Steam turbines and

PWRs operate differendy from BWRs. In PWRs, no boiling takes place in the primary heat-transfer loop. Instead, only heating of highly pressurized water occurs. In a separate heat-exchanger vessel, heat is transferred from the pressurized water circuit to a secondary water circuit that operates at a lower pressure and therefore enables boiling. Because of thermal transfer limitations, ultimate steam conditions in PWR power plants ate similar to those in BWR plants. For this reason, materials used in nuclear plant steam turbines and piping must be more resistant to erosion and thermal stresses than those used in conventional units. 

As of the mid-1990s, many older conventional steam plants have been converted to combined cycle. The old boiler is removed and replaced by a combustion turbine and heat recovery steam generator. Although the cycle efficiency is not as high as completely new plants, substantial capital cost is avoided by the modification and reuse of existing steam turbine and auxiHary equipment. In many combined cycle power plants, steam is injected into the combustors of the combustion turbine to lower peak flame temperatures and consequendy lower NO. ... 

The advent of electric motors, steam turbines, and other drivers has relegated the steam engine to a minor position as an industrial driver. It does have the advantages of reliabihty and operating characteristics that are not obtainable with other drivers but so the disadvantage of bulldness and oily exhaust steam. 

By common usage, the terms turboexpanders and expansion turbines specifically exclude steam turbines and combustion gas turbines, which are covered elsewhere in Sec. 29. 

Turboexpanders are expansion turbines, rotating maehines similar to steam turbines. Commonly, the terms expansion turbines and turboexpanders speeifieally exelude steam turbines and eombustion gas turbines. Turboexpanders (Figure 1-1) ean also be eharaeterized as modern rotating deviees that eonvert the pressure energy of a gas or vapor stream into meehanieal work as the gas or vapor expands through the turbine. If ehilling the gas or vapor stream is the main... 

Smooth starts are possible with steam turbine installations that include an auxiliary boiler because the startup phases of the turbocompressor can be matched to different plant operating conditions. Aside from process-related timing issues, the time elapse or machinery startup duration is generally determined by the temperature gradients admissible on both steam turbine and expander. These factors are influenced by the relative expansion rates of the fixed and rotating components in these machines. 

The second step in developing the process air train is selecting the air blower main driver. This step includes many alternatives such as power recovery expanders, steam turbines, and electric motors. The following example illustrates these alternatives. 

As a first step in the driver analysis, the eapital required to make eaeh alternative operational is estimated. An orifiee ehamber is required to reduee the flue gas pressure for the steam turbine and motor alternatives. In this partieular ease, it is assumed that a third-stage separator is required for the power reeovery alternatives only and that an eleetrostatie preeipitator is used in all eases. Construetion and engineering are estimated as pereentages of total direet material and total material and eonstruetion, respeetively. An allowanee of 15% is made for eontingeney. Beeause the separator often ineludes a royalty fee, this item is added to the power reeovery alternates. As shown in Table 4-7, the motor alternative will require the least eapital. The power reeovery alternatives require additional eapital amounting to 4.63 and 4.75 per million respeetively. 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore the efficiency of the combined cycle. In the case of the overpressure dearation, the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system. 

Table 19-8 shows a load-distribution program for an 87.5-MW power station of steam turbines and gas turbines. The seleetion of equipment and their loading for the most effieient operation ean be programmed when the effieieney of individual units are monitored. The program... 

Hi/ume, A., Transient Torsional Vibration of Steam Turbine and Generators Shafts due to High Speed Reclosing of Electric Power Lines, ASME... 

It is important to seal the glands on the turbine shaft, and a typical arrangement is illustrated in Figure 14-24. Thrust bearing failures can be serious problems for steam turbines and other mechanical drives, as well as the bearings used on the driven equipment. 

STAJNDAKU DATA SHEET FOR STEAM TURBINES (AND GEARS)... 

Figure 14-25. Standard datasheet for steam turbines (and gears). (Used by permission API Standard 615, Mechanical Drive Steam Turiiines for General Refinery Service, 1= Ed., Appendix 1, 1958. American Petroleum Institute.)...

The H-S plot is called a Mollier diagram and is particularly useful in analyzing throttling devices, steam turbines, and other fluid flow devices. A Mollier diagram for steam is presented in Figure 2-37 (standard engineering units) and in Figure 2-38 in SI units. 

The lubricants generally specified for conventional gas turbines invariably fall within the same classification as those used for steam turbines and are often categorized as turbine oils . In those cases where an aircraft type gas turbine has been adapted for industrial use the lubricant is vitally important to their correct operation. Specifications have been rigidly laid down after the most exhaustive tests, and it would be unwise, even foolhardy, to depart from the manufacturers recommendations. No economic gain would result from the use of cheaper, but less efficient, lubricants. 

The compressor speed may be reduced by two-speed electric motors or by electronic variation of the motor speed, down to a lower limit dictated by the inbuilt lubrication system. Many highspeed industrial machines are still driven by steam turbines and this gives the opportunity for speed control within the limits of the prime mover. 

Steam extracted from one or more controlled pressure points on an extraction steam turbine and used for various process applications. Extraction steam may be delivered to an extraction heater. 

Most metals and many refractory compounds can be thermal-sprayed. Applications include coating of gas-turbine components for aircraft and industrial use, components of steam turbines and diesel engines, components for the oil and gas industry, paper and pulp industry, and chemical processing industry. 

On any site it is always worth while considering driving large compressors or pumps with steam turbines and using the exhaust steam for local process heating. 

Figure 23.46 The site power-to-heat curve for steam turbines and export to power. (From Varbanov P, Perry S, Makwana Y, Zhu XX and Smith, 2004, Trans IChemE, 82A 784, reproduced by permission of the Institution of Chemical Engineers.)...

Process machines can, in some cases, be equipped with both a steam turbine and an electric motor, allowing the drive to be switched between the two according to steam demands in the utility system and operating costs. It should be remembered that operating costs might vary significantly as a result of different electricity tariffs, according to the time of day, the day of the week and the season of the year. 

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. 

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