U.S. patent application number 14/780183 was filed with the patent office on 2016-02-18 for methods and systems for controlling turbocompressors.
This patent application is currently assigned to Nuovo Pignone Sr1. The applicant listed for this patent is NUOVO PIGNONE SRL. Invention is credited to Marco BAGGIANI, Andrea BERNOCCHI, Daniele GALEOTTI, Sergio MANNUCCI, Emiliano TOCI, Lorenzo Bazzanti VESTRI.
Application Number | 20160047392 14/780183 |
Document ID | / |
Family ID | 48485280 |
Filed Date | 2016-02-18 |
United States Patent
Application |
20160047392 |
Kind Code |
A1 |
BERNOCCHI; Andrea ; et
al. |
February 18, 2016 |
METHODS AND SYSTEMS FOR CONTROLLING TURBOCOMPRESSORS
Abstract
A method for regulating a turbocompressor to prevent surge, is
described. The method comprises the following steps: providing at
least one surge limit line of turbocompressor; continuously
determining an actual value of a corrected speed of the
turbocompressor; continuously determining at least a maximum
admissible pressure ratio on the surge limit line, corresponding to
the actual value of the corrected speed; continuously determining
an actual pressure ratio; acting upon an antisurge arrangement, if
the actual pressure ratio is equal to or higher than the maximum
admissible pressure ratio.
Inventors: |
BERNOCCHI; Andrea;
(Florence, IT) ; GALEOTTI; Daniele; (Florence,
IT) ; TOCI; Emiliano; (Florence, IT) ; VESTRI;
Lorenzo Bazzanti; (Florence, IT) ; MANNUCCI;
Sergio; (Florence, IT) ; BAGGIANI; Marco;
(Florence, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NUOVO PIGNONE SRL |
Florence |
|
IT |
|
|
Assignee: |
Nuovo Pignone Sr1
Florence
IT
|
Family ID: |
48485280 |
Appl. No.: |
14/780183 |
Filed: |
March 26, 2014 |
PCT Filed: |
March 26, 2014 |
PCT NO: |
PCT/EP2014/055831 |
371 Date: |
September 25, 2015 |
Current U.S.
Class: |
415/1 ;
415/17 |
Current CPC
Class: |
F04D 27/001 20130101;
F04D 27/0246 20130101; F04D 29/462 20130101; F04D 17/10 20130101;
F04D 19/002 20130101; F04D 29/444 20130101; F04D 27/0207 20130101;
F04D 29/563 20130101; F04D 29/542 20130101; F04D 27/0261
20130101 |
International
Class: |
F04D 27/02 20060101
F04D027/02; F04D 19/00 20060101 F04D019/00; F04D 29/44 20060101
F04D029/44; F04D 29/46 20060101 F04D029/46; F04D 29/56 20060101
F04D029/56; F04D 29/54 20060101 F04D029/54; F04D 17/10 20060101
F04D017/10; F04D 27/00 20060101 F04D027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2013 |
IT |
FI2013A000064 |
Claims
1. A method for regulating a turbocompressor to prevent surge,
comprising the following steps: providing at least one surge limit
line and/or at least one choke line of said turbocompressor;
determining continuously an operating point of the compressor
measuring a processing gas temperature at a compressor inlet, the
rotary speed of the compressor, a delivery pressure value and a
suction pressure value; continuously determining an actual value of
a corrected speed of the turbocompressor, the corrected speed being
proportional to a ratio of the rotary speed to a square root of the
processing gas temperature; continuously determining at least a
maximum admissible pressure ratio on said surge limit line and/or
at least a minimum admissible pressure ratio on said choke line,
corresponding to the actual value of the corrected speed;
continuously determining an actual pressure ratio, equal to a ratio
between the delivery pressure value and the suction pressure value;
and acting upon an antisurge arrangement to recirculate a fraction
of a the compressed gas in the compressor through a suction line,
if the actual pressure ratio is equal to or higher than the maximum
admissible pressure ratio or equal to or lower than the minimum
admissible pressure ratio.
2. The method of claim 1, further comprising the step of
calculating a surge parameter defined as a ratio of the maximum
admissible pressure ratio to the actual pressure ratio and using
said surge parameter to control the antisurge arrangement.
3. The method of claim 1, further comprising the step of
calculating a choke parameter defined as a ratio of the minimum
admissible pressure ratio to the actual pressure ratio, and using
said choke parameter to control the antichoke arrangement.
4. The method of claim 1, further comprising the steps of:
providing at least one maximum admissible corrected speed and at
least one minimum admissible corrected speed of the
turbocompressor; if the actual value of the corrected speed is
higher than the maximum admissible corrected speed, reducing the
rotary speed of the compressor; and if the actual value of the
corrected speed is lower than the minimum admissible corrected
speed, increasing the rotary speed of the compressor.
5. The method of claim 1, further comprising the steps of:
determining a parameter indicative of a position of variable stator
vanes located at the inlet of said turbocompressor; and selecting
said surge limit line and/or said choke line as a function of said
parameter, among a plurality of surge limit lines and/or a
plurality of choke lines, each corresponding to a respective
position of the variable stator vanes.
6. The method of claim 1, further comprising the steps of: setting
a maximum admissible corrected speed and a minimum admissible
corrected speed corresponding to a position of variable stator
vanes located at the inlet of said turbocompressor; and if the
actual value of the corrected speed is higher than the maximum
admissible corrected speed or if the actual value of the corrected
speed is lower than the minimum admissible corrected speed, acting
upon the variable stator vanes.
7. The method of claim 1, further comprising the preceding step of
detecting the kind of gas entering in the turbocompressor.
8. An apparatus for providing an antisurge control for a
compression system comprising at least one compressor, said
apparatus being arranged and configured for performing a method
according to claim 1.
9. An apparatus for providing an antisurge control for a
compression system comprising at least one compressor, said
apparatus comprising: a data storage device containing data
defining at least one surge limit line and/or at least one choke
line of the compressor; an arrangement for determining continuously
an operating point of the compressor comprising: a temperature
sensor for measuring processing gas temperature at a compressor
inlet, a rotary speed sensor for measuring rotary speed of the
compressor, and pressure sensors for measuring a delivery pressure
value and a suction pressure value; an arrangement for continuously
determining an actual value of a corrected speed of the
turbocompressor, the corrected speed being proportional to a ratio
of the rotary speed to a square root of the processing gas
temperature; an arrangement for continuously determining at least a
maximum admissible pressure ratio on said surge limit line and/or
at least a minimum admissible pressure ratio on said choke line,
corresponding to the actual value of the corrected speed; an
arrangement for continuously determining an actual pressure ratio,
equal to a ratio between the delivery pressure value and the
suction pressure value; an antisurge arrangement to recirculate a
fraction of compressed gas in the compressor through a the suction
line, acted upon if the actual pressure ratio is equal to or higher
than the maximum admissible pressure ratio or equal to or lower
than the minimum admissible pressure ratio.
10. The apparatus of claim 9, further comprising devices for
calculating a surge parameter defined as a ratio of the maximum
admissible pressure ratio to the actual pressure ratio.
11. The apparatus of claim 9, further comprising devices for
calculating a choke parameter defined as a ratio of the maximum
admissible pressure ratio to the actual pressure ratio.
12. The apparatus of claim 9, wherein said storage device contains
data defining at least one maximum admissible corrected speed and
one minimum admissible corrected speed of said compressor; and
wherein a speed control arrangement is provided, which controls a
rotary speed of said compressor such that: if the actual value of
the corrected speed is higher than the maximum admissible corrected
speed, the rotary speed of the compressor is reduced, and if the
actual value of the corrected speed is lower than the minimum
admissible corrected speed, the rotary speed of the compressor
increased.
13. The apparatus of claim 9, further comprising an actuation
device for controlling a position of variable stator vanes of said
compressor and wherein said storage device contains data defining a
plurality of surge lines and/or choke lines corresponding to a
plurality of respective positions of the variable stator vanes.
14. A compression system comprising at least one compressor and an
apparatus according to claim 9, for antisurge-control of said
compressor.
15. The compression system of claim 14, further comprising an
arrangement for detecting the kind of gas entering in the
turbocompressor.
16. The apparatus of claim 10, further comprising devices for
calculating a choke parameter defined as a ratio of the maximum
admissible pressure ratio to the actual pressure ratio.
Description
BACKGROUND
[0001] The present disclosure relates to compressor systems and
more particularly to turbocompressor systems including axial and/or
centrifugal compressors for processing a gas flow. The subject
matter of the present disclosure concerns methods and systems for
controlling the compressors arrangement to prevent out of operating
envelope phenomena like surge and other undesirable operating
conditions.
DESCRIPTION OF THE RELATED ART
[0002] Turbocompressors are work-absorbing turbomachines used to
boost the pressure of a working gaseous flow. The pressure of the
working fluid is increased by adding kinetic energy to a continuous
flow of working fluid through rotation of a rotor supporting one or
more impellers and/or one or more sets of blades in circular
arrangements. Turbocompressors are frequently used in pipeline
transportation of natural gas, for example to move gas from a
production site to a consumer location, in gas and oil
applications, refrigeration systems, gas turbines, and other
applications.
[0003] The flow of fluid through the turbocompressor can be
affected by various conditions leading to unstable operations which
can result in serious damages of the turbomachine.
[0004] Compressor surge occurs when the pressure of a working fluid
flowing through the compressors increases beyond a maximum
allowable output pressure and/or if the flow rate drops beyond a
minimum limit.
[0005] In general a surge phenomenon occurs when the compressor
cannot add enough energy to the working fluid in order to overcome
the system resistance, i.e. the head drop across the system, a
situation which results in a rapid flow and discharge pressure
decrease. The surge may be accompanied by high vibrations,
temperature increase and rapid changes in the axial thrust on the
bearings of the compressor shaft. These phenomena can severely
damage the compressor and also the components of the system
connected to the compressor, such as valves and piping.
[0006] Other undesirable operating conditions can arise during
operation of the turbocompressor. More particularly, choke (also
sometimes named stonewall) is a condition at which increased flow
results in a rapid decrease in head, i.e. compression ratio a flow
is increased. Operating at a very high flow rate has negative
effects on the compressor performance and can result in compressor
damages.
[0007] To prevent surge and choke phenomena to arise, control
systems have been developed and are currently used in
turbocompressor installations
[0008] FIG. 1 schematically illustrates an exemplary embodiment of
a system 1, comprised of a turbocompressor 3 driven into rotation
by a prime mover 5, for example an electric motor, a gas or steam
turbine, or the like. Reference number 7 indicates a suction line,
where from the working fluid is fed to the suction or inlet side of
the turbocompressor 3. Reference number 9 designates the delivery
pipe, where through the compressed fluid is delivered from the
discharge side of the compressor 3.
[0009] FIG. 2 schematically illustrates a compressor performance
map, typically a compressor performance map of an axial compressor.
The performance map shows the pressure ratio along the vertical
axis and the inlet volumetric flow reported on the horizontal axis.
The inlet flow is indicated with the letter Q. Depending upon the
operating conditions of the compressor, for example the rotary
speed (rpm), a plurality of expected performance curves can be
reported in the performance map. Each curve can correspond to a
different compressor rotary speed. For a given compressor setup,
therefore, a family of performance curves can be reported on the
performance map. Similar curve families can be drawn for different
setup or operating conditions of the turbocompressor, e.g. for
different positions of variable stator vanes (VSVs), the
turbocompressor can be provided with. Each performance curve is
limited by a surge point, i.e. a point where the pressure ratio and
the gas flow through the compressor have achieved a value, beyond
which surge phenomena will be generated. Each performance curve is
further limited by a choke point, beyond which choke phenomena
arise. The line SLL is the so called Surge Limit Line, formed by
the surge points of the various performance curves reported on the
performance map. The line CLL is the choke limit line, formed by
the choke points. The SLL and CLL lines define an envelope, i.e. a
portion of the performance map, within which the operating point of
the compressor is maintained to ensure stable operating conditions
of the turbocompressor and prevent both surge as well as choking
conditions.
[0010] The SLL and CLL thus represent the limit of operation of the
turbocompressor, beyond which the turbocompressor shall not be
operated to prevent the risk of surge and choke phenomena. Known
compressor systems are comprised of control devices and
arrangements to control the turbocompressor so that it will
constantly operate inside the stability area of the performance
map, i.e. between the surge limit line SLL and the choke limit line
CLL.
[0011] In the diagrammatic representation of FIG. 1 a control unit
11 is connected to various instrumentalities surrounding the
turbocompressor to determine the operating conditions of the
turbomachine and provide antisurge control and antichoke control
for preventing surge and choke phenomena from arising.
[0012] More particularly, as shown in FIG. 1, the control unit 11
is connected to a flow measuring device, also called flow element
13 that is designed and configured to determine the inlet volume
flow rate of the turbocompressor 3. A temperature sensor at the
inlet side or suction side provides a temperature value Ts and
pressure sensors provide the delivery pressure value Pd and suction
pressure value Ps or directly the compression ratio Pd/Ps.
[0013] Based on the input data the control unit 11 is capable of
determining inlet volume flow rate and the pressure ratio at each
and every instant of operation of the turbocompressor 3. These two
parameters define the operating point on the compressor performance
map of FIG. 2. As additional parameter the rotary speed N (rpm) of
the compressor can be provided, so that the correct operating curve
can be selected to determine the actual position of the compressor
operating point in the performance map. If the operating point
moves close to the surge limit line SLL, the surge control system
acts upon an antisurge bypass valve 15. The valve 15 is arranged on
a bypass line 17 connecting the delivery side and the suction side
of the compressor 3. A fraction of the working fluid delivered by
the turbocompressor 3 can be recirculated through the antisurge
valve 15, if required, to prevent surge phenomena. When the
delivery pressure increases so that the operating point approaches
surge limit line SLL, the antisurge control arrangement opens the
antisurge bypass valve 15 so that the flow rate through the
compressor can increase and the delivery pressure can decrease.
[0014] Before being recirculated through the antisurge valve 15 the
working fluid can be cooled in a heat exchanger 19.
[0015] In some embodiments, the surge control arrangement can
provide for a bleeding line, along which the antisurge valve is
arranged and which is designed to discharge the process gas in the
environment, if the nature of the gas so permit.
[0016] Choking of the compressor can be prevented by closing an
antichoke control valve arranged along the suction line 7, or along
discharge line upstream or downstream of the turbocompressor 3.
[0017] The actual known solutions require a flow element 13 for
determining the operating point of the compressor for the purpose
of preventing surging phenomena. In some application the flow
element 13 can be cumbersome and requires a relatively long pipe
upstream and downstream thereof, in order to provide a correct
measurement of the inlet flow rate. Providing measuring elements or
devices at the inlet side or suction side of turbocompressor, in
particular air turbocompressor can be difficult.
SUMMARY OF THE INVENTION
[0018] The subject matter disclosed herein concerns an improved
method and apparatus for providing antisurge control of a
compression system comprised of at least one compressor. In some
embodiments, the method and apparatus provide antisurge and/or
antichoke control of the compressor.
[0019] In some embodiments, at least one operating envelope is
defined, the compressor being controlled so that the operating
point thereof falls within the operating envelope. Action is taken,
if the operating point falls outside or on the boundaries of the
operating envelope or if in an embodiment, the operating point
approaches the boundaries of the envelope. The operating envelope
is defined based in a performance map based on two operating
parameters of the compressor: a corrected speed of the compressor
and a pressure ratio. The pressure ratio is the ratio between the
delivery pressure and the suction pressure of the compressor. The
corrected speed is defined as a function of the suction temperature
of the gas being processed by the compressor and the rotary speed
of the compressor. The corrected speed is thus proportional to the
ratio
N Ts ##EQU00001##
[0020] wherein: [0021] Ts is a processing-fluid temperature at
compressor inlet and [0022] N is the rotary speed of the
compressor.
[0023] The corrected speed is defined by the above mentioned ratio
if the gas composition is constant. The method disclosed herein,
therefore, is suitable for antisurge/antichoke control of
compressors processing a gas having a known and constant
composition, e.g. carbon dioxide and the like.
[0024] The operating envelope can be bounded by a suction limit
line, a choke limit line, as well as by a maximum admissible
corrected speed and by a minimum admissible corrected speed.
[0025] If the compressor is provided with movable inlet guide
vanes, i.e. with variable stator vanes, an operating envelope can
be defined for each position of the variable stator vanes. Thus,
according to some, a plurality of operating envelopes are defined
in a corrected speed versus compression ratio diagram or
performance map. Depending upon the actual position of the variable
stator vanes, the corresponding operating envelope is selected for
antisurge and/or antichoke control. Since the position of the
variable stator vanes can vary in a continuous manner, according to
some embodiments, a limited number of operating envelopes are
determined, corresponding to a limited number of different
positions of the variable stator vanes. If the actual position of
the variable stator vanes is different from those for which an
envelope has been determined and the relevant data thereof stored
for control purposes, a new intermediate operating envelope is
calculated, e.g. by interpolating the data of the two nearest
operating envelopes, for which data are available.
[0026] According to some embodiments, therefore, a method for
regulating a turbocompressor to prevent surge is provided,
comprising the following steps: providing at least one surge limit
line and/or at least one choke line for at least one operating
condition of the turbocompressor; determining continuously an
operating point of the compressor measuring a processing gas
temperature at compressor inlet, the rotary speed of the
compressor, a delivery and suction pressure value; continuously
determining an actual value of a corrected speed of the
turbocompressor, being the correct speed proportional to the
ratio
N Ts ; ##EQU00002##
continuously determining at least a maximum admissible pressure
ratio on the surge limit line and/or at least a minimum admissible
pressure ratio on the choke line, corresponding to the actual value
of the corrected speed; continuously determining an actual pressure
ratio, equal to the ratio between delivery pressure and suction
pressure; acting upon an antisurge arrangement to recirculate a
fraction of the compressed gas in the compressor through the
suction line, if the actual pressure ratio is equal to or higher
than the maximum admissible pressure ratio or equal to or lower
than the minimum admissible pressure ratio.
[0027] In the context of the present disclosure and enclosed
claims, the term "continuously determining" a parameter also
encompasses determining in embodiments, the parameter at constant
or variable time intervals during continued operation of the
compressor.
[0028] In embodiments, the surge limit line defines, along with a
choke limit line and maximum and minimum admissible corrected-speed
lines, the operating envelope, within which the operating point of
the compressor will be maintained.
[0029] If the operating point of the compressor is approaching the
surge limit line, an antisurge arrangement can be acted upon. The
antisurge arrangement can be any arrangement known from the art.
Surge is prevented by opening an antisurge bypass valve. In a
particular embodiment, if the process gas is air, surge can be
prevented by venting or bleeding a fraction of the compressor
delivery flow in the environment. In both cases, the delivery flow
is increased, thus shifting the operating point of the compressor
away from the surge limit line.
[0030] The method can also include the preceding step of detecting
the kind of gas or the gas composition entering in the
turbocompressor.
[0031] Since the correct speed is related to the gas composition or
type, the gas detection or prediction could allow an online
continuous setting of the method.
[0032] In particular, when the working gas has a composition not
constant over time, the method requires information about the gas
worked by the turbocompressor. For obtaining this information could
be used any gas detector known in the art, for example a process
gas chromatograph.
[0033] The apparatus can also comprises a suitable database for
containing the operating envelopes associated to respective
gasses.
[0034] If the operating point of the compressor approaches the
upper corrected speed limit or the lower corrected speed limit,
action can be taken to reduce or increase the rotary speed of the
compressor, respectively.
[0035] If the operating point of the compressor is approaching the
choke limit line, an antichoke arrangement can be acted upon. In an
embodiment, the arrangement can be any arrangement known in the
art. For example, an antichoke valve can be closed.
[0036] In some embodiments, variable stator vanes, i.e. movable
inlet guide vanes can be provided at the suction side of the
compressor. The variable stator vanes can be used as a control
means to prevent the compressor operating point from approaching or
moving beyond the lines delimiting the operating envelope. Choke
can e.g. be prevented by reducing the inlet cross section and thus
the inlet flow of the compressed gas.
[0037] The variable stator vanes can be acted upon also to prevent
the compressor operating point to move above the upper corrected
speed limit or to drop under the lower corrected speed limit.
[0038] According to a further aspect, the subject matter disclosed
herein concerns an apparatus for providing an antisurge and/or
antichoke control for a compression system comprising at least one
compressor, the apparatus performing the control method in an
embodiment is defined above. According to yet a further aspect, the
subject matter disclosed herein concerns a compression system
comprised of at least one compressor and the apparatus in an
embodiment for antisurge and/or antichoke control.
[0039] According to a further aspect, the subject matter disclosed
herein concerns a method for regulating a turbocompressor,
comprising the following steps: determining at least one compressor
operating envelope on a corrected speed versus pressure ratio
diagram or performance map, the operating envelope being bounded by
a choke limit line, a surge limit line, a maximum admissible
corrected speed line and minimum admissible corrected speed line;
continuously determining an operative point of the turbocompressor
on the corrected speed versus pressure ratio diagram; determining
whether the operative point is contained in the operating envelope;
acting upon an actuating system to modify at least one operating
parameter of the turbocompressor, if the operative point of the
turbocompressor is not within the operating envelope. Since, the
present can avoid the dispersion of gas in the environment, it's
particularly suitable for polluting gasses.
[0040] Features and embodiments are disclosed here below and are
further set forth in the appended claims, which form an integral
part of the present description. The above brief description sets
forth features of the various embodiments of the present invention
in order that the detailed description that follows may be better
understood and in order that the present contributions to the art
may be better appreciated. There are, of course, other features of
the invention that will be described hereinafter and which will be
set forth in the appended claims. In this respect, before
explaining several embodiments of the invention in details, it is
understood that the various embodiments of the invention are not
limited in their application to the details of the construction and
to the arrangements of the components set forth in the following
description or illustrated in the drawings. The invention is
capable of other embodiments and of being practiced and carried out
in various ways. Also, it is to be understood that the phraseology
and terminology employed herein are for the purpose of description
and should not be regarded as limiting.
[0041] As such, those skilled in the art will appreciate that the
conception, upon which the disclosure is based, may readily be
utilized as a basis for designing other structures, methods, and/or
systems for carrying out the several purposes of the present
invention. It is important, therefore, that the claims be regarded
as including such equivalent constructions insofar as they do not
depart from the spirit and scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] A more complete appreciation of the disclosed embodiments of
the invention and many of the attendant advantages thereof will be
readily obtained as the same becomes better understood by reference
to the following detailed description when considered in connection
with the accompanying drawings, wherein:
[0043] FIG. 1 illustrates a schematic of a compressor system
according to the current art;
[0044] FIG. 2 illustrates a compressor performance map currently
used in antisurge and antichoke control systems;
[0045] FIG. 3 illustrates a schematic representation of a
compressor system according to the present disclosure;
[0046] FIG. 4 illustrates a schematic representation similar to the
one of FIG. 3 in a system comprising a turbocompressor with movable
variable stator vanes;
[0047] FIG. 5 illustrates a compressor performance map according to
the present disclosure showing one operating envelope;
[0048] FIG. 6 illustrates a compressor performance map showing two
overlapping envelopes corresponding to two different positions of
the movable inlet guide vanes or variable stator vanes of the
turbocompressor;
[0049] FIG. 7 illustrates a flow diagram summarizing the control
algorithm according to the present disclosure.
DETAILED DESCRIPTION
[0050] The following detailed description of the exemplary
embodiments refers to the accompanying drawings. The same reference
numbers in different drawings identify the same or similar
elements. Additionally, the drawings are not necessarily drawn to
scale. Also, the following detailed description does not limit the
invention. Instead, the scope of embodiments of the present
invention is defined by the appended claims.
[0051] Reference throughout the specification to "one embodiment"
or "an embodiment" or "some embodiments" means that the 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 phrase "in one
embodiment" or "in an embodiment" or "in some embodiments" in
various places throughout the specification is not necessarily
referring to the same embodiment(s). Further, the particular
features, structures or characteristics may be combined in any
suitable manner in one or more embodiments.
[0052] FIG. 3 schematically illustrates a compressor system 20
embodying the subject matter disclosed herein. The compressor
system 20 comprises a turbocompressor 21, for example a centrifugal
or an axial turbocompressor. The turbocompressor 21 can be driven
into rotation by a mover 23. In some embodiments the mover 23 can
be an electric motor. In other embodiments the mover 23 can be a
gas turbine, for example an aeroderivative gas turbine. In yet
further embodiments different prime movers can be used, for example
a steam turbine. A load coupling 25 connects the prime mover 23
with the turbocompressor 21. A speed manipulation device (not
shown), for example a gear box, can be arranged between the prime
mover 23 and turbocompressor 21.
[0053] A working gas is fed to the inlet or suction side of
turbocompressor 21 through a suction line 25 and the compressed
fluid is delivered from the delivery side of the compressor through
a delivery line, or pressure line, 27. Non-return or check valves
29A and 29B can be arranged in the suction line and/or in the
pressure line 27.
[0054] A heat exchanger 31 can be arranged on the pressure line 27
or, as shown in dotted lines at 31x along a bypass line 33
connecting the pressure line 27 to the suction line 25. An
antisurge, bypass valve 35 is arranged along the bypass line 33.
The antisurge valve 35 is controlled by an antisurge control system
37 that will be described in more detail here below.
[0055] In a particular embodiment, e.g. when the working fluid is
air or anyhow a fluid which can be discharged in the environment,
the antisurge valve 35 can be arranged on a bleeding line
discharging the working fluid directly in the atmosphere, even if a
part is recirculated through the suction line 25.
[0056] To detect the working gas entering in the turbocompressor,
the system could be equipped with an arrangement 50 for detecting
the composition or kind of gas. This arrangement could be a more
particular kind of gas chromatograph.
[0057] A control unit 39 is further provided in the system 20. The
control unit 39 is interfaced with a temperature sensor 41 at the
inlet or suction side of the turbocompressor 21, as well as with
pressure sensors. The pressure sensors directly or indirectly
provide a measure of the pressure ratio between the delivery side
and the suction side of the compressor. By way of example, in the
schematic of FIG. 3 a delivery side pressure sensor 43 provides a
value Pd of the delivery pressure at the discharge side of the
turbocompressor 21. A pressure sensor 45 at the inlet of the
turbocompressor 21 provides a measure of the suction process Ps at
the inlet of the turbocompressor 21. The pressure ratio can be
calculated by the control unit 39, based on the two measured
pressure values Pd, Ps. In other embodiments, the pressure ratio
can be determined outside the control unit 39 and a pressure ratio
value can be directly entered in the control unit 39.
[0058] A rotary speed sensor further provides a rotary speed value
N (expressed e.g. in rpm) to the control unit 39.
[0059] Based on the operating parameters mentioned above, the
control unit 39 is thus capable of calculating the pressure ratio
of the compressor as well as the so called corrected speed of the
compressor, defined as follows:
Nc = N Ts * C 1 ##EQU00003##
wherein [0060] C1 is a function of temperature, pressure and gas
composition [0061] N is the rotary speed of the turbocompressor 21
and [0062] Ts is the absolute temperature at the suction of the
turbocompressor 21.
[0063] The factor C1 is a function of gas composition and it is
assumed constant if the gas has an invariable composition and T and
P are in a restricted range.
[0064] In case of a gas having a known and constant composition the
corrected speed can be simplified as follows:
Nc = N Ts ##EQU00004##
[0065] The corrected speed can be used to define a compressor
performance map, wherein the corrected speed is reported on one of
the coordinates and the pressure ratio is reported on the other
coordinate.
[0066] FIG. 5 schematically shows a performance map of this kind,
wherein the corrected speed is reported on the vertical axis and
the pressure ratio Pd/Ps is reported on the horizontal axis. On
this performance map, a surge limit line SLL can be drawn. To
prevent surging phenomena, the compressor 21 shall operate so that
the operating point thereof on the performing map of FIG. 5 remains
on the surge control line or on the left side thereof, so that the
compressor will never operate on or beyond the surge limit line
SLL.
[0067] On the same performing map of FIG. 5 a choke limit line CLL
can also be drawn, which indicates the limit beyond which choking
phenomena can occur. To operate the compressor 21 free of choking,
the operating point of the compressor shall not move beyond the
choke limit line CLL, on the left thereof.
[0068] On the performing map of FIG. 5 two further lines are drawn,
namely a minimum admissible corrected speed line (Nc)min and a
maximum admissible corrected speed line (Nc)max. The latter are
straight lines parallel to the horizontal coordinate (abscissa) and
represent respectively: the minimum admissible corrected speed
below which the compressor shall not operate; and the maximum
corrected speed, beyond which the turbocompressor shall
operate.
[0069] The four lines defined above form an operating envelope OE.
The compressor control system shall control the compressor so that
the operative point thereof remains inside the operating envelope
OE. In FIG. 5 an exemplary operating point labeled OP is indicated,
corresponding to a corrected speed value Nc and a pressure ratio
PR=Pd/Ps.
[0070] The control system is designed and arranged so that when the
point OP moves towards the right and reaches the surge limit line
SLL the operating conditions of the turbocompressor 21 are modified
to bring the operating point OP back into the operating envelope
OE. This can be obtained e.g. by opening the antisurge valve 35.
When the operating point OP moves to the left until it achieves the
choke limit line CLL, the control system will operate so as to
modify the flow conditions bringing back the operating point inside
the operating envelope OE. This can be done e.g. by acting upon an
antichoke valve 47.
[0071] Moving below the minimum admissible corrected speed value
(Nc)min is prevented by increasing the rotary speed of the
compressor 21, if the operating point OP moves down reaching the
(Nc)min line. Moving of the operating point above the maximum
admissible corrected speed value (Nc)max is prevented by reducing
the speed of rotation of turbocompressor 21 accordingly.
[0072] In the simplified schematic representation of FIG. 3 the
turbocompressor 21 is not provided with movable inlet guide vanes
or variable stator vanes (VSVs). These latter are usually provided
in common turbocompressors to modify the geometry of the inlet
cross section depending upon the operating conditions of the
system. FIG. 4 represents the same system of FIG. 3, with the
addition of variable stator vanes schematically shown at 51. The
same reference numbers as in FIG. 3 indicates the same or
corresponding components or parts, which will not be described
again. In the system of FIG. 4 the control unit 35 further receives
information on the actual position of the variable stator vanes of
the turbocompressor 21. Reference VSV indicates the information
concerning the actual position of the variable stator vanes during
operation of the turbocompressor 21. The VSV position can be set by
suitable actuators, not shown. The actuators can be controlled by
the same control unit 37.
[0073] The antichoke valve 47 has been omitted from the schematic
of FIG. 4, since choking can be prevented alternatively by acting
upon the VSV. The latter are closed to reduce the inlet volume flow
rate of the turbocompressor to avoid compressor choking, without
necessarily using an antichoke valve.
[0074] In actual fact, for each possible position of the movable
variable stator vanes 51 a different performance map and therefore
a different operating envelope can be drawn. This is schematically
represented in FIG. 6, wherein two different operating envelopes
labeled OE1 and OE2 are represented. Each operating envelope is
bounded by four curves which are defined in both instances in the
same way as described above in connection with FIG. 5. Therefore,
each operating envelope is bounded by a surge limit line SLL, a
choke limit line CLL, a maximum admissible corrected speed line
(Nc)max, and a minimum admissible corrected speed line (Nc)min.
[0075] As a matter of fact an indefinite number of operating
envelopes can be provided, one for each position of the variable
stator vanes. Data defining each operating envelope can be stored
in a memory accessible by the control unit 37, and schematically
shown in 38 in FIGS. 3 and 4. In practical embodiments, only a
finite number of operating envelopes will be calculated and stored
for example in the form of lookup tables or the like. During
operation of the turbocompressor 21, the control unit 37 will use
the operating envelope corresponding to the actual position of the
variable stator vanes, if such envelope exists. If the actual
position of the variable stator vanes is different from those for
which an operating envelope has been stored in the control system,
the control unit will calculate an operating envelope, for example
by interpolation of the existing data, using the data corresponding
to the two nearest VSV positions, for which the operating envelopes
are available in the storage memory.
[0076] The operation of the system described so far will become
clearer from the following description with reference to the above
figures as well as referring to the flow chart of FIG. 7. In the
latter the control process is represented as a sequence of steps.
It shall be understood that in order to obtain a continuous control
of the operating conditions of the compressor 21, the sequence of
method steps represented in FIG. 7 will be repeated continuously
and iteratively during operation of the system.
[0077] At start of the control process, the corrected speed Nc is
determined. This is done by detecting the rotary speed N and the
temperature Ts at the suction side of the turbocompressor 21. If
the turbocompressor 21 is provided with variable stator vanes 51 as
described in connection with FIG. 4, the VSV position is
determined. Based on the data concerning the actual position of the
variable stator vanes, the operating envelope is calculated, using
data stored for example in the storage memory 38. As mentioned
above, for some of the variable stator vane positions operating
envelope data can be directly stored in the storage memory 38. For
other intermediate positions the operating envelope can be
calculated by for example interpolating the existing data.
[0078] Once the operating envelope has been determined, the maximum
and minimum pressure ratio for the actual corrected speed Nc can be
determined. This maximum and minimum ratios are indicated PRmax and
PRmin in FIG. 5.
[0079] The actual operating point of the compressor is then
determined based on the corrected speed Nc calculated as mentioned
above and on the actual pressure ratio determined by the pressure
sensors, which measure the delivery pressure Pd and the suction
pressure Ps of the turbocompressor 21. The actual pressure ratio is
indicated PR in FIG. 5.
[0080] At this point the control system has all the data required
to act upon the antisurge and/or the antichoke arrangement to
prevent choking or surging of the system. In some embodiments a
surge parameter and a choke parameter can be calculated as follows.
The surge parameter is defined as
PRmax PR ##EQU00005##
[0081] wherein: [0082] PRmax is the maximum admissible pressure
ratio; [0083] PR is the actual pressure ratio. [0084] The choke
parameter can be defined as follows:
[0084] PRmin PR ##EQU00006##
[0085] wherein: [0086] PRmin is the minimum admissible pressure
ratio.
[0087] The surge parameter and the choke parameter can be used to
generate control signals acting upon actuators controlling the
antisurge valve 35 and the antichoke valve 47 and/or the VSV 51. If
the surge parameter becomes equal to 1, i.e. the compressor
operating point moves on the surge control line, the actuator of
antisurge valve 35 will be acted upon to at least partly open the
antisurge valve 35 on the bypass line 33. Working gas is
recirculated from the delivery side to the suction side of the
compressor to move the operating point OP back into the operating
envelope OE. If the choke parameter becomes equal to 1, the
antichoke valve 47 on the suction line 25 will be partly closed to
reduce the suction flow rate and move the operating point of the
compressor back inside the operating envelope OE.
[0088] The actual corrected speed value Nc being known by the
control system, also correction of the rotary speed N of the
turbocompressor 21 can be performed if required, in order to
prevent the corrected speed to drop below the minimum admissible
value (Nc)min or to increase above the maximum admissible value
(Nc)max. In some embodiments, if the corrected speed value Nc drops
below the minimum or rises above the maximum admissible values, the
position of the VSVs can be modified to move the turbocompressor in
a different operating point of a different operating envelope.
[0089] While the disclosed embodiments of the subject matter
described herein have been shown in the drawings and fully
described above with particularity and detail in connection with
several exemplary embodiments, it will be apparent to those of
ordinary skill in the art that many modifications, changes, and
omissions are possible without materially departing from the novel
teachings, the principles and concepts set forth herein, and
advantages of the subject matter recited in the appended claims.
Hence, the proper scope of the disclosed innovations should be
determined only by the broadest interpretation of the appended
claims so as to encompass all such modifications, changes, and
omissions. In addition, the order or sequence of any process or
method steps may be varied or re-sequenced according to alternative
embodiments.
* * * * *