Rotary kinetic fluid motors or pumps – Bearing – seal – or liner between runner portion and static part
Reexamination Certificate
1999-10-28
2001-05-22
Look, Edward K. (Department: 3745)
Rotary kinetic fluid motors or pumps
Bearing, seal, or liner between runner portion and static part
Reexamination Certificate
active
06234748
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a wear ring assembly for a centrifugal pump.
BACKGROUND
A centrifugal pump may generally include a wear ring assembly for restricting the flow of pumped fluid from a discharge pressure region to a lesser pressure region within the pump. For example, a wear ring assembly may be used to inhibit the flow of fluid that would otherwise occur between a discharge pressure and a suction pressure within a pump.
A wear ring assembly typically has an inner wear ring oriented coaxially with respect to a stationary outer wear ring. The stationary outer wear ring is normally pressed into a pump housing. Variations in radial clearance between the inner wear ring and the outer wear ring result from adding various deviations from ideal dimensions or tolerances within the pump. The inherent limitations of manufacturing processes or materials (e.g., polymers) for manufacturing pumps may lead to deviations from optimum pump tolerances. The inner wear ring does not often have an ideal or entirely concentric rotational relationship with respect to the outer wear ring because of deviations from optimum pump tolerances. Accordingly, during operation of the pump, unwanted frictional contact occurs between the inner and outer wear ring, resulting in heating of the pumped fluid, drag on the drive motor of the pump, and potential contaminants in the pumped fluid from depletion of the surfaces of the wear ring assembly. In addition, if a desired minimal radial clearance between the inner wear ring and the outer wear ring is exceeded because of variations from optimum pump tolerances, the wear ring assembly may provide an inadequate flow barrier between a discharge pressure and a suction pressure of the pump. Thus, a need exists for a pump design or improvement that can compensate for a slightly eccentric relationship or coaxial misalignment between the inner wear ring and the outer wear ring of a wear ring assembly.
In the past, pump designs have featured wear rings pressed onto an outer diameter of an impeller lip. Wear rings pressed onto the outer diameter of an impeller lip typically suffer from two infirmities. First, a wear ring mounted onto the diameter of an impeller tends to be forced off the impeller by hydraulic forces during operation of the pump. The back side of the wear ring experiences discharge pressure with axial forces from fluid that creeps behind the back side. In contrast, the front side of the wear ring experiences suction pressure, which is significantly lower than the discharge pressure. Accordingly, the net hydraulic force acting on the wear ring tries to push it off the impeller toward the inlet of the pump. Even if the net hydraulic force merely slides the wear ring slightly forward toward the pump inlet, a catastrophic failure of the pump may occur. For example, in one failure mode the wear ring jams the rotation of the impeller by contacting stationary casing material which would normally be located axially in front of the wear ring by a clearance dimension.
A second infirmity of the foregoing mounting arrangement, predominately applies in the context of ceramic wear rings. Mounting a ceramic wear ring on the outer diameter of an impeller lip places the wear ring in tension, increasing the chance of failure or breakage of the ceramic wear ring. Ceramic wear rings feature low tensile strength and high compressive strength. Ceramic wear rings can be made radially thicker to compensate for certain tensile forces.
Other pump designs use an elastomer o-ring that engages a groove in a wear ring and an impeller to affix the wear ring to the impeller. The o-ring is composed of a flexible elastomer that has sufficient give to permit assembly. Providing the requisite flexibility of the o-ring typically results in an undesired compromise in structural integrity of the o-ring. Accordingly, the o-ring attachment of the wear ring to the impeller may lack reliability over extended periods of operation. For example, the wear ring may become separated or misaligned with respect to the impeller because the o-ring breaks or yields from a lack of adequate shear or tensile strength during pump operation. Thus, a need exists for a reliable mounting technique for mounting wear rings on an impeller, including ceramic wear rings.
SUMMARY OF THE INVENTION
In accordance with the invention, a wear ring assembly for a pump includes an inner wear ring and an outer wear ring generally coaxially oriented with respect to the inner wear ring. An impeller has an inner recess for receiving the inner wear ring. The inner wear ring has an exterior groove and the inner recess has an interior groove for axial alignment with the exterior groove. The inner wear ring is affixed to the impeller via at least one arcuate retainer for simultaneously engaging the interior groove and the exterior groove.
In one aspect of the invention, the inner wear ring is mounted on an impeller front such that the inner wear ring merely experiences compressive forces, as opposed to tensile forces, from any mechanical interference between the inner wear ring and the inner recess. Accordingly, the above mounting arrangement of the inner wear ring is well-suited for ceramic materials or other materials that offer comparable resistance to compressive forces. Further, during operation of the pump, the inner wear ring on an impeller front is tolerant of hydraulic forces because both a front face and a rear face of the inner wear ring tend to be exposed to the same suction pressure. The mounting of the inner wear ring in the inner recess adjacent to an impeller eye, as opposed to elsewhere on the impeller front, allows for a minimum possible clearance area between the inner wear ring and the outer wear ring to minimize leakage of fluid from the discharge pressure to the suction pressure.
Another aspect of the invention includes an outer recess in a structural support of the pump for receiving the outer wear ring. The recess has a retention channel for accepting an outer retainer having an axially compressible dimension. The outer retainer is adapted to engage the retention channel while protruding radially inward therefrom to retain the outer wear ring. The retainer is biased against the outer wear ring to force the outer wear ring axially into the outer recess. The outer recess preferably has a radial extent greater than that of the outer wear ring to form a radial zone of adjustment for compensating for eccentric misalignment between the inner wear ring and the outer wear ring. The above mounting arrangements lend themselves to fabrication with corrosion-resistant materials, such a ceramic wear rings, and retainers composed of fiber-reinforced polymers, and fiber-reinforced plastics.
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Karassik Krutzsch, Fraser, and Messina,Pump Handbook, 2nd. Ed., McGraw-Hill Book Company, pp. 2.53-2.59.
Robert Neumaier,Hermetic Pumps, Gulf Publishing, Houston, Texas, pp. 154-155; 356-359.
Brown Jeffrey S.
Klein Manfred P.
McAloon Scott A.
Phelps Peter E.
Innovative Mag-Drive, L.L.C.
Look Edward K.
McAleenan James M
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