Abstract:
A main steam relief valve for use in a nuclear power plant. The valve disc includes a cylindrical first end of uniform external and internal circumference, a conical opposite end, an external surface, and an internal surface. The cylindrical first end preferably includes a plurality of groves on the external surface to prevent rotation of the disc. The conical portion is highly polished on the external surface to prevent the formation of cracks and is adapted to receive a push-rod on the internal surface and a valve seat on the external surface. The valve disc is constructed of approximately of 90% or greater silicon carbide.

Description:
This application is a division of U.S. patent application Ser. No. 09/666,580, filed Sep. 21, 2000 now U.S. Pat. No. 6,431,521. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to noncorrosive steam pressure safety release valves, and more particularly, to a valve disc for use in power generating nuclear fission reactor plants and associated equipment. 
     2. Description of the Prior Art 
     Steam pressure relief valves are critical devices in any type of high pressure, hot water, or steam producing units. In a nuclear power plant there are standards and demands that must be applied to the design and fabrication of such valves. The function of the steam pressure relief valve is shown conceptually in the illustration of FIG.  1 . The principles of nuclear power generation are well established and understood. The nuclear reactor  10  is located in a containment building  11  and contains uranium or plutonium fuel elements in a core arrangement. Through neutron absorption and nuclear fission (chain reaction) of the uranium or plutonium, large amounts of energy are released in the form of heat which is utilized to generate electricity. 
     In a boiling water reactor, the chain reaction boils the moderator-water in the core. Steam is thus produced inside the reactor  10 . Steam pipes  12  carry the steam from the reactor  10  to the turbines  13 . In producing electricity, a nuclear plant&#39;s steam turbines and electric generators work like those in a fossil fuel plant. The steam produced by a reactor spins the blades of the plant&#39;s turbines  13 , which drive the generators. Many plants have combination turbines and generators called turbogenerators. 
     After steam has passed through a plant&#39;s turbines  13 , it is piped to a condenser  14 . The condenser  14  changes the steam back into water  15 . Water  15  is then returned to reactor  10  by pump  16 . There are several conditions that may occur in the nuclear plant which require a relief of steam pressure from the system. For this reason, a steam relief safety valve  17  is used in the steam pipe system. The purpose of the valve  17  generally is to relieve steam pressure in excess of 1150 pounds per square inch (“psi”) to prevent overpressurization of the nuclear reactor and subsequent damage and potential shut down. This overpressure could be caused by several events such as a main electrical breaker closure which eliminates the resistance to the power turbines, or any misadventure that might cause overpressure with subsequent excessive revolutions per minute (“rpm”) of the turbines. To prevent excessive rpm to the turbines, the steam power to them is shut down with the pressure then traveling back to the reactor. The valve  17  is designed to then exhaust the excess steam above 1150 psi. 
     The present invention relates to two-stage safety relief valves as part of the nuclear pressure relief system in boiling water reactors to provide automatic depressurization, if needed, for breaks in the nuclear system so that the low pressure coolant injection and the core spray systems can operate to protect the fuel barrier. This two-stage valve is referred to as a main steam relief valve which is installed in the main steam lines. The main steam relief valves are distributed among the four main steam lines so that an accident cannot completely disable a safety, relief, or automatic depressurization function. Of the nuclear plants that use this valve, each reactor (typically 2-3 per plant) has approximately thirteen of these valves along the main steam line. 
     The typical main steam relief valve contains a disc which engages a seat when the valve is in the closed position. The disc and seat are presently made of stellite, a cobalt-based metal containing chromium and carbon. The main problem for the nuclear energy plant is risk of overpressure to the nuclear reactor because of the occasional failure of the main steam relief valve to relieve at the designated pressure. This condition is due to corrosion bonding of the stellite disc to the stellite seat as a result of exposure to radiation, heat, moisture, and oxygen from the steam. Plating the disc with platinum has improved the function of the disc but has not cured the problem of corrosion and bonding of the valve to the seat. 
     Valve discs made of ceramics might be resistant to this corrosive and bonding effect in nuclear plants. However, oxide-based ceramics would not be useful because the presence of oxide causes the ceramic to easily corrode in the presence of oxygen, high temperatures, alkalis, and acids. On the other hand, silicon carbide ceramic is known to be highly resistant to corrosion under extreme conditions. However, it is a complicated process to sinter silicon carbide because it has to be sintered at high temperatures in an inert atmosphere. The stellite valve disc in present use in nuclear reactors has a complex shape which would make construction of a similar valve out of silicon carbide highly impractical. The stellite disc has a conical shape on one end and a cylindrical shape on the opposite end. The cylindrical portion has three different external diameters and two different internal diameters which would make the construction of this valve from silicon carbide unfeasible. To my knowledge, no alternative valve disc composed of silicon carbide has ever been made available for a nuclear power plant heretofore. 
     The present invention has solved these and other shortcomings by providing a silicon carbide valve disc for use in a main steam relief valve. The cylindrical portion of this disc has a uniform external diameter and a uniform internal diameter and is feasible to construct entirely from silicon carbide or similar carbide ceramics. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a main steam relief valve for use in a boiling water reactor nuclear power plant. In particular, the valve of the present invention is a valve for regulating steam pressure in a nuclear power plant, comprising: a hollow disc including a cylindrical first end having a uniform external and internal circumference, a conical opposite end, an external surface, and an internal surface, said conical portion adapted to receive a push-rod on the internal surface and a valve seat on the external surface, wherein said disc comprises a ceramic carbide. The valve disc has a cylindrical first end of uniform external and internal circumference, a conical opposite end, an external surface, and an internal surface. The cylindrical first end has a plurality of groves on the external surface to prevent rotation of the disc and to allow steam to exhaust. The conical portion is highly polished on the external surface to prevent the formation of cracks. The conical portion is adapted to receive a push-rod or a steel plate and push-rod on the internal surface, and a valve seat on the: external surface. The valve disc is constructed entirely of 90% or greater oxide-free silicon carbide, boron carbide, titanium diboride, or boron nitride. The disc is highly resistant to destruction by physical impact by virtue of the thickness of its walls and its conical portion. In a preferred embodiment, the disc is constructed from superfine powders of silicon carbide, hot pressed to rough shape, premachined, sintered at high temperature, diamond wheel ground, and polished. 
     An advantage of the present invention is to provide a valve disc for a nuclear power plant that is resistant to corrosion during operation of the nuclear power plant. 
     Another advantage of the present invention is to provide a valve disc for a nuclear power plant which will not bond to its valve seat during operation of the nuclear power plant. 
     Another advantage of the present invention is to provide a valve disc for a nuclear power plant that is resistant to destruction by physical impact. 
     Yet another advantage of the present invention is to provide a valve disc for a nuclear power plant that is constructed of silicon carbide, boron carbide, titanium diboride, or boron nitride with purity of 90% or greater. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a typical boiling water reactor in a nuclear power plant, known in the art showing the placement of the main steam release valve. 
     FIG. 2 is a perspective view of the silicon carbide valve disc of the present invention. 
     FIG. 3 shows a diagrammatic view of the disc in a valve assembly, with the disc in place on a valve seat. 
     FIG. 4 is a horizontal cross-sectional view of the disc of FIG.  2 . 
     FIG. 5 is a vertical cross-sectional view of the disc of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagrammatic view of a typical boiling water reactor showing the placement of the main steam release valve. FIG. 2 shows a perspective view of an exemplary embodiment of a valve disc in accordance with the present invention. The valve disc  20  has a cylindrical first end  21  and an opposite conical end  22 . The disc is hollow so that it will have an exterior surface  23  and an interior surface  24  (see FIGS.  4  and  5 ). The tip  25  of conical end  22  is a nipple or papilla-like structure to facilitate seating of the disc  20  within the valve. Surface  23  of cylindrical end  21  has a uniform circumference. Conical end  22  forms an approximate 71° flair with the external surface  23  of cylindrical end  21 , ranging between about 60°-80°. Cylindrical end  21  has a plurality of grooves  26  in exterior surface  23 , which accommodate projections within the valve to keep the disc aligned, prevent the disc from rotating about its longitudinal (vertical) axis  36  (see FIG. 5) when in use, and allow steam to exhaust to relieve pressure. 
     FIG. 3 shows disc  20  in a main steam relief valve  30  in place against valve seat  31  on conical exterior surface  22 , thereby closing the valve. The arrows indicate the direction of steam flow if the valve were open. Plunger or push-rod  33  fits within the hollow portion  27  (see FIG. 5) so that it can push disc  20  onto valve seat  31  with whatever force is required. When steam pressure exceeds a certain value, thereby exceeding the force of the plunger, it pushes the disc  20  away from valve seat  31 , opening the valve and allowing steam to escape. 
     FIG. 4 shows a horizontal cross-sectional view of the disc  20  of FIG.  2 . The walls of the cylindrical first end  21  are defined by the internal surface  24  and external surface  23 . The maximum external diameter of cylindrical first end  21 , defined by the circumference of the external surface  23 , is approximately 1.752 inches, ranging between 1.0 and 2.5 inches. This diameter and circumference are uniform along the length of cylindrical end  21 . The internal diameter of cylindrical first end  21  is approximately 1.12 inches, ranging between 0.8 to 1.6 inches. Internal surface  24  of cylindrical end  21  has a uniform circumference along the length of cylindrical end  21 . Likewise, the corresponding internal diameter is uniform. Maximum wall thickness of the cylindrical first end  21  is approximately 0.316 inches, ranging between 0.16 and 0.8 inches. The minimum wall thickness, measured from the center of groove  26 , is approximately 0.16 inches, ranging from 0.08 to 0.4 inches. 
     FIG. 5 shows a vertical cross-sectional view of the disc  20  of FIG.  2 . Cylindrical first end  21  has a base  28  and opposite conical end has a tip  25 . The internal or hollow portion  27  of the disc is contained within the cylindrical end  21 . Conical end  22  is solid so that a flat dome or roof  29  is formed in the internal hollow portion  27 . A soft circular steel plate  35  can be reversibly inserted into hollow portion  27  and placed adjacent to inner surface  24  at dome  29  in between said dome  29  and said push-rod  33  to spread impact load uniformly and to reduce fracture stress from push-rod  33 . The length of the longitudinal axis  36  of disc  20 , extending from base  28  to tip  25  is approximately 2.5 inches, ranging between 1.8 to 3.3 inches. The thickness of the opposite conical end  22  from the external tip  25  to the internal flat dome  29  is approximately 1.28 inches, ranging between 1.0 and 2.0 inches. Internal dome  29  is adapted to accept push-rod  33  (see FIG. 3) or flat plate  35  interposed between push-rod  33  and internal dome  29 . The length of cylindrical first end  21 , denoted by the letters A and B, is approximately 1.39 inches, ranging between 0.6 and 1.8 inches. The angular length of opposite conical end  22 , denoted by the letters B and C, is approximately 1.28 inches, ranging from 0.55 to 1.73 inches. The height of tip  25  is approximately 0.09 inches, ranging from 0 to 0.2 inches, and the diameter of the tip is approximately 0.283 inches, ranging from 0.12 to 0.35 inches. The diameter of circular steel plate  35  is approximately 1.0 inches, ranging between 0.7 and 2.3 inches. The thickness of steel plate  35  is 0.2 inches, ranging between 0.1 and 0.3 inches. The angle D formed between cylindrical first end  21  and opposite conical end  22  is about 71°, ranging between 60°-80°. 
     The valve disc of the present invention is composed of and fashioned from ceramic carbide, preferably from 98.0% (wt %) silicon carbide (SiC), or boron carbide, titanium diboride, or boron nitride, which contain trace amounts of other metals, and which are available commercially, for example, from The Carborundum Corporation (Niagara Falls, N.Y.) or AC Cerama (Sweden). The sintering process results in a single-phase, fine-grain silicon carbide product that&#39;s very pure and uniform, with virtually no porosity. The material is extremely durable, whether submerged in corrosive environments, subjected to extreme wear and abrasive conditions, or exposed to temperatures in excess of 1400° C. This silicon carbide, as well as the other carbides disclosed, are among the hardest, yet strongest, high performance materials available, second only to diamonds (2800 kg/mm2 at room temperature), and have excellent resistance to creep and stress rupture at temperatures up to 1,650° C. They resists corrosion, oxidation and erosion. The high density, low porosity and chemical inertness of these carbides permit them to function in environments of hot gases and liquids, in oxidizing and corrosive atmospheres, in strong acids and bases, even at extremely high temperatures, and in exposure to high radioactivity. 
     Although the disc is fashioned preferably from 98% (wt %) silicon carbide, lesser purity may be used ranging from 90% and higher. The disc is fashioned from superfine silicon carbide powders, hot pressed to rough shape, premachined, and sintered at high temperature under conditions well-known in the art. The disc is then diamond-wheel ground in successively fine (diamond gait size) wheels and final ground by polishing (successively fine grit and polishing papers and cutting fluids). The disc is then exposed to another heat treatment, between 1,500 and 3,000° F., preferably at 2,500° F., for 6 to 12 hours, preferably eight hours, followed by polishing with clear oil, such as, for example, standard commercial baby oil, and with paper tissue, such as, for example, bathroom tissue (toilet paper) or facial tissue. 
     The foregoing description has been limited to specific embodiments of this invention. It will be apparent, however, that variations and modifications may be made by those skilled in the art to the disclosed embodiments of the invention, with the attainment of some or all of its advantages and without departing from the spirit and scope of the present invention. For example, the longitudinal grooves in the cylindrical end may have various shapes to accommodate various interlocking mechanisms, or the cylindrical end may have no grooves. The amount and quantity of trace metals in the silicon carbide or other ceramic carbides may be varied. The internal dome may assume various shapes and forms to accommodate the head of a plunger or push-rod. The external tip on the conical end may take various shapes and sizes or be removed. The conical end may have a plurality of angles relative to the cylindrical end or be flat. 
     It will be understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated above in order to explain the nature of this invention may be made by those skilled in the art without departing from the principle and scope of the invention as recited in the following claims.