US3359732A

US3359732A - Method and apparatus for starting a steam generating power plant

Info

Publication number: US3359732A
Application number: US566999A
Authority: US
Inventors: Wolfram G Schuetzenduebel; William J Deane
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1966-07-21
Filing date: 1966-07-21
Publication date: 1967-12-26
Anticipated expiration: 1984-12-26

Description

1967 w. G. SCHUETZENDUEBEL ET AL 3,359,732

METHOD AND APPARATUS FOR STARTING A STEAM GENERATING POWER PLANT Filed July 21, 1966 (D TEMPERATURE RECORDER PREssuRE RECORDER [I] COMPARER OR CONTROLLER PRESSURE DIFFIERENTIATING DEVICE 3M 5:

= ACTUATOR OF TILTING MECHANISM,

WOLFRAM G. SCHUETZENDUEBEL DAMPER A WILLIAM J. O ANE M @AZLQZ/ United States Patent 3,359,732 METHOD AND APPARATUS FOR STARTING A STEAM GENERATING POWER PLANT Wolfram G. Schuetzenduebel, Avon, Conn., and William J. Deane, Hackensack, N.J., assignors to Combustion Engineering, Inc., Windsor, C0nn., a corporation of Delaware Filed July 21, 1966, Ser. No. 566,999 13 Claims. (Cl. 60-105) ABSTRACT OF THE DISCLOSURE A method and apparatus for hot restarting a forced through flow stream generating power plant wherein the temperature of the start-up steam is raised to the level of the high temperature retained by critical metal parts of the turbine, while at the same time sufficient flow is provided through the furnace walls, the superheater and the reheater to protect these heating surfaces against heat damage that would otherwise be caused by the relatively high rate of fuel firing necessary to produce steam temperatures matching the temperature of critical turbine parts during hot restarts. This is accomplished through the use of a combination of tilting burner or gas recirculation arrangements with fluid recirculation and various fluid bypasses, while keeping to a minimum the heat losses to the condenser.

This application is a continuation-in-part of our earlier application entitled, Method and Apparatus for Starting a Steam Generating Power Plant, Ser. No. 494,492, filed Oct. 11, 1965, now abandoned, which is assigned to the same assignee as the present invention.

The invention relates to steam generating power plants and is particularly concerned with hot restarting of a steam power plant of the forced through flow type operating in the supercritical pressure range and under the steam reheat cycle.

In contrast to subcritical drum type boilers in which a large water storage capacity is available, severe problems are encountered in connection with the starting, specifically hot restarts, of drumless supercritical forced through flow steam generators.

One particular concern is that heat losses to the condenser may be excessive because of the care which must be taken in the protection against overheating and high temperature damage of the tubular heating surfaces lining the furnace Walls as well a those forming the superheater and reheater.

Furthermore, in the operation of supercritical forced through flow boilers, which at frequent intervals must be shut down and restarted, high superheat and reheat steam outlet temperatures are required to match the metal temperature of critical parts of the turbine such as the turbine casing and turbine rotor, so as to prevent distortion due to excessive thermal stresses. If the steam temperature conditions thus required by the turbine manufacturer cannot be fully met during the startup period, prolonged rolling and holding times are needed which will delay the startup of the turbine and of the generation of electric power. Suchdelays are very costly and frequent occurrence thereof seriously reduces the operating efficiency of the power plant.

In forced through flow boilers of the modified through flow type the heat absorbing furnace wall surfaces are effectively cooled during startup operation, by recirculating a suitable quantity of working fluid through these surfaces located in a high heat release zone. Accordingly, the heat losses to the condenser are thereby greatly reduced since a through flow quantity of only percent Patented Dec. 26, 1967 of maximum flow is thus required to be maintained, instead of the 30 percent commonly needed when operating without recirculation. However, when the firing rate at this flow of 10 percent is increased in order to furnish steam to the turbine of a temperature that will match the temperature of critical portions of the turbine, the resulting high temperature of the combustion gases sweeping over the superheater and the reheater may destructively overheat the metal of the heating surfaces thereof.

Accordingly, it is a principal object of the invention to provide a turbine startup system and method which permits the establishment of a wide range of steam temperatures of the steam entering the turbine, for the purpose of matching the metal temperature of critical parts thereof during hot restarts.

A further important object of the invention is to establish, at the time the above turbine temperature conditions are met, a sufiicient flow of the working fluid both through critical portions of the furnace walls as well as through steam heating sections of the steam generator, such as the superheater and the reheater, to protect these heat absorbing parts against high temperature damage, and at the same time to avoid excessive heat losses to the condenser cooling water.

Other objects and advantages of the invention will become apparent from the following description of an illustrative embodiment thereof when taken in conjunction with the accompanying drawing wherein the single figure thereof is a diagrammatic representation of a high capacity steam generator turbine power plant, organized in accordance with the invention.

The steam generating power plant benefited, and normal operation thereof Referring now to the drawing, the furnace and gas passages of the steam generator depicted therein are represented by a box 10 outlined in dot and dash. Tilting burners 12 are mounted in the furnace wall and are organized to discharge fuel and air, received by way of conduit 13, in a conventional manner into the furnace chamber, releasing heat and air for the generation of hot combustion gases. The furnace wall is lined with heat absorbing fluid cooled tubes designated FW which represent the major portion of the radiant heat absorbing section or zone of the steam generator of the power plant. The remaining heat absorbing surfaces may include superheater SH, reheater RH and economizer EC and constitute the convection heat absorbing section or zone. During normal operation the working fluid passes from the hot well 14 of condenser CO through a demineralizer D, a low pressure feedwater heater H1, a feed pump 16, a high pressure feedwater heater H2, and a feedwater valve 18 into economizer EC and by way of conduit 19 into furnace wall tubes FW. From the furnace wall tubes the heated working medium now called steam flows through conduit 20 and boiler throttling valve BT to superheater SH for further heating, and thence through conduit 21 to the high pressure turbine stage HP, where heat energy is extracted by reducing the pressure and temperature of the steam. Leaving the high pressure stage HP the steam passes through conduit 22 to reheater RH for reheating and thence to the low pressure stage LP of the turbine by way of conduit 23. The high pressure and low pressure stages of the turbine may preferably be mounted on a common shaft coupled to an electric generator G for the generation of electricity. The steam exhausted from low pressure stage LP passes to condenser CO, with the condensate being collected in hot well 14, thereby completing the through flow cycle.

Burners 12 are equipped with conventional burner tilting mechanisms 24 to lower or raise the flame in the furnace chamber and to expose more or less furnace wall heating surface to the gases of highest heat intensity. In this manner the heat content of the gases leaving the furnace or radiant heat absorption zone is increased or decrea ed, making available more or less heat in the gases passing over the other heating surfaces or convection heat absorption zone located downstream of the furnace, such as to superheater SH, reheater RH and economizer EC, which are absorbing heat primarily by convection. The same result can be achieved by using conventional gas recirculation, such as for example by recirculating relatively cool combustion gases taken from a low temperature zone of the combustion gas path such as at 25 and returning these gases by way of conduit 26 and gas recirculating fan 27 to a furnace region such as at 28.

In a conventional modified through flow steam generator a working fluid recirculating circuit is superimposed upon the through flow circuit by providing a conduit 30 connecting the outlet of the furnace tubes FW with the conduit 19 leading to the inlet thereof. A recirculating pump 32 is provided in conduit 19 for the purpose of handling this increased flow of the working fluid through the furnace wall tubes PW so as to maintain this flow at or above a safe velocity, such as three feet per second, to prevent heat damage of the tubes.

The load requirements on modern steam power plants are increasingly calling for units which can be shut down for short periods, such as for a few hours and then restarted. During operation of a steam turbine the internal structure assumes the temperature of the steam at the inlet of the turbine stages such as, for instance, 1000 F. in modern power plants. When a turbine is removed from operation, it takes several days for the metal tempcrature inside the turbine to approach room temperature. Accordingly, it is often desired that the turbine be restarted before this cooling is effected. This is called a hot restart, which generally can be accomplished by admitting steam of suitable temperature and pressure first to the high pressure stage and thereafter to the low pressure stage of the turbine. Or, in accordance with the present invention and in cases where the high and low pressure stages are mounted on a common turbine shaft, starting of the turbine can additionally be accomplished by first admitting low pressure steam of suitable temperature to the low pressure or reheat stage of the turbine, and subsequently to the high pressure stage.

Both of these methods, now to be described hereinbelow in greater detail, will utilize the temperature adjusting devices of the invention for the purpose of matching the temperature of the steam with the temperature of critical parts of the turbine.

H or restart by way of high pressure turbine stage Initially, the boiler is filled and pressurized up to the outlet of the furnace wall system FW. With the boiler throttling valve BT closed, a feedwater through flow of approximately. percent of maximum flow is established through the economizer EC and furnace walls PW and out through the boiler extraction valve BE provided in conduit 33 leading to flash tank or separator S. In addition a recirculation flow is established around furnace tubes PW by way of

conduits

30, 19 and recirculating pump 32 of sufficient magnitude to provide a total flow of at least 30 percent through the furnace tubes FW. A considerable drop in pressure occurs (such as from 3500 psi. to 1000 psi.) in passing through valve BE, which causes a quantity of water to flash into steam, with both water and steam being collected in separator S. The steam liberated in the separator S may now flow through conduit 35 and superheater admission valve SA into superheater SH and thence into high pressure turbine stage HP by way of conduit 21. Only a flow of 3 percent of the maximum load flow is generally required at the turbine inlet for starting and rolling the turbine. Accordingly a flow of between 3 percent and 10 percent may pass 4 through superheater SH. Heretofore the remainder, i.e., 7 percent or less was passed to the condenser CO such as by way of conduit 36 and superheater bypass valve SP and conduit 37 shown in dot and dash line.

superheater bypass valve SP automatically controls the pressure in separator S by way of pressure controller 39. This pressure may be established at some relative low value (such as 1000 p.s.i., as earlier stated herein) suitable for starting the turbine with a minimum throttling loss through the turbine inlet valves.

As earlier set forth herein, at hot restarts the turbine chest or other critical parts of the turbine may have retained a temperature of as high as 1000 F. To avoid adverse thermal stresses it is extremely desirable to match the temperature of the incoming steam with this temperature as nearly as possible. With a 10' percent through flow and a corresponding fuel firing rate the temperature at the superheater outlet is generally far below the prevailing turbine chest temperatures. If the firing rate is increased to obtain the desired elevated steam temperature, such as 1000 F. leaving the superheater and entering the turbine, the temperature of the gases entering the reheater would exceed the maximum safe gas temperature such as 1000 F. to which the reheater tubes can be exposed during startup when only the 3 percent flow leaving the high pressure turbine stage HP would pass through the reheater. Accordingly these tubes may sufier.

serious heat damage.

To prevent such damage, in accordance with the in vention, a portion of the 10 percent through flow leaving separator S, such as a flow of between zero percent and 7 percent, bypasses superheater SH and high pressure turbine stage HP 'by way of conduit 36 and superheater bypass valve SP, as earlier described herein. Furthermore, additional means are provided for some of the steam flow, if in excess of 3 percent passing through superheater SH, to bypass the high pressure stage only and flow through conduit 38 and superheater drain valve SD. These flow quantities are controlled by

pressure controllers

39, 40 and 41 in a conventional well-known manner. The flow through

conduits

36 and 38 combine with the steam fiow leaving the high pressure turbine stage HP in conduit 22, to make up a total of 10 percent flow to the reheater. This flow of 10 percent of maximum load flow will be more than suificient in cooling the reheater tubes during hot restart.

Having now provided the means for protecting the reheater while raising the temperature entering the high pressure turbine stage HP to the temperature required by critical turbine parts (such as 1000 F.), the means and method in accordance with the invention to adjust or control the temperature of the steam entering the turbine will now be described.

This method, as indicated earlier herein, consists in adjusting the fuel and air input by controlling valve '42 in response to an error signal received from temperature comparing device or controller 43. This device 43 compares the temperature of critical portions of the turbine stage HP at T1 with the temperature of the steam measured at T2, with the error signal controlling valve 42 to increase the fuel input until temperature T1 and T2 are matched.

While a flow of 10 percent of maximum flow is thus passing through the reheater RH for the protection thereof, the temperature of the steam T3 leaving the reheater likewise must be raised to the temperature T4 of critical portions of the low pressure turbine stage LP. In accordance with the invention there are two ways in'which this can be accomplished. These will now be described.

One method consists in controlling the reheater outlet temperature T3 by raising the tilting burners 12 to cause less heat to be absorbed in the furnace tubes FW and correspondingly more-in the convection heating surfaces such as reheater RH. Thus the temperature T3 of the steam leaving reheater RH is compared at temperature com parer 44 with the temperature T4 of critical portions of the low pressure turbine stage LP. The resulting error signal is then transmitted to tilting mechanism 46 for the purpose of adjusting the tilt of burners 12 and thereby raise the temperature T3 of the steam leaving the reheater RH until it matches the temperature T4 of critical parts of the turbine.

Another method, in accordance with the invention, comprises transmitting the error signal originating at 44 to a controller 48, thereby regulating the speed of gas recirculating fan 27, or transmitting the error signal to controller 50 for controlling positioning of damper 52 in recirculating duct 26. In this manner, by adjusting the recirculating flow, the temperature of the gases leaving the furnace can be raised to increase the temperature T3 of the steam leaving reheater RH until it matches the temperature T4 of critical turbine portions of the low pressure turbine stage LP. I

When the steam temperatures T2 and T3 at the respective turbine throttles 54 and 56 are at a suitable level and with the stop valves SV wide open, the turbine valve SV and intercept valve IV are opened to admit a small amount of steam to roll the turbine. The operation of the intercept valve IV is coordinated or mechanically linked to the high pressure turbine inlet valve CV, so that it will pass approximately the same amount of steam flow through the low pressure turbine as is admitted through the high pressure turbine. By opening the inlet valves the turbine is brought up to speed and synchronized and a minimum load is established.

The pressure leaving the reheater is controlled automatically by valve BP in conduit 58 leading to condenser CO and by pressure regulator 60. This pressure is maintained at a suitable low level for matching the flow through the high pressure and low pressure sections of the turbine during initial rolling and synchronizing.

The feedwater flow to the boiler and the steam flow from the boiler is established at a suitable quantity to provide a reasonable margin over the minimum steam requirements of the turbine. Under this condition the valves in the startup system such as valves BE, SP and SD will automatically maintain the proper pressures during the rolling and initial loading of the turbine. To pick up additional turbine load, the boiler feedwater fiow and firing rate are increased. The valves SP and BP will close automatically, when the steam flow reaches the capacity of the valve BE. The flow is then transferred to the valve BT and the loading of the boiler will proceed in the conventional normal manner.

Hot restart by way of low pressure turbine stage When starting the turbine via the low pressure turbine stage LP in accordance with the invention the pressure drop across valve SP is adjusted to establish a suitable flow through the superheater SH for the cooling of the. tubes thereof. With the turbine admission valves to the high pressure turbine stage HP closed the part of the steam flowing from separator S through the superheater SH passes through conduit 38 and valve SD into conduit 22, joins the remaining part having passed through conduit 36 and valve SP and also into conduit 22. Both parts of the steam then flow through reheater RH and to the inlet of the low pressure turbine stage LP for rolling the turbine after the steam temperature T3 leaving the reheater RH has been raised to T4 the temperature prevailing in critical parts of the low pressure turbine.

This matching of the temperatures T3 and T4 is accomplished in accordance with the invention, as earlier described herein, by regulating the tilt of burners 12, or by controlling the amount -of gas recirculated through duct 19, in response to temperature error signals received from temperature comparer 44.

When the steam temperature at the intercept valve 1V is at a suitable level this valve is opened, to admit steam to roll the turbine via the low temperature stage LP. As

Summary From the foregoing it will be seen that our invention has provided important improvements in the hot restarting of supercritical forced through flow steam generating power plants. These improvements comprise a novel combination of tilting burner or gas recirculation arrangements with fluid recirculation and various fluid bypasses, whereby the temperature of the startup steam is raised to the level of the high temperature (such as 1000 F.) retained by critical metal parts of the turbine.

In accordance with the invention, this increase in steam temperature to match the turbine metal temperature is achieved with a greatly limited through flow to eliminate excessive heat losses to the condenser cooling water. Yet at the same time and in spite of the limited through flow and the high steam temperatures attained, the invention provides for sufiicient flow through the furnace walls, through the superheater and through the reheater to protect these heating surfaces against heat damage that would otherwise be caused by the relatively high rate of fuel firing necessary to produce steam temperatures matching the temperature of critical turbine portions during hot restarts.

It is to be understood that the invention is not limited to the specific embodiment illustrated and described herein, but may be used in other ways without departure from the spirit thereof, and that various changes can be made which would come within the scope of the invention, which is limited only by the appended claims.

What is claimed is:

1. The method of hot restarting a steam generating power plant, in which the temperature of the steam entering the turbine must closely approach the temperature of critical parts of the turbine, said steam power plant including a steam boiler and furnace having a radiant heat absorbing section including furnace tubes and a convection heat absorbing section, including a superheater and a reheater, and further including a steam turbine having a high pressure stage and a low pressure stage, with said reheater being operationally interconnected intermediate said high and said low pressure stages, the steps comprising: establishing a total flow of working fluid through said furnace tubes of a rate suflEicient for cooling said tubes to prevent heat damage; applying heat to said flow by means of a stream of combustion gases and producing a mixture of water and steam flow; the improvement comprising in combination the steps of:

(a) flowing a first part of said separated steam through said superheater and through said high pressure turbine stage;

(b) flowing a second part through said superheater and through said reheater;

(c) adjusting the temperature of the steam passing to said high pressure stage to match the temperature of critical turbine parts by altering the heat input to said total flow.

2. The method as defined in claim 1 including the ditional steps of:

(d) flowing a third part of the separated steam to said reheater bypassing the superheater and the high pressure turbine stage.

3. The method as defined in claim 2 including the additional steps of:

(e) combining the first, second and third flows and passing them through said low pressure turbine stage; and

(f) adjusting the temperature of the steam passing to said low pressure stage to match the temperature of critical turbine parts by altering the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection heat absorbing section.

4. The method as defined in claim 3 including the further step of:

(g) recirculating a major portion of said total flow around said furnace tubes to establish a rate of flow sufficient for cooling said tubes to prevent heat damage, and separating the steam and water from the mixture of the remaining minor portion of said total flow.

5. The method as defined in claim 4 wherein the altering of the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection section is accomplished by tilting the burners.

6. The method as defined in claim 4 wherein the altering of the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection section is accomplished by recirculating combustion gases from a point downstream of said furnace tubes in the gas flow sense to a point upstream thereof.

7. The method of hot restarting a steam generating power plant, in which the temperature of the steam entering the turbine must closely approach the temperature of critical parts of the turbine, said steam power plant including a steam boiler and furnace having a radiant heat absorbing section including furnace wall tubes, and a convection heat absorbing section, including a superheater and a reheater, and further including a steam turbine having a high pressure stage and a low pressure stage, with said reheater being operationally interconnected between said high and said low pressure stages, the steps comprising: establishing a total flow of working fluid through said furnace tubes of a rate sufiicient for cooling said tubes to prevent heat damage; applying heat to said flow by means of a stream of combustion gases and producing a mixture of water and steam flow; the improve ment comprising in combination the steps of:

(a) flowing a first part of said separated steam in bypass relation with said high pressure turbine stage through said superheater and said reheater;

(b) flowing a second part of said separated steam in bypass relation with said superheater and said high pressure stage through said reheater;

(c) combining said first and second flow and passing it through said low pressure turbine stage; and

(d) adjusting the temperature of the steam passing to said low pressure stage to match the temperature of critical turbine parts by altering the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection heat absorbing section.

8. The method as defined in claim 7 including the further step of:

(e) recirculating a major portion of said total flow around said furnace tubes to establish a rate of fiow sufiicient for cooling said tubes to prevent heat damage, and separating the steam and water from the mixture of the remaining minor portion of said total flow.

9. The method as defined in claim 8 wherein the altering of the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection section is accomplished by tilting the burners.

10. The method as defined in claim 8 wherein the altering of the proportion of the heat absorbed in said radiant heat absorbing section to that absorbed in said convection section is accomplished by recirculating combustion gases from a point downstream of said furnace 2% tubes in the gas flow sense to a point upstream thereof.

11. An organization for hot restarting a forced through flow steam generator having a heat input system including a radiant heat absorption zone and a convection heat absorption zone; and having a through flow circuit, including a superheater, a high pressure turbine stage, a reheater and a low pressure turbine stage, which through flow circuit has incorporated therein valve means intermediate said superheater and the remainder of the circuit that is located upstream of said valve means; a first valved bypass around said valve means which bypass has incorporated therein a flash tank with a throttle valve being provided in the bypass intermediate the flash tank and the section of the bypass upstream of said throttle valve; a recirculation system including pump means superimposed on a portion of said upstream remainder of the through flow circuit including the portion that has the highest rate of heat absorption, the improvement comprising in combination:

(1) means for obtaining an indication of the temperature of critical portions of each of said high pressure and said low pressure turbine stages during hot restart operation of said steam generator;

(2) means for regulating the heat input to said steam generator to match the temperature of the steam entering said high pressure turbine stage with the temperature of said critical portions thereof;

(3) means for altering the respective portions of the heat absorbed by said radiant heat absorption zone and said convection heat absorption zone to control the temperature of the steam entering said low pressure turbine stage so as to match it with the temperature of critical portions thereof during hot restart operation;

(4) a second valved bypass around said superheater and said high pressure turbine stage (5) a third valved bypass around said high pressure turbine stage; and

(6) means for controlling the flow through at least one of said second and third valved bypasses to maintain a flow through said reheater and superheater suflicient to prevent heat damage thereto.

12. An organization for hot restarting a forced through flow steam generator having a heat input system including a radiant heat absorption zone and a convection heat absorption zone; and having a through flow circuit, including a superheater, a high pressure turbine stage, a reheater and a low pressure turbine stage, which through flow circuit has incorporated therein valve means intermediate said superheater and the remainder of the circuit that-is located upstream of said valve means; a first valved bypass around said valve means which bypass has incorporated therein a flash tank with a throttle valve being provided in the bypass intermediate the flash tank and the section of the bypass upstream of said throttle valve; 21 recirculation system including pump means superimposed on a portion of said upstream remainder of the through flow circuit including the portion that has the highest rate of heat absorption, the improvement comprising in combination:

(4) a second valved bypass around said superheater and said high pressure turbine stage; and

(5) means for controlling the flow through said second valved bypass to maintain a flow through said reheater sufficient to prevent heat damage thereto.

13. An organization for hot restarting a forced through flow steam generator having a heat input system including a radiant heat absorption zone and a convection heat absorption zone; and having a through flow circuit, including a superheater, a high pressure turbine stage, a reheater and a low pressure turbine stage, which through flow circuit has incorporated therein valve means intermediate said superheater and the remainder of the circuit that is located upstream of said valve means; a first valved bypass around said valve means which bypass has incorporated the-rein a flash tank with a throttle valve being provided in the bypass intermediate the flash tank and the section of the bypass upstream of said throttle valve; a recirculation system including pump means superimposed on a portion of said upstream remainder of the through flow circuit including the portion that has the highest rate of heat absorption, the improvement comprising in combination:

.(1) means for obtaining an indication of the temperature of critical portions of each of said high pressure and said low pressure turbine stages during hot restart operation of said steam generator;

(4) a second valved bypass around said high pressure turbine stage; and

(5) means for controlling the flow through said second valved bypass to maintain a flow through said superheater sufiicient to prevent heat damage thereto.

References Cited UNITED STATES PATENTS 3,021,824 2/1962 Profos.

3,035,557 5/ 1962 Litrvinoff.

3,055,181 9/1962 Argersinger et al. 6073 3,175,367 3/1965 Gorzegno et al 60-73 3,219,018 11/ 1965 Angsburger.

MARTIN P. SCHWADRON, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

Claims

1. THE METHOD OF HOT RESTARTING A STEAM GENERATING POWER PLANT, IN WHICH THE TEMPERATURE OF THE STEAM ENTERING THE TURBINE MUST CLOSELY APPROACH THE TEMPERATURE OF CRITCAL PARTS OF THE TURBINE, SAID STEAM POWER PLANT INCLUDING A STEAM BOILER AND FURNACE HAVING A RADIANT HEAT ABSORBING SECTION INCLUDING FURNACE TUBES AND A CONVECTION HEAT ABSORBING SECTION, INCLUDING A SUPERHEATER AND A REHEATER, AND FURTHER INCLUDING A STEAM TURBINE HAVING A HIGH PRESSURE STAGE AND A LOW PRESSURE STAGE, WITH SAID REHEATER BEING OPERATIONALLY INTERCONNECTED INTERMEDIATE SAID HIGH AND SAID LOW PRESSURE STAGES, THE STEPS COMPRISING: ESTABLISHING A TOTAL FLOW OF WORKING FLUID THROUGH SAID FURNACE TUBES OF A RATE SUFFICIENT FOR COOLING SAID TUBES TO PREVENT HEAT DAMAGE; APPLYING HEAT TO SAID FLOW BY MEANS OF A STREAM OF COMBUSTION GASES AND PRODUCING A MIXTURE OF WATER AND STEAM FLOW; THE IMPROVEMENT COMPRISING IN COMBINATION THE STEPS OF: (A) FLOWING A FIRST PART OF SAID SEPARATED STEAM THROUGH SAID SUPERHEATER AND THROUGH SAID HIGH PRESSURE TURBINE STAGE; (B) FLOWING A SECOND PART THROUGH SAID SUPERHEATER AND THROUGH SAID REHEATER; (C) ADJUSTING THE TEMPERATURE OF THE STEAM PASSING TO SAID HIGH PRESSURE STAGE TO MATCH THE TEMPERATURE