US3358450A - Method and apparatus for steam turbine startup - Google Patents

Description

Dec. 19, 1967 w w SCHRQEDTER EI'AL 3,358,450

METHOD AND APPARATUS FOR STEAM TURBINE STARTUP Filed Dec. 21, 1965 2 Sheets-Sheet 1 FIG! I NVENTORS WILBUR T W. SCHR OED TER WOLFRAM G. SCHUETZENDUEBEL AGENT D136. 19, 1967 w w SCHROEDTER ETAL I 3,358,450

METHOD AND APPARATUS FOR STEAM TURBINE STARTUP Filed Dec. 21, 1965 2 Sheets-Sheet 2 5o I r 7 7' INVE N TORS WILBURT W. SCHROEDTER WOLFRAM G. SCHUETZENDUEBEL AGENT United States Patent Office 3,358,450 Patented Dec. 19, 1967 3,358,450 METHOD AND APPARATUS FOR STEAM TURBINE STARTUP Willburt W. Schroedter, West Hartford, and Wolfram G. Schuetzenduebel, Avon, Conn, assignors to Combustion Engineering, Inc., Windsor, Conn, a corporation of Delaware Filed Dec. 21, 1965, Ser. No. 515,385 Claims. (Cl. 60-105) This invention relates to startup of steam turbines operating in conjunction with high pressure steam generators and in particular to a method and apparatus for controllably matching the temperature of the steam delivered to the turbine to the metal temperature of the high pressure end of the turbine.

The materials of a steam turbine operating on high pressure high temperature cycles are obviously cold when the turbine has been shut down for an extended period of time, and very hot during normal full load operation. Depending on the method used to shut down the turbine, and the time for which the turbine has been down, almost any temperature between the two extreme limits may be found in the high pressure section of the turbine.

The need for matching steam temperature to the metal temperature particularly in the high pressure section of the turbine has been recognized for some time. Several areas of the turbine have received considerable attention. These include the differential temperature between the turbine casing and the turbine casing bolts, the dilferential temperature between the internal and external surfaces of the turbine casing, and longitudinal expansion problems due to the differential expansion between the casing and the rotor. Recently due to the large diameter of the rotor shafts, thermal stress problems occur due to the differential temperature between the outside surface of the shaft and the interior of the shaft. With the many variables to be met, it is seldom that one particular steam temperature is optimum in respect to all the conditions, but it is generally recognized that a desirable steam temperature can be determined which satisfactorily meets all of these conditions. Such problems are discussed in the paper The Keystone Station-Plant Startup and Loading by F. I. Hanzalek and 3'. D. Conrad, Ir. presented at the American Power Conference in April 1964. Our invention is not particularly concerned with those factors which determine the desired steam temperature in the turbine chest, but is directed to a method and apparatus for achieving the desired temperature regardless of the criteria on which that desired steam temperature is based.

The vapor generator itself must necessarily be designed for full load operation and, accordingly, is operating Well below its design point during startup operation. During startup operation of the vapor generator relatively high gas Weights exist through the furnace and gas passes as compared to the steam flow at this time. Accordingly, the temperature of the steam leaving the unit approaches that of the gas temperature and cannot be readily regulated. Formerly once-through supercritical vapor generators were always started up at full operating pressure. This required substantial throttling of the steam as it was introduced into the turbine and consequently a substantial temperature drop. During hot startup of the turbine the low steam temperature resulting from the throttling action had considerable quenching effect on the internal structure of the steam turbine. Therefore, recent designs have incorporated a throttling valve in the steam generator intermediate the furnace wall and superheating sections. During startup the furnace walls only are operated at full pressure with the superheater being operated at low pres sure. This avoids the throttling drop at the turbine inlet and high steam temperatures are achieved in the turbine chest.

Such reduced pressure operation has been used on units with the pressure varying over the load range thereby achieving lower plant heat rates. Such operation is discussed in the paper appearing in March 1965 Electric Light & Power entitled, Reduced Pressure Boiler Operation Involves Many Considerations, by F. J. Cotter and F. I. Hanzalek.

In our invention the pressure within the superheater is regulated in accordance with the desired steam temperature at the turbine so that the throttling drop of the turbine throttle valves produces steam at the proper enthalpy even though the temperature of the steam leaving the steam generator is essentially constant. The pressure is neither constant, nor generally increasing, but varied in such a manner as to obtain the particular desired steam temperature at the turbine,

It is an object of this invention to provide a method and apparatus for supplying steam to a high pressure turbine from a high pressure steam generator at a controllable temperature as required by the particular physical condition of the turbine components at the particular time of startup.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a schematic representation of a power plant system illustrating an apparatus whereby the pressure in the superheater is reduced by withdrawing steam downstream of the superheater; and

FIG. 2 is a schematic representation of a power plant system illustrating an apparatus whereby the pressure in the superheater is reduced by withdrawing steam upstream of the superheater.

Referring to FIG. 1, steam is supplied from the steam generator 12 through steam line 13 to the high pressure section 14 of the steam turbine. Steam stop valve 15 and turbine throttle valve 17 are located immediately upstream of the turbine. This steam passes from the high pressure turbine section serially through the reheater sections 18 and the intermediate pressure 19 and low pressure 20 sections of the turbine. This steam is condensed in the condenser 22 and passed by pump 23 through low pressure feedwater heaters 24 to the deaerator 27. From here the high pressure feed pump 28 takes suction returning the feedwater at high pressure to the steam generator 12 through the high pressure heaters 29 and feedwater control valve 30.

The upstream portion of the generator comprises waterwall section 32 lining the walls of the furnace 33. This boiler contemplates a recirculating pump 34 and a recirculating line 35 operating in a manner as disclosed in US. Patent No. 3,135,252 issued June 2, 1964, to Willburt W. Schroedter.

Boiler throttle valve 37 is located in the main flow path with a small bypass valve 38 in parallel therewith. These valves operate as desired to throttle the flow of the steam in passing through the low temperature superheater 39 and the high temperature superheater 40.

Fuel is fired into furnace 33 through burner 42 with the waterwall section 32 being in radiant heat exchange relation with the combustion gases formed by the burning of the fuel. These combustion gases then pass in convec 3 tion heat exchange relation with the superheaters 39 and 40.

Boiler extraction line 43 containing boiler extraction valve 44 is located so as to withdraw fluid from the steam generator at a location upstream of the boiler throttle valve 37. This line conveys the efiiuent to the flash tank 45 from which Water is removed through Water discharge line 47 containing Water discharge valve 48. The Water thus removed passes to the condenser 22. Steam from the flash tank passes through line 49 and may be introduced into the superheater through steam admission line 50, or passed for deaerating and auxiliary uses through auxiliary steam line 52. Excess steam is passed through the spillover valve 53 to the condenser after passing through desuperheater '54 which operates to avoid excessively high temperatures in the steam entering the condenser 22. Steam drain line 57 is connected to remove water and steam from the steam line 13 at a location relatively near the boiler stop valve 15. This line includes a steam line drain control valve 58 which may be operated to regulate the amount of steam passing to the condenser 22.

During startup of a boiler the circulating pump 34 is put into operation so that circulation occurs through the waterwall section 32. A through-flow of approximately to percent of the steam generator capacity is started by feed pump 28 and controlled together with feedwater valve 30. The boiler throttle valve 37 and boiler throttle valve 38 are closed. Boiler extraction valve 44 is operated to maintain a pressure of 3600 p.s.i. upstream thereof. This 5 to 10 percent water flow passes to the flash tank 45, and when the water is cold all of this water passes directly to the condenser.

When the boiler efiiuent is above 212 F., steam is formed with the steam being passed through the superheaters 39 and 40 and out through steam line drain valve 58. This steam flow serves to cool the superheater surfaces which are exposed to the gas and to warm the external piping which is not so exposed. Additional steam may be required for deaeration purposes or to operate auxiliary equipment. The spillover valve 53 operates to limit the pressure in the flash tank system at this time. This is accomplished by pressure transmitter 59 which senses the pressure upstream of the valve and sends a signal to controller 60 which operates valve 53 to maintain the preselected upstream pressure whenever the pressure tends to exceed this amount.

During this startup period the firing rate in the steam generator 12 is regulated so that the temperature rise of the fluid in the steam generator does not exceed 400 F. per hour and also such that the gas temperature leaving the furnace does not exceed 1000 F. Since the boiler is operating at extremely low loads at this time and all the combustion equipment is designed for maximum load, inherent problems occur due to the difl'iculty of mixing the fuel with the air. Therefore in order to obtain safe operating conditions a high air flow in the order of 30 percent of full load air flow must be maintained through the steam generator even though the firing rate is only in the order of 7 percent. This results in an exceedingly high weight of combustion gases as compared to the steam flow rate even if the entire feedwater flow were being passed through the superheater as steam.

The amount of superheating surface installed in the steam generator is also in accordance with full load heat absorption requirements. Therefore, the steam temperature leaving the steam generator through steam line 13 tends to approach the gas temperature leaving the furnace. Because of the previously mentioned characteristics, changes in steam flow through the superheater do not operate to change this temperature significantly. Due to the same reasons, normal interstage spray desuperheating would be ineffective to control steam temperature even if satisfactory mixing could be obtained at these extremely low steam flows, Neither does the pressure level in the superheater affect the temperature of the steam leaving the superheater, even though it does change the enthalpy of the leaving steam.

While startup of the steam generator with the turbine in a cold condition has been previously described, the same circumstances prevail where the steam generator is already but except for the fact that steam is immediately available from the flash tank 45 rather than the entire flow initially passing through the water discharge line 47.

At the time the steam generator is started up, the temperature of the internal turbine parts can be in any one of a large number of imaginary conditions. All portions of the turbines would be at ambient temperature if the unit has been shut down for an extended period of time while the materials will be at full temperature if the turbine has just tripped off and an immediate h-ot restart is attempted. Depending on the length of shut down, all intermediate temperature conditions may be encountered.

As described in the preamble, there is a need to supply steam to the turbine at a temperature which is a function of the turbine condition in order to avoid thermal shock and expansion difiiculties in the turbine. While there are a number of parameters which dictate this requirement, the selection of a particular one is not a concern of our invention. Our invention simply recognizes that a particular temperature is required and contemplates a method and apparatus for achieving delivery of steam to the turbine at the desired temperature. Accordingly, one particular method of determining the required temperature is illustrated for-simplicity.

Temperature transmitter 62 senses the temperature of the metal within the steam turbine chest through a thermocouple located at a depth of about 1 inch into the steam chest Wall. A control signal indicating this temperature is passed through control line 63 to summation point 64. Since it will be desired that the steam temperature he a preselected value, such as 25 F., above this temperature, set point signal 65 representing 25 F. is added to the signal indicating the sensed temperature, with the summed control signal passing through control line 66. This control signal then represents the desired steam temperature, and it represents that temperature which it is the object of our invention to achieve.

As previously discussed, the firing rate of the unit is passed on the maximum firing within the limits set forth. The temperature of the steam passing through line 13 is determined by this firing rate and cannot be readily controlled. Since throttling through the turbine throttle valve 17 occurs at constant enthalpy, there is a temperature reduction associated with this throttling process. The pressure in the turbine chest is a function of the pressure drop through the turbine sections and the reheater with the condenser pressure being considered as a base. At low startup flows this pressure drop is very low with the pressure in the turbine chest being less than p.s.i. Our invention recognizes that the steam leaving the superheater is constant, and contemplates varying the pressure drop through the turbine throttle valve 17 to achieve a temperature reduction such that the desired temperature is achieved at the turbine 14. This is accomplished by varying the pressure of the steam leaving the superheater 40.

To accomplish this, the desired steam temperature signal passing through line 66 is fed into a function generator 67. The function generator passes through control line 68 a control signal which is representative of the enthalpy corresponding to the desired steam temperature. Since the pressure in the turbine during the period Which the system is in operation is always low, the temperature enthalpy relationship expressed by the function generator 67 is simply the temperature enthalpy relation at 100 p.s.i. The temperature of the steam leaving the superheater 40 is sensed by temperature transmitter 69 which passes a control signal through control line 70 with this signal being representative of the steam temperature.

Computer 72 receives the signal through line 68 representing the desired enthalpy and the signal through line 7 representing the actual temperature, and emits through control line 73 a signal indicating the corresponding desired pressure. This computer may be in the form of an ordinary operational amplifier which can handle a linearized function. In order to use this equipment, the pressure temperature enthalpy relation of the steam tables must be linearized throughout the operating range anticipated, in the form of The actual pressure in steam line 13 is sensed by pressure transmitter 74 with the signal representing the actual pressure being transmitted through control line 75 to controller 77. This controller receiving the desired pressure signal through the control line 73 and the actual pressure signal through control line 75 then operates the steam line drain valve 58 to increase or reduce the amount of steam being extracted from the steam line 18. In this manner the pressure of the steam in the superheater 4G is regulated to that value determined as the required steam pressure so that the temperature drop across the turbine throttle valve 17 produces the proper steam temperature entering the turbine 14.

The flow path and steam generator arrangement illustrated in FIG. 2 is identical to that of FIG. 1 with the only diflerence being the particular means by which the pressure in the superheater is varied. The desired enthalpy signal is again passed from control line 66 to the function generator 67 with a desired enthalpy signal passing through control line 68 to the computer 72. The temperature of the steam leaving the superheater is again sensed by temperature transmitter 69 with the signal representing this temperature passing through control line 70 to the computer 72. A control signal representing the desired pressure is then passed through the control line 83 to the controller 87. Pressure transmitter 84 senses the steam pressure in the piping associated with the flash tank 45 passing the actual pressure signal through control line 85 to the controller 87. This controller compares the actual and desired pressure signals and operates through control line 88 to manipulate the spillover valve 53. While this pressure is sensed at a location upstream of the superheater sections 39 and 40, it should be noted that during this period of low steam flow the pressure drop through the superheater is insignificant. Therefore, the pressure sensed by pressure transmitter 84 is essentially the pressure in the superheater. The controller 87 may also incorporate a fixed pressure limit switch so that the pressure in the boiler system will not exceed a preselected value even though the control system calls for higher pressure due to temperature requirements.

-In either embodiment the steam drain 58 will be manually operated to maintain a nominal steam flow through steam line 13 until such a time until the turbine is taking a satisfactory steam quantity. This amount should be selected so as to accomplish satisfactory warming of piping and headers located external of the steam generator. When satisfactory steam temperature and pressure is achieved for the initial turbine condition, steam is admitted through the steam stop valves 15 and the turbine throttle valve 17. The quantity is regulated to meet turbine demands while the temperature control system continues to operate during the startup operation.

The amount of steam liberated in the flash tank from a fixed temperature eflluent of the upstream portion of the steam generator varies with the pressure existing in the separator. However once the temperature is up to a satisfactory level, the flash tank goes dry and all the steam passes to the superheater. The superheater system then has an inexorable flow entering based on the 5 to percent feedwater flow, while steam is being removed from the system as required for turbine operation and auxiliaries, with the remainder being spilled either through the spillover valve 53 or the steam line drain valve 58. It is apparent, however, that the pressure level at which the superheater is operating may be regulated by manipulation of these valves 53 and 58.

The spillover valve 53 is generally sized to handle a large steam flow and, accordingly, by putting the pressure regulation on this valve, no increase in size of the steam line drain valve 58 is required.

In starting up a steam generator on a cold startup, the turbine is at ambient temperature. It is very diflicult to keep the temperature of the steam leaving the superheater during startup at a temperature less than 750 F. without significantly decreasing the firing rate and, therefore, delaying the startup procedure. In order to achieve low temperature of the turbine during this period, the pressure in the superheater of the steam generator is raised to an extremely high value during the early part of the startup. As the turbine is warmed by this steam, it can take higher steam temperature and, indeed, since an object of the startup is to get this turbine up to full load at maximum temperature, it should receive these higher temperatures. Therefore, the pressure in the superheater is decreased as the startup proceeds, thereby reducing the pressure drop across the turbine throttle valve 17 during the later period of startup. Obviously, as load on the steam turbine is increased, the pressure of the entire systern including the superheater is again increased.

Particularly in this type of operation having the superheater pressure at a high level, a low level, and subsequently back to a high level, the boiler throttle valve 37 is of particular value. This permits the upstream portion of the steam generator to be maintained at a pressure level independent of the superheater pressure. The benefits of this valve for operation of a supercritical generator in avoidance of stability problems is well known. However Where our invention is used on a subcritical unit such as one of the recirculating types, the oscillating of pressure in the evaporating section of the boiler would be undesirable. One of the limits in starting up such a unit is based on the fact that the evaporating section must operate at saturation temperature and the rate of change of this temperature and, therefore, pressure must be controlled to avoid stress problems within the steam generator. Therefore, the steps of serially operating at high pressure and low pressure and again at high pressure would impose considerable temperature transients in the water wall sections and delay startup operation. It is, therefore, recommended that our invention be used in conjunction with boiler throttling valves so that this upstream pressure may be maintained independently of the manipulations of the superheater pressure.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. For instance where startup is achieved by introducing steam only to the reheat turbine, the same approach may be used for temperature matching. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What is claimed is:

1. In a power plant having a steam generator including a radiantly heated water heating portion and a convectively heated steam superheating portion, a steam turbine, and conveying means supplying steam to said turbine from said steam generator including variable throttling means, an apparatus to facilitate startup operation of said steam turbine comprising: means for determining the required steam temperature at the turbine inlet; means for determining the required enthalpy corresponding to the required temperature; means for determining the actual steam temperature of the steam leaving said steam superheating portion; means for determining the required steam pressure corresponding to the required enthalpy and the actual steam temperature; and means for regulating the pressure of the steam in the steam superheating portion to that value determined as the required steam pressure.

2. An apparatus as in claim 1 wherein said means for regulating the pressure of the steam in the snperheater portion comprises: means for determining the actual pressure of the steam in the superheater portion of the steam generator; and means for comparing the required and actual steam pressures.

3. An apparatus as in claim 2 wherein said means for regulating the steam pressure comprises: means for removing steam from said conveying means independent of said turbine and upstream of said variable throttling means; and variable flow control means located in said means for removing steam.

4. An apparatus as in claim 3 wherein said means for removing steam from said vapor generator is located so as to remove the steam from said vapor generator upstream of said superheating portion.

5. An apparatus as in claim 3 wherein said means for removing steam from said vapor generator is located so as to remove steam from said vapor generator downstream of said superheating portion.

6. An apparatus as in claim 2 having also means for maintaining the pressure of the fiuid in said radiantly heated water heating portion at a selected value independently of the pressure in said steam superheating portion.

7. A method of operating a steam generator and steam turbine power system comprising: establishing a flow of 7 hot combustion gases in the steam generator; establishing a flow of steam at a first pressure level; passing said flow of steam in convection heat exchange relation with said hot combustion gases and heating the steam to a first temperature, said flow of gases being at a high rate with respect to the flow of steam such that the temperature to which the steam is heated is insensitive to variations in steam flow; determining the temperature required by the turbine for delivery of steam thereto at low pressure, controllably throttling the flow of steam from said first 8 pressure to said low pressure, and delivering the throttled steam to the turbine; controlling said first pressure level in response to said required temperature such that the required steam temperature is obtained when the steam is throttled from the first pressure level to low pressure.

8. A method as in claim 7 wherein said flow of steam is established by: passing Water at a second pressure level in radiant heat exchange relation with said hot combustion gases; and throttling the flow of water so heated to said first pressure level.

9. A method as in claim 7 wherein said first pressure level is controlled by: determining the enthalpy of steam corresponding to the required temperature at low pressure; determining said first temperature to which the steam is heated; determining the desired steam pressure corresponding to said first temperature and the determined enthalpy; first determining said first pressure level actually existing; comparing said actual pressure level with said desired pressure level, determining a pressure error; and regulating said first pressure level in response to the pressure error.

10. A method of starting a steam turbine-steam generator power plant when the turbine is in a cold condition comprising: establishing a flow of hot combustion gases within the steam generator; establishing a fiow of steam at a first pressure level, and passing said flow of steam in 'convection'heat exchange relation with the flow of hot combustion gases; throttling the flow of steam to low pressure, and delivering the throttled steam to saidrturbine; and varying said first pressure level to a high value during the initial start when the turbine is cold, to a substantially lower value as the turbine warms, and back to a high pressure level as the turbine is loaded.

References Cited UNITED STATES PATENTS 3,172,266 3/1965 Strohmeyer -l05 EDGAR W. GEOGHEGAN, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

Claims (1)

1. IN A POWER PLANT HAVING A STEAM GENERATOR INCLUDING A RADIANTLY HEATED WATER HEATING PORTION AND A CONVECTIVELY HEATED STEAM SUPERHEATING PORTION, A STEAM TURBINE, AND CONVEYING MEANS SUPPLYING STEAM TO SAID TURBINE FROM SAID STEAM GENERATOR INCLUDING VARIABLE THROTTLING MEANS, AN APPARATUS TO FACILITATE STARTUP OPERATION OF SAID STEAM TURBINE COMPRISING: MEANS FOR DETERMINING THE REQUIRED STEAM TEMPERATURE AT THE TURBINE INLET; MEANS FOR DETERMINING THE REQUIRED ENTHALPY CORRESPONDING TO THE REQUIRED TEMPERATURE; MEANS FOR DETERMINING THE

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