Washing And Purging Apparatus For Liquid Seals

Abstract

A washing and purging system for a liquid seal in the flue gas bypass duct of a steam generating facility having an air pollution control system. A series of nozzles positioned within the seal provide a continuous turnover of liquid and are also periodically used to flush accumulated particulate matter from the seal. Disposal of excess liquid from the seal is accomplished through the use of an overflow weir which also serves to skim off surface contaminant particles before they can settle to the bottom of the seal.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to liquid seals and more particularly to a washing and purging arrangement for a liquid seal.

2. Description of the Prior Art

One major source of air pollution is the particulate matter (fly ash) and obnoxious gaseous contaminants, such as SO.sub.2, which are emitted in the flue gas of coal-fired utility boilers used in electric power generating facilities. One approach to removing these contaminants from the gases has been through the use of a gas cleaner having a wet scrubber.

Typically in such a system dust and fume laden gases exit the boiler through an air preheater and continue through ducts to the scrubber where the washing and cleaning of the gas is accomplished. The clean gas then exits from the scrubber and continues through additional ductwork and a booster fan to the stack where it is emitted to the atmosphere.

Many coal-fired steam generating furnaces have the ability to burn fuels other than coal such as low sulfur oil or gas. Such auxiliary fuels do not produce the high level of contaminants and particulate matter which result from the burning of coal and thus permit the boiler units to be operated without the scrubber units on the line. This is accomplished by providing a bypass duct around the scrubber which provides a path directly to the booster fan and the stack.

Because of the very strict governmental air pollution requirements regulating permissible amounts of SO.sub.2 and particular matter emission, water seals are often provided in the bypass ducts of such systems to assure that contaminated gas does not leak through the bypass to the atmosphere when the scrubber is in operation.

One type of liquid seal commonly used in this application is similar to the U-type trap frequently used in sink drains. With such an arrangement a number of problems have been encountered as a result of interactions between dead pocket gases and water which occurs at the inlet plenum water surface of the seal. One such problem is corrosion of the metal walls of the seal caused by an intolerable acid condition of the seal water. This acid condition comes about as a result of corrosive materials in the gases condensing from the gas on the water surface and subsequently being absorbed into the water. Another problem encountered is that of large accumulations of particulate contaminants in the bottom of the seal. This comes about due to particles in the gas contacting and adhering to the water surface and eventually settling to the bottom of the seal. In order to remove such deposits it has been necessary to periodically drain the liquid from the seal and remove them by hand.

SUMMARY OF THE INVENTION

The herein provided invention involves a washing and purging system for a water seal located in a hot gas bypass duct. A series of nozzles is located in the bottom of the seal. Overflow weirs, communicating with overflow discharge means, are provided at the desired liquid level of the seal. During normal operation a constant supply of relatively alkaline water is introduced into the seal by less than the full complement of nozzles. This water effects a continuous purge of the seal water and provides a three-fold advantage: (1) the pH of the seal water is kept at a non-corrosive level, (2) potential loss of the water seal due to water evaporation or level control malfunction is eliminated, and (3) excess water passing from the seal over the overflow weir takes with it surface particulate contaminants before they can settle.

A periodic cleaning operation is performed to remove accumulated particulate matter from the bottom of the seal. This is achieved by activating the entire complement of nozzles and opening the drain connection provided in the bottom of the seal. The seal bottom is inclined with the drain connection at the lower end thereof and the nozzles are so arranged to effect a sluicing operation to guide the accumulated matter through the drains. A distinct advantage of this cleaning operation is that it may be carried out without requiring the water seal to be broken.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view, partly in section, of an air pollution control system having a wet scrubber bypass duct water seal which has the washing and purging features of this invention;

FIG. 2 is an enlarged view, partly in section, of the water seal of FIG. 1;

FIG. 3 is a sectional view taken on lines 3--3 of FIG. 2;

FIG. 4 is a sectional view taken on lines 4--4 of FIG. 2; and

FIG. 5 is a partial view, similar to FIG. 3, showing one end of a seal having an overflow collector in accordance with a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, there is illustrated a steam generating boiler 10 having a pollution control system including wet scrubber 12 for cleansing flue gases prior to passage to the atmosphere. A flue gas scrubber bypass 14 having a water seal 16 is provided to circumvent the scrubber when desired. Under normal circumstances, i.e., when the scrubber is in use, contaminated flue gases produced in the furnace 18 pass into an appropriate duct 20 through an air preheater 22 and into an inlet duct 24 to the wet scrubber 12. The inlet duct 24 has a damper 26 arrangement located therein for selectively opening and closing the duct to the passage of flue gases. Gases passing into the wet scrubber 12 are cleansed of gaseous and particulate contaminants in a well-known manner not forming a part of this invention.

The cleansed flue gases pass from the scrubber 12 through a scrubber outlet duct 28. Located in the outlet duct 28 is a stack gas reheater 30 to minimize steam plume formation and a damper assembly 34 for selectively controlling the passage of gases therethrough. Communicating with the outlet duct 28 is an appropriate exit duct 36 which in turn communicates with the inlet of induced draft fan 38. The fan 38 creates a sufficient pressure differential to insure positive passage of the gases through the ductwork and scrubber into the stack 40 for passage to the atmosphere.

The flue gas scrubber bypass 14 includes a first conduit 42 communicating with the scrubber inlet duct 24 upstream of the scrubber inlet damper arrangement 26. A second conduit 44 communicates at one end with the first conduit 42 and at the other end with the scrubber outlet duct 28 at a location downstream of the outlet damper 34. The passage of gases from the first conduit 42 to the second conduit 44 is selectively controlled by the level of water in the water seal 16.

As best shown in FIG. 2 the water seal 16 includes a first plenum chamber 46 and a second plenum chamber 48 communicating with the first conduit 42 and the second conduit 44, respectively. The plenum chambers 46, 48 are fabricated from appropriate ducting material and have a common bottom plate 50 which is angularly disposed from the horizontal. The plenum chambers 46, 48 are separated by a vertical baffle plate 52 originating at the junction of the upper walls of the first and second bypass conduits 42, 44 and terminating at a level spaced from the inclined bottom 50 to define a horizontal passageway interconnecting the two chambers. Located in the lower end of the ducting forming the front of the seal are two drain connections 54, 56 which are selectively operable to opened and closed position by means of conventional valve arrangements 58, 60.

As best shown in FIGS. 3 and 4, underlying the inclined seal bottom 50 is a network of water pipes 67 communicating through valve arrangements, which will be subsequently further described, to a water supply 64 and adapted to distribute water to a plurality of water nozzles, 66a-f, 68a-f, extending through the bottom plate and which are directed substantially parallel to the bottom and toward the drain connections 54, 56 adjacent the lower end of the bottom. In the preferred embodiment, best illustrated in FIG. 4, a separate series of nozzles is associated with each drain connection. The nozzles are so arranged that the number of nozzles per unit area of the bottom plate becomes increasingly greater as the lower end of the plate is approached. Further in accordance with the invention a portion of the nozzles in each series are disposed in pairs 66c-f, 68c-f laterally offset on opposite sides of and angularly directed toward their associated drain connections.

Referring back to FIG. 3 the valve arrangement interconnecting the water supply 64 and the nozzles 66, 68 comprises a main shutoff valve 70 and a flush valve 72. The shutoff valve 70 directly controls the water supply to one pair of replenishing nozzles 66c, 68c in each series and to the flush valve 72 which in turn controls the supply to the piping 67 communicating with the remainder of the nozzles.

There are located on the interior walls at opposite ends of the first plenum chamber 46 overflow collectors 74 having open upper ends defining generally horizontal upper edges 76. These collectors are positioned so that their upper edges 76 are substantially coincident with the desired normal water level of the seal when it is filled to provide a gas tight seal. Openings 78 are provided in the seal walls for communicating the interior of each collector 74, at the lower end thereof, to discharge pipes 80 extending to the exterior of the seal. The discharge pipes 80 in turn communicate with appropriate conduits 82 which terminate, at a point below the level of the water in the seal, in settling tank or pond 84 of a size capable of handling the overflow liquid. The pond 84 serves as a second liquid seal to insure that atmospheric air is not drawn into plenum 46 through the discharge conduits 82. Such an occurrence would otherwise be probable because the pressure within plenum 46 is less than atmospheric, due to the sucking action of the fan 38.

When the furnace 18 of the steam generating boiler 10 shown in FIG. 1 is burning a low sulfur auxiliary fuel such as gas it is not necessary to pass the products of combustion gases through the wet scrubber 12. Under such circumstances there is no water in the water seal and the scrubber inlet damper 26 and the scrubber outlet damper 34 are placed in a closed position. All flue gases thus pass through the first bypass conduit 42 into the first plenum 46 under the baffle 52 to the second plenum 48 and through the second bypass conduit 44 to the exit duct 36 and ultimately from the stack 40 to the atmosphere.

Under normal circumstances, however, the furnace burns a fuel such as coal which yields a highly contaminated product of combustion gases which must be passed through the scrubber 12 before being released to the atmosphere. During such operation the scrubber dampers 26, 34 are opened and the water seal 16 in the bypass 14 is filled to insure that no contaminated gases are permitted to bypass the scrubber 12. Filling of the water seal is accomplished by opening both the main supply valve 70 and the flush valve 72, water is thus caused to flow into the seal through the full complement of nozzles 66a-f, 68a-f. When the lower edge of the baffle plate 52 is completely immersed in water the seal is effective to prevent the passage of flue gases therethrough.

When the water level in the seal reaches the horizontal upper edge 76 of the overflow collectors the flush valve 72 is moved to its closed position. Only the replenishing nozzles 66c, 68c, located generally midway along the length of the inclined bottom, are now feeding water into the seal. These nozzles 66c, 68c are supplied with water continuously whenever the seal is in use. The water fed into the seal by these nozzles serves two primary purposes. The first is to provide a replenishing of the water in the seal at a rate sufficient to avoid the creation of corrosive conditions in the seal. This is accomplished by making the supply water of a sufficiently alkaline nature to maintain the body of water in the seal in a substantially neutral condition. The second primary purpose of the supply of water is to maintain a continuous flow of water over the upper edges 76 of the overflow collectors 74. This constant overflow results in a skimming off of the surface water in the seal. There is contained in this surface water a high concentration of particulate matter from the gas in the inlet plenum of the seal which has contacted and adhered to the water surface but has not yet settled to the bottom of the seal. By skimming off this surface water this particulate matter is thereby removed.

Despite the skimming off of a large portion of the particulate matter adhering to the water surface, with time, an undesirable amount of such particles settle to the bottom of the seal. These accumulated particles are periodically removed from the seal, without requiring the seal to be broken, by opening the flush valve 72 and opening the drain connections 54, 56 adjacent the lower end of the inclined bottom 50. The positions of the nozzles 66a-f, 68a-f in the bottom, all of which are now injecting water along the bottom, serve to encourage the passage of the accumulated particles from the seal through the drain connections 54, 56. The water supply 64 and nozzles 66, 68 and the drain connections 54, 56 are appropriately sized so that the normal level of liquid in the seal is maintained during this cleaning or sluicing operation. When it is determined that the seal is sufficiently clean, the drain connections and the flush valve are closed and normal seal operation is resumed.

Referring to FIG. 5, an additional embodiment of an overflow collector is shown. A horizontal slot 86 is provided in the seal wall, the lower edge thereof being substantially coincident with the desired normal liquid level. A fully enclosed overflow collector 88 is attached to the outside wall of the seal to completely surround the horizontal slot 86. The collector 88 has a discharge pipe 90, similar to that of the preferred embodiment, attached to an opening in the lower end thereof for carrying off water which has passed into the collector through the slot.

While the invention has been described in conjunction with specific embodiments thereof, it is to be understood that such showings are merely illustrative and that changes may be made without departing from the spirit and scope of the invention as claimed.

Claims

What is claimed is:

1. In combination with a liquid seal for preventing the flow of a contaminated hot gas through a conduit, the seal having a first duct means communicating with a gas supply conduit and defining a first plenum chamber, a second duct means communicating with a gas exhaust conduit and defining a second plenum chamber, said first and second duct means cooperating to define a generally horizontal passageway interconnecting said first and second plenum chambers at the lower ends thereof and means for admitting a liquid into said seal to a level completely immersing said passageway, the improvement which comprises:

a. an inclined generally rectangular plate forming the bottom of said interconnecting passageway;

b. drain means disposed in the lower portion of said duct means adjacent the lower end of said inclined bottom plate, said drain being selectively operable to an open or closed position to drain liquid from or retain liquid in said seal respectively;

c. at least one series of nozzles positioned adjacent the upper side of said inclined plate, a portion of the nozzles in each of said series being angularly directed toward the opening in said drain means whereby liquid ejected from said nozzles is directed toward said drain opening;

d. means for selectively supplying a liquid to said nozzles at a flow rate sufficient to maintain the normal liquid level in said seal when said drain means is in an open position; whereby when said liquid seal is filled to its normal level, said drain means is actuated to an open position, and said nozzle supply means is operating, a sluicing operation is performed on the inclined bottom plate of said liquid seal without requiring the gas tight seal to be broken.

2. The apparatus of claim 1 wherein each drain means has at least one series of nozzles associated exclusively therewith.

3. The apparatus of claim 1 wherein the number of nozzles per unit area of inclined plate becomes increasingly greater as the lower end of said inclined plate is approached.

4. The apparatus of claim 1 including the following additional elements:

a. an overflow collector attached to said first duct means and having an overflow discharge opening in the lower end thereof;

b. liquid discharge means communicating with said overflow discharge opening to carry overflow liquid to an appropriate disposal point exterior of said seal;

c. liquid overflow means defining a generally horizontal upper edge for communicating the interior of said seal adjacent the desired liquid level to the interior of said overflow collector; and

d. means for continuously replenishing the liquid in the seal when the seal is provided with a gas sealing body of liquid.

5. The apparatus of claim 4 wherein said means for continuously replenishing the liquid in the seal comprises at least one of said nozzles, said nozzle being adapted to selectively provide continuous liquid flow to said seal.