GASTECH 2OO2. Everett Hylton, President, Ebara International Corporation. Hans Kimmel, Vice President, Ebara International Corporation

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1 PROGRAMME GASTECH 2OO2 Everett Hylton, President, Ebara International Corporation EVERETT E. HYLTON, President of Ebara International Corporation, after serving in the U.S. Navy, received a B.Sc. from the University of Southern California. He was associated with major cryogenic valve and pump companies prior to forming Cryodynamics in The introduction of Cryodynamics Thrust Equalizing Mechanism (TEM ) dramatically improved the reliability of submerged motor-driven cryogenic centrifugal pumps. Mr. Hylton continues to take an active role in the industry, and has been instrumental in key developments of cryogenic pumps and turbine technology. Hans Kimmel, Vice President, Ebara International Corporation HANS E. KIMMEL is Vice President for Ebara International Corporation, USA and responsible for Research and Development of cryogenic pumps and turbines. Dr. Kimmel obtained his Masters degree in Mechanical and Process Engineering and his Ph.D. in Munich, Germany. He is a member of the German Federal Board of Independent Professional Experts in the field of Cryogenic Turbomachines. Dr. Kimmel s contributions include a variety of patents and technical publications.

2 Exducer Turbines, the Optimized Solution for LNG Expanders Everett H. Hylton President Hans E. Kimmel Vice President Ebara International Corporation Cryodynamics Division Sparks, Nevada, USA

3 Exducer Turbines, the Optimized Solution for LNG Expanders Everett H. Hylton President Hans E. Kimmel Vice President Ebara International Corporation Cryodynamics Division Sparks, Nevada, USA SUMMARY The liquefaction process for natural gas requires LNG expanders capable of handling variable mass flows and differential pressures. The first such expanders were installed at National Helium in Liberal, Kansas, USA and at MLNG Dua in Bintulu, Malaysia. These were designed as hydraulic turbines with variable inlet guide vanes and air-cooled generators. Sealing problems, low reliability and hydraulic efficiency losses due to loose guide vane clearances resulted in the development of a totally new design concept for the next generation of expanders. This second generation of LNG expanders, designed as a variable speed submerged cryogenic turbine generator, eliminated the variable guide vane losses, but required step-down and step-up transformers and electronic frequency converters to properly interface with the power grid and variable fluid flow. For power generation in ranges above 1.5 MW the high cost of the electronic frequency converters make them less than ideal. At these power levels constant speed submerged cryogenic turbine generators become the preferred solution. LNG expanders require a design that permits the turbine generators to adjust their operation according to a variety of input parameters. The latest generation of LNG expanders uses variable pitch Exducer Turbines operating at constant speed, thus eliminating the need for expensive transformers and frequency converters while increasing overall system efficiency and total power output. In this paper, the design and performance of these Exducer Turbines is presented and compared with earlier generations of variable speed cryogenic turbines. Exducer Turbines for liquefied gases are the optimum solution for all applications in new gas liquefaction plants of any size and process, in floating LNG production units, for de-bottlenecking of existing LNG plants, and for two-phase expansion utilizing specialized processes with partial LNG vaporization. In addition, two-phase expanders offer significant improvements in the thermodynamic process of LNG liquefaction or for nitrogen rejection processes. PREFACE The liquefaction technology of natural gas involves a complex process with numerous systems interacting to produce the desired output. The conventional liquefaction process for natural gas and other hydrocarbons is to compress and maintain the gas at a high pressure through the condensation phase, after which the high pressure of the condensed liquefied gas is reduced by expansion across a Joule-Thomson valve (1,4). The Joule-Thomson expansion is essentially a constant enthalpy process, in which the inlet and exit velocities of the fluid are equal and the heat transfer is negligible. A temperature rise of approximately 1-2 C can be expected in the process liquid stream during this type of expansion. Hylton 1

4 Any temperature rise during the liquefaction process is undesirable and efforts are being taken in recent liquefaction plant design to change the isenthalpic expansion process to an almost isentropic expansion process. An isentropic expansion results in a much lower exit temperature as compared to the Joule-Thomson expansion process. Cryogenic LNG expanders are designed to achieve an LNG expansion process as close as possible to an isentropic thermodynamic process. The first generation of LNG expanders with external generators performed with 75% isentropic efficiency (2). The second generation the variable speed LNG expanders with submerged generator are operating at Oman LNG (7) with an isentropic efficiency of 82%. Further improvements in hydraulics and generator design have resulted in isentropic efficiencies on the test stand of up to 88%. Cryogenic liquid expanders with high isentropic efficiencies cool down the LNG production stream (4) (6) by approximately 2-3 C and increase the total LNG output and the overall plant profitability proportionally to the enthalpy reduction. The payback time of LNG expanders is one of the shortest among all investments in petrochemical equipment. LNG EXPANDERS WITH EXTERNAL GENERATOR The first generation of LNG expanders were installed in the mid-nineties at the National Helium Corporation in Liberal, Kansas, USA and at the MLNG-Dua plant in Bintulu, Sarawak, Malaysia. These first LNG expanders included a constant speed turbine mechanically coupled to an external aircooled generator. The turbines were fitted with semi-axial inflow runners and complex, mechanically adjustable inlet guide vanes to accommodate variations in operating conditions. The adjustable inlet guide vanes directed the fluid stream onto the runner to approximate the different fluid velocity vector angles brought about by changes in flow rate These concepts were merely extensions of existing water-power turbine technology. The resultant machines were heavy, large and mechanically complex. Figure 1 shows the first generation LNG expander at the MLNG-Dua plant in Bintulu, Malaysia. The extremely large dimensions of the expander require a platform of three floors. This design, with an external generator, includes a heated double cryogenic mechanical shaft seal, a precision aligned mechanical shaft coupling between the turbine and the generator and a heavy duty thrust bearing to support the weight of the entire rotating assembly. The isentropic efficiency of these first generation expanders was reported to be 75 %, but the downtime of the expanders was exceptionally high due to failures caused by the rudimentary design (2). LNG EXPANDERS WITH SUBMERGED GENERATOR The design of the second generation of LNG expanders used fixed geometry guide vanes, radial inflow turbine hydraulics and submerged induction generators operating at variable speed. The entire unit, both turbine and generator, are totally immersed in the LNG fluid stream. This approach completely eliminated the need for any dynamic mechanical seals or couplings, and eliminated the need for heavy supporting structures and the critical alignment between turbine and generator. Figure 2 compares the size and the weight of first and second-generation LNG expanders. The total weight of the second-generation expander is reduced to less than one third and the height to less than one half that of first generation designs. Second generation LNG expanders are operating at Oman LNG in Sur, Oman and are undergoing installation at MLNG-Tiga in Bintulu, Malaysia. Figure 3 shows the installation of the variable speed LNG expander at the construction site of MLNG- Tiga, Malaysia. As may be seen, the small dimensions of the expander require only a single platform. Figure 4 presents the basic design of a second generation LNG expander showing three turbine stages, an induction generator, rotating shaft, housing, power cables and containment vessel with inlet and outlet piping. The ball bearings are submerged and lubricated by the cryogenic fluid and a special Hylton 2

5 thrust-balancing device is mounted between turbine and generator to eliminate thrust loading and increase the operational life of the bearings. Submerged generator LNG expanders with fixed geometry guide vanes operate at variable speed to adjust the expander performance to the requirements of the liquefaction process. Figure 10 depicts the typical performance for variable speed LNG expanders as a function of differential head and flow. In the case of no-load operation (8) with de-energized generator the expander is operating along the noload characteristic curve N. With increasing differential head or increasing flow the rotational speed is increasing. Assuming that the rotor of the expander is prevented from rotating, then the expander will operate like an orifice without producing any power. The differential head across the expander follows a typical parabolic orifice curve L. If the generator is energized and produces electrical power at a certain rotational speed and frequency, then the expander operates along a certain performance curve T 0 to the right side of N and above L. If the rotational speed of the expander increases or decreases, then the performance curves shift to higher (T 1 ) or lower (T 2 ) differential heads. All of the performance curves T N for different rotational speeds N are approximately parallel to the orifice curve L and are shifted in the vertical direction of the differential head. This principle follows from the application of the affinity laws for hydraulic turbines and pumps. For any given value of flow and differential head within the range between the no-load and orifice characteristics N and L there is a certain rotational speed, which intersects with this given value. This feature enables the fixed geometry variable speed LNG expander to operate over a wide range of flows and differential heads. SUBMERGED GENERATOR RELIABILITY An often-overlooked feature of second generation LNG expanders with submerged generators is the reliability of the generator itself. Unlike in-air generators, a submerged generator is not subject to thermal degradation or environmental contamination. Being submerged in the pure fluid stream, the absence of O 2 precludes the possibility of oxidation and aging weakening the insulation system. Further, by maintaining an operating temperature just slightly above that of the ambient cryogenic fluid, thermal deterioration is virtually nonexistent. Laboratory tests using calibrated dynamometers have shown that the temperature rise of a motor or generator operating fully loaded while submerged in a cryogenic dielectric fluid will be on the order of 5 to 10 O C. Thus the insulation temperature of a generator operating in LNG at 161 O C. will be on the order of 155 O C. Extrapolating from IEEE thermal rating charts (9) these temperatures will result in an insulation system having an almost infinite thermal life. In addition to being in a controlled temperature and contamination free environment, a submerged cryogenic generator is also intrinsically free from corona and partial discharge. (10) This is due to the fact that in a cryogenic dielectric liquid environment, there are no voids in the insulation system where ionization can occur as any void which may exist while in air quickly fills up with the dielectric fluid through either direct or molecular migration. This combination of cryogenic temperatures, a void free insulation and above atmospheric operating pressures prevent PDIV or CIV from occurring. (11) Laboratory tests have proven conclusively that neither corona nor partial discharge can exist in this environment. (10) Lastly, it has been demonstrated that the dielectric strength of many insulation materials actually improve when they are submerged and operated in cryogenic fluids. Two such materials used on submerged generators are polyester films and mica tapes. The following table depicts the difference in breakdown strengths of these two materials in air and in LN 2. (12) Similar results occur in LNG. Material BDS (kv/mm) in air BDS (kv/mm) in LN 2 Polyester film Mica Tape Hylton 3

6 What this all means is that with identically rated units, one operating in air and the other submerged in a clean cryogenic dielectric fluid, the submerged unit will be less susceptible to environmental damage, thermal degradation and transient voltage spikes and will have a greater MTBF than the air unit. Simply put, a submerged generator has greater reliability than a generator of the same rating operating in air so long as proper operating parameters are maintained. The fluid flow through the expander establishes the variable speed of the generator and the frequency of its generated power is a function of this rotational speed. By means of a feedback circuit coupled to a variable speed constant frequency (VSCF) converter, the variable frequency thus generated is transformed to the appropriate power grid frequency. VSCF converters have been in use for variable speed wind turbine control and other applications for many years and are a proven technology in this field. Most power grids supplying large liquefaction facilities operate at relatively high voltages, in the range of 6.6 kv. However, VSCF converters currently are only rated to operate at medium voltages. Thus it becomes necessary to connect the VSCF through step-down and step-up transformers both to the generator and to the power grid. With increasing power demands the size and capital investment for large VSCF converters and their associated equipment grows in direct proportion to the increase in power. However third generation fixed speed LNG expanders have been shown to increase in cost by only the square root of the power change. The crossover point for these two approaches appears to be in the vicinity of 1.5 MW. This would indicate that for applications requiring power generation in excess of 1.5 MW, third generation LNG expanders operating at constant speed are the optimum solution. LNG EXPANDERS USING EXDUCER TURBINES Third generation LNG expanders operate at a constant speed, thus eliminating the need for transformers and frequency converters. As stated previously, the liquefaction process requires flexible mass flow and differential pressure operation, thus necessitating an expander capable of absorbing these variations. To meet this requirement the latest LNG expanders are designed as hybrid machines combining a radial inflow reaction turbine with a variable pitch blade axial propeller turbine. Figure 5 shows a cross section of a third generation LNG Exducer Turbine. Unlike the first and second-generation turbines, the main LNG flow direction is upward, counter to gravity. LNG is slightly compressible and the internal convective forces easily support an upward flow. As with second-generation LNG expanders, the entire Exducer Turbine is completely submerged in LNG to eliminate dynamic mechanical seals and couplings and to take advantage of the previously listed benefits of a submerged generator. In Figure 5, the radial inflow reaction turbine with fixed geometry guide vanes consists of three stages to expand the majority of the fluid pressure and to generate the requisite amount of electrical power. The exducer stage is a unique design variable pitch axial propeller turbine mounted on a common shaft directly above the reaction turbine. The blade pitch of the exducer rotor is continuously electrically adjustable, allowing for continuous operation of the expander at the best efficiency point (BEP) within a specific range of mass flows and differential pressures. Figure 6 shows a cross section of the exducer stage. The particular design of the exducer rotor consists of a concentric hemispherical shroud and hub. The shape of the blades corresponds to the sector of the circular cross-section of shroud and hub and the blade rotational axis intersects with the center of the concentric hemispherical shroud and hub. This hemispherical design is the only rotatory geometric solution for adjustable blades which is able to seal both shroud and hub for all angular blade positions. The adjustment range of the exducer blades extends from horizontal to vertical, corresponding to completely closed to completely open blade positions. The blades are fitted with guide vanes to direct the flow through the hemispherical exducer. These guide vanes are shaped as parts of concentric spherical shells. Hylton 4

7 Figure 11 shows the typical performance of Exducer Turbine Expanders as a function of differential head and flow. In the case of no-load operation (8) with a de-energized generator, the expander is operating along one of the no-load characteristic curves N X. The position of these curves N X depends on the angular position X of the exducer blades. If the blade position for the rated case generates the no-load characteristic curve N 0, then by closing the exducer blades, the no-load characteristic curve moves to the position N 1 towards higher differential pressures. By opening the exducer blades, the noload characteristic curve moves to the position N 2 towards lower differential pressures. The corresponding orifice curve for the rated case is L 0. For the closing and opening blade positions the corresponding curves are L 1 and L 2. If the generator is energized and produces electrical power at the fixed rotational speed, then the exducer turbine expander operates at the rated case along the performance curve T 0 to the right of N 0 and above L 0. If the exducer blade position is closing or opening, the performance curves T 1 or T 2 to the right of N 1 or N 2 and above L 1 or L 2 determine the expander operation. All of the performance curves T X for different blade positions X intersect at one point located at the coordinate axis for the differential head. The location of this point is determined by the rotational speed of the expander. With an increasing flow rate, the performance range is increasing. For any given values of flow and differential head within this range, there is a certain exducer blade position that generates the correct performance curve to meet these values. TWO-PHASE EXPANDERS IN LIQUEFACTION OR NITROGEN REJECTION The thermodynamics of gas liquefaction processes indicate significant advantages if the expansion of liquefied gas across an expansion machine is partially or completely carried out into the vapor phase. The most important advantage is the significantly lower LNG temperature resulting from the twophase expansion process. LNG temperatures below the boiling point decrease LNG boil-off losses for storage and shipping phases. The thermodynamic process of the two-phase expansion explains this temperature reduction as the result of the partial vaporization of LNG, described as an enthalpy increase of the LNG vapor with the corresponding enthalpy decrease of the remaining LNG liquid. The second advantage of two-phase LNG expanders is its application in nitrogen rejection units to enrich the LNG production of existing liquefaction plants. All natural gas mixtures contain some quantity of nitrogen as an undesired component. By expanding the liquefied mixture of nitrogen and methane across a two-phase expander almost all nitrogen can be removed due to its lower boiling point compared to methane. Essentially this process can be described as a type of cryogenic distillation across a two-phase expansion turbine. The design of the Exducer Turbine Expander with the added feature of variable speed is capable of expanding liquefied gas into the vapor phase. The vaporization takes place within the exducer stage and is controlled by the angular position of the exducer blades and the rotational speed of the generator. Figure 7 shows a cross-section of the two-phase expander for nitrogen rejection consisting of one radial inflow turbine runner and one exducer with variable blade pitch. The saturation point of the expanding liquid occurs at the inlet of the exducer stage. Figure 8 and 9 demonstrates the exducer stage with 45 0 and 5 0 blade pitch. Rotational speed and blade pitch determines the total amount of vaporization. Figure 12 depicts the typical performance curve of a two-phase exducer turbine. Let T 0 be the performance curve of the two-phase expander for a given rotational speed and given blade position. By increasing the rotational speed the performance curve moves to T 1 or alternatively by changing the angular blade position the performance curve moves to T 01. For any given flow within a certain range the two-phase expander is able to meet the correct overall differential pressure and the correct intermediate differential pressure to vaporize the liquefied gas to the specified degree by applying the two control parameters of the expander: variable speed and Hylton 5

8 variable blade pitch. Figure 13 shows the performance field of two-phase exducer turbines operating with variable speed and variable blade pitch. Liquid expansion is performed across the radial inflow reaction turbine stages and vaporization is performed across the exducer stage. To meet this requirement the saturation point of the liquefied gas, which is determined by the thermo-physical properties, has to be located between the last stage of the reaction turbine and the exducer turbine. The two control parameters of the two-phase expander, the variable speed and the variable exducer blade pitch, enable the two-phase expander to adjust the saturation point across the stages of the turbine expander. ACKNOWLEDGEMENT The authors wish to acknowledge the generous assistance received from Mr. Norrazak Haji Ismail, General Manager, Technical Division, Petronas, Malaysia LNG Tiga and from Mr. John Edward Bol (PMT), MLNG Tiga Plant Project, Main Site Office, Bintulu, Sarawak, Malaysia, regarding liquid expanders at MLNG Dua and Tiga. REFERENCES 1) Kimmel H.E. "Variable Speed Turbine Generators in LNG Liquefaction Plants" Proceedings of the GASTECH '96, Vienna, Austria, December ) Verkoelen J. "Initial Experience with LNG/MCR Expanders in MLNG-Dua" Proceedings of the GASTECH '96, Vienna, Austria, December ) Baines N.C., Oliphant K.N., Kimmel H.E., Habets G.L.G.M. "CFD Analysis and Test of a Fluid Machine Operating as a Pump and Turbine" IMechE Seminar Publication, CFD in Fluid Machinery, 15 Oct. 1998, London, U.K., ISSN , ISBN X 4) Habets G.L.G.M., Kimmel H.E. "Economics of Cryogenic Turbine Expanders" The International Journal of Hydrocarbon Engineering, December/January 1998/99, Palladian Publications, U.K. 5) Habets G.L.G.M., Kimmel H.E. "Development of a Hydraulic Turbine in Liquefied Natural Gas" IMechE Conference Transactions, Seventh European Congress on Fluid Machinery for the Oil, Petrochemical, and Related Industries, April 1999, The Hague, The Netherlands, ISSN , ISBN , London, U.K. 6) Song M.C.K., Kimmel H.E. "Cooling Cycle Expanders Improve LNG Liquefaction Process" Third Joint China/USA Chemical Engineering Conference, September 2000, Beijing, China 7) Van den Handel R.J.A.N., Kimmel H.E. "A New Generation of Liquid Expanders in Operation at Oman LNG" Proceedings of the GASTECH 2000, Houston, Texas, USA, November ) Hylton E.H., Kimmel H.E. "Upgrading Existing LNG Plants Using Exducer Turbine Expanders" GASEX 2002 Brunei Darussalam, Conference Papers, May ) Shively, R.A. and Miller, H., Development of a Submerged Winding Induction Generator for Cryogenic Applications, Proceedings of the IEEE Electrical Insulation Conference, Anaheim, California, April ) Bulinski, A. and Densley, J., High Voltage Insulation for Power Cables Utilizing High Temperature Superconductivity, IEEE Electrical Insulation Magazine, March/April ) Husain, E., et.al, Dielectric Behavior of Insulation Materials Under Liquid Nitrogen, Proceedings of the IEEE Electrical Insulation Conference, Cincinnati, Ohio, October Hylton 6

9 Figure 1: First generation LNG expander with air-cooled generator and rotating shaft seal. Hylton 7

10 AIR COOLED GENERATOR 7000 mm SEAL, COUPLING & THRUST BEARING TURBINE SUBMERGED TURBINE GENERATOR 3370 mm First Generation Power Output = 900 kw Total Weight = 27,000 kg. (Flowserve Corp.) Second Generation Power Output = 1000kW Total Weight = 7,600 kg. (Ebara Intl. Corp.) Figure 2: Comparison in size and weight between first and second-generation LNG expanders. Hylton 8

11 Figure 3: Second-generation LNG expander with submerged generator and variable speed control. Hylton 9

12 Generator Stator Generator Rotor Thrust Equalizing Mechanism (TEM) Fixed Geometry Inlet Guide Vanes Runners Figure 4: Basic design of second generation submerged LNG expanders. Hylton 10

13 Turbine Exducer Runners Fixed Geometry Inlet Guide Vanes Thrust Equalizing Mechanism (TEM) Generator Stator Generator Rotor Figure 5: Third generation LNG expander with constant speed and variable pitch blade exducer. Hylton 11

14 Figure 6: Cross section showing the radial turbine with the exducer turbine. Hylton 12

15 Turbine Exducer Runner Fixed Geometry Inlet Guide Vanes Thrust Equalizing Mechanism (TEM) Generator Stator Generator Rotor Figure 7: Two-phase LNG expander. Hylton 13

16 PROGRAMME Figure 8: Two-phase exducer with 45 blade pitch. Figure 9: Two-phase exducer with 5 blade pitch. Hylton 14

17 Figure 10: Typical Performance for Variable Speed LNG Expanders Figure 11: Typical Performance for Exducer Turbine Expanders Hylton 15

18 Figure 12: Typical performance of two-phase exducer turbine. Figure 13: Performance field of two-phase exducer turbine. Hylton 16