Patent Number: 048184691
Section: summary

The present invention relates generally to a seal for turning valve bodies and more particularly to valves used in nuclear-engineering plants. From DE-OS No. 2,748,135 it is known to use a flexible graphite laminate as one of the materials for sealings and packings, whereby the laminate is applied to other materials for example a suitable metal. DE-OS No. 2,612,296 and DEP No. 2,039,355 also describe the use of flexible graphite material in the fabrication of seals. Flexible graphite materials are generally known according to the state of the art and are described, in addition to the aforecited publications, in DE-OS No. 2,855,408 and DE-OS No. 3,117,567. The essential advantages of flexible graphite materials may be found in the following properties: The materials are liquid- and gas-tight: they are highly temperature-resistant; they are resistant to radiation; They are resistant to most chemicals; and they have great flexibility and good elasticity and, in some applications, they also act as a self-lubricating seal The latter property, however, is coupled with the disadvantage that in a frequently actuated valve, the graphite material is relatively quickly worn off. In general terms, the useful life of such a seal is relatively short. In valves employed in radioactive plants, e.g., in plants for the reprocessing of nuclear fuel, the useful life of the valve is of special importance inasmuch as the replacement of worn-off parts in radioactive plants is very costly because the operation of the plant must be temporarily stopped, which naturally is extremely undesirable. Sealing materials of polytetrafluoroethylene plastics have not been found satisfactory since PTFE, even though chemically one of the most stable plastics, has very low radiation resistance and also has little resilience. In addition, various copolymers have been developed but their chemical stability, however, is small. For these reasons, it may be said that for the fluidized currents of the Purex process, no plastics are available for seals with a calculable useful life. It is the purpose of the invention to improve the seal of the type described in such a manner that its useful life is increased. This purpose is attained by the distinguishing features indicated in the appended claims. Advantageous embodiments and further refinements of the invention also become apparent from the appended claims. In short, in accordance with the present invention, the seal composed of flexible graphite materials is coated with a thin, flexible layer of tantalum. Tantalum has excellent ductility which is highly desirable for the intended purpose. Tantalum also has very good sliding or lowfriction properties so that a seizing or jamming of the valve body does not occur. In acccordance with the present invention, the tantalum coating layer is sufficiently thin to remain flexible so that the good flexibilty properties of the graphite covered by tantalum are preserved. It should also be stressed that the seal according to the present invention is likewise well suited for chemical plants in that tantalum is chemically very stable.