US3175367A

US3175367A - Forced flow vapor generating unit

Info

Publication number: US3175367A
Application number: US215699A
Authority: US
Inventors: Walter P Gorzegno; Frederick H Weber
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1962-08-08
Filing date: 1962-08-08
Publication date: 1965-03-30
Anticipated expiration: 1982-03-30

Description

March 30, 1965 Filed Aug. 8, 1962 -$E'CO/VDARY SUPER/16747619 FRI/1A1? Y SUPER/1514 TE 1? w. P. GORZEGNO ETAL FORCED FLOW VAPOR GENERATING UNIT 4 Sheets-Sheet 1 28 we PRESSURE CHECK VALVE 74 -H/6H PRESSURE TURBINE INTERMEDIATE MITERMED/A TE PRESSURE PRESSURE REHEA TE)? 42 TURBINE FLU/O (/EC'U/TRY 1 DE'AERA r02 TURBNE 5 TORA GE TAN K ago 5 TURBINE GLAND SEA 1. PEG ULAT'OR 84 66 -64 LOW PRESSURE Eff/EA 1-52 PEESSUR RED PRESSURE HEA 7' 'E'R$ FEED PUMP CONDENSED? HOT WE'LL INVENTORS FEEOEE/(K H. WEBER WALT1$ I? GOEZEG/VO Mal/41 ATTORNEY March 30, 1965 Filed Aug. 8, 1962 4 Sheets-Sheet 2 l,\ I i we,

ill. I"- l i l I INVEN ORS FIEfDE'R/(K WEBER WAL Tip P. GORZEG/VU ATTORNEY March 1965 w. P. GORZEGNO' ETAL 51 3 FORCED FLOW VAPOR GENERATING UNIT Filedl Aug. 8, 1962 4 Sheets-Sheet 5 .sEc'o/vo/IR Y SUPER/15A rae PRIMARY SUPERHEA 727? fi/GH PRE'J'SURE TURBINE INTER/V500 IN 7' E ENE D/A TE PRESSURE PRESSURE 4 EEHEATEE) 42 TURBINE V 32 T0 colvozwsaw TANK 7'0 TURBINE GLAND SEAL V REGULATOR BY MWM ATTORNEY March 30, 1965 w. P. GORZEGNO ETAL 3, 7

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O05 O05! OOSZ 005$ United States Patent York Filed Aug. 8, 1962, 5er. No. 215,699 13 Ciaims. (Ci. 60-43) This invention relates to a start-up system for vapor generators, and in particular to apparatus for starting up and shutting down a subscritical or supercritical vapor generator of the forced flow once-through type.

As compared to conventional drum type boilers, severe problems are experienced in connection with the start-up, including hot restarts, of a once-through forced flow vapor generator. In particular, thermal losses may be excessive, or the time required for start-up may be excessive; but in addition, care must be exercised in the protection of turbine parts and vapor generating and superheating sections against thermal shock and high temperature damage.

Early one-through vapor generator start-up systems provided by-passes for a portion of the generator fluid at a point in the circuitry between the vapor generating and supcrheating sections; and means for bypassing the remainder of the fluid after the superheating section upstream of the turbine. In these systems, although they provided thermal protection for the turbine and superheating section, little consideration was given to heat recovery and the recycling of heat; or to providing more suitable turbine throttle vapor conditions for rolling and bringing the turbine up to speed prior to loading.

It has also been proposed to position a pressure reducing valve between the primary and secondary superheater intended to improve the once-through vapor gen- 7 orator system by permitting lower pressure vapor to be resupcrheated in the secondary superheater. This improvement permitted more suitable throttle vapor conditions to be furnished to the turbine during initial starting Another innovation, designed to provide more suitable throttle vapor to the turbine, and to reduce the time of starting up by permitting earlier warming and starting of the turbine, consisted of providing means for closing off the flow path between the vapor generating and superheating sections of the once-through unit. The fluid leavin the vapor generating section was fed through a pressure breakdown or reducing valve to a flash tank, and flash vapor was then furnished to the final superheating section from the flash tank. The vapor from the final superheating section could be fed either to the con denser through a turbine bypass line, or when starting up the turbine to the turbine throttle.

Systems have also been proposed for maximum recycling of heat. In one such system, the flash tank drains, where the flow is from the vapor generating section to the flash tank during start-up, are returned to a deaerator, and alternatively to a high pressure heater, instead of the condenser for heat conservation. In addition, it has been proposed to pass flash tank steam to a high pressure heater and deaerator for heat conservation.

This latter system greatly reduced the time required for starting up the once-througl1 unit, and when combined with prior innovations, provided for a flow of vapor of optimum enthalpy (pressure and temperature) to the turbine throttle early in the startup cycle.

The main disadvantage of the system was that during initial starting up of the unit, the secondary or final superheatcr is devoid of sufiicient cooling fluid or vapor flow. This condition requires that a maximum flue gas limit of approximately 1,000" F. be maintained within the unit at "a point upstream of the superheater section to prevent 3,l?5,3h7 Patented Mar. 30, 1965 ice overheating of the superheater tube metal. For quick starting up of the oncethrough unit, a fuel input equivilent to 15-20% of the full load input is required. At this input and with a competitive design, flue gas tempering at the furnace exit, or gas recirculation through the entire furnace, is generally required to avoid exceeding the upper limit of 1,000 P.

Accordingly, it is a principle object of the present invention to provide a start-up system which incorporates the advantages of prior systems, lending itself to optimum starting up of a once-through unit, but which in addition permits locating the superheater sections in a hot flue gas temperature zone immediately following the furnace, without the necessity of recirculation and tempering of the flue gas.

in particular, it is a principle object of the invention to provide suflicient cooling fluid or vapor flow throughout the entire starting-up period for maximum protection of portions of the generator exposed to high temperatures, which may include division walls, primary or platen superheaters and secondary or finishing superheaters.

it is a further particular object of the invention in a cold start to provide flash tank vapor to the turbine throttle for early warming and starting of the turbine. in the start-up cycle, while at the same time controlling the enthalpy of the vapor in a novel manner to furnish throttle vapor over a wide range of enthalpy. Towards providing thermal protection for the turbine, particularly in a hot restart, it is an additional object of the invention to obtain a smooth transition in matching turbine throttle vapor enthalpy with the vapor generator enthalpy.

Also, it is an object of the invention to obtain maximum recycling of reclaimed heat.

These and other advantages are accomplished in accordance with the invention by providing, in a oncethrough vapor generator having a plurality of vapor generating sections, vapor superheating sections and reheating sections, the superheating sections being located in a high line gas temperature zone, a start-up system which includes a first valve controlled flow path leading from the outlet of the superheating sections to the throttle of a high pressure turbine by which a fluid flow is first reduced in pressure and then is transmitted to a flash tank, and from there to a portion of one of the reheating sections and to the turbine throttle, the pressure reduction being to a pressure within the design limits of the reheater; and a second flow path to transmit the heated fluid at a pressure exceeding design limits of the reheater leading from the outlet of the superheating sections to the throttle of the high pressure turbine and bypassing a shutoff valve in the main flow line to the turbine, this flow path including a pressure reducing valve means therein.

The reheating section forming part of the first flow path preferably is one part of two of the generator intermediate pressure reheater, which parts are capable of being isolated from each other for simultaneous flow to the high pressure turbine throttle and from the turbine outlet to the intermediate pressure turbine throttle. For this purpose the two inlet and outlet headers of the reheater are employed, this double installation of the inlet and outlet headers, required by other design considerations, conveniently lending itself for use in the start-up system.

Other aspects of the generator include the provision of a superheater bypass line cooperating with the abovementioned first flow path in a novel manner for cold starts or hot restart matching of the vapor generator en thalpy with that required by the turbine, and the provision of a bypass line or bleed line from the intermediate pressure reheater outlet to the condenser for providing low enthalpy steam to the turbine in a cold start and obtaining suitable enthalpy buildup in the steam before 3 supplying it to the turbine in a hot restart. Also proposed in accordance with the invention are means for supply ing flash tank vapor to the deaerator and to a high pressure heater, by which maximum recycling of reclaimed heat is achieved.

Towards providing turbine throttle vapor over a wide range of enthalpy, it is preferred that the reheater section or sections of the generator be located in one of at least two parallel flue gas passageways, with damper control means for proportioning the amount of heat imparted to the vapor. In this respect, the enthalpy of the flashed vapor originating from the flash tank and routed through the reheater is approximately double that of the incoming fluid to the flash tank at the time of initial vapor supply to the high pressure turbine.

Because only five to ten percent of the full load flow is required through the first flow path mentioned above, the piping and valves in this flow path are comparatively inexpensive.

Later in the start-up cycle, the turbine throttle vapor supply is obtained through the second flow path for a smooth transition to full load flow.

It is believed that the invention and other advantages thereof will become apparent upon consideration of the following detailed description, with reference to the accompanying drawings, in which:

FIGURE 1 illustrates schematically a start-up system for a once-through generator and turbine in accordance with the invention;

FIGURE 2 is a schematic section elevation view of a supercritical unit in accordance with the invention;

FIGURE 3 illustrates in part a modification of the start-up system in accordance with the invention;

FIGURE 4 is a temperature enthalpy diagram illustrating a typical start-up sequence in accordance with the invention;

Referring to FIG. 1, liquid from a deaerator 12 drains to a deaerator storage tank 14, and is pressurized by a pump 16 for flow through high pressure heaters 18, 20, and 22 to vapor generating sections 2432 of the unit. From the vapor generating sections, the heated fluid r vapor (vapor liquid mixture in the case of a subcritical unit) is passed to a primary (or platen) superheater 34 and from there to a secondary (or finishing) superheater 35.

Other elements making up the vapor generating and turbine installation of the invention are a high pressure flash tank 36 arranged to receive a heated fluid or vapor from the outlet of the

superheating sections

34 and 35, a high pressure turbine 38 the throttle of which is connected by a main steam line 40 through a stop valve 40a to receive a vapor flow from the superheating sections, intermediate pressure reheating sections 42 and 42a, an intermediate pressure turbine 44, reheating section 46, a low pressure turbine 48, condenser 50, and condenser hot well 52. Condensate from the condenser is returned to the system through low pressure heaters (not shown in FIG. 1) and deaerator 12 by condensate pump 54.

In accordance with the invention, the flash tank 36 is connected to the steam generating section outlet by a line 56 through a superheat'er bypass valve 56a, and to the superheating section outlet by a line 58 through a high pressure turbine bypass valve 58a. Both

valves

56a and 58a regulate control of the flow in the respective bypass lines. Immediately before the inlet 60 to the flash tank, there may be provided a spray attemperator 62 arranged to admit cool water from the condensate system to maintain the enthalpy of the reduced pressure fluid at a temperature within the design limits of the flash tank. In particular, for a hot restart where the enthalpy of the fluid leaving the superheater is high, the pressure reducing valve 58a in cooperation with the attemperator produces a fluid which is at a lower temperature satisfactory for introduction into the flash tank 36.

Further making up the turbine bypass system is line 64,

provided with a pressure reducing valve 64a and spray attemperator 64b, leading from the vapor section of the flash tank to the condenser 50. Since the flash tank vapor is used for early Warming and starting of the turbine, in a manner to be described, the enthalpy of the flashed vapor, and pressure, should be suitable for a cold start turbine condition. For economical sizing of the flash tank and associated piping (and to prevent excessive moisture in the steam exhausting from the turbine), the vapor pressure and temperature supplied to the turbine throttle is well above design operating limits of the condenser. The purpose of the reducing valve 64a and spray attemperator 64b is to provide respectively adia batic pressure reduction and attemperation so that the pressure and temperature of the fluid entering the condenser is within the design limits of the condenser.

For instance, in a supercritical unit, the pressure entering the flash tank for an initial phase of a cold start may be 500 p.s.i.a. reduced from 3,500 p.s.i.a. leaving the superheater. Excess flashed steam routed to the condenser through line 64 would be reduced adiabatically to approximately p.s.i.a. entering the condenser internal break-down battles. The associated unattemperated temperature is approximately 350 R, which is above normal condenser design temperatures. For a hot re-start where the fluid passes through bypass valve 84a (to be described later) to the condenser, the need for the spray attemperator 64b is even more pronounced.

For maximum efliciency and heat recovery, the turbine vapor bypass system includes a line 66 with a shutofl valve 66a arranged to transmit flashed vapor to a turbine gland seal regulator, and a line 68, split into two lines passing through valves 68a and 68b, arranged to transmit flashed vapor to deaerator 12 and high pressure heater 22.

Drains from the flash tank preferably are transmitted through line 70, split into two lines passing through valves 70a and 7%, leading to high pressure heater 20 and the hot well 52 of condenser 50. The high pressure heater drains, from heaters 22 and 2t) cascade through heater 18 to the deaerator storage tank 14 for recycling in the system.

During the start-up sequence, to furnish the high pressure turbine 38 with optimum vapor conditions for rolling, there is provided a line 72 containing a steam valve 72a leading to the turbine throttle through a reheater start-up section 42a and a check valve 74. Also disposed in this line is shutoff valve 76 immediately upstream of the turbine throttle.

An additional bypass line 78 is provided around the main flow line valve 40a leading to the high pressure turbine throttle. This line contains a pressure reducing valve 78a the purpose of which will be described.

For the purpose of transmitting flashed vapor from the flash tank 36 by way of line 72 through the intermediate pressure reheating section 42a, isolation valves 80 and 82 are employed in connecting lines between the two inlet and two outlet headers respectively for the rheating sections to isolate the sections from each other. Accordingly, the flashed vapor from the flash tank is reheated in the section 42a before entering the high pressure turbine throttle through valve 76, while at the same time, exhaust flow from the turbine is reheated in section 42 before entering the intermediate pressure turbine 44.

Primarily for re-starts, but also cold starts, a further bypass line 84 containing bypass valve 84a may be provided leading from the outlet of the reheater section 42a to the condenser 50 through line 64. Although the function of this line will be more fully described, during hot re-starts it operates towards obtaining the desired reheater outlet temperature before passing the flash tank steam to the high pressure turbine throttle, and during cold starts operates to siphon otf excess steam, used for enthalpy control, to the condenser.

FIG. 2 illustrates certain structural aspects of the vapor generator. The proposed once-through steam generator is a supercritical unit fired with bituminous coal using 24 burners. At maximum continuous load, the heat liberation in the furnace, designated by the numeral 90, is 16,000 Btu/cu. ft. and the absorption is 70,600 B.t.u./ sq. ft.

The primary or platen superheater 34 consists of a plurality of platens arranged on 5 foot staggered centers across the width of the upper part of the furnace in a radiant high temperature zone. The secondary or finishing superheater $5 is located immediately before the furnace outlet and is in the form of a bank of U-shaped tubes extending across the width of the upper part of the furnace in a high temperature zone. At maximum continuous load, the gas temperature leaving the furnace is 2.030" F.

Following the hot gas temperature zone, the gas divides into three parallel down flow gas passes 94, 96, and 98. The front pass contains the economizer surface 24, the center and rear passes containing the intermediate pressure reheater dd and the low pressure reheater 46 respectively. At the outlets of the flow gas passes,

dampers

104, and 106 are provided for controlling the heat imparted to the economizer and reheater sections.

Cold start During a cold start-up, in the supercritical unit, the motor driven teed pump 16 is started to pressurize the unit to full pressure at the secondary or finishing superheater outlet 35. Approximately 30% full load fluid flow from the feed pump 16 is established through the high. pressure heaters 13, Zil, and 22 and through the vapor generating sections 2432 and superheating sections 34 35. This flow is first transmitted to the flash tank 36 through the high pressure turbine bypass line 53 and valve Silo by closing off the main flow line valve ida. The bypass valve 58a is a break-down valve reducing the pressure adiabatically in the system from approximately 3,500 p.s.i.a. to less than 15% of full load throttle pressure. Drains from the flash tank are transmitted through line '70 and siutoiii valve 70a to the high pressure heater 20, and from there to the deaerator storage tank 14 through the high pressure heater 18. At the same time, a portion of the drains may be fed through pressure reducing valve 7% to the condenser hot well 52, and from there to the deaerator 12 by condensate pump 54.

At this stage, the furnace for the genenator is fired at a rate of about 15% of full load firing rate raising the enthalpy of the fluid at the maximum allowed rate along an approximate constant pressure line, 3,500 p.s.i.a., shown in FIG. 4, with the furnace exit temperature at about 1,200 F., the maximum permissible. When the enthalpy of the fluid leaving the secondary or finishing superheater outlet (35) in this initial heating period is approximately at about 600 Btu. per lb. or 55% of the enthalpy rise to equilibrium at 15% of full load firing rate (point A, FIG. 4), a level is established in the flash tank and the flash tank pressure is set at approximately 15% of full load throttle pressure. Sufhcient steam is now generated, about 5% of full load steam flow, to begin rolling the turbine. Some flash tank steam is now available for the turbine gland seal regulator through line 66 and valve 66a, for the deaerator 12 through valve 680, and for the high pressure feed heater 22 through valve @815. Some flash tank drain flow is still routed through line '70 and valve 70a to high pressure feed heater 20 for maximum recycling of reclaimed heat. The remainder ,of the flash tank drain flow is dumped to hot well 52 through valve 7%.

The steam which is flashed in the flash tank is made :available from the flash tank through steam valve 72a in line '72 leading to the intermediate pressure reheater section 42a, and from there to the high pressure turbine throttle. Valves 80 and 82 are closed isolating the reheater section 420 from the portion 42 of the reheater.

6 Referring to FIG. 4, the throttle valve of the turbine is set so that the pressure entering the first turbine stage wheel is about 50 p.s.i.a. or that required for initial rolling and bringing the turbine up to rated speed. The flashed vapor supplied from the flash tank at 500 p.s.i.a. is indicated at point B. In the reheater the enthalpy is raised to about point C and then is reduced through the turbine throttle valve to about point D so that the turbine steam chest and first stage inlet parts are initially subjected to 400 F. steam at about 50 p.s.i.a. As the enthalpy of the steam rises during the warming of the turbine, this temperature also rises along an approximately constant pressure, 50 p.s.i.a., line.

When the enthalpy of the fluid leaving the secondary or finishing superheater outlet (35) is approximately 800 Btu. per lb. (point B), the flash tank pressure set point is adjusted to 1,000 p.s.i.a. or approximately 29% of full load throttle pressure.

In the final heating stages of the start-up cycle, still at about 15% of full load firing rate, the generator (sections 24-32) comes to equilibrium (point P). By this time, the pressure in the flash tank has been raised to 1,000 p.s.i.a. (the design pressure of the reheater), and the enthalpy leaving the secondary or finishing superheater is at approximately 78%, about 1090 B.t.u. per 1b., of full load throt-le enthalpy. The 5% of full load flow through. the intermediate pressure reheater for rolling the turbine is maintained, the enthalpy rise following approximately the 1,000 p.s.i.a. constant pressure line to about point G, or a higher enthalpy, if desired, for proper warming of the turbine. in this respect, the amount of flue gas routed through the intermediate pressure reheatei' can be closely controlled to obtain low temperature steam initially in the start-up cycle building up as the turbine temperature increases.

At this stage of starting up, some flash tank steam obtained through the pressure reducing valve 58a is dumped into condenser 50 through valve 64a and line 64. In addition, all flash tank drain flow is dumped to the condenser hot well 52 through valve 70b.

When the turbine is up to temperature (about point H, *lG. 4, obtained by raising the temperature of the steam in the reheater at the constant pressure of 1,000 p.s.i.a., to point B) and ready to be synchronized, the superheater bypass valve Sea is opened to bypass approximately 15% of full load flow rate. This achieves a higher enthalpy input into the steam flowing through the superheater and raises the enthalpy of the steam (at 3,500 p.s.i.a.) at the superheater outlet to about point I on FIG. 4. The flash tank pressure is at approximately 29% of full load throttle pressure, about 1,000 p.s.i.a., and the pressure reducing valve 7hr: is placed on pressure control to hold this pressure at the turbine throttle. The flash tank steam valve 72a is then closed and the intermediate pressure reheater isolation valves and 82 are opened. Since the pressure reducing valve 78a is a constant enthalpy valve, the throttle pressure and temperature, of the steam passing through the valve, may be caused to match exactly, as shown in FIG. 4, conditions supplied from the immediate pressure reheater. As the turbine load increases, the pressure reducing valve 73a is characterized to raise the throttle pressure. At 30% turbine generator load, the full throttle pressure is obtained, but prior to this and at 28% turbine load, the pressure reducing valve 78a is cut out of service and the main steam line stop valve is opened for direct flow to the high pressure turbine. The fiow sequence following the turbine is then through the reheater sections 42 and 42a in parallel, the intermediate pressure turbine 44, reheater section 46 and the low pressure turoine &8.

Advantages of the invention should now be apparent. lrimarily, all portions of the generator exposed to high temperatures are provided with a fluid flow for cooling, but in addition, throttle steam at 500 to 1,000 p.s.i.a. reduced from 3,500 p.s.i.a. and reheated is more ideally a suited for turbine starting. A quicker start is achieved by early gradual heating and rolling of the turbine. Shock to the turbine is prevented in that cold steam, at about 410 F., can be supplied initially to the turbine steam chest and inlet parts, but at the same time, the enthalpy of the fluid entering the turbine can be closely controlled and increased to match perfectly conditions at the superheater outlet in changeover to load flow conditions.

In this latter respect, the amount of flue gas routed to the intermediate pressure reheater can be closely controlled for low temperature steam initially, and carefully increased as the turbine temperature increases. Also control through the fluid side may be achieved by passing or more flow from the flash tank through the reheater during the initial cold start to limit the fluid enthalpy to the turbine throttle. Since the turbine requires only 23% flow, the remainder of the flow through the reheater is siphoned oh? in line 84 through valve 84a to the condenser 50. Later in the cold start cycle, the flow from the flash tank to the reheater may be reduced to about 2 to 3% while warming and bringing the turbine up to speed. This permits a higher enthalpy buildup in the steam, and a temperature increase at the turbine throttle inlet to achieve point G of FIG. 4, or higher enthalpy values.

A further advantage resides in the employment of a sufliciently high pressure in the flash tank and matching level of saturation temperature to achieve a satisfactorily high feed water temperature following the high pressure heaters. A sufliciently high operating pressure in the flash tank also results in a reduced size of tank.

In addition, the heating of the flash tank vapor in the reheater at flash tank pressure rather than at the pressure existing in the once-through generator circuitry results in improved heat transfer efliciency because of the lower level of receiving temperature. Other advantages will be apparent to those skilled in the art.

H at start In a hot start, employing flash tank steam to the intermediate pressure reheater, a slightly diflerent start-up sequence is used. Prior to starting, the turbine steam chest, valve bowls and first stage inlet parts, metal temperatures may be about 900 F. If the maximum temperature of 1,000 E, point K, FIG. 4, is obtained upstream of the pressure reducing station at the superheater outlet at 3,500 p.s.i.a. (for a supercritical unit), the throttling process to 1,000 p.s.i.a. reduces the temperature to approximately 860 F. (point L). This is about the maximum temperature obtainable, and it is likely that some lower temperature would be obtained in actual operation. In further throttling from point L through the turbine throttle valve to 50 p.s.i.a., a temperature of about 800 F. results entering the turbine chest, valve bowls, and first stage inlet parts. Thus, with optimum conditions of operation, the turbine inlet metal temperature can be matched only within about 100 F.

A much easier match of hot-start turbine inlet metal temperatures is obtained by furnishing flash tank steam at 1,000 p.s.i.a., reheated in the intermediate pressure reheater to a suitable enthalpy level (between L and M, FIG. 4), and then reduced through the turbine throttle valve to 50 p.s.i.a., or any suitable pressure, entering the turbine chest and valve bowls.

A smooth changeover can be obtained to the pressure reducing station 78a by applying approximately 5% load to the turbine employing flash tank steam through the reheater 42a. Then a gradual changeover under load flow conditions is achieved by mixing flow through the pressure reducing station with that through the reheater, increasing the former and decreasing the latter, and reducing the turbine metal temperatures at a permissive rate.

By-pass line 84 and valve 84a permit an operator to establish suitable turbine throttle conditions at the reheater outlet by adjustment of the firing rate, flow adjustment and damper control before admitting the steam to the high pressure turbine.

By-pass line 56 and valve 55a around the superheater sections permit obtaining a higher fluid enthalpy at the secondary superheater outlet to satisfactorily match throttle enthalpy obtained through the reheater flow path.

For a cold start, by-pass line 56 and valve 56a also permit passing a lower flow rate through the superheater sections to obtain a higher enthalpy at the superheater outlet if desired.

As an alternative arrangement, for a hot restart it may be advantageous to bypass the relatively thick-Walled flash tank to prevent thermal shock. This is accomplished as illustrated in FIG. 3 by providing lines 108 and 11.0, shown as dashed lines in FIG. 3, and isolation valves x, y, and z. The superheater bypass fluid flow is now routed through line 56, pressure breakdown valve 560, line 110, and then through valve 64a to the condenser. Required fluid flow leaving the secondary (finishing) superheater 35 is routed initially through line 58 and pressure breakdown valve 58a, bypass line 108, and the start-up reheater section 42a.

Subsequent operation of this system is the same as previously described. Additional isolation valves v and w are required in lines 108 and 110 for normal operation of the system.

Although the invention has been described with respect to specific embodiments, many modifications will be apparent to those skilled in the art and Within the scope and spirit of the invention as defined in the following claims.

What is claimed is:

l 1. In a once-through vapor generating and superheating unit designed to provide during operation a high pressure heated fluid to a prime mover, the unit including a main valve controlled conduit from the outlet of the unit to the prime mover, a start-up system comprising a first by-pass line from the outlet of the unit to the prime mover including valve means adapted to adiabatically reduce the pressure of the fluid exiting from the outlet,

a heat exchange surface in the by-pass line arranged to reheat the reduced pressure fluid, the valve means being arranged to reduce the pressure of the fluid to within the design limits of the heat exchange surface,

a second by-pass line from the outlet of the unit to the prime mover including second valve means therein adapted to adiabatically reduce the pressure of the fluid, said second valve means being designed to reduce the pressure to a pressure exceeding design limits of the heat exchange surface.

2. In a once-through vapor generator according to claim 1 and including a third valve controlled by-pass line by which a portion of the heated fluid in said generator by-passes the generator superheating sections, the portion passing through the superheating sections thereby having a higher enthalpy content.

3. In a once-through vapor generating and superheating unit according to claim 1, the unit including a reheater down stream of the prime mover adapted to receive exhaust flow from the prime mover,

wherein said heat exchange surface constitutes at least a portion of the reheater,

the reheater including valve means arranged to isolate the heat exchange surface from the remainder of the reheater when the first by-pass line is functioning.

4. In a once-through vapor generating and superheating unit designed to provide during operation a high pressure heated fluid to a prime mover, the unit including a main valve controlled conduit from the outlet of the unit to the prime mover, and a reheater in flow series with the prime mover adapted to receive the exhaust flow from the prime mover, the vapor generating and superheating unit including sections disposed in a high temperature zone of the generator and the reheating section being in a relatively low temperature exhaust gas zone, a start-up system comprising a first by-pass line leading from the outlet of the unit to the prime mover including valve means adapted to adiabatically reduce the pressure of the fluid exiting from the outlet,

flash tank means arranged to receive the reduced pressure fluid,

a heat exchange surface comprising a portion of the said reheater arranged to receive flashed vapor from said flash tank and to reheat the same, said valve means being designed to reduce the pressure of the fluid to the design limits of the heat exchange surface,

valve means arranged to isolate the heat exchange surface from the remainder of the reheater,

a second by-pass line from the outlet of the unit to the prime mover including second valve means therein adapted to adiabatically reduce the presssure of the fluid, said second valve means being designed to reduce the pressure to a pressure exceeding the design limits of the heat exchange surface,

the first by-pass line functioning during start-up to provide initially a relatively cold vaporized fluid to the prime mover and to raise the enthalpy thereof at a controlled rate.

5. in a once-through vapor generator according to claim 4, said system further including exhaust gas control means to control the heat supplied during start-up to said fluid in said reheater.

6. In a once-through vapor generator according to claim wherein said reheater is a split intermediate pressure reheater having at least two reheater sections one of which constitutes said heat exchange surface, inlet and outlet headers for said reheater sections, and flow conduits between said inlet and outlet headers having valve means therein, said valve meansbeing arranged to close 01f said heat exchange surface from others of said sections.

7. In a once-through vapor generating and superheating unit designed to provide during operation a high pressure heated fluid to a prime mover, the unit including a main valve controlled conduit from the outlet of the unit to the prime mover, and a reheater in flow series with the prime mover adapted to receive exhaust flow from the prime mover, the vapor generating and superheating sections of the unit being located in a high temperature zone of the generator and the reheater being located in a relatively low temperature exhaust gas Zone of the generator, a start-up system comprising a first by-pass line from the outlet of the superheating section to the prime mover including valve means adapted to adiabatically reduce the pressure of the fluid exiting from the outlet,

flash tank means in the first by-pass line arranged to receive the reduced pressure fluid,

a heat exchange surface arranged to reheat the reduced pressure vapor from the flash tank, the reheater comprising at least two sections in parallel, the heat exchange surface constituting one of said reheater sections,

the first by-pass line valve means being designed to reduce the pressure of the high pressure heated fluid to within the design limits of the heat exchange surface,

exhaust gas damper means to control the rate of heat transfer from the generator exhaust gas in the heat exchange surface,

a second by-pass line from the outlet of the superheating section to the prime mover including second valve means adapted to adiabatically reduce the pressure of the fluid, said second valve means being designed orator and prime mover, the former having vapor gencrating and superheating sections, the steps comprising initially establishing through said sections a fluid flow at about prime mover full throttle pressure and at a rate which is a predetermined fraction of full load flow,

upstream of the prime mover, causing a portion of the reduced pressure fluid to flash into vapor, reheating said vapor, sending part of the reheated vapor to the prime mover and bleeding off part, the amount bled off being that required to adjust the temperature and enthalpy level of the reduced pressure and the reheated vapor to that required for initial Warming and rolling of the prime mover,

continuing to bleed a variable portion of the reduced pressure fluid decreasing the amount bled off with increased load on the turbine.

9. A method according to claim 8 including the step of using the fluid which is bled from the fluid flow for preheating of feed flow to the generator.

10. In a method according to claim 8 wherein said fluid flow initially established is approximately at 30% 0t full load flow, the portion made available for reheating being about 2-10% of full load flow.

11. In a method according to claim 8 and further including the step of controlling the heat input during said reheating step by control of the amount of generator flue gas passed in exchange with the flashed vapor.

12. In a once-thorugh vapor generating and superheating unit designed to provide during operation a high pressure heated fluid to a prime mover, the unit including a main valve controlled conduit from the outlet of the unit to the prime mover, and a reheater in flow series with the prime mover adapted to receivethe exhaust flow from the prime mover, a start-up system comp-rising a first by-pass line from the outlet of the unit to the prime mover including first valve means adapted to adiabatically reduce the pressure of the fluid exiting from the outlet,

a flash tank arranged to receive the reduced pressure fluid from said valve means,

reheat means arranged to reheat the reduced pressure fluid, said reheat means comprising a portion of the heat exchange surface of the reheater and second valve means to isolate said portion from the remainder of the reheater and to place said portion in flow series between said flash tank and prime mover,

said first valve means being designed to reduce the pressure of the fluid flow to within the design limits of the reheater,

a second by-pass line from the outlet of the unit to the prime mover including third valve means therein adapted to adiabatically reduce the pressure of the fluid, said third valve means being designed to reduce the pressure to a pressure exceeding design limits of the heat exchange surface,

the first by-pass line being adapted to transmit from 2 to 10% of full load flow during initial start-up.

adiabatically reducing the pressure of the heated fluid 11 a 12 13; In a once-through vapor generator according to 2,900,792 Buri Aug. 25, 1959 claim 12 and further including a third by pass line by- 3,009,325 Pirsh Nov. 21, 1961 passing said superheater sections and adapted to trans- 3,021,824 Profos Feb. 20, 1962 mit approximately 15% of full load flow rate, said by- 3,055,181 Argersinger et a1 Sept. 25, 1962 pass line having a pressure reducing valve therein. 5 FOREIGN PATENTS References Qited a the file of this patent 879,032 Great Britain Oct. 4, 1961 UNITED STATES PATENTS OTHER REFERENCES 2,229,643 De Baufre I an. 28, 1941 German application-4,043,347, printed Nov. 13, 1958,

2,884,760 Buri May 5, 1959 10 (K1 14111/14

Claims

1. IN A ONCE-THROUGH VAPOR GENERATING AND SUPERHEATING UNIT DESIGNED TO PROVIDE DURING OPERATION A HIGH PRESSURE HEATED FLUID TO A PRIME MOVER, THE UNIT INCLUDING A MAIN VALVE CONTROLLED CONDUIT FROM THE OUTLET OF THE UNIT TO THE PRIME MOVER, A START-UP SYSTEM COMPRISING A FIRST BY-PASS LINE FROM THE OUTLET OF THE UNIT TO THE PRIME MOVER INCLUDING VALVE MEANS ADAPTED TO ADIABATICALLY REDUCE THE PRESSURE OF THE FLUID EXITING FROM THE OUTLET, A HEAT EXCHANGE SURFACE IN THE BY-PASS LINE ARRANGED TO REHEAT THE REDUCED PRESSURE FLUID, THE VALVE MEANS BEING ARRANGED TO REDUCE THE PRESSURE OF THE FLUID TO WITHIN THE DESIGN LIMITS OF THE HEAT EXCHANGE SURFACE, A SECOND BY-PASS LINE FROM THE OUTLET OF THE UNIT TO THE PRIME MOVER INCLUDING SECOND VALVE MEANS THEREIN ADAPTED TO ADIABATICALLY REDUCED THE PRESSURE OF THE FLUID, SAID SECOND VALVE MEANS BEING DESIGNED TO REDUCE THE PRESSURE TO A PRESSURE EXCEEDING DESIGN LIMITS OF THE HEAT EXCHANGE SURFACE.