Peaking load steam cycle



United States Patent ice 3,523,421 PEAKING LOAD STEAM CYCLE Wolfram G. Schuetzenduebel, Del Mar, Calif., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed July 24, 1968, Ser. No. 747,149 Int. Cl. F01k 7/34 US. Cl. 60--67 2 Claims ABSTRACT OF THE DISCLOSURE A steam cycle for operating at a peaking load a steam power plant having a feed'water pump turbine which within normal operating load range of the steam power plant is driven with extraction steam taken from the high pressure stage of a multi-stage main turbine. Upon peaking load demand controls are set in motion for discontinuing the withdrawal of extraction steam from the main turbine to the pump turbine, and for supplying the pump turbine instead with high pressure steam directly from an auxiliary source or the main steam line of the boiler, whereby the steam normally withdrawn from the main turbine is free to perform additional work in further expanding through the lower pressure stages of the main turbine. BACKGROUND OF THE INVENTION The invention relates to power plants employing vapor cycles and more particularly to a steam cycle for peaking load operation. For the operation of a power plant at peak load, i.e. above normal full load capacity or at maximum electric power consumption rate, it is desirable to maintain a minimum of additional power equipment necessary for peaking load operation. In many cases gas turbine units are employed for supplying the last few percent of peaking reserve required by the power plant. In other cases, especially designed low cost peaking load boilers have been employed in an effort to minimize the capital investment represented by peaking load capacity requirements. While any one of these methods may be useful in reducing the peaking load capacity capital outlay they still represent a considerable investment and their integration into a standard thermal power generating plant may give rise to undesirable problems of operation. Also when gas turbines are used to provide peaking load capacity a time limiting delay must be taken into account until these machines are started and are able to supply power at full load. SUMMARY OF THE INVENTION The present invention alleviates the above problems by employing the steam which normally is supplied to auxiliary equipment from extraction points provided in the high pressure stage of the turbine, to furnish additional capacity to satisfy peaking load demand. All modern steam turbines have several extraction points from which auxiliary equipment is supplied with steam. One example used hereinafter in describing the invention is the boiler feed pump turbine drive. The steam consumption of the boiler feed pump turbine is appreciable and may amount to as much as 30 mw. in large modern steam power installations. In many cases such steam quantity is suflicient to satisfy sudden small power peak demands. To achieve this in accordance with the invention controls are provided which will close the extraction valve supplying the boiler feed pump turbine with high pressure steam, upon a demand signal which calls for a load increase above full load. Thus, the steam formerly supplied to the boiler feed pump turbine drive now will pass through the lower stages of the turbine to generate 3,523,421 Patented Aug. 11, 1970 more power. Simultaneously to the closing of the extraction valve a steam supply to the feed pump turbine from the main steam line or from an auxiliary source such as the auxiliary startup boiler is opened by interlocking devices to provide the required steam for driving the boiler feed pump turbine. If supplied from the main steam line the load demand signal will automatically call for an increase in feed water and firing which is coordinated with the boiler feed pump demand signal to increase its speed. The main turbine already running at full load would not be capable of passing more than flow through the governor valves. Thus the increase in flow capacity over the 100% design point takes place in the lower stages such as the reheat turbine, downstream of the governor valves. The available increase in generation at peaking loads can be achieved much faster than by starting a separate gas turbine unit. Furthermore, the elimination of additional plant equipment to provide, for instance, for the last 2% of peaking power can be of appreciable economic advantage, especially since the heat rate of a conventional steam generating unit is far better than the heat rate of a gas turbine plant. It is accordingly an important object of the invention to provide a steam power plant cycle with peaking load capacity without the attendant limitations which characterize present peaking load operation. Included in the above are the advantages of a minimum of capital investment needed for realizing peaking load operation, and the minimizing of operational difficulties, in that peaking load capacity is achieved within the main steam cycle. BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages will become apparent from the following description of an illustrative embodiment thereof taken in conjunction with the accompanying drawing showing diagrammatic representation of the cycle of a steam power plant and incorporating the present invention as applied in connection with a steam driven feed water pump. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing the depicted steam cycle includes a steam generator 10. Fuel and air introduced into the furnace of the steam generator by means of burners 12 are reacted to produce high temperature gases from which heat is transferred to a vaporizable fluid to produce highly superheated steam. A steam line 14 delivers the superheated steam to the high pressure stage 16 of a multistage turbine through which the steam expands to produce mechanical energy used in driving an electric generator, not shown. The steam leaving the high pressure stage 16 is reheated in a reheater 17 and returned to the low pressure turbine stage 18 for further expansion and production of mechanical energy. The exhaust steam from the low pressure stage 18 is discharged into a condenser 20. The condensate is recycled to the steam generator 10 by a feed water pump 22 via feed water line 24 and feed water heater 26. Feed water pump 22 is driven by a steam turbine 28. During operation of the cycle in its normal load range the feed turbine pump 28 receives steam bled or extracted from the extraction point 29 of the high pressure turbine stage 16 by way of conduit 30 and extraction valve 32. The exhaust steam from turbine 28 may pass through feedwater heater 26 for imparting heat to the feedwater before being returned to the condenser 20 by way of line 34. When the load demands on the cycle exceed the normal full load rating peaking power may be obtained by decreasing or completely eliminating the withdrawal of steam from the extraction point 29 for driving feed pump turbine 28. Thus the normally extracted steam can further expand through the turbine stages 16 and 18, thereby increasing turbine power output. The use of bleed steam by the feed pump turbine can be controllably reduced or eliminated by manipulation of extraction valve 32. When the quantity of steam extracted through line 30 is reduced or eliminated it is necessary to supplement or replace the steam input to the feed pump turbine 28 by steam from an auxiliary source such as a startup boiler, not shown, or from the main steam line. In the embodiment described herein steam is supplied to the feed pump turbine 28 during peak load operating condition from the main steam line 14 by way of conduit 36 and auxiliar valve 38. Various control elements are provided which when load demand exceeds normal full load, respond to a signal to close extraction valve 32 and open auxiliary valve 38. Thus a load demand signal of conventional origin is received by computer controller 40 which transforms this signal into impulses controlling fuel and air quantities by means of fuel and air control 42, and feedwater quantity by means of boiler feed pump speed control 44. When the load demand exceeds the normal full load rating, i.e. when peaking power is required, a third signal is emitted from computer controller 40 to a proportional controller 46 which sends out impulses to valve contoller 48 to close extraction valve 32, and to valve controller 50 to open auxiliary valve 38. When the load demand subsides again to normal full load a signal is sent forth from the computer controller 40 to proportional controller 46 which will cause valve controller 50 to close auxiliary valve 38 and at the same time cause valve controller 48 to open extraction valve 32. Thus feed pump turbine 28 will again receive extraction steam from the high pressure stage 16 for operation during the normal load range. From the foregoing it will be appreciated that, by use of the invention, a sufficient measure of peaking power within moderate limits can quickly and economically be achieved, and that the capital investment to obtain this peaking power will be appreciably lower than has heretofore been possible with known systems such as those employing a gas turbine unit. What I claim is: 1. The method of operating a steam power plant at peaking load, said power plant having a steam generator, a multi-stage main turbine connected to said steam generator for receiving steam therefrom, a load demand sensing device indicating the current load requirement of said steam generator, an auxiliary turbine associated with said steam generator, and a source of auxiliary steam; and including the steps of: during normal operating load range withdrawing steam from between the stages of said main turbine and passing the withdrawn steam to said auxiliary turbine for operating said auxiliary turbine; the improvement which comprises the additional steps of: during peak load operation and in response to said load demand sensing device discontinuing delivery of steam from said main turbine to said auxiliary turbine and in its place passing steam from said steam generator to said auxiliary turbine to permit the steam normally withdrawn from between the main turbine stages to perform additional work in further expanding through said main turbine. 2. The method according to claim 1, comprising the alternate step of: during peak load operation replacing the steam withdrawn from between the stages of said main turbine with steam from said auxiliary source instead of with steam from said steam generator. References Cited UNITED STATES PATENTS 2,193,863 3/1940 Egloif 60107 2,415,110 2/1947 Pescara 60-67 X 3,163,009 12/1964 Rankin 60-107 3,194,020 7/1965 Hanzalek 60-107 X FOREIGN PATENTS 18,304 3/1956 Germany. 502,002 4/1951 France. CARROLL B. DORITY, JR., Primary Examiner US. Cl. X.R.



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