Protection and Condition Monitoring of the LM2500 Gas Turbine

Application Note Protection and Condition Monitoring of the LM2500 Gas Turbine Overview the LM2500 gas turbine The LM2500 is General Electric s most experienced aeroderivative gas turbine, used mainly in power generation, oil and gas compression and marine propulsion applications. The LM2500 has been in service for decades, and the newer models, the LM2500+ and LM2500 DLE, offer more power and lower NOx emissions. The LM2500 is a single spool Gas Turbine Generator (GG) with a free Power Turbine (PT), and has a power output ranging from approximately 23 to 28 MW. LM2500 monitoring The basic monitoring requirements for the LM2500 are defined by GE in their Installation and Design Manual (IDM). Since the early 1980 s, the LM2500 has been equipped with accelerometer sensors and a basic suite of analog vibration monitor instruments. New generation LM2500 monitoring SKF s and Vibro-Meter s new generation of sensors and monitoring systems provide a complete solution for the LM2500, for increased monitoring reliability and capability: Figure 1. VM600 for LM200 DLE with CMC-16. Accelerometers two per engine The proven vibration sensor of choice for gas turbines, the latest CA series high temperature piezo-electric accelerometers, offer increased reliability and improved immunity to transference vibration from the gas stream and structure. Dynamic pressure sensors two per DLE engine Piezo-electric technology is employed in the CP series sensor to provide a direct measurement of harmful pressure pulsations within the combustion chamber. Displacement probes four per alternator The newest design CMSS series proximity probes offer improved maintainability and reliability. Digital monitoring system The VM600 monitor system provides a compact and programmable solution. For example, the LM2500 DLE employs only three universal MPC-4 monitor modules for protection of the entire machine train ( fig. 1).

Designed for gas turbine use, the MPC-4 permits the consolidation of vibration and combustion protection into a single monitoring system. The programmability of the VM600 allows improved vibration analysis and cross tracking of unbalance components. FWD Accelerometer AFT Accelerometer X Relative Radial Displacement Eddy Current Probes Y X Y Integrated condition monitoring CG PT GEN A single CMC-16 data acquisition interface, fig. 1, provides high performance monitoring, trending and analysis tools integrated in the same rack system for increased reliability. This application note will discuss in detail the recommended installation for protection and condition monitoring of the LM2500, while remaining fully compliant to the needs of General Electric s IDM. Ice Detection (optional) 2x Dynamic Pressure on LM2500 DLE Figure 2. The CMCP 530 transmitter. DE NDE Magnetic speed pickup on multi-tooth target on PT. Optional on GG. Sensors LM2500 Fig. 2 illustrates the basic sensor suite, associated with vibration monitoring, which is commonly fitted by GE to a LM2500 power generation train. Owing to variations in GE purchasing policy, the exact model and make of sensor installed by the OEM will have varied slightly over the years. SKF/Vibro-Meter would recommend its latest generation sensors for an instrumentation upgrade project, and these are listed on the back page. Fig. 3 illustrates the complete measurement component chain recommended for an intrinsically safe (Ex i) application, together with components for non-hazardous area usage. The GE IDM employs English units, and this application note follows suite, with SI Unit equivalents. Let us now consider each of these measurement trains in turn. Figure 3. Sensor measurement chains (Ex i application). 2

Absolute (casing) vibration The vibration of the gas turbine itself, the prime mover, is measured with two piezo-electric accelerometers. The high temperature requirement necessitates the use of an externally charge Figure 4. CA303 accelerometer. amplified accelerometer, the model CA303 from SKF/Vibro- Meter. The unit has a linear frequency range from 5 Hz to 8 khz. Made of high grade stainless steel, the CA303 can operate within specification up to 425 C (800 F). An integral, mineral insulated cable ensures good signal integrity for up to 2 m (6 ft.), to a locally mounted charge amplifier, the IPC704. The charge amplifier conditions the electrical charge output of the accelerometer to a current modulated signal. This method reduces the effect of electrical interference along the field cabling to the monitor, and permits longer cable runs than a traditional voltage signal. A GSI-122 galvanic separation device isolates the field elements in a hazardous area from the monitor system and performs some signal conditioning for the monitor system. The use of a galvanic separation device is not required in a nonhazardous area, unless support of current modulation is needed, in which case a GSI-130 is used. Relative shaft vibration The vibration of the driven component is usually measured by means of non-contacting sensors, known as eddy current probes or proximity probes. These sensors Figure 6. SKF DYMAC eddy current probe. provide a direct measurement of shaft radial displacement. These are typically mounted through the bearing cap and targeted directly onto the shaft. The sensor system consists of a probe, cable and an oscillator/demodulator (known as a driver or proximitor ). A suitable model is the CMSS 68 series from SKF/Vibro-Meter. The probes have a linear frequency range from DC to 10 khz. The RYTON tip material allows the probe to withstand temperatures up to 175 C (350 F), and differential pressures to 415 Kpa (60 psi). Speed sensors A magnetic speed sensor is provided by the OEM at the output of the power turbine, targeted on a multi-tooth wheel. Depending upon age and model, a similar magnetic speed pickup may also be provided by the OEM for the gas generator. Retrofitting of a sensor to provide a phase reference point is desirable, particularly if the condition monitoring option is selected. Dynamic pressure Reduction of emissions requires premixed lean Standard version fuel/air mixture, which in turn can lead to flame instability and excessive dynamic pressures. The DLE version of the EEx i version LM2500 uses two high temperature piezoelectric dynamic pressure Figure 5. Dynamic pressure sensor. sensors to measure pressure pulsation in the annular combustion chamber. The consequences of excessive pulsations can be catastrophic; buckling of the combustion chamber and leaks reduce efficiency, and subsequent component failure can wreck the hot gas path components downstream. Hence, the ability to measure, warn and protect against these pressures is of vital importance. The GE-approved models for mounting on the LM2500 are models CP103 and CP106 from SKF/Vibro-Meter, which can withstand temperatures up to 650 C (1200 F). The CP series employs piezo-electric technology to provide a direct measurement of pressure within the high temperature environment of the combustion chamber itself. This offers a superior solution to indirect measurements utilizing industrial grade pressure sensors, which need to use bleed gas or water-cooling assemblies. Ice detection sensor In order to prevent ice ingestion into the gas turbine engine, an anti-icing system is used. There are a number of possible approaches, but all involve the ingestion of warmer air into the engine inlet, a process that has a significant impact on performance and thermodynamic efficiency. The GE IDM directs that the anti-icing system be engaged when atmospheric conditions favor the formation of ice. This is determined by measurement of humidity and air tem- Figure 7. Ice detection sensor. perature, and subsequent calculation of when ice may be likely to form. These indirect measurements of engine icing tend to be conservative. The model EW140 sensor provides a direct measurement of ice thickness, hence permitting more efficient use of the anti-icing system. The piezo-electric principle used in this sensor was developed for aircraft use, and is available for use on industrial machines such as the LM2500. The sensor is optional, as clearly not all climates suffer from ice buildup. 3

Machine protection LM2500 An IDM compliant machine protection monitor system for the LM2500 is the VM600. The system uses a single MPC-4 universal, digital protection module. This module was specifically designed for gas turbine use, from transducer input, through signal conditioning and processing, to shutdown relay contact closure. Fig. 1 illustrates a system. The entire LM2500 driven train may be protected by two identical MPC-4 cards (three on the LM2500 DLE). Optional redundant power supplies are employed for the highest integrity, and a CPU-M display unit also provides dual serial and/or Ethernet connections to a TCS/DCS/PLC. Digital processing The processing is defined by the GE IDM and is easily configured on an MPC-4 by software. An example LM2500 configuration for vibration protection is illustrated in figs. 8 and 9. In this case, it is assumed that there are two speed signals available, one for the gas generator and one for the power turbine. The FWD accelerometer is terminated, powered and conditioned by input channel 1, the AFT accelerometer by input channel 3. Both FWD and AFT sensor inputs are processed on two parallel paths, in order to isolate the gas generator shaft and/or the power turbine shaft related problems at either sensor location. Owing to high background noise levels on the LM2500 gas turbine engine, narrow band monitoring is required to extract either shaft s unbalance signal component. The power turbine speed signal is used to tune a dynamic filter (tracking filter) to the rotational speed frequency of the power turbine (1x power turbine). All other signal components are filtered out. When the gas generator speed signal is available, the same monitoring technique is employed for the gas generator shaft, i.e., the gas generator speed signal is used to set a tracking filter to eliminate any vibration component with a frequency that does not coincide with the gas generator speed frequency (1x gas generator). In the event that a gas generator speed signal is not available, then broadband processing of the FWD and AFT signals is performed to encompass the gas generator speed range up to 8 500 RPM. Typically this is a band between 75 to 200 Hz (60 db/octave), monitoring above the 50/60 Hz generator speed. Programming this processing is made simple by Microsoft Windows based configuration software. Example VM600 setup screens for the broadband and narrow band processing are shown in figs 10 and 11. A second MPC-4 card is used to monitor the four displacement probes on the alternator itself. Each channel supports a single sensor and is processed for broadband between 10 to 1 000 Hz. Should a phase reference probe be fitted to the generator coupling area, then the same MPC-4 may support this, and provide for narrow band processing and display of nx amplitude and phase angle. A third MPC-4 is used to support the two dynamic pressure sensors on DLE variants. The processing of the pressure signal is identical to that of acceleration vibration, with broadband processing in a band 10 to 3 000 Hz. Slot 3 Slot 5 Slot 7 Slot 12 FWD DE-X DP-1 CMC-16 # DE-Y DP-2 # AFT NDE-X # # # NDE-Y # # SP-1 # # # SP-2 # # 3 Figure 8. Channel layout LM2500 DLE with CMC-16. Figure 9. Vibration protection signal processing LM2500 gas generator and power turbine. 4

Figure 10. Broadband processing Channel 1. Alert and danger (shutdown) Machine shutdown based on any processed variable is actuated via a common relay output, as shown in fig. 9. Each installation will choose whether or not to implement a trip function, and will tailor alert and danger (shutdown) levels for each unit, but the IDM does provide a guide: Gas generator (broadband, < 125 Hz): Alert: 38 mm/sec (1.5 ips) peak Danger: 63 mm/sec (2.5 ips) peak Gas generator (broadband, > 125 Hz): Alert: 44 mm/sec (1.75 ips) peak Danger: 76 mm/sec (3.0 ips) peak Power turbine (1x vector): Alert: 12 mm/sec (0.5 ips) peak Danger: 25 mm/sec (1.0 ips) peak The adaptive monitoring feature of the MPC-4 permits more speed ranges to be defined, each with their own Alert and Danger levels. This gives great flexibility in maintaining protection integrity, yet avoiding false trips. Note the levels are much higher than that expected on other machines; this reflects the relatively flexible nature of aeroderivative gas turbine construction. Machine condition monitoring LM2500 The VM600 permits addition of parallel data acquisition for condition monitoring in the same monitor chassis as that provided for protection, hence ensuring high system integrity without compromising the machine safeguarding function. Any number of 16-channel CMC-16 cards may be placed in any of the 12 available slots ( fig. 1). There is no defined GE standard for condition monitoring of an LM2500, but a likely configuration proposed by SKF/Vibro-Meter would be: Channel 1: SP-1 Channel 2: SP-2 Channel 3: 1X GG FWD Channel 4: RAW FWD Channel 5: ENV FWD Channel 6: 1X PT AFT Channel 7: RAW AFT Channel 8: ENV AFT Channel 9: SPARE Channel 10: DE-X Channel 11: DE-Y Channel 12: NDE-X Channel 13: NDE-Y Channel 14: DP-1 Channel 15: DP-2 Channel 16: SPARE Each channel can process and monitor up to 10 spectral bands. 5

Figure 11. Narrow band processing channel 2. The philosophy of condition monitoring of these critical machines is multi-parameter monitoring. That is, the same input signal is processed in different ways in parallel, in order to determine different characteristic vibration parameters. Each parameter may reveal something different about deteriorating engine condition over time. With the exception of the ENV channels, all signals are bussed over the VM600 backplane, with no interconnecting wiring. This increases reliability and reduces maintenance and troubleshooting tasks. With ENV channels, the input signal is pre-conditioned by SKF Mechanical Condition Monitor-ENV modules, which apply acceleration enveloping to the input signal. The ENV process is used as part of the VEWS application (see section VEWS application). The speed signal(s) is used to detect and track run-up, coastdown and surge conditions. The absence of a once-per-revolution pulse prevents the determination of the phase angle of any vibration component. Overall levels (RMS or peak) and FFT spectra from FWD and AFT sensors would typically be determined in each of the following three bands: 0 to 1 khz 2 to 5 khz 8 to 15 khz Additional overall levels and FFT bands are configured for combustion monitoring, tracking of flame instability (pressure pulsations) caused by low NOx operating conditions ( humming ). Run-up / coast down During transient conditions, the FFT and bands would be monitored closely in order to identify phenomena including: High vibration amplitudes Combustion chamber resonance External equipment stress/fretting Spool nx harmonics Steady state During normal running conditions, the FFT and bands would be monitored periodically to watch for fluctuations in vibration levels, which can occur during low, part or variable load conditions. VEWS application The Vibration Early Warning System (VEWS) application is discussed in a separate application note. VEWS uses acceleration enveloping to enhance the ability of the FFT to highlight frequencies associated with main engine bearing damage, compressor blade problems and internal rubs. Specifically: BPFO Ball Pass Frequency Bearing Outer Raceway BPFI Ball Pass Frequency Bearing Inner Raceway BSF Rolling Element Spin Frequency VPF Blade or Vane Pass of each compressor stage 6

Figure 12. Transient FFT plots CMS software. The radial displacement channels would provide relative shaft vibration levels in microns or mils (peak-peak) and, if a phase reference is present, would be combined to provide polar, shaft centerline and orbit analysis of the generator shaft dynamics. The data would be displayed, trended and analyzed using the CMS software platform, which operates on Windows NT, SCOUNIX or Linux operating systems. A screen example is shown in fig. 12. 7

Sensors 1 2 144-303-000-111 CA303-02 accelerometer, FWD and AFT, equivalent to GE, part number: L34152P01 55 to +425 C ( 65 to +800 F), 50 pc/g, 2,1 m (7.0 ft.) integral MI cable stainless steel with overbraided, 5 /8-24 UNEF-2A connector 144-303-000-111 (optional) CA303-31 accelerometer with side connector (optional) 2 2 143-106-000-011 CP106 dynamic pressure sensor system, DLE, GE, part number: L44723P01 95 to +650 C ( 140 to +1200 F), 232 pc/bar, 1,2m (3.9 ft.) MI cable with overbraid, M83723/89Y10207 connector, special GE flange 3 4 IPC 704 Charge amplifier; symmetrical with configurable low-pass and high-pass filters, optional integrator; voltage or current output 4 4 244-122-000-204 (optional) GSI-122 galvanic separation (not required for non-exi application); 1 V/mA, CENELEC/PTB AND CSA approved Alternator radial vibration (X-Y) and phase reference 5 5 CMSS 668-080-00-12-10 8mm eddy current probe, 3 /8 in. thread; 30,5 mm (1.2 in.) case, 1 m (3.3 ft.) cable, armoured, Exi 6 5 CMSS 958-09-040 Probe extension cable, 4 m (13.1 ft.) length, no armour, Exi 7 5 CMSS 668-16-9 Driver, 200 mv/mil, 5 m (16.4 ft.) system, Exi 8 5 MTL 5000 (optional) Galvanic separation for proximity probe system (not required for non-exi application); CENELEC approved Protection system monitor hardware 9 1 ABE 040 19 in. rack chassis 10 2 RPS 6U Rack power supply modules 90 to 240 V AC input (+24 V DC optional) 11 1 CPU-M-6 Central processing and display unit 1x RS485, 1x Ethernet 12 1 IOC N I/O card for CPU-M 13 3 MPC 4 Machinery protection card, programmable, four dynamic channels, two speed 14 3 IOC 4T Analog I/O for MPC 4 with four SPDT relay 15 1 Config Basic rack configuration and manuals Condition monitoring system hardware 16 1 CMC-16 Dynamic data acquisition module 16 channel 17 1 IOC-16 Analog I/O for CMC-16 Please contact: SKF USA Inc. Condition Monitoring Center San Diego 5271 Viewridge Court San Diego, California 92123 USA Tel: +1 858-496-3400 Fax: +1 858 496-3531 Web: www.skf.com/cm SKF is a registered trademark of the SKF Group. All other trademarks are the property of their respective owners. SKF Group 2011 The contents of this publication are the copyright of the publisher and may not be reproduced (even extracts) unless prior written permission is granted. Every care has been taken to ensure the accuracy of the information contained in this publication but no liability can be accepted for any loss or damage whether direct, indirect or consequential arising out of the use of the information contained herein. PUB CM3081 EN April 2011