44 Wet-Steam Turbines for Nuclear Power Plants
steam wetness for the wet-steam reheat turbines could reach 12–17%, however, the actual values are about 11–14%, thanks to the internal moisture removal in the steam path. Steam wetness after the turbines’ HP section is measured at approximately the same levels as at the LP cylinder’s exit. Thus, the problems of both energy losses due to wetness and erosion of the steam path components require equal attention to both the HP and LP sections. The mean diameters and lengths of the HP section’s blades are smaller than those for the LP cylinder, so they work with smaller circumferential velocities. This circumstance somewhat moderates the energy losses. On the other hand, steam in the HP section stages is much denser, because of its higher pressure, and can impair the turbine performances in terms of energy losses and erosion damages to a greater extent.
Because of the energy losses due to wetness, the internal efficiency values of the wet-steam turbine sections are lower than those for fossil fuel reheat turbines of superheated steam: about 82% for the HP sections of wet-steam turbines versus 86–92% for the HP sections of superheated-steam turbines; and 85–87% for the LP sections of wet-steam turbines versus 89–94% for the intermediate-pressure (IP) sections and 90–91% for the LP sections of reheat steam turbines for fossil fuel power plants.
The most characteristic fossil fuel steam-turbine-based power units in the second half of the 20th century were designed with subcritical main steam pressure—for example, about 16 MPa (2,320 psi), as shown in Figure 2–2, and main/reheat steam temperatures in the range between 530°C (985°F) and 565°C (1,050°F).Along with this, in many countries (the United States, the former Soviet Union, Germany, Italy, Denmark, Japan, South Korea, and China), a large number of supercritical-pressure steam turbine units with a main steam pressure of about 24–25 MPa have been created. In recent years, Germany, Japan, and Denmark have been developing and implementing a new generation of modern units with ultra-supercritical (USC) main steam pressure of up to 31 MPa (4,500 psi) and elevated steam temperatures of up to 600–610°C (1,112–1,130°F). 4
The first fossil fuel steam-turbine power units with USC steam pressure and advanced steam temperatures were designed, constructed, and launched as early as the late 1950s and early 1960s:
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for example, 125-MW Philo Unit no. 6 in Ohio, with a double-reheat turbine of GE and steam parameters of 31 MPa and 621/565/538°C (4,500 psi, 1,150/1,050/1,000°F); 325-MW Eddystone Unit 2 in Pennsylvania, with a double-reheat turbine of Westinghouse and steam conditions of 34.5 MPa and 650/565/565°C (5,000 psi, 1,200/1,050/1,050°F); the 100-MW back-pressure turbine at Kashira power plant near Moscow, with steam conditions of 29.4 MPa and 650/565°C (4,350 psi, 1,200/1,050°F); and 107-MW Hattingen Units 3 and 4 in Germany, with an ordinary supercritical steam pressure, but elevated main steam temperature of 600°C (1,112°C). However, even though all of these units operated successfully, they were somewhat premature, and the design was not copied and did not gain further acceptance.5
In 2000, two new coal-fired USC units exhibited the highest thermal efficiency level yet seen. 6 These two are the German 907-MW unit at the Boxberg power plant, with steam conditions of 26.6 MPa and 545/581°C (3,860 psi, 1,013/1,078°F), and the Japanese Tachibanawan Unit 2, with a capacity of 1,050 MW and steam conditions of 25 MPa and 600/610°C (3,625 psi, 1,112/1,130°F).The Boxberg unit’s net efficiency was recorded at 47.2%, and the gross efficiency of its Siemens turbine was 48.5%.The turbine’s acceptance tests demonstrated an internal efficiency for the HP and IP cylinders of 94.2% and 96.1%, respectively.With a gross efficiency of 49.0%, the Tachibana-wan unit’s steam turbine, produced by Mitsubishi Heavy Industries (MHI), has been acclaimed as the most efficient worldwide.
The previous highest gross efficiency values for modern steam turbines were recorded at the following power units:
47.4%—Japan’s Hekinan 700-MW Unit 3, with MHI’s turbine for steam conditions of 24 MPa and 538/593°C (3,480 psi, 1,000/1,099°F);
47.6%—Germany’s Hessler 720-MW power unit, with ALSTOM’s turbine for steam conditions of 27.5 MPa and 578/600°C (3,990 psi, 1,072/1,112°F), and
48.4%—Japan’s Kawagoe 700-MW Units 1 and 2 , with Toshiba’s turbine for steam conditions of 31 MPa and 566/566/566°C (4,496 psi, 1,051/1,051/1,051°F)
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46 Wet-Steam Turbines for Nuclear Power Plants
Even though the efficiency levels reached at Boxberg and Tachibana-wan look very impressive, they likely represent only interim benchmarks.A new efficiency record could be achieved by the Siemens turbine at Germany’s Niederaussem Unit K, with steam conditions of 27.5 MPa and 580/600°C (3,990 psi, 1,076/1,112°F).This unit’s turbine has a lower condenser pressure compared with the Boxberg unit and a 25% larger exhaust area due to the use of longer LSBs.The unit, commissioned in September 2002, was designed to reach 45.2% thermal efficiency. Even higher efficiency values are targeted for the Avedøre 530-MW Unit 2 in Denmark, with steam conditions of 30 MPa and 580/ 600°C (4,350 psi, 1,076/1,112°F), and Westafalen 350-MW Unit D in Germany, with steam conditions of 29 MPa and 600/620°C (4,210 psi, 1,112/1,148°F). Higher performances are also expected from new Japanese power units to be commissioned by 2005, due to even further heightened steam conditions—up to 30 MPa and 630/630°C (4,350 psi, 1,166/1,166°F)—and turbine steam path advances. 7 It is currently considered technologically feasible, as well as economically advisable, to raise the steam parameters for future fossil fuel steam-turbine-based power units up to 35 MPa and 700/720°C (5,075 psi, 1,292/1,328°F). This promises to increase the net efficiency of such units to as much as 50–51% and to make them effective in combating the rise of fuel prices by lowering the costs of power generation. 8
Against the background of these figures, the thermal efficiency data for nuclear wet-steam turbines, with a best gross efficiency of about 36%, look rather modest. Because nuclear power units of the nearest future, just like their predecessors, are mostly designed with a moderate initial steam pressure of up to 7.5 MPa, it is not likely any radical increases in thermal efficiency will be achieved, except for gains due to improvements in the steam path design and reduction of internal energy losses.
In theory, the thermal efficiency of the wet-steam turbine’s simple cycle (without regeneration, special moisture removal, and steam reheat) would grow by increasing the initial steam pressure up to 16–17 MPa (2,320–2,465 psi), although it would decrease slightly with any further increase in initial steam pressure.9 In reality, the choice of the initial pressure value for nuclear power plants is mainly dictated by the strength of the reactor’s pressure vessel for singlecircuit units and heat transfer conditions for the boiling coolant for double-circuit units with BWRs or LWGRs. The pressure vessels of
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modern PWRs are commonly designed for an internal pressure of about 15–16 MPa (2,175–2,320 psi).With regard to the underheating of the primary circuit water in reference to its saturation temperature and the terminal temperature difference (TTD) between the primary circuit water temperature at the reactor’s outlet and the secondary circuit water’s saturation temperature in the steam generator, the primary circuit pressure creates a steam pressure provided by the steam generator at a level of about 5–7.5 MPa (725–1,085 psi). The best heat transfer conditions for boiling water take place with a pressure of about 7 MPa (1,015 psi), so nuclear power plants with BWRs or LWGRs are commonly designed with an outlet steam pressure of about 6.5–7.3 MPa (940–1,060 psi). 10
An increase of the initial steam pressure entails an undesirable increase in the steam wetness in the HP section stages when the turbine works under partial loads, because of throttling of the main steam in the HP control valves. First-generation wet-steam turbines with lower initial steam pressures (as, for example, TurboAtom’s widespread wet-steam turbines K-220-44 with a rated electric output of 220 MW and initial steam pressure of 44 atm [4.32 MPa; 626 psi] used with Soviet PWR-type VVER-440 reactors) have somewhat more favorable conditions during start-ups and deep load changes, because the initial thermal process point due to throttling extends into the superheated steam region. However, such a low initial steam pressure was derived from quite different reasons—primarily because of the strength of the reactor’s pressure vessel.
Generally, it is desirable to decrease the turbine’s end pressure (pressure in the condenser) in order to increase the turbine’s available energy and improve the thermal cycle’s efficiency. For characteristic wet-steam turbines, reduction of the end pressure from 4 kPa to 3 kPa (from 0.58 to 0.435 psi) could increase the thermal efficiency by approximately 2–3%.11 Because nuclear power plants do not require continual voluminous fuel deliveries, the developers have more freedom in choosing a desirable location for a plant that provides better cooling. As a result, the average end pressure for turbines of nuclear power plants is commonly somewhat lower than that for fossil fuel power plants located in the same region, even though some nuclear power plants in industrial zones have to operate with cooling towers and a relatively high pressure in the condenser. At the same time, deeper vacuum in the condenser (lower end pressure) increases
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48 Wet-Steam Turbines for Nuclear Power Plants
the volumetric steam flow amount at the turbine exit and requires longer LSBs or a greater number of exhaust flows to limit the exhaust energy losses. If the turbine’s exit area is not sufficiently large and the exhaust steam velocity is relatively high, the gain in efficiency reachable due to a lower end pressure decreases considerably. Besides, a lower end pressure means greater steam wetness for the LSBs and hence additional energy losses.
This influence of the end pressure in the condenser, pc, with regard to the steam flow amount through the turbine (or the turbine output, N) related to the exhaust area value, F is illustrated in Figure 2–4.The exhaust area is taken equal to the total annular area of the LSBs:
F=n× dmL, |
(2.1) |
where n is the number of the LP exhaust flows, and L and dm are the LSB’s length and median diameter, respectively.
Fig. 2–4. Influence of the end steam pressure in the condenser, pc, on wet-steam turbine thermal efficiency (initial steam pressure, p0 = 7 MPa
[1,015 psi]; 1: invariable outlet steam velocity, Gc ×vc /F ; 2, 3, 4, and 5: N/F = 5,
10, 15 and 20 MW/m2, respectively)
Source : B. M.Troyanovskii12
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Large steam flow amounts of wet-steam turbines require vast exhaust areas (significantly greater than those for superheated-steam turbines of a comparable output). Wet-steam turbines are designed with more LP cylinders (up to four) or as low-speed machines (with a rotation speed of 1,800 or 1,500 rpm) to have a possibility of increasing the LSB length without sacrificing the LSB strength.
For large wet-steam turbines with several LP cylinders and condensers, additional efficiency gains can be achieved by connecting the condensers successively with the cooling water. As a result, the average end pressure would be lower than if the condensers were connected in parallel.This results in a greater turbine output with the same steam flow amount and cooling water inlet temperature and flow amount. Successive connection of condensers was first proposed for Westinghouse turbines.13 As an example, Figure 2–5 shows how the gains in LP cylinder outputs and total turbine efficiency change with the cooling water inlet temperature, as applied to a 750-MW low-speed wet-steam turbine with three serially connected condensers. Even though the last (third) LP cylinder is underloaded because of warmer cooling water and higher end pressure, this loss is more than compensated by the gain in the output of the first two cylinders, whose condensers are cooled by colder water and provide deeper vacuum. Under favorable conditions, successive connection of the condensers can bring an increase in the turbine’s output or efficiency as much as 0.45%. 14 Unfortunately, this gain is notable only for relatively warm cooling water, although it increases when the turbine operates under partial loads.
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50 Wet-Steam Turbines for Nuclear Power Plants
Fig. 2–5. Gain in the output (a) and efficiency (b) for a 750-MW wet-steam turbine with three serially connected condensers, related to the turbine load and cooling water inlet temperature (I, II, and III: gain in the output for the first, second, and third LP cylinders, respectively, with the turbine working under full load)
Source : R. L. Coit15
If the working steam were to expand within the turbine directly from the initial point to the end pressure, the steam wetness in the last stages would reach inadmissibly high values of about 24% (Fig. 2–2). This would bring about large energy losses and, more importantly, cause intense erosion in the steam path.To reduce the degree of wetness in the LP section and prevent these effects, wet-steam turbines are equipped with external moisture separators and reheaters. Different turbine configurations with the moisture separators and oneand two-stage reheaters are shown in Figure 2–6. Sometimes designers furnish a turbine with an additional MS stage and, in addition to the HP and LP sections, introduce an intermediate-pressure (IP) section (or cylinder) situated between two moisture separators, as shown in Figure 2–6d. However, the most widespread configuration for modern wet-steam turbines comprises a single set of one or more moisture separator and reheaters (MSRs) placed between the HP and LP cylinders (Figs. 2–6b and 2–6c). Sometimes, even with this configuration, the first stages after the MSR are placed in a separate IP section.
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The Thermal Process in Wet-steam Turbines 51
Fig. 2–6. Configurations of wet-steam turbines with different combinations of external moisture separators (MS) and single-stage and two-stage reheaters (R)
The influence of the partition, or splitting, pressure (the pressure after the HP section), related to the initial steam pressure (ppart/p0)
on the turbine efficiency as applied to low-speed wet-steam turbines, according to GE, is shown in Figure 2–7. The choice of the partition pressure also influences the separator’s and reheater’s overall dimensions, so with regard to capital expenditures, power production costs turn out to be less dependent on the aforesaid pressure ratio. In addition, capital expenditures also depend on various other subsidiary circumstances. 16 The choice of the partition pressure also considerably influences the steam wetness in the last stages of both the HP and LP sections: a reduction of the partition pressure increases the exit wetness in the HP section and decreases it in the LP cylinders, and an increase in the partition pressure has the opposite effects.All of these contradictory considerations result in a large spread of values for the partition-to-initial steam pressure ratio for wet-steam turbines of different designs.This ratio normally falls in the range of 0.08–0.2. In the schematic diagram shown in Figure 2–1, this ratio is 0.088, and for the steam expansion process shown in Figure 2–2, it is equal to 0.125.
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52 Wet-Steam Turbines for Nuclear Power Plants
Fig. 2–7. Influence of partition steam pressure (between the HP and LP cylinders) on wet-steam turbine efficiency, according to GE (1: with separator only, as shown in Figure 2–6a; 2: with separator and one-stage reheater, as shown in Figure 2–6b; 3: with separator and two-stage reheater, as shown in Figure 2–6c)
Source : F. G. Baily, J.A. Booth, K. C. Cotton, and E. H. Miller17
Wet-steam turbines’ steam reheat does not directly cause any increase in the thermal efficiency as it takes place for fossil fuel power units, in which steam reheat is provided by external heat. For wetsteam turbines, steam is reheated by highly potential steam extracted from the HP section, as well as a portion of the main steam not used in the turbine. In other words, the heating steam is used ineffectively. Thus, steam reheat for wet-steam turbines is needed only to decrease the steam wetness in the LP section, and it raises the turbine’s efficiency indirectly—by reducing the energy losses because of wetness. Wet-steam turbines are commonly provided with highly developed regenerative systems, which are designed and calculated like those of fossil fuel power units18 (Fig. 2–1).
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The Thermal Process in Wet-steam Turbines 53
Features of Wet-Steam Flow in the
Turbine Steam Path
When superheated steam flows through the turbine’s steam path, its expansion is accompanied by the release of the energy of superheating until it reaches a saturated condition.With further expansion, the flowing steam releases a portion of its latent heat of evaporation. This results in conceiving particles of water that are drawn by the parent steam through the steam path until they either pass to the condenser (or bleedings) or fall onto nearby surfaces. Except for a few stages of the turbine section immediately after the MSR that operate on superheated steam, all of the wet-steam turbine’s rest stages (in both the HP and LP sections) work on wet steam (Fig. 2–2). The working fluid for these stages is a two-phase steam-water mixture. The liquid component of this flow exists in the forms of fog (fine droplets), drops of different diameters, water films and rivulets moving along the solid surfaces, and water streams separated from these surfaces (Fig. 2–8). A portion of this liquid separates from the steam path and passes into bleedings, peripheral water catcher belts, special slots in hollow vanes, and so on.
The two-phase mixture can exist in states of stable or metastable thermodynamic equilibrium or phase transition (that is, condensing or evaporating). In the stable state, the thermodynamic conditions of wet steam are described by: 1) its wetness, y, which characterizes the relative mass of the liquid component in the unit volume as y = m'/m = m'/ (m'+m"), where m' and m" are the masses of the liquid and vapor phases, respectively; or 2) its dryness, x = (1 – y). The water drops drawn by the wet-steam flow vary in their size from fine particles with diameters of about 0.01 mkm (0.4 x 10–4 mil) to coarse-grained drops with diameters of about 100 mkm (4 mil). Small droplets can merge and agglomerate, and on the contrary, large drops can lose stability and divide. The drop stability, characterized by the Weber number, depends on the liquid’s surface tension, density, and flow velocity. For more detailed discussion of the fluid dynamics of wet steam and the motion of two-phase mediums in turbine steam path elements, there are a number of basic monographs on these subjects. 19
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