Seal Materials for Abrasive Slurries.
Operators of submersible pumps are demanding increased reliability while requiring their pumps to work under harsher conditions. The seal material is of crucial importance, not only for its performance in service, but also for its machining properties. The flat faces of the sealing rings, one on the rotating shaft and the other on the stationary stator, are typically pressed together to make the required seal, and materials must be machined flat to within 0.6 micrometer. Since stainless steel and brass ring faces could not stand up to abrasive particle wear, harder materials, such as aluminum oxide, silicon carbide and cemented carbide, must therefore be used in the primary seal faces, where the outer rings come in contact with the pumping medium.
Aluminum oxide has inferior sliding properties, limiting its use to low speeds and pressures, while silicon carbide is brittle and relatively lower in mechanical strength. Conventional cemented carbide, using cobalt as a binder, has poor resistance to corrosion and can only be used in a pH range of 6 to 14, making it unsuitable for use with sea water or acidic liquids. However, Sandvik Hard Materials of Stockholm, Sweden, have developed a new binder for cemented carbide seals that contains cobalt chrome, nickel and molybdenum, and resists corrosion down to pH levels of 3 and even 2. They also have a binderless cemented carbide which brings the corrosion resistance below pH 2.
Cemented carbide has a high degree of hardness, rigidity, thermal conductivity and good sliding properties. Good mechanical strength makes it tolerant of careless handling and its performance is more consistent, predictable and reliable than those of aluminum oxide and silicon carbide. The corrosion-resistant binder expands the range of outer seal rings in pumps for mining and construction applications, and pumps submersed in seawater and acidic liquids. The binderless cemented carbide provides a good alternative to ceramics in highly-acidic liquids.
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|Date:||Jan 1, 2001|
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