Self-compensating clearance seal for centrifugal pumps

A radial seal is provided for installation within a centrifugal pump of the type having an impeller, a pump casing having a suction inlet, a sealing ring groove formed in the pump casing, and means for supplying flushing water to the sealing ring groove. The radial seal comprises a seal body, including a sealing end, a water inlet end, and opposed sides. The sealing end has an outwardly extending lip portion. The seal is adapted for installation within the sealing ring groove, with openings formed through the seal body for the passage of water.

FIELD OF THE INVENTION

This invention relates generally to centrifugal pumps, and, more particularly, to lubricating pump seals for centrifugal pumps, which seals act to reduce wear between the rotating and stationary surfaces of pumps that are used to pump a mixture of solids and carrier liquid, commonly known as slurry.

BACKGROUND OF THE INVENTION

Centrifugal pumps employ centrifugal force to lift liquids from a lower to a higher level or to produce a pressure. Such pumps typically comprise an impeller consisting of a connecting hub with a number of vanes and shrouds, rotating in a volute collector or casing (SeeFIGS. 1 and 2). Liquid is drawn into the center of the impeller and is picked up by the vanes and accelerated to a high velocity by rotation of the impeller. The liquid is then discharged by centrifugal force into the casing and out the discharge branch of the casing. When liquid is forced away from the center of the impeller, a vacuum is created, causing more liquid to flow into the center of the impeller. Consequently there is a continuous flow through the pump.

The rotation of the impeller vanes results in a higher pressure in the volute collector than in the suction, which results in flow. This higher pressure has to be sealed against the lower pressure suction on one side and where the shaft (at a lower atmospheric pressure) on the other side enters the collector, to avoid leakage losses and loss of performance. In the case of the shaft, the most common sealing method is to utilize a stuffing box with rings of packing. On the front, or suction side, the most common method of sealing is to utilize a close radial clearance between the impeller and the casing and to employ radial seal rings. For pumps used to pump slurry, however, the sealing problem is more difficult. While radial seal rings are effective in clean water pumping applications, experience with slurry pumps has shown that the particles (solids) being pushed through the gap between the sealing surfaces are thrown off the rotating radial surface of the impeller seal ring, causing high wear to the wetted surfaces of the pump.

Wear occurs mostly as a result of particles impacting or sliding on the wetted surfaces. The amount of wear depends on the particle size, shape, specific gravity of the solids, and sharpness of the solids.

In order to reduce wear, some pumps employ a water flush to dilute and exclude solids, some utilize semi-axial gaps tapering inwardly at an angle, and some utilize clearing vanes protruding out of the front shroud of the impeller into the gap between the impeller and the suction liner, or combination of the above. Each of these, however, has either not satisfactorily solved the problem of wear, or has reduced wear at the expense of pump efficiency.

What is needed, then, is a pump seal that is simple, effective in reducing wear, and that does not impair the performance of the pump.

SUMMARY

The present invention is directed to a radial seal for centrifugal pumps. Specifically, the sealing assembly is adaptable for use in a centrifugal pump of the type used for pumping an abrasive slurry where wear due to particulate matter is particularly problematic. The seal assembly may be installed in a pump having a sealing ring groove in the stationary pump casing and a means for supplying clean, pressurized flush water into the sealing ring groove. While the present invention may be installed on a variety of pump types, exemplary installation on a single-stage, single-suction centrifugal pump will be explained in detail herein.

One embodiment includes a radial seal that is positioned within the sealing ring groove of the stationary pump casing of a centrifugal pump. The radial seal is dimensioned to be smaller than the groove so that it may move freely within the groove. The radial (circular) seal has a generally rectangular cross section and is formed of a wear-resistant malleable iron, elastomer, or ceramic material.

The radial seal comprises: a flushing water inlet, or outer, end; a sealing, or inner, end; and opposed sides. The radial seal is dimensioned to fit within the sealing ring groove in the pump casing. Multiple openings extend from the water inlet end to the sealing end of the seal for the passage of pressurized water therethrough.

When pressurized water is applied to the water inlet end of the seal, the seal will move to a self-compensating, balanced position between the pump casing and the impeller of the pump. The inventors have found that this balanced condition is approximately defined by the following equation:
PI*AI=PMEAN*AS.

Hydrostatically, as the seal approaches the surface of the impeller, backpressure between the impeller and the radial seal increases. The sealing end of the radial seal also includes a lip portion that extends outwardly so that the area of the sealing end is larger than the area of the water inlet end. This relationship between seal areas and pressures helps to balances the seal so that the seal does not physically contact the impeller.

In another embodiment, the sealing end of the seal of the present invention has a centrally-formed recessed region. Desirably, it creates a “shower head”, or conical, distribution of flush water. Formed in this fashion, the flush water is caused to spread out from the perforations onto an even larger predetermined surface area. When the flush water enters the recessed portion, pressure in the recessed portion builds, again balancing the hydrostatic force between the seal and the impeller surface, so that the seal moves outward, but never actually contacts the impeller.

These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiments when considered in conjunction with the drawings. It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain exemplary embodiments of the present invention are described below and illustrated in the attached Figures. The embodiments described are only for purposes of illustrating the present invention and should not be interpreted as limiting the scope of the invention, which, of course, is limited only by the claims below. Other embodiments of the invention, and certain modifications and improvements of the described embodiments, will occur to those skilled in the art, and all such alternate embodiments, modifications and improvements are within the scope of the present invention.

Referring now toFIG. 3, a close-up sectional view is shown of the impeller nose gap14and pump casing sealing surfaces12hfor a single-suction, single-stage centrifugal slurry pump, designated generally as10.

Pump10comprises a stationary casing, or volute,12that houses the single impeller22. As is conventional for centrifugal pumps, impeller22is rotated by a shaft (not shown) that is coupled to a motive power source (not shown) such as an electric motor. Aligned axially with impeller22is the pump suction inlet13. Suction inlet13is the point of entry for slurry being drawn into the impeller22. Suction inlet13is typically coupled to a suction source via piping (not shown) that mates with a suction flange surrounding suction inlet13. Slurry enters the suction inlet and moves inwardly through the length of the suction branch to the eye22aof the impeller22. The counterclockwise rotation of the impeller22pushes the slurry on the back of the impeller vanes22b, imparting radial motion and pressure to the slurry. The slurry is forced outward through a conventional casing discharge branch (not shown) that is typically connected to discharge piping. Depending upon the size of the pump and the rotational velocity of the impeller22, hundreds or thousands of gallons per minute of slurry are drawn inwardly through the suction inlet13and discharged outwardly under pressure.

As shown inFIG. 3, the radial seal30of the present invention is shown installed in the stationary pump casing12of the centrifugal slurry pump. As shown, the rotating component, i.e., the impeller22, is conventional and requires no modification. The suction side12aof the pump casing12, also commonly referred to as the suction liner, has a continuous and circular sealing ring groove12bformed therein with a generally rectangular cross section for receiving the radial seal30. As used herein, the term “radial seal” refers to any type of seal or gasket that is positioned within a holding groove for sealing the wetted surfaces of the casing12and impeller22. Also, as used herein, the term “sealing” refers to the function of reducing leakage or flow between component surfaces. As will be appreciated by those skilled in the art, a complete elimination of leakage or flow between surfaces is not desirable in certain applications.

Groove12bis preferably dimensioned with a depth that is greater than its width. Thus, the groove stably maintains the radial seal30in position, without the possibility of any substantial distortion or rotation. At least one water inlet connection12cis provided so that a supply of pressurized clean water may be injected into the groove12bduring pump operation or wet layup. As used herein, “clean water” refers to water that is substantially free of solid matter.

The complete radial seal30is best shown inFIG. 4. The seal30is formed of a durable elastomer, ceramic, or malleable metal, such as iron that has a high level of corrosion resistance; however, the selection of materials is not limited thereto. While there is no requirement that the seal material be particularly corrosion resistant because of the continuous flushing with clean water, corrosion resistant materials do, however, increase the service life of the seal30. The seal30is formed as a continuous circular ring. It is sized to be slightly smaller in each dimension than groove12bso that it can move freely laterally within groove12b, but so that it will not twist or otherwise distort. For example, a seal30having a thickness of about 1.000 inch and a depth of about 1.500 inches would be seated in a groove12bhaving a width of about 1.020 inches and a depth of about 2.000 inches. As explained in greater detail below, the sealing end of the seal30has a thickness dimension that is greater than the width of the groove12b.

As noted, the clearance between seal30and the groove12bis such as to allow easy movement and to minimize leakage in the gap. A version of this can have “0” rings in additional grooves (not shown) or other type of side seals to improve sealing. As shown inFIGS. 3 and 4, seal30has an outer, water inlet end30aseated within groove12b, an inner, sealing end30b, and opposed sides30gand30h. The water inlet end30bdefines a water inlet end area, AI. The sealing end30bdefines a sealing end area, As, with respect to the impeller nose surface22a. As best shown inFIG. 3, the sealing end30bfurther includes a lip portion31. As shown in the Figures and as will be described in greater detail below, the sealing end30bdefines an area that is greater than the water inlet end30aarea; i.e., AS>AI. A series of spaced apart openings30care formed around the circumference of the water inlet end30aof seal30and extend through the body of the seal30. These openings30cpermit entry and passage of pressurized sealing water entering through inlet12c. As the sealing water passes through an opening30c, it is forced outward through perforations30fthat are formed through the sealing end30b. Openings30cand perforations30fare sized so that a backpressure is maintained within groove12bbetween seal30and the inner end12eof groove12b. Because the dimensions of openings30cand30fare limited, the application of pressurized flushing water into opening30ccreates a “spray nozzle” effect, forcing seal30inward toward impeller nose surface22a. Opening30cis sized between 10 percent and 80 percent of the width of the seal.

Turning now toFIGS. 5 and 6, two exemplary embodiments of the seal30,50of the present invention are shown in cut-away sections.FIG. 5illustrates a seal50having opening50cfor the entry of pressurized flushing water into the seal50. The embodiment shown inFIG. 5has a sealing end50bthat is substantially flat across the entire width of the seal50. Openings50fare formed through the sealing end50bto communicate with opening50c. A backpressure is created between seal50and groove12bwhen pressurized flushing water is applied through inlet12c.

As shown inFIG. 6, in a second embodiment, a substantial portion of the sealing surface of the seal30is recessed.FIG. 6is a cut-away section of the seal30already shown inFIGS. 3,4, and7. As shown inFIGS. 3 and 6, the sealing end30bof seal30has a recessed portion30ecentrally formed in the sealing surface30b. Recessed portion30eis between about 10 percent and about 30 percent of the width of the seal, or between about 0.2 inches and 1 inch in width. It has been found that with this configuration, pressurized water passing through openings30cand30finto the recessed portion30efills and builds pressure in recessed portion30e. This ensures a substantially greater flushing and pressure balancing surface area between seal30and impeller nose22a. While the recessed portion30eis shown with a generally conical cross-section, it may be hemispherical, parabolic, etc., so long as it is completely surrounded by portions of sealing end30bsuch that a backpressure is created when water fills the recessed portion30e.

As shown inFIGS. 7 through 9, a further aspect and feature of each of the embodiments described herein is directed to a radial seal30having a lip portion31aor31bas described above. While early embodiments of the radial seal did not include a lip portion31extending outwardly from the sealing end30b, the inventors have found that in certain applications, establishing a self-compensating balanced position is more difficult. Varying inlet water and internal pump pressures have caused the radial seals to physically contact the impeller nose surface22a, which is undesirable.

The inventors have now found, however, that a self-compensating balanced position between the pump casing and the pump impeller is ensured when the sealing end area, ASis greater than the water inlet area, AI. The extended lip portion31amay extend inwardly toward the suction inlet13, as illustrated inFIGS. 7B and 9; alternatively, the lip portion31bmay extend outwardly away from the suction inlet13as illustrated inFIGS. 7A and 8. Further, a seal may comprise two lip portions (not shown) that extend inwardly toward the suction inlet13and outwardly away from the suction inlet13.

In further detail, the self compensating feature of the seal, including water being injected through the holes of the seal, is illustrated by the following equations. The variables of the equations are further defined inFIG. 10. The equations assume a linear pressure drop in the impeller nose gap. The axial force, F, acting on the radial seal has been determined as:

F=π3⁢(γG2-γ12)⁢(PG+P1+PG⁢γG+P1⁢γ1γG+γ1)
The corresponding mean pressure within the impeller nose gap has been determined as:

The self-compensating, balanced position of the radial seal is the relative radial seal position that is defined by the following equation:
PC*AI=PMEAN*AS.
Where:PC=pressure on the inlet side of the seal when water pressure is applied.AI=area of the water inlet end of the radial seal.AS=area of the sealing end of the radial seal.

Typically, when pressurized flush water is applied, PI>PMEAN; therefore, the self-compensating balanced position is obtained by compensating with a sealing area, AS, that is greater than the water inlet area, AI. When PI*AS=PMEAN*AS, the force acting on the radial seal is zero.

If PI*AI>PMEAN*AS, the radial seal will be pushed forward and the gap between the pump casing and the impeller nose, as well as the leakage flow rate, will then decrease. The radial seal ring will stop moving forward, when again PI*AI>PMEAN*AS.

The following example, with assumed values, illustrates how the self-compensating, balanced radial seal operates:γ1=525 mm;γG=625 mm;AS=90,320 mm2;AI=40,000 mm2;P1=50 kPa;PG=340 kPa;Pst2;=300 kPa andΔPvol=20 kPa.
Solving for PC, the self-compensating balanced position of the radial seal will occur when PCreaches a value of approximately 438 kPa.

As those skill in the art will appreciate, there are unlimited combinations of pressures, PG, PCand areas, AI, ASthat may be employed in constructing a radial seal according to the present invention.

In operation, pressurized water is injected into groove12bthrough inlet12c. Desirably, the pressure of the water is between about 1 and 20 pounds per square inch greater than the discharge pressure of the pump. The water passes through openings30c,50cand outward through perforations30f,50f. With the sizes of the perforations30f,50frestricted, the pressure of the sealing water forces the seal30,50laterally outward and into gap14, defined by the inner surface12hof casing12aand impeller nose surface22aof impeller22. As seal30,50protrudes outwardly toward surface22a, the seal water forced through the perforations30f,50fcreates a backpressure between seal surface30b,50band impeller nose surface22a. The backpressure between the opposed surfaces keeps the seal30,50from actually contacting impeller nose surface22a. Thus, the pressurized seal arrangement of the present invention creates a self-compensating clearance between the opposed surfaces30b,50band22a. As surfaces30b,50bare not in contact, there is no frictional seal wear on either the casing12or the impeller22caused by solid contact. Further, the pressurized water provides a lubricating and cleaning medium for the wetted surfaces of the centrifugal slurry pump10.

Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims and their equivalents.