EP2128387A2 - Method for reducing or avoiding water drop erosion in steam turbines and corresponding steam turbine - Google Patents

Abstract

The method involves crushing secondary drops in a steam turbine (1) at location of appearance or indirect proximity via acoustic excited vibrations. Smaller, lighter and better accelerated droplets follow a steam flow, and the number of water drops and eroding effects of the drop impacts are reduced to subsequent turbine blade series. The secondary drops are crushed using ultrasonic atomization, where a capacitive sound converter is used for production of the ultrasound. A membrane made of plastic or metal is used for the capacitive sound converter. An independent claim is also included for a steam turbine with an auxiliary device for implementation of the method for decrease or avoidance of water drop impact erosion.

Description

The invention relates to a method for reducing or avoiding drop impact erosion in a steam turbine containing vanes and subsequent blades, at which liquid is condensed and deposited as droplets to form small primary droplets and larger secondary droplets. In addition, the invention also relates to an associated steam turbine with auxiliary devices for carrying out the method.

In steam turbines energy contained in particular in the steam is converted into mechanical energy and thus an electric generator or a working machine driven. In order to be able to use the heat stored in the steam as much as possible for the conversion into mechanical energy, low-pressure steam turbines are operated so that the condensation is already used in the turbine and thus at the outlet of a highly expanded steam with typically 10-16% wetness , Part of this condensate results in the formation of a water film on the turbine blades of the stages which are downstream of the incipient condensation. Since the blades and possibly located on them water film are exposed due to rotation considerable centrifugal forces, an accumulation of water takes place only on the vanes to a significant extent. This water film flows - driven by the vapor flow - to the trailing edges of the vanes and tears off there in the form of drops, which are substantially larger than the primary droplets formed primarily by condensation in the vapor volume. Because these secondary droplets have a high mass compared to the primary droplets, they can follow the steam flow only to a limited extent (inertia) and therefore collide at high relative speeds with the rotor blades rotating downstream of the stator blade. In doing so, they sometimes cause high damage through drop impact erosion.

• 1.: Maßnahmen zur Reduzierung der auf den Leitschaufeln angesammelten Wassermengen;
• 2.: Maßnahmen zur Reduzierung der Abscheidung von Wasser auf den Leitschaufeln;
• 3.: Maßnahmen zur Reduzierung der Sekundärtropfengröße

In order to limit the latter damage, various measures have already been proposed to reduce the amount of water accumulated on the guide vanes. These measures can be divided into three categories:

• 1 .: measures to reduce the amount of water accumulated on the guide vanes;
• 2 .: measures to reduce the separation of water on the vanes;
• 3 .: Measures to reduce the secondary drop size

In the first category fall both measures for the discharge of water through channel-like structures to the outside (suction slots / Absauggebohrungen) as well as measures for the evaporation of the water films by heating the vanes from the inside. The second category includes measures to control condensation in the vapor volume which result in the formation of a larger number of smaller droplets which are better able to follow the flow and therefore do not deposit on the vanes so quickly. The third category includes measures that modify the surface properties of vanes as well as measures that promote atomization by targeted vapor ejection in the area of the trailing edge of the vane.

Measures of the first category have only limited success because structures that lead to almost complete discharge of water from the vane, both affect the flow in the steam turbine sustainable and reduce the available amount of working fluid and thus reduce the efficiency.

Measures to control volume condensation can affect a major source of water film formation on vanes, namely volume condensation. Part of the finest droplets thus formed can still accumulate on the vanes. The surface condensation as a second source of water on the turbine blades can not be prevented by a measure of the second category. In addition, part of the previously proposed measures for controlling the volume condensation rely on the addition of ion-forming substances which can promote chemical corrosion and therefore do not appear desirable.

The heating of the guide vanes requires, in addition to the use of small amounts of higher-energy heating steam increased equipment complexity, which can significantly increase the cost of the product steam turbine despite proven high efficiency.

Previously proposed measures for changing the surface properties of Leitbeschauflungen have particular limitations due to the low resistance z. B. of water-repellent coatings. The steam blow-out at the vane trailing edge, like the measures in the first category, leads to a reduction in the efficiency over the use of higher-energy steam, which is no longer available to the working implementation.

Starting from the prior art, it is an object of the invention to reduce the formation of coarse droplets in a steam turbine or even to prevent and to provide an associated steam turbine. The invention thus falls into the above-mentioned third category.

The object is achieved by the measures of claim 1. An associated steam turbine is the subject of claim 11. Further developments of the method on the one hand and the steam turbine on the other hand are given in the respective subclaims.

With the invention, measures are taken which greatly reduce the size of the secondary drops, either at drop break at the trailing edge of the vanes, or lead to accelerated destruction of large secondary drops before reaching the next row of turbine blades. This is inventively achieved in that on parts of the surface of the guide vanes acoustic vibrations are generated with frequencies in the range of ultrasound. These vibrations lead to a sputtering of the water film before it can tear off in the form of large secondary drops, on the other hand they lead to the formation of ultrasonic waves, which lead to an atomization of drops in the volume.

When utilizing the second-mentioned effect for atomizing droplets with diameters in the sub-millimeter range, the frequency of the ultrasonic waves is inventively chosen so that the wavelength in the steam in the range of a few tenths of a millimeter to a few millimeters. As a result, the noise amplitudes required for atomization and the associated energy requirement can be minimized.

Furthermore, to minimize the sound amplitudes resonant vibrations of the water droplets in the ultrasonic field are utilized. By adapting the frequency of the ultrasound to the resonant frequency associated with a given droplet diameter, droplets whose diameter is close to this diameter can be atomized particularly efficiently.

The invention is based on the recognition that the formation of small primary droplets, which represent one of the main sources of larger secondary drops, practically can not be avoided, because steam turbines are operated for efficiency reasons with Dampfendnässen in the order of about 10-16%. The basic idea of the invention is now to comminute the secondary drops at the place of origin or in the immediate vicinity of them, and thus despite the presence of droplets through Preventing drop impact caused erosion or greatly reduce: The shredded secondary drops can follow the flow of steam easily and therefore bounce on the blade surfaces with very small defects (false inflow). The damage caused thereby is comparatively small because of the fraction of the normally occurring value of the drop mass and the relative speeds.

The main advantage of a mechanical comminution of the droplets compared with other measures such as the heating of guide vanes surfaces is that the energy expenditure is comparatively low: this is essentially the increase of the surface energy plus the electrical losses during the generation of the ultrasound. The constructional / equipment expense of the solution shown also decreases.

Further details and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the drawing in conjunction with the claims.

Figur 1

eine Dampfturbine mit piezoelektrischen Aktuatoren zur Tropfenzerstäubung an der Hinterkante von Leitschaufeln,

Figur 2

ein Schnitt durch eine in Figur 1 verwendete Leitschaufel,

Figur 3

ein Schnitt durch die zu Figur 1 und 2 alternative Gestaltung einer Leitschaufel mit einem piezoelektrischen Aktuator zur Tropfenzerstäubung an der Hinterkante der Turbinenschaufel,

Figur 4

ein Schnitt einer Leitschaufel mit einem kapazitiven Schallwandler,

Figur 5

ein Schnitt einer Leitschaufel mit einem zu Figur 4 alternativen kapazitiven Schallwandler,

Figur 6

ein Schnitt einer Leitschaufel mit einem weiteren zu Figur 4 und 5 alternativen kapazitiven Schallwandler.

It show in a schematic representation

FIG. 1

a steam turbine with piezoelectric actuators for drop sputtering at the trailing edge of vanes,

FIG. 2

a section through an in FIG. 1 used vane,

FIG. 3

a cut through the to FIGS. 1 and 2 alternative design of a guide vane with a piezoelectric actuator for sputtering at the trailing edge of the turbine blade,

FIG. 4

a section of a vane with a capacitive transducer,

FIG. 5

a section of a vane with one to FIG. 4 alternative capacitive sound transducer,

FIG. 6

a section of a vane with another to FIGS. 4 and 5 alternative capacitive transducer.

In the FIG. 1 1 means a steam turbine, of which only one part is shown. In detail, there is in each case a rotor 2 with an axis of rotation I on which there are individual rows of blades 3, 3 ', ..., specifically three rows of blades 3, 3', 3 "are indicated, corresponding to a stator part 4 with associated guide vanes 5, 5 ', 5 ".) Arrangements with more or fewer rows of blades are also conceivable.

About a supply line 8, the steam flow takes place. The steam flow is directed into the arrangement of stator part 4 and rotor 2 with the rotor blades 3, 3 ', 3 ", whereby the rotor is set into rotation via the conversion of kinetic energy into mechanical energy Thus, for example, a mechanically connected generator or a generator Working machine to be operated.

During or after the introduction of the steam in the arrangement according to FIG. 1 The vapor condenses mainly in the form of small droplets. In the figures, for example, a saturation line S (also: "Tau line") is drawn, after which the steam is present in the thermodynamic equilibrium partly in the liquid and partly in the gaseous state of matter.

In FIG. 1 A layer 16 of piezoelectric material is applied on the profile trailing edge of a vane 5 ", on which there is an electrode 12. The electrode can be embodied as a film / thin layer 12 is connected to a voltage source 15 for an alternating voltage U (t). With an AC voltage U (t) which is suitably predetermined in terms of frequency, the piezoelectric vibrator 16 can oscillate be excited in the ultrasonic range (piezoelectric transducer).

In FIG. 2 Fig. 3 is a section through a single vane 50 having the conventional convex-concave profile profile of turbine blades. At the trailing edge of the vane 50, which realizes a liquid water trailing edge AK, the piezoelectric material 56 is applied as a layer, and over it an electrode 52 is arranged flat. To the electrode 52, the voltage from the voltage source 15 according to the FIG. 1 created. It is thus the arrangement after FIG. 1 realized.

In FIG. 3 a section through a single vane 50 is shown, which also has the usual convex-concave profile profile of turbine blades. The trailing edge of the vane 50, which realizes a liquid water trailing edge AK, is formed as a sequence of a solid segment of piezoelectric material 57 and another electrode segment 58 forming the trailing edge.

To the electrode segment 58, the electrical voltage from the voltage source 15 according to the FIG. 1 created.

With FIG. 3 Thus, an arrangement in analogy to FIG. 1 realized. The advantage of this arrangement is that high vibration amplitudes can be generated at the trailing edge AK with moderate voltages, wherein the contraction of the piezoelectric material perpendicular to the electric field, which could significantly reduce the concentration of the layer structure, can be kept substantially smaller than in the case of flat applied material, the corresponding FIG. 2 forms a thin layer.

Alternatively to the piezoelectric excitation of the vane surface, an electrostatically stimulable membrane for generating mechanical vibrations in the ultrasonic range be used, whereby a capacitive transducer is formed.

FIG. 4 FIG. 12 shows an electrically insulating membrane 61, partially provided with an electrode layer 62, which is vibrationally disposed near the trailing edge of the vane 50 flush with the vane surface. The ability of the membrane 61 to vibrate is ensured by a corresponding cavity 60 in the vane 50 forming a cavity. By applying a suitable alternating voltage to the electrode layer 62, the membrane 61 is excited to high-frequency oscillations, which are transmitted to the water film and thus lead to its atomization.

Such a membrane may, for. B. consist of a plastic film. Other materials are possible. The film is embedded in the blade so that a smooth blade surface is formed.

FIG. 5 shows another embodiment of the recessed into the vane 50 capacitive transducer after FIG. 4 in that the electrode 62 'designed as a thin metal film is protected from vapor and moisture by the membrane 61 by being mounted on the side of the membrane 61 facing the cavity 60.

FIG. 6 FIG. 5 shows an alternative embodiment of a capacitive sound transducer embedded in a vane, in which a vibratable membrane 66 is formed of a metal foil, in which case the membrane 66 is carried in an electrically insulating frame 65 of insulating material placed on the cavity 60 , It also results in a smooth blade surface. A separate electrode is not necessary because of the electrically conductive membrane.

In all cases the FIGS. 3 to 6 the design of the membrane is chosen so that oscillation frequencies f of at least 10 kHz, but preferably 100 kHz to 1 MHz, stimulate.

The measures described above for reducing drop impact erosion can be combined with further measures as disclosed in the Applicant's copending patent application with the same application priority and the term "Method for reducing drop impact erosion in steam turbines by controlling the droplet size and associated steam turbine".

Claims (19)

A method of reducing or avoiding drop impact erosion in a steam turbine, comprising vanes and subsequent blades, at which liquid condenses and separates as droplets to form small primary droplets and larger secondary droplets, comprising: the secondary drops are comminuted at the place of formation or in the immediate vicinity thereof by means of acoustically excited vibrations, - the resulting smaller, lighter and thus better accelerated droplets follow the steam flow better, - This reduces the number and eroding effect of the droplet impacts on subsequent turbine blade rows. A method according to claim 1, characterized in that the secondary drops are crushed in the formation of the guide vanes or in the immediate vicinity thereof. A method according to claim 2, characterized in that the secondary drops are comminuted by ultrasonic atomization. A method according to claim 3, characterized in that is used to generate ultrasound piezoelectric transducer. A method according to claim 3, characterized in that is used to generate ultrasonic capacitive transducer. A method according to claim 5, characterized in that the membrane for the capacitive sound transducer is a membrane made of plastic, which is partially provided with a metal film is used. A method according to claim 5, characterized in that for the capacitive transducer, a membrane of a metal, which is used in isolation in the power vane, is used. Method according to one of the preceding claims, characterized in that the vibration actuator is protected by a coating. Method according to one of the preceding claims, characterized in that the vibration actuator is electrically carried out by a arranged between the blade and actuator insulator from the guide blade material. Steam turbine with an auxiliary device for carrying out the method according to claim 1 or one of claims 2 to 9 with guide vanes within the steam turbine housing and a plurality of rows of moving blades, characterized in that at least one guide vane (5, 5 ', 5'', ... 50) piezoelectric material (14) having at least one electrode (12) is provided, wherein the guide vane (5, 5 ', 5'', ..., 50) means (15) for the electrical excitation of the piezoelectric material (16, 56) are assigned. Steam turbine according to claim 10, characterized in that the piezoelectric material (56) in the vicinity of the trailing edge (AK) of the vane (50) is arranged. Steam turbine according to claim 11, characterized in that the piezoelectric material (56) forms a coating on the vane (50) and on the layer of piezoelectric material (56) an electrode (52) is arranged. Steam turbine according to claim 11 or claim 12, characterized in that the piezoelectric actuator and the zu whose excitation electrode used are designed as vane segments. Steam turbine according to one of claims 10 to 13, characterized in that the at least one guide vanes (5, 5 ', 5'',...; 50) is grounded and that the electrodes (12, 52) with a high-frequency AC voltage of predetermined frequency (f) can be acted upon. Steam turbine according to claim 14, characterized in that an AC voltage source (15) is present. Steam turbine with an auxiliary device for carrying out the method according to claim 1 or one of claims 2 to 9, with guide vanes inside the steam turbine housing and a plurality of rows of moving blades, characterized in that at least one guide vane (5, 5 ', 5'', .. 50), a vibratable diaphragm (61) of a recess (60) is provided, wherein the guide vane (5, 5 ', 5'',...; 50)) means (15, U (t)) for electrical excitation associated with the membrane (61, 65). Steam turbine according to claim 16, characterized in that the membrane (61, 65) in the vicinity of the trailing edge (AK) of the guide vane (50) is arranged. Steam turbine according to claim 16, characterized in that the membrane (61) consists of electrically insulating material and that the structure of a guide vane (50) with recess as a cavity (60), membrane (61) and electrode (62) forms a capacitive transducer. Steam turbine according to claim 16, characterized in that a metallic membrane (66) is provided, which is arranged insulated relative to the guide vane.

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