US3530836A - Forced through-flow steam generator - Google Patents

Description

United States Patent Inventor Mario Caravatti Winterthur, Switzerland Appi. No. 744,149

Filed July 11,1968

Patented Sept. 29, I970 Assignee Sulzer Brothers, Ltd.

Winterthur, Switzerland a corporation of Switzerland Priority July 13, 1967 Switzerland FORCED THROUGH-FLOW STEAMGENERATOR 10 Claims, 2 Drawing Figs.

[1.8. CI 122/406 lnt.Cl F22b 29/12 Field of Search 122/4065,

[56] References Cited UNITED STATES PATENTS 2,321,390 6/1943 Juzi 122/406X 3,020,894 2/1962 122/479X 3,021,824 2/1962 122/406 3,368,533 2/1968 Knizia .1 122/406 Primary Examiner- Kenneth W. Sprague Attorney-Kenyon, Kenyon, Reilly, Carr and Chapin ABSTRACT: The steam generator is provided with a tap off point between the evaporator and first superheater and a second tap off point after the first superheater. The tap off points connect with a separator. A by-pass line is also positioned between the second superheater and turbines. Upon starting, working medium is tapped off and returned to the flow path between the feed water tank and evaporator. After starting, up to normal operation. the working medium is not tapped off and the steam generator functions as a once through forced-flow system.

Patented Sept. 29, 1970 I of Sheet Inventor:

' MAP/O RAVATTI 3 3,530,836 Patented Sept. 29, 1970Sheet 2 of 2 v 6? MI FORCED THROUGH-FLOW STEAM GENERATOR This invention relates to a forced through-flow steam generator. More particularly, this invention relates to a forced through-flow steam generator having a combustion chamber formed of gas-tight tubular walls.

Steam generators have been known wherein a combustion chamber is formed of tubes which are welded to one another in gas-tight manner either directly or with strips or fins between adjacent tubes. In some instances, such combustion chambers have been divided into two sections which are arranged in sequence with respect to a flow of working medium passing therethrough. The lower of the sections has formed at least part of an evaporator while the upper section has formed a first superheater. These steam generators have also had at least one other superheater connected in the working medium flow, a steam turbine connected downstream of such other superheater and a by-pass line including a shut-off valve means for by-passing the working medium flow from the superheaters around the turbine.

Where these steam generators are to operate at relatively low pressures, the evaporators are designed to account for quite a large proportion of the tube walls of the combustion chamber. However, as the design pressure increases, this proportion decreases thus necessitating the use of superheater surfaces for lining the combustion chamber. Accordingly, with steam generators of this construction and particularly when they are intended to operate at high pressures. there is a danger that excessive temperature differences will occur at the boundary between the evaporator and the first superheater during starting because no steam will be available for cooling the superheater. This can be particularly damaging where the tubes are welded together in gas-tight manner.

Accordingly, it is an object of the invention to reduce the temperature differences at the boundary between the evaporator and first superheater of a gas-tight combustion chamber of a steam generator.

.It is another object ofthe invention to cool the tube walls of a combustion chamber including an evaporator and first superheater from the beginning of steam generator starting.

Briefly, the invention provides a steam generator having a gastight tubular combustion chamber wall forming an evaporator and superheater with tap-off means for recirculating a portion of the working medium flow through the evaporator and superheater.

The steam generator is supplied with feed water from a feed water tank and is constructed with a combustion chamber which is divided into two sections in series with respect to the flow of working medium. The combustion chamber is formed of tubes which are welded to one another in gas-tight manner so that the lower section of the combustion chamber forms at least part of an evaporator and the upper section forms a first superheater. In addition, at least one further superheater is positioned in the working medium path within the combustion chamber and a steam turbine is connected to the downstream superheater. Also, a line with a shut off means therein leads from "the line connecting the downstream superheater to the steam turbine for by-passing the working medium around the turbine. In order to tap-off the working medium a first working medium tap is positioned between the evaporator and, the first superheater and a second working medium tap is positioned at the end of the first superheater. A water separator. is connected to both tapping positions or a water separator is connected to each tapping position and a connection is made between the separator or separators for returning separated water -to the working medium flow path between the feed water tank and the evaporator.

The steam generator can be started in such a way that there is little risk of danger arising from excessive temperature differences and that steam can be delivered quickly for heating the turbine. That is. during the early stages of starting, working medium is tapped off from the second tapping position and is passed through the water separator associated with that tapping position (or through the common separator if one is employed instead of two individual separators) and is recycled to the working medium flow path between the feed water tank and evaporator. Thereafter, at least during part of the later stages of starting, working medium is tapped off from the first tapping position, passed through the water separator associated with that tapping position (or the common water separator) and returned to the working medium flow path between the feed water tank and the evaporator. With this form of operation, the combustion chamber tube walls are cooled from the beginning of starting since water is circulated through the first superheater in addition to being circulated through the evaporator. In the later stages of starting, and as soon as sufficient steam is generated, the steam is superheated in the superheater downstream of the first superheater and is then used for preheating the turbine system.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

HO. 1 schematically illustrates a forced through flow steam generator of the invention with two liquid separators; and

FIG. 2 schematically illustrates a forced through-flow steam generator of the invention with one liquid separator.

Referring to FIG. 1, the steam generator has acombustion chamber 1 bounded by tubes which are welded to one another in gas-tight manner and which extend rectilinearly from alower inlet header 3 to anupper discharge header 7. As re gards the working medium flow, the combustion chamber tube system is subdivided into two sections in series of which the lower section forms anevaporator 2 and the upper section forms a first high-pressure superheater 5. At the upper end of theevaporator 2 the tubes are bent out of the combustion chamber wall and extend into a header 4. The header 4 is connected by aduct 91 which defines a first tapping position to acentrifugal water separator 20. At the lower end of the first superheater S the tubes are also bent outwardly and are connected to an inlet header 6 which is connected to the steam exit of theseparator 20. The bends of thetubes 2 and 5 are alternatively disposed at different levels to provide an intermeshed transfer from theevaporator 2 to thesuperheater 5. Also, aduct 23 which defines a second tapping position connects theheader 7 to acentrifugal water separator 25. The steam exit of thisseparator 25 is connected by aduct 30 to a second high-pressure superheater 10 of platen construction, the platens being arranged to lie above the flame created within the combustion chamber by the burner assembly.

The steam generator combustion chamber contains, in addition to thesuperheater 10, afinal superheater 12 in the working medium flow path which functions as a convection heat exchanger and which is connected via avalve 33 to ahighpressure stage 34 of a steam turbine plant. A by-pass line 31 with avalve 40 is provided between the final superheaterlZ and the high-pressure stage 34 in parallel with the high-pressure stage 34 for steam to be blown off during the initial stages of starting up. Also, areheater 11 is interposed in the working medium flow path between the high-pressure stage 34 and a low-pressure stage 38 of the steam turbine plant, the reheater being located in the steam generator between thesuperheaters 10 and 12, respectively. The low-pressure stage 38 is further connected by an outlet line to acondenser 39 which in turn communicates with thetank 8. The high-pressure stage 34 and the low-pressure stage 38 of the steam turbine plant are each by-passed through a duct containing a by-pass valve 41, 42, respectively, connected between the inlet and outlet lines of each stage. The duct containing the valve 41 together with the duct containing the valve 42 constitute a by-pass line with shut-off means leading from the line connecting the downstream high-pressure superheater 12 and theturbine 34, 38. Alternatively, where no reheater is employed, this by-pass function is performed by theduct 31 andvalve 40.

A feedwater supply tank 8 supplies the feed water to the steam generator via asupply duct 15 which contains afeed pump 16 as well as a preheater (as shown) which connects to an outlet of the high-pressure stage 34 of the steam turbine.

The supply duct connects with an economiser l3 positioned within the upper end of the combustion chamber to convey the feed water therethrough. Theeconomiser 13, in turn, is connected by aduct 18 to theinlet header 3 of theevaporator 2 to deliver the feed water to theinlet header 3.

Also, the water exit of theseparator 20 at the first tapping position is connected by aduct 36 which contains a circulatingpump 21 and anon-return valve 22 downstream of thepump 21 to theduct 18 upstream of theinlet header 3 in order to recycle the water in theseparator 20 into theevaporator 2. Theduct 36 also communicates with aduct 37 which contains aflow control valve 51 so that a proportion of the water in theduct 36 can be drawn off for purification purposes. In order to control the amount of water drawn off, a level regulator 50 is connected with theflow control valve 51 and theseparator 20 to open and close theflow control valve 51 in response to the sensed water level in theseparator 20. The lever regulator 50 i is also connected to thefeed pump 16 in thesupply duct 15 to control the flow of water fed into theeconomiser 13.

Similarly, the water exit of the separator at the second tapping position is connected by aduct 27 which contains aflow control valve 28 to thesupply duct 15 upstream of thepump 16. Theflow control valve 28 is controlled by the water level in theseparator 25 via a level regulator N connected into theseparator 25. Also, a duct 9 which contains aflow control valve 26 is connected between theduct 23 leading to theseparator 25 and the duct leading from theseparator 25 in order to provide a by-pass for the working medium leaving thedischarge header 7.

In operation. when the steam generator is first started up. thevalve 26 in the duct 9 by-passing theseparator 25 is closed and the by-pass valve or the valves 41, 42 are opened. Next. thefeed pump 16 is actuated to supply feed water through theeconomiser 13 to theevaporator 2 andsuperheater 5 via theseparator 20 and header 6. Also, the circulatingpump 21 is actuated to circulate feed water through theevaporator 2,separator 20 andduct 36. As theseparator 20 is thus flooded, the level regulator is rendered ineffective. The feed water then passes from thedischarge header 7 through theduct 23 into theseparator 25. Excess water is returned from theseparator 25 via theduct 27 to theduct 15 upstream of the feed pump.

The fire is then ignited whereupon saturated steam is gradually generated in thesuperheater 5. This steam is separated in theseparator 25 and, in the early stages, passes via thesuperheaters 10 and 12 and the by-pass line 31 to atmosphere or a degasser (not shown) or via the valve 41, thereheater 11 and the valve 42 into thecondenser 39. ln the later stages of the starting up procedure, the by-pass valve 40 or the valves 41. 42 are closed and the steam is employed for preheating the high-pressure and low-pressure stages 34. 38. respectively. of the steam turbine. Thereheater 11 is either brought into the flow or remains therein at this time. As the firing effect increases, theseparator 25 and thereafter thesuperheater 5 are run dry. When this state is achieved the control ofthe feedpump 16 is transferred to the level regulator 50 on theseparator 20 at the first tapping position. The regulator 50 then maintains the level in theseparator 20 constant while the circulatingpump 21 remains in operation up to approxiamtely 50 percent of full load to superimpose a recirculation of the working medium on the forced-flow of the working medium. When the load increases beyond 50 percent, the steam generator will then function in the manner of a pure once-through forced-flow system.

During normal operation of the steam generator, thevalves 28. 40. 41 and 42 are closed and therecirculating pump 21 is deactivated. The feed water then passes from thetank 8 through the economiser l3 and theduct 18 into theevaporator 2 and from there via the header 4 into theseparator 20. The level regulator 50 regulates the rotational speed of thefeed pump 16 in dependence on the water level and. by adjusting thevalve 51. regulates the amount of working medium discharged for purification through theduct 37. This discharge working medium is returned to the working medium circuit through a desalting system (not shown). The steam separated in theseparator 20 flows through thefirst superheater 5 to theheader 7 from which one part of the steam is passed through thedry separator 25 to theduct 30 and the other part of the steam is passed through the duct 9 to theduct 30. The steam in theduct 30 then passes through theplaten superheater 10 to thefinal superheater 12. Next, the steam flows through the high-pressure stage 34 to do work and, after being reheated in thereheater 11, passes through thelowpressure stage 38 to thecondenser 39. After being condensed, the condensate in thecondenser 39 is returned to thetank 8.

Referring to FIG. 2, wherein like numerals have been used to indicate like parts as above, the steam generator is provided with only a single water separator which is connected to the two tapping positions.

The upper section of thecombustion chamber 1 is formed from tubes which extend rectilinearly upwardly from below and are welded together in a gas-tight manner to form a first superheater-5. The lower section which forms anevaporator 2, however, is formed from tubes in which, in two opposite walls the tubes extend horizontally while in the remaining two walls the tubes are slightly inclined to the horizontal to form quasi-helical windings. These tubes are also welded to each other in gas-tight manner and the lower section is welded to the upper section. As above, the tubes of the evaporator 2 'are bent outwardly where the lower section of thecombustion chamber 1 meets the upper section. However, the tubes of theevaporator 2 return into the interior of thecombustion chamber 1 within aplane 75 above the inlet header 6 of thesuperheater 5 tubes so as to form supportingtubes 76 for theheat exchangers 10'. ll. 12, 13 which are heated by convection.

These supportingtubes 76 lead to a header 77 outside thecombustion chamber 1. The heater 77 is connected by a duct 78 which defines a first tapping position and contains a valve 79 to acentrifugal water separator 63. The steam exit of theseparator 63 communicates with the inlet header 6 via aduct 82 which is connected to the steam exit and leads to anotherduct 100 connected to the inlet header 6 intermediately of a pair ofvalves 83, 95 therein.

Thedischarge header 7 communicates with theseparator 63 via a duct which defines a second tapping position and is connected to the duct 78 leading to theseparator 63 at a point downstream of the valve 79. Theduct 90 also contains avalve 94 for controlling flow of the working medium therethrough. Thus, both tapping positions communicate in common with theseparator 63.

Similarly, in view of the juncture of theducts 78, 90 of the two headers 77. 7, the steam exit of theseparator 63 is provided with two exit paths. That is, theduct 100 is connected downstream of thevalve 95 therein to the second high-pressure superheater 10' so that the steam in theduct 82 can flow to either the header 6 of thefirst superheater 5 or the second superheater 10'. The discharge end of the second superheater 10' is connected to thefinal superheater 12 which in turn is connected to thesteam turbine 34, 38, as above.

Theseparator 63 may be by-passed by aduct 101 which contains avalve 94 and connects theduct 90 upstream of thevalve 94 to the duct between thevalve 95 and thesecond superheater 10. A similar by-pass facility is provided by aduct 102 which contains a valve 96 and connects the duct 78 upstream of the valve 79 to theduct 100 downstream of thevalve 83.

The water exit of theseparator 63 is connected by aduct 36 containing the circulatingpump 21 and thenon-return valve 22 to aheader 60 along one combustion chamber wall from which thedowncomers 65 extend to supply working medium to thelower header 3 oftheevaporator 2. Thedowncomers 65 may also perform supporting functions where appropriate. Theheader 60 is also supplied with feed water from theeconomiser 13 through theduct 18.Headers 61 and 62 corresponding to theheader 60 are provided for the remaining combustion chamber walls anddowncomer tubes 65 extend therefrom into theheaders 3 of theevaporator 2. 1n the interests of clarity, thedowncomer tubes 65 are shown at a greater distance from the combustion chamber wall than is in fact the case.

In normal operation, thevalves 94, 95, and 96 are closed and thevalves 79, 83 and 93 are opened, feed water is passed from thetank 8 by means of thepump 16 through theeconomiser 13, theduct 18, theheaders 60, 61 and 62 and thedowncomer 65 into theevaporator 2. After flowing through the evaporator, the working medium passes via the supportingtubes 76 into the header 77 and from there via the duct 78 into the separator 63 (see the solid triangular arrows). The separated steam passes via theducts 82, thevalve 83 and the header 6 into thefirst superheater 5 and from there via theheader 7 and theducts 90 and 101 into the second superheater 10'. After flowing through thissuperheater 10, the steam is finally superheated in thesuperheater 12 whereupon the steam flows through the high-pressure turbine stage 34 to perform work and, after being reheated in thereheater 11, passes through the low-pressure turbine stage 38. The condensate from thecondenser 39 is returned to thefeed water tank 8.

For the starting up procedure, thevalves 79, 83 and 93 are first closed and thevalves 94, 95 and 96 are opened so that thewater separator 63 is connected to the second tapping point. The working medium is conveyed by means of thefeed pump 16 through the economiser l3 and is subsequently supplied through theducts 18, theheaders 60, 61 and 62 and the supportingtubes 65 to theheaders 3 from which the working medium passes through the supportingtubes 76, is collected in the header 77 and passes via theducts 78 and 102 into the header 6 of thefirst superheater 5 thereby by-passing the separator 63 (see the open triangular arrows). After passing through thesuperheater 5 the working medium is collected in theheader 7 and passes via theduct 90 and thevalve 94 into theseparator 63. From theseparator 63 the separated water is returned by means of the circulatingpump 21 to theheader 60. Where appropriate, excess water can be discharged via thepurification valve 51, as above. Steam which is separated in theseparator 63 passes via theducts 82 and 100 and thevalve 95 into the second superheater 10' and from there via thefinal superheater 12 to the lay-pass line 31 andvalve 40 or to thebypass line 31 and the by-pass line 41, thereheater 11 and the by-pass valve 42 to thecondenser 39. In the later stages of the starting procedure, that is, with increasing temperature rise of the working medium, the water level in theseparator 63 will dropto its normal level whereupon the valve 79 is slightly opened. As soon as slightly superheated steam is present in theduct 90, the system is changed over, that is, the valve 79 is opened still further and thevalves 83 and 93 are also opened while thevalves 94, 95 and 96 are closed. From this moment onwards the separator will be connected to the first tapping point and only vaporized working medium will flow through thesuperheater 5.

lclaim:

1. A forced through-flow steam generator comprising:

a feed water supply means;

a combustion chamber of gas-tight tubular walls, said walls having a lower section forming an evaporator connected to said supply means to receive feed water therefrom and an upper section forming a first superheater;

at least one other superheater disposed in said combustion chamber downstream of said first superheater;

a first tapping means connected between said tubes of said evaporator and said first superheater for receiving working medium passing from said evaporator to said first superheater;

a second tapping means connected to said first superheater at the exit thereof for receiving working medium passed from said first superheater;

a first separator connected in said first tapping means for separating the received working medium into steam and water;

a second separator connected to said second tapping means for separating the received working medium into steam and water;

first duct means connected to said first separator for returning separated water to the feed water flow path between said supply means and said evaporator and for supplying working medium to said first superheater; and

second duct means connected to said second separator for returning separated water to the feed water flow path between said supply means and said evaporator and for supplying separated steam to said other superheater.

2. A forced through-flow steam generator as set forth inclaim 1 further comprising a steam turbine connected downstream of said other superheater, and by-pass means connected in parallel with said steam turbine to said other superheater for selectively by-passing the flow of working medium from said other superheater past said steam turbine.

3. A forced through-flow steam generator as set forth inclaim 2 further comprising a reheater disposed in said combustion chamber, said reheater being connected in the working medium flow path between a high-pressure stage and lowpressure stage of said steam turbine.

4. A forced through-flow steam generator as set forth inclaim 1 wherein said first duct means includes a circulating pump and a non-return valve downstream of said pump for passing the separated water into said feed water flow path between said supply means and said evaporator.

5. A forced through-flow steam generator as set forth inclaim 1 wherein said second duct means includes a flow control valve for controlling the flow of separated water returned to said feed water flow path and means responsive to the level of water in said second separator for controlling said flow control valve.

6. A forced through-flow steam generator comprising:

a feed water supply means;

a combustion chamber of gas-tight tubular walls, said walls having a lower section forming an evaporator connected to said supply means to receive feed water therefrom and an upper section forming a first superheater;

at least one other superheater disposed in said combustion chamber downstream of said first superheater;

a first tapping means connected between said tubes of said evaporator and said first superheater for receiving working medium passing from said evaporator to said first superheater;

a second tapping means connected to said first superheater at the exit thereof for receiving working medium passed from said first superheater;

a separator for separating the received working medium into steam and water;

duct means connected to said separator for returning separated water to the feed water flow path between said supply means and said evaporator;

duct means connectable to said separator for supplying the separated steam to said other superheater when said separator receives working medium from said second tapping means; and

duct means connectable to said separator for supplying working medium to said first superheater when said separator receives working medium from said first tapping means.

7. A forced through-flow steam generator as set forth in claim 6 wherein said first tapping means includes a first header connected to the tubes of said evaporator, a first duct communicating said first header with said separator, a first valve in said first duct for controlling flow of working medium therethrough, and a first by-pass means upstream of said first valve connected in parallel with said separator between said first duct and said first superheater, and said second tapping means includes a second header connected to the tubes of said first superheater, a second duet communicating said second header with said first duct downstream of said first valve, a second valve in said second duct for controlling flow of working medium therethrough, and a second by-pass means upstream of said second valve connected between said second duct and said other superheater.

8. A forced through-flow steam generator as set forth inclaim 7 which further comprises a by-pass means connected between said first tapping means downstream of said separator and said second by-pass means for conveying steam from said separator to said second superheater.

9. A forced through-flow steam generator as set forth in claim 6 which further comprises a by-pass means in said first tapping means in parallel with said separator for by-passing the working medium around said separator and a by-pass means connected between said second tapping means and a second superheater disposed in said combustion chamber for passing steam into said second superheater.

10. The method of starting up a forced through-flow steam generator comprising the steps of:

passing a flow of working medium through an evaporator and at least two superheaters within a combustion chamber of the steam generator;

passing the flow of working medium through a separator interconnected in the working medium flow between the first of said superheaters and the second of said superheaters during the initial phase of operation of the steam generator, and

tapping working medium from the flow of working medium between said first and second superheaters and returning the tapped working medium to the evaporator during said initial phase and tapping working medium from the flow of working medium between said evaporator and said first superheater and returning the tapped working medium to the evaporator at least at intervals during the remaining phases of starting up operation of the steam generator

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