DESCRIPTION
TECHNICAL FIELD
Embodiments of the subject matter disclosed herein generally relate to methods and
equipments for detecting rotating stall in a compressor, in particular in a centrifugal
compressor.
m BACKGROUND ART
"Rotating stall", also known as "rotational stall", is a local disruption of airflow
within a compressor which continues to provide compressed fluid but with reduced
effectiveness.
Rotating stall arises when a small proportion of aerofoils experience aerofoil stall
disrupting the local airflow without destabilizing the compressor. The stalled
aerofoils create pockets of relatively stagnant fluid (referred to as "stall cells")
which, rather than moving in the flow direction, rotate around the circumference of
the compressor. The stall cells rotate with the rotor blades but at a lower speed,
affecting subsequent aerofoils around the rotor as each encounters the stall cell.
^ A rotating stall may be momentary, resulting from an external disturbance, or may
be steady as the compressor finds a working equilibrium between stalled and
unstalled areas. Local stalls substantially reduce the efficiency of the compressor
and increase the structural loads on the aerofoils encountering stall cells in the
region affected.
In many cases, however, the compressor aerofoils are critically loaded without
capacity to absorb the disturbance to normal airflow such that the original stall cells
affect neighboring regions and the stalled region rapidly grows to become a
complete compressor stall which is commonly known as "surge". If surge continues
and no action is taken to stop it, the rotor blades will be severely damaged and,
eventually, the whole compressor will be damaged.
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Therefore, it is important to try to avoid surge in a compressor.
US6092029 discloses a method and an apparatus for diagnosing rotating stall of a
rotating machinery by monitoring dynamic shaft precession of the machine and
comparing this precession with a standard one and altering the precession as the
machine approaches a destabilizing condition when indicated by the comparison
step. Axial vibration monitoring means is also provided for monitoring and
comparing a dynamic axial vibration of the machine with that of a standard one and
altering the axial vibration as the machine approaches a destabilizing condition
when indicated by the comparison step. Furthermore, the complex dynamic stiffness
A of the machine is measured and the direct dynamic stiffness and the quadrature
dynamic stiffness are computed for use as a destabilizing warning.
US6532433 discloses a method and an apparatus for continuous prediction,
monitoring and control of a compressor health via detection of precursors to
rotating stall and surge; at least one sensor is operatively coupled to the compressor
for monitoring at least one compressor parameter; according to the embodiments, a
plurality of sensors are disposed about the casing of the compressor for measuring
dynamic compressor parameters such as, for example, pressure, velocity of gasses
flowing through the compressor, force, vibrations exerted on the compressor casing;
a system is connected to the sensor for computing stall precursors. According to an
^ embodiment, compressor data are measured as a function of time, FFT is performed
on the measured data and changes in magnitudes at specific frequencies are
identified and compared with baseline compressor values.
US2004/0037693 discloses a system and method for detecting rotating stall in a
centrifugal compressor, particularly in the diffuser region of a centrifugal
compressor. The process begins with the detection or sensing of acoustic energy
associated with the onset of rotating stall. A pressure transducer is placed in the gas
flow path downstream of the impeller, preferably in the compressor discharge
passage or the diffuser, to measure the sound or acoustic pressure phenomenon.
Next, the signal from the pressure transducer is processed either using analog or
digital techniques to determine the presence of rotating stall. Rotating stall is
detected by comparing the detected energy amount, which detected energy amount
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is based on the measured acoustic pressure, with a predetermined threshold amount
corresponding to the presence of rotating stall.
US2010/0296914 discloses a stall and surge detection system and method for a
compressor. The system comprises a vibration monitor that monitors radial
vibrations, axial vibrations and axial displacement. According to a first embodiment,
radial vibrations in one fixed and predetermined frequency bandwidth based on the
minimum operating rotating speed of the rotor of the compressor, specifically from
2.5 Hz to 45 Hz, are monitored for detecting incipient surge, i.e. rotating stall.
According to a second embodiment, using a tracking filter, tracked to the rotational
Jfe frequency of the rotor of the compressor, radial vibrations in the range of
frequencies from e.g. 5% of the rotational frequency to e.g. 90% of the rotational
frequency are monitored for detecting incipient surge, i.e. rotating stall.
WO2009/055878 discloses a method to avoid instable surge conditions with
centrifugal compressors. The method provides to measure and/or calculate forces on
the bearings of the rotor of the compressor, and to detect timely exceptional
imbalance of radial forces on the bearings which occurs before the centrifugal
compressor ends up in an unstable condition. According to one embodiment, the
component of the radial forces which is synchronous with the rotational frequency
of the rotor is eliminated.
^ Therefore, there are solutions in the prior art that detect one or more indicators of
an incipient surge in a compressor; some of these known solutions monitor the axial
vibration of the compressor.
Anyway, there is still a need for a solution to the problem of detecting incipient
surge that is accurate, simple and flexible.
SUMMARY
Aspects to the present invention relate to methods and equipments for detecting
rotating stall in a compressor, in particular in a centrifugal compressor.
Rotating stall is considered an indicator of incipient surge.
4
Rotating stall is determined by measuring radial vibration of the compressor
(rotating) rotor relative to the compressor (static) stator that is usually integral with
the compressor casing; it is to be noted that both the stator and the rotor are
typically subject to both radial and axial vibrations. The present invention is
applicable also when the compressor comprises more than one rotor, as explained
afterwards.
According to the present invention, the following steps are carried out:
measuring radial vibration of the rotor relative to the stator and
correspondingly generating a vibration measurement signal,
calculating a frequency spectrum of the vibration measurement signal,
identifying a plurality of frequency bandwidths of the frequency spectrum,
neglecting one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth (depending on the position, number and width of the bandwidths of
plurality as well as the regime of the compressor when rotating stall detection
occurs, there may be nothing to neglect in this step),
neglecting at least one second frequency bandwidth of said plurality of
A frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth (depending on the position, number and width of the
bandwidths of plurality as well as the regime of the compressor when rotating stall
detection occurs, there may be nothing to neglect in this step),
determining the maximum magnitude of the spectrum in each of the nonneglected
frequency bandwidths, and
carrying out a comparison between each of the determined maximum
magnitudes and a predetermined value.
The rotating stall is considered occurring if at least one of the comparisons shows
that the corresponding determined maximum magnitude is greater than the
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predetermined value.
The present invention may be embodied in many different ways.
An exemplary embodiment of an equipment for detecting rotating stall in a
compressor, comprises: at least one sensor arranged to measure radial vibration of
the compressor rotor relative to the compressor stator and correspondingly generate
a vibration measurement signal, and an electronic processing unit connected at least
to this sensor (and any other sensor used for stall detection) configured to receive
and process at least the vibration measurement signal and consequently signal at
least a rotating stall condition when predetermined criteria are satisfied.
w
Such equipment is advantageously associated to a compressor as a safety component.
Such equipment may be integrated into a compressor monitoring and/or controlling
system that monitors many different parameters of the compressor and/or controls
the compressor operation; in this case, the electronic processing unit receives
several and distinct measurement signals and provides several and distinct functions.
Some advantageous features of possible embodiments are set out in the appended
claims and described in the following detailed description.
According to an embodiment of the present invention, a method for detecting
# rotating stall in a compressor comprising a rotating rotor and a static stator, said
rotor and said stator being subject to radial vibration and axial vibration, comprises
the steps of:
A) measuring radial vibration of said rotor relative to said stator and
correspondingly generating a vibration measurement signal,
B) calculating a frequency spectrum of the vibration measurement signal,
C) identifying a plurality of frequency bandwidths of the frequency spectrum,
D) neglecting one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
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bandwidth,
E) neglecting at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth,
F) determining the maximum magnitude of the spectrum in each of the nonneglected
frequency bandwidths, and
G) carrying out a comparison between each of the determined maximum
magnitudes and a predetermined value;
whereby rotating stall is considered occurring if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value.
The frequency bandwidths of said plurality may be fixed.
The frequency bandwidths of said plurality may be non-overlapping.
The frequency bandwidths of said plurality may be adjacent.
The frequency bandwidths of said plurality may have different widths.
~ The method may comprise further the step of:
identifying a further frequency bandwidth below all frequency bandwidths of
said plurality;
wherein said further frequency bandwidth is used for detecting surge of the
compressor.
Step B may be carried out by means of a windowed FFT algorithm.
In step F an average operation may be carried out between magnitudes in a number
of consecutives time intervals.
The number of frequency bandwidths of said plurality may be between four and ten.
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Step A may provide to measure components of the radial vibration according to two
different, preferably perpendicular, directions.
The method may treat separately the radial vibration components;
whereby rotating stall is considered occurring if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value for any of the radial vibration components.
Step A may provide to measure the radial vibration on both sides of the rotor;
j ^ wherein the method treats separately the measurements on both sides of the rotor;
whereby rotating stall is considered occurring if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value for any of the measurements on both sides of the rotor.
A single electronic processing unit may be used for treating different measurements
of radial vibration of the same compressor.
When a set of compressors are coupled together, a single electronic processing unit
may be used for treating distinct measurements of radial vibration of several
compressors.
4fe Step D may provide to measure the rotation frequency of the rotor.
Step D may provide to determine the rotation frequency of said rotor based on the
maximum magnitude of the spectrum in each of the frequency bandwidths of said
plurality.
The method may be adapted to be used for different regimes of a compressor.
The method may be adapted to be applied to different kinds of compressors.
According to an embodiment of the present invention, an equipment for detecting
rotating stall in a compressor comprising a rotating rotor and a static stator, said
rotor and said stator being subject to radial vibration and axial vibration,
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comprises :
at least one sensor arranged to measure radial vibration of said rotor relative
to said stator and correspondingly generate a vibration measurement signal, and
an electronic processing unit configured to :
calculate a frequency spectrum of the vibration measurement signal,
identify a plurality of frequency bandwidths of the frequency
spectrum,
9~ - neglect one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth,
neglect at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth,
determine the maximum magnitude of the spectrum in each of the '
non-neglected frequency bandwidths,
carry out a comparison between each of the determined maximum
A magnitudes and a predetermined value, and
signal a rotating stall condition if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value.
The electronic processing unit may be additionally configured to :
identify a further frequency bandwidth below all frequency
bandwidths of said plurality for signaling surge of the compressor.
The equipment may comprise further :
at least another sensor arranged to measure radial vibration of said rotor
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relative to said stator and correspondingly generate a vibration measurement signal;
wherein the sensors measure the radial vibration according to two different,
preferably perpendicular, directions.
The equipment may comprise further :
at least another sensor arranged to measure radial vibration of said rotor
relative to said stator and correspondingly generate a vibration measurement signal;
wherein the sensors measure the radial vibration on both sides of the rotor.
W The electronic processing unit may be arranged to treat measurements of vibration
of the compressor from distinct sensors.
The equipment may comprise further :
a sensor arranged to measure the rotation frequency of said rotor.
According to an embodiment of the present invention, a compressor comprises at
least one rotating rotor and a static stator, and an equipment for detecting rotating
stall, and this equipment comprises :
at least one sensor arranged to measure radial vibration of said rotor relative
^ to said stator and correspondingly generate a vibration measurement signal, and
an electronic processing unit configured to :
calculate a frequency spectrum of the vibration measurement signal,
identify a plurality of frequency bandwidths of the frequency
spectrum,
neglect one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth,
neglect at least one second frequency bandwidth of said plurality of
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frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth,
determine the maximum magnitude of the spectrum in each of the
non-neglected frequency bandwidths,
carry out a comparison between each of the determined maximum
magnitudes and a predetermined value, and
signal a rotating stall condition if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
^ predetermined value.
The compressor may comprise at least two rotors coupled together and sensors
arranged to measure radial vibrations of said two rotors, wherein said electronic
processing unit is connected to said sensors.
Other advantageous features of possible embodiments can be derived from the
following detailed description.
BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of the
specification, illustrate one or more embodiments and, together with the description,
^ explain these embodiments. In the drawings:
Fig.l shows a first compressor with associated a first embodiment of the
equipment according to the present invention
Fig.2A shows a first spectrum of the radial vibration amplitude of a rotating
compressor in a first regime (rated speed) and a first example of a plurality of
frequency bandwidths used for detecting rotating stall according to the present
invention,
Fig.2B shows a second spectrum of the radial vibration amplitude of a rotating
compressor in a second regime (minimum operating speed) and a first example of a
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plurality of frequency bandwidths used for detecting rotating stall according to the
present invention,
Fig.2C shows a third spectrum of the radial vibration amplitude of a rotating
compressor in a third regime (maximum operating speed) and a first example of a
plurality of frequency bandwidths used for detecting rotating stall according to the
present invention,
Fig.3A shows a fourth spectrum of the radial vibration amplitude of a rotating
compressor in a fourth regime (maximum operating speed) and a second example of
a plurality of frequency bandwidths used for detecting rotating stall according to the
™ present invention,
Fig.3B shows a fifth spectrum of the radial vibration amplitude of a rotating
compressor in a fifth regime (minimum operating speed) and a second example of a
plurality of frequency bandwidths used for detecting rotating stall according to the
present invention,
Fig.4 shows a second compressor with associated a second embodiment of the
equipment according to the present invention that differs from the first embodiment
of Fig. 1 in that it measures the rotation frequency of rotor,
Fig.5 shows very schematically a third compressor with associated a third
^ embodiment of the equipment according to the present invention that differs from
the first embodiment of Fig. 1 in that the compressor comprises two rotors and the
equipment measures radial vibrations according to perpendicular directions - casing,
bearings, inlets and outlet of the compressor are omitted,
Fig.6 shows schematically a detail of Fig.5, and
Fig.7 shows a flow chart of an embodiment of the method according to the
present invention.
It worth noting that these drawings are schematic, simplified and not in scale, as it
is evident for a person skilled in the art.
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DETAILED DESCRIPTION
The following description of the exemplary embodiments refers to the
accompanying drawings. The same reference numbers in different drawings
identify the same or similar elements. The following detailed description does not
limit the invention. Instead, the scope of the invention is defined by the appended
claims. The following embodiments are discussed, for simplicity, with regard to the
terminology and structure of a centrifugal compressor. However, the embodiments
to be discussed next are not limited to this kind of system, but may be applied for
example to axial compressors.
^ Reference throughout the specification to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic described in connection
with an embodiment is included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an
embodiment" in various places throughout the specification is not necessarily
referring to the same embodiment. Further, the particular features, structures or
characteristics may be combined in any suitable manner in one or more
embodiments.
A compressor 1, like the one shown in Fig.l, comprises a rotating rotor 2 and a
static stator 3; in Fig. 1, the stator 3 corresponds to the casing of the compressor 1.
^ The rotor 2 is mounted on a rotating shaft 4 that is supported on one side by first
bearings 7 and on the other side by second bearings 8. The compressor 1 has an
inlet 5 for an uncompressed fluid and an outlet 6 for a compressed fluid; during
normal operation, a fluid enters the compressor 1 through the inlet 5 is compressed
by the rotation of the rotor 2 and exits the compressor 1 through the outlet 6.
During normal operation, both the compressor rotor and the compressor stator are
subject to both radial and axial vibration. When rotating stall occurs at one or more
areas of the blades of the rotor, vibrations establish in the compressor that lead to a
radial vibration of the rotor relative to the stator; the word "radial" refers to the
rotation axis of the rotor and of its shaft. As the stator is static, i.e. fixed to the
ground, most of the movement caused by the radial vibration is with the rotor and
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its shaft. In Fig.6, the radial vibration is measured by two sensors 10 and 11 that
continuously measure the distance of the shaft 4 with respect to the casing 3; a first
sensor 11 is located close to the first bearings 7 on a first side of the rotor 2 and a
second sensor 10 is located close to the second bearings 8 on a second side
(opposite to the first side) of the rotor 2.
In Fig.l, there is also shown an electronic processing unit 9, that may be a computer
(e.g. a Personal Computer). Each of sensors 10 and 11 generates a corresponding
radial vibration measurement signal that is transmitted to the unit 9 through an
appropriate connection (e.g. a wire) for being treated. In this way, radial vibration
A of the compressor 1 is continuously monitored by the unit 9 through the processing
of the signals received from the sensors 10 and 11. The unit 9 comprises
appropriate hardware and software for determining if a rotating stall is occurring in
the compressor 1 based on the signals received from the sensors 10 and 11, or, in
other words, if there is an "incipient surge" in the compressor 1; additionally, the
unit 9 may comprise appropriate hardware and software for determining if "surge"
is occurring in the compressor 1 based on the signals received from the sensors 10
and 11; "incipient surge" and/or "surge" may be signaled by the electronic
processing unit 9 to a human operator and/or to another electronic processing unit
of the same electronic system (e.g. a compressor monitoring and controlling system)
and/or to a remote electronic system - Fig.l does not show any electronic system.
^ The combination of unit 9 and sensors 10 and 11 (not excluding other components)
can be considered an "equipment for detecting rotating stall"; the combination of
compressor 1, unit 9 and sensors 10 and 11 (not excluding other components) can be
considered an "improved compressor"; these two statements are valid in general, e.g.
when number and kind of sensors different from Fig. 1 are used.
Processing within the unit 9 will be now explained with reference to Fig.l and
Fig.7; such processing is used for detecting rotating stall; the first step to be carried
out (step A - reference 700 in Fig.7) is measuring radial vibration of the rotor
(reference 2 in Fig. 1) relative to the stator (reference 3 in Fig. 1) and
correspondingly generating at least one vibration measurement signal and is carried
out by sensors (references 10 and 11 in Fig.l) external to the electronic processing
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unit (reference 9 in Fig.l).
During operation of the compressor 1, considering for the moment only the first
sensor 11 and its vibration measurement signal, the unit 9 carries out the following
steps of:
B) calculating a frequency spectrum of the vibration measurement signal
(reference 702 in Fig.7),
C) identifying a plurality of frequency bandwidths of the frequency spectrum
(reference 704 in Fig.7),
D) neglecting one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth (depending on the position, number and width of the bandwidths of
plurality as well as the regime of the compressor when rotating stall detection
occurs, there may be nothing to neglect in this step) (reference 706 in Fig.7),
E) neglecting at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth (depending on the position, number and width of the
bandwidths of plurality as well as the regime of the compressor when rotating stall
detection occurs, there may be nothing to neglect in this step) (reference 708 in
• Fig.7),
F) determining the maximum magnitude of the spectrum in each of the nonneglected
frequency bandwidths (reference 710 in Fig.7), and
G) carrying out a comparison between each of the determined maximum
magnitudes and a predetermined value (reference 712 in Fig.7);
rotating stall is considered occurring (reference 714 in Fig.7) if at least one of the
comparisons shows that the corresponding determined maximum magnitude is
greater than the predetermined value.
For the sake of clarity, the "frequency spectrum" of a time-domain signal is a
15
representation of that signal in the frequency domain.
The frequency spectrum can be generated via a FT (Fourier Transform) of the signal,
and the resulting values are usually presented as amplitude and phase, both plotted
versus frequency. Due to the fact that the unit 9 is an electronic processing unit, the
Fourier transform is computed as a DFT (Discrete Fourier Transform),
advantageously through the FFT (Fast Fourier Transform) algorithm.
Steps D and E requires that the current rotation frequency of the rotor be known
when the stall detection is carried out; this may be done either by indirect
measurement (embodiment of Fig.l) or by indirect measurement (embodiment of
^ Fig.4) as it will be better explained afterwards; it is to be noted that very often the
rotation speed of the compressor is measured for other reasons and therefore the
same measurement can be used also for stall detection with an precise and effective
result.
In order to detect stall, step F provides to determine the maximum magnitude in
each bandwidth; anyway, for other purposes (e.g. "troubleshooting"), it might be
useful to identify also the frequency corresponding to the maximum magnitude.
The above steps are repeated by the unit 9 (typically periodically) for monitoring
the compressor with regard to rotating stall. In order to avoid considering
^ momentary vibrations peaks, it is advantageous that in step F an average operation
is carried out between magnitudes in a number (e.g. two or three or four) of
consecutives time intervals.
The above method implemented by an electronic processing unit is based on the
observation that when there is a rotating stall in a compressor, radial vibration of
considerable amplitude is created having a frequency between 10% and 85% of the
rotation frequency of the compressor rotor, more typically between 20% and 80% of
the rotation frequency of the compressor rotor.
For better understanding the above steps, a first example will be provided with
reference to Fig.2; each of the three plots of the vibration amplitude "A" versus the
frequency "f" in Fig.2 represents a possible frequency spectrum of the same
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compressor in three different regimes: Fig.2A corresponds to the condition when the
rotor rotates at the rated speed, Fig.2B corresponds to the condition when the rotor
rotates at the minimum operating speed, Fig.2C corresponds to the condition when
the rotor rotates at the maximum operating speed; in the specific case of Fig.2A, no
stall is occurring; in the specific case of Fig.2B, no stall is occurring; in the specific
case of Fig.2C, at least one stall is occurring.
The frequency bandwidths used for detecting rotating stall are five, namely Bl, B2,
B3, B4 and B5. These bandwidths are fixed, non-overlapping and adjacent; this
means that the maximum frequency FM1 of the first bandwidth Bl corresponds to
dfc the minimum frequency Fm2 of the second bandwidth B2 (FB = e.g. 109.6 Hz), the
maximum frequency FM2 of the second bandwidth B2 corresponds to the minimum
frequency Fm3 of the third bandwidth B3 (FC = e.g. 118.4 Hz), the maximum
frequency FM3 of the third bandwidth B3 corresponds to the minimum frequency
Fm4 of the fourth bandwidth B5 (FD = e.g. 132.0 Hz), the maximum frequency
FM4 of the fourth bandwidth B4 corresponds to the minimum frequency Fm5 of the
fifth bandwidth B5 (FE = e.g. 147.1 Hz); the minimum frequency Fml of the first
bandwidth Bl has been appropriately chosen (FA = e.g. 6.0 Hz) in order not to
detect "surge" vibrations; the maximum frequency FM5 of the fifth bandwidth B5
has been appropriately chosen (FF = e.g. 164.0 Hz) in order not to detect the normal
vibration of the rotor when the rotor rotates either at rated speed (FRR = e.g. 183.3
A Hz) or at maximum speed (FMR = e.g. 192.5 Hz).
In the specific example considered with reference to Fig.2, the five bandwidths Bl,
B2, B3, B4 and B5 have different widths even if, in the figure, bandwidths B2, B3,
B4 and B5 look equally wide; in general, using the same width for all bandwidth
will lead to a greater number of bandwidths.
According to this example the same "predetermined value", or "threshold value" TH,
is used for the amplitude comparison in each of the five bandwidths Bl, B2, B3, B4
and B5; the use of different threshold values in distinct bandwidths is not to be
excluded.
In this example, five frequency bandwidths are used. In alternative examples a
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different numbers of bandwidths may be used; the number should be not too small
and not too high; the minimum preferred numbered is four; the maximum preferred
number is ten; the best number to be used depends also on the characteristics of the
bandwidths (i.e. whether fixed-position or moving and whether fixed-width or
variable-width and whether uniform-width or different-width).
It is to be noted that a sixth bandwidth BO, from 0 Hz to the minimum frequency
Fml of the first bandwidth Bl (FA = e.g. 6.0 Hz), is shown in Fig.2; high-amplitude
vibrations in this low-frequency bandwidth are an indicator of an already "existing
surge" and not of an "incipient surge" (independently from the regime of the
A compressor). Therefore, if the unit 9 is able to consider such a low-frequency
bandwidth of the frequency spectrum of the vibration measurement signal, i.e.
below all the other frequency bandwidths, it may signal "surge", or "existing surge".
In Fig.2A, the frequency spectrum comprises four components: CR, CI, C2, C3.
The vibration component CR corresponds to the vibration component directly due to
rotation of the compressor rotor and, therefore, it is centered at the rotation
frequency (in this case the compressor rated frequency FR); the maximum
magnitude (or amplitude) of the component CR is well above the threshold TH, but
this is normal. The component CI falls within the first bandwidth Bl and has a
maximum magnitude below the threshold TH; therefore, this component is not due
to a rotating stall. The component C2 falls partially within the third bandwidth B3
and partially within the fourth bandwidth B4 and has a maximum magnitude below
the threshold TH (in any of the two bandwidths); therefore, this components is not
due to a rotating stall. The component C3 falls within the fifth bandwidth B5 and
has a maximum magnitude below the threshold TH; therefore, this component is not
due to a rotating stall. Considering the steps (from A to G) explained before, there
is no frequency bandwidth to be neglected as none of the five bandwidths (Bl to
B5) comprise or is above the rotation frequency of the rotor (and any of the
frequencies in the limited bandwidth of its vibration component).
In Fig.2B, the frequency spectrum comprises four components: CR, C4, C5, C6.
The vibration component CR corresponds to the vibration component directly due to
rotation of the compressor rotor and, therefore, it is centered at the rotation
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frequency (in this case the compressor minimum operating frequency Fm); the
maximum magnitude (or amplitude) of the component CR is well above the
threshold TH, but this is normal. The component C4 falls within the first bandwidth
Bl and has a maximum magnitude below the threshold TH; therefore, this
component is not due to a rotating stall. The component C5 falls partially within
the first bandwidth Bl and partially within the second bandwidth B2 and has a
maximum magnitude below the threshold TH (in any of the two bandwidths);
therefore, this components is not due to a rotating stall. The component C6 falls out
of any of the five bandwidths (from Bl to B5) and, therefore, is not even considered
by the processing (in any case, its amplitude is below the threshold TH).
W Considering the steps (from A to G) explained before, there are three frequency
bandwidths to be neglected: the third bandwidth B3 as it comprises the component
CR, and the fourth and the fifth bandwidths B4 and B5 as they are above the
rotation frequency Fm of the rotor.
In Fig.2C, the frequency spectrum comprises four components: CR, CS1, CS2, C7.
The vibration component CR corresponds to the vibration component directly due to
rotation of the compressor rotor and, therefore, it is centered at the rotation
frequency (in this case the compressor maximum operating frequency FM); the
maximum magnitude (or amplitude) of the component CR is well above the
threshold TH, but this is normal. The component C7 falls within the first bandwidth
A Bl and has a maximum magnitude below the threshold TH; therefore, this
component is not due to a rotating stall. The component CS1 falls within the fifth
bandwidth B5 and has a maximum magnitude well above the threshold TH;
therefore, this components is considered to be due to a rotating stall. The
component CS2 falls within the third bandwidth B3 and has a maximum magnitude
slightly above the threshold TH; therefore, this components is considered to be due
to a rotating stall. Considering the steps (from A to G) explained before, there is no
frequency bandwidth to be neglected as none of the five bandwidths (Bl to B5)
comprise or is above the rotation frequency of the rotor (and any of the frequencies
in the limited bandwidth of its vibration component).
Therefore, its is clear from the above example that, depending on the rotation
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frequency of the rotor in a specific moment of operation of the (same) compressor,
none or one or more bandwidths are neglected.
For the sake of completeness, according to a very specific exemplary embodiment
of the present invention, the compressor to be monitored has Fmin=119.16Hz
(minimum value of rotation frequency), Frat=183.33Hz (rate value of rotation
frequency), Fmax=l92.50 (maximum value of rotation frequency) and five fixed,
non-overlapping and adjacent bandwidths are used:
First bandwidth : from 6.0Hz to 109.6Hz
£ Second bandwidth : from 109.6Hz to 118.4Hz
Third bandwidth : from 118.4Hz to 132.0Hz
Fourth bandwidth : from 132.0Hz to 147.1Hz
Fifth bandwidth : from 147.1Hz to 164.0Hz
The determination of the bandwidth (in the case of fixed, non-overlapping and
adjacent bandwidths) is advantageously carried out in the following way. A
coefficient K is considered; K is assumed to be in the range from e.g. 0.87 (so to
remain a bit above 85%) to e.g. 0.95 (so to remain a bit below 100%);
4^1 Lower limit of first bandwidth = Fl = any value within e.g. 5.0-10.0 Hz (so to
exclude very low frequencies).
Upper limit of first bandwidth = Lower limit of second bandwidth = F2 = Fmin * K
(so that 85% of Fmin falls within the first bandwidth)
Upper limit of second bandwidth = Lower limit of third bandwidth = F3 = F2 / K
(so not to exclude 85%)
Upper limit of third bandwidth = Lower limit of fourth bandwidth = F4 = F3 / K
(so not to exclude 85%)
20
Upper limit of X bandwidth = Lower limit of X-l bandwidth = F(X) = F(X-l) / K
Further bandwidths are allocated till a frequency is reached comprised between
0.85*Fmax and 0.95*Fmax; ideally F(X) = K*Fmax.
Based on these equations, an appropriate value of K is chosen in the above
mentioned range.
For better understanding the above steps (from A to G), a second example will be
^ provided with reference to Fig.3; each of the two plots of the vibration amplitude
"A" versus the frequency "f' in Fig.3 represents a possible frequency spectrum of
the same compressor in two different regimes: Fig.3A corresponds to the condition
when the rotor rotates at the maximum operating speed (e.g. 190 Hz), Fig.3B
corresponds to the condition when the rotor rotates at the minimum operating speed
(e.g. 120 Hz); in both these two specific cases, no stall is occurring.
In the example of Fig.3, there are two fixed frequency bandwidths B6 and B7 that
are also non-overlapping and adjacent; this means that the maximum frequency
FM6 of the bandwidth B6 corresponds to the minimum frequency Fm7 of the
bandwidth B7; therefore, these bandwidths identify three frequencies FG (e.g. 6 Hz),
FH (e.g. 100 Hz, i.e. 120 - 20, 20 being slightly more than 10% of 190) and FL (e.g.
W 210 Hz, i.e. 190 + 20, 20 being slightly more than 10% of 190); (FB = e.g. 109.6
Hz); there is also a bandwidth B0 identical to that of Fig.2. The bandwidth B7 has
been chosen so that the component CR of frequency spectrum at the rotor rotation
frequency falls always within this bandwidth: in Fig.3A the component CR(A) is in
the upper range of the bandwidth B7 as the rotation frequency is maximum, in
Fig.3B the component CR(B) is in the lower range of the bandwidth B7 as the
rotation frequency is minimum. The bandwidth B6 has been chosen so that a
component CA of the frequency spectrum at half the rotor rotation frequency (so
called "first sub-harmonic") falls within this bandwidth; in Fig.3A the component
CA(A) is in the upper range of the bandwidth B6; in Fig.3B the component CA(B)
is in the lower range of the bandwidth B6 (even if far from the lower limit FG).
21
In this example, both components CR and CA are not to be considered for detecting
stall as they are normal (in some kind of compressors, the rotation of the rotor
generates vibration not only at the rotation frequency but also at half the rotation
frequency), independently from their magnitudes. In order to take this into account,
two fixed-width (the width of BSR is e.g. 40 Hz i.e. slightly more than 20% of 190,
the width of BSA is e.g. 20Hz i.e. BSR/2) and moving bandwidths BSR and BSA
are used; in Fig.3 they correspond to the suppression bandwidths of a two
suppression-band filters tracked to the rotation frequency of the rotor: bandwidth
BSR covers component CR and bandwidth BSA covers component CA.
A The combination of the two fixed-position and fixed-width bandwidths B6 and B7
and the two variable-position and fixed-width bandwidths BSA and BSR may be as
four variable-position and variable-width bandwidths: the first bandwidth ranges
from the frequency FG to the lower limit of the bandwidth BSA, the second
bandwidth ranges from the upper limit of the bandwidth BSA to the frequency FH,
the third bandwidth ranges from the frequency FH to the lower limit of the
bandwidth BSR, the fourth bandwidth ranges from the upper limit of the bandwidth
BSR to the frequency FL. Considering the steps (from A to G) explained before,
there fourth bandwidth must always be neglected as it is always above the rotation
frequency of the rotor (and any of the frequencies in the limited bandwidth of its
vibration component).
In the specific regime of the compressor corresponding to Fig.3A, there are two
components C8 and C9; the component C8 falls within the first bandwidth; the
component C9 falls within the third bandwidth; none of the components C8 and C9
has a maximum magnitude exceeding the threshold value TH and, therefore, no stall
is occurring.
In the specific regime of the compressor corresponding to Fig.3B, there is one
component CIO; the component CIO falls within the second bandwidth; the
component C10 does not have a maximum magnitude exceeding the threshold value
TH and, therefore, no stall is occurring.
Till now the description has considered only one radial vibration of the compressor,
22
or, in other words, one vibration sensor (namely the first sensor 11) and one
corresponding vibration measurement signal.
In the embodiment of Fig. 1, there are two radial vibration sensors, namely the first
sensor 11 and the second sensor 10; each of the two sensors 10 and 11 are located
on a different side of the rotor 2. In this way, a rotating stall may be effectively
detected wherever is located (i.e. in a first end region of the rotor or in a second end
region of the rotor or in a middle region of the rotor). When using such two sensors
and their measurement signals, the above the steps (from A to G) are carried out for
each of the two signals; rotating stall is considered occurring if for at least one of
A the two signals the threshold value is exceeded in any of the non-neglected
bandwidths. The electronic processing unit 9 is able to treat both signals separately
and contemporaneously or substantially contemporaneously.
As already said, the present invention may be embodied in different forms.
The embodiment of Fig.4 differs from the embodiment of Fig.l in that there is a
rotation sensor 12 connected to the unit 9 and adapted to measure the rotation speed
or rotation frequency of the rotor 2 (precisely of the shaft 4); sensor 12 generates a
rotation measurement signal that is received and processed by the unit 9.
The rotation measurement signal may be used by the electronic processing unit for
^ determining one or more bandwidths to be neglected between the set of frequency
bandwidths used for stall detection. For example, in the case of Fig.2B, the signal
from the sensor 12 would indicate that the rotation frequency of the rotor is Fm, the
bandwidth B3 is neglected; alternatively, the electronic processing unit may decide
to neglect the bandwidth B3 considering its very high maximum magnitude (much
higher than the threshold value TH).
The rotation measurement signal may be used by the electronic processing unit for
determining one or more limit frequencies (i.e. lower end and upper end) of one or
more of the set of frequency bandwidths used for stall detection. For example, in
the case of Fig.3, would indicate the rotation frequency of the rotor at any time and
consequently the electronic processing unit may determine the two bandwidths BSA
23
and BSR at any time (two tracking filters may be used in this case).
The embodiment of Fig.5 comprises two rotors 5021 and 5022 mounted on a same
shaft 504 and three sensors couples of radial vibration sensors 5101+5102,
5111+5112, 5131+5132; all the sensors are connected to an electronic processing
unit 509.
In this embodiment, radial two vibration sensors are coupled in order to more
effectively detect radial vibration independently from the vibration direction.
Referring to Fig.6, there are a rotor RO (more precisely the shaft of a rotor) and a
stator ST (more precisely the casing of a compressor); additionally there are a
™ sensor XS arranged primarily to measure radial vibration along the X-axis and a
sensor YS arranged primarily to measure radial vibration along the Y-axis; the
sensors XS and YS form a couple with perpendicularly disposed measurement
directions. When using such sensors couple and their measurement signals, the
above the steps (from A to G) are carried out for each of the two signals; rotating
stall is considered occurring if for at least one of the two signals the threshold value
is exceeded in any of the non-neglected bandwidths. The electronic processing unit
is able to treat both signals separately and contemporaneously or substantially
contemporaneously.
According to the embodiment of Fig.5, a first sensors couple (5111, 5112) is on one
9 side of a first rotor (5021), a second sensors couple (5101, 5102) is on one side of
the second rotor (5022), a third sensors couple (5131, 5132) is in-between the first
rotor (5021) and the second rotor (5022). The electronic processing unit 509 is able
to treat the measurement signals of all the sensors separately and
contemporaneously or substantially contemporaneously.
It is to be noted that an electronic processing unit might be able to treat the
measurement signals of many sensors associated from several compressors
separately and contemporaneously or substantially contemporaneously.
It is apparent from the above description that embodiments of the present invention
are designed to detect rotating stall in a compressor at different regimes and not
24
only when the compressor is operating at rated speed.
Some embodiments of the equipment according to the present invention may be
designed for a specific compressor.
Other embodiments may be designed for being used with different compressors; in
this case, it might be useful to customize the equipment to the specific compressor
at the time of installing the equipment; customization may relate for example to the
number of bandwidths and their characteristics as well as to the one or more
threshold values to be used for comparisons.

WE CLAIM :
1. A method for detecting rotating stall in a compressor comprising a rotating
rotor and a static stator, said rotor and said stator being subject to radial vibration
and axial vibration; the method comprising the steps of:
A) measuring radial vibration of said rotor relative to said stator and
correspondingly generating a vibration measurement signal,
B) calculating a frequency spectrum of the vibration measurement signal,
^ C) identifying a plurality of frequency bandwidths of the frequency spectrum,
D) neglecting one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth,
E) neglecting at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth,
F) determining the maximum magnitude of the spectrum in each of the nonneglected
frequency bandwidths, and
^ G) carrying out a comparison between each of the determined maximum
magnitudes and a predetermined value;
whereby rotating stall is considered occurring if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value.
2. The method of claim 1, wherein the frequency bandwidths of said plurality
are fixed, preferably non-overlapping and adjacent, and preferably have different
widths.
3. The method of claim 1, comprising further the step of :
26
identifying a further frequency bandwidth below ail frequency bandwidths of
said plurality;
wherein said further frequency bandwidth is used for detecting surge of the
compressor.
4. The method of claim 1, wherein the number of frequency bandwidths of said
plurality is between four and ten.
5. The method of claim 1, wherein step A provides to measure components of
the radial vibration according to two different, preferably perpendicular, directions.
6. The method of claim 1, wherein step A provides to measure the radial
vibration on both sides of the rotor.
7. The method of claim 5 or 6, wherein a single electronic processing unit is
used for treating different or distinct measurements of radial vibration of the same
compressor or of several compressors.
8. The method of claim 1, wherein step D provides to measure the rotation
frequency of the rotor or to determine the rotation frequency of said rotor based on
the maximum magnitude of the spectrum in each of the frequency bandwidths of
said plurality.
^ 9. An equipment for detecting rotating stall in a compressor comprising a
rotating rotor and a static stator, said rotor and said stator being subject to radial
vibration and axial vibration; the equipment comprising :
at least one sensor arranged to measure radial vibration of said rotor relative
to said stator and correspondingly generate a vibration measurement signal, and
an electronic processing unit configured to :
calculate a frequency spectrum of the vibration measurement signal,
identify a plurality of frequency bandwidths of the frequency
spectrum,
27
neglect one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth,
neglect at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
frequency bandwidth,
determine the maximum magnitude of the spectrum in each of the
non-neglected frequency bandwidths,
^ - carry out a comparison between each of the determined maximum
magnitudes and a predetermined value, and
signal a rotating stall condition if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value.
10. A compressor comprising at least one rotating rotor and a static stator, and an
equipment for detecting rotating stall; wherein the equipment comprises:
at least one sensor arranged to measure radial vibration of said rotor relative
to said stator and correspondingly generate a vibration measurement signal, and
W - an electronic processing unit configured to :
calculate a frequency spectrum of the vibration measurement signal,
identify a plurality of frequency bandwidths of the frequency
spectrum,
neglect one first frequency bandwidth of said plurality of frequency
bandwidths, if the rotation frequency of said rotor falls within the first frequency
bandwidth,
neglect at least one second frequency bandwidth of said plurality of
frequency bandwidths, if the rotation frequency of said rotor falls below the second
28
frequency bandwidth,
determine the maximum magnitude of the spectrum in each of the
non-neglected frequency bandwidths,
carry out a comparison between each of the determined maximum
magnitudes and a predetermined value, and
signal a rotating stall condition if at least one of the comparisons
shows that the corresponding determined maximum magnitude is greater than the
predetermined value.