The Development and Operating Experience of the SGT-400 ...

The Development and Operating Experience of the SGT-400 Industrial Gas Turbine

Copyright © 2007 Siemens Power Generation, Inc.

POWER-GEN International 2007 – New Orleans, LA December 11-13, 2007

Copyright © 2007 Siemens Power Generation, Inc. 2

The Development and Operating Experience of the SGT-400 Industrial Gas Turbine

Steven Ward, MSc, C.Eng, MI.Mech.E

Brian Igoe, MSc, C.Eng, MI.Mech.E

Siemens Industrial Turbomachinery Limited, Ruston House, P O Box 1, Waterside South, Lincoln, UK, LN5 7FD

Abstract

The 13MW SGT-400 industrial gas turbine was launched in 1997 and is now widely accepted in the market for industrial power generation and oil & gas applications with worldwide sales of over 110 units. The fleet has accumulated 500,000 operating hours and proven itself in a variety of environments, from offshore FPSO mechanical drive duty to base load combined heat and power provision for municipalities. The SGT-400 is available as a stand-alone power provider, or in combined cycle configuration. The focus of this paper is on the development of the SGT-400 into a proven product with the fundamental requirements of high availability and reliability. Described are the methodology applied along with the key changes introduced which resulted in year on year improvements in fleet availability. Introduction of ‘Calm Controls’ has reduced the number of unnecessary engine trips and resulted in increased availability as well as protecting customer operation. In addition, this paper presents details of the process and validation completed to resolve technical challenges identified in early commercial operation. Emphasis is placed on rotor and blading issues through measures taken to minimize rotor vibration and the design improvements introduced to protect blading from environmentally induced corrosion attack. Some current new enhancements to the product are also briefly described, targeting expansion of the fuels capability of the turbine to fuels of a Wobbe Index 25MJ/m3, using the DLE combustion system; changes to the exhaust emissions signature of the engine over a wider range of loads and, finally, the use of Siemens’ own PLC-based control system. This paper demonstrates that the application of robust design and development principles leads to a product suitable for a variety of applications, allowing it to be developed into new areas where fuel flexibility and environmental conditions can be met.


Introduction

The SGT-400 entered commercial service in 1999 in a combined cycle cogeneration facility in Australia. The lead package has now achieved approaching 55,000 hours with the majority of duty on full load. The SGT-400 is configured with a DLE (Dry Low Emissions) combustion system and operates on natural gas or distillate fuels with low NOx and CO emissions across the load range. The fleet has grown to more than 110 units around the world, Figure 1, over 35 being used for Power Generation.

Figure 1 – Geographical Location of SGT-400 Fleet The operating fleet has accumulated in excess of 560,000 hours in commercial operation split evenly between Power Generation and Mechanical Drive applications. Figure 2 shows the operating hours accumulated in various applications.

Data correct as of October 2006

Generator Drive

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Data correct as of October 2006

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Figure 2 – SGT-400 Fleet Operating Hours Availability and Reliability

Siemens have historically used the Siemens Electronic Data Exchange Network (EDEN) to monitor the availability and reliability of their fleet of engines. This system also enables site data to be viewed by Siemens Support engineers to assist in problem analysis without having to visit site. EDEN is currently being upgraded to an improved system called Remote Monitoring System (STA-RMS). All new engines are built with the required data collectors in the control system and customers are encouraged to make the communication connections. Figure 3 shows the Siemens definition of Availability and Reliability. Availability is measured as the percentage of time the engine is operating annually and includes time lost through scheduled and unscheduled downtime. Reliability is a similar measurement but allows removal of time due to scheduled maintenance. Unscheduled downtime and trips are also monitored using the STA-RMS system.

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For illustration purposes only Figure 3 – Availability and Reliability Definition The measurement of Start Reliability, Figure 4, allows repeated initiations of the starting sequence within 30 minutes and repeated initiations of the starting sequence due to successive failures for the same cause are counted as one start attempt. This aligns to the BS ISO 3977-9 standard which allows filtering of repeat start failures if no maintenance has been carried out between start attempts. For illustration purposes only Figure 4 – Start Reliability Definition Early operating experience on the SGT-400 saw engine availability affected by issues with compressor inlet rotor vibration and compressor turbine blade failures. Customers also showed concern over the number of nuisance trips that affected their operation. Through use

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of rigorous Product Development processes Siemens have resolved and implemented solutions to these problems which have resulted in year on year improvements in availability as shown in Figure 5. Figure 5 – SGT-400 Improvements in Monthly Availability Trend of the Operating Fleet To achieve these results Siemens dedicated a team of engineers lead by a Six Sigma Black Belt with sole responsibility of resolving the causes of unreliability. Historical SGT-400 fleet running data was taken from STA-RMS and used for analysis to determine the causes of unreliability and the duration taken to fix the problems when they occurred. This was done by interrogating the engine running trips and engine start-up trips. Six-sigma methodology was adopted and the causes were categorized as: • Start Reliability • Running Reliability Each project had a ‘Black Belt’ project leader assigned with several sub-projects grouped by similar system trips. Start Reliability Three sub-projects for were created: i) ‘Pre-flame on’ determined that the major areas of trip involved the Purge and Fuel Systems. ii) ‘Light-up’ showed that the various engines in the fleet had inconsistent light-up windows. As a result consistent light-up window mapping guidelines were issued.

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iii) ‘Post-flame on’ showed the majority of failures were due to ‘Flame-Out’ but approximately 90% of these trips were from false indications. One solution covered a large number of these issues and was released under the title of ‘Calm Controls’. Calm Control involved a critical examination of the engine control system to determine how the engine trip criteria could be desensitized to prevent unnecessary trips without compromising the integrity of the engine or the safety of the operator. Three approaches were adopted: i) Confidence checking delays were added or extended to detect genuine temperature/pressure/level device failures thus avoiding unnecessary single-sample trips. ii) Downgrading, inhibiting or relaxing operating limit/envelope set-points to improve tolerance to transient device outputs e.g. inhibiting dynamics monitoring during fuel change-over. iii) Improved tolerance of single monitoring device failure. A major success was achieved through allowing engines, if there is a failed start, to have multiple start attempts without the engine dropping below a threshold (spin) speed. This involved automatic relight combined with intelligent start map adjustments. The control system now allows five start attempts and each time it tries one of three different start maps. A project to prevent Combustion Can flame-out highlighted the existing detection algorithms used criteria based upon falling burner temperatures or deviating interduct temperatures. Further analysis shows that neither of these parameters can robustly identify a can-out situation. A revised detection criteria has been developed based upon both parameters as well as the rate of change and variation in interduct and burner temperatures. During testing it was concluded that 90% of Can flame-out trips are unnecessary. Figure 6 – SGT-400 Improvements in Start Reliability of the Operating Fleet

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As a result of these changes the start reliability of the operating fleet, figure 6, has increased significantly and new engines dispatched from Lincoln having all the latest solutions implemented have seen an increase of 12% in the last 12 months. Running Reliability Examination of Running Reliability trips revealed small numbers of components were suffering poor reliability resulting in a significant effect on the overall running reliability figure. These components fell into two categories: i) Design improvement ii) Quality improvement Each component was examined to determine cause and the appropriate action taken to rectify. A number of issues with the original STAR valve system has led to it being replaced by an Integrated Gas Valve (IGV). Issues such as ‘Drifting’ have been addressed by removing the Analogue Position Unit (APU), the actuator is now driven directly by the Engine Control Unit (ECU) and the response times have improved significantly. A digital encoder has also been installed to improve accuracy. A new profile valve has been fitted on the IGV. It has replaced the original V-shaped profile which did not allow for accurate and repeatable fuel flow. The new valve has proven to give repeatability to the required accuracy.

The original Solenoid Valves were improved to address issues with corrosion and speed. During long periods of inactivity there were occasions where corrosion took take place between the ball valve body and the drive coupling. New solenoid valves were fitted to address this issue by replacing the mild steel coupling with a stainless steel one. In addition, the block & bleed valve assembly was reported to be slow to close (outside the one second ruling – IM/24 British Gas Regulation), or would not form a gas tight seal. Introduction of retrofit kit changed the existing pilot solenoid valves with enhanced solenoid valves. This solution has now accumulated over 12 months’ positive experience.

The Variable Guide Vane (VGV) actuator system was a source of poor reliability. The original hydraulic system was replaced by an electrically actuated system. The new Electric VGV system is cleaner, more accurate and easier to calibrate. This new system was validated on the test bed for 12 months prior to being released in service. The original design of the Pilot Burner was in two pieces but internal leakage across the joint allowed liquid fuel into an air passage which subsequently was prone to coking and blockage. Designing a single-piece pilot eliminated the joint altogether and removed the issue. At the same time, a removable fuel nozzle was introduced thus minimizing downtime by allowing quick change of nozzles when fuel spray quality had deteriorated. This design has now accumulated several years running experience on customer sites.


As with Start Reliability, the engine Running Reliability also benefited from Calm Controls. New engines dispatched from Lincoln having all the latest solutions implemented have seen a significant reduction in trips over the last two years. This is predominantly due to the prevention of nuisance trips. Compressor Inlet Rotor Vibration

In 2004 two SGT-400 core engines were prematurely removed from service due to a steady increase in Inlet journal bearing vibration. EDEN was used to monitor and plan the engines removal from service without failure. The increase in vibration was attributed to movement at the Intermediate / Exit Stub shaft joint, Figure 7.

Figure 7 – SGT-400 Cross Section Showing Location of Joint The main characteristic of the vibration was a steady increase in inlet bearing vibration over time which eventually led to engine failure to restart following a shutdown. The vibration was predominantly synchronous, indicating a change in balance. It was also noted that both core engines were operating in high ambient conditions and on full load duty. The mechanism leading to vibration was differential thermal growth at the joint between the Intermediate and Exit Stub shaft components during shutdown and restart of the engine. Under these conditions the joint suffered a reduction in interference due to localized creep of the Exit Stub shaft. This eventually led to a reduction in joint stiffness and changes in rotor balance.


Compressor inlet bearing vibration is amplified during the transient start and shutdown of the engine as the rotor passes through the rigid body critical speeds, Figure 8.

Figure 8 – Inlet Bearing Vibration during Start-up The two core engines that illustrated the vibration trend were dismantled and investigated. The complete compressor rotor and then individual components were geometrically and dimensionally inspected. The rotor disc pack concentricity showed signs of minor movement when compared to the original build geometry. Inspection of the joint between the Intermediate and Exit Stub shaft showed growth on the Exit Stub shaft recess diameter. This resulted in a reduction in interference between the spigot diameter on the Intermediate shaft and recess diameter on the Exit Stub shaft. As a consequence of the reduction in rotor stiffness, inlet bearing vibration increased to levels which gave cause for concern. The dimensional change of the Exit Stub shaft recess diameter was attributed to localized creep due to actual metal temperature being higher than original analytical design prediction. The solution to this problem was to eliminate the joint by redesigning the two-piece shaft joint to a single-piece shaft, figure 9. At the same time, the shaft material was also upgraded to Inco 718 thus providing high strength and improved temperature capability. The new design is physically interchangeable and can be retrofitted.

Joint eliminated from this axial positionJoint eliminated from this axial position

Figure 9 – New Design Single-piece Shaft Key design properties such as Temperature, Stress, Displacement, Creep and Dynamics were verified by analysis and practical tests were performed on an engine to verify Performance, Endurance, Thermal Cycling and Load Cycling (See Figure 10). After the test the engine was stripped and visually inspected for blade tip rubs. Verification was successful and the design released.


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Figure 10 - Waterfall Plot Shows Consistently Low Shaft Vibration during Load Cycle To protect customer operation during the period while the new design shaft was being introduced the following parameters were monitored on the fleet of operating engines to enable an assessment to be made of remaining life on the shaft before the engine could start to suffer vibration problems: • Ambient temperatures • Operating load • Time load • Number of starts Figure 11 shows how this data was used in conjunction with the shaft material properties to determine the rate at which creep life was being used. Vibration data was also monitored for changes in the vibration trend thus providing an early warning of creep.

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From 2005 all new and overhaul core engines were fitted with the new design shaft. Approximately 50 shafts have now been fitted and the new design has accumulated in excess of 150,000 hours with 20,000 hours on the lead unit. The rotor vibration has remained low and stable on these units. CT1 Blade High Cycle Fatigue

During 2001 the original design of SGT-400 Compressor Turbine Stage 1 blades suffered premature failure due to fatigue, Figure 12. Metallurgical inspection of the failed blades confirmed failure by High Cycle Fatigue (HCF) with failure occurring at the interface between the aerofoil and the platform.

Figure12 – CT1 Blade Failure Showing Fracture of the Aerofoil at the Platform Interface The Campbell diagrams were interrogated to determine possible interferences in the running range but as expected, no problems were found. The possibility that the blade frequency may have been modified was then investigated. Close inspection of a failed blade highlighted contact marks which were evident where the blade platform seal wires were located, Figure 13, suggesting that the blade may have been locked at the platform.

Platform sealing design

Blade platforms


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Figure 13 – Original Platform Seal Arrangement on the SGT-400 CT1 Blade


A frequency analysis performed on a blade restrained by the root and locked at the platform to simulate the possible operating condition confirmed a change in frequency. Campbell Diagrams now showed that the frequency of the blade was much higher and an interference was now present between the mode 1 and the 13th engine order, Figure 14.

Figure 14 – CT1 Blade Vibration Analysis Simulating Platform Locking The platform seal was redesigned to eliminate potential platform locking, with a rectangular section seal strip design employed to remove any possibility of the platforms being wedged apart, see Figure 15.

Figure 15 – New Design CT1 Blade Platform Seal


The new design CT1 Blade platform sealing system was validated using a spin-rig with a pulsed air jet. This confirmed that the resonance frequency of seal strip blade had been reduced to acceptable levels. A strain-gauge test confirmed no major resonances were present on the CT1 blades. This design was released in 2002 and all old design blades and seal wires were replaced with the new design. The new design has now achieved more than 300,000 hours with none of the previous issues encountered.

CT1 Blade Stress Corrosion Fatigue Cracking

Two SGT-400 core engines operating in an offshore (platform) Oil and Gas application suffered premature CT1 rotor blade failure. Two incidents involving onshore applications occurred late 2006 and early 2007. All failures were in the extended root neck of the blade and attributed to Stress Corrosion Fatigue. Following extensive metallurgical examination evidence indicated failures occurred in a manner consistent with initiation by environmental attack. The cracks propagated through corrosion fatigue from the internal cooling passage. Corrosion attack was initiated by contamination in the cooling air washing over the base material. This resulted in a reduction in surface material properties and consequential localised cracking. Contaminants known to cause corrosion such as Sulfur (S), Sodium (Na), Phosphorous and Potassium (K) were detected within the failed CT blades. This provided strong evidence that the contamination was indeed airborne. Corrosion Fatigue is driven by a combination of corrosion and cyclic stresses which initiate cracks. Corrosive elements can significantly reduce the threshold level at which fatigue cracking initiates. Trace amounts of contaminant can affect fatigue corrosion causing a reduction in material properties, thus allowing propagation with high stress concentration at the crack tip. Atmospheric contaminates such as S, Na, and K are involved in this process and these elements are commonly found in offshore or coastal environments. A further source includes flare combustion products, giving atmospheric NaSO4. Fracture surfaces resulting from corrosion fatigue are distinct from those seen from High Cycle Fatigue (HCF) or Low Cycle Fatigue (LCF). HCF generally has a smooth fracture surface and LCF fracture surfaces are rougher and show signs of a ductile nature. Corrosion fatigue fracture surfaces have multiple cracks branching off the main failure fracture, Figure 16.


Multiple cracking across fracture surface not seen with typical HCF or LCF fracture surfaces

Multiple cracking across fracture surface not seen with typical HCF or LCF fracture surfaces

Figure 16 – Typical Corrosion Fatigue Fracture Surface High temperature turbine materials are more susceptible to corrosion attack due to hostile operating conditions, with the source of attack found in both fuel and air. Coatings are often used to provide protection from contaminants. The primary function of external coatings is to provide oxidation resistance at high gas temperatures, Figure 17. They also are critical for protection against fuel-bound contaminants to isolate the base material from attack.

Main hot gas streamMain hot gas stream

Figure 17 – Stage 1 Compressor Turbine Arrangement Showing Gas Passage. Airborne contaminants will also wash the turbine blade internal cooling passageways, Figure 18.


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Figure 18 - Stage 1 Compressor Turbine Arrangement Showing Path of Internal Cooling Air. The sensitivity of high strength nickel super-alloys to corrosion was not fully appreciated by the industry until recently. The standard configuration of the SGT-400 did not include coatings to the internal surfaces of the turbine blade since it was believed that the normal levels of environmental contamination did not require this. However, to meet the more severe conditions experienced in offshore applications, an internally coated blade was developed which has now been adopted as the standard for all applications. Siemens now recognize that internal coatings should be applied as standard where required. The CT1 blade new coating system shown in figure 19 has been through extensive validation prior to release for field experience. Corrosion tests were conducted at the research laboratory in Cranfield University, UK. These tests involved the use of different combinations of substrate and coatings with various concentrations of corrosive element. Tests were also performed at temperatures to simulate engine operating conditions. Mechanical tests were performed in the Siemens Materials Laboratory in Lincoln, UK. These tests included strain-to-crack at various temperatures, High Cycle Fatigue comparison with bare material and Low Cycle Fatigue comparison with bare material.


Figure 19 – CT1 Blade New Coating System

Further engine-based validation was performed and involved Infra-red pyrometry to determine blade temperatures and allowed comparison with analytical FE predictions to be made and show sufficient operational margins. A number of sets of internally coated blades have been fitted to SGT-400 engines to date; mostly to offshore Oil & Gas applications with known high levels of environmental contamination. Similar coating systems have been extensively used on the SGT-100 engines and have now accumulated total running experience in excess of 100,000 hours with the lead unit 24,000 hours’ operation. New Enhancements

This section briefly describes some recent new enhancements to the product. Expanded Fuel Capability The SGT-400 is configured with a lean pre-mixed Dry Low Emissions (DLE) combustion system and operates over a range of fuels of Wobbe Index (WI) = 37-49 MJ/m3 (810/1100 BTU/Scf), where:

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and: CV = net calorific value (MJ/m3) at standard conditions SG = specific gravity at standard conditions = �fuel / �air Standard conditions = 288K, 1.013bara A number of gaseous fuels with lower WI have been encountered over recent years where increased level of inert species such as Carbon Dioxide (CO2) and/or Nitrogen (N2) is a common feature. One solution to use such fuels is to remove, for example, CO2 from the raw fuel thus providing a more acceptable fuel for use in the current combustion hardware design. Although the additional processing is well proven, it adds unnecessary complication and cost to a power generation or mechanical drive plant. Siemens’ approach to the problem was to take the existing simple DLE combustion system and modify it through both analytical and practical means to achieve a solution which could be applied to a range of gaseous fuels in the WI range 15-49 MJ/m3 figure 20. To date, validation of design changes to the burner resulted in release of a capability covering a range down to 25 MJ/m3 across the full operational range of load and temperature. Throughout all of this work no increase in the current emission guarantee or turndown capability would be accepted.

Figure 20 - Fuels Expansion Range The first contract covering such a medium calorific value (MCV) fuel application was offshore, SE Asia. The core engine was assembled and tested on natural gas with standard burner hardware and converted to the MCV build to use contract fuel of WI = 28 MJ/m3, figures 21. As part of a package test a full set of ignition tests was completed using bottled gas manufactured to simulate the contract gas specification. Successfully completed this test included a demonstration to the customer, resulting in full release of this gas-only MCV configuration.

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Figure 21: MCV Contract Engine This expanded fuels range has been applied successfully to other applications. Improved Emissions Signature This product has already demonstrated satisfactory exhaust emissions characteristic with normal operation of <25ppm NOx (corrected to 15% O2) and including <10ppm NOx (corrected to 15% O2) for specific application. However, the operability range for the lower level has been limited. Completion of some development has resulted in a wider general operating range for both NOx and CO emissions. The range now available shows a reduction in general NOx signature from 25ppm to 15ppm, with a 10ppm range at full load where required. Part-load CO emissions control has become a bigger requirement and control of CO emissions to <50% load has been achieved and released by the use of a P2 bleed system (common feature with engine load shed control). Siemens Control System The Siemens PCS7 Distributed Controls System (DCS) has recently been introduced to the SGT-400 for process control. Using standard components from the SIMATIC Automation range the system developed offers dual redundant options including the processor itself. PCS7 provides an integrated process control solution that includes Human Machine Interface software, controllers (Automation Stations) and Input/Output (I/O). The system can be configured using various language tools and standard control libraries which comply with IEC 61131. SIMATIC S7 Controller is the undisputed market leader with circa 40% market share.


The new system was released on the SGT-400 in 2006 and has performed exceptionally well. It is currently being developed for all engines in the industrial gas turbine range up to 13MW. Note. In addition to the Siemens Control System, Siemens will continue to offer the Allen Bradley ‘Control Logix’ system where required.

Summary

This paper has demonstrated Siemens commitment to customer focus through improvements in the availability and reliability of the SGT-400. Application of robust design and development principles has delivered effective solutions to design-related issues. New development initiatives on the engine combustion system have delivered increased fuel flexibility and reduced emissions. The Siemens PCS7 control system has provided a sound controls platform for the future.

PERMISSION FOR USE

The content of this paper is copyrighted by Siemens Power Generation, Inc. and is licensed only to PennWell for publication and distribution. Any inquiries regarding permission to use the content of this paper, in whole or in part, for any purpose must be addressed to Siemens Power Generation, Inc. directly.

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