US2078779A - Method of cleaning steam - Google Patents

Description

April 27, 1937. w. T. SHERMAN I METHOD OF.CLEANING STEAM Filed June 25, 19:54

ORV P/PE MW mam ATTORNEY 7 moving these entrained atente pr. 11 5.1? Md William '11. She i w r n, Wbury, N. .lL, agnor to Socony-Vacnnm on Comp t lncorpora i ii New York, N. Y" a corporation of New York Application lie 23,- ldiid, Serial No. I32JW8 5 ilJlaiims.

The present invention relates to a mode of operation of equipment for washing and drying steam, and particularly to an improved method of cleaning steam by the treatment of the steam with water having a low content of material capable of forming solid deposits.

The general field of the invention is the pro.- vision of a process capable of removing entrained liquids and/or solid material from steam, and the more specific field in which the present invention isparticularly applicable is that of rematerials from high pressure steam.

The entrainment of liquid and/or solid materials in steam gives'rise to considerable dimculties in the operation of powerplants. The liquids and/or solids may be entrained in the steam from a number of causes, such as foaming, prim ing and similar dimc'ulties of boiler operation,

and it is frequently possible for serious amounts of such impurities to be present without foaming or priming, "probably from certain causes later discussed. These impurities in the steam give rise to considerable difllcultieatespecially noticeable when the steam from the boiler is passed to a superheater, or when it is used in a turbine. In a superheater, solid materials present in the steam fed to the superheater, are

deposited upon the interior of the super tubes, forming a dense and compact scale which rapidly causes marked decreases in the heat transfer capacity of the superheater, and frequent shut-downs are required for cleaning. This solid material gives rise to considerable onlties in pipes and fittings used for distributing and controlling the steam, as it rapidly e es the inner walls of bends, etc., cuts the seat and discs of partially opened valves, and generallycauses the flowing stream of steam tube of what might be termed an. abrasive nature. If steam carrying a considerable amount of entrained materials is used for driving a steam turbine, .it is.

I ing material.

solids appear in a percentage relative to the total must be dismantled and cleaned. In either case, serious losses of operating time and usefulness of equipment are had. If suficient erosion has.

resulted, the turbine must necessarily be.rebuilt.

The presence of solids in steam, or of materials which will later deposit as solids, does not always appear to be the result of foaming, or priming. Foaming occurs when the concentration of certain materials in the boiler water in the steaming drumis of such a nature that the I frothy steam'which apparently fills the steaming space and is carried away from the boiler. This condition is usually spoken of as priming,

and is a similar source of entrainment. A further source of entrainment, less well understood, isbelieved to be responsible for a considerable portion of the trouble due to carrying over solids or solid-forming material in steam. Apparently when the boiler water hasa relatively high concentration of dissolved solid-forming material, it may allow sumciently free disengagement of steam to avoid frothing or foaming, and the steam coming from the boiler drum may be, for all practical purposes, dry, and yet contain a relatively high proportion of solids or solid-formlit is frequently noted that these steam which is more or less independent of the amount of load or rate of application of load on the boiler, which would argue that they are not placed in the steam by any accident such as priming. Similarly, it has beennoted that the concentration of solids in steamfrequently appears to be somewhat parallel to the concentrationof solid-forming material in the boiler water. Probably the formation of these solids is due to some suchaction as the following: When a hubble of steam,bursts at the surface of the liquid, it throws a small portion of the highly concentrated boiler water into the vapor space. ,This boiler water evaporates in the vapor space without falling back to the liquid, and the solids which were in such water are left within the .vapor as a more or less dry and fine powder, and

as such, are carried from the boiler with the steam. Whether or not this theory as to how impurities'may be carried over is correct is not .known, but I have found that dry steammay functions to remove some of the impurities, it

frequently contain relatively high percentages of solid material capable of causing erosion, etc., or -of being deposited in superheaters or upon turbine blades, that I can remove such solid ma-. .terial by an actual washing of the steam with a water of relatively low concentration of solids, and that the cleaning is not complete unless the wetting of the otherwise dry steam is complete. It-is important that the cleaning be relatively complete for, while the weight percentage of solids present may be minute, the potential cost and damage therefrom may be enormous, becoming especially aggravated with increases of system pressure, and the problem of their substantially complete removal is therefore acute and pressing. It is with a method for the solution of .such a problem of steam cleaning that this application has to do.

Prior methods of cleaning steam which attempt to solve this problem have consisted of various arrangements of baiiiing devices interposed within the steam space of the boiler in such a way that the steam must pass through these baffles before it leaves the boiler, coupled with a spray of boiler makeup waterwhich is distributed somewhat in the form of a curtain upon the entering side of the baflies. These devices have been found to be subject to serious disadvantages, the greatest being that it is quite diflicult to arrange the baflles and sprays so that there is at all times an assurance of positive contact between the steam to be scrubbed and the scrubbing spray. A further disadvantage is that the impurities in the spraying water itself may be relatively concentrated, and, while such water may do so by replacing them with other impurities. Frequently also, the interaction of the spray and the baiiies. is such as to cause deposit of the entrained solid material after wetting upon the surfaces of the baflles, which quickly results in clogged baflles .and failure of means for removing steam from the boiler. A further serious disadvantage is that the-requirements of space within the boiler call for high steam velocity through the baffles, and the spraying water, after taking up material from the steam, frequently may be swept oil the baflies by the action of the stream of vapors and returned to the steam as an imall of the portions of the steam with a scrubbing I agent. It is a further object to develop a process enabling the use of a scrubbing agent of low or relatively. low concentration of impurities capable of forming solids. A further object is to develop such a process in which the scrubbing agent, after contact with the steam, may be positively and effectively removed therefrom. Another object is the provision'of a process wherein the steam scrubbing may be carried out without derangement of the normal heat balances ofv the system. A further object is the devising of a process capable of performing the required cleaning without being hampered by the requirements of space within the steaming drum of the boiler, and which, in its preferred form, is carried on outside of the, drum. Otherobjects and advantages will appear in the descrip tive matter following.

The process of this invention may be briefly described as that of vigorously scrubbing steam with water of low solid concentration, in a manner which requires positive contacting of all of the steam with the scrubbing-water followed by the removal of the water with the impurities,

1 In order that my process of steam cleaning and the' apparatus therefor may be more thoroughly understood, reference is now made to the drawing attached to and forming a part of this specification. In this drawing, Figure 1 is a diagrammatic representation of the preferred form of my invention, while Figure 2 is a. diagrammatic showing of a variation in which the scrubbing device-is placed in the steaming space of the boiler.

In Figure 1 of the drawing, l indicates the steaming drum of a high pressure boiler, the tubes and other drums of the boiler not being shown. 2 indicates the steam main leading from steam 'drum I to scrubber 3. This pipe 2 enters scrubber 3 below a partition 4. In this partitition 4 to a point well below water level 6. Other than for the openings through tube 5 and tube 1, the partition 4 is imperforate. The liquid storage space in the bottom of the scrubber 3 is supplied with water through line 8, and this water is removed from the storage space through line 3 controlled by liquid level valve l0, through the agency of pump II and introduced into the boiler by means of line l2, with auxiliary feed introduced and controlled by some form of boiler water level and feed control as indicated at I3. In the upper portion of the scrubber 3 there is placed a moisture separator I4, of any eflicient type, such as one of the numerous forms of labyrinth ballle separators, and steam passing through this moisture separator is taken from the scrubber through line I5 to the superheater or to a point of use. The water entering through line 8 may be the feed water normally charged to the boiler, or it may preferably be mixed with increasing proportions of returning condensate. For reasons later discussed it is desirable at times to feed nothing but returning condensate through line 8, so this line 8 must be understood to be connected to sources of feed water, treated or raw, condensate, and/or other source of water'- low in solid-forming impurities.

The action of the scrubber is as follows: The steam entering the vapor space between 'water level 6 and partition 4 can escape only by passing under the lower edge of tube 5 and vertically through that tube into the space above partition 4. In so passing into tube 5, it entrains liquids from the storage space, the whole passing through tube 5 in a highly commingled and extremely actively contacting condition whereby each and every portion of the steam is positively and forcibly contacted with scrubbing medium. Above the partition 4, after exit from the tube 5, in the relatively enlarged space provided, the water separates from the steam. Inasmuch as this water is not introduced to the steam under actively steaming conditions in the form of a fine spray, and is usually and preferably at a, temchange.

ao'zawe perature somewhat less than that of. the steam, there is very little tendency for the steam to carry with it particles of water of a size greater than those which may be separated by an ordinarily' good moisture separator placed in location M. I

Figure 2 is a diagrammatic representation of how the apparatus accomplishing this process may be installed within the steam drum of a boiler. In this figure the function of like parts is the same as in Figure l, and such like parts have been given identical numbers in the two figures. v

Due to the reasons pointed out above, the

water carried out of the contacting tube is not difficult to separate from the steam, and consequently the installationshown in Figure 2 is frequently practicable. It may be modified somewhat by installing the moisture remover in an adjacent body, with the contacting tube in the steam drum.

In'the preferred form of-the process, the installation is according to Figure l, where full requirements of space can be made use of. The reasons for preference of the outside installation largely arise from the moisture separator it. Due to the ability to use all of the space required for the properly designed moisture separator, this moisture separator is not called upon to work at high rates, and consequently, is not subject to all of the disadvantages of clo ging, possibility of re-entrainment of separated moisture and the like, to which a moisture separator placed in the steaming drum is subject. In addition, it is not called upon to separate moisture of the type which must be separated by a baffle in the steaming drum working in serieswith a spray, as the nature and form of the contacting action in tube 5 is such'that the relative size of the moisture droplets is greater than with the spray and they tend to separate more easily and are additionally enabled to separate by the possibility, with this process, of being able to scrub the steam at a point relatively remote from the point' of moisture separation, as compared to scrubbing it with a spray immediately in front of the baflles in the steaming 'drum.

In steam generatingplants operating at low "boiler pressures, the difiiculties of solid carryover are frequently not crucial. Similarly, they may be relatively unimportant, if there is an extremely highproportion of condensate return to the boiler. The high generating pressures usual in modern design, in which only a low proportionate return of condensate may be possible, requires some other solution of the problem. Obviously, a pressure fixed at a high level by the design of the plant is incapable of change thereafter. The net return of condensate to the boiler is likewise fixed by the overall conditions of .the installation, and relatively incapable of It is possible, however, by proper operation of my method of steam cleaning, to return enough condensate to cleanse the steam, or to obtain the effects of a high relative pro-. portion of condensate return, in connection with other advantages, as will be explained. Advanciently low, then it must be diluted with condensateto attain a concentration of solids sufficiently low, in the. liquid stored below water level 6, to perform the desired scrubbing. If there is sufiicient condensate availablethis method may be used. If there is insufficient condensate available to reduce the concentration of solids in the entire feed water, a third method may be used, in which the feed to the scrubber through line 8 is only condensate or predominantly condensate, the main feed to the boiler passing to the boiler through a separate system. Then this water of extremely low concentration'is allowed to remain within scrubber 3 until it has reached a concentration by removal of solids from the steam suffi-' ciently high that further removal of solids is not practicable. This water is then passed to the boiler through line 9. While in terms and in accomplishment this last method of operation might be considered to be intermittent, in

actual practice it is continuous, the proportion of drawoff of highly concentrated water through line 9, andfeed of condensate or water of low concentration through line 8, being so proportloned that the concentration of the water withcleaning. This may be done by feeding sufficiently cold feed water so that a portion of the steam is condensed within the scrubber, thus giving a scrubbing water of. desirably low concentration. This method of course does not waste heat, the latent heat of the condensed steam being recovered in the combined condensate and feed which then goes to the boiler.

The efiicacy of this process of steam cleaning is centered around the ability to attain positive and complete scrubbing contact of all of the particles of steam with a scrubbing medium. Collaterally, it is important that this scrubbing action be permitted to take place with water relatively free from impurities which maybe converted to solids upon evaporation of the water. The requirement of relatively solid-free scrubbing water may be met by scrubbing with condensate obtained from subsequent portions of the system, or by use of the feed if that be sufficiently solid-free, or by combinations of feed and condensate return in various proportions, or by adjusting temperature and amount of feed so that sufiicient condensate isformed within the scrubber itself to reduce the scrubbing water to the desired solid free degree.

The requirement of intimate and thorough contacting can be attained in numerous ways. For my preferred operation, I make use of the device shown in the drawing and described herein, but I do not intend by this description to be limited to the use of this device, but consider any method of obtaining such contact to be within the scope of my invention except as limited by the appended claims.

In the diagrammatic drawing attached hereto,

I the contacting of steam and Water is shown as being performed in a single tube. As a matter of actual practice, it will be preferred to use a plurality oftubes, since requirements of space and efficient contacting may be more easily met with several tubes 5 of a convenient size rather than with one large tube of equivalent periphery or of equivalent cross sectional area. Increases of capacity can consequently be made by increasing the number of tubes.

It is to be noted that this method of steam In the previous discussion and theappended claims solids is intended to mean not only solids actually present within the steam as such, but also is intended to include any material capable of appearing as solids at some subsequent point in the steam system, such as droplets of highly concentratedsolutiomand the like. i

I claim:

1. In a process for removing entrained impurities from steam, the steps which comprise liberating steam within a steam generator, removing the steam from the region in which it was liberated at atemperature substantially that of saturation, maintaining a body of water of relatively low content of dissolved solids, intimately contacting the steam with water from the said body, thereafter separating the water from the steam and returning the water to the body, feeding water to said body, withdrawing water therefrom, and proportioning the amount I and temperature of the feed water to cause a suiiicient condensation of steam within the area of contacting to maintain the dissolved solid content of the'said body of water below a predetermined maximum, which is lower than that of the feed water.

2. In a process for removing entrained impurities from steam, the steps which comprise liberating steam within a-steam generator, re,- moving the steam from said steam generating region at a temperature substantially that of 45 saturation, maintaining a body of water of relatively low content of dissolved solids, intimately contacting the steam with water from the said body by passing it through a confined turbulently flowing stream of water droplets, thereafter sepa- 50 rating the water from the steam and returning the water to the body, feeding water to said body,

withdrawing water therefrom, and proportion-' ing the amount and temperature of the feed water to cause a suflicient condensation of steam within the area or contacting to maintain-the dissolved solid content'oi' the said body of water 3. In a process for removing entrained impuriuration, intimately contacting the steam with water relatively low in dissolved solids by passing it through a confined, turbulentlyflowing stream of water droplets, said stream being created and maintained by high velocity contactof said steam with the surface of said body of water adjacent the zone of initiation of said stream, and separating the steam from the water.-

4. In a process for removingentrained impurities from steam, the steps which comprise liberating steam within a steam generator, removing the steam from the region in which it was liberated at a temperature substantially that of saturation, intimately contacting the steam with, condensate water substantially free from dissolved solids by passing it through a confined, turbulently flowing stream of water droplets, said stream being created and maintained by high veloc ty contact of said steam with the surface of sai body of water adjacent the zone of initiation of said stream, and separating the. steam from the water. a

5. In a process for removing entrained impurities from steam, the steps which comprise liberating steam within a steam generator, removing the steam from the region in which it was liberated at a temperature substantially that of saturation, maintaining a body of water of relatively low content of dissolved solids, intimately conta'ctingthesteam with water from the said body by passing it through a confined, turbulently flowing stream of water droplets, said stream being created and maintained by high velocity contact of said steam with the surface of said body of water adjacent the zone of initiation of said stream, thereafter separating the water from the steam and returning'the water to the body, feeding water to said body, withdrawing water therefrom, and proportioning the amount and temperature of the feed water to cause a sufficient condensation of steam within the area of contacting to maintain the dissolved solid content ofthe said body of water,below a predetermined maximum.

1 WILLIAM T. SHERMAN.

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