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74 Wet-Steam Turbines for Nuclear Power Plants
inability to understand the nucleation process in steam turbines is surprising” and emphasized that “it is evident that the nucleation of water droplets in turbines involves phenomena which are not reproduced by laboratory experiments on nozzles and stationary cascades but nevertheless play a dominating role in the process of phase transition in real machines.” 54 He also noted that, despite considerable advances made in aerodynamic design methods, there existed a tendency to consider the wetness loss component as unavoidable. Disputing this point of view, A. Guha declared that more fundamental knowledge of wet-steam flow in turbines could reduce these losses to a minimum. In addition, this should allow more effective removal of water from the steam path and minimize erosion damage. To attain these aims, experimental investigations should be transferred from stationary nozzle rows, cascades, and individual stages to actual or model multistage turbines.
Experimental research of wet-steam flow in turbines
Experimental research of wet-steam flow in turbines in service could be aimed at solving different tasks and using very different techniques. In the simplest case, the wet steam extracted from the steam path is directed into measuring tanks, and measuring the water quantity makes it possible to assess steam conditions by the indirect route. Figure 2–26 shows some results of experimental and computational investigations of wet-steam parameters and moisture separation efficiency for HP stages of the K-220-44 turbine (with a rated output of 220 MW and main steam pressure of 44 atm [4.3 MPa; 626 psi]).55 The use of measuring tanks connected to different cavities for trapping the water could be quite helpful for investigating and comparing different moisture removal techniques. 56
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Fig. 2–26. Variation in wet-steam conditions along the steam path of the HP cylinder of a K-220-44 turbine (1: average water drop diameter, dd; 2: portion of large drops, ignoring moisture separation, λ ; 3: total steam wetness, yd; 4: steam wetness for fine water droplets, y'd; 5: portion of large drops, with moisture separation, λ; ψ2 and ψ5 : moisture separation efficiency for the water traps after the second and fifth stages)
Source:Y. F. Kosyak, G.A. Filippov,Y. E.Yushkevich, et al.57
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76 Wet-Steam Turbines for Nuclear Power Plants
Direct measurement and analysis of the moisture conditions in the steam path are commonly performed with the use of long probes inserted into the intracylinder spaces of the operating turbine through accurately positioned guide tubes. A typical installation of such probes between the blade rows of an LP turbine is shown in Figure 2–27. Traversing across the stage height, a probe can provide detailed data about the radial steam condition distribution.The same or similar probes with other heads are also used to measure total and static steam pressure, steam flow direction, and steam velocity in a 3-D field. These probes can also be used for sampling the primary steam condensate from the flow for chemical analysis. Similarly, endoscopes are input into the steam path to observe the motion of the moisture and visually inspect the steam path itself.
Fig. 2–27. Installation of research probes into an LP turbine section
Source : M. J. Moore58
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Different methods exist for measuring the various wetness fractions, in particular, those of coarse-grain water. By definition, coarse-grain water consists of large water drops (d > 10 mkm) drawn by the steam flow or water films and rivulets on the boundary surfaces, which can leave these surfaces and join the steam stream. Mechanical separation and collection of these fractions are therefore feasible, using various separating elements.A typical coarsegrain water distribution between the next-to-last and last stages of a 500-MW turbine is shown in Figure 2–28. Numerous methods have also been developed for measuring the fog wetness fractions.
Fig. 2–28. Typical distribution of coarse-grain water before the last LP stage of a 500-MW turbine
Source : M. J. Moore59
The simplest method of assessing the steam wetness consists in throttling steam from the two-phase region to a superheated condition. In doing so, the steam wetness is determined by measuring the
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78 Wet-Steam Turbines for Nuclear Power Plants
temperature and pressure of the superheated steam at the end of the throttling process. Some researchers have also used absorption, dielectrics, psychometrics, and other methods, even though they seem to provide less accurate results.
Methods of measuring the drop size also differ for large drops and fog droplets. Among the most widespread and noteworthy is the extinction (also known as light-scattering or light-attenuation) method, which is based on the attenuation of a monochromatic light beam due to its scattering by the drops.The resultant extinction coefficient to be measured can be expressed as a function of the particle size. The indicatrix (a polar diagram) of the scattered light radiation can also be used for measuring the average size of the fog droplets. The main advantages of these and other optical methods are their high degree of sensitivity and lack of inertness.
The steam wetness distribution along the stage height shown in Figure 2–28 is rather typical. It is generally believed that the steam wetness, as well as the modal size of moisture particles and the relative portion of large particles, steadily increases toward the periphery of the stage under the action of centrifugal forces, and experimental data for individual stages commonly confirm this (Fig. 2–29). If the stage has a water extraction belt located after the rotating blades, it promotes this process. Coarse-grain water spray centrifuged from the blade tips can be seen in Figure 2–8. At the same time, there exist numerous experimental data for turbine sections that demonstrate a decrease in the local wetness at the very periphery of the stage, which can be explained mainly by the lower efficiency of the preceding stages at their tip sections, changes of the stage reactivity along the height, as well as some other factors (Fig. 2–30).
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Fig. 2–29. Steam wetness variation over the height of an individual stage (1: without water extraction after the stage; 2: with water drop trap located after the stage)
Source : I. I. Kirillov,Y. F. Kosyak,A. I. Nosovitskii, et al. 60
Fig. 2–30. Wetness distribution along the length of the last turbine stage inlet, according to experimental data from Westinghouse (1) and AEI (2) turbines
Source : B. M.Troyanovskii61
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80 Wet-Steam Turbines for Nuclear Power Plants
Currently, most experimental researches of wet-steam flow in turbines are produced with the use of optical methods. Figure 2–31 depicts an optical attenuation probe used by researchers of Electricité de France (EdF) for investigations of the wet-steam structure in the LP cylinders applied in 900-MW turbines of nuclear power plants and 600-MW turbines of fossil fuel power plants.62 The measurements were performed within a wide light wavelength range of 0.35 to 5 mkm, that is, extending up to infrared frequencies. The optical system was incorporated in a 25-mm diameter probe.The wet steam flows through a slot with a length that can be varied from 0 to 200 mm. Purge air is blown on both ends of the slot to prevent deposition of droplets on the sapphire windows.The light, originating from a tungsten lamp, passes through a monochromatic system of interchangeable grating plates; it is then channeled through the probe by a flexible 1 mm silica optical fiber. The changeable detection component could be a photomultiplier for visible light (up to 0.7 mkm), a PbS cell for infrared radiation (0.8 to 2 mkm), or an InSb cell (2 to 5 mkm).A microvideo probe employing back and side lighting methodology is used for coarse-grained drops. This technique was developed for measuring drop sizes greater than 10 mkm (Fig. 2–32).The measured volume is illuminated from two directions: perpendicular to the optical axis, in order to select drops within the focal distance of the magnifying system, and along the axis, with light shining from the rear, in order to determine directly the drop size. This optical system was also incorporated into the same 25-mm diameter tube. To capture the motion of the drops, a pulse diode laser was used as a light source. The images were recorded on a video recorder and then analyzed by the operator (automatic image processing was an anticipated future development). The ultimate goal was to achieve real-time analysis (at a video frequency of 25 Hz) and obtain realistic histograms over a short recording time (1–2 minutes). Experimental distributions of fog droplets and large drops downstream of the last stage blade are shown in Figure 2–33. On the basis of these experimental investigations, the EdF researchers calculated the structure of the energy losses due to wetness for the 900-MW wet-steam turbine’s LP stages, including the following components: thermodynamic (related to the non-equilibrium state of the steam), drop acceleration, drop collisions with the rotating blades, water film deposition, action of the centrifugal forces, and additional friction (Fig. 2–34). The calculated losses for the last (seventh) stage are compared with those counted on the basis of the Baumann rule.
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Fig. 2–31. Optical attenuation probe for measuring fog droplet size
Source :A. R. Laali, J. J. Courant, and A. Kleitz63
Fig. 2–32. Microvideo probe used for coarse-grain water drop measurements
Source :A. R. Laali, J. J. Courant, and A. Kleitz64
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Fig. 2–33. Experimental distributions of fog droplets and large drops downstream of the LSB
Source :A. R. Laali, J. J. Courant, and A. Kleitz65
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Fig. 2–34. Calculated energy losses due to wetness for LP stages of a lowspeed 600-MW wet-steam turbine (loss components: 1: thermodynamic
(related to the nonequilibrium state of the steam); 2: drop acceleration; 3: drop collisions with the rotating blades; 4: water film deposition; 5: action of the centrifugal forces; 6: additional friction; 7: total energy loss for the seventh stage (calculated on the basis of the Baumann rule)
Source : B. M.Troyanovskii66
Figure 2–35 demonstrates a system developed by Russian scientists at the Moscow Power Engineering Institute, comprising a primary condensate sample trap and a laser probe inserted into the turbine steam flow.67 The drop size and steam wetness were measured using two optical methods—an asymmetric indicatrix of optical dispersion and light attenuation. A helium-neon laser was used as the light source. The experiments were performed with two different types of water chemistry, all-volatile treatment (AVT) and oxygenated treatment (OT), with automatic processing of all the chemical analysis data. The probe facilitated measurements at several positions along the stage height. Some results of these experiments for different impurity levels in the steam at the turbine inlet are shown in Figure 2–36.
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