US3385270A - Steam power plant with forced-flow boiler system, particularly for supercritical pressure, and a superimposed circulating system - Google Patents

Description

May 28, 1968 R. MICHEL 3,385,270

STEAM POWER PLANT WITH FORCED-FLOW BOILER SYSTEM, PARTICULARLY FOR SUPERCRITICAL PRESSURE, AND A SUPERIMPOSED CIRCULATING SYSTEM Filed Feb. 5, 1967 United States Patent 0 3,385,279 STEAM POWER PLANT WITH FORCED-FLGW BGILER SYSTEM, PARTICULARLY FOR SUPER- CRITICAL PRESSURE, AND A SUPERIMPOSED @IRCULATKNG SYSTEli i Ruppreeht Michei, Erlangen, Germany, assignor to Siemens Aktiengesellschatt, Berlin, Germany, a corporation of Germany Continuation-impart of application Ser. No. 445,427, A r. 5, N65. This apphcation Feb. 3, 15367, Sci. No. 513,793

4 Qlaims. (Cl. 122-4ti6) ABSTRACT OF THE DKSQLOSURE A temperature regulating system is combined with a steam power plant having a forced-flow system with a series-connection of working-*ncdium supply source, a preheater, an evaporator, 21 superheater having a finishing superheater surface, a load and intermediate superheater, and a circulating system superimposed upon the forcedfiow system between a high-temperature point arrear of the evaporator and a low-temperature point ahead of the evaporator. The circulating system contains an impeller pump for withdrawing medium from the forced-flow system at the high-temperature point and recycling the medium back to the low-temperature point at least during start-up and partial-load operation of the plant. The regulating system includes a regulator which simultaneously regulates the steam temperature at the intermediate superheater and the temperature of the recycled medium at the pump. The regulator has a heat exchanger connected to the intermediate superheater and connected in the circulating system for cooling the recycled medium by steam substantially at the intermediate superheater temperature and an injector connected downstream of the low-temperature point which supplies feed-water therefrom to the circulating system.

The instant application is a continuation-in-part of my copending application Ser. No. 445,427, filed Apr. 5, 1965, now abandoned, which is based upon German application S 84,554, filed Apr. 4, 1963.

My invention relates to a steam power plant with a forced-flow boiler system upon which a circulating sy tem is superimposed in such a manner that the circulating system withdraws working medium from the forced-flow system at a suitable point of relatively high temperature, for example between preheater and final superheater, and recycles the withdrawn medium back into the forcedflow system at a point of lower temperature preceding the evaporator in the flow direction and located, for example, between the economizer heating surfaces and the evaporator. The circulating system may be thus super imposed upon the forced-flow system for any type of plant operation, or preferably only during startup and partialload operations.

In such a combined forced-flow and circulating system, care must be taken that the impeller pump in the circulating system always delivers the working medium in liquid condition. This, in many cases, leads to the necessity of cooling the working medium supplied to the pump in order to secure a reliable pumping operation.

It is an object of my invention to provide a system which reliably secures proper cooling of the partial flow of working medium passing through the circulating system in a combined forced-flow and circulating system of the abovementioned kind under any occurring operating conditions.

Another object of the invention is to improve the operation of such combined systems by establishing and Fatented May 28, 1968 maintaining a favorable correlation between the cooling of the recirculating working medium and the temperature of the generating steam.

To achieve these objects, and in accordance with a feature of my invention, the cooling of the working medium withdrawn from, and recycled back into, the forced-flow system is effected to a large or preponderant portion with the aid of intermediate-superheater steam. As a result, the invention affords cooling the circulating quantity of working medium simultaneously with utilizing this operation for the temperature regulation of the steam superheated in one or more intermediate superheaters (reheaters).

According to another feature of the invention, the cooling of the recycled working medium in the circulating system of the plant is effected by passing the medium through one or more heat exchangers in which they are thermally coupled with a flow of steam coming from one or more intermediate superheaters.

According to still another feature of the invention the medium in the circulatin system is additionally cooled by coolant supplied from plant components other than the intermediate superheaters. Preferably such additional cooling is employed in cases where the regulating range for the intermediate superheating of the steam does not sutfice for satisfactorily cooling the entire quantity of recycled working medium.

According to another, more specific feature of the invention, the additional cooling of the medium in the circulating system is effected by injection of water. Applicable for this purpose is feed water withdrawn at a suitable locality arrear of the feed-water pump. In plants which have an economizer connected between the preheater portion and the evaporator portion of the forcedflow system, the feed water is preferably withdrawn from this system at a locality between the preheater and the economizer heating surface, this locality of water withdrawal being thus situated ahead of the low-temperature point at which the working medium is recycled back into the forced-flow system.

The invention will be elucidated more in detail with reference to the accompanying drawing illustrating a schematic circuit diagram of a power plant according to the invention by way of example.

The illustrated embodiment comprises a once-through boiler operating at supercritical pressure. The working medium, in the form of feed water, is supplied from a condensator 1 by a condensate pump 2 from which it passes through a low-pressure preheater portion of which two stages 3 and 4 are represented. These two preheater stages 3 and 4 are connected by the lines 50 and 51, respectively, with a low pressure stage 18 of a steam turbine 14. The preheater water then enters into a feedwater tank 5. The feed-water pump 6 for withdrawing the water from the tank 5 is rated for supercritical pressure, although the boiler operation may be in the subcritical range during startup and shutdown of the boiler and, if desired, also during partial-load operation.

On the high-pressure side of the forced-flow system, the working medium passes from the pump 6 through an inlet valve 52 to a high-pressure regenerative preheater portion of which, for simplicity of illustration, only two high-pressure preheater stages 7 and 8 are shown on the drawing. The preheater stage 7 is connected through the line 54- 'with the steam outlet line 53 of a medium-pressure stage 17 of the turbine 14, and the preheater stage 8 receives heatin steam through the lines 57 and 58 from the steam outlet line 56 of a high-pressure stage 16 of the turbine 14. The preheater portion is followed by an economizer 9 to which there are connected in series the evaporator stage 10 and a heating surface 11 which may operate as a post-evaporator. However, the heating Sl11'-- face 11, depending upon the particular operating mode of the plant, may also be looked upon as being a presuperheater heating surface, in which case the next following heating surface 12 constitutes the finishing superheater. If desired, and as illustrated, a pressure holding valve 13 may be inserted ahead of the superheater end stage 12.

The superheated fresh steam passes through a line 19 and an inlet valve to the steam turbine 14 driving an electric generator 15. The steam turbine comprises the aforementioned high-pressure stage 16, medium-pressure stage 17, and low-pressure stage 18. After leaving the high-pressure turbine 16 the steam is reheated in a first intermediate superheater (reheater) 21. The reheated steam then passes into the medium-pressure stage 17. A second intermediate superheater 22 connects the output of the turbine stage 17 with the input of the low-pressure stage 18 from which tthe steam then passes through the line 60 to the condenser 1.

The above-mentioned superimposed circulating system comprises a line 23 which branches off the forced-flow system at a point of relatively high temperature ahead of the finishing superheater 12. The working medium (steam) withdrawn through the line 23 passes through a check valve 24 into a first heat exchanger 25 in which a condensation of the working medium takes place. The coolant in exchanger 25 is constituted by the waste steam, or a partial current of the waste steam, coming from the high-pressure turbine stage 16 through the lines 56, 37 and 59. In heat exchanger 25 the condensed medium passes through a connecting line 25 into a second heat exchanger 26 in which it is further cooled by steam issuing from the medium-pressure turbine 17 and conveyed thereto through the lines 53, 38 and 55. If the circulating line 23 is branched oil at a point of the forced-flow system having a temperature of 400 C. for example, the circulating quantity can be readily kept at approximately 300 C. in the line 27 of the circulating system behind the heat exchanger 26. Consequently when operating with supercritical pressure, the impeller pump 28 connected in the circulating system is not subjected to higher maximum temperatures than about 370 C. In the illustrated embodiment, however, an injection cooler 29 is interposed in the circulating system ahead of the impeller pump 28 so that it receives feed water at a temperature of about 300 C. or less.

The automatic temperature regulation is effected by means of a feed-back control system described presently.

A main regulator 30 receives temperature datum values indicative of the temperature which the reheated steam in the intermediate superheaters (reheaters) is to maintain. Since the illustrated example is provided with two reheaters 21 and 22, two corresponding heat sensors 31 and 32 are provided to furnish respective pilot signals indicative of the steam temperature at the reheaters 21, 22, thus affording a separate temperature regulation for each of the reheaters in accordance with selected datum values respectively.

The temperature for the first reheater 21 is maintained at the selected datum value by a feed-back regulator 33 of any suitable conventional type in which the pilot signal from sensor 31 passing over the connecting line 61 is compared with the selected datum value, and the regulator output controls a three-way valve through the connecting line 62 in accordance with the amount and direction of any departure between pilot and datum values. The temperature of the reheated steam behind the second reheater 22 is maintained at the selected value by means of the regulator 34 which receives its measured value from the location 32 through the line 63 and operates in an analogous manner to the regulator 33. The two regulators 33 and 34 are subordinated to the main regulator 30 and are connected thereto by the signal lines 64 and 65, respectively.

The regulation of the flow rate of steam through the heat exchangers 25 and 26 and through the preheater stages 7 and 8 in the illustrated embodiment is performed by operation of the above-mentioned valve 35 and a corresponding valve 36, which are located at the junctions respectively of steam lines 57, 58, 59 and 53, 54, 55.

The main regulator is also connected through a signal line 66 with another subsidiary regulator 39 which receives temperature signals through a signal line 67 from a sensor 40 located in the circulating system ahead of the impeller pump 28. For additionally cooling the recycled quantity of working medium, the regulator 39 controls valves in one or more injection lines 41, 42, 43 and 44. The illustrated example represents the possibility of taking the quantity of injection water from the forced-flow system at a point ahead of the high-pressure preheater 7 under control by the valve 45, and also at a point ahead of the high-pressure preheater 8 under control by the valve 46, and further at a point ahead of the economizer 9 under control of the valve 47. The regulation is readily adaptable, either to required operating conditions or to highest permissible values, with the aid of conventional means so that, on the one hand, predetermined steam temperatures are maintained or not exceeded at the outlets of the intermediate superheaters and, on the other hand, the temperature of the recycled working medium ahead of the impeller pump 28 is maintained at or below prescribed values.

The main regulator 30 in the illustrated system may be constituted by any conventional boiler-load controller or a component thereof and receives datum commands as to the power to be delivered by the power plant, these datum inputs being set either manually, or automatically from the power distribution system served by the power plant, or also from monitor or programming apparatus operating in accordance with a predetermined working program. The required power-output values thus commanded then correspond to respective temperature and pressure values of the working medium at the various inputs and outputs, as well as the taps of the turbine stages and appertaining heating surfaces. Of these individual datum values only those referring to the temperature datum values for the reheated steam in the intermediate superheaters are of interest to the present invention.

As mentioned, the regulators 33, 34 and 39 are subordinate to the main regulator 30. In the illustrated system embodiment, the reheated steam temperature in the first intermediate superheater 21 and the temperature in the second intermediate superheater 22 are to be regulated, and it may happen under certain operating conditions, or generally, that the reheated steam temperature in the two intermediate superheaters must assume different prescribed magnitudes respectively. This is the reason Why in the illustrated system embodiment two separate regulators 33 and 34 are provided for the intermediate superheaters. Although, as shown and described above, both subsidiary regulators 33 and 34 are connected to the main regulator 30 to receive the same temperature datum value therefrom, it is the purpose of the subsidiary regulators 33 and 34 to perform in each case a modifying control which maintains the temperature of each intermediate superheater at the particular datum value distinct from that of the other intermediate superheater.

If the temperatures of the two intermediate superheaters 21 and 22, sensed at the respective localities 31 and 32, are too high, the regulators 33 and 34 issue to the respective three- way valves 35 and 36 corresponding control signals through the signal lines 62 and 68. These valves then decrease the flow through the lines 59 and 55 from the heat exchangers 25 and 26 so that a larger amount of steam flows from the turbine outlet lines 56 and 53 through the lines 37 and 38, respectively, directly to the respective intermediate superheaters 21 and 22. If the steam temperature is too low at the intermediate superheaters 21 and 22, the three- way valves 35 and 36 receive a signal to increase the flow through the heat exchangers 25 and 26 by opening the junctions of the lines 59 and 55 so that a smaller amount of steam flows through the intermediate superheaters 21 and 22. The more the valves 35 and 36 are closed and the steam flow through the respective heat exchangers 25 and 26 is reduced, the more steam is permitted to directly reach the intermediate superheater 21 or 22 through the lines 56, 37 and 53, 38, respectively.

The regulator 39, also subordinate to the main regulator 30, receives from the main regulator through the signal line 66 the same main datum value as the other subordinate regulator and compares it with the real (pilot) temperature at the sensing location 40. When a sufiicient departure is thus determined, the regulator 39 controls the injection cooler 29 to become elfective to a greater or lesser extent by changing the setting of the valves 45, 46 and 47. The operation is preferably such that the heat exchangers Withdraw from the circulating system as much heat as is required for regulating the reheated steam temperature and the heat exchangers are rated, in relation to the heating of the intermediate superheaters, to leave only a small residual regulating range for operation of the injection cooler 29.

If desired, the sensing location 40 may also comprise a pressure-sensing diaphragm or other pressure gauge which transmits a corresponding pressure signal to the regulator 39. This is in accordance with the main purpose of the regulator 39 to always secure a sufiiciently low temperature of the circulating medium so as to keep it liquid rather than vaporous, at any obtaining pressure of the Working medium. As a rule, the regulator 39 need effect a superpositional control operation only when the cooling of the circulating working medium in the heat exchangers 25 and 26 no longer suffices to prevent the danger that the pump 28 may receive an excessive share of steam. The regulator 39, therefore, may operate as a limit-responsive regulator and become elTective only if the temperature at the sensing locality 4t? approaches the evaporation temperature at the obtaining pressure.

To those skilled in the art, it will be obvious upon a study of this disclosure, that my invention permits of various modifications, for example with respect to layout and number of system components, and hence may be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

I claim:

1. With a steam power plant having a forced-flow system with a series-connection of working-medium supply means, a preheater, an evaporator, superheater means having a finishing superheater surface, a load and intermediate superheater means, and having a circulating system superimposed upon said forced-flow system between a hightemperature point arrear of said evaporator and a lowtemperature point ahead of said evaporator, said circulating system containing an impeller pump for withdrawing medium from said forced-flow system at said hightemperature point and recycling said medium back to said low-temperature point at least during start-up and partial-load operation of the plant, the combination of a temperature regulating system comprising regulating means for simultaneously regulating the steam temperature at said intermediate superheater means and the temperature of the recycled medium at said pump, said regulating means comprising heat exchanger means connected to said intermediate superheater means and connected in said circulating system for cooling the recycled medium by steam substantially at the intermediate superheater temperature and injection means connected downstream of said low-temperature point for supplying feed-water therefrom to said circulating system.

2. In a steam power plant with a regulating system according to claim 1, said forced-flow system having an economizer interposed between said preheater and said low-temperature point, said injection means comprising a feed-water injection line branching off said forced-flow system at a locality ahead of said economizer to receive a flow of feed-water, said line having an injection outlet in communication with said circulating system for additionally cooling the recycled medium and having injection control means connected to said regulating means to be controlled thereby.

3. In a power plant with a regulatin. system, according to claim 1, said regulating means also including control valve means interposed between said load on the one hand and both said intermediate superheater means and said heat exchanger means on the other hand, temperature sensor means responsive to the steam temperature at said intermediate superheater means, and control means responsive to said sensor means and connected to said valve means for controlling said valve means in dependence up on said steam temperature at said intermediate superheater means.

4. In a power plant with a regulating system, according to claim 3, said water injection means communicating with said circulating system between said heat exchanger means and said pump, said injection means having injection valve means for varying the amount of water injected into said circulating system, said regulating means also including further control means having temperature sensor means responsive to the temperature of the medium in said circulating system at a point between said pump and said water injection means, said latter control means being connected to said injection valve means for controlling the latter.

References Cited UNITED STATES PATENTS 3,129,564 4/1964 Brunner 122406 3,183,896 5/1965 Lytle et al 122406 3,186,175 6/1965 Strohmeyer 122-479 XR KENNETH W. SPRAGUE, Primary Examiner.

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