US2468171A - Rotary pump for liquefied gases - Google Patents

Description

April 1949- r R. v. CARLSON 2, 68,171 ROTARY PUMP FOR LIQUEFIED GAS ES Filed Jan. 9, 1947 INVENTOR ROGER V. CARLSON wuved/ ATTO R N EY Patented Apr. 26, 1949 UNITEDVISTATIES PATENT OFFICE ROTARY PUMP FOR LIQUEFIED GASES Roger V. Carlson, Kenmore, N. Y., assignor to The Linde Air Products Company, a corporation of Ohio Application January 9, 1947, Serial No. 721,135

8 Claims. (Cl. 103-'96) This invention relates to rotary pumps for liquefied gases having normal boiling points substantially below.233 K., such as liquid nitrogen, and particularly to inboard bearings for such pumps.

The pumping of liquefied gases having low boiling points involves unusual problems due to the great volatility of the liquid and its extremely low temperature. The volatility of the liquid, which is increased by natural heat flow toward the liquid from the surroundings, makes pumping of such a liquid quite difficult as generation of heat by the operation of the pump may cause gas binding and failure of operation. The low temperature also makes it impossible to employ ordinary lubricants and therefore bearings of the customary type cannot be used.

2 between races out of contact with each other, which means is operable in contact with the liquefied gas without requiring liquid or semi-solid form of plastic ball retainer employed in the bear- To obtain higher delivery pressures when pumping a liquefied gas by rotary pumps it is found necessary to employ two or more stages and when several stages are provided in a single casing with a single drive shaft the difliculties of accommodating the contraction of the parts have previously resulted in the employment of complicated constructions. Multiple staging introduties the problem of interstage sealing, which is necessary because the increased energy of the liquid in the higher pressure stage causes vaporization if it leaks toward a lower pressure stage. Such interstage sealing must be accomplished without incurring excessive friction and also be unaffected by expansion and contraction of the parts. It has also been proposed to use bearings made of bonded carbon or graphite in pumps for low temperature liquefied gas, particularly liquid oxygen. Such material is self-lubricating and is not affected by the low temperature. It does have the disadvantage, however, that it tendsto wear rapidly under the high radial bearing loads which occur in turbine type rotary pumps operating at high discharge pressures. Thus solutions that may suggest themselves for many of the dimculties are conflicting in that they may enhance others.

The principal objects of the present invention are therefore to provide a pump for liquid nitrogen or liquid oxygen embodying an improved combination of features and association of parts that successfully overcomes the difficulties set forth above; to provide in a two-stage rotary pump for liquefied gas an improved combined bearing and interstage seal; and to provide in a liquefied gas rotary pump a means for keeping the rolling elements, such as balls, of an inboard bearing of the type having rolling elements ing according to the invention.

Referring now to the drawing, the pump illustrated is of the two-stage type and has a main frame or casing I'll containing first and second stage annular pumping channels II and I2 with a partition l3 between them. Operating in the pumping channels it and I2 are rotary impellers l4 and I5 respectively provided with suitable blades It at their peripheral portions for propelling the fluid through the pumping channels. Each pumping channel is provided with a suitable inlet and outlet passage, which passages do not appear in the section shown. The outlet of the first stage channel II is connected to the inlet of the second stage channel l2. The arrangement of the pumping channels and their inlet'and outlet passages is similar to that shown and described in U. S. Patent No. 2,340,787, but is not illustrated herein since it forms no part of the present invention. One side of the casing I0 is closed by a cover II which is shaped internally to form part 01 the pumping channel l2 and to provide close clearance between it and one side of the impeller l5 and has a cup-like central depression i8 for enclosing the hub is of the impeller IS. The other side of the casing I0 is provided with a closure 20 similar to the cover I! but having an annular central portion 2| providinga closure for the hub portion 22 of the impeller M. A tubular extension 23 for enclosing an extended drive shaft 24 is secured gas-tightly to the central portion 2|.

The drive shaft 24 extends axially into the casing l0 and the impellers l4 and I! are rigidly secured thereon. To this end the drive shaft 24 has an integral collar 25 adjacent the inner side of the impeller l4, the hub portion 22 of which extends to the. left and is. internally threaded to engage threads 26 formed on the shaft 24. The

3 impeller I4 is thus threaded on the shaft 2| to abut tightly against the collar 25. To the right of the collar 25 the shaft has a cylindrical portion 21 adapted to carry the inner race 28 of a-bearing assembly indicated generally at29. The bearing 29 is of the type having rolling elements between annular races, preferably a ball bearing. The inner side of the impeller I is provided with an annular portion 30 that clamps the race 29 against the collar 25. The rightwardly extending hub portion l9 of theimpeller l5 is internally threaded to engage threads 3| formed on the inner end of the shaft24. If desired, the end of the shaft may be formed with flat areas as at 3| to provide a means for gripping it with a wrench so that the impeller l5 maybe secured tightly.

The ball bearing 29 is preferably of the type I race 28 and an outer race 33. the balls being spaced from each other by an annular retainer 34. The retainer 34 may be a composite ring providing pockets for the balls and is preferably constructed of a material that in effect is selflubricating under the conditions of operation, that is, when subject to the presence'of the liquefied gas being pumped. Self-lubricating as used herein means that the coeilicient of friction between the material and a smooth metal ball is very small and the rate of wear is low. When pumping .liquid nitrogen it will be preferable to form the retainer 34 of a plastic material such as a thermosetting resin bonded structure having a low coeflicient of friction under low temperature conditions as more fully described hereinafter.

The outer race 33 is rigidly secured in the partition l3 by retainingrings 35 and36. To this end the partition l3 has an opening therethrough with a shoulder 31 and an enlarged threaded counterbore 38. The rim of the ring 35 has threads engaging the threads 38 and abuts tightly against the shoulder 31. The ring 35 has a bore 39 into which the outer race 33 is fitted so as to abut against the bottom of the bore 39 formed by an annular flange or shroud portion 40 of the ring 35. The shroud portion 40 extends radially inwardly for a distance suflicient to cover the space between the inner and outer races. The outer race is clamped in place by the ring 36 which has external threads engaging those of the counterbore 38 and has at its inner rim a shroud ll which covers the space between races 28 and 33. The shouds 40 and 4| form a labyrinth type of interstage fluid seal to impede flow of liquid from the high pressure stage through the ball bearing and into the low pressure stage. The retaining ring 34 is also preferably constructed to provide small clearances with the rims of the inner and outer races 28 and 33 in order to cooperate in creating a labyrinth fluid seal.

The shaft 24 passes through'a stuffing box at the outer end of the housing 23, which stufiing box maybe of the customary type or, for example, as shown in U. S. Patent No. 2,296,640. A supporting bearing is also provided at the outer end of the shaft 24 which may be of any customary type provided that it allows for axial 4 expansion and contraction movement of the outer end of the shaft.

From the above description it will be seen that a great simplification of construction has been effected in a rotary multi-stage pump for low temperature liquefied gases. Previously there were provided axially adjustable connections between the drive shaft and the impellers, thrust bearing means for centering each impeller in itspumping channel, means such as balance cylinders for counteracting the resultant radial force of the liquid pressure acting on the high pressure stage impeller, and means for reducing the axial pressures acting on the impeller hub. According to the invention, such complications are avoided by utilizing a rolling element type of bearing positioned in the partition between the impellers, the bearing being constructed to withseparator is found to give good resultsin resisting the combined radial and axial forces because the rotating speeds of the pumps are in excess of 1,500 R. P. M. For heavier duty a double-row ball bearing with two separate ball retainers-may be substituted for the single-row type. v p

The invention relies in part upon the discovery that metal bearings of the rolling element type are successfully operable without customary lubricants when subject to low temperature liquefled gases. It has been universal practice to run ball bearings in an oily or grease lubricant even though there is very little if any sliding friction. The lubricant preserves the fine finished surfaces of the balls and races and removes heat due to the metal resilience at the rolling contact points. In the pump according to the inventionno oily lubricant can be used and although the liquefied gas such as liquid nitrogen does not appear to provide any lubricating qualities for sliding friction, it is inert with respect to the metal of the bearing and it effectively removes the rolling contact heat. For satisfactory operating life it is also necessary that the rolling elements shall not contact with each other so that there is no sliding friction between adjacent elements. Thus to keep the balls spaced from each other a retainer 34 is employed. This may be a ring-shaped member provided with pockets or holes for the balls and be made of two parts for easy assembly of the bearing.

In bearings of this type operating under a radial load that portion of the bearing carrying the load is known as the loaded zone. The rolling elements have to be urged into the loaded zone and the force urging the rolling elements into the loaded zone is exerted by the retainer. In addition the rolling elements entering and leaving the loaded zone travel at different speeds. As a result some friction is produced between the rolling elements and the retainer.

According to the invention it has been found that the retainer should preferably be made of a material that is self-lubricating under the conditions, i. e., one that has a very low coefiicient of friction against polished metal when in the presence of the low temperature liquefied gas. A material found particularly effective is a resinbonded structure that has adequate strength and elasticity at very low temperature and is inert with respect to the liquefied gas being pumped. A suitable material for the retainer is a laminated structure made of fibres bonded with a heathardened phenolic resin. A number of the plasties are suitable, provided they are free of objectionable plasticizing agents. In addition to the phenol-formaldehyde resins there may be used the polyester type such as the glycol esters with added cross-linking agents, melamine and urea type resins provided they are suitably modified to increase their toughness, and vinyl polymers also suitably toughened. Where the resins are not readily machinable, the ball retainers can be made by a casting or moulding process. In any of these structures various fabric and fibrous materials may be employed as the filler and the structure may include self-lubricating agent such as graphite. When liquid nitrogen is to be pumped, any of the plastic materials may be employed but when the liquefied gas is not inert, the plastic must be selected for its resistance to the action of such gas, for example when liquid oxygen is pumped it is preferable 'to select a plastic which is very resistant to combustion in oxygen. An effective retainer for a liquid nitrogen or liquid oxygen pump ball bearing is made from 'a laminated stock consisting of layers of fabric bonded with a heat-hardened phenolic resin. For example a suitable stock is made by impregnating duck fabric with a heat-reactive cresol-formaldehyde resin in alcohol solution in amount to leave a resin content of about 50% on drying to a low volatile content of about 2%. Layers of such impregnated fabric are stacked and pressed at about 1000 pounds pressure at 300 F. for about an hour, and the laminated stock so formed is after-baked to a Rockwell hardness of about 105 to 110 on the M scale. The hardness test is that referred to in procedure D785-44T of the American Society for Testing Materials.

The metal of the bearing should be chosen according to the character of the liquefied gas being pumped. With non-oxidizing gases such as liquid nitrogen the bearing balls and races may be made of the customary carbon or low alloy tool steels provided that care is taken to prevent rusting before the pump is put into service. In a pump for liquid oxygen, rusting during idle periods may be difficult to prevent and it will be preferable to use bearings made-of the hard stainless metals such as stainless steels. Certain hard surface corrosion-resistant surface treatments may also be used. I 1

For efficient operation, excessive leakage 'from the high pressure stage toward the low pressure stage should be prevented. To this end the bearing is constructed to prevent 'or limit such leakage. With a ball bearing, adequate sealing may be accomplished by the shroud flanges 40 and 4| that cover the space between the races 28 and 33 and that are arranged to have a very close running clearance with the sides of the inner race 20. The clearance between the side of the impeller l5 and the ring 36 is also kept small so that in effect a labyrinth type of seal is provided. Such sealing arrangement is found to be adequate as a small amount of leakage can be tolerated without a measurable effect on the pump ing. Such small amount of leakage is advantageous to bring liquefied gas into contact with the rolling elements which are thereby adequately cooled by a clean liquid.

Alternatively suificient sealing may be provided by the retainer 34 which may be constructed to have a radial width that substantially fills the lateral annular spaces between the races 28 and 33. The running clearance between the retainer and the two races can be made quite small because the retainer rotates at one-half the .material is self-lubricating underthe conditions.

By constructing the retainer with close clearances, suflicient interstage sealing may be obtained so that the use of the shroud flanges 40 and 4! may be eliminated to further simplify the pump structure.

Although a preferred embodiment has been described in detail, it is contemplated that changes may be made and certain features of the invention may be used without others without depart ing from the spirit and scope of the invention. For example, inboard ball bearings with plastic ball retainers may be used in single stage pumps and in multi-stage pumps on the sides of the impellers other than intermediately between them.

What is claimed is:

1. A multi-stage rotary pump having a casing, at least two rotary impellers therein, an annular pumpin channel for the peripheral portion of each of said impellers, and an axially extending drive shaft for said impellers, said casing having a partition between the pumping channels, said impellers being secured to said shaft on opposite sides of said partition, and a main inboard bearing for said shaft positioned between the impellers and mounted in said partition, said bearing having rolling elements between inner and outer races and including inter-stage fluid sealing means, and a retainer for keepin the rollin elements separated, said retainer being made of a self-lubricating material.

2. A multi-stage rotary pump according to claim 1 in which said bearing is a ball bearing having its inner race secured to the shaft and the outer race supported in said partition and said retainer is made of a plastic composition.

3. A multi-stage rotary pump according to claim 1 in which said bearing is a ball bearing containing no lubricant and said retainin means is a laminated structure made of fibres bonded with a heat-hardened phenolic resin.

4. In a rotary pump for liquefied gases having boiling points below 133 K., such as liquid nitrogen, said pump having a casing, at least one rotary impeller in the casing, and a drive shaft for the impeller, a main inboard bearing for said shaft in said casing exposed to the liquefied gas being pumped, said bearing havin rolling elements between inner and outer races and having a retaining means to keep the rolling elements out of contact with each other, said retaining means'being'made of a solid self-lubricating material.

5. A rotary pump for liquefied gases according to claim 4 in which said retaining means is made of a laminated fibrous plastic-bonded composition.

6. A rotary pump for liquefied gases according to claim 4in which said bearin is an unlubricated ball bearing and said retaining means comprises a laminated structure made of fibres bonded with a heat-hardened phenolic resin.

7. A m'ulti-stage rotary pump having a casing, at least two rotary impellers therein, an annular pumping channel for the peripheral portion of each of said impellers, and an axially extending drive shaft for said impellers, said casing having a partition between the pumping channels, said impellers being secured to said shaft on opposite sides of said partition, and .a bearin for said shaft positioned between the impellers, said bearing being a ball bearing with its inner race secured to the shaft and its outer race mounted in said partition, and a retainer made of a plastic 7 composition constructed for keeping the bearing balls separated, said retainer being thick enough to substantially close the space between the races and provide a fluid seal for impeding the flow of liquid through the bearing from the hi her Number Name Date 928,327 Prindle July 20, 1909' 2,035,417 Allendorfi Mar. 24, 1936 2,281,971 Goddard May '5, 1942 made of a solid self-lubricating material and be-' ing constructed to have a radial thickness such that the space between the races is substantially closed by the retainer to impede movement of liquefied gas through the bearing.

. ROGER V. CARLSON.

REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS

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