US3226932A - Devices for improving operating flexibility of steam-electric generating plants - Google Patents

Devices for improving operating flexibility of steam-electric generating plants Download PDF

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US3226932A
US3226932A US42194A US4219460A US3226932A US 3226932 A US3226932 A US 3226932A US 42194 A US42194 A US 42194A US 4219460 A US4219460 A US 4219460A US 3226932 A US3226932 A US 3226932A
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steam
turbine
temperature
pressure
superheater
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US42194A
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Jr Charles Strohmeyer
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Gilbert Associates Inc
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Gilbert Associates Inc
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
    • F01K3/00—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein
    • F01K3/18—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein having heaters
    • F01K3/20—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein having heaters with heating by combustion gases of main boiler
    • F01K3/22—Controlling, e.g. starting, stopping
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
    • F01K7/00—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating
    • F01K7/16—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating the engines being only of turbine type
    • F01K7/22—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating the engines being only of turbine type the turbines having inter-stage steam heating
    • F01K7/24—Control or safety means specially adapted therefor

Definitions

  • An object of the invention is to provide a novel apparatus and system providing a complete ⁇ solution to the a-bovernentioned problems.
  • a more specific object of the invention is to provide a throttling valve located after the waterwall section and before the superheater steam outlet of the steam generator.
  • a further specific object of the ⁇ invention is to provide a novel system for controll-ing or supervising steam conditions in the high pressure turbine during shut-down, start-up and low load operation to best suit turbine metal temperatures for both hot and col-d conditions.
  • a still further specific object of the invention is to provide a novel system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation.
  • FIG. 1 is a typical load diversity chart in which thermal generation is plotted against time
  • FIG. 2 shows steam enthalpy plotted against pressure, showing temperature drop for a given pressure drop
  • FIG. 3 shows steam temperatures in the high pressure turbine of a conventional reheat unit plotted against percent of rated ow.
  • FIG. 4 shows steam temperatures in the reheat intermediate and low pressure turbines plotted against percent of rated flow. s
  • FIGS. 5a to 5h inclusive and FIG. 5j show various modifications of throttling valve systems located after the waterwall section and before the superheater steam outlet and between heat absorption conduits of the steam generator;
  • FIG. 6 shows a system forcontrolling or supervising ⁇ Problem background Steam-electric generating -plants presently in use have Patented Jan. 4, 1966 volved in the following description. 'I'he same partially applies to straight condensing designs.
  • FIGURE 1 A typical example of load diversity involving (a) and (c) above is shown in FIGURE 1.
  • one of the main considerations with respect to safe operation relates to metal temperature changes in heavy walled equipment components. Excessive differential temperature across metal cross sections can cause stresses which exceed the elastic limit of the material and result in failures. Metal temperatures depend upon the surrounding environment. If environmental temperature changes are too rapid or are unbalanced, the cross section differential temperatures may be increased to points which produce permanent metal deformation. Where the environmental temperature conditions change with load, safe operation indicates that load changes be made slowly, the rate being influenced by the degree of change. On the other hand, if environmental temperature conditions could be maintained constant for all loads, temperature limitations with respect to load change would be eliminated.
  • shut-down and startup periods make this type of operation uneconomical.
  • the optimum condition is Where the turbine can be shutdown rapidly in a hot state and during restart, steam can be supplied to the turbine at temperatures which will immediately start restoring operating temperature levels at satisfactory rates of rise without causing excessive temperature differentials among components.
  • Partial admission takes maximum advantage of design throttle steam pressure at all loads and increases part load efficiency. For full admission, effective throttle steam pressure roughly decreases proportionally with load.
  • the accompanying temperature drop increases the temperature differential between the resultant fluid and/ or vapor in the superheater tubes after pressure reduction and the hotter gas surrounding the tubes. This increases the transfer of heat from the gas to the fluid and/ or vapor and raises the heat content or enthalpy (B.t.u. per pound) of the superheater outlet steam Where the waterwalls are operated at high pressures.
  • the turbine first stage nozzle chest and downstream steam pressures are a function of flow and are substantially below design values.
  • steam temperature is a function of enthalpy.
  • pressure reduction in the superheating zone of the steam generator above nozzle chest requirements raises steam enthalpy and temperatures in the turbine first stage nozzle chamber and downstream blade path.
  • This is a desirable substitute for high pressure superheater outlet steam during the hot condition where the boiler outlet steam temperature characteristics are such that at design pressure, temperature droops with load.
  • the pressure in the waterwalls may be maximized, minimizing the time required to shut down the turbine hot and subsequently to re-start a hot turbine.
  • rate of metal temperature rise should be restrained to avoid excessive metal stresses.
  • Low superheater outlet steam pressures are desired because downstream metal temperatures rapidly follow saturation steam temperatures as a result of steam condensation. Saturation temperature is a function of pressure. Where metals are cold, raising steam pressure too rapidly will result in high heat transfer rates from the steam to the metal up to the saturation temperature level.
  • the rate of metal heating may then be controlled by regulation of steam flow quantity since heat transfer is a function of both mass flow and temperature differential.
  • rate of metal heating steam pressure control when metal temperatures are below saturation followed by flow control when metal temperatures yare in the steam superheat range are more important than the amount of superheat in the-steam.
  • proper regulation of steam pressure and steam ow are essential.
  • a conventional drum steam generator may be operated at low pressures for cold start-ups. Once-through steam generators must be operated at full pressure or close thereto inthe waterwall circuits to assure proper distribution of flow and density unless variable pressure operation Vis -built into the design at great expense.
  • FIG. 3 shows steam temperatures in the high pressure turbine of a conventional reheat unit plotted against percent of rated flow. From the solid lines it can be seen that where throttle steam temperatureis held constant at 1050 F. and 2400 p.s.i.g. and employing partial admission, the first stage exhaust, first extraction and high pressure turbine exhaust steam temperatures decrease with load. The greatest temperature change occurs in the first stage which is the limiting condition with respect to rate of load change.
  • the dash lines on FIG. 3 indicate high pressure turbine steam temperatures where throttle steam temperature decreases with load; throttle pressure is held at 2400 p.s.i.g. and partial admission is employed.
  • FIG. 4 shows steam temperatures in the reheat intermediate and loW pressure turbines plotted against percent of rated flow.
  • Hot reheat steam flow to the intermediate pressure turbine is of the full admission type. Pressure at any point inthe flow path from the inlet of the LP. turbine to the condenser is dependent upon the downstream ow. From the solid lines on FIGURE 4 it can be seen that if the'reheat turbine inlet temperature is held constant, the downstream temperatures remain approximately constant at all fiows except toward the exhaust end which is affected the greatest extent by leaving losses. The exhaust temperatures would be excessive in the low flow range unless some provision is made to correct this situation. This may be done by injecting low enthalpy steam into the crossover between the intermediateand low pressure turbines. ⁇ The dot-dash lines show operation with injection steam. This enables the reheat turbine to be operated at near constant temperature over the entire load range. The dash lines ⁇ show the effect of decreasing reheat inlet steam temperature with load.
  • Temperature changes in the reheat turbine need impose no restrictions with respect to stop-start and load swing operation providing inlet steam temperatures are held near design values and cooling steam is injected into the cross-over between the I.P. and LP. turbines when throttle admission flows ⁇ are small.
  • FIG. 8 shows the main steam and water cycle of the conventional reheat unit used for FIGS. 3 and 4 and is typical of the steam-electric generating unit described in this specification.
  • the solid lines are water conduits and the dash lines are steam conduits. All components are standard and are produced commercially. Flow control valves and bypasses in the various conduits are omitted for the purpose of simplification. They would be arranged in a conventional manner and are not a part of this invention.
  • Direction of ow is indicated by the arrows in the steam and water conduits.
  • the high pressure and intermediate pressure reheat turbines are connected by a common driving shaft which drives an electric generator.
  • the low pressure turbines also drive their own electric generator. Extraction steam ows are from intermediate stages between turbine blade rows in the turbine steam iiow path.
  • FIG. 8 shows the general relationships of the plant components. It will be understood that details as the arrangement of the turbine elements, extraction points, feedwater heaters, pumps and other auxiliaries can vary to accomplish the same overall results within the intent of the cycle illustrated in FIG. 8. Also, there may be more than one stage of steam reheating in the steam generator associated with the cycle as is known to exist in commercially operating plants.
  • Item 1.--A throttling and shut-off valve system located after the waterwall section and before the superheater steam outlet of the steam generator and between heat absorption conduits, such as shown in'FIGS. 5a and 5b.
  • Various modifications of such System are shown schematically in FIGS. 5a to 5j inclusive.
  • This system per mits the steam generator to be operated at two pressure levels, the lowest level being downstream of the valve. This system may stand on its own merits and does not require the systems described in items 2 and 3 below.
  • Item 2. An automatic control or a supervisory system governing start-up and shut-down of the steam and turbine generator. Such system is shown in FIG. 6.
  • Superheater outlet steam enthalpy is controlled from steam temperature in the turbine first stageexhaust or subsequent downstream point before reheat is added to the steam.
  • Turbine throttle pressure is controlled from the differential between steam and metal temperature in the throttle valve or governor valve chest, or is controlled to a constant pressure, or to a pressure variation programmed with time, load or flow, or is controlled to increase enthalpy of the steam generator superheater outlet steam during low steam flow periods.
  • Item 3. A system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation. ⁇ Such system is shown in FIG.
  • FIGS. 5a and 5b shows two different locations for the throttling valve T in the solid line fluid conduits between the steam generator waterwalls and superheater outlet and between heat absorption conduits indicated by the saw tooth solid line.
  • the steam generator fluid circuits include a steam drum or steam and water separator D for pressures below critical. Such drum or separator D may ⁇ be omitted from the circuit for once-through steam generators designed for subcritical or supercritical pressures.
  • a throttling and shut-off valve T is located in each of one 0r more parallel ow circuits as shown in either FIG. 5a or 5b. The arrow indicates the direction of fiow in the fluid conduits.
  • a conventional drum type steam generator has internal recirculation conduits connecting the steam drum and the steam generating waterwalls.
  • bypass BP may return to the cycle wholly or in part at some point 4between the superheater outlet and the water supply point to the steam generator.
  • Bypass BP may discharge fluid from the drum or steam and 'water separator D, or may discharge fluid from any other downstream point in the main flow line before throttling valve T, in the case where the drum or steam and water separator D is included in the main circuit.
  • Bypass BP may discharge fluid from any point in the main ow line downstream of the ⁇ waterwalls vup to throttling valve T in the case where the drum or steam and water separator is not included in the main circuit.
  • the throttling valve in one single flow line or in multiple parallel flow lines may consist of a single valve in each ow line.
  • FIG. 5c shows a multiple or parallel flow line consisting of a single main line valve V or valves in the solid line uid conduit connected to the arrow and one or more smaller bypass valves in the dash line fluid conduit connected ⁇ to the solid line fluid conduit.
  • FIG. 5d shows a throttling valve V in a single ow line or in multiple parallel flow lines consisting of a valve in all parallel circuits to each end of a main ow line, the parallel circuits being shown as solid and dash lines.
  • valves of the various throttling systems described may be manually or power operated.
  • the power operated valves may be controlled by any ⁇ one or a combination of the following systems using conventional and known components.
  • FIG. 5e shows a power operator P for the throttling valve V governed by an operator controller C through the connecting dash line control circuit.
  • the valve operator controller C is manually set to open -or close the throttling valve to any end or intermediate position.
  • FIG. 5f shows a system in which the throttling valve V is opened or closed by pressure controller PC and the connecting dash line control circuit to control downstream fluid pressure in the solid line fluid conduit to any preset constant pressure or programmed pressure variation with time, ow or generator load which is lower than upstream pressure.
  • Pressure controller PC receives its pressure impulse from pressure tap PT located in the downstream steam pipe and the connecting dash line control circuit.
  • FIG. g shows a system in which the throttling valve V in the serially connected solid line fluid conduit and between the steam generator waterwalls and superheater outlet and upstream of superheating heat absorption conduits (indicated by the saw-tooth solid line) is opened or closed to control downstream uid pressure in the said serially connected fluid conduit which, in turn, controls steam temperature in the said serially connected fluid conduit downstream of the superheater element and in an intermediate portion of the turbine as shown in FIG- URE 8 between turbine stages S.
  • Steam temperature measurement TM through the connecting dash line control circuit to temperature controller TC through the connecting dash line control circuit to C actuates the throttling valve V upstream of all Or a portion of the superheater.
  • FIG. 5h shows a system where two or more power operated valves V in the solid line uid conduits are included in the throttling system, they may be operated in parallel or sequentially by controller SC through the connecting dash line control circuits to control downstream fluid pressure or temperature as described above.
  • FIG. 5j shows a system wherein the throttling valve V located in the same manner as in FIG. 5g above is opened or closed to control downstream fluid pressure in the said serially connected solid line fluid conduit which, in turn, controls downstream steam temperature in the said serially connected fluid conduit to limit the temperature differential between 1) the steam temperature in the said serially connected fluid conduit at some point downstream of the superheater outlet and (2) the uid conduit metal temperature.
  • raising pressure after the throttling valve raises the steam temperature
  • lowering pressure after the throttling valve lowers the steam temperature.
  • the steam temperature is higher than the conduit metal temperature, heat will ow from the steam to the metal. As the metal rises in temperature, the differential temperature will diminish.
  • the differential temperature controller DC receives steam and metal temperature measurements from points TM through the connecting dash line control conduits and will cause the throttling valve to open through the connecting dash line control conduits to C raising steam pressure and temperature. In this manner, rate of metal temperature rise can be controlled. When the steam temperature is below metal temperature, heat will flow from the metal to the steam. Lowering steam pressures by means of the differential controller DC controls rate of metal cooling. Rate of heating or cooling can be adjusted by the degree of temperature differential.
  • the throttling valve can control the steam temperature at some point in the said serially connected fluid conduit downstream of the superheater outlet to any preset constant temperature, or programmed temperature variation with time, llow, generator load or turbine stage pressure without regard to the differential temperature between the steam and metal temperatures. In such case only steam temperature measurement would be required in FIG. 5]'. Differential controller DC would become the temperature controller TC.
  • the throttling valve can supply low pressure steam to the turbine.
  • the pressure in the steam generator waterwalls and evaporating circuits can be maintained at higher pressure and saturation temperature. This accelerates the rate at which load from the steam generator can be increased during hot starts.
  • Steam throttling can be divided between the pressure reducing system in the boiler and the turbne steam admission system, minimizing quenching in the turbine steam admission system for a hot start and saturation pressure and temperature differential across the turbine throttling device for a cold start.
  • the throttling valve permits the upsteam waterwalls to be ⁇ operated at a 'higher pressure than the downstream superheater. Reducing the pressure before the final superheater lowers the steam temperature below what it would have been at a higher pressure without the pressure reducing system. This increases the temperature gradient between (l) the gas and (2) the resultant liquid and/or steam after pressure reduction. This results in greater heat transfer which increases the enthalpy of the superheater outlet steam.
  • the above control can be used to advantage to control cooling or heating in a turbine during shut-down or start-up operations.
  • the bypass BP may be used to establish the necessary flow through once-through steam generator'waterwalls when firing the furnace prior to establishing substantial flow through the throttling and shut-off valve system.
  • the bypass BP may be used to increase firing rate to increase superheater outlet steam enthalpy.
  • the steam generator heat absorption conduits shown on FIGS. 5a and 5b illustrate a steam generator having interdependent components between the steam generator waterwall inlet and superheater steam outlet when steam is discharged through the superheater outlet conduit t-o a steam consumer as the turbine shown in FIG. 8. ⁇ There is no means for bypassing flow around heat absorption components for taking them out of service when the combined unit is in normal operation after startup.
  • the bypass BP as described above may be used for starting up a once-through steam generator or increasing superheater outlet steam enthalpy.
  • the objective of the control system embodying my in- Vention shown in FIG. 6, is to provide a safer steam-electric generating unit when starting up and shutting down by restricting metal temperature rate of change and controlling steam temperatures to eliminate damage from thermal stress.
  • Such a control system can provide the operator with indication of a comparative and supervisory character or it can automatically control part or all of the related boiler and turbine plant equipment.
  • the steam temperature in the turbine steam flow conduits 43 is measured by temperature detector l. 2 is an alternate location for temperature detector 1 and would be connected at 2 in the same manner as is shown for temperature detector 1. All of the control components shown are conventional and known. The signal from detector trol point.
  • a signal from controller 3 is sent through control conduit 45 to 4 to restore temperature at 1 to the control point.
  • 4 may be one or more -of the following: steam generator internal flow bypass valve operator controls, fuel feed controls, gas bypass damper controls, excess air or gas recirculation or gas tempering controls, attemperation or desuperheating spray water controls, or a device to bias heat absorption ratio between evaporating and superheating duty, to suit the specific boiler design involved.
  • the temperature detector 5 measures superheater outlet steam temperature in the serially connected fluid conduit 9.
  • the signal from detector 5 feeds to controller 3 through control conduit 47 and may be used to record, alarm or limit the control action of controller 3 and is optional.
  • the temperature detector 6 measures metal temperature adjacent to detector 1.
  • the signal from detector 6 feeds to controller 3 through control conduit 46 and may be used to record or alarm differential between detectors 1 and 6 above preset values.
  • Detectors 6 is optional.
  • k7 is an alternate location for 6 associated with location 2 above. Detector 6 would be connected in the same manner as shown if installed at location 7. Where supervisory indication only, is required, recorder -or controller 3 just indicates temperatures and/or indicates temperature dilferentials and/or actuates an alarm. Control conduit 45 would be eliminated.
  • Rotating blades 49 are mounted on turbine spindle 50.
  • Stationary blades 51 are mounted on the turbine stationary frame 52. Steam ilow through conduits 43 passes through the blades 49 and 51 performing work and driving the turbine shaft 53.
  • Throttling valve(s) 14 open raising steam pressure and temperature.
  • the rate of metal temperature change can be controlled by adjusting the temperature differential control setting of controller 12.
  • heat will flow from the metal to the steam.
  • the rate of metal cooling can be controlled by lowering the steam pressure with controller 12.
  • Temperature detector 15 is equivalent to detector 10 and would be connected to 12 through conduit 48 and detector 16 is equivalent to detector 11 and would be connected to 12 through conduit 54 when the throttle/ stop valve/s 18 is/ are used as an alternate or substitute con ⁇
  • Power operator 58 actuates the governor valves 54 and t l0 power operator 59 actuates the throttle stop valve/s 18'.
  • Power operators 58 and 59 are associated with the turbine 44 speed controls (not shown).
  • the boiler is normally operated with constant superheater outlet pressure.
  • throttling system 14 When throttling system 14 is used to control steam pressures at 8 or 9, the pressure upstream of 14 can be controlled by steam pressure controller 19. Pressure measurement means for such case is shown at 20. Control conduit 60 connects 19 and 20. Where supervisory indication only is required, 12 just indicates temperatures and/ or indicates temperature differentials and/ or actuates an alarm. Where the throttling valve/s 14 is/are located before the superheater outlet between heat absorption conduits, temperature detector 21 through control conduit 61 to 3 functions in the same manner as 5, to prevent the temperature at 21 from exceeding a preset limit.
  • pressure at points 8 or 9 can be controlled by a pressure tap as a substitute for detector 10 or 15, which tap feeds an impulse to a pressure controller as a substitute for controller 12.
  • Detectors 11 and 16 and conduit 54 are not required.
  • the pressure controller 12 controls steam pressure at 10 or 15 to any preset constant pressure or programmed pressure variation with time, or load, or flow, or boiler outlet steam' high enthalpy capability.
  • Pressure controller 12 feeds signals to controller 13 through conduit 55 and from 13 to 57 through conduit 56 to actuate valve 14.
  • pressure controller 19 may be non-operative while pressure is being raised to design level after firing is commenced. Pressure in the evaporating circuits may be allowed to float below design level as permitted by the steam generator design and water conditions so that increased or decreased firing rate will raise or lower pressure upstream of throttling valve/ s 14as Well as increase or decrease steam enthalpy as required by controller 3 where the firing rate is used as a device for controlling the turbine first sta-ge exhaust steam temperature. Normally, after start-up, the pressure in a conventional drum type steam generator evaporating circuits will drift up to design level. In doing this, small pressure drops in the evaporating circuits, as a result of temporary decreased tiring rates, provide quick response for correcting high supert heater outlet steam enthalpy.. The corrections can be made independently of pressure control downstream of valve/s 14.
  • 1 governor valve nozzle chest 62 o-r other internal portion of the high pressure turbine sends a signal to temperature recorder and/or controller 23 through control conduit 63.
  • the controller 23 operates from rate of temperature change function. I f the rate of ⁇ change exceeds a preset value, controller 2 3 sends a si-gnal to steam temperature controller 3 through control conduit 64 counterbalancing .the signal resulting from temperature change at 1. 26 is an alternate loca.- ton for 22 and would be connected to 23 through conduit 63.
  • the othe-r/s is/ are located in the metal of another/other governor valve nozzle chest/s or other part/ s associated with steam admission to the first stage.
  • the impulse from each of detectors 22 is fed through control conduits 63 to a temperature differential recorder and/ or controller as an alternate for or supplement to 23. If the temperature differential between any two associated points exceeds a preset valve, controller 23 sends a signal through control conduit 64 to steam temperature controller 3 counterbalancin-g the signal resulting from temperature change at 1. Where supervisory indication only is required 23 just indicate-s temperature and/or indicates temperature differentials and/or actuates an alarm control conduit 64 is not required.
  • Controller 23 can be used to control rate of load change. In such case a signal is sent from controller 23 to controller 41 which increases, holds constant or decreases the turbine speed changer or governor setting (not shown). For start-up, increase above controller 23 set point decreases or holds constant turbine steam ilow; decrease below controller 23 set point increases turbine steam tlow. Reverse for shut-down. Controller 23 signal may merely limit another preset rate of load change (not shown).
  • Temperatureload change comparator 25 is equipped with a chart (not shown) having a relatively fast speed such as one inch/ three minutes. Temperature at 22 through conduit 63 to controller 23 through control conduit 65 to comparator 25 and generator electrical output from 24 through control conduiti66 to comparator 25 are separately registered on the chart in comparator 25. The slope of the two plots and, their juxtaposition one with the other indicate acceptable load change with respect to rate of metal temperature change.
  • control systems described above and shown v1n FIG. 6 are convenient ones which can be made automatic or used for supervisory purposes at a time when there are numerous operations taking place in the plant.
  • the operators duties are simplified during start-up or shutdown.
  • FIG. 7 shows a system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation.
  • the turbine reheat inlet steam temperature will also be increased.
  • FIG. 7a shows portions of the physical structures illustrated in FIG. 7.
  • FIGS. 7b and 7c are enlarged views of FIG. 7a.
  • Steam from a high pressure turbine and steam generator reheater (not shown) ilows through an intermediate pressure reheat turbine 31 thence through crossover pipe 30 which is connected to a low pressure turbine 32.
  • Injection steam from the steam generator drum, intermediate or iinal superheater, or from the boiler starting up bypass system, is supplied to crossover pipe 30 through pipe 33 and distribution chamber 35. Holes 36 admit steam uniformly from distribution chamber 35 t0 crossover pipe 30.
  • Injection steam ow to crossover pipe 30 is regulated by a power operated control valve 34, such as an air operated valve.
  • a pair of air or oil operated valves 37 protect the turbine from overspeeding by closing when the turbine trips.
  • Control valve 34 is actuated by controller 38 through control conduit 67, controller 38 receiving temperature detection signals from measuring element 39 located in the exhaust steam flow path or from measuring element 40 located in the metal of the exhaust structure.
  • Control conduit 67 connects 38 and 39.
  • Control conduit 68 connects 38 and 40.
  • controller 38 may be manually set to control the opening of valve 34 to any preset fixed position or be set to automatically control the valve 34 position from controller 69 and connecting control conduit 70 programmed with time, ow, generator load or turbine stage pressure independent of temperature detectors 39 and 40 and control conduits 67 and 68. If the additive steam supply exceeds an enthalpy of 1200 B.t.u./lb. it should be desuperheated beforehand. The best enthalpy value ranges between 1150 and 1200 B.t.u./lb.
  • I have provided ethcient devices and systems yfor improving the operating flexibility of steamelectric generating units including a throttling valve located after the waterwall section and before the superheater steam outlet and between heat absorption conduits of the steam generator to enable the steam generator to be operated at two pressure levels, the lowest level being downstream of the valve system; also including an automatic control or a supervisory system governing start-up and shut-down; also including a system for controlling the steam-temperature of a low pressure turbine exhaust during start-up by injecting controlled amounts of steam into the crossover between the low pressure turbine and the upstream turbine element; also including other systems for improving operation and elliciency of steam-electric generating units.
  • a steam-electric generating plant comprising a steam generator having heat absorption conduits including waterwalls and a superheater, a steam turbine including steam admission controls, fluid conduit means serially interconnecting said waterwalls, superheater and steam turbine, throttling valve means located in said serially connected iluid conduit means between said waterwalls and at least a portion of said Superheater and which is operable from the fully open to the fully closed working position, and control means for effectively regulating the steam temperature in a portion of said turbine for varying load conditions, said control means being responsive to a variable condition of said turbine for selectively opening said throttling valve means so as to selectively vary the pressure in said superheater portion in a range between the pressure which results from the valve means when in the fully open working position, down to the minimum throttled position required to admit only sufcient stea-nrto said turbine to satisfy any required turbine load.
  • a high pressure steam-electric generating plant comprising .a steam generator having heat absorption conduits including waterwalls and a Superheater, a steam 13 turbine drive having a high pressure turbine and ineluding steam admission controls at the inlet to the high prssure turbine, fluid conduit means for ⁇ serially connecting together said waterwalls, superheater and ste-am turbine drive, throttlig Valve means located in said serially Connected Huid conduit means beyond said waterwalls and upstream of at least a portion of said superheater, selec tive opening of said throttling valve means producing variable downstream fluid pressure, control means for regulating iluid pressure upstream of said throttling valve means, and for controlling said throttling valve means and said steam admission controls and being responsive to a variable condition of said turbine drive for controlling said'variable downstream iiuid pressure independently of the upstream fluid pressure in said waterwalls, so as to selectively raise, lower and stabilize steam temperature within said steam turbine drive during shutdown, startup and low
  • a steam-electric generating plant comprising a steam generator having heat absorption conduits includ- ,ing waterwalls .and a superheater, a steam turbine including steam admission Valve means, iluid conducting conduit means serially interconnecting said waterwalls, superheater and steam turbine, throttling valve means located in said serially connected conduit means between said waterwalls and at least a portion of said superheater, the method of controlling the metal temperature of a portion of said steam turbine structure which comprises selectively closing said throttling valve means from its fully open working position to a substantially throttled position While opening said steam admission valve means so as to selectively decrease the duid pressure lin the said superheated portion, whereby the metal temperature of said steam turbine portion is selectively varied to satisfy different load conditions.

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Description

Jan. 4, 1966 c. sTRoHMEYE-'JR 3,226,932
DEVICES Fon IMPRovING OPERATING FLEXIBILITY oF STEAM-ELECTRIC GENERATING PLANTS Fllod June `'7. 1960 5 Sheets-Sheet 1 FLOW, lo OF RATED FLOW his ATTORNEY Jan. 4, 1966 c. sRoHMEYER, JR 3,226,932
DEVICES FOR IuPRovlNG OPERATING FLEXIBILITY 0F STEAM-ELECTRIC GENERATING PLANTS Flled June 7, 1960 5 Sheets-Sheet 2 70o m .J I
z j soo LLI Q oQ H0035 so 25 20 l5 |o 5 o PRESSURE-HUNDREDS OF LBS/SCJN. INVENTOR Charles StrohmeyenJr. F|g.2.
his ATToR-E Y Jan. 4, 1966 c. STRQHMEYER, JR 3,226,932
DEVICES FOR IMPROVING OPERATING FLEXIBILITY OF STEAM-ELECTRIC GENERATING PLANTS Flled June 7, 1960 5 SheetS-Sheet 3 F' -4 IRREHEAT TURBINEINLET |000 soo 3K1 xTRAcTioN 700 4mExTRAcT|oN Lu 600 f n: of lgjechon 50o |.P.REHEAT TuRBl T II (doynsr/eam of injeclo/n point) L 40o 0- E E 30o ZOO lOO
O 2O 40 60 8O |00 |20 FLow, oF RATED FLow sTEAM F y 59, GENERATOR INITIAL FTNAL STEAM g WATERWALLS a suPERHEATER n suPERHEATER GENERATOR his ATTORNEY 7* Jan. 4, 1966 c. STROHMEYER, JR 3,226,932 DEVICES FOR IMPROVING OPERATING FLEXIBILITY OF Filed June 7. 1960 STEAM-ELECTRIC GENERATING PLANTS 5 Sheets-Sheet 4 3 uz-'mp4s ii l I| [54 |l .1146
42l l l l 332 345 INJECTION STEAM REHEAT C ROSSOVER Fi nv vENToR 70 Charles Strohmeyer, Jr. BY l I 58 (turbine exhaust) 40 his ATTORNEY Jan. 4, 1966 c. sTRoHMEYER, JR 3,226,932 DEVICES FOR IMPROVING OPERATING FLEXIBILITY OF STEAM-ELECTRI C GENERATING PLANTS 5 Sheets-Sheet 5 Filed June '7. 1960 United States Patent This invention relates to devices and systems for improving the operating flexibility of steam-electric l generating units including steam generators, turbine-generators and auxiliary equipments. s
Three basic problems that have given riseto consider- `able difficulty in the operation `of large units have been (l) the rather difficult and` prolonged start-up` time, (2) the inability to match steam and metal temperatures in `the turbine during start-up and shut-down and (3) the inability to operate these units satisfactorily at lower load condition-s when this feature becomes desirable due to the` installation of future, more eiiicient generation.
An object of the invention is to provide a novel apparatus and system providing a complete `solution to the a-bovernentioned problems.
A more specific object of the invention is to provide a throttling valve located after the waterwall section and before the superheater steam outlet of the steam generator.
A further specific object of the `invention is to provide a novel system for controll-ing or supervising steam conditions in the high pressure turbine during shut-down, start-up and low load operation to best suit turbine metal temperatures for both hot and col-d conditions. s
A still further specific object of the invention is to provide a novel system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation.
Other objects and advantages of the `invention will become more apparent from a study of the following description taken with the accompanying drawings wherein:
FIG. 1 is a typical load diversity chart in which thermal generation is plotted against time;
FIG. 2 shows steam enthalpy plotted against pressure, showing temperature drop for a given pressure drop;
FIG. 3 shows steam temperatures in the high pressure turbine of a conventional reheat unit plotted against percent of rated ow.
FIG. 4 shows steam temperatures in the reheat intermediate and low pressure turbines plotted against percent of rated flow. s
FIGS. 5a to 5h inclusive and FIG. 5j show various modifications of throttling valve systems located after the waterwall section and before the superheater steam outlet and between heat absorption conduits of the steam generator;
FIG. 6 shows a system forcontrolling or supervising` Problem background Steam-electric generating -plants presently in use have Patented Jan. 4, 1966 volved in the following description. 'I'he same partially applies to straight condensing designs.
It w-as originally planned that steam-electric generating units having the best heat rates would be operated continuously 24 hours a day at high load factor. Therefore, shut-down and start-up economies were not given serious consideration at the time when these units were designed. The older less efficient units were to be operated at lminimum load or were to be shut-down during t-he midnight hours or at other times when system demand was low, such as on weekends and holidays.
The ratio of high eliiciency conventional steam-electric generating plants to total installed generating capacity is increasing, especially since an efficiency plateau based on economic factors has been reached. There are situations developing which require daily load reductions or shutdowns of high efiiciency steam-electric generating unit-s. This requirement may be seasonal or on a continuing basis throughout the year. Some of the responsible factors are: Y
(a) High ratio of high efficiency generating capacit to total generating capacity.
(b) Fuel cost differentials which favor generation in one area of a utility system network vs. another.
`(c) Generation from an alternate source, as hydroelectric generation, where there is inadequate regulation of water flow which would require water to be wasted if steam generation were not curtailed; alternately, hydro generation costs during off peak hours may .be less than thermal generation fuel costs.
(d) Generation from future nuclear power plants. It is anticipated that, while the capital cost of the nuclear plant will ybe high, the fuel cost will be lower than in a conventional steam-electric generating plant.
(e) Increased number of shut-downs required for maintenance of high efficiency units.
A typical example of load diversity involving (a) and (c) above is shown in FIGURE 1.
`In changing load and in starting up or shutting down a steam-electric generating unit, one of the main considerations with respect to safe operation relates to metal temperature changes in heavy walled equipment components. Excessive differential temperature across metal cross sections can cause stresses which exceed the elastic limit of the material and result in failures. Metal temperatures depend upon the surrounding environment. If environmental temperature changes are too rapid or are unbalanced, the cross section differential temperatures may be increased to points which produce permanent metal deformation. Where the environmental temperature conditions change with load, safe operation indicates that load changes be made slowly, the rate being influenced by the degree of change. On the other hand, if environmental temperature conditions could be maintained constant for all loads, temperature limitations with respect to load change would be eliminated.
Temperature problems must find their solution in steam generator and turbine designs which are thoroughly coordinated. There are limits for unit start-up depending upon the condition of the turbine and steam generator at time of start-up. One extreme is where a unit has completely cooled after a maintenance program or prolonged outage, the other extreme is where the unit is in a hot operating state after an unexpected trip-out.
Steam temperature characteristics for many steam generators are such that at rated steam pressure both superheat and reheat steam temperatures droop with load in the lower range. To maintain metal temperature differential within manufacturers tolerances, by control of rate of cooling, it is necessary to shut the unit down slowly. Also, upon restart after the turbine has had time to cool still further, it is necessary to proceed slowly to control the 3 rate of heating. This characteristic is satisfactory when it is desired to overhaul a machine after shut-down as the turbine can be cooled during the shut-down period minimizing the waiting period before maintenance work can commence.
However, if the unit is to be removed from the line for power generation cost or load considerations only and restarted 6 to 8 hours later, prolonged shut-down and startup periods make this type of operation uneconomical. For a short shut-down of this type, the optimum condition is Where the turbine can be shutdown rapidly in a hot state and during restart, steam can be supplied to the turbine at temperatures which will immediately start restoring operating temperature levels at satisfactory rates of rise without causing excessive temperature differentials among components.
It is impossible to accomplish the objective described in the preceding paragraph where steam generator outlet steam temperature at design pressure droops with load, unless some manipulation is employed.
In most turbines steam is admitted individually to separate chambers which supply the nozzles to the initial impulse stage. When steam is fed through the governing valves to each nozzle chamber sequentially and cumulatively, this is called partial admission. When steam is fed through the governing valve or valves to all the nozzles at the same time and at the same rate, this is called full admission. Partial admission takes maximum advantage of design throttle steam pressure at all loads and increases part load efficiency. For full admission, effective throttle steam pressure roughly decreases proportionally with load.
For very low steam flows to the turbine for either partial or full admission, the pressure upstream of the nozzles for the initial stage is very low. Therefore, the turbine does not require design throttle pressure during low flow periods for satisfactory operation.
Pressure reduction of steam at constant enthalpy results in a temperature drop. Temperature drop for a given pressure drop is dependent upon the initial steam temperature before reduction of pressure. These characteristics are shown in FIGURE 2. In reducing 2400 p.s.i. steam at 1050 F. down to 200 p.s.i., there is a loss in temperature of approximately 120 F. In reducing 2400 p.s.i. steam at 750 F. down to 200 p.s.i., there is a loss in temperature of approximately 280 F.
If the pressure reduction of steam occurs in the superheating zone ofthe steam generator during low ow periods, the accompanying temperature drop increases the temperature differential between the resultant fluid and/ or vapor in the superheater tubes after pressure reduction and the hotter gas surrounding the tubes. This increases the transfer of heat from the gas to the fluid and/ or vapor and raises the heat content or enthalpy (B.t.u. per pound) of the superheater outlet steam Where the waterwalls are operated at high pressures. For low loads, the turbine first stage nozzle chest and downstream steam pressures are a function of flow and are substantially below design values. For any given pressure, steam temperature is a function of enthalpy. Therefore, for low iows, pressure reduction in the superheating zone of the steam generator above nozzle chest requirements raises steam enthalpy and temperatures in the turbine first stage nozzle chamber and downstream blade path. This is a desirable substitute for high pressure superheater outlet steam during the hot condition where the boiler outlet steam temperature characteristics are such that at design pressure, temperature droops with load. The pressure in the waterwalls may be maximized, minimizing the time required to shut down the turbine hot and subsequently to re-start a hot turbine.
If the above mentioned pressure reduction of steam occurs across the turbine steam supply admission valves, the accompanying temperature drop will create a metal temperature differential in the valve body above and below the valve seat. For the hot start-up condition where the steam generator outlet steam temperature at design pressure decreases with load, pressure reduction across hot turbine steam supply admission valves can chill downstream metals and damage them. Such type of throttling should be minimized.
For starting a cold unit, rate of metal temperature rise should be restrained to avoid excessive metal stresses. Low superheater outlet steam pressures are desired because downstream metal temperatures rapidly follow saturation steam temperatures as a result of steam condensation. Saturation temperature is a function of pressure. Where metals are cold, raising steam pressure too rapidly will result in high heat transfer rates from the steam to the metal up to the saturation temperature level. This,
'in turn, causes excessive temperature differentials in the metal cross-sections.
When the metal temperature is raised above the steam saturation temperature for a given pressure, condensation will cease and heat transfer from the steam to the metal across a dry surface will greatly reduce the heat transfer rate. The rate of metal heating may then be controlled by regulation of steam flow quantity since heat transfer is a function of both mass flow and temperature differential. For control of rate of metal heating, steam pressure control when metal temperatures are below saturation followed by flow control when metal temperatures yare in the steam superheat range are more important than the amount of superheat in the-steam. For starting a cold turbine, proper regulation of steam pressure and steam ow are essential.
A conventional drum steam generator may be operated at low pressures for cold start-ups. Once-through steam generators must be operated at full pressure or close thereto inthe waterwall circuits to assure proper distribution of flow and density unless variable pressure operation Vis -built into the design at great expense.
Therefore, to economically satisfy the conditions in the paragraph above, I have conceived the novel idea of reducing the pressure'after ythe waterwall'circuit and before the superheater outlet of the steam generator. This system may be used to improve turbine steam conditions for a cold or hot turbine when shutting down, starting up or operating with low load without dependence upon the pressure or temperature level of the waterwalls for both drum and once-through steam generators. The pressure reducing system has other advantages enumerated hereinafter.
FIG. 3 shows steam temperatures in the high pressure turbine of a conventional reheat unit plotted against percent of rated flow. From the solid lines it can be seen that where throttle steam temperatureis held constant at 1050 F. and 2400 p.s.i.g. and employing partial admission, the first stage exhaust, first extraction and high pressure turbine exhaust steam temperatures decrease with load. The greatest temperature change occurs in the first stage which is the limiting condition with respect to rate of load change.
The dash lines on FIG. 3 indicate high pressure turbine steam temperatures where throttle steam temperature decreases with load; throttle pressure is held at 2400 p.s.i.g. and partial admission is employed.
FIG. 4 shows steam temperatures in the reheat intermediate and loW pressure turbines plotted against percent of rated flow. Hot reheat steam flow to the intermediate pressure turbine is of the full admission type. Pressure at any point inthe flow path from the inlet of the LP. turbine to the condenser is dependent upon the downstream ow. From the solid lines on FIGURE 4 it can be seen that if the'reheat turbine inlet temperature is held constant, the downstream temperatures remain approximately constant at all fiows except toward the exhaust end which is affected the greatest extent by leaving losses. The exhaust temperatures would be excessive in the low flow range unless some provision is made to correct this situation. This may be done by injecting low enthalpy steam into the crossover between the intermediateand low pressure turbines.` The dot-dash lines show operation with injection steam. This enables the reheat turbine to be operated at near constant temperature over the entire load range. The dash lines `show the effect of decreasing reheat inlet steam temperature with load.
Temperature changes in the reheat turbine need impose no restrictions with respect to stop-start and load swing operation providing inlet steam temperatures are held near design values and cooling steam is injected into the cross-over between the I.P. and LP. turbines when throttle admission flows `are small.
FIG. 8 shows the main steam and water cycle of the conventional reheat unit used for FIGS. 3 and 4 and is typical of the steam-electric generating unit described in this specification. The solid lines are water conduits and the dash lines are steam conduits. All components are standard and are produced commercially. Flow control valves and bypasses in the various conduits are omitted for the purpose of simplification. They would be arranged in a conventional manner and are not a part of this invention. Direction of ow is indicated by the arrows in the steam and water conduits. The high pressure and intermediate pressure reheat turbines are connected by a common driving shaft which drives an electric generator. The low pressure turbines also drive their own electric generator. Extraction steam ows are from intermediate stages between turbine blade rows in the turbine steam iiow path. FIG. 8 shows the general relationships of the plant components. It will be understood that details as the arrangement of the turbine elements, extraction points, feedwater heaters, pumps and other auxiliaries can vary to accomplish the same overall results within the intent of the cycle illustrated in FIG. 8. Also, there may be more than one stage of steam reheating in the steam generator associated with the cycle as is known to exist in commercially operating plants.
General description of present invention Generally stated, there are three separate but interrelated control elements involved in the present invention, namely:
Item 1.--A throttling and shut-off valve system located after the waterwall section and before the superheater steam outlet of the steam generator and between heat absorption conduits, such as shown in'FIGS. 5a and 5b. Various modifications of such System are shown schematically in FIGS. 5a to 5j inclusive. This system per mits the steam generator to be operated at two pressure levels, the lowest level being downstream of the valve. This system may stand on its own merits and does not require the systems described in items 2 and 3 below.
Item 2.-An automatic control or a supervisory system governing start-up and shut-down of the steam and turbine generator. Such system is shown in FIG. 6. Superheater outlet steam enthalpy is controlled from steam temperature in the turbine first stageexhaust or subsequent downstream point before reheat is added to the steam. Turbine throttle pressure is controlled from the differential between steam and metal temperature in the throttle valve or governor valve chest, or is controlled to a constant pressure, or to a pressure variation programmed with time, load or flow, or is controlled to increase enthalpy of the steam generator superheater outlet steam during low steam flow periods. Temperature differentials between the turbine nozzle chests and/ or rate of metal temperature change of parts in or a part of the high pressure turbine cylinder, which are exposed to primary steam supply to the first stage nozzles, limit the rate of throttle steam enthalpy rise and rate of turbine load increase.
Item 3.--A system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation.` Such system is shown in FIG.
7. Steam from the steam generator drum, low temperature superheater, high temperature superheater after attemperation or desuperheating, or from the steam generator starting bypass system, is injected into the crossover between the low pressure turbine and the upstream turbine element. The increased mass flow through the exhaust decreases leaving losses per pound of steam flow and lowers exhaust steam temperature. The lower enthalpy of the injection steam reduces the temperature of the mixture in the crossover before entering the low pressure turbine. Low pressure turbine exhaust temperature is controlled manually or automatically by a temperature detector or detectors located in the low pressure turbine exhaust steam flow or in the exhaust structure metal. This (or these) in turn control/s injection steam flow. The amount of injection steam flow during low loads may also be controlled at a constant or variable rate by a signal fed to the injection steam supply valve controller from generator load, turbine governor system, turbine stage pressure or ow.
Specific description of present invention FIGS. 5a and 5b shows two different locations for the throttling valve T in the solid line fluid conduits between the steam generator waterwalls and superheater outlet and between heat absorption conduits indicated by the saw tooth solid line. The steam generator fluid circuits include a steam drum or steam and water separator D for pressures below critical. Such drum or separator D may `be omitted from the circuit for once-through steam generators designed for subcritical or supercritical pressures. A throttling and shut-off valve T is located in each of one 0r more parallel ow circuits as shown in either FIG. 5a or 5b. The arrow indicates the direction of fiow in the fluid conduits.
While not specifically shown on FIGS. 5a and 5b, a conventional drum type steam generator has internal recirculation conduits connecting the steam drum and the steam generating waterwalls.
Part of the fluid discharge from the steam generator waterwalls may be drawn off from the main flow circuit before throttling valve T through bypass line BP. Bypass BP may return to the cycle wholly or in part at some point 4between the superheater outlet and the water supply point to the steam generator. Bypass BP may discharge fluid from the drum or steam and 'water separator D, or may discharge fluid from any other downstream point in the main flow line before throttling valve T, in the case where the drum or steam and water separator D is included in the main circuit. Bypass BP may discharge fluid from any point in the main ow line downstream of the `waterwalls vup to throttling valve T in the case where the drum or steam and water separator is not included in the main circuit.
The throttling valve in one single flow line or in multiple parallel flow lines may consist of a single valve in each ow line.
FIG. 5c shows a multiple or parallel flow line consisting of a single main line valve V or valves in the solid line uid conduit connected to the arrow and one or more smaller bypass valves in the dash line fluid conduit connected `to the solid line fluid conduit.
FIG. 5d shows a throttling valve V in a single ow line or in multiple parallel flow lines consisting of a valve in all parallel circuits to each end of a main ow line, the parallel circuits being shown as solid and dash lines.
Any one or all of the valves of the various throttling systems described may be manually or power operated. The power operated valves may be controlled by any `one or a combination of the following systems using conventional and known components.
FIG. 5e shows a power operator P for the throttling valve V governed by an operator controller C through the connecting dash line control circuit. The valve operator controller C is manually set to open -or close the throttling valve to any end or intermediate position.
FIG. 5f shows a system in which the throttling valve V is opened or closed by pressure controller PC and the connecting dash line control circuit to control downstream fluid pressure in the solid line fluid conduit to any preset constant pressure or programmed pressure variation with time, ow or generator load which is lower than upstream pressure. Pressure controller PC receives its pressure impulse from pressure tap PT located in the downstream steam pipe and the connecting dash line control circuit.
FIG. g shows a system in which the throttling valve V in the serially connected solid line fluid conduit and between the steam generator waterwalls and superheater outlet and upstream of superheating heat absorption conduits (indicated by the saw-tooth solid line) is opened or closed to control downstream uid pressure in the said serially connected fluid conduit which, in turn, controls steam temperature in the said serially connected fluid conduit downstream of the superheater element and in an intermediate portion of the turbine as shown in FIG- URE 8 between turbine stages S. Steam temperature measurement TM through the connecting dash line control circuit to temperature controller TC through the connecting dash line control circuit to C actuates the throttling valve V upstream of all Or a portion of the superheater.
FIG. 5h shows a system where two or more power operated valves V in the solid line uid conduits are included in the throttling system, they may be operated in parallel or sequentially by controller SC through the connecting dash line control circuits to control downstream fluid pressure or temperature as described above.
FIG. 5j shows a system wherein the throttling valve V located in the same manner as in FIG. 5g above is opened or closed to control downstream fluid pressure in the said serially connected solid line fluid conduit which, in turn, controls downstream steam temperature in the said serially connected fluid conduit to limit the temperature differential between 1) the steam temperature in the said serially connected fluid conduit at some point downstream of the superheater outlet and (2) the uid conduit metal temperature. For a given steam enthalpy, raising pressure after the throttling valve raises the steam temperature, and lowering pressure after the throttling valve lowers the steam temperature. When the steam temperature is higher than the conduit metal temperature, heat will ow from the steam to the metal. As the metal rises in temperature, the differential temperature will diminish. The differential temperature controller DC receives steam and metal temperature measurements from points TM through the connecting dash line control conduits and will cause the throttling valve to open through the connecting dash line control conduits to C raising steam pressure and temperature. In this manner, rate of metal temperature rise can be controlled. When the steam temperature is below metal temperature, heat will flow from the metal to the steam. Lowering steam pressures by means of the differential controller DC controls rate of metal cooling. Rate of heating or cooling can be adjusted by the degree of temperature differential.
As an alternate or supplement to the above paragraph, the throttling valve can control the steam temperature at some point in the said serially connected fluid conduit downstream of the superheater outlet to any preset constant temperature, or programmed temperature variation with time, llow, generator load or turbine stage pressure without regard to the differential temperature between the steam and metal temperatures. In such case only steam temperature measurement would be required in FIG. 5]'. Differential controller DC would become the temperature controller TC.
The throttling valve can supply low pressure steam to the turbine. The pressure in the steam generator waterwalls and evaporating circuits can be maintained at higher pressure and saturation temperature. This accelerates the rate at which load from the steam generator can be increased during hot starts. Steam throttling can be divided between the pressure reducing system in the boiler and the turbne steam admission system, minimizing quenching in the turbine steam admission system for a hot start and saturation pressure and temperature differential across the turbine throttling device for a cold start. In starting up a unit, to control steam temperatures in the turbine, dual pressure operation of the steam generator (higher pressure upstream and lower pressure downstream yof the throttling) permits changes in tiring rates which increase or decrease upstream steam pressure as well as superheater outlet steam enthalpy at a predetermined downstream pressure. l
Pressure reduction of steam before the superheater outlet permits increase in the energy level of the outlet steam for the :same temperature. Enthalpy of steam at 2400 p.s.i.a. 1050 F.=l494.2 B.t.u./lb., enthalpy of steam at 1000 p.s.i.a. 1050 F.=l533.2 Btu/lb. This can be aC- complished while maintaining saturation temperature upstream of pressure reduction at a levelhigher than that corresponding to the superheater outletvsteam pressure. Saturated temperature at 2400 p.s.i.a.=662.12 F., and at 1000 p.s.i.a.='556.3 l F. This permits the steam generator to produce a high enthalpy, low pressure steam supply for starting a hot turbine without degrading tempera- ,ture level of the waterwalls by operating the entire steam generator at a lower pressure.
The throttling valve permits the upsteam waterwalls to be `operated at a 'higher pressure than the downstream superheater. Reducing the pressure before the final superheater lowers the steam temperature below what it would have been at a higher pressure without the pressure reducing system. This increases the temperature gradient between (l) the gas and (2) the resultant liquid and/or steam after pressure reduction. This results in greater heat transfer which increases the enthalpy of the superheater outlet steam. The above control can be used to advantage to control cooling or heating in a turbine during shut-down or start-up operations.
The bypass BP may be used to establish the necessary flow through once-through steam generator'waterwalls when firing the furnace prior to establishing substantial flow through the throttling and shut-off valve system. The bypass BP may be used to increase firing rate to increase superheater outlet steam enthalpy.
The steam generator heat absorption conduits shown on FIGS. 5a and 5b illustrate a steam generator having interdependent components between the steam generator waterwall inlet and superheater steam outlet when steam is discharged through the superheater outlet conduit t-o a steam consumer as the turbine shown in FIG. 8. `There is no means for bypassing flow around heat absorption components for taking them out of service when the combined unit is in normal operation after startup. The bypass BP as described above may be used for starting up a once-through steam generator or increasing superheater outlet steam enthalpy.
The objective of the control system embodying my in- Vention shown in FIG. 6, is to provide a safer steam-electric generating unit when starting up and shutting down by restricting metal temperature rate of change and controlling steam temperatures to eliminate damage from thermal stress. Such a control system can provide the operator with indication of a comparative and supervisory character or it can automatically control part or all of the related boiler and turbine plant equipment.
The high pressure turbine 44 rst stage exhaust steam temperature at 43, or steam temperature at another downstream location 2 before reheat is added to the steam, controls the enthalpy of the steam to the turbine 44. The steam temperature in the turbine steam flow conduits 43 is measured by temperature detector l. 2 is an alternate location for temperature detector 1 and would be connected at 2 in the same manner as is shown for temperature detector 1. All of the control components shown are conventional and known. The signal from detector trol point.
9 1 is fed through con'trol conduit 42 to` steam temperature recorder and/ or controller 3 which controls steam temperature at 1 to a preset adjustable value which best suits the temperature condition of the turbine 44 during start-up or shut-down.
When steam temperature deviates from the control point, a signal from controller 3 is sent through control conduit 45 to 4 to restore temperature at 1 to the control point. 4 may be one or more -of the following: steam generator internal flow bypass valve operator controls, fuel feed controls, gas bypass damper controls, excess air or gas recirculation or gas tempering controls, attemperation or desuperheating spray water controls, or a device to bias heat absorption ratio between evaporating and superheating duty, to suit the specific boiler design involved. The temperature detector 5 measures superheater outlet steam temperature in the serially connected fluid conduit 9. The signal from detector 5 feeds to controller 3 through control conduit 47 and may be used to record, alarm or limit the control action of controller 3 and is optional. The temperature detector 6 measures metal temperature adjacent to detector 1. The signal from detector 6 feeds to controller 3 through control conduit 46 and may be used to record or alarm differential between detectors 1 and 6 above preset values. Detectors 6 is optional. k7 is an alternate location for 6 associated with location 2 above. Detector 6 would be connected in the same manner as shown if installed at location 7. Where supervisory indication only, is required, recorder -or controller 3 just indicates temperatures and/or indicates temperature dilferentials and/or actuates an alarm. Control conduit 45 would be eliminated.
Rotating blades 49 are mounted on turbine spindle 50.`
Stationary blades 51 are mounted on the turbine stationary frame 52. Steam ilow through conduits 43 passes through the blades 49 and 51 performing work and driving the turbine shaft 53.
Steam temperature on the above seat side of the governor valve(s) 54 in steam chest 17 in connecting conduit 8 (throttle/ stop valve(s) 18 open) is controlled above or below metal temperature at 11 to regulate the rate of governor valve chest 17 heating or cooling, respectively. Since steam enthalpy is controlled at point 1, steam temperature in the governor valve chest 17 may be varied the metal temperature at 11, heat will ow from the steam to the metal. As the metal rises in temperature, the differ-` ential temperature Will diminish. Differential controller 12 will feed a signal through control conduit 55 into operator controller 13 and through control conduit 56 which actuates the power operator 57 on throttling valve(s) 14 which may be located betweenthe lsteam generator Waterwalls and superheater outlet and between heat absorption conduits. Throttling valve(s) 14 open raising steam pressure and temperature. Thus, the rate of metal temperature change can be controlled by adjusting the temperature differential control setting of controller 12. When the steam temperature is below metal temperature, heat will flow from the metal to the steam. The rate of metal cooling can be controlled by lowering the steam pressure with controller 12. t
Temperature detector 15 is equivalent to detector 10 and would be connected to 12 through conduit 48 and detector 16 is equivalent to detector 11 and would be connected to 12 through conduit 54 when the throttle/ stop valve/s 18 is/ are used as an alternate or substitute con` Power operator 58 actuates the governor valves 54 and t l0 power operator 59 actuates the throttle stop valve/s 18'. Power operators 58 and 59 are associated with the turbine 44 speed controls (not shown).
The boiler is normally operated with constant superheater outlet pressure. When throttling system 14 is used to control steam pressures at 8 or 9, the pressure upstream of 14 can be controlled by steam pressure controller 19. Pressure measurement means for such case is shown at 20. Control conduit 60 connects 19 and 20. Where supervisory indication only is required, 12 just indicates temperatures and/ or indicates temperature differentials and/ or actuates an alarm. Where the throttling valve/s 14 is/are located before the superheater outlet between heat absorption conduits, temperature detector 21 through control conduit 61 to 3 functions in the same manner as 5, to prevent the temperature at 21 from exceeding a preset limit.
As an alternate to the above, pressure at points 8 or 9 can be controlled by a pressure tap as a substitute for detector 10 or 15, which tap feeds an impulse to a pressure controller as a substitute for controller 12. Detectors 11 and 16 and conduit 54 are not required. The pressure controller 12 controls steam pressure at 10 or 15 to any preset constant pressure or programmed pressure variation with time, or load, or flow, or boiler outlet steam' high enthalpy capability. Pressure controller 12 feeds signals to controller 13 through conduit 55 and from 13 to 57 through conduit 56 to actuate valve 14.
In starting up a unit where the steam-generator evaporating circuit pressure has decayed during shut-down,
pressure controller 19 may be non-operative while pressure is being raised to design level after firing is commenced. Pressure in the evaporating circuits may be allowed to float below design level as permitted by the steam generator design and water conditions so that increased or decreased firing rate will raise or lower pressure upstream of throttling valve/ s 14as Well as increase or decrease steam enthalpy as required by controller 3 where the firing rate is used as a device for controlling the turbine first sta-ge exhaust steam temperature. Normally, after start-up, the pressure in a conventional drum type steam generator evaporating circuits will drift up to design level. In doing this, small pressure drops in the evaporating circuits, as a result of temporary decreased tiring rates, provide quick response for correcting high supert heater outlet steam enthalpy.. The corrections can be made independently of pressure control downstream of valve/s 14.
The following system is optional regarding the overall control system. As a result of the above described systems, metal temperature changes upstream of steam throttlng to `the initial stages of the turbine and downstream from the first stage during start-up and shut-down can be limited through controls. After 'switching from throttle/stop valve/s steam admission control (full admission) to governor valve control (partial admission), steam temperature at 43 after the rst stage will droop. As steam enthalpy increases to raise temperatures after the irst stage, nozzle chest/s temperature/s will Ie. The following control is intended to limit nozzle chest temperature rise beyond acceptable limits. Temperature detector 22 located in the No. 1 governor valve nozzle chest 62 o-r other internal portion of the high pressure turbine sends a signal to temperature recorder and/or controller 23 through control conduit 63. The controller 23 operates from rate of temperature change function. I f the rate of` change exceeds a preset value, controller 2 3 sends a si-gnal to steam temperature controller 3 through control conduit 64 counterbalancing .the signal resulting from temperature change at 1. 26 is an alternate loca.- ton for 22 and would be connected to 23 through conduit 63.
As an alternate or supplemental control for the system described inthe preceding paragraph, there are two or more temperature detectors 22. One is located in the metal of the No. 1 governor valve nozzle chest, attached or equivalent part and the othe-r/s is/ are located in the metal of another/other governor valve nozzle chest/s or other part/ s associated with steam admission to the first stage. The impulse from each of detectors 22 is fed through control conduits 63 to a temperature differential recorder and/ or controller as an alternate for or supplement to 23. If the temperature differential between any two associated points exceeds a preset valve, controller 23 sends a signal through control conduit 64 to steam temperature controller 3 counterbalancin-g the signal resulting from temperature change at 1. Where supervisory indication only is required 23 just indicate-s temperature and/or indicates temperature differentials and/or actuates an alarm control conduit 64 is not required.
Controller 23 can be used to control rate of load change. In such case a signal is sent from controller 23 to controller 41 which increases, holds constant or decreases the turbine speed changer or governor setting (not shown). For start-up, increase above controller 23 set point decreases or holds constant turbine steam ilow; decrease below controller 23 set point increases turbine steam tlow. Reverse for shut-down. Controller 23 signal may merely limit another preset rate of load change (not shown).
The following system is also optional. Temperatureload change comparator 25 is equipped with a chart (not shown) having a relatively fast speed such as one inch/ three minutes. Temperature at 22 through conduit 63 to controller 23 through control conduit 65 to comparator 25 and generator electrical output from 24 through control conduiti66 to comparator 25 are separately registered on the chart in comparator 25. The slope of the two plots and, their juxtaposition one with the other indicate acceptable load change with respect to rate of metal temperature change. C
The control systems described above and shown v1n FIG. 6 are convenient ones which can be made automatic or used for supervisory purposes at a time when there are numerous operations taking place in the plant. The operators duties are simplified during start-up or shutdown.
FIG. 7 as described hereinabove shows a system for controlling the steam temperature of the low pressure turbine exhaust during start-up or light load operation. Steam from the boiler drum, low temperature superheater, high temperature superhcater after attemperation or desuperheating, or from the steam generator starting bypass system, is injected into the crossover to the low pressure turbine from the upstream turbine element. Where high pressure turbine exhaust steam is maintained close to design values for hot starts, the turbine reheat inlet steam temperature will also be increased. In order to prevent excessive temperature rise in the low pressure turbine exhaust, I devised the system shown in FIGURE 7. It is possible to inject spray water in the exhaust hood as an alternate. Since the spray water enters a Ihigh velocity steam zone, many people are afraid of erosion` from this type of cooling. It is necessary to cool the exhaust hood to prevent excessive expansion in the upward direction which would cause the low pressure turbine bearings to rise and take excessive shaft loading. This would disrupt shaft alignment and cause shaft eccentricity along with vibration. The injection steam to the crossover contains superheat and eliminates dangers `associated with improper mixing of spray water. Also steam injection for cooling purposes into the crossover between the intermediate and low pressure turbines provides proper temperature distribution between the turbine elements. The last stage steam temperature is maintained at the exhaust hood temperature level.
FIG. 7a shows portions of the physical structures illustrated in FIG. 7. FIGS. 7b and 7c are enlarged views of FIG. 7a. Steam from a high pressure turbine and steam generator reheater (not shown) ilows through an intermediate pressure reheat turbine 31 thence through crossover pipe 30 which is connected to a low pressure turbine 32. Injection steam from the steam generator drum, intermediate or iinal superheater, or from the boiler starting up bypass system, is supplied to crossover pipe 30 through pipe 33 and distribution chamber 35. Holes 36 admit steam uniformly from distribution chamber 35 t0 crossover pipe 30. Injection steam ow to crossover pipe 30 is regulated by a power operated control valve 34, such as an air operated valve. A pair of air or oil operated valves 37 protect the turbine from overspeeding by closing when the turbine trips. Control valve 34 is actuated by controller 38 through control conduit 67, controller 38 receiving temperature detection signals from measuring element 39 located in the exhaust steam flow path or from measuring element 40 located in the metal of the exhaust structure. Control conduit 67 connects 38 and 39. Control conduit 68 connects 38 and 40. As a supplement or alternate, controller 38 may be manually set to control the opening of valve 34 to any preset fixed position or be set to automatically control the valve 34 position from controller 69 and connecting control conduit 70 programmed with time, ow, generator load or turbine stage pressure independent of temperature detectors 39 and 40 and control conduits 67 and 68. If the additive steam supply exceeds an enthalpy of 1200 B.t.u./lb. it should be desuperheated beforehand. The best enthalpy value ranges between 1150 and 1200 B.t.u./lb.
Thusit will be seen that I have provided ethcient devices and systems yfor improving the operating flexibility of steamelectric generating units, including a throttling valve located after the waterwall section and before the superheater steam outlet and between heat absorption conduits of the steam generator to enable the steam generator to be operated at two pressure levels, the lowest level being downstream of the valve system; also including an automatic control or a supervisory system governing start-up and shut-down; also including a system for controlling the steam-temperature of a low pressure turbine exhaust during start-up by injecting controlled amounts of steam into the crossover between the low pressure turbine and the upstream turbine element; also including other systems for improving operation and elliciency of steam-electric generating units.
While I have illustrated and described several embodiments of my invention, it will be understood that these are by way of illustration only, and that various changes and modifications may be made within the contemplation of my invention and within the scope of the following claims.
I claim:
1. A steam-electric generating plant comprising a steam generator having heat absorption conduits including waterwalls and a superheater, a steam turbine including steam admission controls, fluid conduit means serially interconnecting said waterwalls, superheater and steam turbine, throttling valve means located in said serially connected iluid conduit means between said waterwalls and at least a portion of said Superheater and which is operable from the fully open to the fully closed working position, and control means for effectively regulating the steam temperature in a portion of said turbine for varying load conditions, said control means being responsive to a variable condition of said turbine for selectively opening said throttling valve means so as to selectively vary the pressure in said superheater portion in a range between the pressure which results from the valve means when in the fully open working position, down to the minimum throttled position required to admit only sufcient stea-nrto said turbine to satisfy any required turbine load.
2. A high pressure steam-electric generating plant comprising .a steam generator having heat absorption conduits including waterwalls and a Superheater, a steam 13 turbine drive having a high pressure turbine and ineluding steam admission controls at the inlet to the high prssure turbine, fluid conduit means for` serially connecting together said waterwalls, superheater and ste-am turbine drive, throttlig Valve means located in said serially Connected Huid conduit means beyond said waterwalls and upstream of at least a portion of said superheater, selec tive opening of said throttling valve means producing variable downstream fluid pressure, control means for regulating iluid pressure upstream of said throttling valve means, and for controlling said throttling valve means and said steam admission controls and being responsive to a variable condition of said turbine drive for controlling said'variable downstream iiuid pressure independently of the upstream fluid pressure in said waterwalls, so as to selectively raise, lower and stabilize steam temperature within said steam turbine drive during shutdown, startup and low load operation by control of the said variable downstream pressure down to the limit required by said steam admission controls to pass suiiicient steam to maintain operating load of said steam turbine drive.
3. A high pressure steam-electric generating plant as recited in claim 2 together with a water inlet to said waterwalls, conduit means having one end connected to said serially connected iluid conduit means downstream of said Waterwalls and before said throttling valve means, and the other end being connected to said water inlet for circulating the necessary flow in said waterwalls prior to establishing substantial llow through said throttling valve means.
4. A high pressure steam-electric generating plant as recited in claim 2, including an intermediate pressure turbine and a low pressure turbine in conjunction with said high pressure turbine, a reheater incorporated in said steam generator, said serially connected fluid conduit means continuing through said high pressure turbine to and through said reheater to and throughsaid intermediate pressure turbine to and through said low pressure turbine, and means for controlling the iluid temperature of said low pressure turbine exhaust during low load operation comprising means responsive to said exhaust temperature for injecting iluidinto said serially connected uid conduit means between said intermediate pressure turbine and said low pressure turbine at lower temperature than the uid exiting from said intermediate pressure turbine, thereby lowering iluid temperature and increasing fluid quantity to said low pressure turbine.
5. In a steam-electric generating plant comprising a steam generator having heat absorption conduits includ- ,ing waterwalls .and a superheater, a steam turbine including steam admission Valve means, iluid conducting conduit means serially interconnecting said waterwalls, superheater and steam turbine, throttling valve means located in said serially connected conduit means between said waterwalls and at least a portion of said superheater, the method of controlling the metal temperature of a portion of said steam turbine structure which comprises selectively closing said throttling valve means from its fully open working position to a substantially throttled position While opening said steam admission valve means so as to selectively decrease the duid pressure lin the said superheated portion, whereby the metal temperature of said steam turbine portion is selectively varied to satisfy different load conditions. n
References Cited by the Examiner UNITED STATES PATENTS 1,767,714 6/ 1930 Stender 122-460 1,832,150 11/1931 Stender 122-448 1,942,861 l/ 1934 Huster 122-1 2,346,179 4/1944 Meyer et al. 60-104 X 2,590,712 3/ 1952 Lacerenza 122-479 2,602,433 7/ 1952 Kuppenheimer 122-479 2,649,079 8/ 1953 Van Brunt 122-479 2,811,837 11/1957 Eggenberger 60-73 2,918,798' 12/ 1959 Schroder 60-7-3 2,989,038 6/1961 Schwarz 122-406 3,009,325 11/1961 Pirsh 60-105 3,019,774 2/ 1962 Beyerlein 122-406 3,035,557 5/1962 Litwinoft 122-479 FOREIGN PATENTS 946,148 7/1956 Germany.
201,304 8/ 1923 Great Britain.
236,253 7/ 1925 Great Britain.
234,093 8/1926 Great Britain.
851,784 10/11960 Great Britain.
879,032 10/ 1961 Great Britain.
116,651 9/ 1926 Switzerland.
OTHER REFERENCES Combustion, December 1956; page 46. German application No. 1,043,347; printed November Mitteilungen: German publication issue of September 1956.
SAMUEL LEVINE, Primary Examiner:
ABRAM BLUM, ROBERT R. BUNEVICH,
Examiners.

Claims (1)

1. A STEAM-ELECTRIC GENERATING PLANT COMPRISING A STEAM GENERATOR HAVING HEAD ABSORPTION CONDUITS INCLUDING WATERWALLS AND A SUPERHEATER, A STEAM TURBINE INCLUDING STEAM ADMISSION CONTROLS, FLUID CONDUIT MEANS SERIALLY INTERCONNECTING SAID WATERWALLS, SUPERHEATER AND STEAM TURBINE, THROTTLING VALVE MEANS LOCATED IN SAID SERIALLY CONNECTED FLUID CONDUIT MEANS BETWEEN SAID WATERWALLS AND AT LEAST A PORTION OF SAID SUPERHEATER AND WHICH IS OPERABLE FROM THE FULLY OPEN TO THE FULLY CLOSED WORKING POSITION, AND CONTROL MEANS FOR EFFECTIVELY REGULATING THE STEAM TTEMPERATURE IN A PORTION OF SAID TURBINE FOR VARYING LOAD CONDITIONS, SAID CONTROL MEANS BEING RESPONSIVE TO A VARIABLE CONDITION OF SAID TURBINE FOR SELECTIVELY OPENING SAID THROTTLING VALVE MEANS SO AS TO SELECTIVELY VARY THE PRESSURE IN SAID SUPERHEATER PORTION IN A RANGE BETWEEN THE PRESSURE WHICH RESULTS FROM THE VALVE MEANS WHEN IN THE FULLY OPEN WORKING POSITION, DOWN TO THE MINIMUM THROTTLED POSITION REQUIRED TTO ADMIT ONLY SUFFICIENT STEAM TO SAID TURBINE TO SATISFY ANY REQUIRED TURBINE LOAD.
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US3403892A (en) * 1967-01-12 1968-10-01 Gen Electric Full arc-partial arc transfer system for electrohydraulic turbine control
US3403891A (en) * 1967-01-12 1968-10-01 Gen Electric Full arc/partial arc admission using control valves
US4047386A (en) * 1976-06-10 1977-09-13 Scm Corporation Process for heating condensate
US4226086A (en) * 1979-05-21 1980-10-07 Westinghouse Electric Corp. Automatic restart control for a power plant boiler
US4561254A (en) * 1984-10-25 1985-12-31 Westinghouse Electric Corp. Initial steam flow regulator for steam turbine start-up
US5621654A (en) * 1994-04-15 1997-04-15 Long Island Lighting Company System and method for economic dispatching of electrical power
US20090260359A1 (en) * 2008-04-16 2009-10-22 Alstom Technology Ltd. Solar thermal power plant

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US5621654A (en) * 1994-04-15 1997-04-15 Long Island Lighting Company System and method for economic dispatching of electrical power
US20090260359A1 (en) * 2008-04-16 2009-10-22 Alstom Technology Ltd. Solar thermal power plant

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