US3187727A

US3187727A - Steam generator

Info

Publication number: US3187727A
Application number: US127216A
Authority: US
Inventors: Ravese Thomas
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1961-07-27
Filing date: 1961-07-27
Publication date: 1965-06-08
Anticipated expiration: 1982-06-08
Also published as: CH396037A; GB1008794A

Description

June 8, 1965 T. RAVESE 3, 7,

STEAM GENERATOR Filed July 27, 1961 2 Sheets-Sheet 1 FIG. I

INVEN TOR. THOMAS RAVESE ATTOR NEY June 8, 1965 Filed T. RAVESE STEAM GENERATOR July 27. 1961 2 Sheets-Sheet 2 INVENTOR. THOMAS RAVESE Maw ATTORNEY United States Patent [and 3,187,727 STEAM GENERATGR Thomas Ravese, Windsor, Conn, assignor to Combustion Engineering, Inc., Windsor, (301111., a corporation of Delaware Filed July 27, 1961, Ser. No. 127,216 Claims. (Ci. 122-406) This invention relates to supercritical once-through steam generators incorporating a circulating system, and in particular to the controls and safety means therefor.

In starting up a turbine, it is necessary to allow only a small amount of steam to pass therethrough at first, so that thermal stresses are not created which could damage the turbine. This is a slow process, and it is normally several hours before enough steam is passing through the turbine so that it can be put on the line, generating electricity.

This low minimum flow of fluid through the turbine is only a small percentage of that required when operating at full load. At the same time, a much higher percentage flow of fluid is required through the waterwalls of the furnace to prevent them from being damaged during start up. One method of accomplishing this in a supercritical once-through steam generator is to provide a circulating system having a circulating pump around the waterwalls of the furnace, so that some of the fluid flowing through the once-through flow system is recirculated back through the waterwalls again, thus maintaining the flow velocity through this section of the through flow system sufiiciently high to prevent the tubes from being damaged.

It is an object of this invention to provide means which safeguard the waterwalls in the event apparatus within the circulating system fails to perform properly.

It is a further object of this invention to provide means which prevents undue stresses and strains on the equipment located within the circulating system, thus prolonging the useful life of such equipment.

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

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination 4 of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of the illustrative embodiments, said embodiments being shown by the accompanying drawings wherein:

FIGURE 1 shows a once-through steam generator utilizing a circulating system and incorporating the novel features of my invention;

FIGURE 2 is a second embodiment of my invention that can be utilized in the system depicted in FIGURE 1; FIGURE 3 is a third embodiment of my invention than can be utilized in the system depicted in FIGURE 1;

FIGURE 4 is an enlarged view of one of the check valves located downstream of the circulating pump.

Looking now to FIGURE 1, it? is a once-through boiler operating at supercritical pressure, or pressure above 3206 psi. Fuel burning means 12 are provided for the furnace 11, and the hot combustion gases flow through the furnace and associated gas pass 12, thus heating the fluid flowing through the various heat exchange members, which fluid will ultimately be used to drive a turbine to generate electricity. Control 14 is used to regulate the rate of firing of the furnace by regulating the flow of fluel thereto. The rate of firing is determined in any wellknown manner, for example by measuring the pressure and temperature of the fluid flowing to the turbine.

Fluid at approximately 3500 p.s.i. is supplied to the 3,l87,727 Patented June 8, 1965 through flow system by feed pump 16. The through iiow system comprises an economizer l8, tubes 20, feed- Water header 22, mixing header 24, tubes 26, inlet header 27, tubes 23 which completely cover the walls of the furnace l1, outlet header 3i), and

heat exchange members

32 and 34 which are conventionally referred to as the primary and final superheaters, respectively, in a subcritical boiler. The fluid leaving heat exchange member 34 is used in driving a turbine 35.

When the turbine is first started up, only a small amount of fluid is allowed to flow through the turbine, this amount being a very small percentage of that required when operating at full load. Yet in order to protect the tubes 28 which line the walls of the furnaces, a much larger flow is necessary, this being maybe five times that which flows through the turbine during startup. If this high velocity flow is not maintained through the tubes 28, they will absorb too much heat from the combustion gases, which heat will not be carried away by the fiuid passing therethrough, thus damaging these tubes.

In order to allow a high velocity flow through these tubes and still just supply a small amount of fluid to the turbine, a circulating system is added to the once-through flow system, which recirculates fluid leaving the outlet of waterwall tubes 28 back through the waterwall tubes 28 again.

The circulating system comprises pipe 36 which extends from outlet header 30 to the circulating pump 38. Outlet lines 40 and 42 allow the recirculated fluid leaving the pump to flow to header 44. These lines contain combined check and control valves 46 and 48 to prevent reverse flow through the circulating system when the pressure in the once-through system exceeds that which the circulating pump is capable of producing.

Motors

50 and 52 are used for moving members into engagement with the combined check and control valves when desired, as later more fully explained, to positively close the combined check and control valves, and thus prevent flow therethrough in either direction. Pressure differential measuring device 54 measures the pressure diiferential across combined check and control valve 46, and is used to control the

motors

50 and 52. Bypass line 56 is connected to the outlet of pump 38, and allows a small flow to pass through the pump at all times, even after combined check and control valves 46 and 48 have been positively closed. The outlet of this bypass is connected to any suitable lower pressure source, such as heat exchange member 32.

Gate vaive 58 is for the purpose of completely isolating circulating pump 38 from the circulating system, when the pump fails and must be replaced, or when maintenance work is to be done thereon. Reserve or standby pump 60 is provided in parallel with the main pump 38, and can be brought into operation when the main pump is not capable of operating. Combined check and control valves 63 and 7d are provided in

lines

64 and 66, and are for the same purpose as the corresponding combined check and control valves 46 and 48. These combined check and control valves can also be positively closed by

motors

72 and 74, which motors are controlled by pressure differential measuring device 80. Bypass line 75 is for the same purpose as corresponding bypass line 56. In order to maintain a small flow of fluid through the reserve pump 60 while the main pump is operating, bleed line 76 is provided. Line 73 bypasses valve 62, so that the flow is through line 76, pump 60, and then back to the suction side of main pump 38 by means, of line 78.

Metering orifice 82 is positioned in one of the tubes 26, and control unit 84 is connected to the tube both upstream and downstream of the device to measure the velocity of the flow therethrough, as an indication of the velocityof flow through the tubes lining the furnace walls. The flow through these tubes 23 must be maintained at a predetermined minimum, for example three feet per second, for below this velocity it is possible the tubes may rupture or be damaged. To prevent this, when control 84 measures a velocity flow which is below the predetermined minimum, it sends out a signal which will shut oif the fuel feed to the furnace, thus preventing the addition of any 7 more heat to the furnace. An alternative control which could be used would be to send out a signal from control 84 to open valve 62 and start up reserve pump 61 when the predetermined minimum velocity is reached.

Another safety feature of the invention is interlock system 86. This mechanism prevents fuel from being supplied to the furnace until pump 38 is operating when the boiler is first being started up. Iri this manner, an operator cannot inadvertently fire the furnace when first starting up until the pump has been started and is operating properly, thus insuring the needed velocity flow through the furnace waterwalls.

The circulating pump should be of such capacity that it will insure a minimum velocity of flow through the tubes lining the furnace walls at all times. As the turblue is gradually brought up to full load, a point will be reached at which the flow through the one-through system will be sufiicient to protect the furnace wall tubes. In one installation, this point was determined to be 70% of full load. However, the percent required may vary between installations, depending on such factors as the size of the feedpump, the size of the tubes lining the furnace walls, the temperature of the burning fuel within the furnace, etc. In most installations today once a turbine has been brought up to full load, it is desired to later run it at some load less than fullload capacity, such as 30%, 50%, or 70% of full load. Obviously, when the turbine is brought down below the 70% load range, then the circulating system must again be utilized to maintain the minimum flow velocity through the vapor generating section, or furnace wall tubes. 7

The operation of the system will now be described. When it is desired to start up the unit, feedwater pump 16 is energized, and forces cold water through the econoinizer, waterwall tubes,- and other sections of the oncethrough flow system. The circulating pump is then started up, recirculating some of the fluid back through the water- Walltubes 28. Fuel is then supplied to the fuel burning means and ignited. If the circulatitng pump 3% fails to start for some reason, then interlock 36 will prevent the fuel from being supplied to the furnace.

The fluid in the system will gradually increase in. temperature, and in pressure, until eventually steam will be created, and also the pressure will be above supercritical pressure, at approximately 3500 p.s.i., which is the pressure the unit has been designed to operate at. It is desirable to operate at pressures above supercritical, since fluids atthis pressure are a-single phase fluid and have substantially the same characteristics throughout, re gardless of the temperature. .If the unit were operated at subcritical pressure, or below 3206 p.s.i., then when a small amount of the liquid in the system started turning to steam, the circulating pump 38 would not be capable of pumping this mixture. When pockets of steam passed through the pump, the steam would be substantially compressed, turning it back into the liquid state, making the pump operate erratically and also causing damage to the As steam is formed in the system, a valve 33 upstream of tion.

the turbine 35 is cracked open, and a very small percentage of fluid is allowed to flow through the turbine. The percent is gradually increased by opening valve 33 more as the turbine warms up. The turbine must be slowly warmed up to prevent undue stresses and strains from being created therein by allowing a large amount of extremely high temperature steam to pass therethrough instantaneously. The period of time to warm up a turbine can be six hours or longer.

During this startup period, aportion'of the fluid flowing through the through flow circuit is withdrawn through the circulating system by means of pump 38, and recirculated back through the waterwall tubes 28. As steam is generated in the circuit, and more and more of it is allowed to flow to the turbine during startup, the pressure on the downstream side of check valves 46 and 48 gradually. approaches and finally exceeds the pressure in the circulating system. The purpose of the combined check and control valves is to prevent reverse flow through the circulating system when this point is reached, which in one particular installation was at approximately 70% of full load capacity.

As is the case with any check valve,.there is a small drop in pressure as the fluid flows through it, this pressure being required to hold the check valve open. When the pressure on the downstream side of combined check and control valves 46 and 48 is just a few p.s.i. less than that on the upstream side, the valves will start to flutter and chatter. The circulating system to be used for a large utility boiler must be capable of handling many thousands of gallons per minute of fluid, it being approximately ten thousand gallons per minute in the earlier mentioned installation. The combined check and control valves 46 and 48 are a foot or greater in diameter to handle this large flow, and if they were allowed to flutter or chatter, they would quickly become worm or damaged by their constant dropping and banging on their respective seats until they would no longer seat properly.

To prevent this from happening, a control is provided to positively close these valves at a point prior to that at which the valves begin to flutter or chatter. Since the flow through the once-through circuit is related to the percent load on. the unit, a measure of the percent of load can be used to control the valves. As illustrated, the pressure difierential across the combined check and control valve 46 is measured, and when this pressure differential reaches a predetermined minimum, the device 54 actuates motors 5t) and 52 which moves members into physical contact with combined check and control valves 46 and 48, forcing them onto their respective seats. This control point is set just below that at which the valves would startto flutter, such as 70% of full load, when the circulating pump 33 is capable of circulating fluid up to maybe 71 or 72% of full load. Although measuring the pressure difl'erential across the combinedcheck and control valve is the most direct and reliable'method to determine the cut off point at which the valves are closed, other means which are also related to load could alternatively be usedas the control. For example, the cut off point of 70% could be measured by the feedwater flow, the steam flow to the turbine, the fuel feed input control, the load requirements on the generator, or the pressure differential acros the pump 38. All of the above vary with the load on the unit and are an indication of it, and could be used to actuate the. motors and 52 when the unit reaches of full load, or whatever other cut off point at which it is desired to have them respond.

The particular construction of the combined check and control valve is not important, so long as it can be positively closed to prevent flow therethrough in either direc FIGURE 4 shows one construction of a combined check and control valve'whichcan be used. Numeral 10$ designatesthe valve inlet, and 102 is the valve outlet. When the pressure of the fluid on the outlet side of the valve exceeds the pressure of the fluid in the valve inlet,

this higher pressure fluid flows to the back side of the valve by means of connection 110 to move the valve head into engagement with the valve seat 306. When there is no flow through the valve, the head 1% rests on seat 106 by means of the force of gravity. When it is desired to positively close the valve to prevent flow in either direction therethrough, the motor is actuated to drive valve stem 108 into physical contact with the back side of valve head 194, thus positively forcing head 104 into engagement with seat 1%.

Most utility boilers today are designed to operate over a wide load range, for example from 30-50% on up to full load. When the unit is brought down from full load to below the 70% point, the circulating system is needed again, and the combined check and control valves must be opened at this point. The point at which they are opened is again set so that they will not flutter or chatter. The pump can be allowed to continue to run above the 70% point, if desired, in order to insure that it is in operation when the unit is brought down below the 7 0% point, after operating at full load. Above 70%, however, it will not recirculate fluid, since the valves are positively closed.

Another method of preventing the check valves from chattering would be to have the control shut off the pump 38 when the 70% cut off point is reached, as shown in FIGURE 2. The pressure on the downstream side or" the check valves would then positively hold the check valves closed. This cut oil? point would still have to be slightly below the maximum of which pump 38 is capable, in order to prevent the check valves from fluttering. Another alternative method of preventing the check valves from fluttering would be to have the control open a valve 99 in a bypass line, such as line 56 or '75, when the control point is reached. This is shown in FIGURE 3. Again, the pressure on the downstream side of the check valves would positively hold them closed. In both of the above alternatives, using the control to shut oil? the pump or open a valve in the bypass line, when the unit comes down from full load to below 70%, the control would again start up the pump or reclose the valve. In both of the above alternatives a normal check valve is all that would be required, and there would be no need for the positive closing member 108 as illustrated in FIG. 4.

Looking again to FIG. 1, the bypass or jumper line 56 is for keeping the pump hot at all times. It connects the outlet of the pump 38 to a lower pressure point, such as heat exchange section 32. These units are designed to supply steam at an extremely high temperature to the turbine, and when at full load the temperature of the fluid leaving the waterwall tubes may be on the order of 725 F. If no bypass line 56 were provided, and the unit operated at full load for a period of time when the circulating system were not in use, the pump 38 would cool off. Then if the unit were brought down below 70%, and hot fluid were allowed to flow through the cold pump, the pump could be ruined by the stresses and strains created by the temperature differential. To prevent this, a small amount of fluid is allowed to flow through the bypass line 56 at all times. This may be on the order of 100 gallons per minute, while the circulating system is capable of handling 9,000-10,000 gallons per minute. All that is necessary is enough to keep the pump hot. It should be pointed out that this small flow through the pump should also be maintained in the alternative design, where the pump is shut off above 70%, as shown in FIGURE 2.

Line 76 allows a small amount of fluid to flow to the reserve pump 66, so that it too is kept hot at all times, in case of failure of main pump 38. Line 78 bypasses gate valve 62, so that the fluid can pass through reserve pump 60 back to the suction side of main. pump 38 even when the valve 62 is closed.

Reserve pump 6i is in parallel with main pump 38, and can be put into operation when the main pump fails. If this happens, gate valve 58 is closed, gate valve 62 is opened, and the reserve pump 6% is actuated. Pump 38 6. is then isolated from the circulating system and can be repaired or replaced, as required.

The reserve pump 60 is provided with combined check and

control valves

68 and 70, pressure differentialmeasuring device 89, and bypass line 75, all of which are for the same purpose as the corresponding members of circulating pump 38.

Velocity measuring device 84 is for the purpose of cutting oil? the fuel feed when the velocity of flow through the waterwall tubes 23 drops to a predetermined minimum, such as three feet per second. An alternative method would be to have this device close gate valve 58, open gate valve 62, and actuate reserve pump 60 when the predetermined minimum was reached, and if this didnt increase the velocity flow after a predetermined short time, then cut off the supply of fuel.

As illustrated, the circulating pump 38 supplies fluid to header 44, from which it flows into mixing header 24, and mixes with the fluid coming from economizer 18. After the unit has been running for a time, the fluid in the circulating system will be hotter than that coming from the economizer, and the mixing header 24 is for the purpose of mixing these two fluids, so that a single fluid of uniform temperature flows through tubes 26 to the waterwall tubes 2%. If desired, the pump 38 could be positioned downstream of mixing header 2 4, and the same controls of this invention utilized therewith. The position of the circulating pump 38 and the combined check and control valves does not matter, and the controls of this invention could be incorporated regardless of the pump and valve locations. A patent application of W. W. Schroedter, Serial No. 127,395, filed on even date herewith and entitled Recirculating System for Steam Generator (now U.S. Patent No. 3,135,252), discloses a number of alternative pump locations, and the inventive features of this application, or portions of them, could be adapted to use with any of them.

While I have illustrated and described the preferred embodiments of my invention, it is to be understood that such is merely illustrative and not restrictive and that variations may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. In a once-through steam generator having a through flow system, a furnace, fuel burning means associated with said furnace, said through flow system comprising tubes lining the walls of said furnace, an inlet and outlet for saidtubes, a feedpump associated with said through flow system for supplying fluid to said inlet, other heat exchange apparatus, the outlet of said tubes being connected to the other heat exchange apparatus, a circulating system comprising an inlet communicating with the outlet of said tubes, an outlet communicating with the inlet of said tubes, a circulating pump associated with said circulating system for maintaining flow through the circulating system, said circulating pump being of sucha capacity that it is capable of maintaining flow through the circulating system only until the steam generator reaches a predetermined percentage of its full load capacity, a combined check and control valve downstream of said circulating pump, activating means for said combined valve including first means for causing closure of the combined valve when the fluid tends to flow from the outlet to the inlet of the circulating system, said activating means including second means for positively closing said combined valve, thereby preventing flow therethrough in either direction, when a predetermined minimum pressure diiferential exists upstream and downstream of said combined valve, this predetermined minimum pressure difierential being slightly greater than that at which the first means would cause closure of the combined valve, so as to prevent the combined valve from fluttering or chattering.

2. The steam generator set forth in claim 1, including a bypas line, the inlet of which is connected intermediate 7 said furnace, said through flow systemcomprising tubes lining the walls of said furnace, an inlet and outlet for said tubes, a feedpump associated with said throughflow system for supplying fluid to said inlet, other heat exchange apparatus, the outlet of said tubes being connected to the other heat exchange apparatus, a circulating system comprising an inlet communicating with the outlet of said tubes, an outlet communicating with the inlet of said tubes, a circulating pump associated with said circulating system for maintaining flow of fluid through the circulating system, said circulating pump being of such a capacity that it is capable of maintaining flow through the circulating system only until the'steam generator reaches a predetermined percentage of its full load capacity, a combined check and control valve downstream of said circulating pump, actuating mean for said combined valve including first means for causing closure of the combined valve when the fluid tends to flow from the outlet to the inlet of the circulating system, said actuating means including second means for positively closing said combined valve, thereby'preventing flow therethrough in either direction, when the steam generator reaches a percentage of its full load capacity slightly less than said predetermined percentage, so as to prevent the combined valve from fluttering or chattering.

4. Ina once-through steam generator having a through flow system, a furnace, fuel burning means associated with said furnace, part of said through flow system residing in said furnace, a feedpump associated with said through flow system for supplying fluid to said through flow system, a recirculating system around a portion of said through flow system, so that some of the fluid flowing in said through flow system can be withdrawn through said recirculating system and made to flow through said portion again, said recirculating system having an inlet and an outlet, a pump associated with the recirculating system for maintaining flow of fluid through said recirculating system, said pump being of such a capacity that it is capable of maintaining flowthrough the recirculating system only until the steam generator reaches a pre+ determined percentage of its full load capacity, a combined check and control valve in said recirculating system for preventing reverse flow therethrough, actuating means for said combined valve including first means for causing closure of the combined valve when the fluid tends to flow from the outlet to the inlet of the recirculating system,

said actuating means including second means for positively closing said combined valve, thereby preventing flow therethrough in either direction, when the steam generator reaches a percentage of its full load capacity slightly less than said predetermined percentage, so as to prevent the combined valve from fluttering or chattering.

5. In a once-through steam generator having a through flow system, a furnace, fuel burning means associated with said furnace, part of said through flow system residing in said furnace, a feedpurnp associated with the through flow system for supplying fluid to said through flow system, a recirculating system around a portion of said through flow system, so that some of the fluid flowing in said through flow system can be withdrawn through said recirculating system and made to flow through said portion again, said recirculating system having an inlet and an outlet, 2. pump associated with the recirculating system for maintaining flow of fluid through said recirculating system, said pump being of such a capacity thatit is capable of maintaining flow through the recirculating system only until the steam generator reaches a predetermined percentage of its full load capacity, a combined check and control valve in the recirculating system, means for sensing the pressure differential across the combined valve, actuating means for said combined valve including first means for causing closure of the combined valve when the fluid tends to flow from the outlet to the inlet of therecirculating system, said actuating means including second means, operatively connected to the sensing means, said second means being capable of positively closing said combined valve, thereby preventing flow therethrough in either direction, when the pressure differentialtacross the combined valve reaches a predetermined minimum, so 'as to prevent the combined valve from fluttering or chattering.

References (Iited by the Examiner UNITED STATES PATENTS 2,495,324 1/50 Griswold 137-497 2,849,990 9/58 Tongret 122.504 X 3,105,468 10/63 Gardam 122-.451 FOREIGN PATENTS 719,753 12/54 Great Britain. 768,201 2/57 Great Britain. 831,175 3/60 Great Britain.

OTHER REFERENCES Combustion Engineering, published by Combustion Engineering, Inc. New York, 1957 (page 16-4 relied on).

PERCY L. PATRICK, Primary Examiner. FREDERICK L. MATTESON, 1a., Examiner.

Claims

1. IN A ONCE-THROUGH STEAM GENERATOR HAVING A THROUGH FLOW SYSTEM, A FURNACE, FUEL BURNING MEANS ASSOCIATED WITH SAID FURNACE, SAID THROUGH FLOW SYSTEM COMPRISING TUBES LINING THE WALLS OF SAID FURNACE, AN INLET AND OUTLET FOR SAID TUBES, A FEEDPUMP ASSOCIATED WITH SAID THROUGH FLOW SYSTEM FOR SUPPLYING FLUID TO SAID INLET, OTHER HEAT EXCHANGE APPARATUS, THE OUTLET OF SAID TUBES BEING CONNECTED TO THE OTHER HEAT EXCHANGER APPARATUS, A CIRCULATING SYSTEM COMPRISING AN INLET COMMUNICATING WITH THE OUTLET OF SAID TUBES, AN OUTLET COMMUNICATING WITH THE INLET OF SAID TUBES, A CIRCULATING PUMP ASSOCIATED WITH SAID CIRCULATING SYSTEM FOR MAINTAINING FLOW THROUGH THE CIRCULATING SYSTEM, SAID CIRCULATING PUMP BEING OF SUCH A CAPACITY THAT IT IS CAPABLE OF MAINTAINING FLOW THROUGH THE CIRCULATING SYSTEM ONLY UNTIL THE STEAM GENERATOR REACHES A PREDETERMINED PERCENTAGE OF ITS FULL LOAD CAPACITY, A COMBINED CHECK AND CONTROL VALVE DOWNSTREAM OF SAID CIRCULATING PUMP, ACTIVATING MEANS FOR SAID COMBINED VALVE INCLUDING FIRST MEANS FOR CAUSING CLOSURE OF THE COMBINED VALVE WHEN THE FLUID TENDS TO FLOW FROM THE OUTLET TO THE INLET OF THE CIRCULATING SYSTEM, SAID ACTIVATING MEANS INCLUDING SECOND MEANS FOR POSITIVELY CLOSING SAID COMBINED VALVE, THEREBY PREVENTING FLOW THERETHROUGH IN EITHER DIRECTION, WHEN A PREDETERMINED MINIMUM PRESSURE DIFFERENTIAL EXISTS UPSTREAM AND DOWNSTREAM OF SAID COMBINED VALVE, THIS PREDETERMINED MINIMUM PRESSURE DIFFERENTIAL BEING SLIGHTLY GREATER THAN THAT AT WHICH THE FIRST MEANS WOULD CAUSE CLOSURE OF THE COMBINED VALVE, SO AS TO PREVENT THE COMBINED VALVE FROM FLUTTERING OR CHATTERING.