Abstract:
A method and a device are for operating a steam turbine in which includes several no-load or light-load phases. All phases are supplied with steam in order to ensure good preheating. The supply of a phase is selected in such a way that the phase produces the least possible output, preferably no output. The enthalpy differential between the entrance to and exit from the phase is thus preferably reduced to zero.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/EP01/05747 which has an International filing date of May 18, 2001, which designated the United States of America and which claims priority on European Patent Application number EP 00111692.0 filed May 31, 2000, the entire contents of which are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention generally relates to a method for operating a steam turbine, which has a plurality of stages. More preferably, it is directed to operating during idling or low-load operation, with steam being admitted to all the stages. It also generally relates to a device for distributing steam to individual stages of a steam turbine, preferably during idling or low-load operation, and in particular for carrying out the method mentioned. 
     BACKGROUND OF THE INVENTION 
     Steam turbines and their design problems are, in particular, presented in Prof. Dr.-Ing. H.-J. Thomas, “Thermische Kraftanlagen” [Thermal Power Installations], 2 nd  Edition, 1985, Springer-Verlag. Details for calculating the enthalpy and further thermodynamic parameters can, for example, be extracted from “Technische Formeln für die Praxis” [Technical Equations for Practical Use], 24 th  Edition, 1984, VEB Fachbuchverlag, Leipzig. 
     Further reduction in the starting times of steam turbines is continuously required. Shorter starting times can only be achieved if all stages have, as far as possible, the largest possible mass flow admitted to them at the same time. It is only by this admission that the preheating of the steam turbine necessary for the shortest possible starting time can be achieved. The power generated by the turbine due to the mass flow being admitted must not, however, exceed the idling load. If the idling load is exceeded, uncontrolled increases in the rotational speed of the steam turbine can occur. The total mass flow which can be supplied overall is, therefore, limited. 
     High windage powers occur at the exhaust-steam end of the high-pressure stage (HP stage) during idling or low-load operation. These high windage powers lead to high temperatures at the exhaust steam end. A large part of the mass flow must therefore be supplied to the high-pressure stage in order to prevent unallowably high temperatures. The low-pressure stage (LP stage), however, also demands a comparatively high mass flow, in particular where large low-pressure stage cross sections and new materials, for example titanium for the blading of the low-pressure stage, are employed. The medium-pressure stage (MP stage) also requires a part of the mass flow. 
     If the necessary, high mass flow is admitted to both the high-pressure stage and the low-pressure stage, the overall power generated is distinctly located above the idling power. Attempts have therefore been made to adjust the distribution of the mass flows, by use of preliminary calculation, in such a way that idling operation becomes possible. In this case, the mass flows through the high-pressure stage and the medium-pressure/low-pressure stage were distributed in such a way that the power was not located above the idling power required. 
     It was only overheating of the high-pressure stage which was avoided by monitoring the temperature occurring at the exhaust-steam end. Only a small mass flow was left for the medium-pressure/low-pressure stage. If the mass flow for the medium-pressure/low-pressure stage was not sufficient or if the temperature at the exhaust-steam end of the high-pressure stage exceeded a specified value, rapid partial shut-down of the high-pressure stage was initiated. In consequence, the high-pressure stage, at least, was only inadequately preheated. Because of this inadequate preheating, a longer starting time was necessarily involved. 
     SUMMARY OF THE INVENTION 
     An object of an embodiment of the present invention may be, therefore, to make available a method and a device which permit good preheating of all the stages of a steam turbine without exceeding the load at idling or that in low-load operation. 
     In a method, this object may be achieved—according to an embodiment of the invention—by an admission to a stage being selected in such a way that this stage delivers as little power as possible. 
     Steam can be admitted to all the stages of the steam turbine by way of the method according to an embodiment of the invention. The admission takes place in such a way that a stage delivers as little power as possible. This stage therefore generates only a small amount of power so that a comparatively large mass flow can be admitted to the remaining stages. All the stages are therefore reliably preheated so that short starting times can be realized. 
     The enthalpy of the steam at inlet into this stage and the enthalpy of the steam at outlet from this stage may be advantageously determined and the enthalpy difference between inlet and outlet may be advantageously minimized. The power delivered by a stage may be directly proportional to the enthalpy difference. By minimizing the enthalpy difference, therefore, the power delivered can be minimized at the same mass flow or even an increased mass flow. 
     According to an advantageous development, the temperature of the steam at inlet into this stage and the temperature of the steam at outlet from this stage may be measured and the enthalpy difference between inlet and outlet is determined, in particular calculated, from these temperatures. The temperature of the steam is easy to measure so that the measurement complexity is reduced. 
     In order to increase the accuracy, the pressure drop between the inlet into this stage and the outlet from this stage may be, advantageously, additionally measured and may be taken into account in the calculation of the enthalpy difference between inlet and outlet. The enthalpy of the steam flowing through the stage depends on both the pressure and the temperature. The enthalpy difference can be more accurately determined, in particular calculated, by taking account of pressure and temperature than it can by taking account of the temperature alone. 
     In another advantageous development, the enthalpy of the steam at inlet into this stage and the enthalpy of the steam at outlet from this stage are measured. A suitable method for measuring the enthalpy of steam is, for example, described in WO 99/15887 by the present applicant. This publication refers to DE-B 10 46 068 for determining the enthalpy of live steam, i.e. of superheated steam. In contrast, WO 99/15887 relates to a measurement and calculation method for determining the enthalpy of wet steam. In order to extract a sample, a partial volume flow of the wet steam is brought together with a reference gas so as to form a mixture and so that the liquid constituents of the partial volume flow evaporate completely. Using measured physical parameters, the enthalpy of the reference gas and the enthalpy of the mixture are determined and the enthalpy of the wet steam is calculated from them. The information revealed by WO 99/15887 and DE-B 10 46 068 is to be expressly encompassed in the content of the present application. 
     In an advantageous embodiment, the mass flow supplied to this stage is modified in order to minimize the enthalpy difference. The mass flow supplied generates power due to expansion in the front part of this stage. At the exhaust-steam end, the mass flow is compressed again and consumes power by this. By modifying the mass flow supplied, a balance can be found between the two processes and the enthalpy difference can be minimized by this. 
     The admission to this stage is advantageously regulated in such a way that this stage does not deliver any power. For this purpose, it is necessary to regulate to zero the enthalpy difference between inlet and outlet. The mass flow through this stage therefore provides no power and is only used for preheating. It is then possible to admit the complete mass flow to the further stages of the steam turbine in order to overcome the idling load. The maximum mass flow is therefore admitted to all the stages and they are preheated in an optimum manner. The starting times can therefore be substantially reduced. 
     In a device, for the achievement of an object, provision may be made according to an embodiment of the invention for the device to have a first measuring station for recording the enthalpy of the mass flow supplied to a stage, a second measuring station for recording the enthalpy of the mass flow emerging from this stage, a comparison unit for determining the enthalpy difference and a unit for adjusting the mass flow supplied to this stage. 
     The device according to an embodiment of the invention permits a determination of the enthalpy difference, either by use of a direct measurement of the respectively present enthalpies or by use of a measurement of parameters relevant to the enthalpy, such as pressure and temperature. The enthalpy difference determined can be regulated by use of the unit for adjusting the mass flow supplied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail below using exemplary embodiments which are represented in a diagrammatic manner in the drawings. In the drawings, the same designations have been used for similar components or components which are functionally identical. In the drawings: 
         FIG. 1  shows a diagrammatic representation of a steam turbine; and 
         FIG. 2  shows an enlarged representation of the high-pressure stage, in a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  represents a steam turbine  10  with a high-pressure stage  11  and a combined medium-pressure/low-pressure stage  12 . The stages  11  and  12  are connected together by means of a shaft  13 , which drives a generator  14  in order to generate electrical current. The shaft  13  and the generator  14  can be decoupled from one another by use of an appliance, which is not represented in any more detail. A steam generator  15  is used for generating the steam necessary for operation and during idling. A condenser  16  for condensing the emerging steam is provided downstream of the medium-pressure/low-pressure stage  12 . The condensate is returned to the steam generator  15  via pumps  17 , a medium-pressure/low-pressure preheater  18  and two high-pressure preheaters  19  and  20 . A reheat system  21  and a feed-water preheating system A, B, C, D, n are provided to increase the efficiency during operation. The components mentioned, and their functions, are known to the specialist so that it is possible to dispense with a more detailed explanation. 
     The steam generator  15  makes available a mass flow {dot over (m)}. The mass flow {dot over (m)} is subdivided upstream of the high-pressure stage  11 . A first mass flow {dot over (m)} 1  is supplied to the high-pressure stage  11 , while the remaining mass flow {dot over (m)} 2  is supplied directly to the reheat system  21 , bypassing the high-pressure stage  11 . A mass flow {dot over (m)} 3  is admitted to the medium-pressure/low-pressure stage  12 . The remaining mass flow {dot over (m)} 4  is guided directly to the condenser  16 , bypassing the medium-pressure/low-pressure stage  12 . Valves  22 ,  23  and  24  are used for adjusting the mass flows {dot over (m)} 1  and {dot over (m)} 3 . The mass flows {dot over (m)} 2  and {dot over (m)} 4  follow automatically from the adjustment of the mass flows {dot over (m)} 1  and {dot over (m)} 3 . 
     A first measuring station  25  is provided upstream of the high-pressure stage  11  and a second measuring station  26  is provided downstream. In the case of the usual assumption of an isentropic expansion, the power P generated by the high-pressure stage  11  is given by:
 
 P={dot over (m)}   1 ( h   2   −h   1 )= {dot over (m)}   1   Δh  
 
where
         {dot over (m)} 1  is the mass flow   h 1  is the enthalpy at measuring station  25     h 2  is the enthalpy at measuring station  26     Δh is the enthalpy difference between measuring stations  26  and  25 .       

     Because the mass flow {dot over (m)} 1  through the high-pressure stage  11  is constant in steady-state operation, the power P is directly proportional to the enthalpy difference Δh. With the exception of mechanical losses, this power is also delivered. In order to minimize the power P delivered, it is therefore necessary to minimize the enthalpy difference Δh, if possible bringing it to Δh=0. 
     In the exemplary embodiment represented in  FIG. 1 , the temperature T 1  of the mass flow {dot over (m)} 1  entering as steam into the high-pressure stage  11  is measured at the measuring station  25 . A temperature measurement takes place downstream at the measuring station  26 , a temperature T 2 , the exhaust steam temperature from the high-pressure stage  11 , being determined at this measuring station  26 . The pressure difference Δp between the measuring stations  25  and  26  is advantageously determined simultaneously by use of suitable pressure measuring appliances (not specified in any more detail). The measured temperatures T 1  and T 2 , together with the measured pressure difference Δp, are supplied to a control unit  27 , which calculates the enthalpy difference Δh between the measuring stations  25  and  26 . 
     The valve  22  is activated as a function of the result of the calculation, so that the mass flow {dot over (m)} 1  is regulated as a function of the calculated enthalpy difference Δh. This balance for the high-pressure stage  11  is essentially achieved by the exhaust steam temperature T 2  being held (by the control circuit  27 , which provides a valve trimming dependent on the enthalpy) to a value which corresponds to the throttled live steam temperature. A mass flow {dot over (m)} 1  with a correspondingly throttled temperature T 1  is therefore made available and supplied to the high-pressure stage  11  by throttling the steam mass flow {dot over (m)} by use of the valve  22 . The throttling action (throttling effect) of the valve  22  is, in this arrangement, employed in a targeted manner in order to adjust the desired temperatures T 1  and T 2 . 
     In this procedure, a calculation of the enthalpy difference Δh is understood to mean not only the actual calculation of this enthalpy difference Δh but also any other appropriate process, by which the enthalpy difference Δh can be minimized. As an example, a comparison can be made with a table which is programmed within the control unit  27 . 
     The enthalpy difference Δh determines the power P generated by the high-pressure stage. By means of the valve  23 , therefore, the control unit  27  controls the mass flow {dot over (m)} 3  through the medium-pressure/low-pressure stage  12 , corresponding to a specified idling load and the power generated by the high-pressure stage  11 . Further measuring stations for recording temperature and/or pressure can be provided downstream of the reheat system or at other suitable positions in order to increase the accuracy. 
       FIG. 2  shows an enlarged representation of the high-pressure stage  11 , together with the associated control of the mass flow {dot over (m)} 1 . In the exemplary embodiment of  FIG. 2 , the enthalpies h 1  and h 2  are measured directly at the measuring stations  25  and  26  and the enthalpy difference Δh is subsequently formed in the control unit  27 . The valves  22  and  23  are activated by the control unit  27  on the basis of the enthalpy difference Δh. By this, the power P delivered by the high-pressure stage  11  is minimized and the mass flow {dot over (m)} 3  through the medium-pressure/low-pressure stage  12  is simultaneously maximized. 
     The admission, provided according to an embodiment of the invention, to the high-pressure stage takes place in such a way that as little power P as possible, and advantageously no power at all, is delivered. The method permits an admission to all the stages  11  and  12  of the respectively maximum possible mass flow {dot over (m)} 1 , {dot over (m)} 3 . By this, good preheating of all the stages  11  and  12  and, therefore, short starting times are achieved. Exceeding the idling load and an unallowable increase in the rotational speed of the steam turbine  10  are reliably avoided. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.