The present invention relates generally to gas seals which surround rotating shafts and, more particularly, to gland seals which utilize a thin film of oil which is maintained between the rotating shaft and a single ring member.
During the past 60 years, the development of hydrogen-cooled dynamoelectric machines has advanced significantly. Since the issuance of U.S. Pat. No. 1,453,083 to Schuller on Apr. 24, 1923, progress in hydrogen cooling of electrical generators has occurred steadily, along with suitable sealing devices which prevent the escape of hydrogen gas along the rotating shafts of the machine at the locations where the shaft extends axially from the gas-tight stator housing. The sealing of the interface between stationary and rotating components the dynamoelectric machine is generally accomplished by the use of a gland seal.
A hydrogen-cooled dynamoelectric machine, such as a generator, requires shaft gland seals at each end of the generator's rotor in order that hydrogen gas is prevented from escaping from the generator housing and also in order that air is not permitted to enter the generator housing. Gland seal assemblies generally comprise a bracket member and one or more sealing rings which are fitted close to the shaft's circumference and are free to move with the shaft in a radial direction. Generally, these rings are also permitted a slight axial movement along the shaft's surface, but are retained so that they cannot rotate around the shaft. Sealing oil acts as a barrier fluid and is provided at the interface between the shaft and rings in order to prevent the disadvantageous passage of gas through this gap. In typical gland seal designs, oil is pumped in a radially inward direction through the ring assembly and toward the rotor surface, where contacting the rotating surface of the rotor, the oil travels in opposing axial directions and produces a thin oil film between the shaft and the gland seal ring.
The oil must be provided at a pressure which exceeds both the hydrogen pressure within the generator and the atmospheric pressure of the air at the axially outboard region of the gland seal assembly. The oil leaving both the air and hydrogen sides of the gland seal assembly is then typically collected and returned to a gland seal oil reservoir. It is generally necessary to remove gas and moisture from the gland seal oil supply by a vacuum treating process in order to prevent the reduction in hydrogen purity within the generator housing.
Gland seal oil systems have been described at length in the technical literature. For example, the early development of gland seals is discussed in "hydrogen cooled turbine generators" by M. D. Ross and C. C. Sterrett, Vol. 59, AIEE Transactions, January, 1940, pps. 11-17. Another discussion of the sealing of hydrogen cooled generators can be found in "The Hydrogen Cooled Turbine Generator" by D. S. Snell, Vol. 59, AIEE Transactions, January, 1940, pps. 35-50. The dynamics of oil seal systems have been analyzed in "liquid film seal for hydrogen cooled machines" by C. W. Rice, General Electric Review, Vol. 30, No. 11, November, 1927, pps. 516-530. An oil purification system used in conjunction with gland seals is described in "continuous scavenging system for hydrogen cooled generators" by D. S. Snell and L. P. Grobel, AIEE Transactions, Vol. 69, 1950, pps. 1625-1636.
When hydrogen pressures are used, the loss of hydrogen can be excessive and expensive. This hydrogen loss is a function of the quantity of oil flowing toward the hydrogen side of the gland seal. In order to minimize this hydrogen loss on larger generators, double oil flow systems were developed in the 1950s. Double flow oil systems use two separate oil flows. One oil flow is directed toward the air side of the seal and the other is directed toward the hydrogen, or generator, side with a small buffer zone in between. These two oil systems are designed so that their pressures can be generally equalized, thus minimizing both the introduction of entrained air into the hydrogen and the loss of hydrogen by entrainment in the oil. A detailed description of single flow and double flow gland seal systems is contained in "Gland Seal Systems for Modern Hydrogen Cooled Turbine Generators", by R. A. Baudry and L. T. Curtis, which was presented at the AIEE Winter General Meeting, New York, Jan. 21-25, 1957, pps. 1-10.
Significant problems are experienced in conjunction with presently known gland seal designs. Excessive oil can be introduced into the generator either by intermittent oil spills or on a regular continuing basis. The normally expected oil flow toward the hydrogen side of the seal ring at any given differential seal oil pressure have been found to be of prime importance in the generator's oil usage. This oil flow creates vapor in proportion to its quantity. It must be drained in order to avoid spills within the generator housing. Therefore, it is significantly advantageous to reduce this oil flow.
In the present designs, it has sometimes been found that more than half the flow of oil to the hydrogen side passes in a radially inward direction between an axial face of the gland seal ring and the adjacent axial surface of an annular groove typically included within the gland seal bracket for containing the ring. The clearance between the gland seal ring and the bracket is generally held to be a nominal 0.007 inches to permit the radial movement of the gland seal ring to prevent rubbing of the ring between the bracket itself and the shaft and, thus, avoiding unstable shaft vibrations. This escaping oil, which does not beneficially aid the primary function of the gland seal system, typically travels a radial distance of approximately 0.37 inches from a hydrogen side oil feed groove to the hydrogen gas atmosphere within the generator frame. The significance of this oil loss can be realized by comparing this oil path to the axial clearance between the ring and the shaft which is typically only about 0.0025 to 0.0035 inches over a length which is approximately 0.625 inches long in the axial direction. Since any oil flow through this type of clearance is proportional to the cube of the clearance and inversely proportional to the length of passage and the oil of viscosity, it can be calculated that this non-functional radial flow exceeds the functional axial flow and can constitute over half the total flow into the generator under these conditions.