Method to avoid instable surge conditions with centrifugal compressors and centrifugal compressors provided with means for automatically applying such a method.

The present invention concerns a method to avoid unstable working conditions in centrifugal compressors .

In particular, the present invention concerns a method to avoid unstable working conditions in centrifugal compressors, which unstable working condition is better known among professionals as "surge" or "surging" conditions .

It is generally known that compressors are used to compress air, for example to drive compressed air tools or to compress gases to be used for example in chemical plants, turbines or natural gas pipes .

In a centrifugal compressor, the fluid to be compressed enters axially, the fluid being compressed in the impeller in a radial direction which is perpendicular to the rotating rotor shaft of the compressor, by means of a centrifugal effect.

As of the impeller the fluid enters what is called the diffuser, in which the fluid is slowed down, such that a static pressure is built up.

In the current compressors, two major flow instabilities occur which result in a high thermal and mechanical load on the compressor which may lead to mechanical failure.

The most important flow instability is what is called "surge", which produces a resonance of the medium resulting from a repeated acceleration and deceleration of the mass flow through the compressor system as a whole.

"Surge" occurs when the flow direction of the fluid in the impeller is suddenly and repeatedly reversed due to a too high pressure at the outlet of the compressor in case of a low flow rate.

When the pressure at the output of the impeller is lower than the pressure at the outlet of the compressor, the flow direction of the fluid will be reversed, such that the fluid flows into the impeller again.

By reversing the flow direction of the fluid, the pressure at the outlet of the compressor will drop again up to a point where said pressure is sufficiently low again to make the fluid flow in the normal direction again.

A compressor is in what is called a "surging" condition when these sudden, repeated reversals of the fluid's flow direction occur.

Major pressure fluctuations hereby occur in the compressor, and the compressor's rotor will start to vibrate violently.

Naturally, this condition is very disadvantageous to the compressor's life, and in particular to the compressor's bearings .

Another, although less disadvantageous unstable working condition of the compressor is what is called "rotating stall" .

In this unstable condition, the flow cannot follow the impeller vanes locally. As a result of separation, cells are formed in which the flow stagnates .

These "stall" cells rotate at a speed of 10-90 % of the rotational speed of the compressor's rotor shaft.

According to the present state of the art, methods are already known with which unstable conditions in compressors, in particular "surging" conditions in compressors can be detected, and these unstable conditions can be remedied through specific actions .

Usually, one or several parameters are measured during the detection procedure which, if they undergo an abnormally high change or follow a certain sequence during a specific length of time, indicate an unstable condition for the compressor.

Typical parameters that are measured are the pressure variations at the compressor's inlet and outlet, the flow rate through the compressor, the temperature at critical points, the power used by the engine driving the rotor shaft, etc.

A major disadvantage of these known methods, however, is

that the compressor is actually already in an unstable "surging" condition before any action is taken, which is disadvantageous because of the high pressure fluctuations occurring in the compressor, as well as the high vibration level of the rotor.

However, also other methods are known whereby unstable "surging" conditions in centrifugal compressors are effectively avoided.

These known methods consist in accurately measuring the working condition of the compressor, whereby it is made sure that this condition remains far enough from the danger zone in which any possible unstable working, in particular "surge", is observed.

Typically, what is called a "compressor map" is hereby made, whereby the flow rate through the compressor is traced on the abscissa and the pressure ratio which is realised in the compressor is traced on the ordinate.

In such a compressor map, the unstable working condition zone is defined by what is called the "surge" line. Above and to the left of this line, "surge" occurs, below and to the right of said line, the working condition of the compressor is stable.

So, these known methods consist in keeping the working conditions of the compressor sufficiently below the "surge" line.

However, a disadvantage of these known methods is that the above-mentioned "surge" line depends on different factors. Indeed, "surge" depends on the system as a whole of the compressor and its load.

For example, a "surge" line measured in a test environment during the manufacturing of the compressor may look different after the installation of the compressor in a client's practical application.

Moreover, due to ageing and contamination of the compressor, the "surge" line may shift in time.

Also manufacturing differences between compressors that are successively produced in series may result in different "surge" lines.

All these uncertainties result in that, in order to be able to work safely with the methods using a compressor map, the margin in relation to the "surge" line must be kept relatively large, which of course seriously restricts the scope of operation of the compressor.

In fact, setting a safe margin implies a missed chance for many machines whose actual "surge" is situated far from said margin.

The present invention aims to remedy one or several of the above-mentioned and other disadvantages.

To this end, the present invention concerns a method to

avoid an unstable working condition in centrifugal compressors, which unstable working condition is also known as "surge" or "surging" condition, whereby the method consists in measuring and/or calculating the forces on the bearings of the rotor; in detecting timely an exceptional imbalance of radial forces on the bearings which occurs , before the centrifugal compressor ends up in an unstable condition; and, when the above-mentioned exceptional radial imbalance is detected, changing the operational parameters of the centrifugal compressor so as to avoid "surge".

By means of extensive tests, the applicant of the present patent application found that, before a compressor ends up in an actual unstable working condition, an exceptional imbalance of radial forces on the bearings can be observed, typically some ten msec before the actual "surge" occurs.

This makes it actually possible to predict an unstable "surging" condition.

An advantage thereof is that one can take action in due time so as to avoid the actual unstable "surge" condition.

This is a great advantage compared to the known methods, where action is always taken when it is in fact too late, i.e. when the actual "surge" with the accompanying rise of pressure fluctuations and vibration levels in the compressor has already started, which of course is disadvantageous to the life of the compressor.

Another advantage of the method according to the invention

is that it makes it possible to have the compressor operating under a condition which is very close to the above-mentioned "surge" line, which considerably enlarges the operational scope of the compressor.

Indeed, with the method according to the invention, it is possible to predict unstable conditions before they actually occur, such that it is no longer necessary to set a very large safety margin for the operational scope of the compressor.

Moreover, such a method according to the invention offers a major advantage in that any shifting of the "surge" line, for example resulting from ageing or contamination, has no negative effect whatsoever on the working of the compressor.

For, by measuring and/or calculating the forces on the bearings while the compressor is operational, up-to-date information is constantly obtained which in case of an exceptional imbalance of radial forces indicates that the "surge" line is being approached, irrespective of whether the situation of the "surge" line has either or not changed .

Thus, this method can moreover be perfectly applied to compressors with manufacturing differences and which thus have different "surge" lines.

According to a preferred method of the invention, in order to support the rotor shaft of the compressor, use is made

of active magnetic bearings, whereby the forces on the bearings are measured and/or calculated by means of a direct or indirect measurement of the electric currents flowing through the coils of the magnetic bearings.

According to an even more preferred method of the invention, the above-mentioned active magnetic bearings are controlled by means of a control unit, whereby the forces on the bearings are measured and/or calculated by measuring the voltage of the control signal generated by the control unit, which control signal is proportional to the reactive forces generated by the magnetic bearings .

An advantage of this method according to the invention is that, by making use of active magnetic bearings, the forces on the bearings can be measured continuously and directly in a simple manner, which measurement is essential according to the method in order to predict any unstable working condition of the compressor.

According to an even more preferred method of the present invention, the radial forces on the bearings are measured and/or calculated according to two directions that are perpendicular to one another; the component of these forces which is synchronous with the rotational frequency of the rotor shaft is eliminated; and the amplitude and frequency of the remaining filtered force is analysed so as to determine whether there is an exceptional radial imbalance.

According to the most preferred method of the invention, the above-mentioned analysis of the remaining forces will

make clear whether there is an exceptional radial imbalance in case the amplitude of the remaining forces exceeds a certain minimum value and the frequency of the remaining forces is situated within a certain range, which is a fraction of the rotational frequency of the compressor's rotor shaft.

What makes the latter methods so interesting, is that the exceptional imbalance of radial forces which is observed, right before the "surge" occurs, is due to a phenomenon which can be compared to the above-mentioned second form of flow instability, namely what is called "rotating stall".

By eliminating the component of the radial forces which is synchronous with the rotational frequency of the rotor shaft, the observation will be more focussed on the phenomenon to be detected, i.e. the exceptional radial imbalance.

Indeed, the synchronous component of the observed forces is only a result of the small geometrical deviations of the rotor shaft that are always present, and thus of the natural imbalance of the rotor shaft.

If, however, the amplitude of the remaining filtered forces exceeds a certain minimum value and the frequency of said remaining filtered forces is within a certain range which is a fraction of the rotational frequency, this will indicate the presence of a "rotating stall" and thus of an exceptional radial imbalance of the bearing forces .

In this way can be predicted in a very accurate manner whether any "surge" will actually occur within a short time span.

The present invention also concerns a centrifugal compressor which is provided with means to automatically perform certain operations according to a method described above in conformity with the present invention so as to avoid an unstable working condition, in particular "surge", of the centrifugal compressor.

Such a centrifugal compressor is preferably provided with a measuring unit, a processing unit and a control unit, whereby the forces on the bearings of the rotor are measured and/or detected by means of the measuring unit, an exceptional imbalance of radial forces on the bearings occurring before the centrifugal compressor ends up in an unstable "surging" condition is detected in time by means of the processing unit and whereby the control unit, as soon as an exceptional radial imbalance has been detected, changes the operational parameters of the centrifugal compressor so as to avoid any "surge".

Such centrifugal compressors are advantageous in that they have a much wider operational scope than the known centrifugal compressors.

Moreover, these centrifugal compressors have a much longer life, since unstable "surging" conditions are constantly being prevented.

In order to better explain the characteristics of the invention, the following preferred methods to avoid unstable conditions in centrifugal compressors are described by way of example only without being limitative in any way, as well as a preferred embodiment of a centrifugal compressor provided with means with which such a method is automatically applied, with reference to the accompanying figures, in which:

figure 1 schematically represents a possibility for measuring the forces on the bearings of a centrifugal compressor, which is a first step of the method according to the present invention, whereby the particular case of a centrifugal compressor with active magnetic bearings is illustrated; figure 2 represents a typical variation of the radial forces on the bearings as a function of time during a time span of 500 msec, right before and while an unstable "surging" condition occurs in the centrifugal compressor; figure 3 is an enlarged detail of what is indicated in figure 2 by means of F3 ; figure 4 represents the same variation of the radial forces as a function of time as in figure 3 , but after the component of the forces that are synchronous with the rotational frequency of the centrifugal compressor's rotor shaft has been filtered out, which variation is preferably analysed in the second step of the method according to the invention so as to detect an exceptional imbalance of radial forces in time; and figure 5 represents an embodiment of a centrifugal

compressor according to the invention which is provided with means to automatically apply a method according to the invention for avoiding an unstable "surging" condition of the centrifugal compressor.

Figure 1 represents a centrifugal compressor 1 which is provided with a rotor shaft 2 on which has been provided an impeller 5 on both far ends 3 and 4.

The impellers 5 are provided close-fitting in a housing 6 which is provided on both far ends 3 and 4 with an inlet duct 7 on the one hand to suck in a fluid and with an outlet duct 8 on the other hand to discharge compressed fluid, and also with a diffuser 9 round the impellers 13 so as to build up a static pressure.

The rotor shaft 2 of the centrifugal compressor 1 is supported in the housing 6, such that it can rotate by means of a pair of bearings 10 and 11, which bearings 10 and 11 are in this case of the electromagnetic type.

Said active electromagnetic bearings 10 and 11 support the rotor shaft 2 without any mechanical contact, whereby the forces to support the rotor shaft 2 are generated in the magnetic coils of the bearings 10 and 11 in which an electric current is injected.

Normally, such electromagnetic bearings 10 and 11 are controlled by means of a control unit which determines how much electric current Il and 12 should be directed to the respective bearings 10 and 11 on the basis of the measured

position of the rotor shaft 2, but this control unit is not represented in the figure for simplicity's sake.

In its centre, the rotor shaft 2 is provided with the rotor 12 of an electric motor 13.

While the rotor shaft 2 and thus the impellers 5 rotate, fluid will be drawn in according to the axial direction AA ¹ .

Said fluid is accelerated by the impellers 5 in a radial direction RR ¹ , after which it leaves the impellers 5 so as to enter the diffuser 9.

As is known, said diffuser 9 is but a conduit for guiding the fluid flow, provided round the impellers 5 in the static housing 6, either or not with vanes, which have to make sure that the kinetic energy which is available in the fluid after said acceleration in the impeller 5, is transformed in potential energy by deceleration of the fluid, i.e. in a static pressure being built up in the fluid.

A user can then take off the compressed fluid under pressure via the outlet ducts 8.

As already explained in the introduction, it is generally known that, in case of low flow demands of compressed fluid, it is possible that the pressure at the outlet of the impeller 5 is smaller than the pressure at the outlet of the compressor 1.

As a result, the flow direction of the fluid will be reversed, after which the pressure at the outlet of the compressor starts to drop again until it is sufficiently low to make the fluid flow in the normal direction again.

Such repeated reversals in the flow direction of the fluid through the centrifugal compressor 1 produce unstable working conditions in the centrifugal compressor, generally known as "surge" or "surging" conditions.

"Surge" is accompanied by a tremendous increase of the pressure fluctuations in the centrifugal compressor 1 and by violent vibrations of the rotor shaft 2, which of course is very disadvantageous to the life of the centrifugal compressor 1 and in particular of the bearings 10 and 11.

The present invention concerns a method for avoiding said "surging" condition in centrifugal compressors 1, whereby it is also made sure that safety margins which are not too large can be implemented in order to maintain the operational scope of the centrifugal compressor 1 as large as possible.

A first step of the method according to the invention consists in measuring and/or calculating the forces on the bearings 10 and 11 of the rotor shaft 2.

In the example as shown in figure 1, this can be realised in a very simple manner.

Indeed, the amplitude of the injected currents Il and 12 is proportional to the forces exerted by the magnetic coils to support the rotor shaft 2.

In other words, by directly or indirectly measuring the electric currents Il and 12 flowing through the coils of the magnetic bearings 10 and 11, it is possible to measure or calculate the forces on the bearings 10 and 11.

In case a control unit is used to inject the electric currents Il and 12, which is usually the case, the forces on the bearings 10 and 11 can be measured and/or detected for example by measuring the voltage of the control signal which is generated by the control unit, which control signal is in this case proportional to the reactive forces generated by the magnetic bearings 10 and 11.

Determining the forces on the magnetic bearings 10 and 11 by measuring the currents flowing through the coils of said bearings 10 and 11 is advantageous in that no separate or special measuring sensors need to be provided, since one can simply use the magnetic bearings 10 and 11 that are already present.

Naturally, many alternatives are possible for realising this first step of the method according to the invention.

When no use is made of active magnetic bearings 10 and 11 to support the rotor shaft 2 of the centrifugal compressor 1, but for example conventional roller bearings are used instead, then the forces on the bearings can be measured

and/or calculated by means of strain gauges placed on the roller bearings .

The use of liquid or air bearings to support the rotor shaft 2 of the centrifugal compressor 1, such as for example oil sleeve bearings or hydrodynamic air bearings, is not excluded either.

In this case, the forces on the bearings can be measured and/or calculated by means of pressure sensors with which the pressure build-up in the fluid round the rotor shaft 2 can be measured.

The essence of the present invention relates to step two of the method, which step will now be illustrated by means of figures 2 to 4 included.

This step consists in analysing the forces on the bearings of the compressor 1 as measured and/or calculated in step one so as to detect timely the presence of an exceptional imbalance of radial forces on the bearings, which occurs right before the centrifugal compressor ends up in an unstable "surging" condition.

Indeed, extensive testing of the "surge" phenomenon has proven that it is always preceded by an exceptional imbalance of radial forces on the bearings 10 and 11.

It should be emphasized that this can be observed before the compressor ends up in the actual "surging" condition.

This is opposed to what happens in the known methods for avoiding "surge" in centrifugal compressors 1.

Figure 2 shows a typical example of the variation of the radial forces being exerted on one of the bearings 10 and 11 as a function of time right before and while a short "surging" condition occurs in the centrifugal compressor 1.

In the example of the figure, a sudden drop P of the radial force can be clearly observed after some 100 msec. A sudden rise in the signal corresponding to the radial forces would be possible as well.

This is what is called the exceptional radial imbalance of forces as mentioned above, which occurs before the centrifugal compressor 1 ends up in a "surging" condition.

This imbalance of radial forces is exceptional as it is different from the natural imbalance of radial forces which is always present due to small geometrical deviations or imbalance of the rotor shaft 2, which can be observed for example during the first 100 msec before the sudden drop P occurs .

Some 10 msec after the exceptional imbalance of radial forces occurs, the radial forces strongly increase all of a sudden during some 150 msec, after which the forces are reduced to zero for a very short period of time, to then increase again during some 150 msec.

During this double increase of forces, the centrifugal

compressor is in the actual "surging" condition, whereby the first increase in force is caused by the fluid flowing back to the impeller 5 and the second increase in force is the result of the flow resuming its original direction from the inlet 7 to the outlet 8.

Detecting timely the exceptional imbalance of radial forces, which occurs right before the compressor ends up in an actual "surging" condition, which is necessary according to the second step of the method according to the invention, can be done in a simple manner, as is clear from figure 2, by continuously measuring the radial forces and by observing a sudden increase or decrease of the measured or calculated radial forces on the bearings 10 and 11.

Figures 3 and 4 show in detail how the radial forces vary while the exceptional imbalance of the radial forces occurs during a short period of ± 50 msec, which will provide a better understanding of what actually happens and which will open the way to more accurate methods for determining the exceptional imbalance of radial forces .

The radial forces Fradl on one of the bearings 10 or 11 in a first direction, as well as the radial forces Frad2 of the same bearing 10 or 11 in a second direction perpendicular to the first direction were hereby traced as a function of time.

The first 5 msec of figure 3 show the normal working whereby the signal for Fradl as well as that for Frad2 constantly varies, be it synchronously with the rotational

speed of the rotor shaft 2 and for both signals with about the same amplitude.

This variation in Fradl and Frad2 is due to the natural imbalance caused by small geometrical deviations in the rotor shaft 2.

The two signals Fradl and Frad2 have a phase difference of 90 degrees which is due to the fact that the directions according to which the forces Fradl and Frad2 are measured are perpendicular to one another.

After some 5 msec, a sudden considerable difference in the amplitudes of both signals Fradl and Frad2 can be observed, and the rise and drop of the signals is no longer synchronous with the rotational speed of the rotor shaft 2.

In the present example, this situation lasts some 20 msec.

This observation corresponds to the short, sudden drop P of the signal seen in figure 2 , which corresponds to the exceptional imbalance of radial forces occurring right before the compressor 1 ends up in a "surging" condition.

By the term "sudden" is indicated a time span of preferably less than 25 msec here, and more in particular of less than 20 msec, or more specifically of less than 10 msec.

The term "exceptional imbalance of forces" hereby indicates that the radial forces acting on the bearings 10 and 11 are at least one and a half times larger than the radial forces

acting on said bearings 10 and 11 when the compressor 1 is idling, without any "surge" occurring. More preferably even, it refers to radial forces that are between one and a half and three times larger than the radial forces which act on the bearings 10 and 11 in case the compressor is idling.

The forces which act on the bearings 10 and 11 in the case of said "exceptional imbalance of forces" rotate at a speed of revolution which is only a fraction of the rotational speed of the rotor of the compressor 1, in particular a rotational speed which is smaller than twenty percent of the rotational speed of said rotor, and which is situated more specifically in a range between 5 and 20 percent of the rotational speed of the rotor of the centrifugal compressor 1.

After this period of 20 msec, there is again less difference between the signals' amplitudes, and the amplitudes will start to vary synchronously with the rotational speed of the rotor shaft 2 again. An increase of both signals' amplitudes is hereby observed. The latter is the beginning of the "surge" or of the first increase of the bearing forces , as shown in figure 2.

In short, another possibility to realise step two of the method according to the invention consists in measuring and/or detecting the radial forces on the bearings according to two directions which are perpendicular to one another, whereby an exceptional radial imbalance on the bearings is recognized on the basis of a sudden, strong

deviation between the amplitudes of the forces measured in said two dimensions.

Another possibility is that an exceptional radial imbalance on the bearings is recognized as the above-mentioned radial forces , measured in directions which are perpendicular to one another, contain a component which is not synchronous with the rotational speed of the rotor shaft.

Naturally, the different possibilities can be combined so as to come to a more precise finding.

An analysis in further detail of the deviation of the two signals Pradl and Frad2 will lead to the conclusion that the exceptional imbalance of radial forces on the bearings right before the actual "surge" is in fact no more or no less than a very short period of unstable operation of the compressor 1, albeit not so worse, i.e. of the "rotating stall" type.

As already explained in the introduction, "rotating stall" is a known phenomenon of flow instability which occurs in a limited section of the impeller whereby the flow can no longer follow the impeller.

This results in radial forces whose amplitude is larger than the natural imbalance of radial forces, which is observed as a result of small geometrical deviations in the rotor shaft.

Further, said radial forces vary according to a rotational

speed which is lower than the rotational speed of the rotor shaft 2.

The phenomenon as observed in figure 3 is indeed of the same nature, which will be illustrated by means of figure 4.

Figure 4 is a copy of figure 3, representing in bold over figure 3 the signal whereby, in the short period before and while the exceptional radial imbalance occurs, the component of the radial forces Fradl and Frad2 , which is synchronous with the rotational frequency of the rotor shaft 2, was eliminated.

This elimination of the synchronous component implies that the image focuses on the phenomenon which actually occurs, whereby the natural imbalance of radial forces on the bearings 10 and/or 11 resulting from small geometrical deviations was filtered out.

The first Fradl signal has a sinusoidal pattern now, with a rising and descending amplitude which is significantly larger than that of the natural imbalance of forces, however, and with a frequency which differs from the rotational speed of the rotor shaft 2.

The second Frad2 signal has an almost analogous amplitude variation, although with some delay, whereby the phase shift amounts to 90°, which results in a cosinusoidal pattern for the same above-mentioned frequency which differs from the rotational speed of the rotor shaft 2.

The frequency of the first Fradl signal and of the second Frad2 signal as observed is clearly smaller than the rotational speed of the rotor shaft 2.

Both signals are in other words the expression of a force, according to two directions that are perpendicular to one another, with an amplitude which is significantly larger than the amplitude of the signal of the natural imbalance of radial forces, which force rotates round the rotor shaft 2 at a frequency which is lower than the rotational speed of the rotor shaft 2.

This is what is characteristic of the known flow instability phenomenon "rotating stall".

In short, we can say that the strength of the invention resides in the discovery of the fact that, before a centrifugal compressor 1 ends up in a "surging" condition, there is a very short period during which "rotating stall" is observed first.

By detecting this "rotating stall", "surging" can effectively be avoided according to the invention.

The illustration of figure 4 further makes clear that the possibility which is preferably applied in the second step of the method according to the invention consists in measuring and/or calculating the radial forces on the bearings according to two directions which are perpendicular to one another; in eliminating the component

of these forces which is synchronous with the rotational frequency of the rotor shaft and in analysing the amplitude and the frequency of the remaining filtered force so as to determine whether there is an exceptional imbalance of radial forces as a result of "rotating stall".

Preferably, it will be decided that an exceptional radial imbalance is present when the amplitude of the remaining filtered force exceeds a certain minimum value and the frequency of the remaining filtered force is situated within a certain range, which range is a fraction of the rotational frequency of the rotor shaft 2 of the compressor 1.

The above-mentioned minimum value, as well as the exact range in which the frequency should be situated, will preferably be determined by means of measurements .

The final step of the method according to the invention consists in taking the necessary measures, after the timely detection of an imminent "surging" condition of the centrifugal compressor 1 through the observation of an exceptional imbalance of radial forces on the bearings, by changing the operational parameters of the centrifugal compressor, such that an unstable "surging" condition of the centrifugal compressor is effectively prevented.

To this end, there are different possibilities. A first possibility is illustrated by means of figure 1, whereby the electric motor 13 is provided with a mechanism to adjust the rotational speed of the electric motor 13.

Such a mechanism may for example consist of what is called a variable frequency drive (VFD) , whereby the rotational speed of the motor 13 can be adjusted by changing the frequency of the supply voltage of the electric motor 13.

In the third step of the method according to the present invention, the aim is to change the rotational speed in time, after an exceptional imbalance of radial forces has been detected, which is an indication for an imminent "surge" .

By changing the rotational speed, the flow will change at the impeller 5 and "surge" can be avoided.

If the rotational speed is temporarily increased, the energy supply from the impeller 5 to the fluid will increase and the local, separated zone as established by the exceptional imbalance of radial forces, which appeared to be a case of "rotating stall", will disappear.

It should be noted that everything must be done fast, since only a few tens of msec elapse between the time an exceptional imbalance of radial forces is observed and the moment the ensuing "surge" occurs.

In other words, the rotational speed must already have been altered in this period.

When "surge" is effectively avoided, the rotational speed can be reset to the original rotational speed afterwards .

A major advantage of this technique compared to the known method whereby use is made of a stationary compressor map

(see introduction) , is that the point of operation whereby "surge" is immediately anticipated is normally situated outside the tolerated operational scope.

Thus, the operational scope is in fact enlarged by means of an appropriate control of the compressor. The limits of this method are reached when the warning signals or detections of exceptional imbalance of radial forces succeed one another faster than the temporary changes of the rotational speed.

In the third step of the method according to the present invention it is also possible to change other operational parameters of the centrifugal compressor 1 in order to avoid "surge" .

A first alternative consists in altering the clearance in the compressor.

Another possibility consists for example in activating a local escape in the compressor.

Yet another possibility consists in altering the geometry of the centrifugal compressor.

This can be done for example by means of variable inlet vanes or variable outlet vanes, or a combination of both.

Yet another alternative consists in opening an exhaust valve in the compressor 1.

Naturally, many other alternatives are conceivable and combinations of the aforesaid examples are not excluded.

The invention also concerns a centrifugal compressor which is provided with means to automatically implement the method according to the invention in order to avoid an unstable working condition, i.e. "surge", of the centrifugal compressor.

Figure 5 schematically represents a possible embodiment of such a centrifugal compressor 1.

This centrifugal compressor 1 is provided with a measuring unit 14, a processing unit 15 and a control unit 16.

By means of the measuring unit 14, the forces on the bearings 10 and 11 of the rotor shaft 2 are measured and/or calculated.

The processing unit 15 continuously checks whether a possible exceptional imbalance of radial forces on the bearings 10 and 11 has occurred.

As soon as an exceptional radial imbalance has been detected, the control unit 16 changes the operational parameters of the centrifugal compressor 1 so as to avoid any "surge" .

In this actual case, the control unit 16 will force up the frequency of the VFD so as to obtain a higher rotational speed for the electric motor 13.

Naturally, many variations of the given examples are possible, as well as more detailed developments of the different units.

Thus, the invention is not restricted for example to compressors with two impellers, but the invention can be applied just as well with one or more than two impellers.

The invention is by no means limited to the embodiments of a method for avoiding an unstable condition in a centrifugal compressor described by way of example and represented in the accompanying drawings; on the contrary, such a method can be realised in many different ways while still remaining within the scope of the invention.

Likewise, the present invention is not restricted to the embodiment of a centrifugal compressor as described in the text and represented in the drawings, which is provided with means to automatically implement the method according to the invention; on the contrary, such a centrifugal compressor can be made in many different forms and dimensions while still remaining within the scope of the invention.