(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. 17 A1 1 00 4 EP 2 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 5 Field of the Invention the incoming gas 40, and the pressure p2 of the gas flow 60output 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-C 4H10), 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 20 weight hydrocarbons, collectively referred to as natural 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 intoa gas stream anda liquidstream. 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 gas25 [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 30 and a compressor 20 have impellers arranged on a same 40, is adjusted. Being able to adjust only the pressure p 1 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 40 gas 40 having a pressure p1 and a temperature T1 when 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 the50 [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 55 having a pressure p4 higher than the pressure p3. 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.