Abstract:
A cryogenic rotary pump for pressurizing a flow of a cryogenic liquid and for dividing the flow into a first lower pressure and a second higher pressure stream has a series of pumping chambers. A single rotary drive shaft carries a rotary inducer located in the chamber, a first impeller in the chamber, and a second impeller in the chamber. A liquid receiving chamber is located intermediate the pumping chambers. A first outlet from the pump for the first lower pressure stream is contiguous to the chamber and a second outlet is provided for the second higher pressure stream. The pump may serve to pressurize a flow of unboiled liquid oxygen from a sump of a lower pressure column of a double column also having a higher pressure column.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a pump, more particularly to a cryogenic rotary pump and to a cryogenic air separation apparatus including the pump. 
     A cryogenic rotary pump conventionally contains one or more pumping chambers. The or each pumping chamber is, in operation, swept by a rotary pumping member. The rotary pumping members are carried on a shaft which is typically driven directly by an electric motor. The number of pumping chambers and/or pump speed depends on the pressure to which it is required to raise a cryogenic liquid by the pump. 
     Such cryogenic rotary pumps may be used to perform any one of a number of different duties. Cryogenic rotary pumps are, for example, widely used in cryogenic air separation plants. Such plants or apparatus typically include a double rectification column, for separating the air, comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing an upper region of the higher pressure column in heat exchange relationship with a lower region of the lower pressure column. The condenser-reboiler is typically located in or above a sump in which a liquid oxygen fraction separated in the lower pressure column collects. Conventionally, the reboiling section operates as a thermosiphon. Therefore no external electrical pump is required to urge the liquid oxygen through the reboiler. One disadvantage of a thermosiphon is that liquid head effects result in a temperature difference between boiling liquid and condensing vapour greater than would otherwise be necessary, thereby adding to the thermodynamic inefficiency of the operation of the condenser-reboiler in operation. Accordingly, downflow reboilers are now used as an alternative to thermosiphon reboilers. In such downflow reboilers the liquid to be boiled is distributed to a header at the top of the boiling passages and flows down these passages. In the case of liquid oxygen, it is considered unsafe to operate the reboiler with dry areas on the boiling surfaces. Accordingly, only a portion of the liquid oxygen is boiled and there is a need to pump to the distributor an appreciable flow of liquid oxygen. A cryogenic rotary pump can be used for this function. 
     Another use for a cryogenic rotary pump in a cryogenic air separation plant is to pump a liquid oxygen product to a relatively high pressure, sometimes above the critical pressure of oxygen. The thus pressurised oxygen is warmed so as to provide an elevated pressure product at approximately ambient temperature. One advantage of such an arrangement is the need for an oxygen gas compressor, the operation of which can be hazardous, is avoided. 
     Modern air separation plants are increasingly designed to produce an elevated pressure gas oxygen product and with downflow reboilers. Typically two separate cryogenic rotary pumps are employed to perform these functions, although when the pressure of the oxygen product is in the order of 10 bar or less, it is known to reduce the pressure of a sidestream of the pumped liquid oxygen and introduce it into the downflow reboiler. Since the recycle flow to the downflow reboiler can exceed the flow rate of oxygen product out of the plant, such a practice is particularly inefficient. 
     It is an aim of the present invention to provide a rotary cryogenic pump which can perform a plurality of pumping duties relatively efficiently. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a cryogenic rotary pump for pressurising a flow of a cryogenic liquid and for dividing the flow into a first lower pressure stream and a second higher pressure stream, including a plurality of pumping chambers in series with one another, a single rotary drive shaft carrying all rotary pumping members, a liquid receiving chamber intermediate a pair of pumping chambers, a first outlet from the pump for the lower pressure stream, the first outlet being contiguous to the liquid receiving chamber, and a second outlet from the pump for the second higher pressure stream downstream of the series of pumping chambers. 
     According to the second aspect of the present invention there is provided cryogenic air separation apparatus including a double rectification column for separating the air, comprising a higher pressure column, a lower pressure column, and a condenser—reboiler placing an upper region of the higher pressure column in heat exchange relationship with a lower region of the lower pressure rectification column, wherein the reboiler is of a downflow kind having generally vertical boiling passages communicating with a sump, there being an outlet for liquid oxygen from the sump, characterised in that the outlet for liquid oxygen communicates with a cryogenic rotary pump according to the first aspect of the invention and that the first outlet of the cryogenic rotary pump communicates with an inlet of the reboiler for liquid oxygen and the second outlet of the cryogenic rotary pump communicates with heat exchange means for warming the oxygen. 
     By appropriate selection of the pumping members and the number of pumping chambers, and/or the pump speed, the first stream can be produced at a pressure not significantly above that required to lift the stream to the top of the reboiler, typically a distance in the range of 10 to 20 meters, and the second stream can be produced typically at a pressure in the range of 10 to 60 bar. An advantage of a cryogenic rotary pump according to the first aspect of the invention is that a single drive shaft (which therefore requires only a single electric or other motor to drive it) carries all the rotary pumping members. Duplication of motors and associated electrical switch gear is therefore avoided. In addition, only one pump inlet liquid line equipped with a shut-off valve is required. When the cryogenic rotary pump according to the first aspect of the invention is used in a cryogenic air separation apparatus according to the second aspect of the invention, the ability to avoid duplication of motors and electrical switch gear and pump inlet liquid lines (equipped with shut-off valves) makes possible a reduction in the size of the insulating housing, known as a “cold box”, in which the cryogenic parts of the apparatus are housed. 
     There is preferably only one pumping chamber upstream of the liquid receiving chamber. The pumping member in this upstream chamber is typically an inducer comprising a helical blade of constant or varying pitch or other axial or radial pumping member dependent on the required pumping duty. For an inducer, the helix preferably performs 1½ to 2½ complete turns, i.e. extends through an angle in the range of 540 to 900° for low NPSH (net positive suction head) requirements. 
     The precise pressure at which the first stream leaves the first outlet depends in part on the pitch or diameter of the blade or the pumping member speed. Accordingly, for an upstream pumping chamber of given size, and for a given pumping member speed (which may be dictated by the speed at which downstream pumping members are intended to operate) the outlet pressure of the first stream can be selected from an albeit relatively small range of pressures by appropriate choice of the precise dimensions of the helical blade. Preferably, the or each pumping chamber downstream of the liquid receiving chamber has associated therewith a radial rotary pumping member, typically taking the form of an impeller having blades which urge the fluid being pumped in a generally radial direction. 
     Preferably, there is an axial or radial diffuser, or three dimensional (axial/radial) diffuser, located downstream of each pumping member. If desired, the blades of the radial diffuser may be of a variable angle kind. 
     The number of pumping chambers and the rotational speed of their pumping members downstream of the liquid receiving chamber depends on the pressure to which it is desired to raise the second stream. If, for example, the second stream is required at a pressure in the order of 10 to 12 bar, there may be a single radial pumping chamber downstream of the liquid receiving chamber. If, however, the second stream is required at a pressure in the order of 60 bar, there may be a series of four to eight radial pumping chambers, or more, downstream of the liquid receiving chamber. 
     Preferably, the drive shaft is driven directly by a single electric motor. 
     A variable speed electrical motor is preferably employed. Such a motor enables the outlet pressure of the second stream to be varied albeit within a relatively narrow range of pressures. However, by appropriate selection of the number of pumping stages downstream of the liquid receiving chamber and appropriate selection of the motor speed it is possible to design a pump according to the invention to give any desired second stream outlet pressure in the range of 10 to 60 bar or more. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Apparatus as according to the two aspects of the invention will now be described by way of example with reference to the accompanying drawings, in which; 
     FIG. 1A is a schematic sectional elevation of the main body of a first cryogenic rotary pump according to the invention; 
     FIG. 1B is a schematic sectional elevation of the pump illustrated in FIG. 1A so as to illustrate the connection of the main body of the pump to an electrical motor which drives the pump; 
     FIG. 2 is a schematic sectional elevation of a second cryogenic rotary pump according to the invention; 
     FIG. 3 is a schematic sectional elevation of a third cryogenic rotary pump according to the invention; 
     FIG. 4 is a schematic sectional elevation of a fourth cryogenic rotary pump according to the invention, and 
     FIG. 5 is a schematic flow diagram of part of an air separation apparatus including a cryogenic rotary pump according to the invention. 
    
    
     The drawings are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIGS. 1A and 1B, and particularly first to FIG. 1B, of the drawings, there is shown a cryogenic rotary pump  2  having a generally cylindrical housing  4  located with its longitudinal axis vertical and having at one end a flange  6  which is secured to the support  8  (sometimes referred to as a “lantern”) of an electric motor  10 . The cryogenic pump has an axial drive shaft  12  which is directly coupled to the electric motor  10 . The coupling engages a labyrinthine seal  14  so as to prevent leakage of fluid from the pump into the motor  10 . 
     With reference now to FIG. 1A, the cryogenic rotary pump  2  has an inlet  16  provided with a flange  18  which is coupled to a complementary flange  20  of an inlet pipeline  22  communicating with a source (not shown) of cryogenic liquid to be pumped. The inlet  16  communicates with a first pumping chamber  26  defined within the housing  4  by a hollow insert  28  in frictional engagement with the inner surface of the housing  4 . Located within the chamber  26  is an inducer  30  in the form of a helical blade which is connected by a key  32  to the drive shaft  12 . The inducer  30  is carried on a hub  34  mounted on the shaft  12 . In operation, rotation of the drive shaft  12  causes the inducer  30  to urge the cryogenic liquid in a generally axial direction through a diffuser  36  having guide vanes  38  which provide communication between the first pumping chamber  26  and a liquid receiving chamber  40  located downstream of and coaxial with the first pumping chamber  26 . The pumping action created by rotation of the inducer  30  is typically sufficient to raise the pressure of the liquid by an amount in the range of 1 to 2 bar depending on the extent and dimensions of the helical blade and its speed of rotation. For such different pressure requirements the diffuser housing  36  and the inducer  30  may be easily exchanged for new parts. 
     The liquid receiving chamber  40  is bounded in part by an appropriately shaped, hollow, generally cylindrical insert  42  which is in frictional engagement with the inner surface of the housing  4 . The liquid receiving chamber  40  communicates at its side with an intermediate outlet  44  (“the first outlet” referred to above) for cryogenic liquid from the pump  2 . The outlet  44  has a flange  46  which is coupled to a complementary flange of a pipeline  50  leading to apparatus (not shown in FIG. 1) in which the cryogenic liquid is employed. The liquid receiving chamber  40  also has at its end remote from the diffuser  36  an axial outlet  52  communicating with a second pumping chamber  54 . A baffle  56  is provided in the liquid distribution chamber  40  so as to prevent straight line flow from the diffuser  36  to the outlet  52  of the liquid that does not pass to the outlet  44 . The second pumping chamber  54  is bounded by appropriately shaped inserts  58  and  59  which are in frictional engagement with the inner surface of the housing  4 . The insert  58  is integral with the baffle  56 . A first impeller  60  is mounted on the drive shaft  12  and is held in position by a pair of keys  62 . The first impeller  60  is located within the second pumping chamber  54 , and is formed as an integral casting which has a lower disc  64  and an upper disc  66  spaced axially apart from one another such that an annular recess is defined therebetween. One or both of the discs  64  and  66  are formed with integral curved blades  68  which extend thereacross and project into the recess. The blades  68  are shaped and arranged in a manner well known in the art such that, in operation, rotation of the first impeller  60  by the drive shaft  12  causes liquid entering the second pumping chamber  54  from the liquid receiving chamber  40  to be urged by centrifugal force radially outwardly along progressively narrowing passages defined by the discs  64  and  66  and blades  68 . The liquid is thereby raised in pressure. Typically an increase in pressure in the order of 8 to 12 bar can be achieved. 
     The first impeller  60  is provided with an upwardly extending collar  70  and a downward extending collar  72 . The collar  70  is provided with an annular labyrinthine bearing  74  which is pinned or otherwise secured to the insert  56 . Similarly, the collar  74  is provided with an annular labyrinthine bearing  76  which is pinned or otherwise secured to the insert  58 . 
     The inserts  58  and  59  are shaped so as to define an axial annular channel in which is located a diffuser  78  having guide vanes  80 . The diffuser  78  is positioned so as to receive liquid, in operation of the pump  2 , from the periphery of the first impeller  60 . Pressurised liquid flows from the diffuser  78  to an outlet  82  of the second pumping chamber  54  communicating with a third pumping chamber  84 . The outlet  82  is positioned coaxially with and below the outlet  52  from the liquid receiving chamber  40 . 
     The third pumping chamber  84  is bounded by the insert  59  and another insert  86  which is in engagement with the bottom of the housing  4 . A second impeller  88  is mounted on the drive shaft  12  and is held in position by a pair of keys  90 . The second impeller  88  is located within the third pumping chamber  84 , and is generally identical to the first impeller  60 , being formed as an integral casting which has a lower disc  92  and an upper disc  94  spaced axially apart from one another such that an annular recess is defined therebetween. One or both of the discs  92  and  94  are formed with integral curved blades  96  which extend thereacross and project into the recess. The blades  96  are shaped and arranged such that, in operation, rotation of the second impeller  88  by the drive shaft causes liquid entering the third pumping chamber  84  from the second pumping chamber  54  to be urged by centrifugal force radially outwardly along progressively narrowing passages defined by the discs  92  and  94  and blades  96 . The liquid is thus raised in pressure, typically by a further 10 to 12 bar. 
     The second impeller  88  is provided with an upwardly extending collar  98  and a downwardly extending collar  100 . The collar  98  is provided with an annular labyrinthine bearing  102  which is pinned or otherwise secured to the insert  59 . Similarly the collar  100  is provided with an annular labyrinthine bearing  104  which is pinned or otherwise secured to the insert  86 . 
     The inserts  59  and  86  are shaped so as to define an axial annular channel in which is located a diffuser  106  having guide vanes  108 . The diffuser  106  is positioned so as to receive liquid, in operation of the pump, from the periphery of the second impeller  88 . Pressurised liquid flows from the diffuser  106  to an axial outlet  110  (referred to hereinabove as “the second outlet”) at the bottom of the pump  2 . The outlet  110  has a flange  112  which is coupled to a complementary flange  114  of a pipeline  116  leading to an apparatus (not shown in FIG. 1) in which the pressurised liquid, now typically at a pressure in the range of 16 to 25 bar is used. 
     The bottom of the drive shaft  12  is provided with a nut  118  which can be removed to enable the impellers to be removed from the drive shaft  12 . 
     Typically, the housing  4  is formed of stainless steel. The drive shaft  12  is also formed of stainless steel, but of a martensitic kind. Internal parts of the pump are preferably formed as bronze castings. 
     The impellers  60  and  88  typically have balancing holes  120  formed therethrough. 
     In operation of the pump shown in FIG. 1, the drive shaft  12  is typically driven at a velocity of 3000 revolutions per minute (or 3600 rpm for 60 Hz net) and the pump  2  is operated continuously. A 150 to 400 kilowatt electric motor generally suffices for this duty. Typically, the pump is arranged such that about two-thirds of the incoming cryogenic liquid, for example liquid oxygen, leaves through the intermediate outlet  44  and the remainder through the bottom outlet  110 . If there is but a single impeller, the motor may drive the shaft  12  at a higher velocity, eg 4000 to 7000 revolutions per minute. 
     The cryogenic rotary pump  125  shown in FIG. 2 is very similar to the one shown in FIG. 1 of the drawings. Whereas the pump shown in FIG. 1 has two high pressure pumping stages downstream of the liquid receiving chamber  40 , one such stage being located in the second pumping chamber  54 , the other in the third pumping chamber  84 , the pump shown in FIG. 2 has five high pressure stages  120 ,  130 ,  140 ,  150 , and  160  generally similar to the two high pressure stages of the pump shown in FIG.  1 . The pump shown in FIG. 2 is able to deliver the second stream of cryogenic liquid at a pressure in the order of 40 to 60 bar, the precise pressure depending on the speed at which the motor  10  drives the shaft  12  and the diameter of each impeller. Any delivery pressure of the second stream of cryogenic liquid in the ranges of 20 to 60 bar can therefore be raised depending on the number of high pressure stages that are incorporated into the pump and their hydraulic design. 
     In FIG. 3 of the accompanying drawings there is shown a cryogenic rotary pump  175  which delivers the second stream of cryogenic liquid at a pressure in the order of 10 bar. In this pump, there is a single high pressure stage. Further whereas in the pumps shown in FIGS. 1 and 2 the inlet is at a greater elevation than the two outlets, in the pump shown in FIG. 3 the inlet is at a level below those of the two outlets. 
     The pump  175  has at a bottom region of the housing  170  an inlet  176 . The inlet  176  communicates via an intermediate chamber  177  with a first pumping chamber  178  located within the housing  170 . The first pumping chamber  178  has an inducer  180  in the form of a helical blade which is keyed to the drive shaft  174 . In operation, rotation of the drive shaft  174  causes the inducer  180  to urge the cryogenic liquid upwardly in a generally axial direction through a diffuser  182  having guide vanes  184  which afford communication between the first pumping chamber  178  and a liquid receiving chamber  186  located thereabove. The pumping action created by the rotation of the inducer  180  is typically sufficient to raise the pressure of the liquid by an amount in the range of 1 to 2 bar (or to lift it to a height in the range of 10 to 20 meters) depending on the extent and dimensions of the helical blade and its speed of rotation. 
     The liquid receiving chamber  186  communicates at its side with a first outlet  188  for cryogenic liquid from the pump  175 . The liquid receiving chamber  188  also communicates through guide vanes  191  at its top with a second pumping chamber  190  formed in the housing  170 . An impeller  192  having upper and lower discs is keyed t o the drive shaft  174  within the second pumping chamber  190 . The impeller  192  is generally similar to the impeller  60  of the cryogenic rotary pump  2  shown in FIG.  1 . It has integral curved blades  194 . The blades  194  are shaped and arranged in a manner well known in the art such that, in operation, rotation of the impeller  192  by the drive shaft  174  causes liquid entering the second pumping chamber  190  to be urged by centrifugal force radially outwards along progressively narrowing passages defined by the upper and lower discs of the impeller  192  and the blades  194 . The liquid is thereby raised in pressure. The second pumping chamber has an outer spiral shaped annular peripheral region  196  which receives pressurised liquid from the impeller  192  and which communicates with a second outlet  198  from the pump  175 . 
     Typically, in operation of the pump  175  the shaft  174  may be driven at a higher velocity (eg up to 7000 rpm) than the pump  2  shown in FIG.  1 . 
     In FIG. 4 of the accompanying drawings there is shown a cryogenic rotary pump  199  which delivers the second stream of cryogenic liquid at a pressure in the order of 10 bar. In this pump, there is also a single high pressure stage. Further whereas in the pump shown in FIG. 3 the inlet is at a level below those of the two outlets, in the pump shown in FIG. 4 the inlet is a greater elevation than the two outlets, that is the same configuration as in the pumps shown in the FIGS. 1 and 2. The individual components of the pump  199  and their operation are essentially the same as in the pump shown in FIG. 3, so will not be described further below. 
     Referring now to FIG. 5, there is illustrated part of an air separation plant which incorporates a cryogenic rotary pump  200  according to the invention. The plant includes a double rectification column  202  comprising a higher pressure rectification column  204 , a lower pressure rectification column  206  and a condenser-reboiler  208  placing the top of the higher pressure rectification column  204  in a heat exchange relationship with the bottom of the lower pressure rectification column  206 . The reboiling passages (not shown) of the condenser-reboiler  208  are of the downflow type. For ease of illustration, only a top section of the higher rectification column  204  and a bottom section of the lower pressure rectification column  206  are shown in FIG.  5 . The condenser-reboiler  208  is located in the sump  210  of the lower pressure rectification column  206 , above a volume of liquid oxygen which may be pure or impure. 
     In operation, nitrogen vapour separated in a higher pressure rectification column  204  is condensed in the condenser-reboiler  208  and at least part of the resulting condensate is returned to the higher pressure rectification column  204 . The condensation is effected by indirect heat exchange with liquid oxygen separated in a lower pressure rectification column  208 . The liquid oxygen collects in the sump  210  of the lower pressure rectification column  206 . The lower pressure rectification column  206  has an outlet  214  for the liquid oxygen communicating with the sump  210  and, via shut-off valve  212 , with the inlet to the pump  200 . The pump  200  has a first low pressure stage  220  and one or more high pressure stages  222 . There is a first outlet  232  for the liquid oxygen which communicates via flow control valve  234  with a header  236  at the top of the condenser-reboiler  208 . The liquid oxygen flows down the reboiling passages and a part of it is vaporised. The remaining liquid oxygen falls under gravity into the volume of liquid in the sump  210 . 
     The pump also has a second outlet  224  from the high pressure stage or series of high pressure stages  222  which communicate via a flow control valve  226  with a heat exchanger  228  which is employed to vaporise the liquid oxygen (assuming the oxygen is at a sub-critical pressure). Typically, the heat exchanger  228  may be either the main heat exchanger of the air separation plant and not only is the oxygen vaporised therein, it is also warmed to approximately ambient temperature. 
     If there is a single high pressure stage the pump according to the invention is typically able to raise the pressure of the liquid oxygen to a pressure of 20 bar. However, by using, two, or three to eight such stages, it is possible to produce oxygen at pressures of up to 60 bar or above. 
     Although one use of a cryogenic rotary pump according to the invention is in simultaneously pumping a first stream of liquid oxygen to the head of a downflow reboiler and a second stream of liquid oxygen to a higher pressure to enable it to be taken as an elevated pressure product, the cryogenic rotary pump may be put to any other use in which a cryogenic liquid is simultaneously required at two different pressures.