US3089469A - Apparatus for maintaing liquid level within a steam and water drum constant - Google Patents

Description

May 1963 A R. MUMFORD APPARATUS FOR MAINTAINING LIQUID LEVEL WITHIN A STEAM AND WATER DRUM CQNSTANT Filed April 27, 1961 Ma d- ATTORNEY United States Patent 3,089,469 APPARATUS FOR MAHNTAINING LIQUID LEVEL WITHIN A STEAM AND WATER DRUM CON- STANT Albert R. Mumford, Bogota, N.J., assignor to Combustion Engineering, Inc, Windsor, Conm, a corporation of Delaware Fiied Apr. 27, 1061, Ser. No. 105,936 1 Claim. (Cl. 122-491) This invention relates to apparatus for maintaining the liquid level within a steam and Water drum substantially constant, regardless of varying conditions.

In many boilers utilizing a convection bank of steam generating tubes in the gas outlet pass of a furnace, it is dilficult to maintain a constant water level within the steam and water drum that supplies these steam generating tubes as the load requirements of the boiler change. Prior to this invention, a load change of 20,000 lbs. of steam per hour in one particular instance was accompanied by a water level change within the steam and water drum of about 16 inches. This greatly varying water level within the drum is highly undesirable. If the water level rises too high within the drum, it may cause water to carry over into the superheater or turbine, depending on the type of boiler employed. A high Water level also reduces the efficiency of the steam and water separators normally employed within the drum. Too low a water level enhances the possibility of steam being generated in the downcomer tubes, which would reduce the natural circulation of fluid out of the drum through the downcomers and back into the drum through the riser tubes. With a reduced circula tion through the downeomers and risers, some tubes may become too hot and burn out, requiring shut down of the unit and replacement of such burner out tubes.

It is an object of this invention to enable a substantially constant liquid level Within the steam and water drum to be maintained at all times.

When the load requirements of a boiler increase, and more steam leaves the drum to flow either to the superheater or to the turbine, the amount of feedwater flowing from the economizer to the steam and water drum is also increased. The temperature of the feedwater being introduced into the steam and water drum is well below the saturation temperature within the boiler. When the subcooled feedwater is thus injected into the drum in large amounts, it will lower the over-all temperature therein, and will cause condensation of the steam, thus raising the water level.

In accordance with my invention, a substantially vertical baflie which is parallel to the longitudinal axis of the drum is placed within the drum, which baffle extends from the bottom of the drum to an upper point six to eight inches below the normal liquid level. Depending on conditions, it may in some instances be desirable to extend the baffle to an upper point approximating the normal liquid level. The battle separates the lower side of the drum where the downcomer tubes are connected thereto from the rest of the drum internal. Within this area confined or segregated by means of the baffle, the feedwater pipe extends. The openings in the feedwater pipe are positioned such that the incoming feedwater is directed towards the area of the drum just above the downcomer tube outlets. In this manner the greater portion of the comparatively subeooled feedwater is restricted to the vicinity of the downcorner outlets, and hence does not mix with the steam and Water within the drum so as to cause condensation and resultant increase in the liquid level.

A second advantage of such a construction, and probably the more important advantage, is that by restricting the comparatively cooler feedwater within the area just above the downcomer tubes, the water that flows through .these tubes will be at a much lower temperature than it would otherwise be without utilization of my invention. This lower temperature increases the density of the water flowing through the downcomer tubes, and results in a downcomer flow such that no steam will be generated in the downcomers. In a natural circulation flow, where the difference in density between the water in the downcomers and the mixture of water and steam within the riser tubes is utilized to maintain circulation, the greater density of the water in the downcomer tubes will result in an in creased circulation.

In one boiler unit where the steam and water drum was used both With and without my invention, a change in load of 20,000 lbs. of steam per hour brought about a change in water level within the drum of 16 inches without utilizing my invention. With the baflle plate and suitably positioned openings in the feedwater pipe, a change in load of 100,000 lbs. of steam per hour was accompanied by a Water level change of only four inches.

The foregoing will be more readily understood after reading the following description of a preferred embodimerit when considered in conjunction with the accompanying drawings wherein:

FIGURE 1 is a cross-sectional side view of a steam and water drum incorporating my invention;

, omizer (not shown) through pipe 12 which is connected to pipe 14. Water flows in restricted paths from the drum 10 down through the gas pass in the outlet of a furnace by means of rows of downcomer tubes '16 which extend along the entire length of the drum 10, and is circulated back into the drum by means of risers =18 which also extend along substantially the entire length of the drum as shown in FIGURE 2. A mixture of steam and water is returned to the drum through risers 18, after absorbing heat from the hot gases flowing through the furnace outlet gas pass. Since the risers tubes are upstream of the downcomer tubes in the gas pass, these tubes will be exposed to hotter gases, and hence will absorb more heat, so that steam is formed within these tubes. The difference in density between the water in downcomers 16 and the mixture of steam and water in risers 18 sets up a natural circulation flow of fluids therethrough.

The mixture of steam and Water from risers 18 enters the drum 10 and is guided by means of housing 20, which completely surrounds the riser inlets, into ducts 22. Duets 22 direct the high velocity flow of the mixture into two rows of separators 24. Brackets 25 may be used to help support the ducts 22 and separator structure. The separators 24 comprise an inner cylinder wall 26 containing fixed vanes or blades 28, which tend to throw the water contained in the mixture outwardly against the Walls 26, due to centrifugal force created by the shape of the vanes. This separated water continues upwardly along the wall and is directed into the annular passage between wall 26- and wall 30 by means of upper curved end 32. The steam flows upwardly in the central part of cylinder 26, and leaves the separator through screened hoods 34, where more water is separated out of the steam. The steam is withdrawn from the drum 10 through pipes 38, after first passing through drying means 36, which consists of a nurnber of screens. The separated Water falls by means of gravity to the bottom of the drum, the level of such water being indicated at 40. Although my invention has been illustrated in conjunction with a drum containing centrif ugal separators, it is to be understood that any separating means could be used equally as well.

As mentioned earlier, the temperature of the feedwater entering the drum through pipe 14 is at a much lower temperature than the temperature of the steam and water within the drum. To confine this cooler feedwater, so as to allow cooler water to flow into the downcomers, and also to prevent its tendency to cause condensation of the steam and ultimate rise in the liquid level, battle 42 is provided. As illustrated, baffle 42 extends from the bottom of the drum to a point just below the liquid level 43. As mentioned earlier, it may sometimes be desirable to have the bafile extend up to the normal liquid level. Bafile 42 extends substantially the entire length of drum 10 as best shown in FIGURE 1, with the left and right ends being closed by means of end plates or bafiles 50 and 52 respectively. These end plates help enclose the area just above the downcomers 16, thereby forming an open topped tank within the drum 10. The end walls also aid in supporting the bafile 42. The baifie 42 has cut out portions 43 along the length thereof to accommodate the passage of pipes 22, as shown in FIGURE 1. As illustrated in FIGURE 2, the drum 10 is provided with an end man hole cover opening 46 to permit access to the drum internal.

Pipe 14 is provided with a plurality of openings 44 along the length thereof through which the feedwater is discharged into the drum. As illustrated, these openings direct the feedwater towards the area just above downcomers 16, at approximately a 45 angle from the vertical. In this manner, the relatively cool feedwater is confined within the portion of drum 10 above the downcomers 16, thus preventing to a great extent condensation within the drum, and more importantly, allows cooler water to flow through the downcomers, thus increasing circulation.

As mentioned previously, the baffle construction and the position of openings 44 in the pipe 14 so as to direct the incoming feedwater towards the wall containing downcomers 16 results in a more stable water level within the drum, regardless of changes in load. This arrangement also increases the velocity of flow through the natural circulation system consisting of downcomers 16 and risers 18.

It is to be understood that the above description is intended for the purpose of illustration only and that modifications such as will occur to those skilled in the art are possible and are embraced within the scope and spirit of the invention. For example, all of the rows of downcomers 16 need not be positioned within the area confined by the baffie, if desired. The number of downcomer rows to be included under the segregating baffle can be varied depending on the heat transferred to these rows, the temperature of the feedwater as it enters the drum, the amount of boiler water recirculated at saturation temperature, and other related factors. Not only is the invention applicable to single upper drum boilers, but to all multiple drum boilers and to all convection bank boilers.

What I claim is:

In combination, a cylindrical drum adapted to house steam and water at very high temperatures, riser tubes having open ends extending through one side of the drum through which a mixture of water and steam is discharged into the drum, separating means for separating the steam from the water that is discharged into said drum through said riser tubes, openings in said separating means through which the separated water flows to the bottom of the drum, housing means enclosing all of the riser tube ends and connecting them to the separating means, an outlet in the upper portion of said drum through which steam is discharged, downcomer tubes through which water flows from said drum connected to the side of said drum opposite the connection of said riser tubes, a feedwater pipe extending parallel to the axis of said cylindrical drum above said downcomer tubes, said feedwater pipe containing openings along its length to allow feedwater, at a lower temperature than the temperature of the steam and water within said drum, to enter said drum, a substantially vertical baille means extending from the bottom of said drum to a point near the water level therein but above the position of said feedwater pipe, said baffie means in conjunction with a portion of the drum forming an open topped tank within said drum, said open topped tank enclosing all of said downcomer tubes and all of said openings in said feedwater pipe, and said openings in said feedwater pipe being positioned such that the incoming feedwater is directed away from said one side of said drum containing the riser tube connections, the greater portion of the comparatively cooler incoming feedwater being entrapped within the open topped tank portion of said drum, and thus flowing from said drum through said downcomer tubes before substantially mixing with a major portion of the separated water from the separating means.

References Cited in the file of this patent UNITED STATES PATENTS 2,264,248 Schrader Nov. 25, 1941 2,594,490 Patterson Apr. 29, 1952 2,743,709 Armacost May 1, 1956 2,763,245 Place Sept. 18, 1956 2,954,014 Rehm Sept. 27, 1960

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