Steam Turbine Optimization

Thermal solar farm generated heat used for electricity generation by steam turbine. 

Rotational speeds vary from approximately 1,800 to 14,000 rpm and can be modulated over a wide range. Such variable speed is an advantage if the turbine is used to drive pumps and compressors or if it is to convert a variable steam flow into electricity, as is the case with thermal solar systems. When provided with the appropriate speed governor, steam turbines can provide excellent speed stability, which is desirable when the turbine serves as the prime mover in electric generators. 

The efficiency of the turbine, neglecting mechanical losses, is found by 

The optimization of steam turbine systems is well understood and more widely practiced than the power consumption optimization of alternatives such as electric motors and gas turbines. In addition to the initial cost, the major disadvantage of steam turbines is their low tolerance for wet or contaminated steam. Wet steam can cause rapid erosion, and contaminants can cause fouling. Both will reduce the turbine s efficiency and will shorten its life. Steam quality monitoring is therefore important to maintain the reliability and to reduce the operating cost of steam turbines. 

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A range of industrial steam turbines with a ehoiee of reaetion and impulse blading are available to satisfy these needs. They virtually guarantee an optimal solution to the various problems eneountered when eombining eompressors, expanders, and turbines to form an effieient, reliable nitrie aeid train. A typieal train is depieted in Figure 4-26. 

Once the highest steam level is set, then intermediate levels must be established. This involves having certain turbines exhaust at intermediate pressures required of lower pressure steam users. These decisions and balances should be done by in-house or contractor personnel having extensive utility experience. People experienced in this work can perform the balances more expeditiously than people with primarily process experience. Utility specialists are experienced in working with boiler manufacturers on the one hand and turbine manufacturers on the other. They have the contacts as well as knowledge of standard procedures and equipment size plateaus to provide commercially workable and optimum systems. At least one company uses a linear program as an aid in steam system optimization. 

Since 1900, manufacturers have made many step changes in the basic design of steam turbines. New technology and materials have been developed to support the industry s elevation of steam conditions, optimization of thermal cycles and unit capacity. Steam turbines will continue to be the principal prime mover for electricity generation well into the twenty-first century. 

Figure 23.49 Steam turbine networks and let-down stations provide degrees of freedom for optimization. (From Varbanov PS, Doyle S and Smith R, 2004, Trans IChemE, 82A 561, reproduced by permission of the Institution of Chemical Engineers.)...

Site composite curves can be used to represent the site heating and cooling requirements thermodynamically. This allows the analysis of thermal loads and levels on site. Using the models for steam turbines and gas turbines allows cogeneration targets for the site to be established. Steam levels can be optimized to minimize fuel consumption or maximize cogeneration. A cost trade-off needs to be carried out in order to establish the optimum trade-off between fuel requirements and cogeneration. 

A proposal is made to use a geothermal supply of hot water at 1500 kPa and 180°C to operate a steam turbine. The high-pressure water is throttled into a flash evaporator chamber, which forms liquid and vapor at a lower pressure of 400 kPa. The liquid is discarded while the saturated vapor feeds the turbine and exits at lOkPa. Cooling water is available at 15°C. Find the turbine power per unit geothermal hot-water mass flow rate. The turbine efficiency is 88%. Find the power produced by the geothermal power plant, and find the optimized flash pressure that will give the most turbine power per unit geothermal hot water mass flow rate. 

Except for the remotely located sea water pump, all the pumps in the process area (shown in Figure 2) are driven by steam turbines. Because large quantities of relatively low-pressure steam are required in the process, the use of steam-driven turbines instead of electric motors results in a savings of several cents per thousand gallons of product. Because of this, the economic analysis and optimization presented below have been based on the use of steam-driven turbine drivers for the pumps located in the process area. 

It is important to have the correct set of variables specified as independent and dependent to meet the modeling objectives. For monitoring objectives observed conditions, including the aforementioned independent variables (FICs, TICs, etc.) and many of the "normally" (for simulation and optimization cases) dependent variables (FIs, TIs, etc.) are specified as independent, while numerous equipment performance parameters are specified as dependent. These equipment performance parameters include heat exchanger heat transfer coefficients, heterogeneous catalyst "activities" (representing the relative number of active sites), distillation column efficiencies, and similar parameters for compressors, gas and steam turbines, resistance-to-flow parameters (indicated by pressure drops), as well as many others. These equipment performance parameters are independent in simulation and optimization model executions. 

In the case of "flash steam" power plants, the steam is either generated directly by the production wells or the wells produce hot water from which steam can be separated to drive conventional steam turbine generators. The size of these plants ranges from 100 kW to 150 mW. Figure 2.104 illustrates the optimizing control system for such a geothermal facility. 

The control of steam turbines will be discussed separately in Section 2.19. Therefore, here the focus will be on the optimization of gas turbine (GT) operation. The GT system consists of three main parts the air compressor (axial type), the burners, and the turbine itself. The mechanical power gener-... 

The problem must be set up such that the objective function and the Lagrange constraint equations are functions of the state and decision variables (Equations 4 and 5). A major deviation from the procedure outlined by Tribus and El-Sayed (5) is in the selection of the Lagrange constraint equations and state variables. The added complexity of having steam as the working fluid (compared to an ideal gas in the gas turbine optimization performed by Tribus and El-Sayed) makes it impractical to select state variables that correspond to available-energy flows. Consequently, this requirement was relaxed entirely. This gives the designer the opportunity to use any variable as a state variable,... 

Capacity of steam turbine power generation system to be optimized... 

Compression in the larger plants is currently carried out with turbocompressors. Plants with capacities <600 t7d still use the previously widely used piston pumps. With turbocompressors, which are almost always driven by steam turbines, plants with a capacity of 1500 t/d can operate in the preferred and economically optimal pressure range of 250 to 350 bar, a single turbocompressor providing the fresh gas compression and the circulation. 

Optimize use of steam turbines for maximizing power generation and also minimizing steam venting... 

From a scientific report "The efficiency can be increased from yesterday s promising 54 % in two steps to values around 60 % at the end of the decade. In each step three parameters are important, these are higher effiencies by optimalization of the design of the blades, increased gas turbine-inlet temperature and improvements in the steam-turbine process, e.g. with a sub-critical 3-pressure-process including re-heat steps", lit. RBEDLE-1994, S. 39. 

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