MONITORING CRYOGENIC TURBINES USING NO-LOAD CHARACTERISTICS

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1 8th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC-8, March 26-30, 2000, Honolulu, HI, USA MONITORING CRYOGENIC TURBINES USING NO-LOAD CHARACTERISTICS Gilbert L.G.M. Habets Shell International Oil Products B.V. Carel van Bylandtlaan 30, The Hague 2501 CM, The Netherlands Fax KEY WORDS: Hydraulic Turbines, Performance Monitoring, No-Load Characteristics, Cryogenic Fluids. ABSTRACT Described is a typical characteristic of hydraulic turbines, which is the basis for determining the condition of this category of equipment, and in particular for cryogenic turbines in liquefied gases. This typical characteristic of a hydraulic turbine operating under no-load conditions provides a unique manner of reliably measuring the performance of the hydraulic turbine. No sophisticated instruments are required to accurately monitor the condition of the turbine; its mechanical parameters and hydraulic performance. These no-load characteristics apply to hydraulic turbines in general and the method presented is unique, however generally applicable to turbines without restrictions in type or power range. For cryogenic turbines in particular it provides significant advantages, because no special cryogenic sensors are required. The benefits of being able to accurately verify the performance condition of hydraulic turbines on-line can be significant. Maintenance activities can be based on an on-line condition measurement resulting in a better decision process for reducing frequency, duration and scope of scheduled downtimes. The risk for unexpected failures can be reduced when the characteristic is verified at regular intervals. The validity of this unique method is demonstrated on a hydraulic turbine in cryogenic service. These hydraulic turbines are applied in Liquefied Natural Gas plants to enhance the process efficiency and operate in a cryogenic liquid at minus 164 degrees C. The overall thermodynamic efficiency of these units is important and a degradation thereof will have a direct adverse effect on the plant performance. The typical no-load characteristics of these cryogenic turbines were measured during performance tests with modified mechanical turbine configurations. The analysis of the different no-load characteristics confirmed the validity of the method. This method can be implemented with low capital investment and provides a powerful tool for maintenance and operations to determine the health of hydraulic turbines online. INTRODUCTION In Liquefied Natural Gas plants the process efficiency and equipment reliability are of paramount importance. Treated natural gas is condensed and sub-cooled in a Main Cryogenic Heat Exchanger. In the final process step the Liquefied Natural Gas, which is under pressure, is expanded and subsequently pumped down to large LNG tanks that are at atmospheric pressure. This expansion usually takes place by letting down the liquid pressure over Joule-Thompson valves. Cryogenic hydraulic turbines have been introduced to improve the thermodynamic process of the pressure letdown and present an important economic advantage. (Habets, Kimmel, 1998). The first application of these turbines dates from 1995 (Verkoelen, 1996 and Johnson, Renaudin 1996 and 1995). TURBINE DESCRIPTION The hydraulic turbines for cryogenic processes consist of a vertical turbine with several stages. The turbine is a radial flow turbine runner with a fixed geometry diffuser between stages. Existing are two designs: one with a variable geometry inlet (wicket gates) that control the flow by directing the flow angle into the turbine runners. These units operate at a fixed speed and are directly coupled to an air-cooled generator. The other design with fixed geometry operates at variable speed and is directly coupled to a generator that is submerged in the cryogenic fluid. Units are typically in the range of 1MW generator output and may have up to 8 stages. The turbine is mounted in a pressure vessel with inlet and outlet piping connections. The power generated is supplied into the plant power grid. The liquid pressure is in the range of 45 bar at the inlet and 2.5 bar at the outlet for LNG applications. A typical hydraulic turbine in LNG of the variable speed design is shown in Figures 1 and 2. Not shown is the Variable Speed Constant Frequency System that converts the frequency of the generated electrical power which corresponds to the turbine operating speed into the required grid frequency. Depending on the plant throughput and operational needs

2 Figure 1: LNG Hydraulic Turbine at Completion of Performance Tests in Liquid LNG

3 Figure 2: Hydraulic Turbine Cross-section

4 the operating point of the turbine can be adjusted either via the wicket gates or the turbine operating speed. The turbine outlet pressure is usually maintained at an adequately sufficient pressure to avoid excessive vapor formation at the final stages. TURBINE PERFORMANCE CHARACTERISTICS The performance characteristic of a typical variable speed hydraulic turbine for a LNG plant that is currently under commissioning in the Middle East is shown in Figure 3. It concerns a unit that will serve in Heavy Mixed Refrigerant (HMR) liquid and is rated for 986 m3/hr at 990 m differential head. HMR is a liquid that is part of the cooling steps in the LNG process. The number of stages is three with three turbine runners, each with an individual nozzle (Figure 4) at the runner inlet. Figure 3 shows the predicted overall performance of the unit in terms of flange to flange units. The performance map consists of various performance lines depicting the relationship between head and flow at a constant speed. Performance lines for various speeds are initially parallel to each other and for increasing speed the required head has to be increased. When following a performance line at constant speed towards the higher flow range, the power output increases and ultimately a limit line is approached (higher flow range not shown in Figure 3). As a matter of fact all speed lines asymptotically approach this limit line of zero speed. At this line the unit achieves maximum torque however the speed is zero. It can be seen as if the entire flow is throttled over a fixed orifice. Another limit line of the performance map exists in the lower flow range. All performance lines for a unit in turbine mode start at this line, which is the zero torque line. This is the performance map boundary line that corresponds with a noload condition of the turbine. The turbine will operate with head, flow and speed parameters that correspond with the noload characteristic. These relationships can be mathematically confirmed (Refer Kimmel, 1997). Subject no-load characteristic is very much a specific attribute of the hydraulic turbine internal geometry and turbine condition, as will be demonstrated. At this no-load line the hydraulic energy presented to the turbine is entirely converted into kinetic energy of the entire rotating inertia and the losses associated with the turbine. Ultimately the energy is dissipated in heat via turbulence in the fluid and heat loss via the pressure vessel. There is no energy output from the generator as the unit is disconnected from the electrical grid. For variable inlet geometry units a similar line exists although here the speed is fixed but the wicket gate position varies. The dissipation of energy and its distribution will be slightly different due to differences in configuration but the same physical principle exists. The no-load characteristic can be described as the condition at which the hydraulic energy is dissipated over a rotating orifice. MONITORING THE NO-LOAD CHARACTERISTICS The no-load characteristic of hydraulic turbines is a typical signature that exhibits the inherent condition of the unit. Changes in geometry, bearing losses, opening of internal clearances, erosion of internals, increase of internal leakage passages, incorrect assembly, etc will exhibit itself as a difference of the no-load characteristic. Due to a shift between kinetic energy and losses directly related to the hydraulic turbine, a different no-load characteristic will result. As a matter of fact the no-load characteristic will shift its position within the performance map. Figure 5 shows the no-load characteristic of the HMR three stage hydraulic turbine as described above. Two different noload characteristics of the same HMR turbine are shown. The difference is a small geometrical modification at only one of the three stages. The turbine, indicated by nozzle 1, contains one turbine stage with modified vanes of its inlet nozzle ring (Figure 4). The nozzle vane angle has been decreased by 1 (one) degree compared to the two other nozzle ring vane angles. The turbine indicated by nozzle 2 has three identical nozzle rings (no decreased angle). The HMR turbine with nozzle 1 will have one nozzle ring that is more closed and the nozzle ring has a reduced flow section. This results in an increased angular flow velocity at the inlet of one stage and consequently its inlet angular momentum is increased. Although the difference in geometry appears insignificant, the no-load characteristics illustrate the difference clearly. Because at the no load curve no net power output exists, the inlet minus outlet angular momentum shall remain zero. For nozzle 1 the inlet angular momentum has been increased and consequently the outlet angular momentum shall also increase. This results in an increase of no-load speed for nozzle 1 compared to nozzle 2 at equal flow. This is also evident from Figures 5, 6 and 7 that show both no-load characteristics with head, flow and speed variables along different axis. For the same flow rate e.g. 350m3/hr nozzle 1 yields a much higher speed, N1=3200 rpm compared to nozzle 2 with N2=3000rpm. The practicality of this unique monitoring technique for hydraulic turbines is to accurately establish the no-load characteristic of hydraulic turbines at the manufacturers performance tests and thereby the baseline no-load machine signature is determined. The verification of the no-load characteristic in an operating plant can be performed before the unit is energized. This requires no additional test instrumentation, apart from existing and available plant instrumentation; i.e. flow, pressure difference, speed or wicket gate position and fluid density. It suffices to determine one point of the no-load characteristic in order to evaluate a difference between shop test and operating plant characteristics. In case during operation bearing damage, rotor-rubbing etc. have occurred, than the no-load speed at a given flow and wicket gate position, if applicable, will be reduced compared to the baseline no-load speed. The method demonstrates to have adequate sensitivity in order to be applied with existent plant instrumentation. Refer Figure 8 that shows the delta head between nozzle 1 and nozzle 2 configuration versus flow for various speeds over the entire operating range. CONCLUSION Monitoring the no-load characteristic of hydraulic turbines in cryogenic services is a powerful and sensitive diagnostic method to detect a machine condition. Particularly small differences in nozzle vane angles can be rapidly detected on line. It is a unique method that can be applied to any hydraulic turbine irrespective size or type. Because the method is simple and can be applied before a unit is energized, an early warning of even minor deviations is obtained. The measurement can be repeated any time with the unit in its operational installation without prior maintenance effort. When combined with appropriate vibration measurements the complete turbine hydraulic performance and mechanical dynamic condition can be rapidly verified at low cost. To record the no-load characteristic as part of the manufacturers shop test shall be standard practice in order to establish the initial baseline condition and record this as the unit s fingerprint, together with other performance data.

5 VARIABLE SPEED HYDRAULIC TURBINE 10TG-153 Head (m) Zero-Torque Curve Maximum Speed rpm 3600 rpm 3300 rpm 3000 rpm 2700 rpm 2400 rpm Minimum Speed rpm Zero Speed Curve Capacity (m 3 /hr) Figure 3: LNG Hydraulic Turbine - Performance Map Figure 3: LNG Hydraulic Turbine - Performance Map Figure 4: Nozzle Vanes

6 COMPARISON OF TWO SETS OF NOZZLES No-Load Characteristics Nozzle 2 Nozzle 1 H = Head (m) Q = Flow (m 3 /hr) Model EBARA 10TG-153 Figure 5: LNG Hydraulic Turbine COMPARISON OF TWO SETS OF NOZZLES No-Load Characteristics Nozzle Nozzle H = Head (m) N = Speed (rpm) Model EBARA 10TG-153 Figure 6: LNG Hydraulic Turbine

7 4800 COMPARISON OF TWO SETS OF NOZZLES No-Load Characteristics N = Speed(rpm) Nozzle 1 Nozzle Q = Flow (m 3 /hr) Model EBARA 10TG-153 Figure 7: Hydraulic Turbine 70 COMPARISON OF TWO SETS OF NOZZLES Difference in Head Over the Operating Envelope 66 H = Delta Head (m) rpm 3000 rpm 2400 rpm Q = Flow (m 3 /hr) Model EBARA 10TG-153 Figure 8: LNG Hydraulic Turbine

8 REFERENCES The International Journal of Hydrocarbon Engineering Habets G.L.G.M., Kimmel H.E. December 1998, Economics of Cryogenic Turbine Expanders. Proceedings of GASTECH 96 Conference Vienna Austria Verkoelen J. December 1996, Initial Experience with LNG/MCR Expanders in MLNG-Dua. Oil and Gas Journal Johnson L.L., Renaudin G., 1996, Liquid Turbines improve LNG Operations. Proceedings Eleventh International Conference on Liquefied Natural Gas Birmingham UK. Johnson L.L., Renaudin G., 1995, Improvement of Natural Gas Liquefaction Processes by using Liquid Turbines. Proceedings of 7 th International Fluid Machinery Conference, The Hague Netherlands Habets G.L.G.M., Kimmel H.E. April 1999, Development of a Hydraulic Turbine in Liquefied Natural Gas. Hydrocarbon Engineering Kimmel H.E., May 1997, Speed controlled turbine expanders. World Pumps Kimmel H.E., June 1997, Speed controlled turbines for power recovery in cryogenic and chemical processing.