Selecting a mechanical shaft seal
Mechanical seals are used throughout industry to minimize or eliminate leakage in centrifugal pumps, mixers, and other rotating equipment. The ability of a mechanical seal to meet its performance objectives depends upon a wide range of factors involving equipment design, operating conditions, and support systems. Included in this list is the selection of the materials of construction of the seal. Mechanical seals are constructed of a wide range of materials including metals, elastomers, and ceramics. Each of these materials plays an important role in the operation of the seal. Seal manufacturers have narrowed the list of available materials through years of successful field experience. Still, the selection of the correct materials remains a critical step in the successful application of a mechanical seal.
A container holding a liquid or a gas has a rotatable shaft extending through its housing. This housing is shown in Fig. 1. Fluid is to be kept from escaping where the shaft extends through the housing, especially as the shaft rotates. A ring, part 1, with an O-ring, part 4, is sealed against the housing of the container. It is called the mating ring. Another ring, part 2, with an O-ring, part 3, is mounted onto the shaft. It is called the primary sealing ring. The contacting faces of these rings are lapped flat, within light bands. Initial contact between the faces is maintained by a spring, part 6, which pushes them together. The spring reacts against a retainer, part 7, which is locked to the shaft, perhaps with set screws. As the shaft rotates the primary sealing ring rotates with it. To assure that the primary sealing ring does not slip, the retainer may have drive lugs (not shown) that engage corresponding slots of the primary sealing ring and provide positive drive. With the mating ring stationary, there is relative motion between it and the primary sealing ring. The primary or “dynamic” sealing is done at the faces of the primary sealing ring and its mating ring. In a properly designed seal they slide relative to each other on a fluid film. The secondary or “static” sealing is done by O-rings or any other suitable gasketing arrangement. They are called secondary seals. The remaining components are referred to as hardware.
Material selection for mechanical seals can be a challenge. This is true not only because seals are used under so many different operating conditions but also because they contain such a wide variety of materials. Between elastomers, nonelastomer gaskets, metals,ceramics, and carbons, the end user must be knowledgeable about many different materials with significantly different properties. Fortunately there are many references available to support this endeavor. Besides printed materials,the Internet offers access to websites from seal OEMs, educational groups, and material suppliers. By far the most useful resource though remains the seal OEMs and their technical support staffs. These groups not only maintain a comprehensive set of reference materials, they also have data on thousands of successful field applications that can be used as a basis for future material selections.