Patent Application
20080178953
The present invention relates to rotary valves and an assembly related thereto for handling abrasive materials which is characterized by the use of a plurality of vanes that are made out of a unitary ceramic composition. More specifically, the present invention relates to the use of a unitary ceramic composition for vanes in a rotary valve or similar device comprised of a ceramic composition having a high hardness value, such as silicon carbide. A preferred embodiment of a system including the features of the claimed invention would include a blower or similar device for creating a pressure differential across a rotary valve with vanes comprising a unitary ceramic component.
Rotary valve assemblies are integral components for handling particulates in a variety of industrial applications, including but not limited to cement, limestone, chemical processing, food processing and coal fired power plant operations. In the past, certain particulates, such as fly ash produced from coal combustion, were simply taken up by flue gases and dispersed into the atmosphere. This created significant environmental concerns and health risks. These days, most power plants are required by law to reduce their fly ash emissions to less than 1% of ash produced. Thus, there is a need to provide the efficient removal of particulate from a variety of applications. Additionally, environmental laws are requiring reductions in sulfur emissions from power plants. Most reduction methods involve the use of limestone, an abrasive bulk material. Limestone is commonly handled using pneumatic conveying systems which employ rotary valves. Failure of a rotary valve can result in non-compliance with environmental permits and the plants must be shut down or limit load to stay within permitted levels.
Rotary valve assemblies have been used as a part of the solution to process such particulates as an airlock. The rotary valve acts like a revolving door inside a casing, with the revolving door typically spinning around a horizontal axis. The rotary valve is typically placed between a lower pressure bin or hopper and a higher pressure pipe or conduit. The goal in such an application is to transfer the particulate from the bin to the pipe (where it is transferred to subsequent collection and processing or disposal), but without leakage of air back into the bin. Thus, the rotary valves used in such airlock applications have a narrow gap between the vanes of the rotary valve and the valve casing.
However, when the particulate being handled is fine and/or abrasive, additional problems must be addressed. The pressurized line is a pneumatic conveyor which is capable of delivering high pressures to move large quantities of bulk materials. The high pressures can drive the abrasive particulates through the narrow gaps between the rotor and valve casing, at high velocities, eroding the rotary valve assembly, reducing the efficiency of the rotary valve and shortening or ending its effective lifespan, resulting in a loss of total pneumatic conveying system functionality or reduced system capacity. Thus, a problem exists in maintaining the efficiency of rotary valve assemblies in abrasive operating environments.
A variety of approaches have been tried previously with limited success, and with the creation of separate problems in the upkeep of rotary valve assemblies in the field. One such approach is the use of a “shoe” or adjustable member on the casing to create a user-adjustable gap between the rotary valve and the casing or body. The gap can be adjusted or narrowed as the vane and the casing start to wear. Similar minded designs use an adjustable rotor, where the centerline of the rotor can be adjusted to compensate for wear. However, such approaches have a variety of limitations. First, the use of an adjustable shoe or rotor limits the sealing area of the valve to approximately 120 degrees of the rotor circumference. This limit reduces the number of vanes which can be used in a rotor design to prevent air leakage through the valve. Second, the body or casing and rotor wear is caused by abrasion. Abrasive wear, especially in a sealing application such as an airlock, is never smooth. Instead, the wear is typically in the form of “wire draw,” which looks like grooving on the sealing surface. The adjustable shoe and rotor designs are dependent upon a smooth or uniform wear on the sealing surfaces, and thus cannot compensate for this type of wear. Third, the adjustable rotor/shoe approach is dependent upon continuous preventative maintenance to maintain proper operating efficiency. If the valve clearance is not checked on a regular basis, it will wear out and eventually fail. Such maintenance may be time consuming and highly undesirable depending upon the nature of the processing involved.
Another approach for dealing with abrasion problems is the use of harder materials on the vane and shroud tips of the assembly, through the application of attached hard iron or steel alloy edges, or through the attachment of tiles comprising various, ceramics or carbides. Such an approach reduces the wear rate of the mating components in the valve. However, this approach also has a variety of drawbacks. First, component wear is caused by air leakage through the valve due to differential pressure. The leaking air picks up the abrasive particles being conveyed in the system and draws the particles past the vane and shroud tip surfaces in the valves. The hard particles in the air stream impact the surfaces of the vanes themselves, causing wear beyond the vane and shroud tips. Second, the use of cast iron or steel edges that are modified with different alloys to improve hardness are of limited lifespan in abrasive environments because the microstructure and overall hardness of such materials provide limited wear resistance. Third, while the known use of ceramics and carbides in such applications (typically alumina and tungsten carbide tiles) result in harder and more abrasion resistance than the iron alloys listed above, such an approach still requires the tiles to be mounted to the cast iron body and steel rotor with different types of adhesive. The unprotected joints between the tiles are prone to abrasive failure. This has especially been seen at the joint between the rotor tips and the rotor sidewalls. The adhesives used to mount the tiles also can fail from temperature or by failure of bond to the smooth ceramic surface. Additionally, while alumina is significantly harder than the iron alloys listed above, the inventors of the present invention have found that there are materials such as silicon carbide that perform better in high velocity, abrasive environments, and when cast into a one piece unitary rotor can offer improved performance at a more economical cost.
The following terms are used in the claims of the patent as filed and are intended to have their broadest plain and ordinary meaning consistent with the requirements of law.
Body: this refers to the casing or shroud which generally surrounds the circumference to the rotor, although various openings may exist, such as openings to a bin, pipe, conduit or other source or destination for particulate, or a vent for pressure release or other openings as needed.
Rotor: this generally refers to a body which surrounds a rotating shaft and a number of vanes extending generally radially from the body, although it should be understood that the term “rotor” or “rotors” may also be referring to the vane or vanes, depending upon context.
Mohs Hardness Value: a value which characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It is one of several known definitions of hardness in materials science, and may be translated over to other known scales. It is an objective measure for the hardness of various materials used to accomplish the goals of the present invention.
Where alternative meanings are possible, the broadest meaning is intended. All words used in the claims set forth below are intended to be used in the normal, customary usage of grammar and the English language.
The apparatus of the present invention generally comprises a rotor assembly for handling of an abrasive material. The rotor assembly comprises a shaft for driving the rotor, while the rotor itself comprises a rotor body with a bore for surrounding the shaft circumference, and a number of vanes projecting radially from the rotor body. The assembly further includes a collar assembly for mating the rotor to the shaft. Furthermore, the rotor is formed as a unitary ceramic component having a Mohs hardness value of at least about 9. A further application of the present invention in the context of a rotary valve would include the valve assembly discussed above in combination with a body which creates an annular bore for receiving the rotor and the surface of the body bore is comprised of at least one unitary ceramic component which covers at least one of the rotor pockets (i.e., the space between two adjacent vanes). Still another application of the present invention in the context of an abrasive handling material system would include the rotary valve in combination with a blower, compressor or other means to supply a pressure differential to the valve. In addition, the system would include a bin or similar abrasive supply container communicating with the valve to supply particulate to be processed, and a motor or similar drive means for rotating the rotor and a vent for releasing pressure in the rotary valve.
As can be shown from the discussions above, an object of the present invention is to provide an improved wear life system efficiency which is maintained for a greater duration of time.
A further object of the present invention is to provide a rotary valve with increased durability and decreased maintenance time or down time.
Still yet another object of the present invention is to provide a rotor with a less complex design having improved abrasion resistance.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, and as illustrated in the accompanying drawings
a and 3b show cross sections of the rotary valve assembly and body.
a-4c show front, side and top views, respectively, for an embodiment of a rotary valve of the present invention in combination with a reduction gear motor for driving the valve.
Set forth below is a description of what is currently believed to be the preferred embodiment or best examples of the invention claimed. Future and present alternatives and modifications to this preferred embodiment are contemplated. Any alternatives or modifications which make insubstantial changes in function, in purpose, in structure or in result are intended to be covered by the claims in this patent.
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The above description is not intended to limit the meaning of the words used in the following claims that define the invention. Rather, it is contemplated that future modification in structure, function or result will exist that are not substantial changes and that all such insubstantial changes in what is claimed are intended to be covered by the claims. For instance, the selection of a particular number of vanes in the rotor assembly is for illustrative purposes with reference to the example drawings only. Likewise, it will be appreciated by those skilled in the art that various changes, additions, omissions, and modifications can be made to the illustrated embodiments without departing from the spirit of the present invention. All such modifications and changes are intended to be covered by the following claims.
