necessary. Additional heat transfer surface cleaning equipment can be added such as steam sootblowers, water wall blowers and water cannons. In addition, more efficient burners and rotating classifiers for coal pulverizers can be added. Boiler designers can predict the effectiveness of these boiler modifications using computer modeling programs based on computational fluid dynamics.
Turbine manufacturers have designed highly efficient turbine blades and nozzles so that a turbine can generate more power at the same steam flow. Still more power can be generated if the associated boiler can produce additional steam. For existing turbines, the inner cylinders and rotors can be replaced by the newly designed components and the power outputs can be increased while reusing the outer cylinders. As a result, piping and foundation changes are not required.
These major equipment changes affect the balance-of-plant equipment and systems. For example, an increase in boiler steaming capacity requires a corresponding increase in feedwater flow. As a result, the boiler feed pumps and condensate pumps must be analyzed to determine if sufficient pumping capacity is available. The additional steam flow through the turbine increases extraction steam flow, heater drain flow and, finally, the low-pressure turbine exhaust steam flow to the condenser. The effects of these additional steam and water flows must be assessed. Upgrades may be required to realize the maximum potential associated with the boiler and turbine upgrades.
Depending on a unit’s original design; past retrofits; past and present maintenance and operating practices; age and anticipated operating regime, various optimization approaches are available to the owner. These may include the following.
Thermal performance audits— These are used when a unit’s heat rate is significantly higher than the original design. Thermal performance audits are also useful when it is necessary to evaluate the improvement in a unit’s
heat rate when planning the replacement of deteriorated equipment with new equipment of a different design and/or modifying the operating regime.
Bottleneck relief studies—Studies are necessary when a unit’s owner needs greater output and suspects existing limitations can be overcome by upgrading or replacing equipment. Economic analyses are performed to allow selection of the most viable alternatives.
Turbine upgrades—To gain addit ional capacity and enhance reliability and efficiency, entire turbine sections may be replaced. Reasons for retrofitting various sections of turbines may include the following.
• High-pressure turbines—The main reasons for retrofitting HP turbines with new, inner cylinders and rotors are to increase efficiency and power output. In addition, if the boiler steaming capacity can be increased, the design steam-s wallowing capacity of the HP turbine will be increased for even greater power output. Other reasons for include reducing the risk of solid particle erosion by utilizing enhanced blade designs.
• Intermediate-pressure turbines— Reasons for retrofitting IP turbines are to increase steam fl ow capacity and power output, moderately increasing the efficiency and increasing the interval between overhauls.
• Low-pressure turbines—Replacing a shrunk-on rotor with a welded drum rotor equipped with new blades eliminates the risk of LP turbine rotor stress corrosion cracking. Greater LP turbine efficiency and reliability is achieved by utilizing free-standing “L-0” blades. These blades do not use lashing wires thus avoiding coupling interaction bet ween adjacent blades. Blade/rotor interaction does not occur due to the stiff welded design of the drum rotor, allowing accurate tuning of the blades.
Boiler uprates—Uprating a boiler first requires a complete analysis of the existing operational issues and constraints, including emissions and Q-fired limits and identification of
available solutions. Next, changes to the boiler main steam, reheat steam and feedwater flows, temperatures and pressures brought about by modifications to the steam turbine must be evaluated and potential solutions identified.
For example, installing a new and more efficient HP turbine inner cylinder and rotor will decrease the cold reheat steam temperature by 20 degrees Fahrenheit or more for the same main steam flow. The existing boiler reheater may have to be enlarged to maintain the design hot reheat steam temperature entering the IP turbine. Likewise, if a boiler has excess steaming capacity, then new, more efficient turbine inner cylinders with a greater steam flow capacity would need consideration. Finally, boiler efficiency improvements can be obtained by upgrading components with better designs and materials, optimizing boiler subsystem performance and eliminating conditions that reduce boiler efficiency between maintenance outages. This review needs to consider fuel preparation and transport, firing system design, furnace wall cleaning, convective heat transfer surface arrangement and cleaning, regenerative air heater upgrades and so on.
Boiler/turbine/balance of plant comprehensive study—Optimizing the heat rate and power output of electric generating units requires balancing the boiler, steam turbine-generator and balance of plant equipment. Balance of plant analysis identifies both the ability to support any planned boiler/ turbine uprates as well as the equipment technology enhancements that could improve unit capacity and/or efficiency.
Utilizing a consistent methodology to assess the balance-of-plant equipment and systems for existing and uprated conditions is key to the overall success of an uprate program.
The balance-of-plant assessment methodology consists of several steps.
First are the preliminary discussions with the power plant personnel regarding information about the plant equipment
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