US2895566A

US2895566A - Method of washing high pressure steam

Info

Publication number: US2895566A
Application number: US546335A
Authority: US
Inventors: Earl E Coulter
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1955-11-14
Filing date: 1955-11-14
Publication date: 1959-07-21
Anticipated expiration: 1976-07-21

Description

July 21, 1959 E. COULTER METHOD OF.WASHING HIGH PRESSURE STEAM Filed Nov. 14, 1955 2 Sheets-Sheet 1 INVENTOR.

Earl E.Coul+er ATTORNEY July 21, 1959 E. E. COULTER METHOD OF WASHING HIGH PRESSURE STEAM Filed Nov. 14, 1955 2 Sheets-Shec. 2

FIG.3

FIG. 2

WASH WATER Row, M LB/HR v INVENTOR.

Earl E. Coulfer ATTORNEY METHQD ()F WASHING HIGH PRESSURE STEAM Earl E. Coulter, Alliance, Ohio, assignor The Babcoc k & Wilcox ompany, New York, N.Y., a corporation of New Jersey Application November 14, 1955, Serial No. 546,335

2 Claims. (Cl. 183-421) The present invention relates to a method of purifying steam and more particularly to steam washing applicable to the removal of silica vapor from high pressure steam.

It is known that high pressure steam usually contains solid impurities that are either in the vapor state or mechanically entrained in the vapor as minute particlm of solids. These impurities pass with the steam from the vapor generator and the impurities are usually subsequently released when the vapor is cooled during subsequent use in, for example, expansion engines. Some of the mechanically entrained impurities cause difficulties from deposition in superheater tubes or in other tubular conduits leading to the expansion engine. Wlren the expansion engine is of the turbine type the impurities are deposited in the turbine, resulting in a gradual loss turbine capacity and efficiency.

Heretofore mechanical means had been used with success in the removal of the mechanically entrained impurities in the steam. Such means have taken the form of cyclone separators, tortuous path separators, and the like. However, when the steam is generated at high pressures, as is common today, the mechanical removal of the entrained solids does not effectively remove the vaporized impurities, such as silica and siliceous compounds, which are present in the steam delivered by the vapor generator.

It is known that silica vapors can be removed from high pressure steam by 'washing the steam with a relatively pure wash water. Apparently the wash water contacting the steam absorbs the silica vapors and adsorbed silica can be removed from the steam with wash water. The effectiveness of the wash water removal of the impurities is largely dependent upon the intimacy of contact between the steam and the wash water, and the efiective removal of the wash water from the steam flow path after washing contact with the steam.

In the generation of high pressure steam the higher the pressure under which the steam is generated the more nearly the density of the steam is to the water with which it is washed. At the critical pressure of steam, the densities of steam and water are equal. Whenever steam is washed by projecting a stream of steam at high velocity through a body of wash water the degree of atomization of the water, i.e. the dispersion of the water into droplets, by reason of the steam passing there through is primarily dependent upon the velocity of the steam streams and the density of the steam in use. At the higher steam pressures of, for example, 1800 to 2400 lbs. per square inch, and higher, the steam being of high density has a greater ability to suspend the atomized wash water and the suspension of the water in steam is more effective.

In accordance with the present invention high pressure steam is washed by high purity Water for the removal of silica vapors in the steam. To insure an effective and efficient washing eifect the apparatus of the present invention includes a plurality of closely spaced perforated States Patent 0 'ice trolled rate. For best silica removal the flow rate of the wash water to the washing apparatus is regulated for a substantially uniform rate of delivery even though the rate of steam flow through the apparatus may vary between relatively wide predetermined limits. At a low steam flow rate below the predetermined lower limit the Wash water flow is reduced so that the effect of the wash water added to the steam drum will not adversely afiect the conditions within the drum insofar as water level therein is concerned.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is an elevation, in section, of a steam and water drum containing the steam washing apparatus of the present invention;

Fig. 2 is a perspective view of the perforated trays in the steam washer of the present invention;

Fig. 3 is an enlarged end section of the washer shown in Fig. 1; and

- Fig. 4 shows a family of curves illustrating the wash water flow control system of the present invention.

As shown in the drawings, the steam and water drum 10 of a steam generator is provided with a plurality of riser tubes 11 which discharge a mixture of steam and water into the drum. Ordinarily the riser tubes are positioned to discharge into the lower segment of the drum. A steam outlet 12, consisting of a row of tubular connections is positioned in the upper portion of the drum for the discharge of saturated steam to the customary superheater' headers of a vapor generating and superheating unit. The drum 10 is provided with suitable downcomer connections (not shown) at the opposite ends thereof for the downward movement of water into the circulatory system of the steam generator. In the embodiment of the invention shown the steam and water mixture entering the drum 10 through the riser tubes '11 is passed through a plurality of steam and water separators 13 with the water discharging from the separators into the water space 14 in the lower portion of the drum 10 and the steam passing upwardly from the separators 13 into plates which are supplied with wash water at a conthe steam space 15 in the upper portion of the drum.

The drum 10 is provided with a plate structure 16 spaced from the inner surface of the drum 10 to provide a chamber 17 which collects the steam and water mixture discharged by the riser tubes 11 for distribution of the mixture into tangential inlet ducts 18 individually connected to each of the steam and water separators 13. The separators 13 disclosed in the drawings are of the cyclone type and are described in US. Patent 2,368,211. As shown, the separators are arranged in rows for parallel flow of steam from the chamber 17 and are each provided with a corrugated plate scrubber 20 upwardly adjacent the upper steam outlet 21 from the separators 13. With the construction described substantially dry steam is discharged from the upper outlet portion of the steam separators 13 and is passed toward the steam outlet 12, While the water is discharged through a bottom outlet 22 from each separator, into the water space 14 of the drum.

The corrugated plate scrubber 20 is inclined downwardly toward the interior of the drum 10 and is provided with a film of wash water for contact purification of the steam leaving the separators 13. The detailed construction of the corrugated plate scrubber is shown in the Raynorv US. .Patent Number 2,715,451, issued 3 August 16, 1955. As shown in Fig. 1 and by said patent, generally pure wash water is introduced through a water manifold 23, branch conduits 24 and passes over a distributing weir 25 to flow along the surfaces of the scrubher and discharge through a channel 26 to'the interior of the drum. p I

The steam purifying unit 27 of the invention is disposed within the steam drum downwardly adjacent the steam outlet 12, in the steam space 15 of the drum 10. The unit includes depending plates 28 attached at their upper ends to the drum and forming a closed structure open at its lower end in communication with the steam space and enclosing the lower end of the outlet 12.

As shown, structure 30 of inverted V shaped section is positioned beneath the lower end of the steam purifying unit 27 so that the steam entering the unit must pass between the lower edge portions of the depending plates '28 and the upper surface of the bafile 30. The baffle also serves to receive the downward flow of wash water discharging from the steam purifying unit 27, and returns that water to the water space 14 of the drum with a minimum of agitation of the water surface. This battle structure arrangement effectively avoids the entrainment of drum water in the steam passing into the steam purifying unit 27.

The steam washing unit of the invention is provided with a series of horizontally disposed perforated plates or trays 31 extending between the depending side walls of the apparatus. The perforated plates or trays are vertically spaced with the spacing therebetween being in the range of from 1" to 3". As shown, each of the trays is constructed of a steel plate of, for example, thickness, and each plate is provided with a series of circular perforations therein where the perforations are of the order of V in diameter and are formed on a centerline triangular to centerline spacing of This structure provides a net opening in the trays of approximately 33 percent of the total transverse area of the trays.

With steam passing upwardly through the perforated trays wash water is distributed to the uppermost tray 32 with the water gravitating downwardly through the perforations countercurrent to the steam. Wash water is delivered to the washer from an outside source through a manifold 33 and a series of branch conduits 34 spaced longitudinally of the drum 10. Each of the conduits 34 terminates in a nozzle 35 which is spaced above the tray 32 and substantially midway the plates 28.

As shown in Figs. 1, 2 and 3, the washer is provided with a corrugated plate scrubber 36. In the embodiment shown, the closely spaced, parallel, corrugated plates are substantially upright and extend longitudinally of the drum 10. The lower edge of the scrubber is spaced above the conduits 34 and the nozzles 35, with the assembled scrubber supported from the depending plates 28. The scrubber is ordinarily constructed as a plurality of sections for ease of installation in the drum. The washed steam passing upwardly through the scrubber is substantially dried and enters a perforated dry pipe 37 before discharge from the drum through the steam discharge 12.

With the construction described a proper flow of steam passing through the perforations will disperse the wash water into droplets and suspend the droplets between the trays. At a steam flow rate of the order of 14,000 lbs. per hour per square foot of tray cross sectional area the steam will disperse the wash water and maintain the water in suspension in the intertray space. With steam flow rates in excess of this value there will be a tendency for the water suspension in the steam to carry over from one intertray space into and through the perforations of the tray immediately above. When this condition occurs, and for convenience, this condition is termed the flood point, the silica removal efliciency of the washer is drastically reduced. When the steam flow rate through the washer is reduced to a value above 2,000 to 3,000 pounds per hour per square foot of tray area the velocity of the steam in passing through the perforations is sufiicient to disperse the water maintained upon the tray surface. This condition for convenience is termed the dispersion point. Thus between fairly definite steam flow rates the steam in passing through the perforations of the trays will disperse the wash water and maintain it in suspension in the intertray spaces. This condition leads to an intimate contact between the steam and the wash Water and results in an effective removal of silica vapor from the steam.

It will be understood that the size of the perforations in the trays and the total open area of the perforations with respect to the total transverse area of the trays will be dependent upon the pressure of the steam which is being generated within the unit with higher pressure steam due to its increased density being more effective in dispersion of the wash water.

It has been found that the rate of wash water delivery to the steam washing apparatus can also effect the flood and dispersion points of the washer with steam flowing therethrough. This is illustrated in Fig. 4 by the dotted line entitled capacity limit which illustrates the flood point of the wash trays when operated with steam of 1300 p.s.i.a., for example. The lower limit of the dispersion point does not display as sharp a condition as that of the flood point but could be considered as occurring along the line indicated by the solid line in Fig. 4 designated 50 percent silica reduction.

Heretofore it has been considered desirable to proportion the flow of wash water to the tray in direct proportion to the rate of flow of steam through the unit. When such a procedureis followed the silica reduction, or the removal of silica from the steam, is indicated by the dotdash line A shown in Fig. 4. It has been determined that the efficiency of the apparatus, insofar as silica removal from the steam is concerned, is best when the water flow rate is reduced in a selected ratio with respect to the steam flow rate such as indicated by the dot-dash curve B shown in Fig. 4.

It will be appreciated that a control apparatus for regulation of the water flow rate can be of the manual or automatic type and can be regulated for any desired relationship between steam flow and water flow. However, to follow a ratio such as indicated by the curve B in Fig. 4 it is necessary to provide the equivalent of a camming control system to regulate water flow in the selected ratio to the steam flow through the unit. For purposes of simplicity of control it has been found desirable to maintain the water flow to the steam washer substantially uniform through a major portion of the steam flow rates through the apparatus. This is indicated by the curve marked C in Fig. 4. In comparing curves A, B and C it will be noted that the differences in efiiciency of the apparatus, with water flow rates controlled according to curves and C, is small. For reasons of simplicity a practical control system can be easily supplied to follow the procedure illustrated by the curve C in Fig. 2.

Ordinarily steam washing apparatus of the type disclosed is provided with boiler feed water as a source of wash water. It is understood that high purity condensate could be used in place of boiler feed water, but for most practical purposes boiler feed water is of sufficient purity to effectively be used for the purpose described. Regardless of the source of the wash water the water used in washing the steam is delivered eventually to the circulatory system of the generating unit and even though the amount of such water may be only a fraction of the total make up water required in the steam generating unit this water nevertheless necessitates a reduction in the total amount of the water delivered in the normal way to the steam and water drum for make up purposes.

With the wash water feed rate controlled according to the system indicated by the curve C in Fig. 4 at low steam operating rates, the wash water may reach a point where it forms a major part of the total make up water delivered to the steam and water drum. Under these conditions it is desirable to gradually reduce the proportion of water delivered as wash water so as to avoid unnecessary and undesirable fluctuations in the water level within the drum. For this reason, as indicated in Fig. 4, the rate of wash water delivery at low steam output rates of the unit is reduced proportionately with the steam output rate. This is indicated by the curve D in Fig. 4.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation or the invention now known to me, those skilled in the art will understand that changes may be made in the process without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of purifying high pressure steam which comprises, subjecting said steam to a preliminary purifying step of contact with films of wash Water, passing a countercurrent flow of said partially purified steam through a plurality of relatively closely spaced steam and wash water contact stages wherein the wash water is dispersed into said steam, limiting the maximum quantity of steam flow through said washer whereby wash water droplets dispersed in said steam do not carry over from one stage to an upwardly adjacent stage, and regulating said Wash water flow to said washer to a substantially uniform rate of less than 10% by weight of said steam at the maximum steam flow through an upper range of steam flow rates, and reducing said water flow rate proportionally to said steam tflow rate throughout a lower range of steam flow rates.

2. The method of purifying high pressure steam which comprises passing a ccruntercurrent flow of steam through a plurality of steam and wash water contact stages wherein the wash water is dispersed into said steam, limiting the maximum quantity of steam flow through said washer whereby wash water droplets dispersed in said steam do not carry over from one stage to an upwardly adjacent stage, regulating said wash water flow weight rate to said washer to a substantially uniform rate of less than 10% by weight of said steam at the maximum steam flow through an upper range of steam flow weight rates, reducing said water flow rate proportionally to said steam flow rate throughout a lower range of steam fio'w rates, and collecting the Wash water discharging from said washer out of entraining contact with the steam entering said wash water contact stages.

References Cited in the file of this patent UNITED STATES PATENTS 708,028 Crawley Sept. 2, 1902 1,671,110 Gibson May 29, 1928 1,879,930 Gibson Sept. 27, 1932 2,712,929 Wilson July 12, 1955 2,715,451 Raynor Aug. 16, 1955