US3504655A - Natural circulation start-up system for once-through steam generator - Google Patents

Description

April 7, 1970 w. P. GORZEGNO NATURAL CIRCULATION START-UP SYSTEM FOR ONCE-THROUGH STEAM GENERATOR Filed Oct. 11. 1967 INVENTOR. WALTER I? GORZEG/VO RICHARD H. THOMAS ATTORNEY United States Patent 3,504,655 NATURAL CIRCULATION START-UP SYSTEM FOR ONCE-THROUGH STEAM GENERATOR Walter P. Gorzegno, Florham Park, N.J., assignor to Foster Wheeler Corporation, Livingston, N.J., a corporation of New York Filed Oct. 11, 1967, Ser. No. 674,550 Int. Cl. F22d 7/00 U.S. Cl. 122406 15 Claims ABSTRACT OF THE DISCLOSURE A once-through vapor generator including means for operating the generator during start-up by natural circulation subsequently converting to forced circulation during high pressure normal operation of the generator.

This invention relates to improvements in the starting of a once-through vapor generator.

To start-up a once-through boiler, it has heretofore been necessary to provide a by-pass line around the turbine or point of use, and/or portions of the generator to handle the flow during the start-up period. The reason for this is that during start-up, the fluid is in either a liquid state or liquid-vapor mixture state neither of which can be handled by the turbine, and perhaps portions of the generator. Towards reducing start-up time, it has been the practice to provide a reduced pressure flash tank in the by-pass line designed to provide a vapor flow earlier in the start-up period for warming and rolling the turbine for other uses.

Many disadvantages are experienced with these conventional systems, a principal one being that a separate source of power is required to drive the feed pump during the initial stage of start-up and before vapor is produced in the generator. Another disadvantage is that the conventional by-pass and heat recovery system and controls therefor are complex and expensive, and by virtue of the by-pass, some surfaces of the generator may not be cooled during the startup period. A further disadvantage is the difliculty of matching fluid enthalpy in conventional units during changeover from flow through the by-pass line to flow to the point of use or turbine. Usually a higher quality flow is obtainable from the flash tank than the main flow path at the time of switchover to the main flow path, the result being a sudden temperature drop at the point of use.

In accordance with the invention, these disadvantages are overcome by providing a means for natural circulation of the boiler during start-up with change-over to forced circulation at a predetermined load. To accomplish this, the tubes of the generator in the furnace section thereof are parallel to each other and vertically oriented. Circuit inlet and outlet headers are provided from which riser tubes connect to manifolds at the top of the generator; a drum is positioned connected to these manifolds to receive the flow therefrom. Separation means in the drum separate the flow into vapor and liquid streams, and an unheated downcomer disposed outside of the furnace section returns the liquid stream to the tube inlet headers. The downcomer is sized large enough so that the total circuit resistance to flow during start-up of the generator is sufiiciently less than the head created by the difference in densities of the flows in the riser tubes as compared to the unheated downcomer to obtain unassisted natural circulation of the flow. At a predetermined load point, for instance approximately 30% load, natural circulation flow in the unheated downcomer changes to a forced flow; that is suitable feed pump means forces the flow through the circuits of the generator. A by-pass line is provided by-passing the drum.

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The invention and advantages thereof will become apparent upon consideration of the following specification with reference to the accompanying drawings, in which the figure is a vertical perspecive and partial section view of a multi-pass vapor-generator in accordance with the invention.

Referring to the figure, there is illustrated a vaporgenerator 12 having a furnace section 14 defined by riser tubes 16. Suitable inlet headers connected to tubes in the front and rear walls (18a and 18b respectively) of the generator make these walls a first flow pass in the generator, and suitable inlet headers feeding the side walls (20a and 20b) of the generator (plus portions of the front and rear walls) make these walls the second furnace flow pass. A main feed pump 22 is operatively connected (in a way to be described) to the first pass walls 18a and b, and downcomer 24 recycles the flow from the top of the furnace front and rear walls to the side walls 20a and b via valve 26; for two-pass furnace operation of the furnace.

The normal once-through flow in the boiler is then from the feed ump 22 to the economizer 28; to a generator drum 30 (to be described) via line 31, then via downcomer 32 to the front and rear walls (18a and b) of the boiler, and finally through downcomer 24 to the side walls (200 and b) of the boiler, with valve 26 open.

A once-through steam generator during start-up must maintain a minimum circuit flow in the furance for adequate cooling. Initially the turbine cannot accept this minimum flow (about 25 to 30% of full load flow), either in quantity or fluid state (enthalpy, pressure), and a startup by-pass system must be employed from initial start-up to 25 or 30% of full load.

The described invention establishes a natural circulation loop for the furnace circuits during initial start-up and to 25 to 30% load, with the remainder of the circuits cooled by steam generated as a result of burner input to the furnace. During this start-up load period, as with a natural circulation steam generator, operating pressure may be maintained at low valves (about 1000 p.s.i. for a 3500 p.s.i. full load operating pressure), and steam may be generated by control of firing rate to satisfy turbine and other system demands. In thi way, the start-up bypass system (required of conventional once-through steam generators) is not needed, nor are the complicated controls usually associated with one-through by-pass systems. Feed pump 22 output is only required to maintain throughput steam flow to meet demand requirements. At the same time steam cooling of surfaces other than the furnace riser tubes is easily achieved using conventional natural circulation steam generator principles.

Operation during start-up is described as follows: A water level is established in drum 30, and shunt line 34 between downcomers 32 and 24, with valve 36, is opened so that the flow in downcomer 32 is into all of the passes 18 and 20 (a and b), feeding in parallel the side, front and rear walls. For this purpose valve 26 in line 24 (connecting the passes in series) is closed. On firing the burners in the furnace section of the generator, an upward natural circulation for the fluid in the furnace walls is established, the flow passing from the walls to drum 30 positioned at the top of the generator. This is accomplished through collecting headers 38 and 40 at the top of the furnace and valves 42 and 44 between these headers and the drum 30. At this stage of the generator operation, the valves 42 and 44 are of necessity open, as well as valve 36 in shunt line 34, mentioned in the foregoing.

In the drum 30, vapor-liquid separators are disposed designed to produce separate vapor and liquid streams, with the drum having a liquid space and a vapor space; Liquid is returned from the drum 30 by means of the downcomer 32, and the vapor flows via line 46 and valve 58 to remaining heat recovery portions of the generator, including roof 48, convection wall passes 50 and superheater 52.

A significant feature of the present invention is that the downcomer 32 and other components, for instance the drum 30, are sized sufficiently large to lower the resistance in the circuits such that up to about 30% load, the difference in densities between the flows in the riser furnace circuit tubes and the flow in the downcomer is sufficient to achieve unassisted natural circulation of the flow in the boiler.

To achieve forced circulation at the 30% load point, programmed manipulation of circuitry valves 36, 26, 42, 44, 56 and 58 will gradually change furnace circuit flow from a natural circulation characteristic cooling all four walls, 18a, 18b, 20a, 20b, in parallel to a forced circulation characteristic cooling walls 18a and 18b in series with walls 200: and 20b.

Referring to Table I, the programming for valve operation is evident.

TABLE I.PRO GRAMMED VALVE OPERATION This programmed manipulation of valves is as follows:

During natural circulation operation up to 30% load, valves 36, 42, 44 and 58 are open, and valves 26 and 56 are closedas listed in Table I.

At 30% load or the load corresponding to the minimum circuit flow required by the steam generator for oncethrough operation, the turbine-generator can accommodate 100% of the through-put flow, and the turbine by-pass requirement is zero. At this load point, it is then desirable to change to one-through operation for the steam generator.

To accomplish this, valve 36 (in shunt line 34), valve 42 between first pass header 38 and drum 30, valve 44 between second pass header 40 and drum 30, and valve 58 in transfer steam line from drum 30 to the roof circuit 48, are programmed closed at a desired rate. Simultaneously valve 26 in downcomer 24 and valve 56 in line 54 are programmed open. For once-through operation feed flow in conduit 31 flows through drum 30 to enter downcomer 32 which feeds the first furnace pass, 181: and 18b.

Functional considerations during this transition from natural circulation to once-through operation are as follows: At the 30% load point with the furnace circuitry still cooled using natural circulation, the furnace circuit pressure has reached approximately 2500 psi. The fluid (steam-water mixture) exiting from the furnace circuits at this stage of operation is about 25% steam by weight at an enthalpy of approximately 825 B.t.u. per pound. The circulation ratio for this natural circulation loop is approximately four to one; that is, the weight of steamwater mixture circulating in the furnace cincuit loop is four times the through-put steam leaving drum 30 through line 46.

When the circuit valves listed in Table I are programmed opened and closed respectively to change from natural circulation to once-through circuit flow, the circulation ratio through the furnace circuits decreases from four to one, and the exit fluid enthalpy increases to approximately 1130 B.t.u. per pound. During this changeover period, the furnace circuits, and other circuits of the steam generator, are also pressurized to the full pressure level set by design 3500 psi. for a supercritical unit. The once-through fluid mass flow rate in the furnace circuits is now approximately 600,000 lb./hr. sq. ft. at the 30% load point. Operation to full load is now in accordance with conventional procedures for a once-through steam generator.

The 30% load point is usually recognized as the point in once-through circulation where a minimum mass flow for proper furnace cooling is established. As is well known, the unheated external downcomers in a natural circulation boiler carry saturated water from the drum to distribution pipes and headers feeding the furnace wall risers, which receive heat. Natural circulation occurs since the risers contain a steam and water mixture which is less dense than the saturated water in the downcomers.

As the pressure in the boiler is increased, the difference in densities of the mixtures in the risers and downcomers becomes smaller, with less available force or head to promote natural circulation. At lower pressures, for instance 2,000 pounds per square inch throttle pressure there is more than adequate natural pressure available and circuit design is not highly critical. At higher pressures, such factors as steam in the water in the downcomers, and improper sizing of the downcomers and/or valves, can more easily retard natural circulation. Accordingly, sizing of the downcomers and valves, or calculation of resistance in the unit is critical towards maintaining sufficient head up to the pressure at 30% load and switchover.

It is, of course, possible to design the boiler components for natural circulation up to loads exceeding 30% but for high subcritical pressures costs become prohibitive. For instance, drum 30 would have to be inordinately large for minimal resistance to flow. A changeover at or about 30% load appears most economical in most instances. Changeover below 30% load is feasible, but would require the by-pass system which a purpose of the present invention is to avoid.

In the foregoing, it was mentioned that a very substantial and expected reduction in fluid mass flow rate is experienced during changeover from natural to forced circulation. Accordingly, it may be desired to dispose in at least some of the furnace circuits orifices or turbulators in the tubes to obtain equal distribution of the flow and adequate cooling. In the present example, by dividing the furnace section into two passes in series relationship for once-through operation, the control of distribution of flow becomes less critical, but it is still feasible, if the boiler is of substantial perimeter, to use distribution devices. As these devices increase the resistance in the circuit, the extent to which natural circulation occurs is also decreased. However using a larger downcomer can compensate for increased riser rsistance to obtain a desired circulation ratio.

Although the invention has been described with respect to specific embodiments, many variations within the scope of the following claims will be apparent to those skilled in the art.

What is claimed is:

1. A method for starting-up and operation of a vapor generator of the type which includes a flow circuitry including primary radiant heated surfaces, secondary primarily convection heated surfaces in series flow relation-- ship with the primary surfaces, the primary surfaces at least in part comprising parallel vertically oriented tubes defining the generator furnace section; comprising the steps of establishing a natural circulation flow through said primary surfaces;

separating the flow into a vapor stream and a liquid stream;

recycling said liquid stream to the primary surfaces for obtaining said natural circulation therein;

passing said vapor stream to said secondary surfaces;

and

at a predetermined load point during start-up causing all of the flow from said primary surfaces to pass to said secondary surfaces, circulation following said predetermined point being pump assisted.

2. A method for starting-up and operation of a vapor generator of the type which includes a flow circuitry including a primary radiant heated surface, and secondary primarily convection heated surface in series flow relatiOnShip with the primary surface, the primary surface at least in part comprising parallel vertically oriented tubes defining the generator furnace section, the tubes having lower inlet ends and upper outlet ends; comprising the steps of establishing a fluid level in said primary surface;

imparting heat to said fluid to establish a natural circulation rising flow in the tubes of the primary surface;

separating the flow from the tube outlet ends into a.

vapor stream and a liquid stream;

downwardly flowing said liquid stream to said tube inlet ends to maintain in part said liquid level; passing the vapor stream to said secondary surface;

and

at a predetermined load point during start-up causing all of the flow from said primary surface to pass directly to said secondary surface, circulation in said generator following said predetermined point being forced flow pump assisted.

3. A method for starting-up and operation of a vapor generator of the type which includes a flow circuitry including vapor generating and superheating surfaces, the generating surfaces comprising at least in part parallel vertically oriented tubes defining the generator furnace section, the tubes having lower inlet ends and upper outlet ends; said circuitry further including a fluid collecting and separation drum disposed above said vertically oriented tubes and connected to the tube outlet ends to receive the flow therefrom; comprising the steps of establishing a liquid level in said drum and vertically oriented tubes;

heating said tubes to cause a rising flow of the liquid therein;

collecting the flow in said drum from the outlet ends of the tubes and separating the flow into separate vapor and liquid streams;

downwardly flowing said liquid stream to the tube inlet ends, the resistance to flow upwardly in the tubes and downwardly to the tube inlet ends being maintained sufliciently low so that the flow is by natural circulation;

passing the vapor stream from said collecting and separation drum to remaining surfaces of the generator; and

at a predetermined point during start-up causing all of the flow from said primary surface to pass directly to remaining surfaces of said generator, circulation in said generator following said predetermined point being pump assisted.

4. A method according to claim 3 wherein said predetermined point of transition from natural circulation to pump assisted circulation occurs when the resistance to adequate flow starts to exceed the head provided by the difference in density of said rising furnace section tube flow and said downwardly flowing liquid stream.

5. A method according to claim 4 wherein the mass flow and pressure in the generator during natural circulation of the flow are maintained at a portion of normal operation mass flow and pressure, the mass flow and pressure increasing with increase of enthalpy of the fluid being heated, the increase in mass flow correspondingly increasing said resistance to flow, the increase in pressure reducing the head available.

6. A method according to claim 3 wherein said point of transition is at or above approximately 25% load.

7. A method according to claim 3 wherein said furnace section tubes are divided into at least two discrete upflow flow passes, said method further comprising the steps of causing the flow following transition to pump assisted circulation to pass through said upflow flow passes in series.

8. A once-through generator circuit comprising a plurality of vertically oriented parallel riser wall tubes having lower inlet ends and upper outlet ends defining a furnace section;

burner means in the furnace section;

drum means positioned above the furnace section connected to the riser tube outlet ends to receive the heated fluid from said tubes;

separation means in said drum means to separate the heated fluid into separate vapor and liquid streams, said drum having vapor and liquid spaces to receive said vapor and liquid streams;

a feed system connected to the riser tube inlet ends including pump means to force a cooled flow through the riser tubes;

unheated downcomer means outside said furnace section connected between the drum liquid space and the tube inlet ends;

said downcomer means being sized with respect to the resistance to flow in said circuit whereby the resistance to flow therein is sufficiently less during start-up of the generator than the head created by the difference in density of the flows in the riser tubes and downcomer means to obtain an unassisted natural circulation of the flows therein;

valve means associated with at least said drum means to prevent the flow of heated fluid therein during pump assisted operation of the generator.

9. A once-through vapor generator circuit according to claim 8 wherein the wall riser tubes are welded together to form gas-tight panels;

header means for said panels connected to predeter mined numbers of tubes dividing the panels into at least two distinct upflow furnace flow passes;

conduit means connecting said header means so that the passes are in series flow relationship;

valve means in the conduit means operable during natural circulation start-up of the generator to prevent the flow therein so that the flow passes are in parallel flow relationship.

10. A once-through generator circuit comprising a plurality of vertically oriented parallel riser wall tubes having lower inlet ends and upper outlet ends defining a furnace section;

burner means in the furnace section;

the riser tubes being welded together to form gas-tight panels;

inlet and outlet header means for said panels connected to predetermined numbers of tubes dividing the panels intoat least first and last distinct furnace upflow flow passes;

drum means positioned above the furnace section connected to the riser tubes outlet header means to receive the flow from said riser tubes;

separation means in said drum means to separate the flow into separate vapor and liquid streams, said drum having vapor and liquid spaces to receive said vapor and liquid streams;

a feed system to supply flow to said riser tubes;

pump means connected to said feed system for forced circulation of flow through said riser tubes;

unheated downcomer means outside said furnace section connected between the drum liquid space and the tube inlet header means such that unassisted natural circulation can be obtained;

conduit means connecting said header means so that the upflow passes are in series flow relationship during normal operation of the generator;

valve means operatively associated with said conduit means and feed system so that the flow in said upflow passes is in parallel relationship during start-up of the generator.

11. A circuit according to claim 10 wherein said furnace passes are two in number.

12. A circuit according to claim 10 wherein said pump means disposed in flow communication with said drum means liquid space and including valve means to operatively isolate said drum means from the circuit except for the flow of feedwater during pump assisted operation of the generator.

13. A vapor generator comprising a primary circuit which includes vertically oriented parallel furnace tubes, collecting and separation means adapted to receive the OW from said furnace tubes and to divide the flow into separate liquid and vapor streams, and downcomer means to return the liquid stream to said furnace tubes;

means to impart heat to said furnace tubes;

said circuit being sized so that the total circuit resistance during start-up is less than the head created by the diiference in density of the flow in said furnace tubes and said liquid stream to obtain an unassisted natural circulation of the flow in said circuit during start-up;

pump circulating means for said generator; and

conduit and valve means associated with said circuit and pump means for once-through circulation of the flow during normal operation of the generator.

14. A vapor generator circuit comprising a plurality of vertically oriented parallel riser t-ubes having lower inlet ends and upper outlet ends defining a furnace section; means to impart heat to said riser tubes; collecting and separation means above the furnace section connected to the riser tube outlet ends to receive the heated fluid from said tubes, and to separate the heated fluid into separate vapor and liquid streams;

means to recycle the liquid stream to said tube inlet ends;

said circuit being sized so that the total circuit resistance during start-up is less than the head created by the difference in density of the flow in the riser tubes and the liquid stream being recycled to the tube inlet ends, to obtain an unassisted natural circulation of the flow;

pump circulating means for said generator;

valve means for said circuit whereby the flow during pump assisted circulation is once through the circuit.

15. A vapor generator comprising a plurality of vertically oriented parallel riser wall tubes having lower inlet ends and upper outlet ends defining a furnace section;

burner means in the furnace section;

the riser tubes being welded together to form gas tight panels;

inlet and outlet header means for said panels connected to predetermined numbers of tubes dividing the panels into at least first and second distinct furnace upfiow flow passes;

drum means positioned above the furnace section;

separation means in the drum means producing separate liquid and vapor streams;

downcomer means connected to said drum means to receive said liquid stream;

vapor conduit means connected to said drum means to receive said vapor stream;

a feed system including pump means;

said generator including first circuit connections comprising conduit means to transmit the flow from said furnace section outlet header means to said drum means, and conduit means to transmit the flow from said downcomer means to said furnace section inlet header means;

said drum means, furnace section downcomer means and first circuit connections being sized so that the circuit resistance during start-up is less than the head created by the difference in density between the flow in said furnace circuit and said liquid stream to obtain an unassisted natural circulation of the flow during start-up;

said generator including second circuit connections including second downcomer means connecting predetermined header means so that said upfioW passes are in series flow relationship, bypass conduit means transmit the flow directly from One of said outlet header means to said vapor conduit means bypassing said drum means, and means connecting said feed system to one of said inlet header means;

valve means for said downcomer and conduit means so that the flow is in series in said furnace passes and once-through in said generator during normal operation thereof.

References Cited UNITED STATES PATENTS 1,707,920 4/1929 Norton l22406 2,869,517 1/ 1959 Leeberherr 122-406 3,003,479 10/ 1961 Bock et a1.

3,135,243 6/ 1964 Schroedter.

3,215,126 11/1965 Sprague l22406 3,234,920 2/ 1966 Kemmetmuller et a1. l227 3,299,860 l/ 1967 Svendsen l22406 3,368,533 2/1968 Knizia l22406 KENNETH W. SPRAGUE, Primary Examiner

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