Air-cooled shaft seal

Details Air-cooled shaft seal Air-cooled shaft seal

2000-10-11

2001-07-24

Denion, Thomas (Department: 3748)

Rotary expansible chamber devices

Shaft or trunnion lubrication or sealing by diverted working...

C418S101000, C418S104000, C277S429000, C277S430000, C277S563000, C277S377000, C184S061000

Reexamination Certificate

active

06264447

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to apparatus and methods for sealing the rotating shaft of a fluid-containing housing, such as the drive shaft of a fluid-conveying pump. More particularly, the present invention relates to apparatus and methods for cooling a seal that utilizes the fluid conveyed by a pump to improve the performance of the seal.
BACKGROUND ART
Various types of pumps are utilized in fluid transporting systems in order to develop and maintain a desired amount of flow energy in the fluid. Many of these pumps require for their operation at least one rotatable shaft to drive a mechanical energy-transferring device such as a piston, impeller, or gear. Typically, the rotational power or torque transmitted to the shaft is generated in a motor disposed in remote relation to the pump housing. Thus, a portion of the shaft necessarily extends outside the housing through, for example, a bore in a wall of the housing, for direct or indirect linkage to the motor. The shaft is supported or mounted in the housing, but must be free to rotate at the interface of the housing and shaft in accordance with the operation of the pump.
A clearance of operationally-significant magnitude therefore exists between the bore of the housing wall and the shaft, even in a case where a bushing or like element is employed at the shaft housing or pump/atmosphere interface. It is recognized that over the range of operating pressures of the pump, this clearance presents a potential leakage point. Depending on the direction of the pressure gradient between the interior of the pump housing and the atmosphere, the leakage point may be characterized by fluid leaking out of the pump or air infiltrating into the pump. The leakage may contribute to a variety of undesirable conditions, including reduced pump efficiency, reduced economic life of the pump and related components, increased maintenance costs, and contamination or non-uniformity of the fluid being pumped. Accordingly, it is well understood that the pump must include some means for sealing the shaft at the interface.
The approach taken in the design of the shaft seal is especially critical in the context of gear pumps, which are utilized in a number of well-known applications to meter and discharge various types of fluids. A gear pump may generally be described as being a rotary, positive displacement pump. In its most basic design, the gear pump includes a pair of intermeshing spur, single-helical or double-helical (i.e., herringbone) gears disposed in a housing having narrow internal dimensional tolerances. One gear serves as the driving gear and is rotatable with a drive shaft, i.e., the shaft powered by a motor. The other gear serves as the driven gear and is rotatable on an idler shaft. The shafts are mounted in journal bearings on each side of gears. In operation, the gears create a pressure differential between a suction side and a discharge side of the gear pump housing. The working fluid is drawn into the housing at the suction side, is carried by the teeth of each gear in spaces defined by the teeth and one or more internal surfaces of the housing, and is squeezed out on the discharge side. This design results in a relatively constant rate of fluid flow with a minimum of drifting or slippage. The flow rate is dependent on gear rotational speed, but is largely unaffected by viscosity variations and pressure differential variations across the gear pump.
The performance characteristics of the gear pump make it especially useful in the processing of high-shear polymers such as rubber, PVC, and EDPM, where pressure, volume and uniformity of the flowing material must be controlled. For example, the gear pump may be used to transport synthesis polymeric material from a reaction vessel. The gear pump may also be used in connection with an extruder. A typical extruder includes an elongate barrel containing a rotating auger or screw. A hopper feeds pellets or granules of the polymeric material to the barrel, where the material is heated and melted as it is forced along the length of the barrel by the screw. In such an application, the gear pump is installed between the extruder and an extrusion die to pressurize and meter the polymer melt flow, and to dampen any pressure fluctuations or surges caused by the rotating screw of the extruder. Because the gear pump moves fluid more efficiently than the extruder and reduces the load on the extruder, the gear pump itself can be used to develop the high pressure needed in the fluid line. This enables the discharge pressure of the extruder to be separately adjusted to a reduced level in better accord with the extruder's own optimal operating point. Finally, the gear pump may be installed in line with two or more extruders as part of a compounding or mixing process to obtain similar advantages.
In view of the foregoing, it is readily apparent that the gear pump may produce not only a high pressure differential between the inlet and outlet fluid conduits communicating with the gear pump, but also a high pressure differential between the interior of the gear pump and the atmosphere. Thus, the problem of leakage in gear pumps may be potentially significant.
The leakage problem is further exacerbated when the gear pump is used to process viscous fluids. For example, in polymeric material processing the bearings selected for the gear pump are typically hydrodynamic and self-lubricating. That is, instead of using a separate lubrication method such as a forced oil circulation system, the gear pump and bearings are designed with flow paths for diverting a portion of the incoming polymer melt flow and circulating that portion between the bearings and shafts prior to discharge from the gear pump. The radial clearance provided in the bearing permits a wedgeshaped polymeric film to develop between the journal and the bearing as the shaft rotates. As a result, a hydrodynamic pressure is generated in the film that is sufficient to float the journal portions of the shafts and support the loads applied to them. And since the journal portion of the rotating shaft does work on the polymeric film and induces shear stresses therein, the frictional heat energy produced raises the film temperature. Consequently, the heated and pressurized polymer melt flowing in the vicinity of the shaft/housing interface has a high tendency to leak out from the pump.
Previous sealing solutions have not adequately controlled the leakage problem observed in gear pumps. In one application typical of the prior art, the sealing means took the form of a packing seal. A packing seal is constructed of one or more layers, windings or gaskets constructed of packing material such as graphite-impregnated cotton. The packing material is compressed within a packer or stuffing box. The stuffing box is usually disposed adjacent to the main pump housing. The main shaft of the gear pump extends outside the housing and through the stuffing box, such that the compressed packing material is squeezed against the shaft.
Apart from its general ineffectiveness in environments marked by high pressure differentials, the packing seal suffers from several other problems The compressed packing material, although treated with graphite, is nonetheless abrasive enough to produce substantial frictional contact with the shaft and thereby accelerate wear and deterioration of the shaft as well as the packing material itself, inviting frequent replacement of both. Additionally, the excessive frictional contact engendered by the packing material causes the pump to work harder, which lowers output and efficiency.
An attempt to improve the utility of the packing seal in the context of polymer processing is disclosed in U.S. Pat. No. 4,515,512 to Hertell et al. The gear pump disclosed in the Hertell patent includes a stuffing box attached to an end wall of the main pump housing. The stuffing box is thus adjacent to and outside of the housing. The drive shaft of the gear pump extends through a bore in the end wall of the housing, through the stuffing box, an

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