IPT CHENG CYCLE SYSTEM KNOW HOW
INTRODUCTIONInternational Power Technology was founded in October 1974 on the basis of a core of technology which has grown and evolved continuously since then. IPT has developed technological know-how in optimized steam-injected gas turbine engines and their subsequent development into high-efficiency generation and cogeneration cycles. Below is a summary of the IPT know-how gained in over 1,200,000 operating hours and 26 years of experience.
METHODS AND SOURCES OF KNOW HOW DEVELOPMENTIPT's developed technology and know-how are derived from many sources. These various paths of know-how development, which cover the full spectrum of development and application, are combined to produce an interlocked and well-rounded technology base. Original steam injection research was conducted with computational models developed and executed by IPT. IPT has designed, instrumented, and operated its own small-scale engine test cells and laboratory facilities for basic research, product development, pilot testing, and product support. The bulk of IPT's practical know-how is derived from IPT's experience with applying IPT's steam injection technology to commercial products. IPT took complete "turnkey" responsibility for providing all the key components for the first commercial applications of large scale gas turbine steam injection in the world. The designs evolved over successive installations to the point where IPT was able to develop a standardized design for a cogeneration system. Continuous feedback on the standardized system design is also a critical source of know- how for IPT. IPT is directly responsible for the day-to-day operation, maintenance, and upgrade of cogeneration plants incorporating IPT-supplied equipment; this provides direct feedback to the design staff and an opportunity to conduct field performance and evaluation programs. On going expansion of IPT's Know How base has resulted from IPT's participation in installation and operation and maintenance of Cheng Cycle systems delivered by Licensees throughout the world. IPT has often added supervision and/or key engineers to the efforts of Licensees and vendors in resolving design, production, and maintenance problems. IPT staff have provided the design basis and key equipment specification for the 501-KH5 Cheng Cycle cogeneration plant modules and often oversees their manufacture and factory test. IPT also participates in and/or directs the installation, commissioning, and acceptance testing of new Cheng Cycle cogeneration plants.
CHENG CYCLE OVERVIEWUntil recently, use of steam injection had been limited to NOx control and/or simple power augmentation. The critical and interrelated nature of steam-to-air ratios, steam-to-fuel ratios, and other cycle parameters in optimizing the efficiency of the steam injected gas turbine cycle was first realized by Dr. Dah Yu Cheng, a professor at the University of Santa Clara. In 1974, Dr. Cheng formed International Power Technology to develop and commercialize what is now called the Cheng Cycle. Since that time IPT has been granted over 50 US and international patents on the concept and associated hardware. The early Cheng Cycle development work focused on applications where only electricity generation was the goal. In a "pure power" application of the Cheng Cycle, all the steam produced by waste heat is recycled through the gas turbine. For cogeneration applications, the Cheng Cycle system is modified to the extent that steam may be used for process needs and/or sent to the gas turbine. A duct burner is generally added between the gas turbine and the waste heat boiler to increase the total steam producing capability of the system. The flexibility of the system makes it applicable to a wide range of sites with little or no design change required of the major equipment package.
SERIES 7 PRODUCT AND SUCCESSOR 501-KH5 CHENG CYCLEThe first commercial development of the Cheng Cycle concept for cogeneration applications was called the Cheng Cycle Series 7 System, and was based on the 501-KH industrial gas turbine. In 1994 IPT renamed the system the 501-KH5 Cheng Cycle system to better define the systems relationship to the Allison turbine. The 501-KH5 Cheng Cycle System consists of pre-engineered, modular, component groups: a gas turbine generating set, a matched heat recovery steam generator, boiler auxiliaries, and system controls responsible for coordinating and optimizing operation of the complete system.
TECHNOLOGY AND KNOW HOWThe following sections describe the extent of 501-KH5 Cheng Cycle System Know How that was incorporated into IPT's products and services. This Know How has evolved in three distinct layers: ENGINE/GENSET - the innermost layer, which focuses on the gas turbine engine and its supporting subsystems. In addition to the engine, this layer includes the main reduction gear, generator, starter, skid, genset enclosure, lubrication and combustion air systems, and a portion of the distributed digital control system. HRSG/CYCLE - the middle layer, which adds the matched HRSG to the genset, completing the core Cheng Cycle hardware. Included here are the transition duct, superheater, duct burner, evaporator, economizer, feedwater heat exchanger, deaerator, steam purity control equipment, boiler auxiliaries, and a portion of the control system. PLANT/ENVIRONMENT - this outermost layer adds the balance of plant systems and several other features which tailor the Cheng Cycle for successful operation in a cogeneration environment. Examples include the water treatment system, fuel(s) preparation equipment, switchgear and protective relays, economic operating strategies, performance monitoring systems, and the remainder of the control system, including the operator interface. Also discussed in the following sections, but distinct from the above three layers, are additional areas of Know-How development which unify these three above layers into a cohesive product: INSTRUMENTATION AND CONTROLS - IPT has implemented a distributed digital control system based on the Bailey INFI 90tm hardware. The result is a unified approach across all three layers to both sequential and real-time control, as well as support for the important functions of monitoring, reporting, trending, and archiving. IPT has also implemented the plant controls in an Allen Bradley PLC and it is possible, with additional engineering work, to implement the control scheme in other control hardware. INTEGRATION AND PACKAGING - IPT has developed a means of streamlining plant installation by pre-packaging and testing all equipment in logical modules. This provides for uniform equipment interfaces, reduced construction time, and standardized plant layouts. OPERATIONS AND MAINTENANCE - The technology behind the 501-KH5 Cheng Cycle product has been extended to include many of the real-life requirements which are often overlooked in a less well-developed collection of equipment. Included are proven operating, diagnostic, and maintenance procedures as well as specific hardware and software which enhance operations and maintenance.
ENGINE/GENSET TECHNOLOGYThe 501-K engine was chosen for a number of technical reasons including its compressor's large surge margin, combustion section design, efficiency, fuel flexibility, serviceability, and experienced base of engine packagers. PERFORMANCE MODELING - Computer programs have been developed which allow engine performance prediction over the full range of ambient conditions, firing temperatures, and steam injection rates. The regenerative nature of the cycle requires iterative techniques since the engine exhaust characteristics both influence, and are influenced by the total steam injection rate. The heat recovery model and overall cycle model makes iterative calls to a specially modified version of Allison Engine Company's PC Performance turbine model. COMPRESSOR - Steam injection back-pressures the compressor and there is a need to re-match the load line characteristics of the compressor/expander as a function of the injection steam flow. Re-matching is complicated due to the molecular weight ratio of air and steam. Analysis of the compressor characteristics allowed IPT to select optimum operating speed in consideration of the equilibrium pressure ratio and the subsequent change in compressor work function and surge margin. Surge margin retention is further enhanced by IPT's development of real-time control algorithms which monitor surge margin and automatically make adjustments for adverse ambient conditions or hardware malfunction. OPERATING SPEED - Cheng Cycle adaptation benefitted from re-evaluation of the operating speed choice. IPT's recommended speed is based upon consideration of shaft torque capabilities, compressor and expander efficiencies, surge margin, airflow changes, stress rupture lifetimes, and rotor characteristics. COMBUSTOR - Steam injection is arranged to take advantage of the airflow patterns and pressure drop characteristics around the combustion liners to ensure rapid, uniform mixing of steam without perturbing the original airflow distribution. The burner outlet profiles were examined with special instrumentation to confirm similarity with the non-injected profiles, ensuring a hot section lifetime similar to that of the standard engine. Emissions characteristics of the engine have been another area of technology development associated with the combustor. An IPT-developed procedure called "staged steam injection" works in conjunction with the dual injection manifolds, further enhancing NOx suppression. WATER INJECTION - The need for water injection can occur when little or no injection steam is available due to a large process steam demand and/or due to economic considerations. Coordinated water injection for the 501-KH required development of hardware and control strategies which provide for water flow in proportion to fuel and steam flow. Dual fuel (liquid and gas) applications produce an additional level of complexity since injection water passes through the unused-fuel passageway of the fuel nozzle. In addition, fuel changeover must occur "on the fly" with no power reduction or shutdown required. NOZZLE STEAM - Nozzle steam injection is now available as a functional equivalent to water injection as a means of NOx control. Nozzle steam injection requires use of the "dual gas" nozzle originally designed for low heating value gasseous fuels. It is not available for dual fuel applications. Coordinated nozzle injection for the 501-KH required development of hardware and control strategies which provide for nozzle steam flow in proportion to fuel and case steam flow. EXPANDER - The addition of steam to the cooling airflow results in the blades and vanes being cooled by a mixture of air and steam. This required revisions to the heat transfer calculations which predict metal temperatures, and hence lifetimes, of the critically loaded first stage hardware. A test engine fitted with special instrumentation, including optical pyrometers and vane temperature thermocouples, was operated by IPT under actual field conditions to confirm these predictions. MOISTURE CONTROL - Specific attention has been paid to the control of the added moisture imposed by steam injection on various engine systems. IPT has developed and tested turbine seal designs and materials for improved immunity to moisture. Special startup and shutdown procedures are implemented in the Cheng Cycle control system to prevent the entry of condensed steam into the engine. STEAM PIPING - The mechanical and thermal design of the injection steam piping between the engine and genset frame required special expertise to deal with a multitude of complex trade-offs. The piping bridges two contrasting design worlds; on one side is industrial power piping design, with its appropriate standards, emphasis on bulk and weight, and generous safety margins. At the engine side, the design is derived from aircraft standards which favor lightweight construction, high strength materials, compact layouts, rigorous quality controls, and smaller safety margins. This interconnecting piping must handle the complex thermal growths of the engine without undue loading either on the steam manifold flanges or the engine itself. At the same time, the piping must accommodate its own thermal growth at a design operating temperature. The design chosen by IPT also addresses maintenance access, engine removal, engine interchangeability, and thermal loading of the compartment ventilation system. FUEL SYSTEMS - The increased heat release rate requirements of the Cheng Cycle version required modifications of the 501-K fuel gas control valve and its associated electronics. As with most 501-KH engine developments, these fuel system changes have been applied by Allison across the 501-K product line. EQUIPMENT SIZING - Since the 501-K engine had an established record of reliable industrial applications, many component selection and arrangement "habits" were almost automatic. It was necessary to confirm or reestablish these choices as being appropriate to the requirements of 501-KH5 Cheng Cycle System. The "standard" main reduction gearbox, for example, had to be re-sized, and the reduction ratio was altered for greater compatibility with Cheng Cycle operation. Starter pad provisions were examined for compatibility with torque input and starter dropout speed. The low-speed shaft, shear pins, and low-speed coupling were also re-sized. The generator required the obvious capacity increase as well as fresh consideration of the process of matching generator and turbine derating curves. The generator's part-load efficiency curve was modified after consideration of the economics of continuous cogeneration service. Starter sizing, dropout speed, motoring service, and start cycle purge were also modified. LUBRICATION SYSTEM - IPT's genset specification recommends a lubrication system design which makes provision for the increased heat rejection from the larger components, and for pre-lube and post-lube. In Cheng Cycle operation, steam injection of the 501-KH unavoidably substitutes water vapor for some of the normal air flow. This air and steam mixture is discharged from the engine along with the lube oil flows. Special IPT design features control foaming and enhance moisture extraction from the discharge flows. GOVERNOR - Cheng Cycle adds a significant energy input source to the turbine which is not under the direct control of the governor. IPT has developed the control extensions required to accommodate this additional degree of freedom, and has encompassed the governor within a larger shell of distributed digital control. The engine, via its governor, is managed as one of the many subsystems coordinated by the Cheng Cycle system controls.
HRSG/CYCLE TECHNOLOGYWithin this layer, Cheng Cycle technology addresses the heat recovery steam generator (HRSG), duct burner, and the remainder of the components required to complete the thermodynamic specification of the cycle. A major consideration throughout all levels of the technology is the large turndown capability of the 501-KH5 Cheng Cycle equipment. Especially in cogeneration service, it is important to follow, smoothly and efficiently, both thermal and electric loads over the full range from zero to maximum capability. The Cheng Cycle approach to cogeneration delivers this rangeability without a complicated equipment arrangement. BASIC DESIGN - After examining several original approaches to a boiler designed specifically for enhanced operation with Cheng Cycle, IPT chose to modify and extend the capabilities of "standard" HRSG designs. As with the genset equipment selection process, this required that entrenched "rules of thumb" be scrutinized for appropriateness to Cheng Cycle and cogeneration service. Steam conditions required by this application are characterized by the temperatures found in utility power boilers; at the same time, pressures are characteristic of commercial process boilers. IPT has retained the suitable features of both boiler types, and has avoided inclusion of inappropriate design requirements from either type. The boiler is required to provide large turndown and accommodate various operating modes, with various combinations of turbine firing rate, duct burner firing rate, and steam injection rate. The natural circulation versus forced circulation choice considered pump requirements, tolerance to drum chemistry variations, transient and upset capability, wide turndown, conventional fabrication requirements, and repairability. IPT specifies boiler auxiliary components in order to control plant overall energy balance, plant construction uncertainty, and plant reliability. PERFORMANCE MODELING - IPT has developed specialized computer programs which are used for both synthesis and analysis of the HRSG. These same programs link to the turbine performance models to provide analysis of the overall plant performance. Specific attention is paid to handling mixed-fluid properties correctly, and the regenerative properties of the overall cycle are correctly iterated. Performance prediction also includes consideration of the deaerator requirements, flash steam production, and blowdown losses, since these elements influence the total heat balance and are part of the standardized scope of supply. Modeling has also been used to clarify the availability of energy from the stack gases at temperatures less than the lower pinchpoint temperature. SIZING - The process of sizing the heat recovery components is complicated by the various operating modes provided by Cheng Cycle cogeneration. Base sizing is derived from requirements of pure power operation (no duct burner or process steam). Iterative calculations involving economic/cost recovery techniques and the various pinchpoints, injection rates, etc. are used to establish evaporator and superheater surface areas. Alterations are made to this basic sizing to accommodate the extended capabilities provided by the duct burner, site specific requirements for process steam pressure, site condensate return conditions, deaerator consumption, blowdown rate, and to balance cold end heat recovery. COLD END DESIGN - Those portions of the HRSG located in the gas path downstream of the evaporator, referred to as the cold end, are responsible for recovering sensible heat from the stack gases in order to maximize the recovery of latent heat (steam production) in the evaporator. However, the design criteria for these sections are contradictory at different operating modes. IPT has designed a simplified approach to cold end heat recovery which sidesteps this conflict and ensures optimal heat recovery under all operating modes. MATERIALS - Materials selection for the steam injection piping and superheater play an important role in maintaining steam purity and avoiding turbine erosion damage. TRANSITION DUCT - IPT has applied expertise in flow diffusers to the design of the transition section between the turbine tailcone and inlet face of the superheater. The design includes proper consideration of the increased flow rate and altered fluid properties under Cheng Cycle operation. DUCT BURNER - A duct burner is included in the 501-KH5 Cheng Cycle System design to enhance steam production either for increased power production or for increased process steam production. The burner location in the exhaust gas stream was selected by IPT for various cycle-related reasons. Combustion in the duct burner depends on the residual oxygen contained in the turbine's exhaust gasses. Under Cheng Cycle operation, injection steam reduces the partial pressure of oxygen and increases the specific heat of the gas stream. The burner is specifically modified to account for these effects. The burner's heat release rate limits are chosen to take advantage of the increased heat capacity of the exhaust gases when steam injecting, and provide for the increased capacity available under these conditions. Other technical features of the duct burner have also been developed for Cheng Cycle service. STEAM PURITY - Injection steam, like other fluids entering the engine, must be clean. In addition, the process of maintaining steam purity must be robust so that the inevitable upsets do not allow unacceptable impurities into the engine or into the superheater. IPT has evolved a multilevel approach to steam purity control which emphasizes simplicity and reliability. The first step is control of drum chemistry, next step is drum sizing, and the final step is steam purification. INJECTION CONTROL - Injection steam control is provided by a specially developed system of block and modulating control valves. The location of these valves, as well as their function at the air/steam - turbine/HRSG interfaces is also unique to the IPT-developed Cheng Cycle technology. PROCESS CONTROL - Operating experience has resulted in reliable algorithms for rate-of-change (to control drum upsets and steam purity), anticipatory drum level control, reverse flow prevention, and process load sharing among parallel steam generators.
PLANT/ENVIRONMENT TECHNOLOGYTechnology development in this layer results from IPT's direct participation in the end use of the 501-KH5 Cheng Cycle product. The experience gained in installing, managing, operating, and maintaining plants has led to improvements and extensions to the basic technology which specifically address the unique cogeneration environment. The major driving force has been the recognition that cogeneration is a specialized "business" unlike that of a traditional power plant or boiler plant. Cogeneration often must be financially self-supporting, and special emphasis is placed on availability and repairability. Cogeneration is often viewed as a source of profit rather than an overhead expense. IPT-developed technology and know-how helps this business be responsive to a complex regulatory, contractual, and permitting environment. PERMITTING - IPT has designed in features aimed specifically at streamlining plant permitting. Examples include acoustical enclosures, equipment guards, fire protection systems, ladders and platforms, piping and piping supports, wiring practices, exposed surface insulation, and venting provisions - all aimed at ready compliance with a broad range of national, state, and local standards and codes. Nevertheless, air quality issues are usually a most troublesome aspect of permitting a gas turbine plant; hence IPT has developed specific technology and permitting expertise in this area. Cheng Cycle's intrinsic use of massive amounts of steam injection results in very low NOx emissions - lower than any similarly sized commercially available, proven, gas turbine plant without the use of external add-on catalytic converters. The use of various combinations of steam injection, staged-steam injection, direct saturated steam injection, minimum steam injection, and/or coordinated water or nozzle steam injection enables IPT to tailor the NOx characteristics for each application. Permitting often stands in the critical path of a cogeneration project's development. Because of IPT's unified approach to the 501-KH5 Cheng Cycle product, permitting assistance, well-established data, and application procedures are all available early in a project, often obviating this timing problem and reducing project development uncertainties. CCEM - Many air quality districts currently require the installation of continuous emissions monitoring (CEM) equipment. Because of the operational problems associated with these systems, IPT has taken advantage of the equipment uniformity at 501-KH5 Cheng Cycle sites and developed a computational model of pollutant production. This Calculated Continuous Emissions Monitor (CCEM) is implemented within the Cheng Cycle control system, and has been accepted as an equivalent method for monitoring and reporting continuous emissions by several air quality districts. ON-LINE EMPHASIS - Specific design attention has been paid to hardware and software features which extend the plant's ability to stay safely on-line in spite of partial equipment failure and throughout maintenance activities. WATER TREATMENT - Steam injected into the turbine is lost from the cycle as water vapor discharged through the boiler stack. Make up water is also required to replace process steam losses. The proper choice of water treatment methods depends on the quantity and quality of water to be treated, the decision to treat externally versus internally (via boiler blowdown), and the overall plant economics. Water treatment requirements can be very site-specific, IPT offers licensees know how on how best to optimize water treatment for each site. OPERATING STRATEGIES - One of the most attractive features of 501-KH5 Cheng Cycle System is the ability to vary independently electrical production over a wide operating range while simultaneously tracking process steam requirements. Such electrical production freedom provides the ability to respond favorably to economic variations in utility electric rate schedules, site electrical consumption, capacity payments, demand charges, and fuel prices. IPT has developed extensions to the Cheng Cycle control system which enhance the usefulness of this capability.
INSTRUMENTATION AND CONTROLSControls represent a unique area of IPT's technology development. Throughout each layer discussed above, the controls embody, extend, and unify Cheng Cycle expertise. Since the foundation of successful controls is instrumentation, IPT also places special emphasis on the correct selection, installation, and use of instruments in the 501-KH5 Cheng Cycle product. INSTRUMENTATION - In addition to basic support of the control system, instrumentation selection is tailored to several other requirements of the 501-KH5 Cheng Cycle product. Operating flexibility and turndown are key features of the product; this demands instrumentation systems throughout the plant which provide more than the usual two to three times turndown capability. Reliable performance tracking depends on quality measurements of small differences over time; this implies high accuracy and low drift characteristics. Regulatory compliance, maintenance costs, commodity billing, contractual obligations, and the intrinsic profit-oriented nature of the cogeneration business are other examples of demanding instrumentation requirements. IPT has developed an experience base for the selection of instrumentation systems designed to satisfy these requirements. CONTROL SYSTEM - The 501-KH Cheng Cycle product incorporates a distributed microprocessor-based control system. This design allows a high degree of automatic plant operation and provides sophisticated operator displays. The distributed architecture places control hardware close to the sensors and final control elements, while providing all essential plant data to the operator. There is a minimum amount of field wiring, thus reducing installation costs and startup problems. The control logic for the system has been developed by IPT and successfully proven on several operating plants. IPT provides complete support for the system during initial plant start-up and subsequent operation. CONTROL SYSTEM COMPONENTS - The Cheng Cycle control system is primarily based on a Bailey Controls Company INFI 90tm distributed control system. The control logic and operator displays were developed specifically for the 501-KH Cheng Cycle System. Other control system components have been integrated into the system so that most plant functions can be operated through the console(s). The main control system modules pass data among themselves over a communication system called a "plant loop". The major modules of the system are:
CONTROL SYSTEM FUNCTIONS - The control system operates on several levels to provide integrated plant-wide control. These operating levels are:
INTEGRATION AND PACKAGINGIPT's unified design approach to the 501-KH5 Cheng Cycle System has produced technological features which result from the synergistic combination of various aspects of the product. Experience has proven that a successful cogeneration project must have its major equipment, as well as all subsystems and components, designed and built as an integrated system. Integrated protective systems for fire, vibration, electrical fault, and other plant alarms enhance safety and improve availability. SCOPE OF SUPPLY - Consistent interface definition and design provisions throughout the product allowed the Licensee to satisfy the customer requirements for scope of supply. TRAINING - IPT has developed specific operator training lessons and materials for the 501-KH5 Cheng Cycle which are derived from total familiarity with the product. These standard courses take two weeks, are given on-site, and provide owners and operators with component, system, and procedures training. Uniformity of specifications, drawings, and design philosophy greatly enhance the programs. This is in contrast to the usual approach which might simply gather specifications sheets and generic equipment bulletins into an uncoordinated presentation. CONTROLS - One of the best examples of beneficial integration comes from IPT's total system controls experience. When possible, consistent instrument types, interface levels, suppliers, calibration methods, wiring diagrams, tag names, and engineering units apply throughout the product and throughout the finished plant. The benefits include reduced maintenance costs, rapid familiarization, spares flexibility, and improved availability.
OPERATIONS AND MAINTENANCEOperations and Maintenance provisions have been included by design throughout all of the 501-KH5 Cheng Cycle System products. IPT is in a unique position to incorporate maintenance features because IPT also operates on 24 hour 7 days-a-week basis some of the same plants it designed and installed. Operation and maintenance are treated as the logical end result of the entire process, not as an afterthought. HARDWARE PROVISIONS - Careful selection of proven components and design integration ease maintenance throughout the plant. Many of the benefits of established aircraft engine maintenance practices apply to the Allison 501-KH engine. IPT further enhanced maintainability of the engine with specific genset design features including an IPT designed, AGT approved engine mount tailored to maintenance inspections. IPT's injection steam piping design includes supports to ease handling and prevent inadvertent damage. Similar thought goes into the remainder of the plant design. SOFTWARE PROVISIONS - Specific controls development is aimed at easing operations and maintenance. Auto/manual provisions are included for most devices to aid in troubleshooting, calibration, and emergency operation. In addition, remote displays and control stations acknowledge the reality of special operating circumstances. Tested formats for logging, archiving, alarms, and data reports are available from startup, and are easily modified. The system controls incorporate a high degree of automation, such that semi-unattended operation is possible; the result is time available for actual maintenance rather than plant controls manipulation. This is especially true since the Bailey controls are often extended to include existing boilers and other balance of plant systems into an integrated set of equipment. OVERHAUL - Based on ongoing experience, IPT has developed an overhaul and repair specification for the 501-KH engine. The use of this specification ensures engine repairs which incorporate appropriate upgrades, avoid unnecessary expense, and draw upon the experience of the entire fleet of 501-K engines. CONTROLS - Although the 501-KH5 Cheng Cycle control algorithms have been specially developed, the control hardware was standardized (Bailey INFI 90tm), and was chosen deliberately for serviceability. An operator thus enjoys all the service and support benefits associated with a worldwide controls supplier. SPARES - IPT's design uniformity and operating experience reduces required spare parts stocking requirements and increases parts availability. Whenever possible, the same manufacturer, style, or part number is used throughout the plant. Suggested spare parts inventories are established based on prior experience with identical systems. Because of IPT's involvement in plant ownership and operation, shared spares can be made available for expensive and/or critical parts.
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