Patent Publication Number: US-6655932-B1

Title: Pressure impacted cooling and lubrication unit

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device for use with a subsea pump module which includes a housing, a pump disposed in the housing and having a pump inlet and a pump outlet, a pump shaft with mechanical seals and bearings for supporting the pump shaft in the housing, an electric pump motor arranged in the housing and having a motor shaft drive-connected to the pump shaft, which pump module has a pump motor lubrication/cooling circuit (a lubrication and coding circuit) exposed to the surrounding seawater and where the said pump inlet is at the side of the pump that is adjacent to the pump motor. 
     DESCRIPTION OF THE RELATED ART 
     One of the requirements for subsea devices, such as a subsea pump module as mentioned, is that they should have a useful life of maximum length and with full functioning ability, even after they have been out of service for longer or shorter periods of time. In this connection, the integrity and functioning ability of the lubrication and cooling circuit will be of vital importance. 
     Clearly, therefore, there is a need for a subsea device of the said type that is reliable and has a long operative useful life. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a device for cooling and lubricating a subsea pump module as mentioned, and a specific object of the invention is to provide barriers and pressure differences relative to the fluid pressure in the pump in order thereby to ensure that any leaks are in the direction of the pump. In this way, in the case of a subsea pump module, the pump medium is prevented from contaminating the lubricant and coolant. 
     Another specific object of the invention is to provide a device for cooling and lubricating a subsea pump module in the form of an autonomous passive lubrication and cooling system without the need for external control systems or instrumentation under normal operating conditions, apart from condition monitoring which requires instrumentation, and means for refilling lubricant/coolant. 
     According to the invention, a device is proposed for use with a subsea pump module which includes a housing, a pump disposed in the housing and having a pump inlet and a pump outlet, a pump shaft with mechanical seals and bearings for supporting the pump shaft in the housing, an electric pump motor arranged in the housing and having a motor shaft drive-connected to the pump shaft, which pump module has a pump motor lubrication/cooling circuit exposed to the surrounding seawater, and where the said pump inlet is at the side of the pump that is adjacent to the pump motor, characterised in that the lubrication/cooling circuit is pressure-impacted by the pump medium pressure in front of the pump inlet via a separating means pretensioned towards the pump motor, and that an impeller is arranged on the pump shaft at the end adjacent to the pump inlet. 
     In this way a favourable balance of pressure is obtained, and by means of a suitable layout of the lubrication-cooling circuit with an impeller drive-connected to the electric motor there can advantageously be obtained a certain overpressure in the lubrication/cooling circuit towards the pump across the mechanical seals. It is especially advantageous, according to the invention, for the lubrication/cooling circuit to be pressure-impacted by said medium pressure via a separating diaphragm. 
     A separating diaphragm of this kind will provide a physical separation of the lubricant/coolant and the pump medium at the external point of pressure application. 
     It is particularly advantageous to provide/increase the said pressure application using a weight load which acts in the direction of the lubrication/cooling circuit. 
     A weight load of this kind will ensure/provide an overpressure in the lubrication/cooling circuit relative to the medium pressure. 
     It is particularly advantageous for the weight load to be in the form of a liquid column. 
     A heavy liquid column of this kind can be provided in a simple manner by using a heavy, preferably inert liquid. 
     A particularly favourable embodiment according to the invention is one where the liquid column is provided between an upper and a lower separating diaphragm. 
     In this way a double barrier is obtained. 
     It is especially advantageous for each individual separating diaphragm to be provided with a damper piston. 
     In one practical embodiment, the damper piston has a control rod attached thereto which passes through a seal that will interact with the damper piston if and when the diaphragm and thus the damper piston move towards an end position upon loss of volume in the separating diaphragm means. The use of a damper piston of this kind will protect the diaphragm in the event of such loss of volume. 
     In another embodiment according to the invention, the liquid column may be separated from the said medium and from the lubricant/coolant in the lubrication/cooling circuit by a respective float valve. In this connection, it is particularly advantageous to split the liquid column into two liquid columns which are separated from one another by a lubricant/coolant column, which is bounded at one of the liquid columns by a float valve. 
     It is especially advantageous to provide a seal lubrication circuit for the pump. 
     This seal lubrication circuit can advantageously be connected to the lubrication/cooling is circuit in such manner that the seal lubrication circuit has a pressure that is lower than that of the lubrication/cooling circuit, but higher than the said medium pressure. 
     A particularly favourable embodiment is one where the seal lubrication circuit is connected to the lubrication/cooling circuit via a separator for separating water from the lubricant/coolant, the separator being connected to the lubrication/cooling circuit at some point in the cooler. 
     A design of this kind allows a slight suction pressure to be obtained in the direction of the seal lubrication circuit, so that it is automatically refilled from the lubrication/cooling circuit via the separator. 
     In one embodiment, the seal lubrication circuit can be connected to the lubrication/cooling circuit via a separating diaphragm that is weight-loaded towards the lubrication/cooling circuit. 
     According to the invention, when separating diaphragms are used, each individual separating diaphragm can advantageously be associated with an external level sensor. 
     It is especially advantageous for the lubrication/cooling circuit to include an impeller in the electric motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in more detail with reference to the drawings, wherein: 
     FIG. 1 is a purely schematic cross-sectional view of a subsea pump module having a device according to the invention; 
     FIG. 2 is a purely schematic cross-sectional view of a subsea pump module according to the invention; 
     FIG. 3 is a purely schematic partially cutaway view of a subsea pump module having a device according to the invention; 
     FIG. 4 shows a variant of the weight load system in the device according to the invention; 
     FIG. 5 shows another variant of the weight load system in the device according to the invention; and 
     FIG. 6 is another partially cutaway view of a subsea pump module having a device according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The pump module  1  shown in FIG. 1 has a pump and pump motor in a common pressure shell. The pump motor  4  is an electric motor having a motor shaft  3 , which by means of a flexible connection  5  is drive-connected to a pump shaft  6 , which carries three pump impellers  7 . The pump inlet is indicated by means of the reference numeral  9  and the pump outlet is indicated by means of the reference numeral  10 . The bearings for the motor shaft  3  are indicated by means of reference numerals  16  and  17 . 
     A lubrication/cooling circuit  2  is provided for the electric motor and includes a lubricant/coolant impeller  15  on the motor shaft  3 . The motor space, where the coolant circulates (the actual motor housing acts as a cooler) is, via a transverse drilled hole  28  in the motor shaft  3 , connected to a line  14  which runs to a diaphragm housing  20 , where there is indicated a separating diaphragm  21 , so that the diaphragm housing  20  is divided into two chambers. The separating diaphragm  21  is pretensioned in the direction of the line  14 , i.e., towards the electric motor  4 . In FIG. 1 the upper chamber is connection by a line  22  to the space around the flexible connection  5 , for pressure balancing. 
     The lubrication circuit  2  is connected to the pump inlet line  18  at  19  via the diaphragm housing  20  by line  23 , which opens into the lower chamber in the diaphragm housing  20 , i.e., in this case inside the diaphragm/bellows  21 . 
     A balance line  24  runs from the high-pressure end of the pump to the inlet line  18 . 
     Via the separating diaphragm  21 , the lubrication/cooling circuit will be pressure-impacted by the fluid pressure at the inlet of the pump. The pressure application via the separating diaphragm ensures a balancing of pressure, with a small pressure difference from the motor towards the pump via the bearings  25 ,  26  and a mechanical seal  27 . The pressure difference arises because the pressure falls in the pump inlet  9  relative to the pressure at  19  when the pump is running, and because there is an impeller function (not shown in any detail) in/at the seal  27 . 
     The system shown in FIG. 2 differs from the embodiment in FIG. 1 primarily in that an external cooler  11  is used. The same reference numerals are therefore used for corresponding parts. 
     The pump module  1  shown in FIG. 2 has a pump and a pump motor in a common pressure shell. The pump motor  4  is an electric motor having a motor shaft  3 , which by means of a flexible connection  5  is drive-connected to a pump shaft  6 , which carries two pump impellers  7 . The pump inlet is indicated by means of the reference numeral  9  and the pump outlet is indicated by means of the reference numeral  10 . 
     For the electric motor  3 ,  4  there is provided a lubrication/cooling circuit having an external cooler  11  which is connected to the motor through line  12 , connected to the motor at connecting piece  13  on the pressure shell  2 , and a line  14 , which runs to the other end of the electric motor. 
     There is a lubricant/coolant impeller  15  on the motor shaft  3 . This impeller  15  sends lubricant/coolant out through the connecting piece  13 , through the line  12 , the cooler  11  exposed to the surrounding seawater (the pump module  1  is a subsea device) and back to the motor through the line  14 . The bearings for the motor shaft  3  are indicated by means of reference numerals  16  and  17  respectively. The lubricant/coolant flows in a known way through the whole motor. 
     The lubrication/cooling circuit  12 ,  11 ,  14  is connected to the inlet line  18  of the pump at  19 , via a diaphragm chamber  20 , containing a pretensioned separating diaphragm  21 . As shown, a line  22  runs from the diaphragm chamber  20  to the space around the flexible connection  5 , for pressure balancing. 
     A line  23  runs from the high-pressure end of the pump to the inlet line  18  of the pump, at  19 . A line  24 , connected to the line  23 , runs from the end of the pump shaft  6   
     The lubrication/cooling circuit will be pressure-impacted by the fluid pressure at the inlet of the pump via the separating diaphragm  21 . Pressure application via the separating diaphragm  21  ensures a balancing of pressure, with a small pressure difference from the motor towards the pump via the bearings  25 ,  26  and a mechanical seal  27 . The pressure difference is caused in that the pressure, as mentioned above in connection with FIG. 1, falls in the pump inlet when the pump is running, and in that there is an impeller function in/at the seal  27 . 
     The system shown in FIG. 3 includes a pump module  31  which has a pump and a pump motor in a common pressure shell  32 . The pressure shell  32  is in this case in the form of a substantially cylindrical body with open ends and an inner through passageway between these ends. The passageway is designed and sized to slidably and snugly receive respectively a pump insert  33  and an electric pump motor  34  from a respective end. The substantially cylindrical pressure shell  32  has a casing having two openings  35 ,  36  associated with respectively an inlet and an outlet in the pump insert. The pump insert inlet is arranged at the end of the pump insert  33  facing the pump motor  34 . 
     The illustrated pump insert  33  is a centrifugal pump, with a rotor or pump shaft  37 , whilst the pump motor  34  is, as mentioned, an electromotor, with a drive shaft  38 . The motor shaft and the pump shaft are drive-connected at  39  in a manner not shown in more detail. 
     The pump module  31  shown in FIG. 3 is intended for use as a water pump and is connected to a water inlet line  40  and a water outlet line  41  at the inlet  35  and the outlet  36  respectively. 
     The illustrated pump module  31  has a unique lubricating and cooling system which will now be explained in more detail. The inner space of the motor housing  32  is part of a bearing lubrication and motor cooling circuit, and the flange of a thrust bearing is, in a manner not shown in detail, made in the form of an impeller  42  which, when the motor is running, will pump lubricant from the inner space of the motor out through the outlet  43  and through an external cooler  44  and from there through a line  45  back to the other end of the motor space. Here, some of the lubricant will pass through the bearing at hand and then through the motor space to the other motor bearing and back to the impeller  42 . The rest of the lubricant will spread radially and cool the end windings in the motor. 
     A separate lubricant-carrying line  46 , which opens out between a mechanical seal and the bearing, runs to the pump shaft bearing at the high-pressure end of the shaft. A lubricant-carrying line  47  runs from the outside of the bearing to the area of connection between the passageway  43 , the cooler  44  and the line  46 , thus forming a circulation circuit for lubricant to the bearing at the high-pressure end. The circulation in the circuit formed by the lines  46 ,  47  is supported by a non-illustrated impeller formed by the pump It should be mentioned that the main impeller  42  and the non-illustrated impeller at the high-pressure shaft end are elements which will be well-known to the skilled person, and such impellers can be made using simple means, for example, with the aid of radial drilled holes in the flange  42  and small blade cut-outs on the end of the shaft  37 . 
     As shown, the line  47  is connected to a separator  48  for separating water which might have infiltrated into the lubricant. This water separator  48  may be made in the form of a sump where water can be deposited. 
     At each end, the pump has a known double mechanical seal. A separate circulation circuit is provided for lubricating these two double mechanical seals. The mechanical seal on the low-pressure side has an inlet  49 , only schematically indicated, which is connected to an external circulation line  50 , and an outlet  51 , also only schematically indicated, which is connected to an external circulation line  52 . Similarly, a mechanical seal arranged in the high-pressure end has an inlet and an outlet (not shown) connected to the lines  52  and  50  respectively. This lubrication circuit for the seals has a separate circulation which is provided by pump rings incorporated in the mechanical seals, in a known way per se. 
     When in operation, the seal lubrication circuit  50 ,  52  has slightly lower pressure than the bearing lubrication and motor cooling circuit, because it is connected to this circuit through a line  53  and a separating diaphragm means  54 . The separating diaphragm means  54  consists essentially of two hemispherical shells that are bolted together with a rolling diaphragm  55  fixed between them. The rolling diaphragm  55  is weight-loaded  56  and has a control rod  57  which passes through the seals  58  in the two hemispherical shells. The control rod  57  is in a manner not shown in detail made in the form of a damper piston, intended for interaction with the seals  58  if the diaphragm should sink to the bottom of the vessel formed by the spherical shells. The weight load  56  and the impeller function in/at the mechanical seal in the ends of the pump will be instrumental in providing the said lower pressure in the line  59  and thus in the circulation lines  50 ,  52 . 
     The pressure load of the lubrication/cooling circuit is provided via a line  60  from the circuit, in the embodiment in FIG. 2, from the inner space of the motor. The line  60  is connected to the inlet end  40  of the pump via a pressure compensation arrangement that comprises two diaphragm separating means  61 ,  62  having a weight column  63  connected therebetween, and a line  64  from the upper of the two diaphragm separating means. A balance line  64  runs from the pressure or discharge side of the pump to the inlet or suction side. 
     The lower diaphragm separating means  61  comprises a rolling diaphragm  65  fixed in a two-part vessel and having a damper piston/control rod  66  which passes through a seal  67 . The upper separating diaphragm means  62  also has a rolling diaphragm  68  fixed in a two-part vessel and a damper piston/control rod  69  running through a suitable seal  70 . 
     Any liquid loss in the cooling circuit and the seal circuit can be detected by reading the position of the respective control rod with the aid of an external level gauge  71  and  72  respectively. Each control rod  66 ,  57  may, for example, have a magnetic or radioactive device for this purpose. 
     A pipe  63  runs between the two separating diaphragm means  61 ,  62 . In the embodiment in FIG. 3, this pipe  63  is filled with an inert oil having a density that is considerably greater than that of the pump medium (water). As an example, the density of water is about 1000 kg/m 3 , so as inert oil in the pipe  63 , it would be advantageous to use an oil which has a density of about 2000 kg/m 3 . The lubricant has, for example, a density of 870 kg/m 3 . Needless to say, heavy media and lubricants other than oil may be used. 
     If, for example, there is a pressure of 90 bar in the pump inlet  35  and a pressure in the pump outlet  36  of 140 bar, the illustrated and described system will be able to operate with a pressure on the diaphragm  68  of just below 90 bar. The pressure on the diaphragm  65 , in the lower separating diaphragm means  61 , will be just above 90 bar. When the motor shaft  38  runs at 3600 rpm, there will, for example, be a pressure of about 97 bar in the outlet  43  associated with the impeller  42 . As a consequence of the connection to the lubrication/cooling circuit via the diaphragm separating means  54 , the prevailing pressure in the seal lubrication circuit (the lines  50 ,  52 ) will be slightly lower under running conditions, about 96 bar. 
     When the pump module  31  has been installed, the interior of the pump will have contact with the surrounding pressure via inlet  35  and outlet  36 . This means that the motor and the lubricant circuits are pressure-balanced against the surroundings. The inert oil system (the line  63 ) will ensure that there is always a slight overpressure towards the pump, i.e., across the mechanical seals at the drive end of the pump. 
     When the pump module is stationary, where the pump module  31  is connected to inlet and outlet lines, there are similar pressure conditions. The motor and the lubricant circuits are pressure-balanced against the process system, i.e., the inlet and outlet lines. The compensation system with the liquid column of inert oil in the pipe  63  will ensure that there is always a slight overpressure towards the pump. 
     When the pump module is pumping, and the motor is running, the pump will be pressurised in conformity with the pump inlet pressure. The main impeller  42  will increase the pressure towards the mechanical seals in the pump and circulate the lubricant (oil) through the cooler  44  and back to the motor, i.e., the inner space of the motor. 
     The pump bearing at the high-pressure side of the pump is supplied with lubricant by means of the action that the pump shaft end produces, since, as mentioned, this shaft end is made in the form of an impeller. 
     The lubrication circuit for the seals will have a pressure that is lower than the pressure around the mechanical seal at the ends of the pump. This is due to the effect of the impeller function in/at the mechanical seal and the compensator system including the separating diaphragm means  54 . 
     Refilling the lubrication/cooling system can take place with the aid of two valves  73 ,  74 . Refilling can take place through these valves from an external source, for example, an oil-filled pressure tank connected to the pump module or a so-called umbilical from a platform. 
     The pump medium, in the embodiment in FIG. 3 water, in particular so-called produced water, will in the event of a leak in the diaphragm  68  be unable to penetrate through or mix with the inert oil in the pipe  63 . 
     In the event of a leak in the diaphragm  65  arranged closest to the motor, the inert oil will penetrate into the lowermost part of the line  60  which runs to the motor. An appropriate dimensioning of this line will prevent the inert oil from penetrating into the motor. 
     A leak in the diaphragm  55  in the separating diaphragm means  54  will cause a slight increase of pressure, accompanied by increased leakage. Loss of this barrier might mean that the motor is contaminated with water. This can only happen if water leaks past the seal facing the pump medium. This will normally only happen in the event of adverse pressure transients or substantial damage to the seal. If water infiltrates into the lubrication/cooling circuit of the motor, the water separator  48  will effect a change of such water. The water separator, made in the form of a simple sump, should have a capacity of several litres. Water that is collected in the water separator will not have any consequences for the operation of the pump module. 
     Leaks through the seals will call for refilling, but this has no negative consequences as long as the refilling system is dimensioned and designed for such refilling. Leaks inside the seals will result in a need for refilling in the seal lubrication circuit. This will not have any effect on the operation of the pump module as long as the refilling system (through the valves  74 ) is dimensioned and designed for necessary refilling. Seals of this type always have a normal leakage that is very small. What is special about the arrangement in this case is that the buffer medium which circulates between the seals is continuously replaced in that it leaks into the pump whilst the motor medium always leaks into the buffer circuit. 
     Normally, the buffer medium leaks in both directions. If the buffer medium has been contaminated with water, it will also leak into the motor. 
     FIG. 4 shows another possible design of the weight load system in the lubrication/cooling circuit. In FIG. 4 only the actual pump module and the weight load system are shown. The same reference numerals are used as in FIG. 3 for the same components. In the embodiment shown in FIG. 4, between line  64  and line  60  a length of pipe  75  has been installed between two float valves  76  and  77 . In this length of pipe  75  there is provided a heavy medium which thus separates the motor coolant (in the line  60 ) from the pumped medium (in line  64 ). As both the motor coolant and the pump medium are lighter than the heavy medium in the length of pipe  75 , they will float on the top. In the length of pipe  75  there is provided a gas and liquid trap  78  approximately halfway along the length of pipe  75 . 
     FIG. 5 shows yet another variant of the weight load system. Here too, only the actual subsea device (the pump module) with the weight load system are shown, and the same reference numerals are used as in FIG. 3 for the same components. 
     In the embodiment in FIG. 5, between the line  64 , which carries pump medium, and the line  60 , which carries coolant, a length of pipe  79  has been installed which has a float valve  80  arranged approximately halfway. In the line  60  there is coolant/lubricant all the way up to a vessel  81 . The length of piping going out from the upper end of the float  80  is filled with coolant all the way up to a second vessel  82 . From there the length of pipe  83  is filled with a heavy medium, up to a third vessel  84 . 
     The level of liquid in each of the vessels  81 ,  82 ,  84  is indicated by means of a transverse line. The heavy medium in all three embodiments in FIGS. 3,  4  and  5  is inert liquid which does not react with either the coolant or the pump medium. 
     FIG. 6 shows yet another variant of the invention. Here too, the same reference numerals as those used in FIG. 3 are used for the same components. 
     In contrast to the embodiment in FIG. 3, instead of the lower diaphragm separating means  61 , a vessel  85  is used where the heavy medium in the line  63  and the lubricant/coolant in the line  60  meet. 
     In the embodiment in FIG. 6 the diaphragm means  54  from the embodiment in FIG. 3 has been omitted. Instead the reference pressure is taken out after (or at a point on) the cooler  44 , as shown by the lines  86  and  87 , and the refilling line  59  is connected via a separator  88 . In the separator  88  water is separated out. The arrangement is such that there is a slightly lower pressure in the line  59 , so that the water can be sucked up from the separator  88  through the line  59 . Water that is sucked in through the line  59  will disappear in the pump. This gives a continuous cleaning of the system. 
     It may be desirable to have a local increase in pressure at the seals in the pump with the aid of an impeller (not shown). 
     The diaphragm separating means  62  is provided with a heat exchanger  89 . Heat comes from a branch  90  of the drainage pipe  65 . The water there will be a few degrees hotter than the temperature in the inlet line  40  because of hydraulic loss in the pump  33 . 
     In a pump medium containing hydrocarbons there is often the possibility of the deposit of waxy substances. An increased temperature reduces or prevents this. 
     It is advantageous to insulate the diaphragm separating means  62  against the sea in order to prevent great heat loss. Such insulation is not shown. 
     The diaphragm separating means is filled with an inert oil having high specific gravity. The oil may, for example, be a silicone fluid. The requirement for this medium is that it has a considerably higher specific gravity than the pumped medium and the substances this medium contains, and will be capable of being deposited in the diaphragm separating means  62 . The medium must also be inert to these. This provides an extra barrier against the coolant/lubricant and also ensures that deposits do not block the diaphragm movement, but will float on top of the inert medium. 
     The refilling of the system in FIG. 5 can be carried out through the line  91 . 
     In the embodiment in FIGS. 3 to  6  liquid columns are used as weight loads. In principle, the same effect can be achieved by weighting down the diaphragms. Pretensioned diaphragms are also a possibility. However, it is preferable to use a heavy inert medium because this gives the advantage that there is an extra barrier which will be virtually unaffected in the event of a leak. 
     The invention is not limited to the illustrated design of the pump module. Thus, the invention is generally useful for all subsea devices which have an electric machine and a fluid machine drive-connected thereto.