Controlling variables in boiler pressure vessels

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.

1 . A method of controlling stress in a boiler pressure vessel, the method comprising:
 limiting the diameter of a drum of the boiler pressure vessel; and   preheating at least a portion of the wall of the drum;   wherein limiting the diameter of the drum allows pressure in the drum to be increased for a given mechanical stress; and   wherein preheating the wall of the drum reduces peak thermally induced stresses in a material from which the drum is fabricated.   
     
     
         2 . The method of  claim 1 , wherein limiting the diameter of the drum comprises using a drum having an inside diameter of less than about 1,775 mm. 
     
     
         3 . The method of  claim 1 , wherein preheating at least a portion of the wall of the drum comprises locally preheating penetrations in the wall of the drum. 
     
     
         4 . The method of  claim 3 , wherein locally preheating penetrations in the wall of the drum comprises heating nozzles extending into the wall of the drum. 
     
     
         5 . The method of  claim 3 , further comprising locally preheating areas of the wall of the drum adjacent to the penetrations. 
     
     
         6 . The method of  claim 1 , wherein preheating at least a portion of the wall of the drum is undertaken at at least one of a startup of the boiler pressure vessel and during an operation of the boiler pressure vessel. 
     
     
         7 . The method of  claim 1 , wherein preheating at least a portion of the wall of the drum is undertaken during a shutdown of the boiler pressure vessel. 
     
     
         8 . A method of operating a boiler pressure vessel, the method comprising:
 applying local heating to a portion of the boiler pressure vessel during at least one of prior to a startup operation of the boiler pressure vessel, during an operation of the boiler pressure vessel, and during a shutdown operation of the boiler pressure vessel;   wherein applying local heating to the boiler pressure vessel reduces thermally induced stresses in the boiler pressure vessel.   
     
     
         9 . The method of  claim 8 , wherein applying local heating to the portion of the wall of the boiler pressure vessel comprises at least one of,
 heating a penetration extending into a surface of a drum of the boiler pressure vessel; and   heating an area surrounding the penetration extending into the surface of the drum.   
     
     
         10 . The method of  claim 8 , further comprising limiting a diameter of a drum of the boiler pressure vessel, wherein limiting the diameter of the drum reduces mechanical stresses in the boiler pressure vessel. 
     
     
         11 . A method of controlling variables in a boiler pressure vessel, the method comprising:
 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; and   wherein the heating of the portion of the steam drum is effected by preheating at least one of penetrations in the steam drum and 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.