Efficacité supérieure Coût inférieur Plus grande fiabilité et robustesse. PO-6.1

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1 AN ADVANCED CRYOGENIC EXPANDER FOR NATURAL GAS LIQUEFACTION PLANTS AMELIORATION DES PROCEDES DE LIQUEFACTION DE GAZ NATUREL PAR UTILISATION DE TURBINES HYDRAULIQUES S. Gopalakrishnan Pump Division Flowserve Corporation Los Angeles, California, U.S.A. W. Hardeveld Shell Global Solutions International B.V. The Hague, The Netherlands ABSTRACT In modern LNG plants, the pressure reduction of the LNG and the mixed refrigerant liquid from the main cryogenic heat exchanger is achieved through two expanders instead of the Joule Thompson valve in the previous process designs. The use of such expanders unloads the main refrigeration compressors. Alternatively, for the same refrigeration compressor power, the LNG throughput of the plant can be increased significantly. This increased capacity is a function of the expander efficiency and hence there is a strong customer interest in having the highest possible expander efficiency. In this paper, the development of an advanced cryogenic expander, that will be the successor to the expander types currently applied in the Malaysia LNG and Nigeria LNG plants, is presented. The objective of this new expander design is: Increased efficiency Reduced cost and Enhanced reliability and robustness. RESUME Dans les cycles de liquéfaction modernes de gaz naturel, la détente du GNL et du mélange réfrigérant en phase liquide à la sortie de l'échangeur cryogénique est effectuée au travers de deux turbines hydrauliques au lieu des vannes Joule Thompson utilisées auparavant. Ces turbines hydrauliques soulagent les compresseurs réfrigérants principaux. Donc, pour un compresseur de puissance de réfrigération identique, la production de GNL sera augmentée de façon significative. Cette augmentation de capacité est fonction de l'efficacité de la turbine hydraulique et, de ce fait, il est indispensable de sélectionner une turbine hydraulique ayant le rendement le plus élevé possible. Dans cet article, le développement d'une turbine hydraulique nouvelle génération, successeur des turbines hydrauliques actuellement installées sur les GNL de Malaysie et de Bonny au Nigeria, sera présenté. Les objectifs recherchés lors du développement de la turbine hydraulique nouvelle génération sont : Efficacité supérieure Coût inférieur Plus grande fiabilité et robustesse. PO-6.1

2 INTRODUCTION The use of liquid expanders in the LNG process was first discussed in the 1960 s, when the first LNG plants were designed. Inspite of its evident advantage, the expanders could not be installed in the early plants due to technology limitations of the expanders. Flowserve identified the markets in the late 1980 s and developed the first cryogenic expanders based on its past experience with turbines used in power recovery from water storage dams. Currently the following expanders have been in operation in cryogenic service, the first units since the early 1990's: TABLE 1: CRYOGENIC LIQUID EXPANDER APPLICATIONS Location Power (KW) Speed (RPM) Operating Temperature ( C) Efficiency (%) Number of Units National Helium Kansas USA MLNG Dua (HMR) MLNG Dua (LNG) Nigeria LNG (HMR) Nigeria LNG (LNG) Key Dates: 1991 First cryogenic expander installation at Liberal Kansas 1995 MLNG Dua Expander start up 1999 NLNG Expander start up 1999 Completion of design and testing of Advanced Cryogenic Expander 2000 Design and Testing of Dry Gas Seal for Expander HEAT DISSIPATION WITH EXPANDERS In the natural gas liquefaction cycle, the liquefied natural gas and the mixed refrigeration liquid emerge from the main cryogenic heat exchanger at high pressures of the order of 45 to 50 bars. For the cycle to continue, the pressure has to be let down to about 5 to 10 bars. When this pressure reduction is achieved through J-T valves, all of the pressure energy is converted to heat and introduced into the process liquid, thereby raising its temperature. When an expander is used as the main pressure reduction device, much of the pressure energy is removed from the process and can be converted to electrical energy, which can be put back into the grid. Only a very small portion of the pressure energy goes into the liquid as heat. The following example shows the heat dissipation effect using expanders. Figure 1 shows a calculated enthalpy entropy diagram for various expansion processes with the inlet stream represented by point A. The line AB represents an iso-enthalpic process as would occur with a J-T valve. Line AC represents an isentropic process through an ideal expander. Line AD represents an expansion process that would occur through a real world expander. The temperature differences for the three processes are shown in Table 2. PO-6.2

3 EXPANSION PROCESSES A B C D E A-B : J-T VALVE A-C : IDEAL EXPANDER. A-D : ACTUAL EXPANDER (EFF.=85%) D-E : HEAT ADDITION WITH INTERNAL GENERATOR ENTROPY (kj/kg C) Figure 1. Expansion Processes TABLE 2 PROCESS T C (inlet outlet) A-B Joule Thompson valve +1.5 A-C Ideal Expander -1.0 A-D Actual Expander -0.7 It can be seen that using a good turbine (Line A-D) allows the temperature at the exit to be 2.2 C below what would be achieved using a J-T valve (Line A-B). GENERATOR LOSSES Cryogenic Expanders can be built with internal and external generators. In the above calculations, it is assumed that the generator is outside the process stream and hence at ambient temperature. Therefore, the heat losses produced by the generator are not removed by the cold process liquid, but are removed by convection into the surroundings. This adds extra efficiency benefits to the LNG process compared to a design with an internal generator that is cooled by the process liquid. EFFECT ON LNG THROUGHPUT The value of an expander for an LNG plant lies not in the power put back into the grid but in the unloading of the refrigerant compressors. This effect can be easily quantified by the following analysis. Power Impact: The compressor power required to remove heat energy from the process stream is a function of the temperature of the process stream, the colder the stream the higher the power requirement. A simplified equation relating these quantities can be stated as: PO-6.3

4 P c / E = [(T rej / T liq ) - 1 ] / η where P c = Refrigeration compressor power required, E = Heat energy to be removed from the stream, T rej = Temperature in K of the heat sink, T liq = Temperature in K of liquid stream, and η = Overall cycle efficiency. The above equation can be applied for an actual field condition by using the data from an operating plant (MLNG Dua at Bintulu, Malaysia, Reference 1). Assuming T rej = 303 K, and η= 0.4, the following values result T liq ( K) P c /E E(MW) P c (MW) LNG MCR TOTAL The combined power saving is calculated to be 3.71 MW. (Reference 1 cites the actual saving to be 4.1 MW. This suggests that the actual overall cycle efficiency is somewhat less than 0.4). It is important to note that the energy recovered in the expander has a double effect: it adds power to the grid it unloads the compressor by a multiple of the amount of power to the grid The expander efficiency plays a direct role in the above computation. Every point increase in efficiency yields dividends of multiples of the expander power for the process energy gains. Production Benefits: Another way of looking at the decrease in compressor power requirement is to see it as a gain in LNG throughput for the same compressor power. In the above example, the total compressor power requirement is about 100 MW. A saving of 3.7 MW translates to about 4% gain in compressor power. Now, if the compressors can be operated at the same power levels as when the expanders are not in operation, the LNG throughput can be increased by about the same percentage (viz. 4%) when the expanders are used. To get an approximate idea of this benefit, Flowserve has made a computation using the specifications for a new LNG plant. The following key assumptions are made: LNG Throughput = 3.4 mil tons / year / module Compressor Power = 138 MW Cost of LNG = 150 $ / ton Overall Refrigeration System Efficiency = 45% Cost of Electricity = 0.03 US$ / kwhr Turbine Availability = 90% PO-6.4

5 The computation was made for a series of postulated turbine efficiencies and Figure 2 shows the income to the user in $ / year / module. For the existing designs with average efficiencies of about 83%, the income to the user would be $20. Million per year per module. For the advanced design with 88% efficiency, the income is $21.25 million. This analysis shows the key benefit to be an increase in income of 1 million US$/year/module for every five points improvement in expander efficiency MIL$/YR/MODULE TURBINE EFF. Figure 2. Total User Income As stated in Reference 1, the use of expanders in MLNG Dua would allow an increase of 4% in LNG production capacity. Assuming that the front-end treating facilities can handle the additional throughput, it is stated that the expander capital investment can be easily justified as the pay back period works to about one year, when the increased capacity is taken into consideration. ADVANCED EXPANDER DESIGN As Flowserve continued to accumulate operating experience in LNG services, it became increasingly clear that the energy recovery in the expander provides benefits in compressor power requirements in multiples. The lessons learned from the first years of operations at the MLNG Dua plant suggested that improvement could be made to the expanders. Therefore, it was decided to develop a second-generation expander with the aim to maximize the benefits of the Owner through: 1. Efficiency optimization 2. Cost reduction 3. Reliability and robustness enhancement The method anticipated to achieve these benefits was the development of optimized hydraulics that would result in an expander with high efficiency and consist of fewer parts. This design would need to be verified by a design verification test, basically a test run on water. PO-6.5

6 In parallel the development of a dry gas seal was started as an alternative to the oil buffered mechanical shaft seals. DEVELOPMENT OF NEW HYDRAULICS i. Optimization of Specific Speed To decide on the optimum specific speed (and hence the number of stages), one has to compare the effect of such a choice on efficiency, runaway speed and choke flow. Figure 3 shows this dependency derived from empirical data. Discussions with customers revealed that modern LNG plants have sophisticated control systems that minimize the potential of choke flow disturbances during expander runaway conditions. The acceptability of lower choke flow allowed the designers to choose the lowest specific speed that would be consistent with maximum efficiency. From Figure 3, one can deduce that for a specific speed of about 1800, the following performance measures can be anticipated: Efficiency 86% Runaway speed 150% Choke flow 60 % For comparison, the first-generation expanders have runaway speed ratio of about 200% and choke flow ratio of about 80% RUNAWAY SPEED CHOKE FLOW ANDERSON EFFICIENCY TURBINE SPECIFIC SPEED Figure 3 Expander H-Q Curve It can be seen that the ability to chose lower choke flow leads to two distinct advantages: 1. Reduced runaway speed 2. Reduced number of stages resulting in a rotor dynamically more robust expander PO-6.6

7 ii. Computational method: The detailed hydraulic design of the advanced expander was carried out using a Computerized Fluid Dynamic (CFD) code. The CFD analysis computes the entire flow field and since the viscous effects are modeled, an accurate picture of the real flow inside the flow passages can be obtained. The analysis, therefore, produces realistic estimate of hydraulic losses from which the efficiency can be computed. Further, one can also judge the quality of the flow field, which leads to optimization of flow passages. In order to develop an accurate prediction of the performance of a single stage of the expander, the geometry included three components. These were: the runner, the wicket gates, and the casing that routes the flow from the runner discharge of one stage to the wickets of the next stage. The overall geometry is shown schematically in Figure 4. A total of nodes defined the entire flow passage in the computerized model. iii. Design of advanced design expander: Figure 4 Turbo Expander Topology With the specific speed of the advanced design optimized around 1800, an initial design (Design #1) was created from past experience with expanders in terms of choice of inlet and discharge angles, vane numbers, etc. The CFD analysis for this design showed a hydraulic efficiency for a single stage to be 91.7%. The parasitic losses (leakage, mechanical, disk friction, etc.) were estimated from empirical data and determined to be about 4.9%. There are additional losses due to the flow in the passage from the exit of one stage to the entrance of the wicket gates of the next stage. By placing the casing struts behind the wicket gate stems, these losses could be reduced compared to the baseline design. The overall efficiency of Design #1 in the multistage configuration was thus calculated to be 84.9%. From this initial configuration, the design was altered in a few steps. Table 3 shows this evolution. Design #2 optimized the incidence to the runner resulting in a significant efficiency pick up. The final design involved further optimizations of the runner geometry to minimize residual swirl. This configuration resulted in an expected efficiency of 88%, which amounts to a five-point improvement over the existing design. PO-6.7

8 TABLE 3. EVOLUTION OF THE ADVANCED DESIGN Single Stage Eff. (%) Parasitic Losses (%) Stage Losses (%) Overall Eff. (%) Design # Design # Final DESIGN VERIFICATION TEST The design was verified by testing a two-stage full size version in water at 1800 RPM. A single stage charge pump was used to deliver the water to the expander. Since the water was taken from an underground tank, a booster pump was used ahead of the charge pump. A venturi meter was used to measure the flow to the expander. Pressures were recorded at several locations to allow an accurate estimate of the inlet and discharge configuration losses. In order to directly measure the generated torque, the generator was mounted on a set of ball bearings placed between the generator casing and the support stand. The torque introduced on the casing was directly measured with a set of load cells RPM Efficiency Total Turbine Output 100% 90% 80% 70% 60% 50% 40% 30% Head 20% 10% 0 0% Flow (l/sec) Figure 5 Advanced Expander Bowl Performance This measurement allowed the power output and the efficiency of the expander to be calculated directly without needing to know the generator efficiency. Figure 5 shows the composite curves of head, efficiency and output power for three different wicket gate settings. It can be seen that a peak efficiency of 89% is reached at 189 l/sec with the design wicket gate opening. At this point, the net head across the turbine is 133 m. These conditions of head and flow very closely duplicate the design expectations on this unit. The efficiency compares very well with the CFD prediction of 88%. PO-6.8

9 CONCLUSIONS It is being well recognized that using cryogenic expanders for pressure let down can increase the throughput of LNG liquefaction plants significantly. As the initial expander installations are operating well in the field, their application in future plants and for retrofits will continue to increase. Flowserve has developed an advanced design cryogenic expander with improved efficiency and greater inherent reliability to serve this growing market. The anticipated objectives have been achieved with the following benefits: Efficiency - Increased by about five point from 83% for current operating units to 88% for the advanced design, potentially resulting in 1 million US$ extra revenue /module/year. Cost - A reduction of 10% in cost is achieved due to a design with fewer parts. Reliability and Robustness Lessons learned have been incorporated and design has been simplified. Runaway speed has been reduced leading to increased safety margins. Reduced number of stages will improve overall robustness. In addition a dry gas seal with inherent improved reliability will become available early ACKNOWLEDGMENTS The authors would like to acknowledge the guidance provided by Shell Global Solutions OGBE/1 in The Hague towards the development of the advanced design. The compressor power equation used in this paper was obtained through personal communication from G. Renaudin, Technip, Paris. The authors would like also to acknowledge the efforts of R. Ferman, M. Cugal, L. Hanson, P. Ho, and M. Eftychiou of the Flowserve Technology Department for the design and testing of the new expander. The authors also want to express their gratitude to G. Pieters and I. Robbins of Flowserve for the close participation and guidance in the execution of this project. REFERENCES J. Verkoelen: Initial Experience with LNG/MCR Expanders in MLNG-Dua, Oil andgas Journal, November 18, PO-6.9