same rate of temperature change. This is described by the equation: 
         [0000]      σ t   =f ( T   r   t   2 )
 
         [0000]    where 
         [0029]    σ t  is the thermal stress; 
         [0030]    T r  is the rate of temperature change; and 
         [0031]    t is the drum wall thickness. 
         [0032]    Starting a boiler that is initially at ambient conditions results in rapid temperature changes in the drum  10  as well is in other components of the drum  10  (e.g., nozzles  14  and the like). These temperature changes can generate thermal stress within these components. Such stresses can cause crack initiation and growth in the material of which the component is fabricated and in some cases in a magnetite layer that forms on the inner surface  15  of such drums  10  that contain water  26 . Preheating at least portions of the drum  10  or other components of the pressure vessel in a controlled manner can reduce the rate of temperature change, thereby reducing the thermal stresses within the component. Preheating the drum  10  can be effected by electrical resistance heating or other means readily available. 
         [0033]    Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill 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, 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 embodiments disclosed in the above description, but that the invention will include all embodiments falling within the scope of the appended claims.