WITH ONLY ONE AXIAL THRUST BEARING

The present invention relates to a turbocompressor or a motor compressor, and particularly to an integrated motor compressor unit.

More particularly, the present invention relates to a motor compressor unit for subsea applications .

Motor compressor units o f the integrated type comprise a motor means, generally an electric drive motor, and a compressor having one or more compression stages, depending on the applications, the motor means and the compressor being mounted in a common casing that is sealed against the gas handled by the compressor unit.

Each compression stage comprises a bladed impeller mounted on a shaft driven by a rotor, which is driven by the motor means.

According to the state of the art, when used under special operating conditions, especially for underwater operations, the motor compressor is usually arranged vertically to facilitate draining o f the interior of the casing.

A compressor unit o f this type is already known from US 2009/0 220 362 in which, in addition, the shaft o f the compressor and the rotor driven by the motor means are coupled to form one common shaft that rotates around a vertical axis. The shaft and the rotor are coupled using a twistable shaft that transmits the rotational moments between the motor means and the compressor and, as far as possible, mutually decouples the driven shaft and the rotor from vibrations. Besides, in operation, the pressure difference on the opposite sides o f the compression stages generate a thrust on the compressor that is transmitted via the twistable coupling to the rotor and to a thrust bearing provided at one end of the common shaft, namely on the upper end of the o f the motor rotor. The common shaft being arranged vertically, only one common thrust bearing is required for the motor and the compressor.

The thrust is directed in opposition to the weight forces o f the common shaft, including the twistable coupling and the compression stages, such that a thrust balance is in addition carried out by the weight of the common shaft.

However, this type o f arrangement in which the rotor is axially supported by one thrust bearing has the disadvantage that the coupling between the rotor of the motor and the compressor shaft may be prejudicial to the accuracy of the axial position of the rotating elements o f the compressor which may impact the motor compressor efficiency, in particular during transient phases .

This problem may in particular incur at high loads, due to a thermal expansion o f the compressor shaft driven by the motor means and may arise for compressor units having a high pressure ratio , namely above 3 , and having an increased number o f compression stages, equal or higher than 3.

Other arrangements o f motor compressor units are known, having a common casing in which are mounted a compressor having one or several compression stages mounted on a compressor shaft and a motor having a rotor driving the shaft of the compressor around a vertical axis, and in which the compressor shaft and the rotor are assembled together using a rigid coupling to form a common shaft supported by three radial bearings . Such an arrangement does not permit, however, to increase the number of the compression stages such that the motor compressor unit cannot be equipped with more than three compression stages .

In view of the foregoing, one obj ect of the present invention is to alleviate the above-mentioned drawbacks of the motor compressor units according to the state of the art.

A further purpose of the present invention is to provide a centrifugal compressor unit for subsea app lications in which the problem o f axial displacement of the bladed wheels mounted on the compressor shaft are overcome.

The invention thus proposes a subsea centrifugal compressor unit, comprising :

-a motor having a rotor,

-at least one compressor having a shaft driven by the rotor, and -a set of bladed wheels fitted on the driven shaft.

According to a general feature, in operation, the motor, the compressor and the casing having the motor and the compressor are arranged horizontally, one axial thrust bearing only being in addition provided on a compressor side .

According to another feature of the invention, the rotor and the driven shaft form one common shaft, said compressor unit having a set o f three radial bearings for supporting said shaft, said set of radial bearings comprising two bearings being provided on the motor side and one bearing being provided on the compressor side. According to another embodiment, the rotor and the driven shaft are connected by means o f a flexible coupling that is tortionally flexible and axially unflexible, the compressor unit further comprising a set of four radial bearings for supporting each end o f the rotor and the driven shaft.

For example, the bearings, namely the axial bearing and the radial bearings for supporting the driven shaft or the rotor are magnetic bearings .

According to a further feature, the gas cooler is part of a closed loop extending partly outside the casing.

Besides, a seal may be provided between the motor, downstream the cooling means, and the compressor.

Advantageously, the casing is pressurized.

Other obj ects, features and advantages of the invention will become apparent from the fo llowing description, given only by way o f example and in view of the following drawings in which:

-Figure 1 illustrates the general features of a centrifugal compressor unit according to the invention; and

-Figure 2 illustrates a further aspect of a centrifugal compressor unit according to one embodiment of the invention.

Figure 1 shows a centrifugal compressor unit 1 according to the invention, for subsea applications, having a motor 2 and one or more compressor(s) 3 mounted in a common casing 4 leak tight to the gas manipulated by the compressor 3. The casing 4 comprises two casing elements fixed together by appropriate attachment means, the one, denoted by numeral reference 5 , accommodating the motor 2 and the other, denoted by reference 6, accommo dating the compressor. The casing elements are intended to provide support and protection for the motor and the compressor, respectively. As illustrated, the casing 2 is provided with a gas inlet orifice I facing the first compression stage of the compressor and an outlet orifice O facing the last compression stage. In operation, the casing element assembly is intended to be pressurized and immersed and is arranged on a base such that the compressor unit is arranged horizontally.

In the exemplary embodiment, the motor 2 is formed by an electric motor, for examp le a high rotation speed motor (6000 to 16 000 rpm) supplied by a frequency variator and comprising a stator 7 and a rotor 8 supported by two radial end radial bearings 9 and 10.

As illustrated, the compressor 3 comprises a shaft 1 1 driven by the motor rotor and supported by a set of two radial end bearings 12 and 13 and a set of bladed wheels 14 mounted on the driven shaft and defining each a compression stage for the compressor.

Therefore, the rotor of the motor turns in two radial end bearings 9 and 10, whereas the compressor shaft turns in two radial end bearings 12 and 13.

Advantageously, the bearings are rigidly fixed to the casing and are formed by active magnetic bearings that are controlled such that they can adapt to the dynamic behavior of the rotor or the driven shaft that they support. As illustrated, the rotor 8 and the driven shaft 1 1 are linked via a flexible coupling that is radially flexible and axially unflexible to dynamically reduce the amplitude of the noise generated by ventilation when the compressor is rotated while efficiently propagating an axial thrust generated by the bladed wheels to the rotor of the motor.

As a matter of fact, in operation, the pressure differences generated at each compression stage generate a thrust on the driven shaft which is transmitted to an axial thrust bearing 1 6, for example an axial active magnetic bearing that is provided at the free end o f the driven shaft 1 1 , namely at the end opposite to the rotor of the motor 2.

As illustrated, preferably, the axial bearing 16 is located on the compressor side, namely as close as possible o f the active portion o f the compressor shaft such that the axial bearing 16 is more appropriate to compensate for the thermal expansion o f the shaft and to more accurately guarantee the axial position of the driven shaft and of the rotating bladed wheels within the second casing element 6.

It should be appreciated that thanks to the use of the flexible coupling that is tortionally flexible and axially unflexible, each of the rotor and the driven shaft can keep it on vibration behavior while the thrust generated during operation of the completion stages is supported by one first bearing only provided on the compressor side o f the compressor unit.

In addition, with such an arrangement in which the rotor and the shaft are connected by a flexible coupling and having four radial bearings and one axial bearing, the compressor unit may comprise a large number of compression stages, namely above 3 , and provide a high pressure ratio , namely above 3. It has been noted that such a subsea compressor unit may provide a pressure between 80 and 150 bars .

For example, the compressor may comprise up to 8 compression stages and provide a pressure ratio up to 3 or 3 ,5. Referring now to figure 2, in one embodiment, the subsea centrifugal compressor unit may be provided with cooling means for cooling the motor as well as the bearings.

In one embodiment, the subsea compressor unit may be provided with a cooling system having a cooling loop lying between the first compression stage and one side o f the motor opposite to the compressor or lying between a both sides of the rotor, as illustrated, to provide cooling in closed loop .

In such a case, the cooling system comprises a gas coo ler 1 8 lying outside the casing 2 and therefore in a contact with the co ld deep water at constant temperature.

According to another embodiment, the cooling means may be arranged as an open loop with a clean gas supp ly 19 entering the first casing element 2 at one side of the rotor opposite to the compressor.

In such a case, the rotor may be provided with an additional bladed wheel 20 such that the clean gas supply is directed to the motor, through the radial bearing 9 and to the compressor through a barrier seal 21 provided between the flexible coupling 1 5 and the compressor.

Referring back to figure 1 , an additional axial thrust bearing 17 may be used, in such a case, to compensate for the thrust generated by the motor internal pressure drop to the cooling flow. At last, it should be noted that the invention is not limited to the disclo sed embodiment.

As a matter of fact, according to the exemplary embodiments disclo sed with reference to figures 1 and 2, the compressor unit comprises a rotor and a shaft supporting the compression stages that are linked using a flexible coupling and having four radial bearings for supporting the respective free ends of the rotor and of the driven shaft.

According to another embodiment, the compressor unit may comprise a common rigid shaft or a rotor and a stator that are linked using a rigid coupling, namely a coupling that is axially and tortionally unflexible.

With such an arrangement, the common shaft may be supported by two radial bearings on the motor side and by one radial bearing at the compressor side, with one axial bearing only. This arrangement also permits to have a large number o f compression stages, namely above 3 , and provide a high pressure ratio , for example above 3.