Abstract:
The invention provides a method for operating a power generating assembly which assembly comprises a steam generator and a turbine, and said method comprises controlling the quantity of heat released by the steam generator by a signal which is representative of the difference between the predicted stresses which will exist at the instant to+T on that part of the turbine which undergoes the greatest thermal stress and the maximum permissible stress on this same part at the same instant, to being the present instant and T being a period of time close to the time constant of the power production assembly.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for operating a power generating assembly. 
     BACKGROUND OF THE INVENTION 
     A power generating assembly comprises for example a steam generator (nuclear boiler or reactor) associated with a turbine. An alternator which transforms the mechanical power of the turbine into electric power is generally coupled to this turbine. 
     A steam generator and turbine assembly is controlled by varying the rate at which heat is supplied to the generator or steam is supplied to the turbine, or by varying both of these factors simultaneously, to adjust the power supplied by the assembly to match the power required by the user. 
     This problem of the adjusting the power supplied by the assembly to the power requirement gives rise to difficulties. 
     Indeed, all transient operating states of the assembly such as a change in the rate of steam delivery or a change in the temperature of the steam give rise to mechanical and thermal stresses at various components of the turbine. 
     In particular, the disks of the first wheels of the turbine, or their extension in the central shaft of the rotor, are subjected to the greatest stresses; transient states which are too sudden can generate thermal stresses which are too high, causing the machine to break, particularly at the disks. 
     Until quite recently, operators of such assemblies only had empirical guide lines, based on experience, for operating such assemblies under operating conditions not envisaged in the operation manuals. 
     In practice, the said operators allowed for large safety margins; the result of this was that the power production assembly did not respond to the user&#39;s power requirements as quickly as would have been possible without exceeding the maximum permissible stresses for the equipment. 
     To facilitate the task of the operators of power production assemblies, means have been devised which make it possible to calculate at any instant the difference between the current stress at the most vulnerable point of the machine and the maximum permissible stress at the same point. But knowledge of this difference at a present instant is insufficient for operating an installation properly; indeed, the fact of having the value of such a difference available at a present instant does not make it possible to predict with certainty that the stress will remain acceptable without damage to the equipment in a near future, since the time constants of such assemblies are generally fairly large (about ten minutes). 
     The increased power demand of the modern world makes it essential that power production assemblies operate at the limits of their capabilities; in particular, it is necessary for an assembly to start up as quickly as the maximum permissible stresses allow. It is also desirable that each change in the power required should give rise to a transient state of the machine which lasts for as short a time as possible while guaranteeing that the stresses which the equipment undergoes remain within the permissible limits. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of operating a power generating assembly, said assembly comprising a steam generator and a turbine. The method comprises controlling the quantity of heat released by the steam generator by a signal which is representative of the difference between the predicted stress which will exist at the instant to+T on that part of the turbine which undergoes the greatest thermal stress and the maximum permissible stress on this same part at the same instant, to being the present instant and T being a period of time close to the time constant of the power production assembly. 
     The invention will be explained hereinbelow by a detailed description of an example of the method applied to a generator-turbine set and with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a power production assembly; 
     FIG. 2 is a graph illustrating the method of the invention; and 
     FIG. 3 is a graph illustrating a variant of the method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a power production assembly comprising a boiler 1 supplying at its output a fluid (steam) whose temperature and flow rate can be modified by means schematically shown respectively by a cock 2 for the input of fuel 3 and an inlet cock 4 to a turbine 5. The first blade wheel of the turbine is shown schematically at 6. 
     The greatest stresses are developed at the disks of this wheel. The turbine rotates at a speed of N rpm and produces a power W. 
     A signal representative of the operation state of the assembly is formed as described hereinbelow with reference to the graph of FIG. 2. In this graph, time t is plotted along the X-axis and stress is plotted along the Y-axis. 
     At the present instant to, the point Ao is representaive of a present stress on the disk of the wheel (6) and the point Bo is representative of the maximum permissible stress at the same point at the same instant. 
     Although the disk is not accessible to direct measurement, it is possible to calculate the Y-axis values of Ao and Bo. For this purpose, the pressures and temperatures upstream of the turbine and the pressure downstream from the wheel (6) are measured. From these measurements, it is easy to determine, for example, by means of Stodola&#39;s equation, the flow rate of the fluid and the characteristics of this fluid (pressure, temperature and coefficient of temperature exchange with the metal of the disk). 
     The knowledge of the above data makes it possible to calculate the temperature distribution in the rotor and to calculate therefrom an average temperature. The thermal stress can then be calculated from the data supplied by the manufacturer of the machine, such as the Young&#39;s modulus of the material, its coefficient of thermal expansion and its Poisson&#39;s ratio. 
     The time which elapses after the present instant to until the instant to+T, where T is of the order of magnitude of the time constant of the installation, (for example 10 minutes) is divided into n equal intervals t, of about 10 seconds each for example; the probable future stresses Ak and Bk are determined for each of the instants to+kt. Thus, for each instant after to, it is possible to calculate the difference Bk-Ak between the maximum permissible stress Bk and the predicted stress Ak. In particular, importance is attached to the difference between the maximum permissible stress Bn at the instant to+T and the predicted future stress at that instant. 
     The future stresses (predicted and maximum permissible) are determined by assuming, at each step of the calculation, that the change of the parameters used (temperature, pressure and flow rate of the input fluid) can be deduced from their change since the preceding instant to. Of course, account must be taken of any predicted change in these parameters that is expected to occur during the period to to to+T because of some variation in the load or in the power supplied. 
     The signal used in the method is proportional to K where: ##EQU1## where Bn is the maximum permissible stress at the instant to+T; to is the present instant; T is equal to or close to the time constant of the installation; An is the predicted future stress at the instant to+T, and δ is equal to +1 or -1 according to whether the installation is respectively being heated or being cooled. 
     The knowledge of the signal K is used as follows by the operator at the instant to: 
     (1) In a period during which the installation is being heated (starting up or increasing the power supplied): 
     Where K is positive, the operator knows that he has a safety margin which allows him to increase the flow rate of the fuel to the boiler and/or steam to the turbine; 
     Where K is zero, the operator knows that he has no safety margin and that he should in no case increase the power of the machine; and 
     Where K is negative, there is a danger of damage at the instant to+T. This case will not occur in practice if the operator has taken care to use the operating method of the invention from the moment the installation was started, providing however that the steam generator has not been subjected to some accidental transient state. 
     (2) In a period during which the installation is being cooled (reducing the power): 
     Where K is positive, the operator knows either that he has a safety margin allowing him to accelerate the rate of power reduction and to reduce the flow rate of fuel to the boiler and/or steam to the turbine; 
     Where K is zero, the rate of power reduction is correct; and 
     Where K is negative, the operator must slow down the rate of power reduction and/or of temperature reduction, but as previously explained this situation should not arise. 
     To operate the power generating assembly properly, this signal is generated periodically, for example 3 to 4 times at regular intervals during a period T which is substantially equal to the time constant of the installation. 
     As a function of the modifications made by the operator to the operation of his power generating assembly, the curves A(t) and B(t) could begin by converging and then diverge during a period to to to+T. 
     An example is given in FIG. 3. 
     The true stress increases up to Am, then decreases. The maximum permissible stress decreases down to Bm, then increases. It is interesting for the operator to know the instant Tm of the minimum difference Bm-Am and the value of this difference. 
     If this time is close to to, the operator&#39;s actions can have little useful effect; if this time is close to T or after T, his freedom of action is great. 
     The signal K can be used in an automatic control unit of a power generating assembly. 
     During the heating period, the amplitude of the appropriate modification to the heating rate is an increasing function (for example directly proportional) to the signal K and a decreasing function (for example inversely proportional) to the time Tm defined above. 
     The direction of the action is reversed during cooling. 
     Further, it is possible for the operator of the assembly to replace future behaviour estimated on the basis of past changes with a future behaviour which he is going to impose by his actions. The parameters will be estimated during the period to to to+T taking into account the data supplied by the operator and the present known data multiplied by the present rate of change for those parameters which have a time constant of more than 30 seconds.