EP0602040B1 - Cooling a low-pressure steam turbine in ventilation mode - Google Patents

Abstract

The invention relates to a process for cooling a low-pressure steam turbine (1) in ventilation mode. The low-pressure steam turbine (1) has a closable inlet (2) through which steam can be admitted in the power mode and blocked in the ventilation mode, an outlet (3) communicating with a condenser (5) for condensing the steam into a condensate and a tapping (4) between the inlet (2) and the outlet (3). A tap line (6) is connected to the tapping (4) to evacuate steam and/or condensate in power mode. According to the invention, steam is supplied via a steam by-pass (12) to the tap line (6) to cool the low-pressure steam turbine (1) in ventilation mode. Condensate is also preferably supplied to the tap line (6). According to the invention, the cooling effect in the low-pressure steam turbine (1) is largely restricted to the heavily loaded components in ventilation mode and the cooling media are taken from resources which in any case, are available.

Description

The invention relates to a method for cooling a low-pressure steam turbine in ventilation mode, the rotor of the steam turbine being rotated without being subjected to steam to be expanded. Such ventilation operation comes, for. B. before in a multi-housing turbo set, in which a possibility for dissipating the steam otherwise to be relaxed in the low pressure turbine into a heating heat exchanger or the like is provided in front of a low pressure steam turbine.

In a multi-housing turbo set, it is common to couple the rotors of the individual turbines to one another and to connect them rigidly to the shaft of a generator or the like. Accordingly, all turbines of the turbo set rotate synchronously, including turbines that, for example, do not operate in power mode due to a different use of the steam.

In a low-pressure turbine operating in ventilation mode, there is no absolute vacuum, but rather a vapor atmosphere is present, the static pressure of which corresponds to the pressure prevailing in the condenser connected to the low-pressure turbine. The friction of the turbine blades on this steam (ventilation) can lead to considerable heat development, as a result of which the turbine can be heated up strongly, possibly even inadmissibly high. Cooling is therefore necessary to ensure safe ventilation operation.

As part of known cooling measures, condensate is injected into the outlet or, if the cooling capacity to be used must be particularly high, into the inlet of the turbine with atomization. The condensate evaporates with a drop in temperature and is therefore able to cool the ventilating turbine. Disadvantageous is that the cooling effect of the condensate injected at the outlet of the turbine is severely limited, or that the injection of condensate at the inlet of the turbine leads to an undesirable strong cooling of the turbine shaft. On the one hand, this greatly increases the cooling capacity to be used, and on the other, the turbine shaft is subjected to undesirable stresses due to the cooling.

If the injection takes place at the outlet, the cooling effect is also often restricted to parts of the turbine in the vicinity of the outlet; If the injection is carried out at the inlet, condensate, which agglomerates in the area of the inlet, can endanger the blading of the turbine through surge formation.

Thermal power plants with steam turbines are described for example in DE-OS 14 26 887 and DE 34 06 071 A1; the latter document relates specifically to cooling measures in a steam turbine, but cooling measures that aim at the power operation of the steam turbines. Information on the design of multi-housing steam turbine sets can be found, for example, in EP 0 213 297 B1, relating specifically to connecting means between the housings of a turbo set. General information on the implementation of steam power plants can be found in the "Handbook Series Energy", published by Thomas Bohn, Technical Publishing House Resch, Graefelfing, and Publishing House TÜV-Rhineland, Cologne - see in particular Volume 5, "Concept and Construction of Steam Power Plants", published in 1985. A condenser for the water-steam cycle of a power plant is described in DE 37 17 521 A1.

US Pat. No. 3,173,654 shows a method for cooling a steam turbine in ventilation mode, condensate being injected into the steam turbine through a special distributor pipe arrangement for cooling.

In view of the prior art, the invention is based on the task of specifying the most efficient and gentle cooling of a steam turbine in ventilation mode.

The inventive method for cooling a low-pressure steam turbine in ventilation operation, which low-pressure steam turbine has a shut-off inlet through which steam can be delivered for power operation and which is shut off in ventilation operation, an outlet which communicates with a condenser for condensing the steam to condensate, and has a tap between the inlet and the outlet, to which a tap line for discharging steam and / or condensate to a preheater is connected in power operation, is characterized in that by steam transfer of the tap line, and thus the tap, steam is delivered.

The steam introduced into the low-pressure steam turbine at the tap advantageously carries a certain proportion of finely divided condensate drops with it, since such condensate drops evaporate in the low-pressure steam turbine and can absorb considerable amounts of heat. Such a steam-condensate mixture can be obtained directly by removing the steam to be fed to the low-pressure steam turbine at a suitable location in the thermal power plant, by expanding the steam on the way to the tap or by providing the steam with condensate.

It is not necessary that the inlet of the low-pressure turbine to be cooled according to the invention immediately has a shut-off device - the inlet of the low-pressure turbine can also be shut off by connecting a medium-pressure turbine upstream of the low-pressure turbine and communicating with it. Turbine is shut off (and accordingly also ventilated). The turbine to be cooled according to the invention can also have a plurality of taps.

An essential feature of the invention is that the cooling steam or the cooling steam-condensate mixture of the turbine is not supplied at the inlet or at the outlet, but at a tap. In this way, the cooling in the turbine particularly benefits the radially outer ends of the blades, which are most heavily loaded by the friction on the steam in the turbine. According to the invention, the cooling effect is thus largely restricted to the areas of the turbine where it is desired; cooling of other components of the turbine, which is generally undesirable for the reasons mentioned, is avoided.

Another advantage of the invention results in steam turbine systems with nozzle lines which are guided vertically downwards by the tapped turbines. If a mixture of steam and condensate is added to such a tap line, only steam and sufficiently small condensate drops carried by the steam reach the turbine. Larger drops and condensate, which is deposited on the walls of the tap line, are carried downwards and do not reach the turbine. Accordingly, it is not necessary to provide special drainage devices in the turbine cooled according to the invention, with the tap line leading approximately vertically downwards, with which the condensate originating from the large drops and which hardly evaporates would have to be discharged from the turbine.

It is always particularly favorable to additionally supply condensate to the tap line in addition to the steam, in particular by injecting condensate into the steam line and / or into the bleed line through a condensate line. It is particularly advantageous to mix the condensate with the steam in an atomizer nozzle and out of this atomizer nozzle inject into the tap line. Condensate distributed in fine droplets - a droplet diameter smaller than about 0.1 mm is desirable - has a particularly high cooling effect due to the evaporation taking place in the turbine to be cooled while absorbing heat.

Condensate for delivery into the tap line is advantageously branched off from the main condensate line behind a condensate pump that conveys the condensate; in this way, a special conveying device for the condensate to be used in the context of the invention can be dispensed with.

The method according to the invention is particularly advantageously controlled in such a way that a temperature is measured in the ventilating, low-pressure turbine cooled according to the invention between the tapping and the outlet at a measuring point and, depending on this temperature, the delivery of the steam or the delivery of the steam -Condensate mixture, is regulated to the tap.

Advantageously, the delivery of steam or steam and condensate to the tap line is limited within the scope of the invention so that a steam flow is generated in the low-pressure turbine, which corresponds to a portion of the order of about 1% of the steam flow during power operation. A steam flow of this magnitude enables the turbine to be cooled to a sufficient extent according to the invention, but does not do so much work that the speed control of the turbo set, of which the cooled turbine is a component, could be impaired.

It is advantageous to use the steam used for cooling the low-pressure steam turbine (which advantageously contains a certain proportion of finely divided condensate drops) can be found in a condensate container, which is already provided in steam power plants and is used for collecting, heating and degassing the condensate. Heating steam of this type is generally to be supplied for the purpose of degassing the condensate; As a result, the thermodynamic conditions in the condensate container are always kept very constant. Therefore, the condensate container is a preferred reservoir for steam that can be used according to the invention, since the steam removed from the steam chamber of the condensate container is always replaced immediately by evaporation of the condensate, with no significant changes in the thermodynamic conditions in the condensate container due to the small amounts of steam required according to the invention. Steam from the condensate container is saturated due to the coexistence of steam and condensate, possibly even with finely divided condensate, and is therefore particularly suitable for use in the context of the invention.

It is also advantageous to remove the steam to be fed to the tap line according to the invention from a steam discharge line, through which the steam is conducted around the low-pressure turbine during ventilation operation. Such a steam discharge leads, for example, the steam from a high-pressure steam turbine upstream of the low-pressure steam turbine or arrangement of a high-pressure steam turbine and a medium-pressure steam turbine around the low-pressure steam turbine to a heating device or the like, where the steam may be cooled and is condensed. To obtain a steam-condensate mixture, it is particularly favorable to take the steam to be fed to the dispensing line from such a heating device.

It is also advantageous to directly or indirectly supply the steam to be supplied to the nozzle of a high-pressure or medium-pressure steam turbine upstream of the low-pressure steam turbine (For example, a preheater fed by this or the like). The steam removed from a point of the steam-condensate circuit upstream of the low-pressure steam turbine usually has a sufficiently high intrinsic pressure and can therefore be supplied to the nozzle without special pumps or the like being required for this. Steam that is under sufficiently high pressure can also be converted into a steam-condensate mixture by expansion, which is particularly favorable for the cooling of the low-pressure steam turbine according to the invention.

The further explanation of the invention is based on the exemplary embodiment shown schematically in the drawing. The only figure shows a section of a thermal power plant in which a working fluid, especially water, is guided in a closed cycle. The circuit comprises a high-pressure steam turbine 17, a low-pressure steam turbine 1, a condenser 5, a preheater 7 and a condensate tank 8; other components of the circuit, for example a boiler, are not shown. For the sake of clarity, only a single high-pressure steam turbine 17 is shown; Of course, the invention can also be used in circuits in which there are three or more turbine housings, or in which a turbine is not single-flow as shown, but is double-flow. The representation of a single preheater 7 is also not intended to preclude the applicability of the invention to circuits in which a plurality of preheaters 7 are provided. The components of the circuit shown are interconnected by steam connecting lines 18 or main condensate lines 9. A condensate pump 15 is inserted into the main condensate line 9. This condensate pump 15 is also shown as representative of a possibly existing plurality of such condensate pumps 15. Between the high-pressure steam turbine 17 and the low-pressure steam turbine 1 is in the steam connection line 18 a switch 19, which is usually formed with flaps, with the help of which the steam flowing out of the high pressure steam turbine 17 can be discharged through a steam discharge line 20 to a heating heat exchanger 21, so that depending on the setting of the Switch 19, the low-pressure steam turbine 1 is not subjected to steam. The heat exchanger 21 symbolizes a multitude of possibilities for using the steam that flows out of the high-pressure steam turbine 17. In the example shown, the steam supplied to the heat exchanger 21 is condensed therein and flows as condensate via a condensate return line 22 to the main condensate line 9 upstream of the preheater 7.

The low-pressure steam turbine 1 should be rigidly coupled to the high-pressure steam turbine 17, so that the rotors of both steam turbines 1 and 17 run synchronously. Thus, when the steam flowing out of the high-pressure steam turbine 17 is discharged through the steam discharge line 20, the low-pressure steam turbine 1 rotates at idle; since there is a static pressure in this low-pressure steam turbine 1 which corresponds to the pressure of the steam in the condenser 5, friction occurs. However, there is no heat dissipation due to the energy loss of the steam released in power operation in the low-pressure steam turbine 1. Accordingly, the provision of cooling may be necessary to enable ventilation operation of the low-pressure steam turbine 1.

The low-pressure steam turbine 1 is pressurized with steam at an inlet 2, and the expanded steam leaves the low-pressure steam turbine 1 at an outlet 3 to the condenser 5. To discharge condensate, which is present in the low-pressure steam turbine 1 during power operation by relaxing the work performing steam already arises, or for tapping steam for heating a preheater 7 a tap 4 is provided between inlet 2 and outlet 3, where a tap 6 is connected. The tap 6 leads from the tap 4 to the preheater 7, where the tapped working fluid is used to preheat the condensate from the condenser 5. There are several options for removing the working fluid drawn off at the tap 4 from the preheater 7; For example, after flowing through the preheater 7, it can flow through further preheaters (not shown) and finally be combined with the condensate in the main condensate line 9. The condensate flows through the main condensate line 9 to a condensate container 8 (which is sometimes also referred to as a "degasser"). In the condensate container 8, the condensate is heated by means of steam which is introduced into the condensate through a heating steam line 10 below the condensate level 26. This heating serves, among other things, from the condensate gases, such as. B. remove oxygen. Above the condensate level 26 there is a steam space 11 filled with steam in the condensate container 8. Steam is removed from this steam space 11 and fed to the dispensing line 6 through a steam transfer line 12. Furthermore, the tap 6 flows through a condensate transfer line 13 condensate; Steam and condensate are injected together into the nozzle 6 through a schematically indicated atomizer nozzle 14. A mixture of steam and fine condensate drops is formed in the nozzle 6, which flows into the low-pressure steam turbine 1 for the purpose of cooling at the tap 4. The condensate transfer line 13 opens behind the condensate pump 15 into the main condensate line 9. It is not necessary to supply the condensate and steam to the dispensing line 6 through a single atomizing nozzle 14; Steam and condensate can also be fed to the tap 6 independently of one another. To limit the steam flow in the low-pressure turbine, a critical orifice may be provided in the steam transfer line 12 be provided. To regulate the cooling of the low-pressure steam turbine 1 during ventilation operation, without work, a measuring point 16 is provided in this between the tap 4 and the outlet 3, at which a temperature measurement is carried out; this temperature measurement is evaluated by means (not shown, known per se) and fed via a control line 25 to a steam control valve 23 in the steam transfer line 12 or a condensate control valve 24 in the condensate transfer line 13.

Finally, it should be noted that the steam transfer line 12 and the condensate transfer line 13 do not necessarily have to be completely shut off during the power operation of the low-pressure steam turbine 1; Via small bypass lines with which steam control valve 23 or condensate control valve 24 are bypassed, a small flow of steam or condensate to the tap line 6 can be maintained, which leads to keeping the steam transfer line 12 and the condensate transfer line 13 and. U. can be advantageous.

If a condensate container 8 is not available for the extraction of steam for feeding into the tap 4 of the low-pressure steam turbine 1, such steam can be supplied to a steam connecting line 18 between the high-pressure steam turbine 17 and the low-pressure steam turbine 1 or the steam discharge line 20, possibly even be removed from the heat exchanger 21; removal from a preheater (not shown) associated with the high-pressure steam turbine 17 is also conceivable. Since the high-pressure steam turbine 17 also operates in the ventilation mode of the low-pressure steam turbine 1 in the power mode, it can be assumed that the thermodynamic conditions in the high-pressure steam turbine 17 as well as in the auxiliary devices communicating directly therewith are very stable, so that they for inclusion in the invention System for cooling the ventilating low-pressure steam turbine 1 can be easily included.

The method according to the invention for cooling a low-pressure steam turbine in ventilation operation is particularly energy-saving, since it largely uses available resources and avoids material stresses in that the cooling effect only affects areas of the low-pressure steam turbine where it is desired.

Claims (10)

1. Method for cooling a low pressure steam turbine (1) operating in ventilation mode, which low pressure steam turbine (1) has a closable inlet (2) through which steam can be delivered when operating in power generation mode and which is blocked off when operating in ventilation mode, an outlet (3) which communicates with a condenser (5) for condensing the steam to condensate and, between the inlet (2) and the outlet (3), a bleed port (4) to which is connected a bleed pipe (6) for diverting steam and/or condensate to a preheater (7) during operation in power generation mode, in which method steam is supplied to the bleed pipe (6) through a steam transfer pipe (12).
2. Method according to Claim 1, in which condensate is additionally supplied to the bleed pipe (6) through a condensate transfer pipe (13).
3. Method according to Claim 2, in which the condensate is sprayed into the steam transfer pipe (12) and/or into the bleed pipe (6).
4. Method according to Claim 3, in which the condensate is sprayed through an atomizing nozzle (14) into the bleed pipe (6), and is mixed with the steam and atomized in the atomizing nozzle (14).
5. Method according to one of Claims 2 to 4, in which the condensate to be supplied to the bleed pipe (6) is diverted from the main condensate pipe (9) behind a condensate pump (15).
6. Method according to one of the preceding claims, in which :

   a) a temperature is measured at a measuring station (16) in the low pressure turbine (1) between the bleed port (4) and the outlet (3);

   b) the supply of steam, or alternatively the supply of steam and/or condensate, to the bleed pipe (6) is regulated as a function of the temperature.
7. Method according to one of the preceding claims, in which the supply of steam, or steam and condensate, to the bleed pipe (6) is limited in such a way that a flow of steam is produced in the low pressure turbine (1) which is, at most, approximately 1% of the flow of steam in the low pressure turbine (1) when operating in power generation mode.
8. Method according to one of the preceding claims, in which the condensate is supplied from the condenser (5) through the preheater (7) via a main condensate pipe (9) to a condensate tank (8), in which the condensate can be heated by the introduction of steam through a heating steam pipe (10) and from which steam is extracted from a steam space (11) and supplied to the bleed pipe (6).
9. Method according to one of the preceding claims, in which the steam to be supplied to the bleed pipe (6) is extracted from a steam by-pass pipe (20) through which steam is supplied during ventilation operation of the low pressure steam turbine (1).
10. Method according to one of the preceding claims, in which the steam to be supplied to the bleed pipe (6) is extracted from a high pressure steam turbine (17) upstream of the low pressure steam turbine (1).

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