Method for preventing adhesion of scale and nozzle of a geothermal steam turbine

Abstract: This invention relates to a nozzle for use in a geothermal steam turbine and method for preventing adhesion of scale thereto which are adapted to prevent precipitation and adhesion of scale onto the surface of the nozzle. A coolant water passageway (22) is provided in an initial stage nozzle (21) of a geothermal steam turbine or the like, an inlet side passageway (22a) of this coolant water passageway is communicated with coolant water inlet passageways (25, 26) provided in inner and outer turbine casings (23, 24), respectively, and also an outlet side passageway (22b) is communicated with coolant water outlet passageways (27, 28). Owing to the above-mentioned construction, coolant water (30) is made to pass through the coolant water passageway (22) within the nozzle (21) from a coolant water feed source to lower the surface temperature of the nozzle, and thereby reevaporation and condensation of drain on the blade surface are prevented to prevent precipitation of scale.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: The present invention relates to a nozzle, especially a nozzle of a geothermal steam turbine, and more particularly, to a nozzle of a geothermal steam turbine which is adapted to prevent precipitation and adhesion of scale onto a nozzle surface, and a method for preventing adhesion of scale onto the same nozzle. Description of the Prior Art: A purity of steam in a geothermal power plant is a factor exerting a very large influence upon a reliability of the power plant. More particularly, SiO₂, Fe, Na, Cl and the like contained in steam have a tendency of precipitating and adhering as scale 4 mainly to a rear surface 2 and a concave front surface 3 of a first stage nozzle 1 as shown in Fig. 3. Adhesion of scale onto the first stage nozzle does not only bring about lowering of an output power caused by reduction of a nozzle area, but also could possibly result in a breakdown accident of a rotor due to contact between the scale and a rotor (moving blades or a disc). Accordingly, adhesion of scale onto a nozzle is a factor largely governing an overhaul interval of a geothermal power plant. From the above-mentioned reasons, establishment of a technique for preventing adhesion of scale to a nozzle is an essentially necessary problem in view of both the aspects of insuring a reliability of a geothermal power plant and improving an availability factor of the same. On the other hand, as a currently practiced technique for preventing adhesion of scale, a method illustrated in Fig. 4 has been known. More particularly, this method is a method consisting of the steps of extracting a part of steam 13 fed to a geothermal steam turbine 12 through an inlet steam pipe 11 and condense it in a condenser 14, then injecting the thus condensed water 15 into the steam 13 flowing through the inlet steam pipe 11 by pressurizing it with a pump 16, and thereby water-washing out the scale adhering to the nozzle within the turbine 12. It is to be noted that reference numerals 17 and 18 designate valves. As described above, the method known in the prior art is a method of water-washing scale adhered to a nozzle by injecting condensed water 15 prepared from geothermal steam into the inlet steam pipe 11 of the geothermal steam turbine 12. This method is generally called "water-washing method". However, according to practical results in a practically used machine, there are some plants in which this method is not always effectively practiced. Considering the reasons of the unavailability, in the prior art, it appears that attention was paid solely to only removal of the precipitated and adhered scale, and a fundamental countermeasure for removing the cause was not taken. Therefore, the inventors of the present invention at first investigated on the mechanism of "adhesion of scale". The results of examination of a composition of moisture mixed in the steam of the investigated geothermal power plant were: and it was investigated with respect to a rear surface and a concave front surface of the nozzle how these components adhere to the nozzle surfaces. The results were as follows: 〈Rear Surface of Nozzle〉 As shown in Fig. 5, at the portion downstream of Point A ○ which is about 0.55 (width) apart from the front edge of the nozzle, a metal surface temperature is higher than a steam temperature at the blade surface. Consequently, drain would reevaporate from the metal (blade) surface, hence NaCl and SiO₂ would condense and would be precipitated as scale. However, it is to be noted that whereas a percentage content of NaCl is 0.55 ppm, that of SiO₂ is as small as 0.066 ppm, and moreover, since SiO₂ is liable to transfer to the vapor phase in view of the distribution rate which is characteristic to SiO₂ and is liable to disperse into the steam flow, a principal component of the scale is NaCl. From the above-mentioned reasons, the following conclusion is obtained: (1) Scale would precipitate on the downstream side of the position about 0.55 (width) apart from the front edge. (2) A principal component of the scale is NaCl. 〈Front Surface of Nozzle〉 As shown in Fig. 5, since a metal (blade) surface temperature is lower than a steam temperature at the surface of the blade in nearly all the range, condensation of steam into drain would proceed on the surface of the blade, but reevaporation of the drain would not occur. Since a solubility of NaCl is large, if drain is present, NaCl would not precipitate as scale. Accordingly, on the front (concave) surface of the nozzle where reevaporation of drain would not occur, NaCl could never precipitate as scale. On the other hand, with respect to SiO₂, at the location behind the Point B ○ (the position about 0.95 (width) apart from the front edge) where condensation of drain commences gradually, it precipitates as scale by the amount exceeding its solubility. From the above-mentioned reasons, the following conclusion is obtained: (1) Scale would precipitate in the proximity of the rear edge. (2) A principal component of the scale is SiO₂. Further, with respect to Fe, it precipitates at the location where drain condensates. In US-A-4 183 456 relating to a method of manufacturing turbine buckets and nozzles for water-cooled high temperature gas turbines there is described a rotary blade of such turbine wherein a coolant water passageway for cooling the surface of the nozzle as a countermeasure for corrosion is provided. SUMMARY OF THE INVENTION: The present invention has been worked out in order to resolve the problems in the prior art, and has it as an object to provide a nozzle having a structure for directly preventing precipitation of scale, in which precipitation and adhesion of scale onto a nozzle themselves are prevented without relying upon the method of removing scale adhered to a nozzle. In order to resolve the above-mentioned problems, in the nozzle of a geothermal steam turbine according to the present invention, a coolant water passageway for cooling the surface of the nozzle has been formed in the inside of the nozzle. Also, for the purpose of effectively carrying out cooling of the nozzle surface, an inlet side of the above-mentioned coolant water passageway has been disposed on the upstream side of a nozzle width, a bore diameter of the above-mentioned coolant water passageway has been chosen to be nearly 1/3 of a nozzle thickness, or the above-mentioned coolant water passageway has been connected to a coolant water feed source via a coolant water passageway formed in a turbine casing. In addition, in the method for preventing adhesion of scale to a nozzle of a geothermal steam turbine according to the present invention, provision has been made such that coolant water may be made to pass through the inside of the nozzle to make a nozzle surface temperature lower than a steam temperature at the nozzle surface and thereby precipitation of scale onto the nozzle surface may be prevented. Precipitation and adhesion of NaCl onto the rear surface of a nozzle would occur due to the fact that drain on the blade surface reevaporates. While, precipitation and adhesion of SiO₂ onto the concave front surface would occur due to the fact that drain is condensed in the proximity of the rear edge of the blade and its concentration exceeds a solubility of SiO₂. Accordingly, if these reevaporation and condensation of the drain are prevented, then precipitation and adhesion of NaCl and SiO₂ could be prevented. To that end, a surface temperature of a nozzle is lowered by externally leading coolant water into a coolant water passageway provided within the nozzle, thereby reevaporation and condensation of drain onto the blade surface can be prevented and precipitation of scale itself can be prevented. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of one preferred embodiment of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a cross-section view of an initial stage portion of a geothermal steam turbine making use of a nozzle according to one preferred embodiment of the present invention; Fig. 2 is a cross-section view taken along line II-II in Fig. 1; Fig. 3 is a schematic view showing a state of adhesion of scale onto a nozzle in the prior art; Fig. 4 is a system diagram showing a method for removing scale in the prior art; and Fig. 5 is diagrams showing temperature difference distributions on the nozzle blade surfaces. DESCRIPTION OF THE PREFERRED EMBODIMENT: In the following, detailed description will be made on one preferred embodiment of the present invention with reference to Figs. 1 and 2. Fig. 1 is a cross-section view of an initial stage portion of a geothermal steam turbine making use of a nozzle according to one preferred embodiment of the present invention, and Fig. 2 is a cross-section view taken along line II-II in Fig. 1. In these figures, reference numeral 21 designates an initial stage nozzle, within this nozzle 21 is formed a coolant water passageway 22, an inlet side coolant water passageway 22a of this coolant water passageway 22 is communicated with coolant water inlet passageways 25 and 26 formed respectively in an outer turbine casing 23 and in an inner turbine casing 24, and likewise, an outlet side coolant water passageway 22b of the coolant water passageway 22 is communicated with coolant water outlet passageways 27 and 28 formed respectively in the outer turbine casing 23 and in the inner turbine casing 24. It is to be noted that in Fig. 1, reference numeral 29 designates a moving blade. In the above-described construction, a metal surface temperature of the nozzle 21 is lowered by making coolant water 30 flow through the coolant water passageway 22 (22a, 22b) provided within the nozzle 21 from an external coolant water feed source not shown through the coolant water inlet passageways 25 and 26 and the coolant water outlet passageways 27 and 28 provided in the inner and outer turbine casings 23 and 24, respectively, and thereby drain 31 on the surface of the nozzle 21 can be prevented from reevaporating or condensing as shown in Fig. 2. Describing this in more detail, generally on a rear surface 21a of the nozzle 21, on the downstream side of the nozzle surface, since the metal temperature is higher than the steam temperature at the nozzle surface, NaCl and the like would precipitate and adhere to the nozzle surface due to the fact that drain produced by condensation of steam on the inlet side of the nozzle would reevaporate. Whereas, on the concave front surface 21b of the nozzle 21, on the downstream side of the nozzle surface, since the metal temperature is lower than the steam temperature at the nozzle surface, condensation into drain of the steam on the nozzle surface would proceed, but in the proximity of the rear edge 21c, the metal temperature becomes higher than the steam temperature at the nozzle surface, and condensation of the drain proceeds. As a result, impurities such as SiO₂ and the like would reveal the tendency of precipitation and adhesion by the amount exceeding their solubilities. Accordingly, by lowering the metal surface temperature by cooling the nozzle as per the present invention, reevaporation and condensation of drain on the nozzle surface can be prevented, and thereby precipitation and adhesion of scale itself can be prevented. It has been confirmed through demonstration tests that whereas in the conventional nozzle such a large amount of scale adheres to the nozzle within a short period of time that a throat portion of the nozzle is blocked, in the nozzle according to the present invention, scale does almost not adhere to the nozzle, and thus cooling of a nozzle is very effective as a countermeasure for preventing adhesion of scale. It is to be noted that preferably in order to fully reveal the effect of preventing adhesion of scale, it is desirable that with respect to the coolant water passageway 22 to be provided within the nozzle 21, the inlet side coolant water passageway 22a and the outlet side coolant water passageway 22b thereof should be disposed respectively at the central portion of the blade thickness (on the mean camber line), also the inlet side coolant water passageway 22a should be disposed on the upstream side by 50% or more of the blade width in view of the relation of the metal (blade) surface temperature versus the blade surface steam temperature shown in Fig. 5, and the bore diameter of the coolant water passageway is chosen to be about 1/3 of the blade thickness. As described in detail above, according to the present invention, by lowering a surface temperature of a nozzle by externally making coolant water pass through a coolant water passageway provided within the nozzle, reevaporation and condensation of drain on a blade surface can be prevented, thus precipitation of scale can be prevented, and so, scale would never adhere to the nozzle. Thereby, the present invention offers the advantages that lowering of an output power caused by reduction of a nozzle area and damage of a rotor caused by contact between scale and the rotor can be prevented.

Claims: Method for preventing adhesion of scale to a nozzle of a geothermal steam turbine, characterized in that coolant water is made to pass through the inside of the nozzle to make a nozzle surface temperature lower than a nozzle surface steam temperature, and thereby precipitation of scale onto the nozzle surface is prevented. A geothermal steam turbine nozzle, characterized in that within the nozzle (21) is formed a coolant water passageway (22) for cooling the surface of the nozzle. A geothermal steam turbine nozzle as claimed in claim 2, characterized in that said coolant water passageway (22) is divided into an inlet side coolant water passageway (22a) and an outlet side coolant water passageway (22b), and the inlet side coolant water passageway (22a) is disposed on the upstream side of a nozzle width. A geothermal steam turbine nozzle as claimed in claim 2 or 3, characterized in that a bore diameter of said coolant water passageway (22) is chosen to be about 1/3 of the thickness of the nozzle (21). A geothermal steam turbine nozzle as claimed in claim 2, 3 or 4, characterized in that said coolant water passageway (22) is connected to a coolant water feed source via coolant water passageways formed in turbine casings (23, 24).

Inventors: KOUICHI YOSHIDA C O NAGASAKI T, KOUICHI, YOSHIDA, C/O NAGASAKI TECHN.INST.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: F01D5
IPC codes: F01D5, F01D9