US3857344A

US3857344A - Furnace seal

Info

Publication number: US3857344A
Application number: US00404537A
Authority: US
Inventors: E Coulter
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock International Ltd; Babcock and Wilcox Co
Priority date: 1973-10-09
Filing date: 1973-10-09
Publication date: 1974-12-31
Anticipated expiration: 1991-12-31
Also published as: CA1008733A; JPS5751014B2; JPS5077940A

Abstract

An improved furnace seal comprising an endless skirt and a shielding means extending into the liquid within a trough and including means for supplying a gas to the shielding means.

Description

United States Patent [191 Coulter' Dec. 31, 1974 2,720,177 10/1955 Barber 110/165 3,228,362 1/1966 Jarvis, Jr... 110/1'65 3,580,226 5/1971 Blackburn, .lr., et a1. 110/171 Primary ExamineF-Kenneth W. Sprague Attorney, Agent, or Firm-.1. M. Maguire, Esq.,

R. J. Edwards, Esq.

[ ABSTRACT An improved furnace seal comprising an endless skirt and a shielding means extending into the liquid within a trough and including means for supplying a gas to the shielding means.-

10 Claims, 3 Drawing Figures 7s A 66 i 69 1 72 so 74 fl PATENTED EH13 1 I974 sum 1 or 2 FIG.I

PAIENTED SE83 I 1974 SHEET 2 OF 2 FURNACE SEAL BACKGROUND OF THE INVENTION The present invention relates to vapor generating units and more particularly to an improved furnace seal arrangement. Large vapor generating units, such as those operated by public utilities, include furnaces for burning ash bearing fuels and are formed with floor openings which discharge the ash, in its solid or liquid state, into a hopper or slag tank located beneath the furnace. Vapor generating units are either suction or pressure fired, the former being of the type wherein the combustion gases contained in the furnace are operated at lower than atmospheric pressure while the latter define operation at higher than atmospheric pressure. A seal is provided between the ash hopper and the furnace so as to prevent the leakage of air into a suction fired furnace or the leakage of combustion gases out of a pressure fired furnace.

The furnace of a large vapor generating unit is topsupported, that is to say the tubular boundary walls associated therewith have their upper ends connected to hangers which attach to structural steel to provide a fixed support. The ash hopper or slag tank is bottomsupported, that is to say it rests on a foundation at ground level or is supported thereon by means of stanchions. As the unit is brought up to operating tempera ture the furnace wall tubes undergo thermal expansion in a downward direction from the point of restraint at the hangers; conversely, thermal expansion, if any, experienced by components of the ash hopper or slag tank will be in an upward direction from the point of restraint at the base. Thus it can be readily seen that any furnace to ash'hopper seal arrangement must be capable of accommodating a difference in magnitude and direction of thermal expansion and contraction between the furnace and the ash hopper. A seal arrangement of the type which has gained wide commercial acceptance was described in US. Pat. No. 2,275,652-issued on Mar. 10, 1942 to Harold R. Purcel. The invention therein disclosed includes a'trough of water on the outside of and near the top of the ash hopper and a water lever being on the gas side of the seal skirt. With this arrangement, the metal exposed to the flue gas is continuously being cooled by the outside water level thereby providing a condensing surface that combines with certain sulphur compounds in the combustion gases to form acidic solutions which corrode the seal skirts. Since the fuel contributes the sulphur, a change of fuel and/or method of firing may change the sulphur compound concentrations in the flue gas and vary the dew point temperatures as well as the acid strength. Under these conditions no commercially used metal will successfully resist the acid concentration and temperature conditions imposed on the seal skirt.

SUMMARY OF THE INVENTION The present invention is directed at an improved furnace seal arrangement wherein the problem of seal skirt metal wastage has been substantially eliminated.

Accordingly, there is provided an improved arrangement for sealing the space between the furnace bottom outlet and the inlet to the ash hopper associated therewith. The improved'seal comprises a liquid containing trough extending around the outer periphery of the ash hopper inlet and an endless skirt depending from the furnace bottom and extending into the liquid in spaced relation to said trough and cooperating therewith to preclude leakage 'therethrough. The distal end of the skirt includes an inwardly projecting ledge which supports a shield formed of packed material and extending along the inner periphery of the skirt and covering at least a portion of the height thereof. The shield inmetal sheet having its upper end attached to the furnace and a lower portion submerged in the water contained within the trough.

A serious problem encounteredwith the above furnace seal arrangement is the limited use life of the metal skirt associated therewith..The failure of the skirt 7 is due to metal wastage which results from the formation of acidic condensate on 'the inside surface of the metal sheet. The. corrosive condensate may contain both sulpuric and sulphurous acids over a wide range of concentrations. Sulphuric acid occurs when some of the sulphur dioxide in the flue gas is converted to sulphur trioxide, which, in turn, raises the dew point temperature at the metal sheet and condenses thereon to form. a high concentrationof sulphuric acid. Sulphurous acid occurs whensome of the sulphur dioxide in the flue gas diffuses in'the corrosion zone and is absorbed by the moisture film which covers the metal sheet.

The problem of metal wastage has been encountered with both suction and pressure fired units; however, it has been most prevalent of the pressure fired units. In the single seal skirt arrangement of the present state of the art as applied toa pressure fired vapor generator, there are two water levelsin the trough with the lower cludes a first and second layer, the first layer being disposed subjacent to the second layer and being comprisedof'a material, eg stainless steel wool which, when tightly packed, results in a relatively high porous construction. The second layer is comprised of a material, e.g., crushed rock which, when tightly packed, results in a relatively low porous construction. The shield fills the space between the'skirt and the inner side of the trough to a level which is above the highest water level in the trough. A gas, preferably air, in introduced through the skirt and into the first layernear the top thereof and is supplied in sufficient quantity so that its velocity through the second layer will prevent the pene-' trationof furnace gas into the shield thereby precluding the gas and its sulphurous constituents from contacting the cooled area of the skirt andforming corrosive acid solutions thereon.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS The furnace to ash hopper seal of the present invention is illustrated with respect to a vapor generator having a pressure fired furnace. However, it is to be understood that this furnace to ash hopper seal is equally well adapted for use with'a vapor generator having a suction fired furnace.

.unit including an upright furnace chamber 12 comprising fluid heating tubes 14 arranged to form a gastight boundary. Furnace chamber 12 is of substantially rectangular horizontal cross section defined by a front wall 16, a rear wall 18 and side walls 20. The upper end of the furnace chamber 12 is divided into an uptake gas pass 24 and a down-flow or rear gas pass 26. The rear wall tubes are alternately arranged to form the front and rear boundaries of the downflow gas pass 26 and include an upper screen section 28 and a lower screen section 30. The furnace roof 32 is formed by inwardly inclined segments of the front and

rear walls

16 and 18 which thence project vertically upward for vapor-fluid discharge into a drum 34. The furnace floor 36 is formed by inwardly sloped segments of the front and real walls rear and 18 which thence form a furnace discharge opening 38 and bend outwardly to connect to respective supply headers 40 in fluid receiving communications there-with. Gas passes 24 and 26 will normally contain super reheater and other heating surfaces (not shown). The fuel firing equipment consists of independently operable fuel burners (not shown) extending horizontally along the center line of burner ports 42 which are located in the front and

rear walls

16 and 18 at the lower portion of furnace chamber 12. A hot air duct 44 (partially shown) connects to the windboxes 46 to supply combustion air to the burners from a forced draft fan (not shown). A hot gas duct 48 (partially shown) connects to the gas pass 26 at the screen 30 to receive the outflow there-from for passage to an air heater and stack (not shown).

The vapor generating unit-10 is top-supported by structural steel members including columns 50 and cross beams52. The supports for the unit is provided by a plurality of hangers 54 extending downwardly from the beams 52, some of which rigidly attach onto the furnace roof 32 while others connect to a cradle which supports the drum 34. The weight of the vapor generating unit .10 is transmitted through the hanger 54 and through the beams 52 to the upright support col; ,umns 50 which rest on the ground denoted as G. The tubes 14 have their fixed points with respect to thermal expansion at the hanger connections and will thus expand downwardly as the metal is heated to its operating temperature r The furnace discharge opening 38 communicates with an ash hopper 56, the latter being situated beneath the floor 36 for receiving ash, slag or scale being discharged fromthe furnace. The ash hopper 56 willnormally include a refractory lined metal casing 58 having an inwardly sloped floor 60 and a roof 62 formed with shoulder portions 64 and a neck 66 defining an inlet opening 68 which is aligned with the furnace opening 38 and is adequately spaced therefrom to accommodate the thermal expansion of the vapor generating unit 10. The ash hopper 56 is supported by a plurality of stanchions 70 which rest on the ground G. A discharge trough 71 extends along the apex of the floor 60 and provides the means for removing the combustion residue collected in the ash hopper 56. Water jets (not shown) are generally used to direct the residue toward the discharge trough 71. v

A sea] trough72rests on the ash hopper roof 62 and is fixedly attached thereto at the shoulder portions 64. The trough 72 is of generally rectangular cross-section and extends as an endless channel around the outer peleading edge of the bar and the oppositely adjacent side-- riphery of the ash hopper'neck portion 66. The trough 72 is partially filled with a liquid 74, preferably water, which is keptat a level capable of maintaining a seal throughout the operating furnace pressure range of the unit. The water is preferably kept in constant circulation through the provision of inlet and outlet nozzles coupled with a continuous supply of water (not shown). 1

An endless seal skirt 69 depends from the outwardly bent tube segments below the furnace floor 36 and extends at all times, during normal operation, into the water circulating within the seal trough 72 and cooperates therewith to provide a gastight seal around the space between the furnace and

ash hopper openings

38 and 68, respectively. Adequate spacing is provided between the skirt surfaces and the trough to accommodate movement due to thermal expansion. The distal end of the skirt 69 is provided with an inwardly projecting endless ledge 73 supporting a shield 75 which extends along the inner periphery of the skirt 69 and is' formed of two superposed porous layers of packed material. Pressurized heated air is introduced through the skirt and into the lower layer at the upper end portion thereof. The heated air is at a pressure higher than the furnace pressure and is taken from windbox 46 where temperatures average 600F and is conveyed through supply pipes 76 to respective manifold headers 77 for distribution to the skirt through branch pipes '78. The

supply pipes are equipped with flow control valves 80 to regulate the quantity of heated air being delivered to the skirt 69 for passage through the shield 75.

It is to be appreciated, however, more elaborate flow control means may be achieved with the addition of such known devices as flow meters, alarms, check valves, etc. Moreover, it will be understood that the supply of air may be any suitable source'other than that described, e.g., the source of air may be the outlet of the forced draft fan (not shown) in which case the air would not be preheated. r

Referring to FIG. 2, there is shown adetail view of the embodiment of the invention depicted in FIG. 1 and including a lower end portion of one of the tubes 14 associated with the front wall of the unit and a 'sup- I port plate 84 which extends throughout the width of the front wall and is fixedly connected to theundersid'e of the tubes 14. Depending from the support plate 84 and welded thereto are a plurality of elongated flat plates 79 which extend the entire width of the front wall and are weldably joined with like corresponding furnace through the space between the furnace and ash hopper openings. The trough 72 is supportedly connected to the shoulder portion 64 of the ash hopperroof.

An elongated flat bar 81 is connected along the bottom edge of each plate 79and extends over the entire width thereof. The bar 81 lies along a horizontal plane and projects inwardly with respect to the periphery of the skirt 69; The projecting dimension of the bar 81 is such that a predetermined space is allowed between the of the trough 72 to accommodate thermal expansion of the unit. It should be understood that the bar 81 could be replaced by an arrangement wherein the plate 79 would be of sufficient height to accommodate the bending of the lower end portion thereof into a horizontally projecting segment having the same dimensions as the bar 81. The bars or segments 81 cooperate with one another and with the respective plates 79 to form a ledge 73 projecting inwardly from the seal skirt 69. The skirt 69, the ledge 73 and the inner side of the trough 72 define the boundaries of an endless channel 83 which houses a shield 75 formed of packed material. The shield 75 is formed of a first layer 86 and a second layer 88. The first layer 86 is disposed subjacent to the second layer 88 and is comprised of stainless steel wool which, when tightly packed, results in a construction of relatively high porosity. The second layer is comprised of crushed rock which, when tightly packed, results in a construction of relatively low porosity. The shield fills the channel 83 to a level which is above the highest water level attained in the trough 72 under normal operating conditions. W.L. 1 represents the water level at the side of the trough which opens to atmosphere and W.L. 2 represents the water level which opens to the space between the furnace and ash hopper openings. The water levels shown are those for a pressurized unit.

lt should be understood that there are other materials, i.e, gravel, stainless steel knitted wire mesh, etc., which are suitable for packing in the form of a porous construction and may be used to make the shield of the present invention.

Pressurized heated air is conveyed through supply pipe 76 into the manifold header 77 for distribution among a plurality of branch pipes 78, only one being shown, and which penetrate through the plate 79 to discharge the heated air into the upper end portion of the first layer 86 of the shield 75, As the air flows upwardly through the less porous second layer 88 it prevents the corrosive, sulphur bearing gas from penetrating the shield.

Referring to FIG. 3, there are shown the tubes 14 being weldably joined to one another by gastight web plates 15. The support plate 84 is fixedly connected to the underside of the tubes 14 and scalloped plate members 85 seal the spaces between the tubes 14, webs 15 and support plate 84. Depending from the support plate 84 and welded thereto is a skirt plate 79 which forms part ofthe seal skirt 69 and is perforated to admit the respective discharge ends of the branch pipes 78. A horizontally oriented bar 81 projects from the bottom of the skirt plate 79 to form the ledge which supports the shield 75, the latter including a lower layer 86 comprised of steel wool and upper layer 88 comprised of crushed rock. The trough 72 is supportedly connected to the shoulder portion 64 of the ash hopper roof.

In the operation of the apparatus of the present invention, heated air is obtained from the windbox and is supplied to the lower layer whose high porosity construction permits lateral distribution before passage to the upper level. A sufficient quantity of air is supplied to maintain its velocity at approximately 3 feet per second through the upper level, this having been determined as adequate to prevent furnace gases from infiltrating the shield. The low porosity construction of the upper level also acts to limit the amount of air required for the apparatus.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. The combination of a furnace chamber having a bottom outlet, a hopper disposed thereunder and including a top opening spaced from said outlet for receiving discharges therefrom, means for passing combustion gases through said chamber, an improved seal for preventing leakage from between the chamber and hopper and comprising a liquid containing trough extending around the outer periphery of said opening, plate means depending from the furnace bottom and forming an endless skirt extending into the liquid within said trough, shielding means disposed within said trough and comprised of packed material forming a porous construction extending along the inner periphery of the skirt in contiguity therewith, and means introducing a gas into said shielding means to prevent combustion gas infiltration therein.

2. The combination according to claim 1 wherein said gas is preheated.

3. The combination according to claim 1 wherein said gas is air.

4. The combination according to claim 1 wherein said shielding means is formed of a first and second porous layer of. packed material, and first layer being disposed subjacent the second layer and being comprised of a material differentthan that of said second layer.

5. The combination according to claim 4 wherein the porosity of the first layer is of a higher degree than that of said second layer.

6. The combination according to claim 4 wherein the gas enters the shielding means at said first layer.

7. The combination according to claim 1 wherein said shielding means is supported from an endless ledge extending inwardly from the lower end of the skirt.

8. A method of sealing combustion gases including a liquid containing trough, and endless skirt and a shielding means extending into the'trough, said shielding means being comprised of packed material forming a porous construction extending along the inner periphery of the skirt in contiguity therewith and comprising the steps of:

maintaining the liquid within said trough at a level wherein the skirt and shielding means have their-

Claims

1. The combination of a furnace chamber having a bottom outlet, a hopper disposed thereunder and including a top opening spaced from said outlet for receiving discharges therefrom, means for passing combustion gases through said chamber, an improved seal for preventing leakage from between the chamber and hopper and comprising a liquid containing trough extending around the outer periphery of said opening, plate means depending from the furnace bottom and forming an endless skirt extending into the liquid within said trough, shielding means disposed within said trough and comprised of packed material forming a porous construction extending along the inner periphery of the skirt in contiguity therewith, and means introducing a gas into said shielding means to prevent combustion gas infiltration therein.

2. The combination according to claim 1 wherein said gas is preheated.

3. The combination according to claim 1 wherein said gas is air.

4. The combination according to claim 1 wherein said shielding means is formed of a first and second porous layer of packed material, and first layer being disposed subjacent the second layer and being comprised of a material different than that of said second layer.

8. A method of sealing combustion gases including a liquid containing trough, and endless skirt and a shielding means extending into the trough, said shielding means being comprised of packed material forming a porous construction extending aLong the inner periphery of the skirt in contiguity therewith and comprising the steps of: maintaining the liquid within said trough at a level wherein the skirt and shielding means have their respective lower end portions immersed in the liquid at all times, and introducing a gas into said shielding means to prevent combustion gas infiltration therein.

9. The method according to claim 8 including the step of preheating the gas.

10. The combination according to claim 1 wherein the skirt and shielding means have their respective lower end portions immersed in the liquid at all times.