Abstract:
A method and apparatus for extending the life of a thermowell tube positioned in a hot gas duct to contain a thermocouple capable of measuring the temperature of a hot gas in the duct.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for extending the life of a thermowell tube positioned in a hot gas duct to contain a thermocouple capable of measuring the temperature of a hot gas in the duct. 
     BACKGROUND OF THE INVENTION 
     The use of burners positioned to generate hot gas for passage through a hot gas duct to heat water or other substances contained in heat exchanger tubes positioned in the duct is well known and is a widely used technique for generating steam and the like. Frequently, the gases passed through such hot gas ducts are at temperatures of up to at least 1100° F. to about 1800° F. It is desirable to measure the temperatures of these gases reliably since excessive temperature can damage the heat exchange tubes and the like. These temperature measurements are frequently made by positioning thermowell tubes across the height of the duct to contain a thermocouple, which then senses the temperature of the hot gas flowing through the duct. In many instances, these tubes are suspended from the top of the duct and extend to a bottom restraint in the bottom of the duct so that the tube is restrained at both the top and the bottom of the tube. The tube is typically slideably restrained at the bottom of the duct so that it may thermally expand and contract. 
     In the use of such thermowell tubes it has been found that when temperatures above about 1200° F. are used, the thermowell tubes degrade thermally over relatively short periods, i.e., typically one year or less. This degradation is the result of a number of factors, but a significant factor is the fact that elevated temperatures in the range of up to 1100° F. to 1800° F. are well above the creep range for even high temperature stainless steels and the like. As a result, it has been necessary to replace such thermowell tubes relatively frequently. 
     Since the replacement of these tubes is relatively expensive and since it results in an interruption of operations, methods have been sought to extend the life of such thermowell tubes. 
     SUMMARY OF THE INVENTION 
     It has now been found that the life of such thermowell tubes can be extended by the use of an extended life thermowell tube comprising: (a) a thermowell tube adapted to contain a thermocouple and having a top and a bottom, and an upstream side and a downstream side relative to a gas flow past the thermowell tube; (b) a surface near the thermowell tube bottom positioned to engage a restraining surface on a gas duct containing at least a major portion of the thermowell tube; and, (c) a thermowell stiffener positioned on the downstream side of the thermowell tube. 
     The invention further comprises; a method for extending the life of a thermowell tube having a top and a bottom and a downstream side relative to a hot gas flow through a hot gas duct and positioned in the hot gas flow in the hot gas duct to contain a thermocouple capable of measuring the temperature of the hot gas flow in the hot gas duct; the thermowell tube being fixed at its top and its bottom relative to the hot gas flow past the thermowell tube; the method comprising positioning a thermowell tube stiffener on the downstream side of the thermowell tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art hot gas duct, including a thermowell tube, for heating fluids in heat exchange tubes; 
     FIG. 2 is a schematic diagram of the hot gas duct of FIG. 1 showing a thermowell tube stiffener of the present invention positioned on the thermowell tube; 
     FIG. 3 is a schematic diagram of the bottom of a thermowell tube; 
     FIG. 4 is a schematic diagram of the top of a thermowell tube; and, 
     FIG. 5 is a top cross-sectional view of a thermowell tube, including a thermowell stiffener according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the discussion of the Figures, the same numbers will be used throughout to refer to the same or similar components. Further, additional components required for the operation of such systems as known to those skilled in the art have not been shown for simplicity. 
     In FIG. 1, a hot gas duct  10  is shown. Hot gas duct  10  includes a top  12  and a bottom  14  and a first and second side, one of which is shown as side  20 . Hot gases are produced by combustion of carbonaceous or hydrocarbonaceous fuel in burners schematically shown at  16  which generate gases which are discharged as shown by the arrows  16 ′. The hot gases flow through duct  10  as shown by arrow  18 . The hot gases are at a suitable elevated temperature to heat fluids contained in heat exchange tubes  22 . A plurality of tubes  22  are used as required to extract the desired heat from hot gases in duct  10 . Typically, tubes  22  are heat exchange tubes for the generation of steam, which may be used to drive a turbine, or for other process purposes in a variety of chemical, refining or other applications. To control the temperature of the hot gases in duct  10 , thermowell tubes are positioned in duct  10 . Thermowell tubes  24  as shown are commonly used. These thermowell tubes typically extend across substantially the entire height of duct  10  and are typically suspended from top  12  as shown in FIG. 1 by a support  30 . Support  30  as shown includes a flange  30 ′, which mateingly engages a flange  32  as known to those skilled in the art. A thermocouple signal sensor  34  is positioned above flange  32  and typically contains connections to measure signals from a thermocouple positioned inside thermowell tube  24 . Thermowell tube  24  also includes near its lower end wear sleeves  28  which are sized to slideably engage a guide pipe  26  on bottom  14  of duct  10 . In operation, hot gases are produced by burner  16 , and flow past thermowell tube  24  and tubes  22  to produce the desired steam at a desired temperature. It will be understood that one or more rows of tubes  22  can be used or a variety of other configurations as known to those skilled in the art may be used. 
     In many applications, hot gases having a temperature of up to about 1100° F. to about 1800° F. may be generated and passed into contact with tubes  22 . At these temperatures, the walls of tubes  22  remain at a temperature well below the temperature of the hot gases by reason of the heat exchange with the fluids contained in tubes  22 . By contrast, thermowell tube  24  does not contain a fluid for the removal of heat. Accordingly, the temperature of thermowell tube  24  quickly approaches the temperature of the hot gases flowing through duct  10  so that the temperature of the gases flowing through duct  10  can be accurately determined by the thermocouple in tube  24 . Typically, thermowell tubes  24  are fabricated of stainless steel, such as  316  stainless steel or the like. At temperatures above about 1200° F., the creep range of such materials is exceeded. Accordingly, thermowell tubes  24  begin to deform as a result of the elevated temperature and the continued pressure against tubes  24  as a result of the flow of hot gases through duct  10  as shown by arrow  18 . Tube  24  is restrained at both its top and its bottom by connector  30  and by guide pipe  26 . Accordingly, thermowell tubes  24  begin to bend and deform and become more susceptible to mechanical deterioration. Harmonic motion in the thermowell tubes may also cause damage to the thermowell tube, the thermocouple, or both. For instance, a slight amount of bending of thermowell tube  24  results in an inability for sleeves  28  to slide upwardly and downwardly in guide pipe  26 . The thermowell tubes as so restrained then become quickly vulnerable to mechanical deterioration with possible destruction of not only the tube, but the thermocouple as well. Further, the accuracy of the readings can be adversely affected. As a result, it has been necessary in the past to replace thermowell tubes frequently. 
     In FIG. 2, an embodiment of the present invention is shown. A thermowell tube stiffener  36  has been positioned on thermowell tube  24  on its downstream side relative to the gas flow. The stiffener is positioned to extend over a substantial portion and preferably substantially all of the length of tube  24 . The stiffener may extend to within a foot of the top of the duct  10  and within a foot of the bottom of duct  10 , or it may extend even closer to the top and bottom of duct  10 . Preferably, the stiffener extends along the length of tube  24  over at least two-thirds of its length inside duct  10 . The stiffener not only results in stiffening tube  24 , thereby reducing possible harmonic motion and creep, but as a result of its configuration, it reduces the drag on tube  24  as a result of gas flow past tube  24 . Since the gas flow velocity may be 50 feet per second or higher, a substantial force is exerted on tube  24  by the hot gas flow. Typically, tubes  24  may be as long as 30 feet or longer. Since these tubes are restrained at both ends, it is desirable that these tubes be reinforced and that some way be found to minimize and resist the drag forces on tube  24  as a result of the flow of hot gas past tube  24 . 
     Typically, stiffener  36  is made of a high temperature stainless steel alloy such as 316 stainless steel and comprises an angle member. This angle member will be described in more detail in conjunction with FIG.  5 . The angle member as positioned in FIG. 2, has been found to extend the useful life of tube  24  substantially and provides a useful life of up to triple the life previously achieved using such tubes without stiffeners. 
     FIG. 3 shows the bottom of tube  24 , including sleeves  28  as positioned in guide pipe  26 . As shown, it is clear that tube  24  can longitudinally expand and contract within guide pipe  26 . Bottom  14  of duct  10  is fabricated as shown with an outer shell  38 , which typically may be a material such as 0.25 inch thick carbon steel with a ceramic fiber insulating material  40  being positioned between outer shell  38  and an inner shell  42 , which may typically be a material such as 0.95 inch thick stainless steel. Such construction details are considered to be well known to those skilled in the art. 
     Guide pipe  26  may be of any suitable height, for instance, from about one to about three feet or higher. 
     In FIG. 4, a top mount  30  is shown for supporting tube  24 . The top of duct  10  comprises an outer shell  46  which may comprise 0.25 inch thick carbon steel with a ceramic insulation layer  48  being positioned between outer shell  46  and an inner shell  50  which may comprise 0.95 inch thick stainless steel. Tube  24  is desirably supported in support  30 . Support  30  ends in a flange  30 ′, which is adapted to mateingly, join a second flange  32 , which supports a thermocouple sensor  34 . Leads  52  are shown schematically depicting the electrical contacts necessary to determine the temperature in duct  10  via the thermocouple positioned in thermowell tube  24 . 
     In FIG. 5, a cross-section of thermowell tube  24  is shown. A thermocouple  54  is shown positioned centrally in tube  24 . Thermocouple  54  may be equipped with fins or other devices for centralizing thermocouple  54  in tube  24  if desired. Stiffener  36  comprises sides  55 , which are joined to form an angle member at an angle  56 . Typically, stiffener  36  is welded at welds  58  to tube  24 . Desirably, an angled tip  60  of the stiffener  36  is directed downstream with respect to gas flow. This configuration results in a reduction of the drag on a downstream side of tube  24 , thereby reducing the gas drag load on tube  24 . Desirably, stiffener  36  is formed of a suitable high temperature alloy such as  316  stainless steel or the like. Stiffener  36  may be of any suitable size but is desirably sized to have an extension at its outer ends at welds  58  such that neither the welds nor the ends of stiffener  36  extend beyond the outer diameter of tube  24 . Typically, when the diameter of tube  24  is from about 2.0 to about 3.0 inches in outer diameter, the outer dimensions of sides  54  will be from about 1.75 to about 2.50 inches. Typically, angle  56  is from about 60 degrees to about 120 degrees and is preferably at about 90 degrees. 
     EXAMPLE 
     In a hot gas duct having a bottom to top height of 25 feet, and a width of 15 feet, a plurality of thermowell tubes were used. These tubes included wear sleeves and a guide pipe on the bottom of the hot gas duct generally as shown in FIG.  1 . The thermowell tubes had an outside diameter of 2.5 inches and an overall length inside ducts  10  of 22 feet. 
     A thermowell tube, including a 1¼ inch by 1¼ inch by 20 feet 316 stainless steel angle positioned on the downstream side of the thermowell tube was also installed. The hot gas duct was then used for the flow of gases at a temperature from about 1100° F. to about 1800° F. over an extended period of time. After about one year, the thermowell tubes of the invention were checked and it was found that there was no downstream deflection of these thermowell tubes and they appeared to be in the same condition as when installed. Tubes without the stiffener showed extreme downstream deflection and had to be replaced after one year. The tubes of the invention showed no downstream deflection after two years and were left in service. 
     In view of the foregoing example, it is clear that the use of the stiffener has surprisingly and unexpectedly extended the life of the thermowell tube. This extension in the life of the tube is considered to be the end result of a number of factors, not all of which are fully understood, but which include the strengthening of tube  24  in combination with at least one of creep reduction, and harmonic motion reduction on tube  24 . 
     Having thus described the invention by reference to certain of its preferred embodiments, it is pointed out that the embodiments described are illustrative rather than limiting in nature and that many variations and modifications are possible within the scope of the present invention. Many such variations and modifications may be considered obvious and desirable based upon the foregoing description of preferred embodiments.