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The pump assembly includes an impeller rotatably mounted in a pipe of the reactor having blades that generate a flow of fluid slurry through the pipe, a nosecone protruding in an upstream direction in front of the impeller blades that rotates along with the blades, and a plurality of stationary guid

The pump assembly includes an impeller rotatably mounted in a pipe of the reactor having blades that generate a flow of fluid slurry through the pipe, a nosecone protruding in an upstream direction in front of the impeller blades that rotates along with the blades, and a plurality of stationary guide vanes connected to the housing upstream of the impeller blades and having free ends that are radially spaced apart from the nosecone. The leading edges of the stationary vanes are shaped to direct any stringy material into the radial gaps between the free ends of the stationary vanes and the rotating nose cone to be shredded and thus prevent fouling and clogging of the pump assembly when such stringy material is generated during the polymerization process.

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1. A clog resistant pump assembly particularly adapted for circulating fluid slurry through the pipes of a loop reactor, comprising: an impeller rotatably mounted in a pipe of said reactor that generates a flow of fluid slurry through said reactor pipes, said impeller having a hub and plurality of b

1. A clog resistant pump assembly particularly adapted for circulating fluid slurry through the pipes of a loop reactor, comprising: an impeller rotatably mounted in a pipe of said reactor that generates a flow of fluid slurry through said reactor pipes, said impeller having a hub and plurality of blades radially extending from said hub;a nosecone protruding in an upstream direction in front of said blades that rotates relative to said pipe; andat least one stationary guide vane connected to the reactor pipe upstream of the impeller blades and having a free end radially spaced apart from said nosecone to define a gap therebetween that forms a fluidic pinch point that cuts stringy material entering said gap. 2. The pump assembly defined in claim 1, wherein said stationary guide vane is shaped to direct stringy material into said gap. 3. The pump assembly defined in claim 2, wherein said stationary guide vane includes a tapered leading edge that directs stringy material entrained in said flow of fluid slurry into said gap. 4. The pump assembly defined in claim 3, wherein the tapered leading edge of the stationary vane includes a portion that is substantially straight and tapered between about 20° and 40° relative to an axis of rotation of said pipe. 5. The pump assembly defined in claim 1, wherein a radial extent of said gap is less than about 0.5 inches (12.7 mm). 6. The pump assembly defined in claim 1, further comprising a drive shaft, wherein both said impeller and said nosecone are connected to said drive shaft. 7. The pump assembly defined in claim 1, wherein said stationary guide vanes are shaped to impart a rotation direction to said fluid slurry that is opposite in direction to a rotation direction of said impeller blades. 8. The pump assembly defined in claim 1, wherein said nosecone includes a portion of said impeller hub upstream of said impeller blades and a rounded front portion, and wherein said nosecone includes grooves on a surface opposite from said free ends of said vanes to increase the effectiveness of the fluidic pinch point in cutting stringy material entering said gap. 9. The pump assembly defined in claim 8, wherein said surface of said nosecone opposite from said vanes is substantially cylindrical, and said grooves are helically oriented on said substantially cylindrical surface in a direction that pulls stringy material through said fluidic pinch point when said impeller rotates. 10. The pump assembly defined in claim 1, wherein said nosecone includes a substantially cylindrical surface opposite to said free ends of said vanes, and a radial extent of said nosecone is at least half of a maximum radial extent of said impeller. 11. A clog resistant pump assembly particularly adapted for circulating fluid slurry in the pipes of a loop reactor, comprising: an impeller rotatably mounted in a pipe of said reactor that generates a flow of fluid slurry through said reactor pipes, said impeller having a hub and plurality of blades radially extending from said hub;a nosecone protruding in an upstream direction in front of said blades that rotates with said blades; anda plurality of stationary guide vanes connected to the pipe upstream of the impeller blades, wherein said blades circumscribe and are radially spaced apart from said nosecone to define gaps between said nosecone and free ends of said vanes that provide fluidic pinch points for cutting stringy material flowing through said gaps, and wherein said stationary guide vanes include tapered leading edges that direct said stringy material into said gap. 12. The pump assembly defined in claim 11, wherein a radial extent of said gap is less than about 0.25 inches (6.4 mm). 13. The pump assembly defined in claim 11, wherein said stationary guide vanes are shaped to impart a spin direction to said fluid slurry that is opposite in direction to a spin direction of said impeller blades. 14. The pump assembly defined in claim 11, wherein said nosecone includes grooves on a surface opposite from said free ends of said vanes to increase the effectiveness of the fluidic pinch point in cutting stringy material in said slurry. 15. The pump assembly defined in claim 14, wherein said surface of said nosecone opposite from said vanes is substantially cylindrical, and said grooves are helically oriented on said substantially cylindrical surface in a direction that pulls stringy material through said fluidic pinch point when said impeller and nosecone rotate. 16. The pump assembly defined in claim 11, wherein said nosecone includes a substantially cylindrical portion opposite to said free ends of said vanes, and a radial extent of said nosecone is at least half of a maximum radial extent of said impeller. 17. The pump assembly defined in claim 11, wherein the leading edges of the stationary vanes include a first portion that is substantially straight and tapered between about 15° and 60° relative to an axis of rotation of said pipe. 18. The pump assembly defined in claim 11, further comprising a drive shaft coupled at one end to both said hub of said impeller and said nosecone, and coupled at an opposite end to a drive motor. 19. The pump assembly defined in claim 18, wherein said housing is located adjacent to an elbow in said loop reactor pipe, and wherein said drive motor is located outside of said pipe and a mid-portion of said drive shaft extends through said pipe. 20. The pump assembly defined in claim 11, wherein said nosecone includes an annular base portion and a distal tapered portion that protrudes upstream away from the cylindrical portion, and wherein the outer diameters of the impeller hub and the base portion of the nosecone are substantially equal. 21. A loop reactor comprising the pump assembly defined in claims 1 or 11. 22. A process for producing polymer comprising: polymerizing in liquid diluent at least one monomer in a loop reactor to produce a slurry of polymer solids and under certain conditions stringy material;inducing flow of the slurry in the loop reactor using a pump assembly comprising: an impeller rotatably mounted in a pipe of the reactor having blades that generate a flow of fluid slurry through the pipe;a nosecone protruding in an upstream direction in front of the impeller blades that rotates along with the blades;a plurality of stationary guide vanes connected to the housing upstream of the impeller blades and having free ends that are radially spaced apart from the nosecone, wherein the stationary vanes have leading edges shaped to direct any stringy material into the radial gaps between the free ends of the stationary vanes and the rotating nose cone; andshredding any stringy material generated during the polymerization process through shearing force generated by the relative motion between the stationary free ends of the vanes and the rotating nosecone.