Abstract:
A solar thermal energy transmission system having a bellows that sealingly interconnects a first structure to a second structure is disclosed. The bellows includes a surface that is compatible with a molten salt and is configured to resiliently deform in response to contraction and expansion of at least one of the first and second structures, to thereby accommodate relative movement of the structure(s) to which it is coupled.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to thermal energy transmission systems and more particularly to the accommodation of cyclic thermal expansion and contraction in the piping and valves of such systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Molten salts, such as a potassium nitrate (KNO 3 ) and sodium nitrate (NaNO 3 ) mixture, are used as a thermal transfer medium in solar power plants. Typically, reflectors are used to concentrate solar energy at a receiver. The receiver operates as a heat exchange device to raise the temperature of the molten salt as the molten salt is pumped through the receiver. The outside surface of the receiver can reach temperatures that exceed 1200° F. (650° C.). The molten salt is then transferred to a thermal storage tank and then to a heat exchanger to generate steam.  
           [0003]    Molten salts are chosen for their heat transfer characteristics and handling requirements. Mixtures of KNO 3  and NaNO 3  salts are in the liquid phase while in the operational temperature range of a solar receiver facility. The operational temperature range for a solar receiver facility is typically 550° F. to 1050° F. (290° C. to 560° C.). For power generation, the molten salts are necessarily heated in the solar energy receiver and cooled in the heat exchanger.  
           [0004]    The heat exchanger typically transfers this thermal energy to a working fluid such as water. The heated water is converted to steam in the heat exchanger which can be used to power a steam turbine-generator to produce electricity.  
           [0005]    The piping and equipment used to transport the molten salts experience thermal expansion and contraction as the system is started, operated and shut down. As with most land-based solar power applications, this cycle is repeated daily as the sun rises to heat the receiver. Most of the piping and equipment of a solar facility are allowed to cool to ambient temperatures at night. Thermal expansion of equipment and piping will lead to induced stresses as pipe lengths increase between fixed points. With a receiver located atop a tower several hundred feet in height, several inches of axial thermal expansion must be accommodated. Expansion loops, such as a series of pipe sections connected by 90° ells or a piping spiral, can be installed in piping to relieve some of these induced stresses, but require more piping which increases material costs and thermal losses. Additionally, expansion loops require more space, maintenance and installation costs and time.  
           [0006]    Another issue that arises when using molten salt in a solar power application is leak prevention. Both KNO 3  and NaNO 3  are severe oxidizers and contact with fuels at elevated temperatures can cause combustion. Penetrations into the molten salt equipment, such as valve stems, are potential areas for leaks. Periodic inspection and maintenance are required to ensure that any leaks are minimized.  
           [0007]    Molten salt is typically required to be at temperatures in excess of 500° F. (260° C.) to remain in the liquid phase since its freezing temperature is approximately 430° F. (220° C.). Equipment used to transfer molten salt is maintained above this minimum temperature to ensure that the molten salt does not freeze. Electrical heat tracing is typically used to maintain this minimum temperature. Freezing of the salt in equipment can cause reduced flow up to a total flow stoppage. In the event of a freeze out of salt in a length of piping or equipment, special care must be exercised when thawing the salt. Upon thaw, salt expands and can damage pipe and equipment if a free surface is not available. If the molten salt cannot be fully drained from pipes or equipment, solidification will occur as the salt is cooled. These solid portions of salt can restrict flow or damage equipment upon restart.  
           [0008]    Conventional valve stem seals for molten salt applications use dynamic friction seals. Most valve stem seals experience a high short term failure rate at an operating temperature of 1050° F. (560° C.). In previous solar molten salt facilities, the valve stem and bonnet were lengthened in order to separate the valve stem seal from the flow of molten salt. This modification provided a lower operating temperature for the valve stem seals and resulted in a longer service life. With longer valve stems, valve failures such as plastic stem twisting and stem buckling become more likely. Additionally, valve locations are limited when greater clearance is needed for the lengthened valve stem/bonnet.  
           [0009]    What is needed is a containment device for molten salt that will accommodate axial expansion, withstand thousands of cycles, reduce the potential for leaks, and allow maximum drainage while providing a long operating life at elevated temperatures.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to an expandable metal bellows for accommodating axial movement in molten salt containment equipment. In one preferred form the present invention provides a metal bellows that is located within a length of piping. The metal bellows will contract and expand axially with thermal growth of the piping. The metal bellows can also accommodate lateral movement of the attached equipment and piping.  
           [0011]    In another preferred form the present invention provides a metal bellows to seal a valve stem. The metal bellows permits axial movement of the valve stem while providing a positive seal. This embodiment of the bellows will also accommodate any limited lateral movement of the valve stem. The bellows valve stem seal obviates the need for lengthened valve stems and bonnets.  
           [0012]    In another preferred form the present invention provides a metal bellows constructed from an appropriate material such as a low cycle fatigue Inconel 625 alloy. The bellows wall has a generally circular cross-section taken perpendicular to the axis of the bellows. The bellows wall is contoured along the axis of the bellows. Thus formed, the wall thickness of the metal bellows is capable of withstanding a high temperature, high pressure environment while providing sufficient elasticity to allow the bellows to expand and contract along the axis of the bellows.  
           [0013]    In another preferred form the present invention provides an external heat source to provide a minimum desired temperature for a metal bellows used in a thermal transfer system.  
           [0014]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limited the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a schematic view of a solar power facility;  
         [0017]    [0017]FIG. 2 is a sectional view of a preferred embodiment of the bellows of the present invention;  
         [0018]    [0018]FIG. 3 is a view of a piping system with the bellows of FIG. 2 in accordance with a preferred embodiment;  
         [0019]    [0019]FIG. 4 is a view of a prior art piping system;  
         [0020]    [0020]FIG. 5 is a sectional view of a valve utilizing the bellows of FIG. 2 in a preferred embodiment; and  
         [0021]    [0021]FIG. 6 is a sectional view of a prior art valve.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0023]    With reference to FIG. 1 of the drawings, an exemplary solar power plant is generally indicated by reference numeral  10 . The solar power plant  10  includes a plurality of heliostats  12 , a receiver  14  mounted atop a tower  16 , and a solar thermal energy transfer system  18  constructed in accordance with the teachings of the present invention.  
         [0024]    Aside from the solar thermal energy transfer system  18 , the solar power plant  10  is conventional in its construction and operation and as such, need not be described in significant detail. Briefly, solar energy  20  produced by the sun  22  is reflected by the heliostats  12  and concentrated at the receiver  14 . The receiver  14  absorbs the energy reflected by the heliostats  12  in the form of heat. The receiver  14  includes one or more molten salt solar absorption panels  26 , each having a plurality of tubes  28 .  
         [0025]    Solar thermal energy transfer system  18  includes pumps  30  to transfer a molten salt  32  in a closed loop system. Solar thermal energy transfer system  18  transfers the hot molten salt  32   a  from the tubes  28  of receiver  14  to a hot storage tank  34 . Hot storage tank  34  is sized to accumulate hot molten salt  32   a  until hot molten salt  32   a  is desired for electrical generation. In this manner, hot storage tank  34  can be used in a thermal storage capacity. To generate electrical power, hot molten salt  32   a  is transferred by pumps  30  from hot storage tank  34  to a heat exchanger  36  which, in the particular example provided, is employed to heat water and generate steam for use in a conventional steam turbine (not shown). The temperature of hot molten salt  32   a  is reduced from approximately 1050° F. (560° C.) to 550° F. (290° C.) as it flows through heat exchanger  36 . A cold storage tank  38  accumulates molten salt  32  from heat exchanger  36 . Using pumps  30 , solar thermal energy transfer system  18  transfers molten salt  32  from cold storage tank  38  to tubes  28  of receiver  14  to complete the thermal transfer cycle. While the working fluid utilized in solar thermal energy transfer system  18  is identified as a molten salt, it would be recognized by one skilled in the art that other fluids, such as liquid sodium (Na) or sodium-potassium (NaK) could be substituted.  
         [0026]    In FIG. 2, a portion of the piping of the solar thermal energy transfer system  18  is illustrated to include a first conduit  40  and a second conduit  42 . The conduits  40  and  42  are coupled together by an expansion bellows  44  in accordance with a preferred embodiment of the present invention. The first and second conduits  40  and  42  are conventional in their construction and may be of, for example, a seamless, welded or welded and drawn construction and formed from a material that is compatible with the molten salt  32  contained therein. In the particular example provided, the first and second conduits  40  and  42  are formed from low-cycle fatigue  625  alloy disclosed in U.S. Pat. No. 5,862,800 to Marko entitled “Molten Nitrate Salt Solar Central Receiver of Low Cycle Fatigue 625 Alloy”, and U.S. Pat. No. 4,765,956 to Smith, et al., entitled “Nickel—Chromium Alloy of Improved Fatigue Strength”, the disclosures of which are hereby incorporated by reference as if fully set forth herein.  
         [0027]    The bellows  44  is shown to include a pair of annular connecting portions  50  and one or more convolutions  52 . The bellows  44  may be unitarily formed from a material that is compatible with the molten salt, such as the aforementioned low cycle fatigue  625  alloy, or may have a composite construction wherein, for example, various materials are formed into annular rings that are layered in a concentric manner. In the particular example provided, the bellows  44  is of a composite construction having an inner layer  54  (not shown) that is formed of a material that is compatible with the molten salt, such as low cycle fatigue  625  alloy, and an outer layer  56  (not shown) that is formed from stainless steel. As those skilled in the art will appreciate, the composite construction provides the bellows  44  with the characteristics of the materials from which it is constructed. In the example provided, the low cycle fatigue  625  alloy provides chemical compatibility with the molten salt, while the stainless steel provides added strength.  
         [0028]    The annular connecting portions  50  are sized to receive an associated one of the first and second conduits  40  and  42 . A continuous weld bead  60  is illustrated to sealingly couple each of the annular connecting portions  50  to an associated one of the first and second conduits  40  and  42 .  
         [0029]    Each convolution  52  is illustrated in cross section, having an upper wall  64  that is interconnected to a lower wall  66  at a pivot interface  68 . The upper and lower walls  64  and  66  may be skewed or generally parallel to one another and intersect at the pivot interface  68 . The convolution or convolutions  52  resiliently interconnect the annular connecting portions  50 . In the particular embodiment illustrated, the upper wall  64   a  of a first one of the convolutions  52  intersects a first one of the annular connecting portions  50  at an upper pivot interface  68   a  and the lower wall  66   a  of a last one of the convolutions  52  intersects a second one of the annular connecting portions  50  at a lower pivot interface  68   b . Preferably, convolutions  52  are constructed such that lower walls  66  will not pool molten salt  32  when solar thermal energy transfer system  18  is drained.  
         [0030]    During start-up of the solar thermal energy transfer system  18 , thermal energy is absorbed by the solar thermal energy transfer system  18  causing the various components thereof, including the first and second conduits  40  and  42  and the bellows  44 , to expand in their overall length. As the distance between the receiver  14  (FIG. 1) and the storage tanks,  34 ,  38  (FIG. 1) remains constant, convolutions  52  of bellows  44  axially contract in response to the axial expansion of first and second conduits  40  and  42 . In this manner, a portion of the thermal expansion of solar thermal energy transfer system  18  is absorbed by bellows  44 , thereby reducing a portion of the associated forces, stresses and moments.  
         [0031]    With reference to FIG. 3, a portion of solar thermal energy transfer system  18  is shown in accordance with a preferred embodiment of the present invention to include a bellows  44  and a first and second conduits  40  and  42 . Solar thermal energy transfer system  18  is located within an insulation  80  and a heat source  82 . Heat source  82  is provided to maintain a minimum temperature of the solar thermal energy transfer system  18  to prevent a freeze out of molten salt  32 . Heat source  82  is preferably a conventional electrical resistance element that is contoured to the exterior surface of solar thermal energy transfer system  18 . Bellows  44  is oriented in a generally vertical manner within solar thermal energy transfer system  18  to allow for maximum drainage of the molten salt  32  during each daily shutdown.  
         [0032]    [0032]FIG. 4 shows a prior art piping arrangement  90 . Piping arrangement  90  utilizes a series of sections of pipe  92  that are connected via a series of ells  94 . In this manner, thermal expansion of piping arrangement  90  is absorbed by lateral deflection of pipes  92 .  
         [0033]    With reference to FIG. 5, a valve  100  in accordance with a preferred embodiment of the present invention is shown. Valve  100  includes a stem  102 , a gate (or plug)  104 , a plurality of seats  106 , a bonnet  108 , a body  110  and an operator (not shown). The operator can be a hand wheel, pneumatic actuator, electric motor-gearbox, or equivalent. Preferably, bonnet  108  is enclosed in insulation  80  with a heat source  82  to maintain valve  100  above the freezing temperature of molten salt  32 .  
         [0034]    In operation, the operator causes stem  102  to move axially within bonnet  108 . This movement causes gate  104  to move toward or away from seats  106  which are connected to body  110 . When gate  104  contacts seats  106 , valve  100  is in the closed position and flow of molten salt  32  ceases through body  110 . When the operator moves stem  102  and causes gate  104  to lift from seats  106 , flow of molten salt  32  is permitted through body  110 . Typically, gate  104  can be lifted into bonnet  108  to allow unobstructed flow of molten salt  32  through body  110 . Bellows  44  is located within bonnet  108 . A first end  120  of bellows  44  is attached to stem  102 . A second end  122  of bellows  44  is attached to a distal end  124  of bonnet  108 .  
         [0035]    Preferably, both the first end  120  and second end  122  of bellows  44  are welded at their attachment locations to stem  102  and bonnet  108  distal end  124  to provide a leak resistant seal. In this manner, bellows  44  can expand and contract as stem  102  moves axially while providing a reliable seal within bonnet  108 . It should be noted that molten salt  32  will be on the exterior surface of bellows  44  within valve  100 . As shown in FIG. 5, bellows  44  is contracted when valve  100  is opened. One skilled in the art will recognize that molten salt  32  is not allowed to pool on the surface of bellows  44  when solar thermal energy transfer system  18  is drained.  
         [0036]    [0036]FIG. 6 shows a prior art valve  130  used for molten salt applications wherein stem  102 ′ is sealed within bonnet  108 ′ with a prior art seal  132 . Stem  102 ′ and bonnet  108 ′ are lengthened to protect prior art seal  132  from the extreme temperatures and oxidizing affect of the flow of molten salt  32  through body  110 ′. Stem  102 ′ slides within prior art seal  132  as prior art valve  130  is operated. As one skilled in the art will recognize, bellows  44  provides valve  100  (FIG. 5) with a leak resistant seal that is more reliable than the frictional seal between stem  102 ′ and prior art seal  132 .  
         [0037]    The present invention thus provides a bellows  44  for use in piping systems where a degree of expansion and contraction of pipes or conduits, which affects the lengths of these components, needs to be accommodated.  
         [0038]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.