US3313111A - Startup system for a once through steam generator including a startup balancing heatexchanger - Google Patents

Description

pril 6 CTSTROHMEYER, JR

y STARTUP SYSTEM FOR A ONCE THROUGH STEAM GENERATOR INCLUDING A STARTUP BALANCING HEAT EXCHANGER Filed April 50, 1965 5 Sheets-Sheet l WATER LEVEL INVENTOE. CHARLES STROH MEYER,JR. BY

H ATTORNEY April 11, 1967 c. STROHMEYER, JR 3,313,111-

STARTUP SYSTEM FOR A ONCE THROUGH STEAM GENERATOR I INCLUDING A STARTUP BALANCING HEAT EXCHANGER Filed April 30, 1965 5 Sheets-Sheet 2 FEG,4

INVENTOR. CHARLES STROHM EYERJR.

HIS TORNEY pr 1967 c. STROHMEYER, JR 3,313,111

' STARTUP SYSTEM FOR A ONCE THROUGH STEAM GENERATOR INCLUDING A STARTUP BALANCING HEAT EXCHANGER Filed April 30, 1965 3 Sheets-Sheet 5 INVENTOR- HIS TT ORNEY CHARLES STROHMEYER, JR.

United States Patent STARTUP SYSTEM FOR A ONCE THROUGH STEAM GENERATOR INCLUDING A START- UP BALANCING HEAT EXCHANGER Charles Strohmeyer, Jr., Reading, Pa., assiguor to Electrodyne Research Corporation, Reading, Pa. Filed Apr. 30, 1965, Ser. No. 452,143 1 Claim. (Cl. 60-105) This invention relates to steam-electric generating units having a steam generator of the once-through type and wherein it is desired to co-ordinate the startup of the steam generator to best suit the requirements of the turbine generator.

Known startup systems for once-through generators include throttling means located intermediately between heat absorption conduits connected at one end of the feedwater inlet and at the other end to the superheater steam outlet. This enables the steam generator heat absorption conduits to be operated at two pressure levels, the highest pressure being upstream of the throttling means.

The generating section upstream of the throttling means, requires a certain cumulative parallel minimum fluid flow through the circuits when the steam generator is fired, to properly proportion flow through the individual circuits in the burner zone which is the area of highest heat input, thus preventing overheating of the individual fluid conduits. Minimum flow required may range from 17 to 70 percent of full load flow. In order to maintain such minimum flow in the generating section circuits, a bypass is usually provided for withdrawing fluid from the steam generator circuits, upstream of at least a portion of the superheater, at times when the superheater outlet flow drops below the upstream minimum value. The bypass flow may be recovered in the feedwater collection system and recycled to the feedwater inlet. Appreciable heat losses are usually associated with such use of the bypass.

In addition to use of the superheater bypass during startup, throttling means are used to reduce fluid pressure in the downstream superheater portion. Where the superheater bypass is upstream of the throttling means, it

. perature instability at the superheater outlet also follows.

The firing rate cannot be increased to minimize moisture entering the superheater, without excessively heating the tube elements at the superheater outlet. The difliculty arises from the unbalance in flows upstream and downstream of the throttling means when initially flowing steam directly from the generating section to the superheater outlet.

An object of the invention is to provide a means for better distributing heat absorbed upstream of the throttling means between the flow entering the downstream superheater after passing through the throttling means and the flow exiting from the superheater bypass.

When steam is throttled at constant enthalpy, temperature decrease-s correspondingly with a pressure decrease. Thus, the fluid temperature downstream of the throttling means decreases although its heat content per unit of flow may be equivalent to that of the fluid upstream of the throttling means. Since the upstream flow is greater than the superheater flow by the amount of flow passing through the superheater bypass, heat in the bypass flow may be effectively exchanged with the fluid discharging from the throttling means to raise the fluid enthalpy and reduce moisture in the fluid entering the downstream superheater. The pressure drop through the throttling superheater reduces the thermal shock to the tube elements. The specific volume of the fluid entering the downstream superheater is increased, increasing flow stability in the superheater, permitting flow through the su perheater and firing rate to be increased with minimum thermal upsets at the superheater outlet.

According to the invention, there is provided a startup heat exchanger for a steam-electric generating plant, said plant comprising a steam generator and turbine generator, said turbine generator having a high pressure turbine, said steam generator having a feedwater inlet, steam generating and superheating heat absorption conduits in series, a superheater steam outlet, fluid conduit means inter-connecting said feedwater inlet, heat absorption conduits, superheater steam outlet and high pressure turbine, throttling means for reducing fluid pressure intermediately between portions of said heat absorption conduits between said feedwater inlet and said superheater steam outlet, the portion of said heat absorption conduits downstream of said throttling means being operated during startup of said plant at a lower pressure than is in the portion of said heat absorption conduits upstream of said throttling means, a startup bypass conduit connected to said upstream portion and downstream of at least a portion of said heat absorption conduits which are directly connected to said feedwater inlet of said bypass conduit including means to flow fluid away from said upstream portion and for establishing circulation through at least a portion of said upstream portion independently of flow through said downstream portion during startup of said plant, thereby providing an independent circulation circuit including said bypass conduit and at least a portion of said upstream portion of said heat absorption conduits, said startup heat exchanger being located in the high temperature portion of said circulation circuit, said throttling means including conduit means for passing fluid from said upstream portion through a separate circuit in said startup heat exchanger after throttling and pressure reduction and .for discharging the eflluent from said separate circuit to said downstream portion, said startup heat exchanger being adapted to transfer heat in the fluid from said circulation circuit to said fluid passing through said separate circuit after pressure reduction.

The invention will be described in detail with reference to the accompanying drawings wherein:

FIG. 1 is a schematic diagram of the steam and water cycle for a steam-electric generating plant embodying the startup heat exchanger, and

FIGS, 2 through 7 inclusive are modified arrangements for the startup heat exchanger.

In FIG. 1, steam generator 1 is provided with feedwater inlet 2 and superheater outlet 3. Fluid passes from 2 through heat absorption conduits 4, through valve A, through heat absorption conduits 5 to superheater outlet 3. Conduits'4 may comprise steam generating circuits only and conduits 5 may comprise superheating circuits only. Conduits 4 alternatively may comprise both steam generating and superhea-ting circuits. Conduits 5 alter,- natively may comprise b-oth steam generating and superheating circuits.

During operation of the unit in the full load range, pressure drop across valve A is to be maintained at a minimum value to avoid excessive pumping power. Essentially all of the flow entering feedwater inlet 2 passes directly out through superheater outlet 3 in a single pass. Steam from superheater outlet 3 passes through conduit 6 to steam admission valve/s 7 which control flow of steam to high pressure turbine 8. Steam from turbine 8 After reheating, steam passes through conduit 11, through interceptor valve/s 12' to reheat turbine 13.

High pressure turbine 8 and reheat turbine 13 are connected through shaft means to electric generator 14.

Steam from reheat turbine 13 exhausts through conduits 15 to steam condenser 16. Cooling water passing through conduits 17 condenses the exhaust steam which collects as condensate in hotwell 18. l

Condensate pump 19 takes suction from hotwell 18 I ,through conduit 20 and discharges through conduit 21 to water purification equipment 22 and from there through regulator valve 23, through low pressure feedwater heater 24 and conduit 25 to deaerator 26.

Water enters deaerator 26 through spray pipe 27 and flows over trays 28 and through conduit 29 to deaerator storage tank 30. Conduit 31 equalizes pressure between deaerator 26 and storage tank 30. Water level controller 32 controls the opening of valve 23.

High pressure feedwater pump 33 takes suction from deaerator storage tank through conduit 34 and raises the discharge pressure to the working level of the steam generator. Feedwater flow control valve 35 at the pump discharge, regulates flow of water to steam generator 1 and is co-ordinated with firing .rate controls (not shown). Valve 35 discharges through high pressure heater 36, and conduit 37 to feedwater inlet 2.

Turbine extraction steam is fed to high pressure feedwater heater 36 through conduit 38, to deaerator 26 through conduit 39 and to low pressure heater 24 through conduit 40.

Steam generator 1 is provided with combustion means. Burners 41 fire oil or coal in a furnace (not shown) which supplies heat to heat absorption conduits 4 and 5. Fluid enthalpy progressively increases from the feedwater inlet 2 to superheatcr outlet 3. At constant pressure, fluid temperature increases as enthalpy increases.

During startup, minimum fluid flow circulates from feedwater inlet 2 through conduits 4, through conduit 42, through startup heat exchanger 43, through conduit 44 and pressure reducing valve R to flash tank 45. Valve R controls pressure in circuits 4 by means of controller 59. Liquid draining from flash tank 45 passes through conduit 46, through valve J to deaerator storage tank 30. Level controller 58 maintains constant flash tank water level.

Conduit 46 terminates in a spray pipe inside storage tank 30 which discharges into compartment 47. Compartment 47 isolates the flash tank drain flow from the remainder of the storage tank. Compartment 47 is cross connected with the remainder of the storage tank on the steam and water side, to maintain a common water level by means of gravity.

Conduit 48 is arranged to selectively draw flash tank drain flow from compartment 47 up to the limit of the drain flow collected in compartment 47. Flow through conduit 48 in excess of that collected from conduit 46 in compartment 47, is drawn from the remainder of storage tank 30. Conduit 48 discharges through flow con trol valve L to steam condenser 16.

Flow through conduit 46 enters the remainder of the storage tank 30 when there is no flow through conduit '48 or may be proportioned with respect to flow through conduit 48 to the condenser. 7

Thus, fluid in conduit 46 entering storage tank 30 may be discharged to condenser 16 through conduit 48 and returned to storage tank 30 through pump 19, conduit 25 to deaerator 26 and conduit 29 before passing through conduit 34, feedwater pump 33, high pressure heater 36 and conduit 37 to feedwater inlet 2. Alternatively, fluid in conduit 46 entering storage tank 30 may flow direct from compartment 47 to .conduit 34 to feedwater pump I deaerator 26 through conduit 39. Valve N controls pressure in deaerator 26. Conduit 49 also supplies steam to high pressure feedwater heater 36 through conduits 51 and 38 at a time when turbine extraction steam pressure is very low. Valve M controls steam pressure in heater 36. Conduit 49 also discharges surplus flash tank steam to condenser 16 through flow control valve P. Pressure controller 52 regulates valve P opening.

When starting up the unit, prior to lighting burners 41, circulation is established in conduits 4 through the flash tank and feedwater supply system as outlined above, using the feedwater pump 33 as a circulator. Valves A and B are closed. The conduits 4 are pressurized and valve R reduces the downstream pressure to the working pressure of the flash tank.

Burners 41 are fired. Fluid exiting through conduit 42 gradually rises in temperature. Steam is flashed in vessel 45. Flashed steam in deaerator 26 raises the fluid temperature entering storage tank 30 through conduit 29. Fluid temperature at the suction of pump 33 rises. Flashed steam in heater 36 raises the fluid temperature at the feedwater inlet 2. The fluid temperature continues to rise in conduits 4 as the firing rate of burners 41 is increased. Surplus steam becomes available in flash tank 45.

The surplus steam may be discharged to condenser 16 7 duit 53 to heat absorption conduits 5. Valve(s) 7 is/ are closed and valve 54 is opened to draw steam through conduit 6 to warm up the steam supply conduits upstream of Valve(s) 7. Valve 54 discharges to atmosphere or alternatively may discharge to condenser 16.

To purify the cycle Water, fluid is passed through conduit 48 to condenser 16 and returned to deaerator 26 through pump 19 and water purification equipment 22.

When the cycle is clean, steam is admitted through steam admission Valve(s) 7 to high pressure turbine 8, reheater 10 and reheat turbine 13 to bring the turbinegenerator unit up to speed. Firing of burners 41 is con trolled to regulate steam availability and temperature in high pressure turbine 8, co-ordinated with pressure regulation in flash tank 45 and conduits 5.

When the turbine generator unit is at synchronous speed, the unit is synchronized and the generator output is increased to a pre-determined minimum Value (approximately 5 percent of rating). Firing of burners 41 is increased; flow from flash tank 45 through valve V to conduits 5 and turbine 8 is increased. However, flow from flash tank 45 through valve V to conduits 5 and turbine 8 is substantially below the required minimum flow circulating in conduits 4, 42 and through valve R to flash tank 45. As a result, at'this time, considerable moisture is present in the fluid upstream of Valve R.

In order to continue loading the turbine generator unit, 1t 1s necessary to pass fluid directly from conduits 4 to 5 without passing through the flash tank. Up to this time 7 dry steam has been passing from flash tank 45 through quantity of moisture in the fluid passing through valve A would quench a substantial portion of conduits 5 at the inlet end. After a time delay, temperature at the superheater outlet 3 would droop substantially causing difliculty in the turbine unless the operation were carefully scheduled and spray water attemperation was used to minimize superheater outlet temperature transients. In any case, temperature transients within the superheater are severe.

The present invention overcomes difiiculties associated with differential enthalpies when transferring flow to conduits 5 from the flash tank source direct to the conduits 4 source. During the transfer, the enthalpy of the fluid entering conduits 5 is held relatively constant. This is accomplished by means of startup heat exchanger 43.

During the transfer, valve A remains closed. Valve B is gradually opened. Fluid from conduits 4 is reduced in pressure in conduit 55 by means of throttling action of valve B.

The pressure reduction is accompanied by a substantial temperature drop. Conduit 55 connects to one end of a closed circuit 56, passing through startup heat exchanger 43. The other end of closed circuit 56 connects to conduit 57 which in turn connects to conduits 5. Thus, as fluid passes through valve B, conduits 55, 56 and 57 to conduits 5, the fluid enthalpy is increased by means of heat received from flow passing through conduit 42 to heat exchanger 43. Since pressure reduction in the superheater bypass flow circuit through valve R occurs downstream of startup heat exchanger 43 and pressure reduction in the valve B circuit occurs upstream of startup heat exchanger 43, a temperature gradient exists for transferring heat from the valve R circuit to the valve B circuit across closed circuit 56.

For example, if the fluid pressure upstream of valves A and B is 2700 p.s.i.a. and the fluid temperature is 680 F., reducing pressure in conduit 55 to 500 p.s.i.a. will reduce fluid temperature to 467 F. This will produce a temperature differential of 213 F. across closed circuit 56 for evaporation of moisture in the effluent of valve B, assuming saturated fluid temperatures exist on both sides of conduits 56. As a result of heat transfer, the fluid passing through the valve R circuit Will increase in moisture content and the fluid passing through the valve B circuit will decrease in moisture content, as it passes around and through circuit 56. However, as the transfer is being made, displacing valve V flow to conduits 5 with flow through valve B, less and less steam is required from the flash tank. In effect, the transfer can be made holding enthalpy of the fluid entering conduits 5 essentially constant.

As flow through valve B is gradually increased above the maximum flow through valve V, firing of burners 41 can also be increased. Increased firing rate raises fluid enthalpy upstream of the throttling means, valve B. Less heat transfer is required through circuit 56 as flow through valve R is substantially decreased below the latter condition.

As load is increased by means of flow through valve B, pressure in conduits 5 can also be increased. Pressure rise in conduits 5 can be coordinated with requirements for heat exchange in startup heat exchanger 43. Increas- ,ing' pressure in conduits 5 will reduce heat transfer across closed circuit 56. Pressure rise in conduits 5 is controlled by coordination of valve(s) 7 opening with valve B opening. When heat exchanger 43 is no longer required for raising enthalpy of fluid downstream of valve B before it enters conduits 5, valve A opening may be coordinated with valve B opening to produce the required flow to conduits 5 and turbine 8. Coordination of valve A, B and 7 opening is then required to regulate pressure downstream of valve A and B and upstream of valve(s) 7.

The configuration of startup flow circuits shown on FIG. 1 is not limiting with respect to the present invention. For example, conduit 53 and valve V could be eliminated. In such case, initial flow to conduits 5 would be through valve B, conduit 55, through closed circuit 56 and conduit 57 to conduits 5. The enthalpy of the fluid upstream of valve B would be raised sufliciently by firing burners 41 in conjunction with circulation through con duits 4, valve R flow to the flash tank system and return flow to the feedwater inlet 2 through the feedwater cycle,

before valve B was opened. Heat recovery in the deaerator 26, storage tank 30 and high pressure feedwater heater 36, will permit this to be done with sufficiently low firing of burners 41, so that combustion gas temperatures in the circuits 5 zone will not be excessive for the condition prior to the opening of valve B. When valve B is opened to establish flow through conduits 5, 6 and valve 54, heat exchange with the valve R circuit flow in closed circuit 56 will assure reasonably dry steam entering conduits 5.

Also, the valve B circuit may comprise the main flow circuit and valve A and its connecting conduits could be eliminated. In such case, valve B may consist of a single valve or multiple valves designed to handle full load flow with minimal pressure drop for the full load condition.

As shown by FIG. 1, valve A serves as a bypass for startup heat exchanger 56 and may be used in conjunction with valve B to proportion flow through valves A and B to regulate the enthalpy of the fluid entering conduits 5, increasing or decreasing'the heat transfer rate through closed circuit 56.

The flash tank 45 shown in FIG. 1 is not essential to this invention, as it is obvious that the discharge from bypass conduit 44 could be connected to some other part of the feedwater cycle and accomplish similar results. The essential elements of the invention are that heat in the circulation flow through conduits 4 is exchanged with the flow entering conduits 5 after pressure reduction and that the flow through conduits 4 is of greater magnitude than the flow through conduits 5 at time of heat exchange.

FIG. 2 is a modified arrangement of startup heat exchanger 43. Heat exchanger 43 is located in the main flow circuit between conduits 4 and valve A. The superheater bypass conduit 44a and valve R are independent of heat exchanger 43 location.

FIG. 3 is a modified arrangement of startup heat exchanger 43. Heat exchanger 43 is an extension of the heat absorption conduits 4 and receives heat from burners 41. Closed circuit 56 is located internally in the heat absorption conduits 43.

FIG. 4 is a modified arrangement of startup heat exchanger 43. Both the valve B and valve R circuits are in the main flow line.

FIG. 5 is a modified arrangement of startup heat exchanger 43. The valve B and valve R circuit main line take off points, are separated by heat absorption conduits 4a. The take off points from the main flow circuit for conduits 42 and 55 alternatively could be reversed.

FIG. 6 is a modified arrangement of startup heat exchanger 43 in conjunction with a pumped recirculation circuit for heat absorption conduits 4. Circulation through conduits 4 is accomplished by means of pump 62. Recirculation flow is from pump 62 discharge, through conduits 4, 42, heat exchanger 43, reverse flow preventer 61 and conduit 60 to the suction of pump 62. Pump 62 could be located elsewhere in the recirculation loop.

FIG. 7 shows how conduit 57 could discharge intermediately between heat absorption conduits 5 and 5a between valve A and superheater outlet 3.

The comments above relative to valve V and conduit 53, valve A and the flash tank 45, also apply for FIGS. 2 through 7 inclusive.

Thus, it will be seen that I have provided an eflicient embodiment of the invention, whereby heat absorbed by the conduits in the high heat input zone of the steam generator, can be properly proportioned between the flow which discharges through the steam generator superheater outlet and the flow which bypasses at least a portion of the superheater during startup and at times when flow quantity in the conduits of the high heat input zone substantially exceeds that passing through the superheater outlet. This is accomplished by means of a startup heat exchanger, the temperature gradient for heat tnansfer resulting from reduction of fluid pressure upstream of the 'heatexcha'nger for that portion of the fluid flow which While I have illustrated and described several embodiments of my invention, it will be understood that these are by way of illustration only, and that various changes and modifications may be made within the contemplation of my invention and within the scope of the following claim.

What I claim is:

A startup heat exchanger for a steam-electric generating plant, said pl ant comprising a steam generator and turbine generator, said turbine generator having a high pressure turbine, said steam generator having a feedwater inlet, steam generating and superheating heat absorption conduits in series, a superheater steam outlet, fluid conduit means inter-connecting said feedw'ater inlet, heat absorption conduits, superheater steam :outlet and high pressure turbine, throttling means for reducing fluid pressure intermediately between portions of said heat absorption conduits between said feedwater inlet and said superheater steam outlet, the portion of said heat absorption conduits downstream of said throttling means being operated during startup of said plant at a lower pressure than is in the portion of said heat absorption conduits upstream of said throttling means, a startup bypass conduit connected to said upstream portion and downstream of at least a portion of said heat absorption conduits which are connected directly to said feedwater inlet, said bypass conduit including means to flow fluid away from said upstream portion and for establishing circulation through at least a portion of said upstream portion indeseparate circuit in said startup heat exchanger after throttling and pressure reduction and for discharging the efiiuent from said separate circuit to said downstream portion, said startup heat exchanger being adapted to transfer heat in the fluid from said circulation circuit to said fluid passing through said separate circuit after pressure reduction.

No references cited.

MARTIN P. SCHWADRON, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

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