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| Controlled Gap Seals | |||||||||||||||||||||
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The standard labyrinth seal (i.e., controlled gap seal) consists of one or more carbon-steel composite which is designed to minimize loss of process air pressure by using a closely controlled gap between the floating carbon ring assembly and a rotating shaft. By maintaining positive pressure differential, oil migration to the air side is also prevented. This seal will operate with unlimited axial movement, and will give long life and very low heat generation. The labyrinth ring assembly utilizes a carbon ring which is shrink fitted into a low expansion stainless steel alloy. The assembly of these two parts is designed to match the thermal and centrifugal growth of the shaft that is to be sealed. Consequently, at elevated temperature the floating labyrinth ring assembly grows at a rate similar to that of the shaft. |
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In operation under normal conditions, there is a pressure differential across the seal with the air side pressure being higher than the oil side of the seal. With the air pressure being higher, there will be some air leakage through the controlled gap between the carbon ring and shaft. The differential air pressure along with the spring load provide the closing force necessary to maintain contact between the carbon and housing secondary sealing face, while the shaft turns freely within the carbon ring. At high-pressure condition, the air pressure primarily holds the carbon ring assembly in place while the shaft turns freely within the carbon ring. Under these conditions there is no rubbing contact and, consequently, no heat is generated by the seal and no carbon or metal wear takes place. Since the seal gap is held to a small value, there will be times when, due to loading or temperature expansions, the shaft will shift slightly in a radial direction with respect to the housing and will contact the carbon ring. As this happens, the shaft will apply a radial load to the carbon ring. When this load becomes large enough to overcome the static friction of the carbon ring assembly against the housing face, the carbon ring assembly will shift its position to a new location where the shaft again turns freely with no rubbing contact. This condition can only be maintained when the shaft run-out is smaller than the clearance between the shaft and carbon ring. When shaft run-out exceeds this clearance value, a static carbon ring is no longer maintained and heat generation and wear start to take place gradually. The amount of heat and wear will depend on shaft run-out, seal clearance, shaft size and speed, and pressure across the seal. |
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