(19)

& 
(11) EP 2 400 117 A1
(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.:

28.12.2011 Bulletin 2011/52 F01D 17/16 (2006.01) F02C 9/20 (2006.01)
F25B 11/02 (2006.01) F25J 3/06 (2006.01)
(21) Application number: 11170560.4

(22) Date of filing: 20.06.2011

(84) Designated Contracting States: • Scotti del Greco, Alberto

AL AT BE BG CH CY CZ DE DK EE ES FI FR GB 50127 Firenze (IT)
GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO • Ghiraldo, Stefano
PL PT RO RS SE SI SK SM TR 50127 Firenze (IT)
Designated Extension States:
BA ME (74) Representative: Illingworth-Law, William
Illingworth
(30) Priority: 24.06.2010 IT CO20100034 Global Patent Operation - Europe
GE International Inc.
(71) Applicant: Nuovo Pignone S.p.A. 15 John Adam Street
50127 Florence (IT) London WC2N 6LU (GB)

(72) Inventors:
• Mariotti, Gabriele
50127 Firenze (IT)

(54) Turboexpander and method for using moveable inlet guide vanes at compressor inlet

(57) A turboexpander-compressor system (200) in- pressor (224) and configured to control a pressure of the
cludes an expander (210) configured to expand an in- gas input into the compressor (224), and a controller
coming gas (214), a first set of moveable inlet guide (240) configured to acquire information about a rotating
vanes (218) configured to control a pressure of the in- speed of the shaft (230), a pressure and a temperature
coming gas, a compressor (224) configured to compress of the incoming gas, a pressure and a temperature of the
a gas received from the expander (210), a shaft (230) gas output from the expander (210), and to control the
configured to support and rotate an expander impeller second set of moveable inlet guide vanes (232) to max-
(212) and a compressor impeller (226), a second set of imize a ratio between the rotating speed of the shaft (230)
moveable inlet guide vanes (232) attached to the com- and a drop of an enthalpy across the expander (210), in
off-design conditions.
EP 2 400 117 A1

Printed by Jouve, 75001 PARIS (FR)

 1 EP 2 400 117 A1 2

Description moveable input guide vanes (not shown in Figure 1) may

be used to control the pressure of the incoming gas 40
BACKGROUND OF THE INVENTION entering the turboexpander 10.
[0007] Ideally, at design conditions, the pressure p1 of
Field of the Invention 5 the incoming gas 40, and the pressure p2 of the gas flow
60 output from the turboexpander 10 have predetermined
[0001] Embodiments of the subject matter disclosed values (i.e., within a range around the predetermined val-
herein generally relate to methods and systems for ues). When the pressures p1 and p2 have the predeter-
achieving an enhanced operation of an expander using mined values, a speed u of the shaft is close to a design
moveable inlet guide vanes at a compressor inlet. 10 value. However, the turboexpander-compressor system
at times functions in conditions different from the design
Description of the Prior Art conditions.
[0008] Generally, the turboexpander efficiency is re-
[0002] Turboexpanders are widely used for industrial lated to a ratio of (i) the shaft speed u and (ii) the isoen-
refrigeration, oil and gas processing and in low temper- 15 tropic enthalpy drop across the turboexpander 10. How-
ature processes. Turboexpanders are used, for example, ever, that a real transformation occurs in the turboex-
to extract heavier hydrocarbon gases such as ethane pander 10. The real transformation is determined when
(C2H6), propane (C3H8), normal butane (n-C4H10), isob- knowing a gas composition, the pressure p1 and the tem-
utane (i-C4H10), pentanes and even higher molecular perature T1 of the incoming gas 40, and the pressure p2
weight hydrocarbons, collectively referred to as natural 20 and the temperature T2 of the gas flow 60 output from
gas liquids (NGL), from natural gas. A gas-liquid mixture the turboexpander 10. The isoentropic enthalpy drop
resulting from an expansion of a raw gas in an expander across the turboexpander 10 can be calculated knowing
is usually separated into a gas stream and a liquid stream. the gas composition, the pressure p1, the temperature
Most of the natural gas liquids are removed by outputing T1, and the pressure P2.
the liquid stream separately from the remaining gas 25 [0009] The compression in the compressor 20 pas-
stream, which is usually then compressed to be sent to sively affects the turboexpander efficiency by altering the
downstream users. speed u of the shaft 30. Therefore, in off-design condi-
[0003] Figure 1 illustrates a conventional turboexpand- tions, the turboexpander efficiency is not optimized when
er-compressor system 100 in which a tuborexpander 10 a single parameter, the pressure p1 of the incoming gas
and a compressor 20 have impellers arranged on a same 30 40, is adjusted. Being able to adjust only the pressure p1
shaft 30. The turboexpander 10 is typically a centrifugal of the incoming gas 40 limits an operator ability to opti-
or axial flow expander inside which an incoming gas 40 mize the turboexpander efficiency.
is expanded. The gas expansion produces mechanical [0010] If no additional source of energy is used, the
work causing a rotation of an expander impeller 50. The compression is a by-product of the expansion in the ex-
expanded gas is output as a gas flow 60. The gas flow 35 pander 10. The compression efficiency is determined by
60 output from the turboexpander 10 may be input to the the pressure p3 of the gas input in the compressor 20,
compressor 20 (i.e., the gas flow 70). and a rotation speed of the compressor impeller, which
[0004] After an expansion (an isoentropic expansion is the same as the rotation speed u of the shaft 30.
may be used for calculation purposes) of the incoming [0011] In the conventional turboexpander-compressor
gas 40 having a pressure p1 and a temperature T1 when 40 system capable to adjust only the pressure p1 of the in-
entering the turboexpander 10, the gas flow 60 has a coming gas 40, an operator has no leverage to fully con-
pressure p2 and a temperature T2 which are respectively trol the rotating speed u of the shaft 30 for off-design
lower than the pressure p1 and the temperature T1. conditions.
[0005] Since a compressor impeller 80 is mounted on [0012] Accordingly, it would be desirable to provide
the same shaft 30 as the expander impeller 50, the rota- 45 systems and methods that avoid the afore-described
tion of the expander impeller 50 causes the rotation of problems and drawbacks.
the compressor impeller 80. In this manner, the mechan-
ical work produced in the turboexpander 10 is transferred SUMMARY OF THE INVENTION
to the compressor 20. The expander impeller 50, the
compressor impeller 80 and the shaft 30 rotate at the 50 [0013] According to one exemplary embodiment, a tur-
same speed. The energy of the rotation of the compres- boexpander-compressor system includes an expander,
sor impeller 80 is used in the compressor 20 to compress a first set of moveable inlet guide vanes, a compressor,
the gas flow 70 input at a pressure p3 in the compressor a shaft, a second set of moveable inlet guide vanes and
20. The compressor 20 outputs an output gas flow 90 a controller. The expander is configured to expand an
having a pressure p4 higher than the pressure p3. 55 incoming gas, and has an expander impeller. The first
[0006] The pressure of the incoming gas 40 entering set of moveable inlet guide vanes are attached to the
the turboexpander 10 is often controlled to be maintained expander and are configured to control a pressure of the
around a design value. For example, a set of standard incoming gas. The compressor is configured to compress

2
 3 EP 2 400 117 A1 4

a gas received from the expander, and has a compressor termine when the turboexpander-compressor system
impeller. The shaft is configured to support and rotate functions in off-design conditions, and generate the com-
the expander impeller and the compressor impeller. The mands for the compressor set of moveable inlet guide
second set of moveable inlet guide vanes are attached vanes in order to maintain a ratio between the rotating
to the compressor and are configured to control a pres- 5 speed of the shaft and a drop of enthalpy across the
sure of the gas input into the compressor. The controller, expander within a predetermined range, in off-design
which is connected to the second set of moveable inlet conditions.
guide vanes, is configured to acquire information about
a rotating speed of the shaft, the pressure and a temper- BRIEF DESCRIPTION OF THE DRAWINGS
ature of the incoming gas, and a pressure and a temper- 10
ature of the gas output by the expander. The controller [0016] The accompanying drawings, which are incor-
is also configured to control the second set of moveable porated in and constitute a part of the specification, illus-
inlet guide vanes in order to adjust the pressure of the trate one or more embodiments and, together with the
gas input into the compressor to maximize a ratio be- description, explain these embodiments. In the drawings:
tween the rotating speed of the shaft and a drop of an 15 [0017] Figure 1 is a schematic diagram of a conven-
enthalpy across the expander in off-design conditions. tional turboexpander-compressor system;
[0014] According to another exemplary embodiment, [0018] Figure 2 is a schematic diagram of a turboex-
a method of controlling a turboexpander-compressor pander-compressor system according to an exemplary
system having an expander with an expander impeller embodiment;
connected via a shaft to a compressor impeller of a com- 20 [0019] Figure 3 is a turboexpander-compressor sys-
pressor that compresses a gas flow output by the ex- tem according to another exemplary embodiment;
pander is provided. The method includes receiving first [0020] Figure 4 is a flow diagram of a method of con-
information on a pressure and a temperature of an in- trolling a turboexpander-compressor system according
coming gas entering the expander, receiving second in- to another exemplary embodiment;
formation on a pressure of a gas output by the expander, 25 [0021] Figure 5 is a flow diagram of a method of con-
and receiving third information on a rotating speed of the trolling a turboexpander-compressor system according
shaft attached to the compressor impeller and the ex- to another exemplary embodiment; and
pander impeller. The method further includes determin- [0022] Figure 6 is a controller according to another ex-
ing when the turboexpander-compressor system func- emplary embodiment.
tions in off-design conditions based on the received first, 30
second and third information. The method further in- DETAILED DESCRIPTION OF THE INVENTION
cludes comparing a ratio between the rotating speed of
the shaft and a drop of an enthalpy across the expander [0023] The following description of the exemplary em-
with a predetermined value, when determined that the bodiments refers to the accompanying drawings. The
turboexpander-compressor system functions in the off- 35 same reference numbers in different drawings identify
design conditions. The method also includes controlling the same or similar elements. The following detailed de-
a compressor set of moveable inlet guide vanes connect- scription does not limit the invention. Instead, the scope
ed to a compressor inlet to adjust a pressure of the gas of the invention is defined by the appended claims. The
input in the compressor, to bring the ratio between the following embodiments are discussed, for simplicity, with
rotating speed of the shaft and the drop of the enthalpy 40 regard to the terminology and structure of turboexpander-
across the expander in the off-design conditions closer compressor systems. However, the embodiments to be
to the predetermined value. discussed next are not limited to these systems, but may
[0015] According to another embodiment, a controller be applied to other systems that transfer work generated
configured to control an turboexpander-compressor sys- by an expander to a compressor compressing gas output
tem has an interface and a control unit. The turboexpand- 45 by the expander.
er-compressor system has an expander with an expand- [0024] Reference throughout the specification to "one
er impeller, and a compressor with a compressor impel- embodiment" or "an embodiment" means that a particular
ler, the expander impeller and the compressor impeller feature, structure, or characteristic described in connec-
being rotated by a same shaft, and the compressor com- tion with an embodiment is included in at least one em-
pressing a gas output by the expander. The interface is 50 bodiment of the subject matter disclosed. Thus, the ap-
configured to receive information on a pressure and a pearance of the phrases "in one embodiment" or "in an
temperature of an incoming gas entering the expander, embodiment" in various places throughout the specifica-
a pressure and a temperature of the gas output by the tion is not necessarily referring to the same embodiment.
expander, and a rotating speed of the shaft, and to output Further, the particular features, structures or character-
commands to a compressor set of inlet vanes configured 55 istics may be combined in any suitable manner in one or
to control a pressure of the gas input in the compressor. more embodiments.
The control unit is connected to the interface and is con- [0025] Figure 2 is a schematic diagram of a turboex-
figured to receive the information from the interface, de- pander-compressor system 200 according to an exem-

3
 5 EP 2 400 117 A1 6

plary embodiment. An expander 210 has an expander the incoming gas flow 214 entering the expander 210,
impeller 212. The expander 210 receives an incoming (ii) the pressure p2 of the gas flow 216 output from the
gas flow 214. Inside the expander 210, the gas may ex- expander 210, and (iii) a gas composition. The gas com-
pand and thus, cause rotation of the expander impeller position may be a constant, input manually or provided
212. The expanded gas is output from the expander 210 5 as an output of a gas composition analyzer.
as a gas flow 216. [0032] In reality, the gas expansion in the expander
[0026] When the turboexpander-compressor system 210 is not an ideal isoentropic process. The drop of the
200 functions at design conditions, a pressure p1 and a enthalpy ∆H across the expander 210 may be calculated
temperature T1 of the incoming gas flow 214, as well as using (i) the pressure p1 and the temperature T1 of the
a pressure p2 and a temperature T2 of the gas flow 216 10 incoming gas 214 entering the expander 210, (ii) the pres-
have values close to predetermined values. However, at sure p2 and the temperature T2 of the gas flow 216 output
times, the turboexpander-compressor system functions from the expander 210, and (iii) the gas composition.
in off-design conditions. When off-design conditions oc- [0033] The characteristic parameters (i.e., p1, T1, p2
cur, the pressure p1 of the incoming gas flow 214 may and T2) of the gas expansion in the expander 210 and
be adjusted to become again close to the respective pre- 15 the rotating speed u of the shaft 230 may not vary inde-
determined value, using, for example, a first set of move- pendently. Therefore, in off-design conditions, in order
able input guide vanes (IGV1) 218. The IGV1 218 are to maximize the expander efficiency, the pressure p3 of
attached to an inlet of the expander 210. the gas flow 216 input in the compressor 224 may be
[0027] If the incoming gas flow 214 is a mixture of gas- controlled, for example, by a second set of moveable
es including heavier hydrocarbon gases, most of the 20 inlet guide vanes (IGV2) 232 provided at the compressor
heavier hydrocarbon gases liquefy at the low tempera- inlet. By modifying the pressure p3 of the gas flow 216
tures achieved due to the expansion. In one application, input in the compressor 224, the rotating speed u of the
the liquefied heavier hydrocarbon gases may be re- shaft 230 is modified and, therefore, the expander effi-
moved from the expander 210 as a separate liquid stream ciency in the expander 210 can be maximized.
by a separator (Sep) 220. 25 [0034] The rotating speed u of the shaft 230 may be
[0028] In the turboexpander-compressor system 200 measured at a location between the expander 210 and
illustrated in Figure 2, a compressor 224 has a compres- the compressor 224, next to the shaft 230, for example,
sor impeller 226. The compressor 224 receives the gas by a speed sensor (Su) 234. The pressure p1 and the
flow 216 from the expander 210 and outputs a com- temperature T1 of the incoming gas flow 214 entering the
pressed gas flow 228. However, between the expander 30 expander 210 may be measured, for example, by a sen-
210 and the compressor 224, the pressure of the gas sor (Sp1) 235 and a sensor (ST1) 236, respectively.
flow 216 may be altered due to other process compo- [0035] The pressure p2 and a temperature T2 of the
nents (e.g., separators, coolers, valves) and pressure gas flow 216 may be measured at the output of the ex-
losses, the gas flow 216 having a pressure p3 when input pander 210, for example, by a sensor (Sp2) 237 and a
in the compressor 224. 35 sensor (ST2) 238, respectively. The pressure p3 of the
[0029] The mechanical work generated due to the ex- gas flow 216 at the input of the compressor 224 may be
pansion of the gas rotates the expander impeller 212. measured, for example, by a sensor (Sp3) 239.
The expander impeller 212 is mounted on the same shaft [0036] A controller 240 acquires information regarding
230 as the compressor impeller 226. Due to this arrange- the pressure p1 and the temperature T1 of the incoming
ment, the compressor impeller 226 also rotates due to 40 gas flow 214 entering the expander 210, the pressure p2
the mechanical work generated during the expansion of of the gas flow 216 entering the compressor 224, and
the gas in the expander 210. The rotation of the com- the rotating speed u of the shaft 230, from the sensors
pressor impeller 226 provides energy used to compress 234, 235, 236, and 237, respectively.
the gas in the compressor 224. Thus, if no additional [0037] In one embodiment, the controller 240 may
source of energy is used, the compression is a by-product 45 send commands C1 to IGV1 218 in order to adjust the
of the expansion in the expander 210. pressure p1 of the incoming gas flow 214 to be within a
[0030] Conversely, the mechanical work necessary to predetermined range.
rotate the compressor impeller 226 also named load, af- [0038] Based on monitoring the acquired information,
fects the rotating speed u of the shaft 230, and, thereby, the controller 240 determines when the turboexpander-
indirectly affects the process of expanding the gas inside 50 compressor system 200 functions in off-design condi-
the expander 210. tions. When the controller 240 determines that the tur-
[0031] The expander efficiency is related to a ratio of boexpander-compressor system 200 functions in off-de-
the rotating speed u of the shaft 230, and a drop of an sign conditions, the controller 240 sends commands C2
enthalpy ∆H across the expander 210. The gas expan- to the second set of IGV2 232 to adjust the pressure p3
sion in the expander 210 may be approximated as being 55 of the gas input into the compressor in order to maximize
an isoentropic process. The isoentropic drop of the en- a ratio R between the rotating speed u of the shaft 230
thalpy ∆H across the expander may be estimated as a and the drop of the enthalpy ∆H across the expander 210.
function of (i) the pressure p1 and the temperature T1 of [0039] Figure 3 illustrates an exemplary embodiment

4
 7 EP 2 400 117 A1 8

of a turboexpander-compressor system 201 having the at S445.

expander 210 with the expander impeller 212, and the [0044] The method further includes controlling a com-
compressor 224 with the compressor impeller 226 pressor set of moveable inlet guide vanes connected to
mounted on the same shaft 230. The pressure p1 of the a compressor inlet through which the gas output by the
incoming gas flow 214 entering the expander 210 is ad- 5 expander enters into the compressor, to adjust the pres-
justed by the set of moveable inlet guide vanes 218. The sure p3 of the gas input into the compressor, such as to
pressure p3 of the gas flow input in the compressor 224 bring the ratio R closer to the predetermined value V at
is controlled by the set of moveable inlet guide vanes S460.
232. The expander 210, the shaft 230 and the compres- [0045] The method may further include adjusting an
sor 224 are encased in a casing 250. The casing 250 10 expander set of moveable inlet guide vanes such as IGV1
holds the expander 210, the shaft 230 and the compres- in Figure 2 located at the input of the expander to maintain
sor 224 at predetermined locations relative to each other. the pressure p1 of the incoming gas entering the expand-
[0040] According to an exemplary embodiment, Figure er within a predetermined range.
4 illustrates a method 400 performed by a turboexpander- [0046] When the incoming gas is a mixture including
compressor system having an expander with an expand- 15 heavy components, the method may further include sep-
er impeller connected via a shaft to a compressor impeller arating the heavy components that are liquefied in the
of a compressor that compresses a gas flow output by expander from the gas output from the expander and
the expander. The method, which may be performed by input into the compressor, for example, by a separator
a controller similar to the controller 240 in Figure 2, in- such as Sep 220 in Figure 2.
cludes receiving first information on a pressure p1 and a 20 [0047] The method may calculate the drop of the en-
temperature T1 of an incoming gas entering the expander thalpy across the expander using a function of (i) the
at S410, receiving second information on a pressure p2 pressure p1 and the temperature T1 of the incoming gas,
and a temperature T2 of a gas output by the expander at (ii) the pressure p2 of the gas output from the expander
S420, and receiving third information on a rotating speed and (iii) a gas composition, assuming that the gas ex-
u of a shaft attached to the compressor impeller and the 25 pansion in the expander is an isoentropic process.
expander impeller, at S430. [0048] According to another exemplary embodiment,
[0041] The first information on the pressure p1 and the Figure 5 illustrates a method 500 of controlling a turboex-
temperature T1 of an incoming gas entering the expander pander-compressor system having an expander with an
may be based on measuring the pressure p1 and the expander impeller connected via a shaft to a compressor
temperature T1 at S405, for example, by a sensor such 30 impeller of a compressor that compresses a gas flow
as Sp1 235 and ST1 236 in Figure 2. The second infor- output by the expander.
mation on the pressure p2 and the temperature T2 of the [0049] The method includes receiving first information
gas output by the expander may be based on measuring on a pressure and a temperature of an incoming gas
the pressure p2, at S415. The pressure p2 and the tem- entering the expander at S510, receiving second infor-
perature T2 may be measured, for example, by sensors 35 mation on a pressure of a gas output by the expander at
such as Sp2 237 and ST2 238 in Figure 2, at an exit of S520, and receiving third information on a rotating speed
the expander 210. The third information on the rotating of the shaft attached to the compressor impeller and the
speed u of the shaft may be based on measuring a ro- expander impeller at S530.
tating speed u at S425. The rotating speed u may be [0050] The method further includes determining when
measured, for example, by a sensor such as Su 234 in 40 the turboexpander-compressor system functions in off-
Figure 2 at a location next to the shaft 230, between the design conditions based on the received first, second
expander 210 and the compressor 224 therein. and third information at S540.
[0042] The method illustrated in Figure 4 further in- [0051] The method also includes comparing a ratio be-
cludes determining when the turboexpander-compres- tween the rotating speed of the shaft and a drop of an
sor system functions in off-design conditions at S440. If 45 enthalpy across the expander with a predetermined val-
determined that the turboexpander-compressor system ue, when determined that the turboexpander-compres-
does not function in the off-design conditions, the method sor system functions in the off-design conditions at S550.
loops back receiving information on p1 and T1 (S410), p2 [0052] The method further includes controlling a com-
(S420), and u (S430), and determining whether the tur- pressor set of moveable inlet guide vanes connected to
boexpander-compressor system functions in off-design 50 a compressor inlet and through which the gas output by
conditions (S440). the expander enters into the compressor, to adjust the
[0043] If determined that the turboexpander-compres- pressure of the gas input into the compressor such as to
sor system functions in off-design conditions, the method bring the ratio closer to the predetermined value at S560.
further includes calculating and comparing a ratio R be- [0053] Figure 6 illustrates a controller according to an-
tween the rotating speed of the shaft u and a drop of an 55 other exemplary embodiment. The controller 600 is con-
enthalpy ∆H across the expander with a predetermined figured to control a turboexpander-compressor system
value V at S450. The enthalpy drop may be calculated such as 200 in Figure 2. The controller may include an
as a function of p1, T1, p2, T2, and the gas composition interface 610, a control unit 620 and a memory 630.

5
 9 EP 2 400 117 A1 10

[0054] The interface 610 receives information on the comprehensive understanding of the claimed invention.
pressure p1 and the temperature T1 of the incoming gas However, one skilled in the art would understand that
entering the expander, the pressure p2 of the gas output various embodiments may be practiced without such
by the expander, and the rotating speed u of the shaft, specific details.
for example, from sensors such as Sp1 235, ST1 236, 5 [0059] Although the features and elements of the
Sp2 237, and Su 234 in Figure 2. The interface 610 may present exemplary embodiments are described in the
also receive information about the temperature T2 of the embodiments in particular combinations, each feature or
gas output from the expander and the pressure p3 of the element can be used alone without the other features
gas input in the compressor from sensors such as ST2 and elements of the embodiments or in various combi-
238 and Sp3 239 in Figure 2. The interface 610 is also 10 nations with or without other features and elements dis-
configured to output commands C2 to the compressor closed herein.
set of inlet vanes such as IGV2 232 in Figure 2. The [0060] This written description uses examples of the
interface 610 may also be configured to output com- subject matter disclosed to enable any person skilled in
mands C1 to the expander set of moveable inlet guide the art to practice the same, including making and using
vanes such as IGV1 in Figure 2. 15 any devices or systems and performing any incorporated
[0055] The control unit 620 is connected to the inter- methods. The patentable scope of the subject matter is
face 610 and monitors the information received through defined by the claims, and may include other examples
the interface, to determine, for example, when the tur- that occur to those skilled in the art. Such other examples
boexpander-compressor system functions in off-design are intended to be within the scope of the claims.
conditions. For example, the controller may determine 20
whether the turboexpander-compressor system func-
tions in off-design conditions by comparing values or Claims
functions of the pressure p1, the temperature T1, the pres-
sure p2, and a rotating speed u of the shaft with prede- 1. A turboexpander-compressor system, comprising:
termined values stored in a memory 630. A composition 25
of the gas used to calculate the drop of the enthalpy may an expander configured to expand an incoming
also be stored in the memory 630. gas and having an expander impeller;
[0056] If the control unit 620 determines that the tur- a first set of moveable inlet guide vanes attached
boexpander-compressor system functions in the off-de- to the expander and configured to control a pres-
sign conditions, the control unit 620 generates com- 30 sure of the incoming gas;
mands to be transmitted to the compressor set of move- a compressor configured to compress a gas re-
able inlet guide vanes, to adjust the pressure p3 of the ceived from the expander and having a com-
gas input in the compressor in order to maintain the ratio pressor impeller;
R between the rotating speed u of the shaft and the drop a shaft configured to support and rotate the ex-
of enthalpy ∆H across the expander, within a predeter- 35 pander impeller and the compressor impeller;
mined range. a second set of moveable inlet guide vanes at-
[0057] The control unit 620 may be configured to esti- tached to the compressor and configured to con-
mate the drop in enthalpy ∆H using the information on trol a pressure of the gas input into the compres-
the pressure p1, the temperature T1, the pressure p2 and sor; and
the composition of the gas. The control unit 620 may also 40 a controller connected to the second set of
generate commands to be transmitted to the expander moveable inlet guide vanes and configured to:
set of inlet guide vanes in order to maintain the pressure
p1 within a predetermined range. The interface 610 may acquire information about a rotating speed
then output these commands to the expander set of inlet of the shaft, the pressure and a temperature
guide vanes. The control unit 620 may be implemented 45 of the incoming gas, and a pressure and a
in hardware, firmware, software or a combination thereof. temperature of the gas output by the ex-
[0058] The disclosed exemplary embodiments provide pander, and
a system, a method and a controller which maximize a control the second set of moveable inlet
turboexpander-compressor system efficiency in off-de- guide vanes to adjust the pressure of the
sign conditions using a set of moveable inlet guide vanes 50 gas input into the compressor to maximize
at an inlet of the compressor. It should be understood a ratio between the rotating speed of the
that this description is not intended to limit the invention. shaft and a drop of an enthalpy across the
On the contrary, the exemplary embodiments are intend- expander in off-design conditions.
ed to cover alternatives, modifications and equivalents,
which are included in the spirit and scope of the invention 55 2. The turboexpander-compressor system of claim 1,
as defined by the appended claims. Further, in the de- wherein an expander efficiency is related to the ratio
tailed description of the exemplary embodiments, numer- of the rotating speed of the shaft and the drop of the
ous specific details are set forth in order to provide a enthalpy across the expander.

6
 11 EP 2 400 117 A1 12

3. The turboexpander-compressor system of claim 1 receiving second information on a pressure of a

or claim 2, further comprising: gas output by the expander;
receiving third information on a rotating speed
a first sensor configured to measure the rotating of the shaft attached to the compressor impeller
speed of the shaft; 5 and the expander impeller;
a second sensor configured to measure the determining when the turboexpander-compres-
pressure of the incoming gas; sor system functions in off-design conditions
a third sensor configured to measure the tem- based on the received first, second and third in-
perature of the incoming gas; formation;
a fourth sensor configured to measure the pres- 10 comparing a ratio between the rotating speed of
sure of the gas output by the expander; and the shaft and a drop of an enthalpy across the
a fifth sensor configured to measure the temper- expander with a predetermined value, when de-
ature of the gas output by the expander, termined that the turboexpander-compressor
wherein the drop in the enthalpy is estimated system functions in the off-design conditions;
using the information achieved from the first, 15 and
second, third, fourth and fifth sensors. controlling a compressor set of moveable inlet
guide vanes connected to a compressor inlet
4. The turboexpander-compressor system of any pre- through which the gas output by the expander
ceding claim, further comprising: a sensor located enters into the compressor, to adjust a pressure
at an inlet of the compressor, connected to the con- 20 of the gas input into the compressor such as to
troller and configured to measure a pressure of the bring the ratio closer to the predetermined value.
gas input in the compressor.
10. A controller configured to control a turboexpander-
5. The turboexpander-compressor system of any pre- compressor system having an expander with an ex-
ceding claim, wherein the controller is configured to 25 pander impeller, and a compressor with a compres-
determine when the turboexpander-compressor sor impeller, the expander impeller and the compres-
system functions in off-design conditions. sor impeller being rotated by a same shaft, and the
compressor compressing a gas output by the ex-
6. The turboexpander-compressor system of any pre- pander, the controller comprising:
ceding claim, wherein the controller is configured to 30
control the first set of moveable inlet guide vanes to an interface configured to,
maintain the pressure of the incoming gas in a pre- receive information on a pressure and a temper-
determined range. ature of an incoming gas entering the expander,
a pressure and a temperature of the gas output
7. The turboexpander-compressor system of any pre- 35 by the expander, and a rotating speed of the
ceding claim, wherein the controller is configured to shaft, and
estimate the drop of the enthalpy across the expand- output commands to a compressor set of inlet
er using a function of the pressure of the incoming vanes configured to control a pressure of the
gas, the temperature of the incoming gas, a compo- gas input in the compressor; and
sition of the incoming gas, and the pressure of the 40 a control unit connected to the interface and con-
gas output by the expander. figured to,
receive the first, second and third information
8. The turboexpander-compressor system of any pre- from the interface, determine when the turboex-
ceding claim, further comprising: pander-compressor system functions in off-de-
45 sign conditions, and
a casing configured to hold the expander, the generate the commands for the compressor set
shaft and the compressor at predetermined lo- of moveable inlet guide vanes to maintain a ratio
cations. between the rotating speed of the shaft and a
drop of enthalpy across the expander within a
9. A method of controlling a turboexpander-compres- 50 predetermined range, in off-design conditions.
sor system having an expander with an expander
impeller connected via a shaft to a compressor im-
peller of a compressor that compresses a gas flow
output by the expander, the method comprising:
55
receiving first information on a pressure and a
temperature of an incoming gas entering the ex-
pander;

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