The present invention relates to pressure seal assemblies and, more specifically, pertains to low pressure differential seals between a gas and a buffer oil and the like for rotary machinery such as centrifugal compressors.
In most compressor applications it is undesirable and often unsafe to have any leakage of the compressed gas from the compressor to the surrounding environment. To prevent this from happening, a buffer oil at a higher pressure than the gas is introduced adjacent to the gas seal. A small portion of oil is allowed to leak across the gas seal to an annulus on the inside of the compressor adjacent to the seal. From this annulus the oil is collected in a cylinder and drained at regular intervals. The larger portion of the buffer oil, by far, crosses the air side seals dropping in pressure from a level which in some applications can be several thousand pounds per square inch to approximately atmospheric pressure.
The leakage of oil across the gas side seal should be as low as possible but still be sufficient to provide lubrication for the gas seal ring to prevent heat buildup and eventual seizure of the seal ring. Bushing seals of the type heretofore used have two significant disadvantages, firstly, the small clearances required for the seal rings makes the oil flow from the oil side to the gas side marginal, particularly at a low pressure differential. This is compounded when the seal ring rides eccentrically on the shaft. In the area of smallest clearance of an eccentrically riding ring, there may be virtually no oil film at all. Furthermore, changes in temperature between the seal ring and the shaft on a fast shutdown; for example, can cause the seal ring to cool faster than the shaft and thus contract to the point where seizure occurs. Secondly, when larger clearances are used in order to mitigate the aforementioned drawbacks there is an increase in the amount of oil leaking across the seal into the compressor collection cavity. Oil leakage is a cubic function of the clearance and a linear function of the axial length of the seal ring.
In accordance with the invention, a seal assembly for rotary machinery includes a seal ring in which the sealing oil first passes through the gas side seal which provides both cooling for the ring and lubrication for the bearing pads which are integral parts of the seal ring. The bearing pads center the seal ring so that it operates in the minimum leakage position for that clearance. The oil flow remains constant regardless of the pressure differential on the gas side seal ring since oil flow depends only on the pressure drop across the air side seals.
Accordingly, it is an object of this invention to provide an improved pressure seal assembly for rotating machinery.
It is another object of this invention to provide an improved pressure seal assembly utilizing a buffer oil.
It is another object of this invention to provide an improved pressure seal assembly capable of operating at high pressures and large peripheral shaft speeds.
It is another object of this invention to provide an improved pressure seal assembly having minimal oil leakage into the pressure side.
It is another object of this invention to provide an improved pressure seal assembly capable of operating at a small oil differential pressure.
Still other objects of this invention will become apparent upon a reading of the detailed specification to follow.