Patent Publication Number: US-6220037-B1

Title: Cryogenic pump manifold with subcooler and heat exchanger

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to manifolds for pumps used in handling cryogenic liquids, and more particularly, to a cryogenic pump manifold that utilizes a subcooler and heat exchanger to cool liquid entering the pump. 
     2. Description of the Prior Art 
     Cryogenic liquids are those which must be greatly refrigerated to keep them in a liquid state under reasonable pressures. Liquid nitrogen is one example. Various equipment has been developed for the handling and storage of such liquids, including pumps for transferring the liquid from one location to another, such as from a storage container to another area in which the liquid will be utilized. One type of pump which has been used for this purpose is a reciprocating piston or plunger pump, such as the Halliburton Triplex pump. Typically, an inlet or suction manifold is mounted on the pump connecting the pump suction to a source of the cryogenic liquid. It is desirable to provide the coldest possible cryogenic liquid to the pump inlet because this is necessary to meet the most efficient net positive suction head (NPSH) requirements of the pump. In all cryogenic pumps, the lower the suction fluid temperature the better will be the overall performance of the pump. 
     Insulating the suction manifold and inlet piping for the pump keeps the incoming liquid cool. This has the limitation of the capabilities of the insulation depending upon ambient conditions, and, of course, provides no additional cooling. One device which has been developed and which has had success in providing some cooling is a cryogenic subcooler on the pump inlet. A cryogenic subcooler is a device that takes the pressurized cryogenic liquid and uses a portion of it to produce a low temperature within the subcooler. This subcooler temperature is lower than the inlet liquid temperature because the portion of the inlet liquid that is released from the liquid flow to the subcooler is passed through an expansion device. This expansion usually causes the liquid to evaporate or “flash.” The expansion and evaporation of the liquid into a gaseous state causes the temperature to drop and lowers the subcooler temperature. The lower temperature expanded gas reduces the temperature of the pressurized inlet liquid entering the pump, producing a refrigerated or “conditioned” liquid. 
     Previous subcoolers may not be able to create enough heat transfer in some cases, so that the liquid entering the pump is not adequately cooled to meet the pump NPSH requirements to obtain optimum pump performance. Therefore, there is a need for greater subcooling. 
     The present invention solves these problems by incorporating a heat exchanger in conjunction with the subcooler to increase heat transfer and provide more cooling of the liquid entering the pump through the suction manifold. 
     Another problem with inadequate subcooling is that prolonged ambient heat gain may mean that the pump cannot function for a long period of time. Therefore, there is also a need for more cooling to overcome this problem. The present invention addresses this in that the greater exchange of heat in the apparatus results in an elimination of, or at least reduction in, ambient heat gain that provides longer running periods for the pump. 
     An alternate embodiment of the present invention increases the cooling even more by increasing the evaporation of liquid through the subcooler and heat exchanger through use of a fluid ejector or jetting device. 
     SUMMARY OF THE INVENTION 
     The present invention is an inlet or suction manifold or system for a cryogenic pump. The manifold comprises a subcooler and a heat exchanger which uses expanded gas to cool the cryogenic liquid entering the suction of the pump. 
     The invention may be described as an inlet system for a cryogenic pump which comprises an inlet header connectable to an inlet of the pump and a heat exchanger having a cooling side and a coolant side. The cooling side is in communication with the inlet header. The apparatus further comprises an expansion device in communication with the inlet header and the coolant side of the heat exchanger, such that some cryogenic liquid may be flowed out of the inlet header to the expansion device, expanded or evaporated into a gas through the expansion device whereby a temperature of the gas is lowered, and flowed through the coolant side of the heat exchanger, thereby lowering a temperature of the cryogenic liquid flowing thorough the cooling side of the heat exchanger. 
     The heat exchanger is preferably a shell and tube heat exchanger. The tube side of the heat exchanger is the cooling side. The shell side of the heat exchanger is the coolant side. 
     The system further comprises a jacket disposed around the inlet header forming a subcooler, and the jacket is in communication with the expansion device and the coolant side of the heat exchanger. The jacket and shell side are preferably integrally attached, and the inlet header and the tube side are also preferably integrally attached. 
     The system further comprises a coolant outlet in communication with the coolant side of the heat exchanger through which evaporated gas may be discharged. In one embodiment, the gas is exhausted or vented through the coolant outlet to the atmosphere. 
     In an alternate embodiment, the system further comprises an ejector having an inlet port or fluid inlet in communication with the coolant outlet, a jetting port or inlet connectable to a secondary gas source, and an ejector outlet port or fluid outlet. The secondary gas may be air, another gas from a separate gas source, or waste gas vented from the pump. 
     The expansion device may comprises an orifice or may be characterized by other devices such as a valve. 
     The present invention may also be described as a method of cooling liquid flowing through a cryogenic pump inlet header, the method comprising the steps of (a) connecting a cooling side of a heat exchanger to the inlet header, (b) diverting a portion of the liquid through an expansion device, (c) expanding the gas through the expansion device and expanding the liquid into a gas, thereby reducing a temperature of the gas, and (d) flowing cooled gas from the expansion device through a coolant side of the heat exchanger such that liquid flowing through the cooling side thereof is cooled. Step (d) preferably comprises flowing the cooled gas through a shell side of a shell and tube heat exchanger and flowing liquid to the inlet header through a tube side of the heat exchanger. 
     The method may further comprise the step of (e) exhausting the gas from the heat exchanger. Step (e) may comprise venting the gas to the atmosphere and/or increasing exhausted gas flow using a gas ejector. Step (e) also may comprise connecting the ejector to a secondary gas supply. The secondary gas is preferably selected from the group consisting of air or nitrogen. The secondary gas may also be supplied by venting the secondary gas from the pump. 
     In the method, step (d) may additionally comprise flowing the cooled liquid through a subcooler in communication with the coolant side of the heat exchanger. 
     Numerous objects and advantages of the invention will become apparent as the following detailed description of the invention is read in conjunction with the drawings that illustrate such embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows the cryogenic pump manifold with subcooler and heat exchanger of the present invention connected in the inlet or suction piping of a cryogenic pump. 
     FIG. 2 shows a cross section of a first embodiment of the manifold exhausted to the atmosphere with a portion of the piping connections associated therewith and a second embodiment utilizing an ejector. 
     FIG. 3 is a cross section taken along lines  3 — 3  in FIG.  2 . 
     FIG. 4 is a third embodiment of the invention which is a variation of the second embodiment but in which the ejector is connected to waste high pressure gas vented from the pump. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and more particularly to FIG. 1, the cryogenic pump manifold with subcooler and heat exchanger of the present invention is shown and generally designated by the numeral  10 . Manifold  10 , also referred to as an inlet or suction manifold or system  10  is shown schematically as a portion of inlet or suction piping  12  for a cryogenic pump  14 . Manifold  10  is well adapted for use with any cryogenic liquid, such as liquid nitrogen, and is not intended to be limited to any particular material. 
     Pump  14  is illustrated herein as a known piston or plunger pump, such as the Halliburton Triplex pump, but the invention is not intended to be limited to any particular pump configuration. Pump  14  is driven by a prime mover (not shown) and has a plurality of cylinders  16 , each of which is connected to a manifold outlet port  18  on manifold  10  by a pump inlet or suction line  20 . 
     The cryogenic liquid is supplied to an inlet  22  of manifold  10  through a supply line  24  which is a portion of suction piping  12  extending from a cryogenic storage tank  26 . A control valve  27  controls flow of cryogenic liquid from storage tank  26 . Other conventional items in suction piping  12  are omitted for clarity. 
     Referring now also to FIG. 2, the details of a first embodiment of manifold  10  will be discussed. Manifold  10  comprises an inlet or suction header portion  28  and a heat exchanger portion  30 . Heat exchanger  30  is preferably adjacent to suction header  28  as illustrated, but can actually be spaced therefrom and connected thereto by piping. 
     Heat exchanger  30  is preferably of conventional, shell and tube construction having a first, tube side  32  and a second, shell side  34 . Tube side  32  comprises a plurality of tubes  36  extending between an inlet end plate  38  and an outlet end plate  39 . See also FIG.  3 . Tubes  36  are integrally attached to end plates  38  and  39  such as by welding or braising. Shell side  34  comprises an outer shell  40  which extends between end plates  38  and  39  and is also integrally attached thereto such as by welding or braising so that a shell chamber  42  is formed therein. It will be seen by those skilled in the art that shell chamber  42  is not in communication with tubes  36 . 
     An inlet nipple or reducer  44  is attached to an inlet end  46  of heat exchanger  30  adjacent to inlet end plate  38 . Inlet nipple  44  is in communication with tubes  36  but is prevented from communication with shell chamber  42  by inlet end plate  38 . 
     An outlet nipple or reducer  48  is attached at one end to an outlet end  50  of heat exchanger  30  adjacent to outlet end plate  39 . Outlet nipple  48  is in communication with tubes  36  but is prevented from communication with shell chamber  42  by outlet end plate  39 . 
     The other end of outlet nipple or reducer  48  is attached to one end of a cylindrical portion  49  of suction header  28 , and the outlet nipple may be considered a part of suction header  28 . An opposite end of suction header  28  is closed by an end cap  52 . An access port  54  may be attached to end cap  52  to provide access to suction header  28 , as necessary, such as for instrumentation. Previously mentioned manifold discharge ports  18  are attached to cylindrical portion  49  of inlet header  28 , forming an integral portion thereof. It will be seen that discharge ports  18  on manifold  10  are in communication with storage tank  26  through supply line  24 , inlet  22 , inlet nipple  44 , tube side  32  of heat exchanger  30 , outlet nipple  48  and inlet header  28 . In other words, cryogenic gas in storage tank  26  is communicated to pump suction lines  20 . 
     Manifold  10  further comprises a subcooler portion  56  which is characterized in the preferred embodiment by a cylindrical portion  58  attached at one end to outlet end  50  of heat exchanger  30  and closed at the opposite end by an end cap  60 . Subcooler  56  thus substantially encloses cylindrical portion  49 , outlet nipple  48  and end cap  52  of suction header  28 . Thus, subcooler  56  may also be referred to as a jacket  56  defining a subcooler chamber  62  around suction header  28 . 
     Subcooler chamber  62  is in communication with shell chamber  42  of heat exchanger by means of a plurality of ports  63 . Ports  63  are preferably formed in the lower half of end plate  39  so that the cooled gas is forced into the lower part of shell chamber  42  to then flow up across tubes  36  to increase cooling. A single port  65  is defined in the upper portion of end plate  39  to act as a relief which prevents formation of a stagnant area of gas in the upper part of shell chamber  42 . Manifold discharge ports  18  extend through subcooler chamber  62  and cylindrical portion  58  of subcooler  56  but are not in communication with subcooler chamber  62 . Access port  54  extends through end cap  60  of subcooler  56  but also is not in communication with subcooler chamber  62 . 
     A header vent or expansion port  64  is attached to cylindrical portion  49  of suction header  28 , extending through subcooler chamber  62  and cylindrical portion  58  of subcooler  56 . Header vent port  64  is not in communication with subcooler chamber  62 . A subcooler vent or expansion port  66  is attached to cylindrical portion  58  of subcooler  56  and is in communication with subcooler chamber  62 . 
     A vent line  68  is connected between header vent port  64  and subcooler vent port  66  thereby placing the vent ports in communication with one another. An expansion device  70  is disposed in vent line  68 . Thus, as will be further described herein, a portion of liquid in suction header  28  will flow through vent line  68  to subcooler chamber  42  while being expanded, and correspondingly evaporated into a gas and cooled, through expansion device  70 . In the preferred embodiment, expansion device  70  is characterized by a known orifice, interchangeable with other orifices of various sizes. However, expansion device  70  may also be a controllable device such as a valve. 
     A gauge port  72  may be attached to cylindrical portion  58  of subcooler  56 . Gauge port  72  is adapted for connection with a vacuum gauge  74  and/or other instrumentation for monitoring vacuum and/or other conditions in subcooler chamber  62 . 
     An instrumentation port  76  is attached to inlet nipple  44  and is connectable to a gauge or instrument panel  78  by an instrument line  80 . 
     A shell outlet  82  is attached to shell  40  of shell side  32  of heat exchanger  30  and is in communication with shell chamber  42 . A heat exchanger outlet or discharge line  84  is connected to shell outlet  82 . See FIGS. 1 and 2. 
     In a first preferred embodiment, discharge line  84  is simply exhausted or vented to the atmosphere. As will be further discussed herein, this allows flow of vented gas through subcooler  56  and heat exchanger  30 . 
     A second embodiment is also illustrated in FIG.  2 . In this embodiment, discharge line  84  is not exhausted directly to the atmosphere. Instead, discharge line  84  is connected to a fluid inlet  86  of a fluid ejector or eductor  88  of a kind known in the art. Ejector  88 , which may also be referred to as a jetting device  88 , further has a fluid outlet  90  which is exhausted or vented to the atmosphere and a jetting inlet  92 . When a high-pressure secondary gas is supplied to jetting inlet  92  of ejector  88 , the flow rate of fluid therethrough is substantially increased. Such a high-pressure gas may be supplied from a separate gas source, such as a gas storage tank  94 , through a gas line  96 . Gas line  96  may have a control valve  98  therein. This gas can be any non-hazardous gas, such as air or nitrogen. FIG. 2 illustrates an example of gas line  96  for an air source. In this case, an air dryer  100  is disposed in gas line  96  to knock out moisture from the air stream. A heat exchanger  102  may also be included in discharge line  84  to warm the discharged gas as necessary to prevent freezing in ejector  88 . 
     Referring now to FIG. 4, a third embodiment of manifold  10  and its associated piping is shown. Actually, the third embodiment is a variation on the second embodiment in that the third also utilizes ejector  88 . In this case, a portion of pump  14  is connected to jetting inlet  92  of ejector  88  by a pump vent line. Thus, waste high pressure gas may be communicated or vented from pump  14  to jetting inlet  92 . 
     Operation of the Invention 
     In operation, manifold  10  is installed in one of the ways previously shown and described. The cryogenic liquid is flowed from storage tank  26  by opening control valve  27  in supply line  24 . Pump  14  is operated in a known manner. Thus, the cryogenic liquid will flow from storage tank  26  to the suction of pump  14 . Any of inlet piping  12 , including manifold  10  may have insulation installed thereon in a known manner. Such insulation is not shown in the drawings for clarity. 
     A portion of the liquid is vented from suction header  28  to subcooler  56  through expansion device  70 . As is well known, rapid expansion of a liquid into its gaseous state will result in a decrease in temperature thereof. The cooled gas passes through subcooler  56  and heat exchanger  30  and is discharged from manifold  10  through shell outlet  82 . Thus, cooled gas enters subcooler  56  and provides some direct cooling to suction header  28  and the cryogenic liquid flowing therethrough. Because the cooled, expanded gas also passes through shell side  34  of heat exchanger  30 , additional cooling is provided to the cryogenic liquid flowing to pump  14  though tube side  32  of the heat exchanger. In fact, because of the heat transfer efficiency of heat exchanger  30 , most of the cooling will be done in it rather than in subcooler  56 . 
     For the first embodiment, the gas discharged from manifold  10  is simply exhausted to the atmosphere through line  84  as previously described. The gas could also be scavenged by a compressor (not shown) or similar apparatus if it is undesirable to vent it to the atmosphere. 
     The first embodiment will provide significant cooling to the cryogenic liquid entering pump  14  which results in improvement in the performance of pump  14  by meeting, or coming close to, the NPSH requirements of pump  14 . This keeps the cryogenic liquid in its liquid state. 
     If additional cooling is desired, the use of ejector  88  may be incorporated as in the second and third embodiments previously described. In either case, the pressurized gas in gas line  96  enters jetting inlet  92  of ejector  88  with relatively high velocity which results in a significantly increased pressure drop of the cryogenic gas though the ejector. The general operation of ejectors is known. The increased pressure drop, of course, causes a greater and more rapid expansion of the cryogenic liquid in expansion device  70  so that it is even cooler as it passes through subcooler  56  and heat exchanger  30 , thereby further cooling the cryogenic liquid flowing to the suction of pump  14 . The pressure in a typical subcooler is approximately atmospheric, or 14.7 psia at sea level. For nitrogen, this pressure limits the temperature of the expanding gas to approximately −320 degrees F. The use of ejector  88  causes the exhaust pressure to drop below what is shown on vacuum gauge  74 . This lowered pressure will force the refrigerated gas temperature to drop well below −320 degrees F., and in turn, further lower the temperature of the liquid entering the suction of pump  14 . This enhancement not only improves the efficiency of manifold  10 , but also increases the ambient temperature range of operation for pump  14 . 
     It will be seen, therefore, that cryogenic pump manifold with subcooler and heat exchanger of the present invention is well adapted to carry out the ends and advantages mentioned as well as those inherent therein. While presently preferred embodiments of the invention have been shown for the purposes of this disclosure, numerous changes may be made n the arrangement and construction of the parts. All such changes are encompassed within the scope and spirit of the appended claims.