Turbo Machinery Dynamics Part 3 ppt

In order to improve thermal efficiency, a turbine’s hot path components are exposed to gases at extraordinarily high temperatures, sometimes beyond the capability of the mater- ial. Sophisticated cooling methods are required in the combustor and the turbine to avoid degradation of the material’s strength and durability characteristics. New breeds of superalloys are constantly under development to deliver the required characteristics, and still can be formed to the required component dimensions during manufacturing. The dif- ferential thermal growth between mating rotating and static parts must be predicted with a high level of precision. Besides normal operating conditions, dynamic loads arise from rotating unbalance, fluid flow forces, and misalignment. Manufacturing techniques have also seen vast changes in the past 20 years to allow machining and fabrication of highly contoured airfoils to close tolerances. 3.2 SIMPLE-CYCLE GAS TURBINE A gas turbine operates on the Brayton cycle. The Brayton principle consists of two isobaric and two isentropic processes, as shown in Fig. 3.1. The former take place in the gas tur- bine’s combustor and in the steam generator’s gas side, and the latter represents the com- pression of air and expansion of gases in the turbine. Gas turbine cycle efficiency depends on the compression ratio and turbine firing temperature, increasing with both parameters according to the relation shown in Eq. (3.1). It is assumed in the derivation that the pres- sure ratios in the compressor and the turbine remain the same, the fuel flow rate is consid- erably smaller than that for air, the specific heat of the gases stays constant, and that all components operate without incurring any losses: (3.1) where h is the ideal cycle efficiency, r represents the compression ratio, and g is the ratio of specific heats at constant pressure and constant volume. Turbine and compressor efficiencies η γ γ =−       − 1 1 1 r INDUSTRIAL GAS AND STEAM TURBINES 63 FIGURE 3.1 Standard Brayton cycle for gas turbine. will have a moderating effect on the overall efficiency. Figures 3.1 and 3.2 demonstrate the impact of the two parameters. Work performed per pound of air occurs at a lower pressure ratio than the point of maximum efficiency for a given firing temperature. The overall cycle efficiency is improved with the increased pressure ratio, cooler compressor inlet tempera- ture, and higher turbine inlet temperature (Fig. 3.3). Evaporative cooling, direct water fog- ging, and refrigerated cooling are some of the methods used to cool the air at the inlet. The impact of efficiencies in the compressor, combustor, and turbine, as also system-related pressure losses in a simple-cycle turbine is shown in Fig. 3.4. Since the gas temperature at the turbine exit is higher than that at the compressor exit, the insertion of a regenerator to preheat the air between the compressor and the combustor with the turbine’s exhaust gases will reduce the fuel requirement. If T 1 , T 2 , T 3 , and T 4 rep- resent inlet temperatures of the compressor, regenerator, combustor, and turbine and T 5 is the exit temperature from the turbine, ideal regenerator efficiency (assuming no pressure loss) may be expressed in the following form: (3.2) η Re g TT TT = − − 32 5 2 64 APPLICATIONS FIGURE 3.2 Compression ratio and thermal efficiency. FIGURE 3.3 Optimized pressure and temperature for maximum thermal efficiency. System cycle efficiency then takes the form (3.3) Regenerator efficiency will depend on the available surface area for heat transfer, but increased area will call for higher cost, pressure loss, and space. Heat exchangers may be of regenerative or recuperative type. Regenerative systems call for another medium to effect the transfer of heat between the turbine exhaust gases and compressed air. Thus, heat flows into and out of the intermediate fluid. Recuperative heat exchangers have heat- transferring elements at a constant temperature, with the air and gas paths arranged in coun- terflowing directions (Boyce, 2002). Power output from a gas turbine can be increased in a number of ways. Intercooling of air between the stages in a compressor may be used to reduce the work done to pressurize the air. Temperature reduction of the partially compressed air causes its volume to shrink, consequently less work is required to compress it to the next pressure level. Thermal effi- ciency of a simple cycle is decreased by the addition of an intercooler, but the addition of an intercooler to a regenerative gas turbine cycle increases thermal efficiency and power output. The explanation is that a larger portion of the heat required for regeneration now comes from the turbine exhaust instead of additional fuel consumption. When turbine expansion is split into two or more steps, with constant pressure heating taking place before each expansion, the process is referred to as reheat cycle. As in inter- cooling, the thermal efficiency of the simple cycle is reduced by reheat, while work output rises. In combination with a regenerator, reheat can be made to increase thermal efficiency. Performance curves for a simple-cycle gas turbine that includes provision for intercooling, regeneration, and reheat are shown in Fig. 3.5. Figure 3.6 provides details of a single-shaft gas turbine supported on two bearings. η Re ()() () g Cyc TT TT TT − = −−− − 45 21 43 INDUSTRIAL GAS AND STEAM TURBINES 65 FIGURE 3.4 Simple-cycle gas turbine performance map. 66 APPLICATIONS FIGURE 3.5 Simple-cycle gas turbine with intercooling, regeneration, and reheat per- formance map. FIGURE 3.6 W501F compressor and turbine components on a single shaft. (Courtesy: Siemens Westinghouse) 3.3 INDUSTRIAL COMBUSTION TURBINE Gas turbines are a practical and economic way for utility and industrial service. Advanced design units offer high firing temperatures, low NO x , and improved efficiency. For example, the 60 Hz 200 MW class W501G engine has been jointly developed by Westinghouse Electric Corporation, Mitsubishi Heavy Industries, and FiatAvio (Southall and McQuiggan, 1995). This machine continues a long line of large heavy-duty single-shaft combustion tur- bines by combining the proven efficiency and reliability concepts of the W501F with the latest advances in the aero technology. Designed for both simple- and combined-cycle applications, the turbine can operate on all conventional turbine fuels and on coal-derived low-Btu gas produced in an integrated gasification plant. The general configuration of the combustion turbine calls for a bolted construction rotor supported in two bearings (Fig. 3.7). The 18-in-diameter bearings have compounded form, with tilting pads in the lower half and a fixed arc element on the upper side. The compres- sor segment of the rotor is assembled from spigotted disks bolted together by 12 through bolts. Alignment and torque transmission are assured by employing radial pins between the disks. The turbine rotor section is made up of disks provided with curvic clutches (see Fig. 3.8), and are bolted together by 12 through bolts. The curvic clutch is machined in the form of uniformly spaced teeth protruding axially from the flat face of the disk. The teeth engage and interlock with a similar pattern machined on the face of the adjacent disk, pro- viding a slippage-free joint under the action of the clamping load from the through bolts. The combustion system consists of 16 cans arranged in an annular pattern. Stability of the flame and uniformity in the distribution of fuel flow between the combustors are mon- itored by thermocouples located downstream of the last turbine stage. Malfunctions in the combustor when at load and sensing of ignition during the startup mode can also be detected with the aid of ultraviolet sensors. The casings are split horizontally to permit maintenance with the rotor in place. The inlet and compressor casings are made of nodular cast iron and cast steel, while the com- bustor, turbine, and exhaust casings are of alloy steel. The inlet end bearing housing is sup- ported from eight radial struts. At the exhaust end the bearing is supported by tangential struts that respond gradually during transient conditions, and maintain alignment between the rotor and the bearing through rotation of the housing about its axis to accommodate thermal growth. The arrangement provides an additional benefit of reducing thermal stresses in the struts. The fairings around the struts are configured in the form of an airfoil to enhance aerodynamic performance. Individual blade rings are employed for each com- pressor and turbine stage to control leakage past the blade tips. The rings have a relatively high thermal response independent of the outer casing, and are provided with features to obtain concentricity with the rotor to prevent rubbing from the blade tips, minimize radial clearance, and thus maximize performance. Since the turbine operates at extremely high inlet temperatures, cooling air for the rotor is extracted from the compressor discharge, and is externally cooled and filtered before returning to the torque tube casing to cool the disks and the first-, second-, and third-stage INDUSTRIAL GAS AND STEAM TURBINES 67 FIGURE 3.7 Westinghouse W501G combustion turbine (Southall and McQuiggan, 1995). blades and vanes (Fig. 3.9). Filtration is deemed essential for eliminating blockages by sus- pended particles of the intricate cooling passages inside the blades. Bleed air from the com- pressor is also used to cool the blade ring cavities and to cool and purge the interstage disk cavities to prevent ingestion of hot blade path gases. Compressor diaphragms are coated to improve aerodynamic performance and to obtain protection from corrosion. The stationary vanes and rotating blades for the first two stages of the turbine are provided with a thermal barrier coating. The operating firing temperature level of 1425°C is selected to be commensurate with the capabilities of the superalloys for the components in the hot path and with the cooling schemes. The cycle pressure ratio is chosen to maximize power output during simple-cycle operation and efficiency in the com- bined cycle mode. With a compression ratio of 19.2:1, the potential-combined cycle effi- ciency is 58 percent. The cycle airflow rate is controlled through the annular flow area at the turbine exit, since the last-stage blade stress level is directly proportional to it. A flow rate of 1200 lb/s results in a conservatively stressed blade and higher power output. The 17-stage axial flow compressor is patterned from the proven design for the W501F engine. Flow and pressure coefficients are similar in the two designs, with the mean diam- eter of the stages increased to accommodate the 25 percent increase in flow. Bleeds for starting and cooling flows are located at the 6th and 11th stages, and the 14th-stage bleed is used for the hot path components. The compressor is also equipped with variable inlet guide vanes (Fig. 3.10) for improving the low-speed surge characteristics and to enhance part-load performance in combined cycle applications. The rotating blades are controlled diffusion airfoils, made of multiple circular arc forms. Stationary blades are fabricated in two 180° segments for easy removal, and are provided with sealing at the inner shroud. Moderate aerodynamic loads are used for the four turbine stages operating at a higher peripheral speed than the W501F engine. Aerodynamic airfoil shapes are obtained from a fully three-dimensional viscous analysis code. The third- and fourth-row blades are shrouded. The first-row stationary vanes are individual precision cast of IN939 alloy, and can be removed from access manways without lifting the cylinder cover. Inner shrouds are supported from the torque tube casing to limit flexural stresses and distortion. Vane seg- ments for the other rows are supported in a separate inner ring. The cooling scheme maintains the NiCrMoV turbine disks under 400°C, within the creep range for long life. Row-1 vane cooling is done by the methods of impingement, convection, and film cooling, as shown in Fig. 3.9. Impingement inserts are used in combination with an 68 APPLICATIONS FIGURE 3.8 Turbine rotor disks with curvic coupling. (Courtesy: Siemens Westinghouse) FIGURE 3.9 Turbine row-1 vane-cooling scheme (Southall and McQuiggan, 1995). Film hole (Typ) Impinge- ment hole (Typ) Precise alignment of seal and shroud Shaped film holes All shroud cooling holes Impingement plates Containment plate Transition mouth seal Core printout Impingement plate Shower head Pin fins Trailing edge slot Outer shroud impingement and film holes Cooling for shroud Vane cooling 69 array of film cooling holes and a pin fin at the trailing edge. Pin fins help to increase tur- bulence and the surface area. Film cooling is provided at the leading edge on the pressure and suction sides. This limits thermal gradients and external surface temperatures at the walls of the vane. Special attention is paid to the inner and outer shrouds because of the flat temperature profile from the dry low NO x combustor. The shrouds are cooled by impinge- ment plates, film cooling, and by convection through drilled holes. The cooling arrangement for the row-1 blades consists of serpentine passages with angled turbulators (Fig. 3.11). Film cooling uses fan-shaped cooling holes, and is used extensively at the tip to reduce the metal temperature of the squealer tip. The airfoil is coated with a vapor deposited thermal barrier coating. For the row-3 blade the cooling is unique in that it positively cools the blade tip shroud (Fig. 3.12). Because of the flat profile from the combustor and because of leakage past the tips of row-1 and row-2 blades, posi- tive cooling for the tip shroud is deemed essential. 70 APPLICATIONS FIGURE 3.10 Compressor inlet variable guide vanes. (Courtesy: Siemens Westinghouse) FIGURE 3.11 Turbine row-1 blade cooling scheme (Southall and McQuiggan, 1995). The rotating blades are made from CM247 for all four rows. The blades are provided with long root extensions, or transitions, to reduce the stress concentration as the load trav- els through the airfoil into the shank. The blade roots are of multiple serration type, with four serrations on the first three rows and five on the last-stage blades. The dry low NO x combustor operates at 25 ppm NO x level at 1260°C turbine inlet tem- perature. Steam cooling is used for reduced emissions at higher firing temperatures. By eliminating the transition cooling air virtually all the combustion air is introduced into the primary zone of the combustor to maintain the flame temperature at nearly the same level as in the W501F engine. 3.4 CLASSIFICATION AND CHARACTERISTICS OF STEAM TURBINES Steam turbines remain the workhorse of power generation worldwide. Hero of Alexandria is credited with developing the first steam turbine 2000 years ago. Dr. de Laval demon- strated driving of a paddle attached to a shaft by expanding steam through a trumpet-shaped steam jet in the latter half of the nineteenth century. In 1894, Sir Charles Parsons invented the multistaged steam turbine. Today, steam turbines are the favored choice in the driving of electric generators, mostly due to the extensive availability of coal. A steam turbine converts the thermal energy of steam into kinetic energy by expansion in nozzles, the resulting jet then forcing rows of blades mounted on a rotor. Steam turbine power plants may be split into three groups: (1) heat sources such as boilers or steam gen- erators, feed water pump, and heater; (2) power generation components that include turbine and generator; and (3) condensers and condensate pump. Rankine cycle is most commonly employed, with water-steam as the working medium. Water is pressurized isentropically by the feed-water pump before entering the boiler, where it evaporates to steam and is even- tually superheated. The cycle requires substantial amount of heat to raise the temperature of water at pump discharge to steam temperature at turbine inlet. Using high-temperature INDUSTRIAL GAS AND STEAM TURBINES 71 FIGURE 3.12 Cooling for row-3 blade shroud (Southall and McQuiggan, 1995). Shroud cooling feed hole Shroud cooling hole Airfoil cooling hole exhaust gas for this purpose partially offsets the amount of external heat required by mini- mizing the temperature difference between the two points. This is the concept behind regenerative heating. A more common practice calls for intermediate pressure steam to per- form the function of heating the feed water. Steam tends to increase in moisture content as it progresses through the stages of a tur- bine. Wet steam tends to affect the buckets, since the higher density of water impacting its leading edges causes erosion. A reheat of steam withdrawn after partial expansion is required to overcome the situation (Boyce, 2002). A regenerative and reheat steam arrangement is shown in Fig. 3.13 and the correspond- ing temperature-entropy diagram in Fig. 3.14. The compressed liquid at D is heated to sat- uration point (D1), evaporated to steam (D2), and finally superheated (D3). After isentropic 72 APPLICATIONS FIGURE 3.13 Regenerative—reheat steam turbine plant schematic. FIGURE 3.14 Regenerative—reheat steam turbine temperature/entropy diagram. [...]... (see Fig 3. 27) 55 iate soc d as 170 ir an ry a 180 45 160 80 40 er lb 150 140 tu p 75 130 halp 35 120 Ent 70 110 80 60 70 55 60 50 15 50 45 40 40 0 32 35 30 Zone II 30 20 Zone III 25 10 0 30 40 50 10 60 70 80 Dry bulb temperature °F 15 90 100 20 110 75 70 65 Dewpoint temperature °F 90 80 60 55 50 45 40 35 30 25 20 15 10 5 0 115 25 Enthalpy, Btu per lb of dry air and associated moisture FIGURE 3. 27 Psychrometric... 65 20 85 1 .30 0 .35 35 30 0.60 0.55 0.50 0.45 0.40 0 .30 0.25 0.20 0.15 0.10 0.05 Vapor pressure, inches of mercury of d 190 85 30 25 0 .30 200 Humidity ratio (or specific humidity grains of moisture per pound of dry air y, B 0.25 Enthalpy, Btu per lb of dry air and associated moisture 50 dm oist ure 0.20 Sensible heat ratio = Qx−QI 0 .35 90 86 APPLICATIONS 3. 8 COGENERATION Since a greater part of a fuel’s... cooling 71 50 50 116 87 93 8 639 8 639 9560 160.5 176.2 176.2 127 .3 53. 6 54.6 54.6 48.7 7.0 9.2 9.4 6.7 207.1 221.6 221.4 169 .3 Table 3. 1 provides a numerical comparison of plant performance for the four different cooling methods The figures are calculated for ambient conditions set at 116°F dry-bulb and 71°F wet-bulb temperature The chiller is rated at 2200 tons, delivering 33 00 gal/min for two gas turbines... moments transferred from 1 FIGURE 3. 33 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 Compressor rotor assembly (Courtesy: Siemens Westinghouse) 92 APPLICATIONS FIGURE 3. 34 Inlet bellmouth and support frame (Courtesy: Siemens Westinghouse) the rotor Stiffness characteristics of support struts play a major role in determining the vibration aspects of the rotating system Figure 3. 35 provides illustrations of the... first law of thermodynamics, work done by the compressor and gas turbine is WC = (dma / dt )(h2 − h1 ) (3. 5) WGT = (dma / dt + dm f / dt )(h3 − h4 ) (3. 6) Work done by the steam turbine is WST = (dms / dt )(h5 − h6 ) (3. 7) Steam generator heat input is QSG = h5 − h8 (3. 8) Total work output is WP = (dms / dt )(h8 − h9 ) / ηP (3. 9) WCyc = WGT − WC + WST − Wp Work done by the pump is (3. 10) Q = (dm f /... relating relative and absolute flow and blade velocities is shown in Fig 3. 32 Cooling air for hot path sections in the turbine is extracted at two or more points from the compressor For example, the sixth-stage bleed air may be used to cool the third row of Inlet guide vanes α2 α1 V2 Rotor U Stator α4 3 V1 V V3 V4 U FIGURE 3. 32 V Typical velocity diagram for axial compressor INDUSTRIAL GAS AND STEAM... FIGURE 3. 22 Forty-inch titanium last-stage buckets (Cofer, 1995) 3. 6 COMBINED CYCLE MODE The Brayton-Rankine cycle combines a gas turbine with a steam turbine and has many benefits for electric utilities and for process industries requiring steam The hot gases from the exhaust of the gas turbine are employed in a fired boiler to generate superheated steam to drive a steam turbine (Fig 3. 23) FIGURE 3. 23. .. and imparted pressure head As the air travels from the hub to the outer tip, changes in the flow direction lead to variations in the velocity and are accompanied by changes in density as the air compresses Blade types for impellers are dictated by the angle at entry and discharge points, which also define the developed head (see Fig 3. 36) FIGURE 3. 36 Impeller blade styles 94 APPLICATIONS TABLE 3. 2 Impeller... blades Figure 3. 31 shows a picture of the components of an axial compressor FIGURE 3. 31 Axial compressor rotor and stator (Courtesy: Siemens Westinghouse) 90 APPLICATIONS A stationary row in the form of inlet guide vanes is located at the start of the passage Many manufacturers offer variable inlet guide vanes to modulate the flow for protection against surge, especially when operation is at part load... three-admission reheat steam turbine has 33 -in last-stage buckets The gas turbine, steam turbine, and electric generator are solidly coupled to generate 480 MW of power The gas turbine is equipped to carry thrust created in both units An auxiliary 2 .3 MW diesel generator set is used to start the rotating train Figure 3. 25 shows a picture of the gas turbine rotor during assembly 3. 7 COMBINED CYCLE FOR PERIODIC . gas turbine is (3. 5) (3. 6) Work done by the steam turbine is (3. 7) Steam generator heat input is (3. 8) Work done by the pump is (3. 9) Total work output is (3. 10) and input is (3. 11) Then overall. turbine (Fig. 3. 23) . 80 APPLICATIONS FIGURE 3. 22 Forty-inch titanium last-stage buckets (Cofer, 1995). FIGURE 3. 23 Combined gas and steam cycle plant. From the first law of thermodynamics, work. may be expressed in the following form: (3. 2) η Re g TT TT = − − 32 5 2 64 APPLICATIONS FIGURE 3. 2 Compression ratio and thermal efficiency. FIGURE 3. 3 Optimized pressure and temperature for