Spiral seal

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Spiral seals are used in many advanced technology fields, such as the sealing of gas-cooled reactor compressors and the sealing of sodium pumps in breeder reactors. It is sometimes also used for the sealing of the high-speed shaft in reducers. Its greatest advantage is that it can effectively provide sealing even when there is a large gap between the sealing components. If properly designed, its service life can be infinite. Due to the wide range of materials available for selection and the extreme simplicity of manufacturing, when the pressure difference is not high, the power consumption and heat generation of spiral seals are very low, and cooling with a water jacket is sufficient. Spiral seals often require additional shutdown seals, which complicates the structure and increases its size, thus often limiting their application. Spiral seals can be used for liquids under high temperatures, extreme cold, in corrosive environments, or those containing particles; they perform well even in harsh sealing conditions. 1. Sealing mechanism of the helical seal: The shaft surface of a helical seal features spiral grooves, while the bore has a smooth surface; this is similar to the grooved structure of a labyrinth seal. Therefore, the helical seal can be regarded as a special type of labyrinth seal, known as a helical labyrinth seal. However, the teeth of the spiral labyrinth are continuous, unlike the continuous teeth of the various labyrinths mentioned earlier. Due to the continuity of the teeth, the flow state of the medium through the teeth changes. The helical groove no longer functions as an expansion chamber to generate vortices that consume flow energy; instead, it acts as a propulsion device that exchanges energy with the medium, thereby creating what is known as a \"pumping effect\" and generating a pumping head. This pumping head balances the pressure of the sealed medium, meaning the pressure difference p=0, thus preventing leakage. Therefore, its sealing mechanism is slightly different from that of a labyrinth seal. However, as the medium passes through the gap, a portion of it remains above the tooth tips and does not flow along the groove, resulting in a permeation effect, which is the same as in labyrinth seals. Based on the spiral structure, the sealing mechanism of spiral seals varies slightly. It is a single-screw type; it utilizes the principle of screw pump operation, employing the pumping action of the screw to push the medium along the leakage gap back, thereby achieving sealing. It is suitable for sealing liquids or gas-liquid mixtures without the need for an external sealant, and is commonly used for bearing lubrication. It should be noted that the direction in which the spiral pushes the oil must be opposite to the direction of oil leakage; otherwise, not only will sealing not be achieved, but the amount of leakage will also increase sharply. Two helices with opposite rotation directions push the sealing fluid toward the center, creating a liquid seal. A pressure in the liquid seal that is slightly greater than or equal to the pressure of the medium being sealed is sufficient to achieve sealing. It is commonly used to seal gases or vacuums. Two helices with opposite rotation directions expel gas to both sides at high rotation frequencies, creating a high-vacuum trap in the middle to achieve sealing. This seal can be used as a vacuum seal. Theoretically, a smaller gap in a spiral seal is more conducive to ensuring a good seal. If the gap is large, the liquid medium cannot adhere to the surface of the shaft at the same time. Assuming that the liquid medium adheres only to the wall of the hole and is separated from the shaft, the spiral seal does not exert any force to push the medium away, resulting in seal failure. However, the gap is too small, and there is a risk of the shaft touching the hole wall. To avoid friction and wear of the sealed metal components, a layer of graphite can be applied to the surface of the hole walls. 2. Labyrinth helical seal: The labyrinth helical seal has been in use in industry only for a relatively short time. Its difference from a regular helical seal lies in the fact that spiral grooves are machined on the surface of the shaft, and spiral sleeves are also machined on the sealing hole; these spiral sleeves have a thread direction opposite to that of the shaft, thereby creating intense turbulence in the flow between the shaft and the spiral sleeves. Furthermore, the helical movement speed of the labyrinth spiral seal is higher than that of the screw seal, and it is used for low-viscosity liquids under turbulent conditions. Spiral seals are generally used for liquids with high viscosity under laminar flow conditions (such as liquids with a viscosity greater than that of water). Working principle: Within the working space between the screw and the screw sleeve, the liquid is located in several honeycomb-shaped spaces formed by the tooth surfaces of the screw sleeve and those of the screw. The gap between the surface of the screw and that of the nut is a grooved toroidal cylindrical surface. As the liquid passes through these threads, vortices are formed in a direction opposite to the flow direction. As a result of the momentum exchange between the screw and the flowing liquid, the screw transfers energy to the liquid. The screw and the nut interact with the liquid, resulting in frictional forces at the nominal interface passing through the gap between the screw and the nut. The friction generated in the liquid creates pressure between the screw and the screw sleeve.