Abstract:
A corrosion sensor for use within a fluid flow path includes a sensing element and circuitry coupled to the sensing element for detecting corrosion of the sensing element. Another embodiment of a corrosion sensor for use within a fluid flow path comprises a hermetic housing, a sensing element attached to an external portion of the housing for exposure to the fluid flow path, and circuitry disposed within said hermetic housing, which circuitry is coupled to the sensing element for detecting corrosion of the sensing element.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates to the detection of corrosion of structural materials within a fluid flow path. More particularly, the invention relates to a method and apparatus for detecting corrosive contaminants in combustion gases in gas turbines. Even more particularly, the invention relates to a method and apparatus for detecting corrosive alkali metal contaminants in combustion gases in gas turbines.  
           [0002]    Many structures, such as aircraft and power turbine assemblies, reactors and cracking columns in chemical plants and refineries, blast furnaces, and nuclear reactors, include structural components that are exposed to fluid flows containing potentially corrosive contaminants. For example, advanced gas turbines, designed for power generation, use metal alloys that are selected to provide maximum strength at the high temperatures encountered in the first stage buckets. As a result, such alloys are less resistant to hot corrosion than alloys operating at lower temperatures.  
           [0003]    The presence of sodium and other alkali metals in fuel, air, water, or steam supplied to the combustion chambers of gas turbines can create a gaseous mixture that is corrosive to the high temperature alloys used in a hot gas path. Alkali metals may initially be present as salts in either the fuel or air ingested by a turbine assembly. Seawater, for example, may mix with fuel transported in tanker ships. Similarly, turbine assemblies sited on or near a coastline ingest air having a high salt content. Alkali metal salts, once ingested, may combine with sulfur in the fuel to form alkali metal sulfates that condense or impinge upon turbine components. Metals present within the alloys, such as nickel, cobalt or iron, may react with the molten alkali sulfates to form a corrosive liquid over a wide temperature range, resulting in the phenomena known as hot corrosion. Hot corrosion leads to a considerable reduction in the life of turbine components.  
           [0004]    Techniques are presently available for detecting and measuring the concentrations of corrosion-causing species in turbine supply lines. The concentration of sodium in fuel oil, for example, can be measured by atomic absorption, inductively coupled plasma (ICP)/atomic emission, and mass spectroscopy. These analytical methods are designed for use under laboratory conditions and are not commonplace in industrial settings, such as those encountered at power generation facilities. In addition to the above-mentioned techniques, sodium ion selective electrodes can also be used to measure sodium concentrations in aqueous media, and ion chromatography has also been employed for alkali metal detection. None of these technologies has been applied to monitor the combustion gases in a gas turbine, and none provide a direct indication of the corrosivity of the gases.  
           [0005]    Although sensors are currently being used to monitor sodium levels in water and other systems are being developed to measure sodium in fuels, it is impractical to measure the presence of sodium or other potentially corrosive contaminants in a fluid flow path, such as the air that is supplied to a gas turbine. Therefore, what is needed is a sensor capable of detecting the total amount of corrosive contaminants, such as sodium or other alkali metals, in a fluid flow path. What is also needed is a sensor capable of measuring the degree of corrosion of structural components exposed to such a fluid flow path.  
         SUMMARY OF INVENTION  
         [0006]    A corrosion sensor for use within a fluid flow path includes a sensing element and circuitry coupled to the sensing element for detecting corrosion of the sensing element. Another embodiment of a corrosion sensor for use within a fluid flow path comprises a hermetic housing, a sensing element attached to an external portion of the housing for exposure to the fluid flow path, and circuitry disposed within said hermetic housing, which circuitry is coupled to the sensing element for detecting corrosion of the sensing element.  
           [0007]    These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic representation of a turbine assembly having a corrosion sensor of the present invention;  
         [0009]    [0009]FIG. 2 is a schematic representation of another embodiment of the corrosion sensor of the present invention;  
         [0010]    [0010]FIG. 3 is a schematic representation of a wire sensing element of the present invention; and  
         [0011]    [0011]FIG. 4 is a flow chart showing a method of using the corrosion sensor of the present invention to detect corrosion in a fluid flow. 
     
    
     DETAILED DESCRIPTION  
       [0012]    In the following detailed description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms.  
         [0013]    Referring to the drawings in general and to FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto.  
         [0014]    [0014]FIG. 1 is a schematic diagram of a turbine assembly  100  of the present invention. The turbine assembly  100  may either be a land-based turbine, such as those widely used in power generation, or an aircraft engine. Air enters the turbine assembly  100  and is first compressed in the compressor section  12  of the turbine assembly  100 . The high-pressure air then enters the combustor  14  where it is combined with a fuel, such as natural gas, diesel fuel, or jet fuel, and is burned. The hot, high-pressure air exiting the combustor  14  is then expanded through a series of turbine stages and then through an exhaust section  16  of the turbine assembly  100 , where energy is extracted to generate power. Components found within the combustor  14  and exhaust section  16 , such as wheels, rotors, vanes, buckets, nozzles, and the like, are most susceptible to corrosion caused by contaminants found in either the fuel or air ingested by the turbine assembly  100 .  
         [0015]    The turbine assembly  100  comprises components that are formed from alloys, such as Rene N5 and other superalloys, having low chromium content. While such alloys have the required strength for turbine applications, they are susceptible to corrosion by gaseous species that contain alkali metals, such as sodium or potassium. Alkali metals may initially be present as salts in either the fuel or air that is ingested by the turbine assembly  100 . Seawater, for example, may mix with fuel transported in tanker ships. Similarly, turbine assemblies sited on or near a coastline ingest air having a high salt content. Alkali metal salts, once ingested, combine with sulfur in the fuel to form the corresponding alkali metal sulfates, such as Na 2 SO 4  and K 2 SO 4 . At the temperatures encountered within the combustor  14  (about 1600° F.), the alkali metal sulfates are molten. The molten alkali metal sulfates condense or impinge upon turbine components formed from metallic alloys. Nickel, cobalt or iron present within the alloys reacts with the molten alkali sulfates to form a corrosive liquid over a wide temperature range. For example, cobalt present within the alloys reacts with the molten sulfates to form a liquid eutectic mixture, such as CoSO 4 /Na 2 SO 4 , thus corroding the turbine component.  
         [0016]    The corrosion sensor  10  of the present invention provides in situ, real time measurement of the degree of corrosion of such alloys within the turbine assembly  100 . Whereas other means of detecting contaminants detect contamination from a single upstream source, such as fuel or air, the corrosion sensor  10  offers the advantage of measuring the collective effect of the contaminants under conditions that closely approximate those under which corrosion is most likely to occur. A corrosion sensor  10  may, for example, be placed in the exhaust section  16  of the turbine assembly  100 , such that the corrosion sensor  10  protrudes into the turbine exhaust stream. In this configuration, the corrosive species exiting the combustor  14  will directly condense or impinge on the corrosion sensor  10 . Alternatively, the corrosion sensor  10  may be located in a slipstream  11 , which diverts a portion of the turbine exhaust from the exhaust section  16  and directs the diverted exhaust onto the corrosion sensor  10 . In a third embodiment, the corrosion sensor is placed in the hot gas path within the combustor section  14  of the turbine assembly  100 .  
         [0017]    In one embodiment of the invention, the corrosion sensor  10  of the present invention detects the degree of corrosion by imposing an oscillating electric field across a sensing element and directly measuring the degree of impedance of the oscillating electric field. The sensing element is at least as susceptible to corrosion as the materials present in the fluid flow path that are to be monitored. As the sensing element corrodes, the impedance changes. The change in impedance can be calibrated against a standard to provide a quantitative indicator of the degree of corrosion of selected materials within the fluid flow path.  
         [0018]    [0018]FIG. 2 shows one embodiment of the corrosion sensor  10  of the present invention. The corrosion sensor  200  comprises a probe  210  that is insertable into a fluid flow, such as an exhaust stream within the exhaust section  16  of the turbine assembly  100 . The probe  210  includes a sensing element  20  formed from a metallic alloy and located at one end of the probe  210 . In order to accurately gauge the corrosion of alloy components within the turbine assembly  100 , the sensing element  20  is preferably a substantially planar coupon, having a thickness of between about  10  mils and about  50  mils, and is formed from a metallic alloy, such as a superalloy, that is used in components in either the combustor  14  or exhaust section  16 . Alternatively, the sensing element  20  is formed from a nickel-base superalloy or the sensing element  20  is formed form either the alloy Udimet  700 , an alloy quite sensitive to hot corrosion or the alloy Rene N5.  
         [0019]    Typically, the surface of the planar sensing element  20  is oriented normal (90°) to the impinging fluid flow  39 . However, the planar surface may be oriented such that the angle  38  between the surface and the fluid flow  39  is between 0° and 90°.  
         [0020]    The probe  210  has an outer housing  24  formed from an alloy that is resistant to chemical attack at temperatures normally encountered in the fluid flow path. The sensing element  20  is separated from the outer housing  24  by a ceramic insulator  22 . The ceramic insulator  22  prevents the sensing element from making electrical contact with the outer housing  24  and prevents material in the fluid flow path from entering the interior space of the outer housing  24 .  
         [0021]    The sensing element  20  is connected by a connector  32  to a processor  30  that is capable of detecting a signal therefrom and potentially to provide power to the sensing element  20 . The signal generated by the sensing element  20  may be either an impedance signal or resistance signal. The signal may be either a time dependent signal or an on/off signal. The processor  30  may further include an alarm, shutdown switch, or the like, which is triggered when the signal generated by the sensing element  20  falls below a predetermined level.  
         [0022]    As shown in FIG. 2, connector  32  typically comprises cables that are fed through the inner space and extend the length of the outer housing  24 , exiting the outer housing  24  to connect the sensing element  20  to the processor  30 . It is also contemplated, however, that a wireless means, such as a transmitter and receiver, may also be used to couple the sensing element  20  to the processor  30 , thus permitting corrosion to be remotely monitored.  
         [0023]    In the embodiment of the invention shown in FIG. 2, the corrosion sensor  200  is designed to be placed in the exhaust section  16  of a turbine assembly  100 , where the temperature of the exhaust stream is typically between about 900° F. to about 1250° F. In order for the corrosion sensor  200  to accurately determine the degree of corrosion occurring at the higher temperatures within the combustor  14 , the sensing element  20  itself needs to be heated to these higher temperatures. If the corrosion sensor  200  is not placed directly into a location exposed to these higher temperatures, the corrosion sensor  200  is additionally heated to the higher temperature by a heating module  26 . The heating module  26  is preferably located within the outer housing  24  and contacts a surface of the sensing element  20  opposite the surface of the sensing element  20  exposed to the fluid flow  39 . Preferably, the heating module  26  includes a resistance heater and a thermocouple for measuring the temperature of the heater and sensing element  20 . Feedback from the thermocouple is provided by heater module connector  34  to a temperature controller and power supply  28  to regulate the temperature of the sensing element. Insulation, including, but not limited to, heat shields, tubing, and fibers formed refractory materials such as ceramics or refractory metals, may be placed in the free space within the interior of the outer housing  24  to increase the efficiency of the heater module  26  and prevent excessive damage to the probe  210 .  
         [0024]    In operation, the probe  210  is fitted into a wall of the exhaust section  16  of the turbine assembly  100  such that the sensing element  20  protrudes into the exhaust stream  39 . A fitting  40 , preferably a threaded fitting, located on the end of the probe  210  opposite the sensing element  20 , is used to secure the probe  210  to the wall of the exhaust section  1   6 .  
         [0025]    In another embodiment of the invention, shown in FIG. 3, the sensing element  50  is formed from an alloy wire, typically having a diameter of between about 1 mil and about 5 mils. In this embodiment, the sensing element  50  acts as a fuse, corroding until electrical continuity is completely interrupted. In order to heat the sensing element  50  to the temperature range at which corrosion occurs, an electrical current is passed through the sensing element  50 . Rather than detecting the variation of impedance over time, the processor  30  detects the sudden change in resistivity of the sensing element  50  due to corrosive failure.  
         [0026]    A method of using the corrosion sensor  10  of the present invention is outlined in FIG. 4. In step S 1 , the sensing element  20  is heated to the temperature T at which corrosion is to be measured. An oscillating electric field is then imposed on the sensing element  20  (step S 2 ) and the impedance signal generated by the sensing element  20  is measured (step  53 ) to obtain a baseline value for the sensing element  20  in the absence of any substantial corrosion. The sensing element  20 , which is maintained at temperature T in the presence of the oscillating electric field, is then exposed to the fluid flow in step S 4 . Contaminants within the fluid flow react with the sensing element  20 , resulting in corrosion of the sensing element  20 , which in turn causes a change in the impedance signal generated by the sensing element  20 . The change in impedance of the sensing element  20  is measured (step S 5 ), thereby providing an indicator of the degree of corrosion of the sensing element  20 , which is indicative of the degree of corrosion experienced by other components within the fluid flow path. The degree of corrosion can be further quantified be separately performed tests in which the degree of corrosion is closely correlated with the impedance signal generated by a particular sensing element  20 .  
         [0027]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, sensing elements having a geometry such as, but not limited to, tubular sections may be used. Also, the corrosion sensor  10  of the present invention may be used in structures other than a turbine assembly, in which structural materials are subjected to corrosion caused by the presence of contaminants in a fluid flow path. These structures include, but are not limited to, reactors and cracking columns in chemical plants and refineries, blast furnaces, and nuclear reactors. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.