US20080080988A1

US20080080988A1 - Pump with electric motor, immersed in the fluid to be pumped

Info

Publication number: US20080080988A1
Application number: US11/580,160
Authority: US
Inventors: Fabien Wahl; Francois Danguy; Patrice Fayolle; Jean-Yann Luend
Original assignee: SNECMA SAS
Current assignee: Safran Aircraft Engines SAS
Priority date: 2006-09-28
Filing date: 2006-10-13
Publication date: 2008-04-03
Also published as: FR2906580B1; EP1908960A1; FR2906580A1; JP2008082318A; KR20080029772A

Abstract

Pump with electric motor, immersed in the fluid to be pumped Circuits for the auxiliary circulation of fluid in the pump for the axial balancing and the cooling of specific portions of the pump. The pump comprises a centrifugal region (28) with a fluid axial balancing system (60) and the supply circuit of this balancing system is independent of the or each fluid cooling circuit (90, 50, 48, 52, 86) of specific elements of the pump such as the motor (16) and/or a rolling bearing (38).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a pump with electric motor, completely immersed, including the motor, in the fluid to be pumped. A pump of this type is, for example, used for pumping a cold liquid such as liquefied natural gas wherein said liquid is used as a fluid for cooling specific elements of the pump, in particular the electric motor, and also for the dynamic axial balancing of the rotational parts. The invention relates, more specifically, to a new arrangement of circuits for the auxiliary circulation of fluid allowing these operations to be carried out.
2. Description of the Related Art
A type of pump is known in which the suction stage, located in the lower portion of a casing, is surmounted by an electric drive motor accommodated in the same casing. Said casing is designed to be plunged into the fluid to be pumped. For example, a pump of this type can be used for the pumping of liquefied natural gas contained in a tank. The pump is immersed into the liquefied gas, preferably generally vertically down to the bottom of the tank. A pump of this type is, for example, described in U.S. Pat. No. 3,652,186. The common shaft of the electric motor and the suction stage is supported by rolling bearings.
The suction stage is followed by a centrifugal wheel connected to the shaft and the structure of said wheel is utilised in order to form an axial balancing system defined by a circuit for fluid circulation, of the actual fluid to be pumped, provided between the centrifugal wheel and the casing. During operation, this balancing system relieves the mechanical rolling bearings. A small portion of the pumped fluid is therefore removed in order to feed this balancing system.
It is also known to remove pumped fluid in order to cool specific elements of the pump, in particular the electric motor and at least one mechanical rolling bearing, especially the main rolling bearing, located in proximity to the fluid balancing system.
Although the pump is able to operate in any position, it is particularly designed to be positioned vertically, with the suction stage at the bottom. This is why, for the remainder of the description, the terms “lower” or “upper” will be used, for example, in order to define the position of one element in relation to another.

SUMMARY OF THE INVENTION

The invention proposes to optimise the internal auxiliary circulations of fluid in the pump in order to improve the performance levels of the balancing system and those of the cooling stream.
The invention therefore relates primarily to an electric motor pump immersed in the fluid to be pumped, comprising a casing intended to be plunged into said fluid and accommodating, on the one hand, at least one centrifugal wheel and, on the other hand, a coaxial electric motor surmounting said centrifugal wheel and comprising a stator which is integral with said casing and a rotor which is integral with a shaft coupled to said centrifugal wheel, a first rolling bearing mounted between said shaft and said casing being installed between said centrifugal wheel and said motor, and an axial balancing system being defined by a fluid circulation circuit provided between said centrifugal wheel and said casing, characterised in that it also comprises at least one fluid cooling circuit, in particular for said motor and/or said first rolling bearing, and in that said supply circuit of said axial balancing system is independent of the or of each cooling circuit.
Under these conditions, there is maximum pressure available for each operation (dynamic balancing and cooling) and minimum temperature. These two conditions combined have been found to be important for effective operation of the axial balancing system and the cooling circuits. The invention also allows elimination of the risks of internal vaporisation (cavitation) of the fluid circulating in the auxiliary circuits defined hereinbefore.
Moreover, the various fluid circulation auxiliary circuits are easy to install in the pump.
Advantageously, the fluid cooling circuit of the electric motor passes through the air gap thereof.
According to one embodiment, the motor is installed in an inner jacket of the casing, defining in said jacket a lower chamber (between said first rolling bearing and the motor) and an upper chamber above the motor. The cooling circuit of the motor comprises inlet orifices formed in the wall of the inner jacket and establishing communication between an evacuation conduit downstream of said centrifugal wheel and the lower chamber. At least one discharge conduit communicating with the upper chamber is provided in order to reintroduce the fluid used for the cooling into the reservoir in which it is stored.
The axial balancing system comprises a high-pressure flow space defined between the casing and a lower (front) face of said centrifugal wheel and a low-pressure flow space defined between the casing and an upper (rear) face of this same centrifugal wheel, said low-pressure flow space being limited radially internally by an annular passage forming an axially variable flow restriction. Reinjection channels are provided in said centrifugal wheel between an annular discharge space and the inlet of the centrifugal wheel (where the fluid is at a lower pressure). The annular discharge space is defined around the shaft, radially internally in relation to said annular passage forming the variable flow restriction.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and further advantages thereof will become clearer in the light of the following description of a pump in accordance with the principle of the invention, given solely by way of example and with reference to the appended drawing, in which:

the single FIGURE shows a cross-section and elevation of the pump in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrated pump comprises a, generally cylindrical, casing 12 accommodating a suction stage 14 and an electric motor 16. Said motor comprises a stator 18 which is integral with the casing and a rotor 20 equipped with a drive shaft 22, the axis of which coincides with that of the suction stage. The shaft 22 is mechanically coupled to the suction stage 14 which comprises mainly an Archimedes' screw drive unit, referred to hereinafter as the impeller 26. The suction stage is surmounted by a centrifugal wheel 28 which is known per se. A diffuser 30, the fixed blades 31 of which are integral with the casing, is installed coaxially between the impeller 26 and the centrifugal wheel 28. Under normal operating conditions, the pump is designed to be positioned at least approximately vertically and immersed into the fluid to be pumped, with the end 34 of the casing that is closest to the impeller being positioned in the bottom portion. The pumping inlet 35 is defined in the vicinity of this lower end 34. Under these conditions, i.e. viewing the pump set up vertically, there is successively found in the casing, from bottom to top, the impeller 26, the diffuser 30, the centrifugal wheel 28 and the electric motor 16 driving the shaft 22. This shaft and also the rotating parts of the pump which are integral therewith are supported by two rolling ball bearings which are coaxial and set apart on either side of the motor: a first, main, rolling bearing 38 located between the suction stage 14 and the motor 16, more precisely just above the centrifugal wheel 28, and a second rolling bearing 40 located above the motor. For each rolling bearing, the inner cage is fixed to the shaft whereas the outer cage is mounted so as to slide in a bore in the casing 12 in such a way that all of the rotating elements of the pump are able to move in the longitudinal direction of the shaft.
Said first rolling bearing 38 is installed at the centre of a transverse wall 44 of the casing that separates the suction stage from the motor. The stator of the electric motor is integral with an inner jacket 46 of the casing, which jacket is coaxial with the shaft, in such a way that the air gap 48 of the motor, defined between the exterior of the rotor and the interior of the stator, establishes communication between a lower chamber 50 of said inner jacket (extending between said first rolling bearing and the lower end of the motor) and an upper chamber 52 of said inner jacket, extending between the motor and said second rolling bearing. During operation, said inner jacket is filled with the fluid to be pumped.
An annular conduit 54 communicating with the outlet 56 of the centrifugal wheel 28 extends around said inner jacket 46 and opens, at its lower portion, into a conduit 58 for evacuating the pumped fluid.
A dynamic balancing system 60 defined by a double circuit for the circulation of pumped fluid is provided between the centrifugal wheel and the casing. This balancing system comprises a high-pressure flow space 62 defined between the casing and a lower face of the centrifugal wheel and a low-pressure flow space 64 defined between the casing (more specifically, said transverse wall 44 in which said first rolling bearing is mounted) and an upper face of the centrifugal wheel.
The high-pressure flow space 62 defined between said casing and said lower face extends annularly between the high-pressure outlet 56 of the centrifugal wheel and the low-pressure inlet 66 of this same wheel. A first labyrinth seal 68 (or similar calibrated annular flow restriction) is provided between the innermost radial end of this high-pressure flow space and said low-pressure inlet. The inlet of this high-pressure flow space is located at the actual outlet of the centrifugal wheel, without flow restriction.
The low-pressure flow space 64 defined between said transverse wall 44 of the casing and said upper face of the centrifugal wheel extends annularly between a second labyrinth seal 70 (establishing calibrated communication with the high-pressure outlet of the centrifugal wheel) and an annular passage 72 forming a variable flow restriction, defined radially internally in relation to said second labyrinth seal 70.
This annular passage is formed between two mutually facing coaxial annular ribs: a rib 74 protruding from the upper face of the centrifugal wheel and a rib 76 protruding from the transverse face of the casing carrying said first rolling bearing. The annular passage forms a variable flow restriction owing to the fact that the shaft, to which the centrifugal wheel is fixed, is movable to a limited extent in its longitudinal direction, as indicated hereinbefore.
An annular discharge space 78 is defined radially internally in relation to the annular passage 72, i.e. between said passage and the shaft. Said space communicates with the low-pressure inlet 66 of the centrifugal wheel via reinjection channels 80 provided in this wheel. Said annular discharge space is thus at all times substantially at the low pressure prevailing at the inlet of the centrifugal wheel.
The axial balancing system operates as follows:
At rest, the mass of all of the rotational elements connected to the shaft causes the annular passage 72 to open to its maximum extent.
During operation, the respective positions of the labyrinth seals 68 and 70 in relation to the high-pressure flow space 62 and the low-pressure flow space 64, respectively, produce a rising force on the centrifugal wheel and accordingly on all of the rotational elements connected to the shaft. This force tends to reduce the opening of the annular passage 72, and this in turn tends to increase the pressure in the low-pressure flow space 64 and therefore to drive the centrifugal wheel back downward.
A balance is established, and this relieves the axial loading of the rolling bearing 38, the two annular ribs remaining set apart from each other by a variable distance producing a regulating effect.
Moreover, said second rolling bearing 40 holds the upper end of the shaft and is installed in an axial cavity 84 upwardly extending the upper chamber 52. At least one discharge conduit 86 (returning to the reservoir into which the pump plunges) communicates with this cavity and therefore with said upper chamber 52.
The or each discharge conduit is equipped with a flow rate calibration element 88.
In the described example, said second rolling bearing 40 is therefore installed between the upper chamber and the or each discharge conduit.
Inlet orifices 90 formed in the jacket 46 accommodating the motor establish communication between the annular conduit 54 for evacuating pumped fluid, at the outlet 56 of the centrifugal wheel, and said lower chamber. The pressure prevailing in said chamber is therefore substantially equal to the high pressure prevailing at the outlet of the centrifugal wheel.
The recess in the first rolling bearing 38 communicates with the annular discharge space 78 via a seal 92 forming a calibrated annular flow restriction.
According to an important characteristic of the invention, fluid cooling circuits (using a portion of the pumped fluid) are independent of the double fluid circulation circuit of the axial balancing system defined hereinbefore.
The fluid supply circuit of the balancing system is basically composed of the high-pressure flow space 62 and the low-pressure flow space 64 as defined hereinbefore. The low-pressure annular discharge space 78 plays no part in the balancing system. Furthermore, the cooling circuit of the motor is established between the inlet orifices 90 formed in the jacket and the flow restriction or restrictions 88 and therefore comprises the lower chamber 50, the air gap 48 of the motor, the upper chamber 52 (and the axial cavity 84), the discharge conduit or conduits 86 and the flow restriction or restrictions 88. The fluid is reintroduced at the inlet of the pump. Moreover, the cooling circuit of the first rolling bearing 38 is fed via the orifices 90 and comprises the lower chamber 50, the seal 92, the annular discharge space 78 and the reinjection channels 80 bringing the fluid back to the inlet of the centrifugal wheel.
Finally, the cooling circuit of the second rolling bearing 40 is the same as that of the electric motor 16. The cooling of the second rolling bearing is less crucial than that of the first rolling bearing and can therefore be combined with that of the electric motor.

Claims

1. Pump with electric motor, immersed in the fluid to be pumped, comprising a casing intended to be plunged into said fluid and accommodating, on the one hand, at least one centrifugal wheel (28) and, on the other hand, a coaxial electric motor (16) surmounting said centrifugal wheel and comprising a stator which is integral with said casing and a rotor which is integral with a shaft (22) coupled to said centrifugal wheel, a first rolling bearing (38) mounted between said shaft and said casing being installed between said centrifugal wheel and said motor, and an axial balancing system being defined by a fluid circulation circuit provided between said centrifugal wheel and said casing, characterised in that it comprises at least one fluid cooling circuit (90, 50, 48, 52, 86), in particular for said motor and/or said first rolling bearing, and in that said supply circuit of said axial balancing system is independent of the or of each cooling circuit.

2. Pump according to claim 1, characterised in that said fluid cooling circuit of said motor passes through the air gap (48) of said motor.

3. Pump according to claim 2, characterised in that said motor (16) is installed in an inner jacket defining therein a lower chamber (50) between said first rolling bearing (38) and the motor (16) and an upper chamber (52) above said motor, and in that the cooling circuit of the motor extends between inlet orifices (90) establishing communication between a conduit for evacuating said pumped fluid and said lower chamber and at least one discharge conduct (86) communicating with the upper chamber.

4. Pump according to claim 3, characterised in that the or each discharge conduct is equipped with a flow rate calibration element (88).

5. Pump according to either claim 3 or claim 4, characterised in that a second rolling bearing (40) is mounted between said shaft and said casing, in said upper chamber.

6. Pump according to claim 5, characterised in that said second rolling bearing (40) is installed between said upper chamber (52) and the or each discharge conduit (86).

7. Pump according to any one of the preceding claims, characterised in that said axial balancing system (60) comprises a high-pressure flow space (62) defined between said casing and a lower face of said centrifugal wheel (28) and a low-pressure flow space (64) defined between said casing and an upper face of said centrifugal wheel, and limited radially internally by an annular passage (72) forming an axially variable flow restriction, reinjection channels (80) being provided in said centrifugal wheel between an annular discharge space (78) and the inlet (66) of said centrifugal wheel, said annular discharge space being defined around said shaft, radially internally in relation to said annular passage (72) forming the variable flow restriction.

8. Pump according to claims 3 and 7 in combination, characterised in that said first rolling bearing (38) communicates with said annular discharge space (78) in such a way that the cooling circuit of said first rolling bearing extends between said inlet orifices (90) and the inlet of said centrifugal wheel via said annular discharge space (78).

9. Pump according to claim 8, characterised in that a seal (92) forming a calibrated annular flow restriction is inserted between the recess in said first rolling bearing (38) and said annular discharge space.