US3399656A - Circulation system for a steam generator - Google Patents

Description

Se t. 3, 1968 c. STROHMEYER, JR 3,399,655

CIRCULATION SYSTEM FOR A STEAM GENERATOR Filed Jan. 19, 1967 2 Sheets-Sheet 1 INVENTOR. CHARLES STROHMEYERJK p 1968 c. STROHMEYER, JR 3,399,556

CIRCULATION SYSTEM FOR A STEAM GENERATOR Filed Jan. 19, 1967 2 Sheets-Sheet 2 Fi.2A

Fig.2B

INVENTOR. CHARLES STROHME YER, Jr:

te tate 3,399,656 CIRCULATION SYSTEM FOR A STEAM GENERATOR Charles Strohmeyer, Jr., Wyomissing, Pa., assignor t Electrodyne Research Corporation, Reading, Pa. Filed Jan. 19, 1967, Ser. No. 610,414 Claims. (Cl. 122406) ABSTRACT OF THE DISCLOSURE This invention relates to steam-electric generating units having a steam generator incorporating pumping means for recirculating fluid through at least a portion of the steam generator heat absorption circuits, which portion includes at least a portion of the walls for a combustion furnace and wherein it is desired to improve the distribution and/or quality of the fluid circulated through parallel conduits of said portion of said heat absorption circuits to accommodate variations in heat absorption among said parallel conduits from said combustion furnace. This invention is a continuation-in-part of US. patent application Ser. No. 452,143 filed Apr. 30, 1965.

An object of this invention is to provide a high pressure steam-electric generating plant having a steam generator comprising a feedwater inlet and superheater steam outlet and heat absorption circuits connected by first fluid conduit means therebetween, wherein conduit and pumping means are provided to recirculate fluid in at least a portion of said heat 'obsorption circuits, said portion comprising at least a portion of walls for a combustion furnace and having multiple parallel tubes connected at the fluid inlet end to a segmented header forming a portion of said first fluid conduit means, said recirculation conduit means outlet connecting individually to segments of said header, the upstream portion of said first fluid conduit means also connecting individually to said segments of said header through fluid supply conduits, orifice means located in said fluid supply conduits to control the distribution of fluid from said feedwater inlet to and selectively among portions of said segmented header, selective sizing of said orifice means providing the means for selectively proportioning said fluid from said feedwater inlet among said portions of said segmented header as a consequence of fluid pressure drop across said orifice means whereby fluid dynamic head requirements for said pumping means are essentially independent of the pressure drop across said orifice means and selective control of fluid enthalpy at the inlet to said multiple parallel tubes is upstream of the point where recirculated fluid is blended with fluid flowing directly from said feedwater inlet.

Another object is to provide a means for selectively proportioning recirculated fluid with fluid flowing directly from the feedwater inlet among the multiple parallel tubes connecting to a segment of the inlet header.

A further object is to provide a means for cooling the recirculated fluid before it enters the pumping means, thereby decreasing the specific volume of the pumped fluid and reducing pumping power requirements for equivalent pumping head and mass fluid flow rates.

A still further object is to provide a means for bypassing fluid around the fluid cooling means and to selectively distribute fluid flowing between the fluid cooling 3,399,656 Patented Sept. 3, 1968 means and the fluid bypass means by selective arrangement of the fluid cooling means, the pumping means and the fluid bypass means.

A still further object is to provide a means for controlling the mass quantity of fluid recirculated by means of introducing fluid from a point upstream of the recirculation loop into the pumping means inlet circuit.

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 said circulation system for a steam generator improvements, and

FIG. 2 is a typical detail of a segment of an inlet header supplying fluid to tubes forming a portion of the walls for a combusting furnace.

In FIG. 1 the steam generator is disposed between feedwater inlet 1 and superheater steam outlet 2. Feedwater flows from 1 through conduit means to and through economizer 3 to fluid supply conduits 4. Conduits 4 feed fluid to segments A, B, C, and D of composite header 5.

Multiple parallel tubes 6 are divided into groups and connect individually to segments of header 5 as shown on FIG. 1. Fluid from header 5 passes up through tubes 6 to header 7. Tubes 6 form walls for at least a portion of a combustion furnace (not shown). Firing means are provided for said combustion furnace (also not shown).

From header 7 fluid flows through conduit means to and through multiple tube wall enclosure 8 for the gas stream resulting from the products of combustion in said combustion furnace. From enclosure 8 fluid passes through shutofl and throttling valve means 9 to and through primary superheater 10, to and through secondary superheater 11 to superheater steam outlet 2. Heat absorption surfaces, 3, 6, 8, 10 and 11 may all receive heat from the products of combustion generated in the same said combustion furnace.

Conduit 12 connected to superheater steam outlet 2 conveys steam through steam admission valve means 13 to turbine 14. Warm up conduit 15 and valve 16 are used to heat up conduit 12 during plant startup prior to admitting steam to turbine 14. Turbine 14 connects to electric generator 17 through shaft means 18. Turbine 14 exhausts through conduit 19 to condenser 20. Circulating water conduit 21 provides the means for condensing exhaust steam which is collected in hotwell 22.

Conduit 23 conveys hotwell 22 fluid to pump 24. Pump 24 discharges through conduit 25 to water purification equipment 26, to and through low pressure feedwater heaters 27 and 28 in series to deaerator 29. Deaerator 29 discharges through conduit 30 to deaerator storage tank 31. The vapor sides of deaerator 29 and storage tank 31 are cross connected by means of conduit 32. Valve 33 controls the discharge from pump 24 to maintain a constant water level in storage tank 31.

Conduit 34 feeds liquid from deaerator storage tank 31 to feedpump 35. Feedpump 35 is driven by variable speed steam turbine driver 36. Feedpump 35 raises the fluid pressure to the working pressure in the steam generator. The variable speed of driver 36 regulates flow quantity to feedwater inlet 1.

Feedwater pump 35 discharges through conduit 37 to and through high pressure feedwater heaters 38 and 39 in series to feedwater inlet 1. Check valve 40 prevents reverse flow through pump 35. Feedwater heaters 27, 28, 29, 38 and 39 receive extraction steam from turbine 14 in ascending order of pressure (not shown).

Turbine 14 in conjunction with the steam generator may incorporate one or more reheat steam circuits (not shown).

Means are provided for recirculating fluid in multiple parallel tubes 6. Eflluent from enclosure 8 passes through conduits 40, 41 and 42 to mixing chamber 43. Recirculation pump 44 driven by motor 45 takes suction from chamber 43 through conduit 46 and discharges through check valve 47 and conduit 48 to segments A, B, C and D of header 5. Pump 44 provides the dynamic head required to overcome fluid resistance and differences in fluid density and static head throughout the recirculation circuit.

When first firing the unit during startup and when raising temperature of the fluid in the circuits to the normal working level, valve means 9 is shut. Feedwater pump 35 supplies make up fluid to the recirculation circuit at the working pressure of the system. Pump 44 maintains a minimum fluid circulation flow rate through multiple parallel tubes 6. To increase the makeup flow rate fluid may be drawn from conduit 40 through conduit 49 to and through heat exchanger tubular surface 50 in heat exchanger 51, through conduit 52 and through conduit 53 and flow control valve 54 to deaerator storage tank 31 or alternatively through conduit 55 and flow control valve 56 to condenser 20. Heated fluid passing to deaerator storage tank 31 through conduit 53 flashes providing a source of auxiliary steam for feedwater heating and other services not shown. Fluid passing to condenser 20 through conduit 55 is condensed and cooled in condenser 29 after which it is collected in hotwell 22 and passed through water purification equipment 26 for cleanup and return to the steam generator circulation circuits.

As the fluid temperature and enthalpy increase in conduits 40 and 49, fluid may be passed from conduit 41 through conduit 57 and pressure reducing valve 58 to the shell side of heat exchanger 51, through conduit 59 to superheaters and 11 in series, through conduit 12 to and through warm up conduit and valve 16. The fluid from valve 16 may discharge to waste or be recovered elsewhere in the cycle.

Fluid internally in tubular surface Sfl is at the high working pressure level of the recirculation circuit. Fluid in heat exchanger 51 shell downstream of valve 58 is low in pressure. As a result of pressure reduction through valve 58 there is a substantial reduction of fluid temperature entering heat exchanger 51 shell compared with the temperature of the fluid passing through tubular surface 50. As a result, heat i transferred from tubular surface 50 to the fluid passing through heat exchanger 51 shell. There is a fluid temperature and enthalpy decrease from conduits 49 to 52 and a fluid enthalpy increase from conduit 57 downstream of valve 58 to conduit 59. By adequately sizing tubular surface 50 and control of pressure drop across valve 58, essentially dry saturated steam may be passed through conduit 59 to superheater 10 during the period when initially flowing steam to turbine 14 through steam admission valve means 13. The operation of heat exchanger 51 is more completely described in US. patent application Ser. No. 452,143 filed Apr. 30, 1965.

Circulation is initially established in tubes 6 at a pressure sufficient to supress liquid and vapor separation, fires are ignited in the combustion furnace, fluid is heated in the boiler recirculation circuit, heated fluid is drawn off through conduit 52 for feedwater heating and cooling in condenser 20, liquid from condenser hotwell 22 is purified in 26 and returned to the boiler circulation circuit through feedwater inlet 1. As fluid in conduit 41 approaches 550 F., a portion of the flow stream may be passed through valve 58 to superheater 10 and out through valve 16 to Warm conduit 12. After conduit 12 is warmed and there is superheated steam of adequate quality upstream of steam admission valve means 13, steam may be passed to turbine 14 to roll it up to working speed. The enthalpy in conduit 40 is in range of 900 B.t.u./lb. or higher at the time generator 17 is ready to be synchronized with the connected electrical system (not shown). After synchronizing firing rate in the combustion furnace is increased, flow to the superheater through valve 58 is increased and pressure in superheaters 10 and 11 is increased.

Flow through valve 58 continues up to the time that fluid enthalpy in conduit 40 reaches a range of from 1100 to 1200 B.t.u./lb. at which time flow may be passed from enclosure 8, through valve means 9 to superheater 10. Pressure in superheaters 10 and 11 is increased up to the working level of the system by coordinated increase of port area of valves 9 and 58. Flow through conduits 53 and is discontinued. Recirculation through conduits 40, 41, 42, 46, pump 44 and conduit 48 is continued as required to maintain minimum circulation in multiple parallel tubes 6.

Heretofore, steam generators employing pumped recirculation means have not incorporated features which provide for selective blending of recirculated fluid with lower enthalpy fluid direct from the feedwater inlet with respect to different segments of multiple parallel tubes 6. Fluid of a uniform quality entered all of tubes 6. Orificing to the individual tubes regulated the flow quantity to each tube so as to adjust the total flow among the tubes in proportion to the total heat absorption in that portion of the parallel circuit as well as to accommodate differences in absorption among individual parallel tubes. Such a system had fixed characteristics which cannot be readily adjusted to accommodate changing heat absorption patterns among portions of tubes 6 (Le, slag formation decreasing heat absorption in certain circuits and increasing heat absorption in other circuits). An unbalanced condition tended to amplify itself. Should heat absorption in a tube increase above its normal value, then fluid within the tube expanded a greater amount as it passed through the circuit, the increased fluid volume per pound of flow increased the fluid resistivity through the circuit with the net result that mass flow decreased causing the tube metal temperature to increase as a consequence of reduced mass flow. To limit the degree of thermal unbalance among tubes 6, high pressure drop was required at the orifice inlet to tubes 6. This increased the fluid dynamic head requirements for pump 44, increasing the horsepower rating of motor 45.

My invention overcomes past difficulties in that orifice means for regulating fluid flow from feedwater inlet 1 to tubes 6 is upstream of the recirculation loop for tubes 6. The fluid enthalpy in segments A, B, C and D of header 5 to tubes 6 is selectively adjusted to best accommodate changing heat absorption patterns among groupings of tubes 6. Thus, as heat absorption in a particular tube 6 segment increases at constant feedwater inlet 1 fluid flow rate, fluid flow rate to that tube segment from the feedwater inlet 1 remains relatively constant, the increase in fluid expansion only decreases the quantity of recirculated fluid from pump 46. This tends to reduce fluid enthalpy entering the said tube 6 segment which partially compenstates for the decrease in mass fluid flow rate so as to minimize the rise in tube metal temperature as a result of increased heat absorption. Since changes in heat absorption for any tube 6 segment are accompanied by a fluid enthalpy change in the inverse direction entering that same tube 6 segment, variations in fluid mass flow and metal temperatures among the different tube segments are minimized. The circuits tend to be self-compensating.

Location of orifice means 60 in fluid supply conduits 4 reduces the dynamic head required for pump 44 as no orificing would normally be required at the inlet to tubes 6. The construction is economical and permits a furnace wall to be designed and built within known parameters. As a result of balancing inlet fluid flows and adjusting the quality of the fluid circulated through segments of tubes 6, a greater enthalpy rise may be tolerated from the inlet header 5 to outlet header 7, thus reducing the requirement for intermediate mixing headers from feedwater inlet 1 to superheater outlet 2.

FIGURE 2 is an extension of the FIGURE l principles to provide compensation for individual tube circuits 6 connected to a segment of header 5. Segment A has been selected for purposes of illustration. Other segments could employ similar construction. The position of conduits 4 and 48 with respect to header 5A are modified for FIG- URE 2 compared with FIGURE 1.

Conduit 4 is extended internally within header 5A through tube 61 which connects to distribution tube 62. Tube 61 is welded to the body of header 5A and dis tribution tube 62 as shown. Support lugs 63 are welded to tube 62 and center tube 62 axially in header 5A.

Initially tube 61 and lugs 63 are welded to tube 62 complete with end plates 64. This assembly is then inserted in header 5A, end plate 65 removed. Tube 61 is inserted in the hole in header 5A prepared for connecting to conduit 4. Tube 62 is properly aligned with header 5A so that lugs 63 provide a solid support for tube 62. Tube 61 is welded in place to header 5A. Tubes 6 below welded joint (a)) are joined to header 5A. Orifice holes 66 are drilled in tube 62 through the hollow core of the tube 6 stubs. The tube 6 stubs could be assembled after drilling orifices 66 providing the connecting holes 67 are first drilled in header 5A. Orifice holes 66 are selectively sized to accommodate expected heat transfer rates in the individual tube 6 circuits. End plate 65 is welded in place. Conduits 4 and 48 or portions thereof are welded to header 5A.

Fluid from feedwater inlet 1 enters through conduit 4, through tube 61 to tube 62. Orifices 66 in tube 62 direct the fluid toward tubes 6 through plenum chamber 68 in the space between header 5A and tube 62. There is pressure drop across orifices 66 which distribute feedwater to tubes 6. Orifices 66 also serve as nozzles for directional control of fluid flow. Thus, fluid from orifices 66 flows preferentially to tubes 6 compared with recirculated fluid flow from conduit 48 to header 5A.

Should one of tubes 6 connecting to header 5A receive more heat from the combustion furnace than the other tubes, fluid expansion in that tube would tend to reduce the mass of fluid flowing through that tube. Since fluid flowing direct from feedwater inlet 1 enters tubes 6 preferentially, any reduction of fluid mass flowing to any tube compared with the remainder of the tubes tends to reduce the quantity of recirculated fluid flowing in that tube from conduit 48. The reduction in mass flow through said tube is minimized by the reduction in average enthalpy entering said tube. Since there is a common pressure drop from header 5A plenum chamber 68 to header 7, there is no need for orificing at the inlet ends ofstube 6 for flow control purposes. Thus, tubes 6 may be sized for maximum advantage in furnace wall construction. Each tube is assured its proportionate share of fluid flowing direct from feedwater inlet 1. Pressure drop through tubes 6 may be low and dynamic head requirements for pump 44 are minimal.

On FIGURE 1 heat exchanger 51 is used during times when steam is first admitted to turbine 14 to roll it up to speed, when synchronizing initially loading the turbine generator set up to a predetermined loading value which may be substantially below the rating of the unit. All of the fluid flowing through conduit 40 could be directed through tubular surface 50, conduit 52, conduit 69 and reverse flow preventer 70 to pump 44. Isolation means (not shown) may be required for conduit 42 to do this. Reduction of fluid temperature and enthalpy through tubular surface 50 reduces the fluid specifiic volume. Where heat exchanger 51 is located at an elevation substantially higher than pump 44, the reduced specific volume of the fluid in conduit 52 provides additional fluid static head to overcome the resistivity of the fluid through tubular surface 50. Where the fluid static head equals or exceeds the resistivity (friction loss) of the fluid, fluid will flow preferentially through conduits 52, 69 and reverse flow preventer 70 to mixing chamber 43 and fluid flowing through conduit 42 will be zero. In other cases flowing fluid will be proportioned between conduits 42 and 69. Flow of fluid through valves 54 and 56 will also influence the proportioning of fluid flowing between conduits 42 and 69 as pressure drop through the respective circuits as varied thereby.

Flow of fluid through conduit 69 to mixing chamber 43 to pump 44 reduces the specific volume of the fluid entering the pump and for a given set of pump characteristics increases the mass of the fluid flowing through tubes 6, especially during the startup process, thereby assuring tubes 6 greater protection from development of hot spots.

At times when average fluid temperature and enthalpy exiting from tubes 6 exceeds a desired preset value and as measured at some downstream point in the recirculation circuit, the fluid cooling effect in tubes 6 may be increased by increasing mass flow of fluid through tubes 6. This is accomplished by directing fluid from a point upstream of the point where conduits 4 connect to header 5 to the suction side of pump 44. On FIGURE 1 conduit 71 connects to conduit 37 at the outlet of high pressure heater 39. Flow control valve 72, responsive to fluid temperature in conduit 48, controls fluid flow rate to mixing chamber 43. When fluid temperature in conduit 48 exceeds a preset value this is sensed by themocouple 73 which is connected through conductor means to temperature controller 74 which in turn positions valve 72 through conduit and power actuator means to restore fluid temperature in conduit 48 to a predetermined value established in set point setter 75.

For any given pump characteristics, mass flow (pounds/hr.) is increased as fluid specific volume is decreased. Thus, if the fluid in conduits 42 and 71 at 3800 p.s.i.g. have respective enthalpies of 1150 and 550 B.t.u./ lb. and a flow ratio of 3:1, the ratio of specific volumes will be 12:7 with a resultant increase of 71 percent in mass flow through the pump. This results in a recirculation flow rate increase of 28 percent.

The cooling effect may be utilized permanently or during a transient control process wherein feedwater flo-w and/or firing rate are adjusted to restore the circuits to equilibrium conditions.

Thus, it will be seen that I have provided an eflicient embodiment of my invention, whereby a means is provided for improving circulation in a steam generator by selectively proportioning fluid flowing direct from the steam generator feedwater inlet with recirculated fluid and among parallel conduit segments of a furnace wall, reducing fluid dynamic head requirements for the recirculation pumping means, and controlling fluid enthalpy entering the furnace wall segments downstream of orificing means. In addition a means is provided for cooling the recirculated fluid before it enters the pumping means. A bypass conduit is provided for the cooling means incorporating selective proportioning of fluid through the bypass conduit and cooling means. A means is also provided for increasing the recirculation flow rate by means of introducing fluid from a point upstream of the recireulation loop into the pumping means inlet circuit.

While I have illustrated and descriebd 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 claims.

I claim:

1. A high pressure steam-electric generating plant having a steam generator comprising a feedwater inlet and superheater steam outlet and heat absorption circuits connected by first fluid conduit means therebetween, said heat absorption circuits including walls for a combustion furnace, said walls having multiple parallel tubes connected at the fluid inlet end to a segmented header forming a portion of said first fluid conduit means, conduit and pumping means to recirculate fluid in at least a portion of said walls and said segmented header, said recirculation conduit means outlet connecting to individual segments of said header, the upstream portion of said first fluid conduit means also connecting to said individ ual segments of said header through independent fluid supply conduits, orifice means located in said fluid supply conduits and adapted to control distribution of fluid flow from said feedwater inlet to and selectively among said individual segments of said segmented header separately from said fluid recirculating through said conduit and pumping means, selective sizing of said orifice means providing the means for selectively proportioning said fluid flowing from said feedwater inlet among said individual segments of said segmented header as a consequence of fluid pressure drop across said orifice means whereby fluid dynamic head requirements for said pumping means are essentially independent of the pressure drop across said orifice means and selective control of fluid enthalpy at the inlet to said multiple parallel tubes is upstream of the point where recirculated fluid from said pumping means discharge is blended with fluid flowing directly to said multiple parallel tubes from said feedwater inlet.

2. A high pressure steam-electric generating plant as recited in claim 1 and wherein one or more of said fluid supply conduits extends internally within said connected portion of said segmented header, said associated orifice means comprising at least multiple orifices, said tube connections and said multiple orifices being arranged for distributing fluid discharge from each of said multiple orifices selectively to said tube connections through a common plenum chamber within the associated portion of said segmented header supplying recirculated fluid to said parallel tubes.

3. A high pressure steam-electric generating plant as recited in claim 1 wherein said conduit and pumping means to recirculate fluid incorporate a heat exchanger located upstream of said pumping means in said conduit circuitry, said heat exchanger reducing fluid enthalpy and temperature to said pumping means.

4. A high pressure steam-electric generating plant as recited in claim 1 and wherein conduit and flow control means are provided to inject fluid flowing from a point upstream of said fluid supply conduit connections to said segmented header into said recirculation conduit means upstream of said pumping means and downstream of said multiple parallel tubes.

5. A high pressure steam-electric generating plant as recited in claim 3 wherein said heat exchanger is located substantially higher in elevation than said pumping means, said conduit circuitry upstream of said pumping means including a bypass conduit for said heat exchanger, said difference in elevation between said heat exchanger and said pumping means and said fluid temperature reduction at said heat exchanger outlet providing the means to flow fluid to said pumping means selectively through the portion of said conduit circuitry incorporating said heat exchanger with respect to said bypass conduit.

6. A high pressure steam-electric generating plant as recited in claim 4 wherein said flow control means is responsive to fluid temperature in said conduit and pumping means.

7. A high pressure steam-electric generating plant having a steam generator comprising a feedwater inlet and superheater steam outlet and heat absorption circuits connected by first fluid conduit means therebetween, wherein conduit and pumping means are provided torecirculate fluid in at least a portion of said heat absorption circuits, said portion having multiple parallel tubes connected at the fluid inlet to said recirculation conduit means outlet and to the upstream portion of said first fluid conduit means and adapted, orifice means located in said upstream portion of said first fluid conduit means and adapted to control distribution of fluid from said feedwater inlet to and selectively among portions of said multiple parallel tubes separately from said fluid recirculating through said conduit and pumping means, selective sizing of said orifice means providing the means for selectively proportioning said fluid from said feedwater inlet among said portions of said multiple parallel tubes as a consequence of fluid pressure drop across said orifice means whereby fluid dynamic head requirements for said pumping means are essentially independent of said pressure drop across said orifice means and selective control of fluid enthalpy at the inlet to said multiple parallel tubes is upstream of the point where recirculated fluid from said pumping means discharge is blended with fluid flowing directly to said multiple parallel tubes from said feedwater inlet.

8. A high pressure steam-electric generating plant as recited in claim 7 wherein said conduit and pumping means to recirculate fluid incorporate a heat exchanger located upstream of said pumping means in said conduit circuitry, said heat exchanger reducing fluid enthalpy and temperature to said pumping means.

9. A high pressure steam-electric generating plant as recited in claim 7 and wherein conduit and flow control means are provided to inject fluid flowing from a point upstream of said orifice means into said recirculation conduit means upstream of said pumping means and downstream of said multiple parallel tubes.

10. A high pressure steam-electric generating plant as recited in claim 8 wherein said heat exchanger is located substantially higher in elevation than said pumping means, said conduit circuitry upstream of said pumping means including a bypass conduit for said heat exchanger, said dilference in elevation between said heat exchanger and said pumping means and said fluid temperature reduction at said heat exchanger outlet providing the means to flow fluid to said pumping means selectively through the portion of said conduit circuitry incorporating said heat exchanger with respect to said bypass conduit.

References Cited UNITED STATES PATENTS 3,185,136 5/1965 Cozza 122-406 3,213,835 10/1965 Egglestone 122406 KENNETH W. SPRAGUE, Primary Examiner.

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