New solutions for the improvement of turbocompressor availability

Transcription

1 New solutions for the improvement of turbocompressor availability By Filippo Cinelli, Riccardo Valorosi, Gianni Mochi, Antonio Pumo, Dante Sabella / GE Oil & Gas GE imagination at work 1

2 New solutions for the improvement of turbocompressor availability By Filippo Cinelli, Riccardo Valorosi, Gianni Mochi, Antonio Pumo, Dante Sabella / GE Oil & Gas Abstract In recent years, market demand for plant uptime has progressively increased. Turbocompressor technology usually represents the core equipment of a plant and its availability is one of the key factors in optimizing plant uptime. OEMs have concentrated significant engineering efforts on developing new hardware to extend the mean time between maintenance (MBTM) and to reduce the need for downtime inspection activities. The new generation of material technology for hot gas path parts is focused on enhancing the durability of machines. The combination of proven materials and advanced technology from other industries, such as the aircraft engine industry, accelerates the introduction of improved materials, coatings and design solutions into new machines as well as into retrofit kits to uprate installed base units. Uprate kits are designed to be fully interchangeable with original parts. Hence, they can be installed during planned shutdowns of a unit with no additional impact on the plant downtime, and in some cases, even significantly reducing the downtime. In addition, substantial improvements in specialized software to manage and analyze data on operating equipment have been made, which provide faster and more accurate troubleshooting. Predictive capabilities have also been embedded into software tools and in the near future it is expected that they will be able to cover residual life assessment of hot gas path components as well. These advances are all key to extending equipment availability, which translates into additional days of production as well as reduced life cycle cost. Introduction Uptime is one of the areas where oil and gas OEMs are concentrating their effort, investing in new technology for products which address this key need. Maximizing up time means reducing outages, extending the mean time between maintenance, shortening site activities and reducing unplanned shutdowns due to failures. Reliability and availability are the metrics used to measure up time, and are specified by the ISO definition [1]: FOH = Forced Outage Hours: period of time during which the maintenance team is actually working during forced outages POH = Planned Outage Hours: period of time during which the maintenance team is actually working during planned outages Key factors critical to enhancing up time include: Reduction of site activities turn around time Reduction of planned maintenance Reduction of unplanned maintenance Reliability Availability R = [(PH-FOH)/PH]x100 A = [(PH-(FOH+POH))/PH]x100 The cases addressed by this paper concentrate on the FR5/2 D model gas turbine. Where F = number of failures PH = Period Hours: cumulative hours of monitoring The FR5/2 is an heavy-duty, double-shaft gas turbine. The 17-stage axial compressor is a high efficiency model. The 12 combustion chambers, which have individual burners, This paper was prepared for presentation and awarded at the SAOGE 2008 in Dammam, Saudi Arabia, November 15-17,

3 are set parallel to the machine s centerline. Expansion is achieved in 2 turbine stages with impulse rotor buckets: the first stage drives the axial compressor and the two together comprise the high-pressure shaft; the second stage drives the low-pressure shaft and load. The partition airfoils of the first stage fixed nozzles are cooled with air from compressor bleed, while the partition airfoils of the second stage fixed nozzles have an adjustable angle which enables broad speed range and load flexibility. Owing to their flexibility, high capacity, simplicity and low maintenance requirements, these units are ideal drivers for pipeline centrifugal compressors. The FR5/2D is also used in natural gas re-injection plants to drive high pressure re-injection centrifugal compressors An overview of the case study presented is shown in Table 1. Cycle Fuel NO X emission reduc. system Type of application Type of operation Type of installation site Inlet plenum Exhaust plenum Type of enclosure simple Natural gas Not installed Mechanical drive Continuous On shore side side acoustic enclosure under shed (with an overhead crane) Table 1 Case study gas turbine configuration Maintenance Operation of a gas turbine, like any other rotating machine generating mechanical power, must include a maintenance program for repair and/or replacement of parts when needed to ensure maximum availability and reliability of the unit. Naturally, this maintenance program will begin with minor operations, which will increase in scope over time and according to the severity of operation up to the point of a major inspection. This maintenance pattern is repeated cyclically. A gas turbine maintenance program can be split into: Preventive maintenance Inspection during operation This consists of logging the key operational parameter during start-up and while the unit is in service. Subsequent analysis of this data establishes the general conditions of the unit and auxiliary systems. Standby inspection This is conducted when units are out of operation due to the plant schedule and therefore does not interfere with availability but increase reliability. Activities may include changing filters, checking the lube oil level, checking and regulating instrumentation, etc. Scheduled maintenance Scheduled maintenance is performed with machines out of service, as the unit has to be disassembled so that internal components can be inspected. The scope of the area to be disassembled depends on the type of inspection. Inspections start from the most critical zone, which is the point where the temperature is highest, and continue to cover the entire machine at subsequent increased inspection levels: Combustion inspection = CI Liners and transition pieces inspection = LTPI Major inspection = MI The zones covered by the above mentioned inspections are shown in the cross-section drawing of the turbine in Figure 1. figure 1 FR5/2D Gas turbine maintenance sections

4 Scheduled maintenance frequency Based on a wide variety of actual operating experience of heavy-duty gas turbines installed worldwide, the scheduled maintenance spacing method has been developed as an initial guideline. Table 2 OEM recommended maintenance plan The frequency of scheduled maintenance indicated in Table 2, expressed in terms of fired hours accumulated by a machine, is to be considered as a standard calculated based on the following operating conditions: Natural gas fuel Continuous operation Base load (100%) No water/steam injection Best maintenance practice A day of site activities equals 12 working hours Optimization of Site Activities GE s first efforts to reduce the duration of site activities during an outage focused on the major overhaul (MO). The target set by one of the leaders in the LNG industry for execution Standard Maintenance Extendor Modular Replacement Year 2 Year 4 Year 6 Year 8 16,000 32,000 48,000 64,000 CI 5days CI 5days HG 14 days HG 14 days FR5/2D inspections scheduled maintenance standard frequency - whichever occurs first CI LTPI HGPI MI CI 5days CI 5days MR 12 days Table 3 Duration of planned maintenance site activities of the MO on site challenged GE to complete it in 12 days vs. the standard 24 days considering that the site team was organized for double shifts of 12 hours each. The major overhaul of the FR5/2 gas turbine was on the critical path of the entire LNG plant shutdown. A 100% reduction of time could not be achieved with the standard maintenance approach of stripping the unit and replacing the hot gas path parts at the site as recommended by the maintenance policy. The breakthrough came directly form the aeroderivative approach of swapping the entire engine with a fully refurbished engine. Table 4 LNG plant shut down plan As a result of the introduction of the engine swap concept, the duration of site activities decreased to 13 days (26 shifts). The major overhaul turn around was cut from an average 634 hours and Best record 560 hours, to average 428 hours and best record 332 hours. MI 24 days firing hours: 0 12,000 24,000 36,000 48,000 firing starts: ,200 2,400 Turn around day LNG OFF COLD WORKS ONLY MHE WARM CONDITIONS APRIL STARTUP GE SCOPE: ENGINES EXCHANGE AUX INSPECTION COOLING DOWN CHEMICAL CLEANING BLINDING HOT WORKS / OPEN FLAME ACTIVITIES PROCESS SYSTEM INSPECTION INITIAL COOLING DOWN OF THE MHE TO REACH -68º LNG ON DE-BLINDING NO WORKS START PUNCH LISTING OF RE-INSTALLED EQUIPMENT Implementing the lessons learned and fine tuning the procedures, a major overhaul using modular exchange could potentially be further improved by an additional 3 days. The areas to explore are: team relationship and communications, and the 3

5 selection of the team members based on skills and experience. Lessons learned Relationships and communication channels are key to the success of the effort. Table 4 reports the highlights of specific experience from this case study. PROS Daily progress meetings Continuous intern between parties Customer very sensitive to EHS matters Accident-free campaign Toolboxes Stop cards CONS Permit to work system too complex Delays & confusion Arbitrary interpretation of safety rules Unnecessary suspension of work Recertification of tools and devices Table 5 Relationships and communication lessons learned Additional outage experience provides the following lessons learned: Improve preliminary agreements with customer (outage excellence) Clarify operating and safety procedures to avoid delays due to misunderstandings or arbitrary interpretation of rules Improve the issuing of work permits Single interface between GE and the customer It is estimated that by applying the lessons learned, the team could save an additional two shifts in the next modular replacement. People are the other key to success. The lesson learned is to carefully select the personnel and all contractors specifically for the shutdown, ensuring that each individual possesses all of the necessary skills for the execution of his role in the project. The estimated savings by precise staffing based on the specific project is about three shifts. PROS Skillfull P Flexible Team players Optimized CONS Table 6 Contractors selection lessons learned Finally some low-impact engineering modification proposed by the outage field team and maintenance engineering can further facilitate the modular replacement operation speed. Instead of delivering rigid pipes for lube oil adduction and cooling air line, enhanced alignment devices, and pre-packaged new turbine modules, these kits will be prepackaged with flex pipes to reduce outage time by one day or more. The engineered solution addresses the following areas: Cooling & sealing air piping - Air manifolds pre-installed on new engines - Use of flexible piping Exhaust plenum - Use insulation mats planning problem VISA problems tools and certifications not in place Manpower not enough skilled (lack of experience) Slow pace of mechanical workers Bad English speakers Communications problems Alignment - Jacking bolts to facilitate positioning on the 2 axes - Guiding frame for reduction of turbine misalignment Lube oil piping - Use of flexible pipes for lube oil line inlet New engine preparation - Tailored spare parts list for engine exchange procedure Lighting system - Redesign of lighting system in turbine compartment 4

6 Figure 2 From left to right: bearing #1 flexible lube oil piping, bearing #4 flexible lube oil piping, extended pin for engine positioning These improvements optimize the hardware for modular replacement and allow the team to save one shift. In total, based on the FR5/2 modular replacement experience, a reduction of 10 days is potentially achievable. To further reduce the turn around time, another breakthrough in the maintenance approach would be required, but associated with dramatic change in the GT package design. the maintenance intervals recommended by the OEM. Departing from standard maintenance intervals can have a very positive impact on plant production but, at the same time, might compromise machinery reliability. Careful and rigorous management of reduced maintenance activities can help in extending the maintenance intervals while at the same time avoiding the deterioration of reliability. Days Standard major overhaul 26 Days Engine swap Jet Events Experience Figure 3 Left: GE Oil & Gas modular replacement experience. Right: Modular replacement learning curve initiation phase (planning) early phase mature phase breakthrough needed Enhancing availability of hot gas path parts by departure records Technical risk analysis of the critical turbo compressor components allows the OEM to assess by how much the maintenance intervals can be extended. The aim is to manage the technical risk when extending Required information for this assessment includes: Full remote monitoring (torquemeter and fuel flow rate measurements are normally required) Presence of site engineer 5

7 Supervision of inspection activities Tracking of repair activities Laboratory analyses including destructive testing of removed parts (depending on the type and severity of damage) Unit history and spare parts tracking Phases of the assessment The various phases of the project are described below: 1. Analysis of operating conditions A detailed analysis of the actual operating conditions is needed in order to identify factors impacting the aging of turbine components, such as fuel type and quality, over/under-firing, etc. 2. Analysis of unit configuration A detailed analysis of the actual machinery configuration is performed in order to identify the part numbers of the installed components 3. Analysis of the fleet maintenance history When working on defining the potential failure modes of a specific unit, GE Oil & Gas takes advantage of the extensive feedback from sites which is continuously collected and which includes various applications and all possible ambient conditions. A fleet analysis is always conducted to identify the failure modes. 4. Analysis of unit maintenance history The above-mentioned fleet experience shows that in specific installations some failure modes may predominate. For this reason, the fleet experience is compared with the specific history of the unit over time, in terms of events, part damage, inspections, findings, etc. 5. Run-test plan On the basis of the above analyses, a gradual increase in planned maintenance intervals is usually defined. During planned maintenance shut downs, RAM Engineers participate in the site inspection to collect all the information needed for defect maps, accumulated damage assessment, etc. 6. Lab test plan As a part of the interval extension program, a residual life assessment of the critical parts is performed to provide quantitative information on how much additional operation the parts might sustain without maintenance. Such an analysis, together with the site inspection findings and the operating parameters record from RM&D, are used to validate the program s assumptions and manage the risk of stretching the intervals. As an additional step, the need for the injection of new technology into the parts design may be raised, leading to the initiation of research and development programs dedicated to the successful achievement of interval extension targets. GE Oil & Gas has substantial experience in the use of departure records for maintenance planning and diagnostic purposes. One of the most significant availability improvement successes was with a series of 10 turbocompressor trains driven by FR5/2C and D model gas turbines. The analysis of operating data combined with the information obtained through laboratory analysis of the metallurgy of similar components and the findings from parts removed in previous inspections provided the necessary information for assessing the risk of extending the combustion inspection interval The risk of failure due to skipping the combustion inspection of these particular turbocompressors while maintaining the operating conditions the same was determined to be negligible, and the owner accepted the OEM recommendation to move the combustion inspection from the original 12,000 hours to 24,000. As a result of the use of the departure record, the availability expected over the full life of the turbocompressor trains improved by 0.6 points. 6

8 Data from RM&D Firing temp. Trip trends Pulsations Info from laboratory Destructive analyses Residual life assessment Info from inspections Statistical analyses Defect maps Figure 4 Departure record enablers MTBM extension Figure 5 The Power Crystal kit Enhancing availability by injecting the latest design technology GE Oil & Gas over the years has consistently invested in new technology and new materials to improve the mean time between maintenance of its products. The focus of the research and development team has also extended to the installed fleet, seeking solutions that can be easily retrofitted into a machine that has been running for years, without increasing the plant complexity and the management of the turbotrain. In those plants where plant down time is critical, plug and play is a winning solution. A heavy-duty machine plus centrifugal compressor, customized to meet the required plant pressure and service configuration is the most common type of train for oil and gas applications. In these designs, maintenance activities are scheduled to match the plant production cycle and the life cycle of the critical components of the machinery that is driven by the hot gas path of the gas turbine and the compressor wear parts. 7

9 GE Oil & Gas R&D efforts strive to create products and solutions that closely match the MTBM targets demanded by applications, the market and customer expectations. The Power Crystal kit, for the GE frame 5 two shaft machine is one of the outcomes of this effort. The Power Crystal kit builds on the long history of successful Frame 5 upgrades since the introduction of the first A model in The features of the kit allow the maintenance schedule to be extended with no impact on the reliability and availability of the gas turbine. The Power Crystal kit is composed of the turbine first stage buckets made of single crystal material that have given the name to the kit, and of other hot gas path components such as combustion hardware, shrouds and first stage nozzles that complete the supply. Power Crystal technology The latest advance in hot part technology has seen the introduction of single crystal buckets, which offer the potential of further improving the strength of high temperature materials through the control of crystal orientation. With a single crystal material, all grain boundaries are eliminated from the structure and a single crystal with controlled orientation is produced in an airfoil shape. By eliminating all grain boundaries and with associated strengthening additives, a substantial increase in the melting point of the alloy can be achieved, thus providing a corresponding increase in high temperature strength. The creep and fatigue strength are also increased, compared to equi-ax or directionally solidified structures. This technology has been applied in GE, especially in aero engine applications, for more than 15 years. Almost all aviation engines produced after 1990, including the famous CFM, CF6 and the latest GE90, rely on the outstanding properties of single crystal alloys. GE also has extensive experience in the use of single crystal material in aeroderivative gas turbines such as the LM2500+, LM2500+G4, LM6000PD and LMS100 along with all FB and H class industrial turbines. The first stage nozzle and shroud complete the path of the turbine components that have been analyzed to rejuvenate the product, and light modifications have been included. A new material has been applied to the shroud and a new cooling system design is now embedded into the final machined first stage nozzle. Combustion hardware is a critical area for the operating life of a gas turbine that requires periodic inspection. The duty cycle and type of fuel used are the key factors in determining the recommended combustion intervals since these factors directly influence the amount of thermal barrier coating erosion, material creep, thermal stress and wear of the combustion components. The Power Crystal kit includes extendorized combustion parts that can increase inspection intervals by significantly reducing combustion component wear. Wear is the most critical failure mode of combustion hardware in HD gas turbines. The new extendor kit consists of the latest hard coating material and the redesign of parts to reduce wear on various combustion system components. CL OF CROSSFIRE TUBE Figure 6 FR5/2 liner showing redesigned areas to improve wears resistance The redesign was implemented to reduce the relative movement between combustion components and to reduce the forces and vibrations at wear interfaces. 8

10 Figure 7 FR5/2 transition piece showing redesigned areas to improve wear resistance By incorporating proven wear-resistant materials developed by the GE Global Research Center, the new design provides critical clearance control at the various wear interfaces such as the liner stops, fuel nozzle tip to combustion liner, fuel nozzle collar, crossfire tube to combustion liner tube collar, combustion liner hula seal to transition piece forward sleeve, transition piece forward supports and bracket, and transition piece aft picture frame seal. As a consequence of the redesign of the wear contact points, some of the gas turbine combustion inspections can be skipped, increasing the availability of the whole machine by about 0.6 points over the life cycle. With these proven advanced materials derived from jet engine technology and from our research centers, the life of the critical turbine components and hence the MTBM can be extended to 24,000 hours for the combustion parts and 72,000 for the other components. This kit delivers to customers increased availability and production with a moderate investment and a simple plug and play installation that is, in terms of time and complexity, the equivalent of a normal hot gas path inspection. Data analysis All the recent technology developments have been conducted in partnership with Customers, fully exploiting the Engineering content Abstraction Figure 8 Data layers extensive operating and laboratory data available, and represent incremental evolution of the products in accordance with the conservative philosophy of the oil and gas industry. Recently available communication technologies have dramatically increased the capability of technical groups to share data which has been very useful in pushing the availability boundaries with controlled risk. Considering that the majority of site technicians are focused on daily operations, the remote support services and highly specialized diagnostic capabilities provided by gas turbine and centrifugal compressor manufacturers have strengthened the relationship between industrial plants and original equipment manufacturer technical teams. The main components of a distributed diagnostic system are site data acquisition systems and sensors, connectivity devices, and finally, remote data centers staffed with experts. The principle services delivered by remote diagnostic centers are anomaly management and engineering analysis of performance. Recent studies have shown that the availability of monitored units increases compared to unmonitored units. The benefits of improved communication between the site and remote support teams include solving technical issues proactively and quickly, and also, better coordination of site activities. Starting from the data available at the site, the value of the information increases with the data analysis and abstraction that is achieved through the use of specialized tools and expertise available from the equipment manufacturer. Figure 8 illustrates the different levels of data abstraction starting from site data. Post processed data Normalized data Site raw data 9

11 Examples of raw data are the parameters acquired by the unit control system to control and protect the machine during site operations; normalized data are typically ratios between raw data to decrease the level of variation of the phenomena and enable easier data management both at the unit and fleet level. Postprocessed data represent the maximum level of abstraction and are normally derived parameters or performance parameters including polytropic and isentropic efficiencies, power at reference conditions and nondimensional coefficients or internal design flows. As an example of a set of data, we can start from an axial compressor discharge temperature, then normalize it to the inlet temperature, and calculate the expected values at the same thermodynamic conditions. Comparing data at the same inlet guide vane angle and plotting the data against the compression ratio, the following results can be obtained: and diagnostic tools. Applying this approach systematically to data acquired remotely, the analysis of the diagnostic engineer can be concentrated at higher levels to increase the effectiveness of service and to support maintenance technicians in making decisions during scheduled maintenance activities or in case of emergencies. Remote monitoring and diagnostic tools represent a means of increasing the value of information from raw data; using site data acquired and processed by an RM&D system and taking into account the added accuracy [2] that this process provides, a set of systematic analysis can be started and used to help in the troubleshooting of rotating equipment under real operating conditions. The following diagram represents the link between site data sources and intermediate tools used to define the symptoms of performance losses. Statistic feedback Set of symptoms Machine system or component Weighted RCA Figure 10 Definition of symptoms PRT-GT:E2D3 [-] Axial Compressor is. eff. vs. corrected speed NHP_CORR [RPM] Figure 9 Data analysis (actual data vs. baselines) In general, data are available as raw measurements and the following levels of abstraction are normally reached through the use of design Once the symptoms have been defined, a sophisticated system is used to define the link between the symptoms and root cause analysis. Recent development efforts have been dedicated to developing the root cause analysis (RCA) process. On the anomalies management side, diagnostic engineers are assisted by a prognostic engine designed to manage anomalies by exceptions, based on diagnostic rules. By analyzing site data and using the historical unit behavior or signatures, a set of prognostic rules is used to trigger anomaly notifications. The prognostic engine for anomaly evaluation and an expert system to expedite the RCA are important tools in used by the RM&D team. 10

12 Site data Expected performance feedback Current performance symptoms (increased information) the condition as an anomaly. As soon as advised, the customer bypassed the faulty sensor to avoid a trip and maintained the unit in production avoiding an availability loss of about 5 hours. Figure 6 shows the trend and main events relevant to this anomaly. Figure 11 Root cause analysis As a first example of anomaly detection, a deviation from a system signature is considered. When running at normal conditions, a centrifugal compressor driven by an FR5/2D gas turbine presented the following ratio between the inlet and discharge temperatures vs. the compression ratio. After some period of operation, the rule processor identified a step change and highlighted it with colors in the pictorial representation shown in Figure 13. The colors are associated with the timing of the event in hours, the reddish ones are closer to the event. A step change in the efficiency is strongly linked with the step highlighting wear of an internal component. TT OUT/TT IN PT OUT/PT IN Figure 12 Compressor anomaly identified by variation of historical baseline After notification, the compressor was opened to replace the component, and efficiency was recovered thereby avoiding loss of production at the same available power. In another case, a temperature sensor started to deviate from its normal behavior. The change was detected by the prognostic engine below the trip limit of the machine and identified Figure 13 Trend captured around the event; DIAG is the diagnostic threshold used to trigger notification All the above processes and analyses are used to help maintain high availability by enabling a continuous link between the site equipment and diagnostic engineers, providing a continuous process to improve machine performance in a controlled risk environment both for customers and external service providers. Significant MTBF improvements derive from this approach and availability is significantly enhanced by the short anomaly resolution time. Conclusions We saw how the availability of a FR5/2 turbocompressor can be dramatically improved. The modular replacement maintenance approach can result in savings of nearly 50% of the major overhaul down time. This means an average of an additional 2 days of up time per year over the life of the gas turbine. We also saw that the introduction of the Power Crystal kit allows the combustion inspection to be pushed from 12,000 to 24,000 hours and the Major Overhaul from 48,000 to 72,000 hours, with an expected availability improvement of 2 days per year over the entire gas turbine life. 11

13 Finally, exploiting the departure record can improve the availability by extending the MTBM of components through analysis of the information that operating data provides in combination with the OEM know-how. The data and analysis available through RM&D can result in as much as 1 point of additional availability As an overall result of the initiatives discussed, the up time of an FR5/2 can theoretically be improved by 4-7 days considering the combination of modular replacement to shorten site activities, maximizing the exploitation of the operating data for proactive identification of incipient faults and faster diagnosis of failures, as well as by modifying the hardware with the application of Power Crystal or through the use of departure record initiatives. The specific FR5/2 case initiatives discussed in this article can be easily extended to other gas turbine models. For more information Via Felice Matteucci, Florence, Italy T F customer.service.center@ge.com Nuovo Pignone S.p.A. ge.com/oilandgas The information contained herein is general in nature and is not intended for specific construction, installation or application purposes. GE reserves the right to make changes in specifications or add improvements at any time without notice or obligation General Electric Company All Rights Reserved References [1] IEEE Transactions of Industry Application vol 31, no. 4 July/ August 1995 [2] K.Brun, R.Kurz; Measurement Uncertainties Encountered During Gas Turbine Driven Compressor Field Testing