EP0537307A1

EP0537307A1 - Method for maintaining warm turbines

Info

Publication number: EP0537307A1
Application number: EP19920907519
Authority: EP
Inventors: Cruz Antonio Lara
Original assignee: SEVILLANA DE ELECTRICIDAD SA
Current assignee: SEVILLANA DE ELECTRICIDAD SA
Priority date: 1991-03-26
Filing date: 1992-03-25
Publication date: 1993-04-21
Also published as: ES2029430A6; WO1992017687A1

Abstract

System for keeping warm high power turbines, essentially steam turbines, based on the proper capactity of predicting the possible thermal behaviours of the machine as a function of temperature variations during cooling or heating, the system manipulating the temperature of the rotor and of the external shell of the turbine in order to obtain and keep within the turbine an order temperature appropriate to perform a fast start-up without requiring a preheating phase. The system is comprised of an assembly of heating blankets provided with electric resistances and placed against the external surface of the turbine, a data acquisition and automatic and computerized control system, and an electric power supply and distribution system.

Description

The present invention, as indicated in its title, is related to a system for maintaining warm high power turbines, essentially steam turbines.
A system which by means of electric heating blankets attached to the exterior surface of a turbine and controlled by an automatic and computerized mechanism capable of predicting the possible behaviours of the machine when said machine undergoes changes of temperature, offers the possibility of maintaining a determined thermal level in the rotor, so that the start-up time in cold may be reduced, saving thereby fuel and electrical energy, and also reducing or even eliminating cycles of warming up-cooling off, which are normal in the operation of the machine thus making longer the life of the same.

BACKGROUNG OF THE INVENTION AND PRIOR STUDIES

The operating conditions of Thermal Power Plants have been substantially changed as economic and energetic diversities are the criteria which establish the planning of electric energy production.
These criteria have transformed the regime of exploitation of the plants, such that from operating in base with constant charge have passed to supply demand peaks, thus operating with variable charge. This implies that the number of start up-shutdown cycles of the plant is higher than that initially planned, moreover the efficiency factor as a result of operation based on partial charges, is negatively influenced.
Studies carried out on the case show that the Thermal Groups are not prepared to endure this mode of operation (cyclic operation and rapid start-ups), since they are not equipped with the necessary elements which make possible the prevention or reduction of the anomalous effects which are produced in the equipments by the continuous and abrupt variations of temperature and flow involved in such operations.
The Thermal Groups not originally designed for cold start-ups, consume the time and the fuel during start-up in two clearly different phases.
In the first phase which may be called "boiler phase" the necessary energy is supplied to the boiler in order to increase its energetic level based on heating a large mass up to a determined value, the efficiency of which is certainly improvable, although it can be considered as acceptable.
In the second phase which may be called "turbine phase" the turbine of a relatively small mass is heated, with a small quantity of value (approximately 3% of the nominal) proceeding from the boiler, attending to the thermal gradient, especially to that of rotor, and the differential expansion between fixed and movable parts.
During all the time of heating the turbine the boiler keeps burning with the mere aim of supplying the required steam, which involves the consequent costs of fuel due to the low thermal efficiency of the process where the turbine is included.
At present there is a considerable amount of electric power generation which is inactive, such inactivity affects especially the fuel-oil and the gas groups and occasionally the carbon groups.
Such power, available as reserve in Switching Rooms, may be needed either by demand peaks, or by problems in interconnection lines or by unavailability of other groups; and in the majority of the cases when needed, it is required as fast and as safe as possible.
The objective of the invention is to implant a system for maintaining warm the high and medium pressure bodies of steam turbines, thus avoiding heating by cyclic steam, and by means of an arrangement of electric resistances adequately adapted.
With the system for maitaining warm turbines the following adavantages are obtained:

reducing the cold start-up time, while maintaining in the rotor of the turbine the thermal level which corresponds to a warm start-up.
prolonging the useful life of the turbine by controlling the thermal tensions and the deformations produced during the shutdown of the turbine, monitoring the external temperature differences and requiring conection in the resistances which thermally homogenize the turbine.

A turbine of this type has been tested and installed in the United States, although only for low power turbines, which can not satisfy the demands of higher power turbines. The extrapolation from low power to high power applications, which is the object of the present invention is the result of an extensive program of research and investigations for the development of the new system, intensively performed on the basis of an exhaustive thermoelastic analysis which has served as a basis so that the new system has the sufficient data on the nature and functioning of the machine and thus is capable of foreseeing or predicting the possible behaviours of the same. The system has, as well, a computer program specially elaborated in order to satisfy the practical demands of the turbine.

DESCRIPTION OF THE INVENTION

The general rules of operation of a turbine indicate that in order for it to come to an adequate state of start-up, the rotor has to reach certain level of temperature. Said temperature is a set point in such a manner that in lower temperatures the machine is considered to be cold, and before being coupled to the network it is necessary to warm up and homogenize temperatures by running steam during the time and by the velocity that are adequate for the case.
On the other hand, if the temperature is higher than that of said set point, the machine is considered to be warm, and it does not need any delay time; (the only requirement in this case is that the temperature of the principal steam admission valves are higher than or equal to that of the steam saturation in the admission pressure).
The need to warm up the rotor before the star-up, arises from the fact that once started, the different fixed and moveable parts of the turbine undergo different longitudinal stretchings or radial strains, for which a starting in such conditions can result in frictions between said parts producing serious damages in the machine. On the other hand said warm-maintaining has to obey the requirements established by the behaviours of the machine as for the stretching of heated pieces, which behaviours varying from one piece to another. This fact implies that the system is provided with certain intelligence; in the first place in order to predict the behaviour that the pieces would demonstrate undergoing increase and decrease of temperature, and in the second place, to launch the relevant orders to one or the other part of the machine according to its own conclusions based on foreseeing said behaviours.
It has to be noted that the dynamic thermal behaviour of the turbine presents two peculiarities that condition the design of the control strategy:

The exterior shell warms-up (or cools down) much more rapidly than the rotor and the interior housing.
The lower part (base) of the machine presents higher heat loss than the upper part (cover).

While the first question tends to provoke differential expansion problems, the second is related to radial strains (humping).
The hereby proposed system for maintaining turbines warm is ment to satisfy such requirements maintaining the temperature of the machine on the adequate level so that said machine is continuously kept in the appropriate condition for a rapid start.
The system consists of three different parts:

1.- Heating blankets
2.- The electric energy supply and distribution system
3.- The data acquisition and control system.

1.- The heating blankets consist of flexible electric resistances located inside a metalic cord made of inconel thread . Said heaters are clamped to a metalic mesh which forms part of the inner isolating blanket and are placed on the shell of the turbine. Upon connecting the electric energy, the resistances are heated and by the aid of the isolating blankets the resulting temperature is maintained on the machine.
2.- The supply and distribution system essentially consists of a tarnsformer and a switching module capable of activating and deactivating a number of relays connected to the resistances of the electric energy blankets. The characteristics of the system are determined as a function of the necessary heating energy for heating the turbine and maintaining warm the same at the set point temperature. This temperature is established with regard to the manufacturer data related to start-ups as a function of the temperature of the rotor.
The requirements of the heating flux are defined, in addition to the available data, by the results of a prior thermoelastic analysis carried out on the turbine in order to study the response of the machine in situations called short rotor (average temperature in the shell higher than that in the rotor) and by the cooling of the lower part of the shell which is faster with respect to the higher part; this effect being called humping.
3.- The data acquisition system (DAS) allows to know in any instant the thermal and mechanical state of the turbine, regardless of the operative situation of the same.

The DAS consists of an automatic and computerized system , specifically useful for such an application, and provides flexibility and simplicity in handling the same.
The equipment for acquisition of data and control comprises essentially of a computer capable of running the relevant program either for the acquisition of data or for the control having its peripheral units such as printer, and plotter.
The temperatures are read by means of thermocouples which are distributed on the shell, on the main steam and reheated steam valves and on the piping instalations.
Based on read temperatures as well as prefixed set points and the utilized control rules, the DAS decides in each interval of control which zones should have voltages and which ones should not. Based on this information it gibes orders of activating the relays corresponding to the resistances of the selected zones.
The DAS shows all the electric signals proceeding from sensors upon a specific frequency, it digitizes said signals and converts them into engineering units, it further updates them automatically and presents them using the screen, the printer and the plotter. Its function also consists of supplying data to the system in order to control the heating power which is to be applied to the exterior shell, and identifying faults when some anomalous circumstance is produced due to erroneous temperature signals, activating alarms and even disconnecting completely the resistances when the number of the erroneous thermocouples reaches a predetermined value.
The DAS provides thus everything that is strictly necessary for the control of the heating blankets, and permits to follow the development of the thermal and mechanical state of the turbine in a start-up, during a normal operation or during the cooling after the triggering of the machine.
In order to specially distribute the heating power, the outer surface of the turbine is divided into zones. Such zonal distribution is caused by the different characteristics which the surface of the turbine presents (thickness, use of pipings, etc...). Each zone may have one or various resistances due to different heating requirements of each zone, although there exists only one thermocouple for the control of each zone.
The control procedure is carried out in cyclic form. Each control cycle is the period of time which has elapsed between two consecutive samplings and presents two clearly different parts. One of which parts for the acquisition of data, checking and calculation by means of the control algorithms, during which the blankets are disconnected in order to avoid interferences with the measured signals and, a second part, available for the heating which ends with the next sampling instant.
The control system thus regulates the space and time distribution of the heating power applied to the outer shell with the basic objective of maintaining said shell at a temperature so that a sufficient temperature for a "warm" start-up is assured in the rotor of the turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a diagram of installation of the system for maintaining warm high power turbines, the object of the invention.
Figure 2 shows a detail of installation of the heating blankets over the external surface of the turbine with electric heating elements and a thermocouple.
Figure 3 shows a block diagram representing the function of zonal control of the system.
Figure 4 shows a block diagram representing the "on-off" control strategy with master PID link.
Figure 5 shows the response of the system with respect to a simulation with the thermal and dynamic model in heating condition from ambient temperature.
Figure 6 represents the response of the system with respect to a simulation with the thermal and dynamic model in cooling condition from the triggering condition.
Figure 7 shows a front view of a device for installation, securing and pulling out a thermocouple.
Figure 8 shows the qualitative behaviour of the master PID link strategy in heating condition from ambient temperature.
Figure 9 shows the qualitative behaviour of the master PID link in cooling condition from the triggering condition.
Figure 10 represents a real situation of a cooling cycle of the internal metal of the trubine from a disconnection from the network resulting from a triggering and the response of the system for taking the temperature to the set point.
Figure 11 represents the temperature variations on the surface of the turbine and the response of the control system.

EXAMPLE OF PREFERED EMBODIMENT

In figure 1 it can be observed that the system, object of the present invention, consists of a turbine (1) on the exterior surface of which there are a number of resistances (2) in order to carry out the heating function and a number of thermocouples (3) which serve for acquiring data as for the state of the machine. The data acquisition and control equipment (4) consists of a computer with its peripheral equipments. The system also has equipments for supply of electric energy which essentially comprises a transformer (5) and a switching module (6).
In figure 2 the disposition of the heating blankets (7 and 8) can be observed as well as the resistances (2) a thermocouple (3) and the enveloping corrugated mesh (9) inside of which a layer of heating blanket (7) and the electric resistance are situated. The isolating assembly is, in a practical case, composed of three layers of detachable blankets ( the third layer is not shown in the figure). The thickness of each layer of blankets varies from the first to the third according to the special function of each as for insulating capacity which is required from each one. The enveloping mesh (9), due to its special shape, confers great flexibility to the elctric resistances (2). The blanket is directly installed on the exterior shell of the turbine by means of anchorages soldered to the same and represents the first layer of insulation. For the installation of thermocouple (3) a certain type of holder has been designed so that it is perfectly extracable form outside without having to dismantle the blankets while assuring at the same time a permanent contact between the end-point of the thermocouple and the surface to be measured. said holding element consists of a hollow protecting tube (31) of cylindrical shape (see figure 7) which in one end has a tip of conical shape and in the other end presents a flat braodening as of a disc with a slightly larger diameter than that of the tube. Inside said holding element (31) the thermocouple (3) is fitted being protected against possible damages.
The signal received by the data acquisition equipment (4) from the thermocouple is adequately processed and as a result allows a code to the zone selection module (6). By this code, order is given as to which resistances are to be activated by said module. The module decodes the received signal and activates the corresponding solid state relays incorporated in the module. Said relays permit the passing of the current coming from the supply transformer (5) to the resistances through the general switch and the magnetothermics also incorporated in the switching module (6). Finally the selectioned resistances warm up the required zones. As for the relays it should be mentioned that there is one for each pair of resistances (connected in series) and either allows or closes the passing of electric intensity towards the resistances.
Two clearly different parts are herewith presented concerning the functional aspects of the data acquisition system (DAS) on the one hand, and the control system on the other. The software which carries both systems is completely adapted to the operational and reliability requirements of their application.
Although both systems reside in the same computer and in the same computer program, it is preferable to describe them separately since their objectives are different; moreover the DAS can operate independently, thus constituting itself as a first order auxiliary element for monitoring the turbine in any operative condition of the same.

a.- The data acquisition system is developed in order to allow to know at any moment the technical and mechanical state of the turbine, regardless of the operative situation of the same. The DAS provides thus everything that is striclty necessary for the control of the heating blankets, and allows to follow the development of the technical and mechanical state of the turbine in a start-up, during a normal functioning or during the cool-off after the triggering of the machine.
The data obtained by the system are:
- Surface temperatures.
- Turbine metal temperatures.
- Temperatures and pressures of the steam.
- Variables of strain of the turbine.
- Variables of operation of the turbine.
The DAS shows the electric signal coming form the sensors with a frequency (sampling period) specified by the operator, it digitizes siad signals and converts them into engineering units. The data are stored in an array type form in the main memory of the computer; later it is verified to see if the data is inside the limits defined by the range of the sensor. Any failure which may be detected during the checking is registered as an incidence for its posterior treatment. The data shown on the screen are automatically updated and recorded in the hard disk if that is the case of concern, according to the recording period specified by the operator. With said data, the corresponding absolute time is also recorded which is measured by the internal clock of the computer.
The punctual data corresponding to any group of variables previously difined may be presented in tabular form by printer or by screen. By these peripherals the development of any group of data in time in graphical form may be presented. The time period which involves said representation depends on the period of recording and on available RAM memory. Each variable can be represented in percentages (of the range of the sensor) or in its own units. The presentation on the screen allows the use of a zoom with automatic change of scales in order to be able to observe details. Finally the map of surface temperatures may be presented on the screen; over a schematic figure of the turbine the surface of the same is colored as a function of its thermal level.
The DAS supplies temperature data to the control system so that this latter manipulates the heating power dissipated in each zone. It is thus necessary to have at all times reliable data available for which the system has to warn about any failure in the sensors and protect itself immediately against said failures without waiting for their repair.
Each zone has at least two thermocouples, one for the control and the other as a reserve which is called "gradient"; in the case of failure in the thermocouples they produce out of limit signals which are easily detecable, or they produce partially erroneous signals within said limits. The protection system is designed to recongnise a failure when any of the following circumstances is produced: out of limit signals; change of signal higher than a predetermined limit between two consecutive samplings; difference between the control and the gradient thermocouples higher than a predetermined limit; difference between the signal of thermocouple and that measured by the adjacent thermocouples higher than a predetemined limit.
In any of said circumstances the system produces an alarm and substitutes the signal of the thermocouple which has had the failure by another according to a pre-established sequence. The system counts the number of thermocouples which fail and once said number surpasses a certain limit, specified by the operator of the system, produces a total alarm and the disconnection of the heating blankets.
b.- Control procedure of the heating blankets.
In order to distribute specially the heating power, the external surface of the turbine is divided into zones. Although to each zone, various heating blankets may correspond, there only exists one single temperature representing each zone and one single signal as common command to all said blankets. In order to control the cover-base temperature difference, such zones which having the same axial position are symmetrical with respect to the axis of the turbine are defined as corresponding zones.

For each zone a feedback control link is established which has as controlled variable the surface temperature measured by the thermocouple of the zone, and as manipulated variable the heating power yielded by the blankets in said zone.
The set point may be local or remote, according to the implemented strategy, but it is always the same for all zones of the outer shell, and it is equal to the outer shell temperature which is necessary for reaching the sufficient temperature in the rotor for a warm star-up, when a steady condition has been reached.
In figure 3 the zonal control process is shown in which it can be observed that on the basis of the information received from the thermocouple and the one established by the set point (TSET) on the one hand and the corresponding zone temperature (TEMP1) on the other, the control computer sends the pertinent orders through the energy supply equipment to the heating blankets (blanket), turning on and off the resistances according to each case, which results in a temperature adjustment in the surface zones of the shell (zone). This new temperature (TEMP2) is detected by the thermocouple corresponding to the zone which sends the respective signals to the computer.
The control cycle is the period of time elapsed between two consecutive samplings. It presents two clearly differenciated parts: One part for data acquisitions, checking and calculations by means of control algorithms during which the blankets are disconnected in order to avoid interferences with measuring signals; and a second part available for the heating which is terminated by the next sampling instant. The control algorithm calculates in each cycle the fraction of time of the cycle during which each blanket has to be connected; in this manner, the average heating power for each zone in each cycle is regulated.
In order to avoid the connection of all blankets at the same time which produces intensity peaks at the beginning of each cycle, an algorithm is implemented which uniformizes the total power yielded to the turbine by distributing the total time available in each cycle among all zones.
The control system regulates the space and time distribution of the heating power applied to the external shell with the basic objective of maintaining said shell at a temperature such that a sufficient temperature for a "warm" start-up of the turbine is assured in the rotor.
The strategy of control permits that the natural cooling process isstopped from any triggering condition, or permits for the heating of the turbine from the ambient temperature until a technical state which would allow a warm start-up.
Figures 5 and 6 show the qualitative behaviour of this strategy, obtained with a simple model of simulation of the turbine.
From the observation of the cited figures the following conclusions are obtained: in a cooling from triggering conditions the differential TCTB (cover and base temperatures) is rapidly reduced to a value equal to BAND, this latter being the amplitude of the surface temperature oscillations which affect the frequency of turn-on and turn-off of the blankets; in a heating from ambient temperature TC oscillates around the ascending trajectory of TB but always inside the specified band (±BAND); in both cases the system manages to reach and maintain TC and TB in the neighbourhood of the spceified set point (TSET ± BAND); in a heating, the maximum of the differential expansion is produced by reaching the steady condition and does not depend on the maximum power.
This strategy is specially indicated for stopping the cooling off of the machine after a triggering as long as the temperature to be maitained in the outer shell so that the rotor is at SOAK temperature is previously known; or for the cases in which the desired temperature for the rotor does not change and that the changes in the ambient temperature are not excessive.
However there exists another control strategy which we call "on-off master PID". By this strategy the control system is capable of finding, by itself, the set point for the zonal control links corresponding to the outer shell of the turbine. Additionally the power that the blankets yield is modulated as a function of the differential expansion measured at each moment.
Figure 4 shows the block diagram corresponding to this strategy. The master link controls the temperature of the rotor manipulating the set point of the zonal links. The control algorithm contains proportional, integral and derivative actions. The control changes the set point of zonal links while there exists a difference between the desired temperature for the rotor and the measured temperature by means of a thermocouple, which assumes (simulates) the temperature of the rotor. When a steady condition is reached, the rotor will have the desired temperature and the output of the controller will indictate the surface temperature required in such conditions.
The power modulator permits the machine to reach an elevated heating velocity limiting at the same time the differential expansion. In a heating, the system starts up with the maximum specified power in order to later reduce it as a function of the measured differential expansion. The applied power is eliminated before reaching the triggering limit, which makes it impossible for such limit to be reached.
Figures 8 and 9 show the qualitative behaviour of the strategy of master PID link.
From the observation of said figures the following conclusions are obtained: The strategy "on-off" in zonal links eliminates the cover-base temperature difference and takes the surface temperature to the set point calculated with the master PID link. The master PID link takes the temperature of the rotor to the desired temperature for a warm start-up. The power modulator reduces the initial velocity of the heating in order to limit the differential expansion.
It should be mentioned that there is no inconvenient for applying the master PID link to all heating processes. For the processes of cooling the adequation of the PID algorithm must be different.
The real results of the system as for the connection and the disconnection of the heating elements in order to maintain a homogeneous and stable cooling process in the operation of the turbine are shown in figures 10 and 11 from a disconnection from the network produced by a triggering. In said figures the response of the control system can be observed towards the set temperature which is the maintenance temperature for a posterior warm start. The series of connections (↑) and disconnections (↓) of the blankets, and the response of the system in order to take the temperature to the set point can also be observed.
Finally it should be added that the system object of the invention, which is essentially designed to be applied in high power steam turbines is perfectly compatible for application in every type of turbines being especially appropriate in units whose functions are :

Cold reserve.
Stop and start cycles.
Steam turbines in combined cycle units.
Large nuclear turbines in general.
Other machines such as boilers and the like.

Once sufficiently described the nature of the present invention, as well as a form of putting it into practice it only remains to be added that changes of form, material and disposition may be made in the whole invention or in the parts it is composed of as long as said alterations do not substantially vary the characteristics of the inventions which are claimed as follows:

Claims

System for maintaining warm high power turbines, essentially steam turbines, which based on the capacity of predicting or foreseeing the possible thermal behaviours of the machine as a function of temperature variations during a cooling or a heating, manipulates the temperature of the rotor and the exterior shell of the turbine in order to obtain and mainatin in the turbine a set temperature appropriate for performing a rapid start without the necessity of pre-heatings, characterized in that it is composed of an assembly of electric heating blankets attached to the external surface of the turbine; an automatic and computerized data acquisition and control system which is capable of making possible the maintaining of a determined thermal level in the rotor; and an electric energy supply and distribution system.
System, according to claim one, characterized in that the heating assembly is composed of various layers of electric blankets, normally three layers, a series of resistances situated in a corrugated and enveloping mesh in the interior of which the first layer of the blankets is placed.
System, according to previous claims, characterized in that the data acquisition and control system is composed of a computer which in relation to a series of thermocouples situated on the exterior surface of the turbine, receives the electric signals which said thermocouples send towards it in order to determine infromation on the temperatures of different zones of the turbine, and after having processed said information sends the pertinent orders for turning on or turning off the resitances of the heating blankets. Said computer is also comprised of respective peripheral equipments such as printer and plotter.
System, according to previous claims, characterized in that the electric energy supply and distribution system is essentially composed of a transformer and a switching module which can activate or deactivate a series of relays incorporated in the same and governed by the computer for turning on and turning off the electric resistances on the surface of the turbine.
System, according to calims 1, 2 and 3, characterized in that each thermocouple is placed in a hollow holding device of cylindrical shape with a conical and acute end which assures a permanent contact between the end of the thermocouple and the surface to be measured, and that it can be pulled out without a need to dismantle the blankets, the objective of which is to protect the thermocouple against possible damages.