US2842103A - Method for washing out the conduits of supercritical pressure, water tube, vapor generators - Google Patents

Description

July 8, 1958 v P. PROFOS 2,842,103

METHOD FOR WASHING OUT THE CONDUITS OF SUPERCRITICAL PRESSURE, WATER TUBE, VAPOR GENERATORS Filed March 25, 1954 2 Sheets-Sheet 1 R P m m ATTORNEK 2,842,103 TICAL I v P. PROFOS July s, 1958 METHOD FOR WASHING OUT THE CONDUITS OF SUPERCRI PRESSURE, WATER RATORS Filed March 25, 1954 TUBE VAPOR GENE 2 Sheets-Sheet 2 INVENTOR. P401. Peer-vs.

BY l ATTOR/VEK atent 2,842,103 Patented July 8, 1958 METHUD 1 R WASHKNG @UT THE CONDUITS fill SUPERCRITICAL PRESEURE, WATER TUBE, VAPOR GENERATQRS Paul llrofos, Wiuterthur, Switzerland, assignor to Sulzer Freres, Eloeiete Anonyme, Winterthur, Switzerland, a corporation of Switzerland The present invention relates to a forced flow steam generator in which the steam is produced at supercritical pressure and in which sediments produced during the steam generation at supercritical pressure are washed out by wet steam which is produced by temporarily reducing the pressure of the operating fluid to below the critical pressure.

It is an object of the present invention toprovide a forced circulation tubular supercritical pressure steam generator in which a plurality of individual tubular conduits are arranged for conducting the operating tluid in a plurality of separated parallel streams from an inlet header to an outlet header, the latter being arranged upstream of a superheater with respect to the flow of the operating fluid.

ln contradistinction to steam generators which operate at sub-critical pressure, in steam generators which are operated at supercritical pressure there is no possibility to concentrate the salts contained in the feed Water in the water remaining during the evaporating process, and to remove salts from the boiler by means of a continuous or temporary blow-down operation. When producing vapor at the critical point, there is no mixture of two phases of the operating fluid because transition from the liquid phase to the steam or gas phase is continuous. The operating fluid which is removed from any part of the steam generator cannot have a higher salt concentration than the water which is fed into the generator. Unless special measures are taken, the salts contained in the feed water will appear in the generated steam, except for the salts which adhere in the form of salt crusts to the interior of the tubes forming the heating surfaces along which the liquid is transformed into vapor. These sedimentations on the heating surface of the boiler reduce heat transfer and may cause scaling and corrosions, as well as swelling and bursting of tubes due to excessively high tube wall temperatures.

It is an object of the present invention to provide a method and means for avoiding the aforementioned undesirable conditions and disadvantages. In a steam generator according to the invention, the tube lines of the steam generator extending from the inlet header of the generator are separated from each other until they reach the superheater. In a further development of the invention, a pressure-reducing device is connected with each tube line downstream of the zone in which the liquid is transformed into vapor. It may be desirable to arrange a length of tube between the point where the pressurereducing device is connected and the outlet header in which all tube lines terminate. It may be of advantage 2 to provide a throttling device in the conduit between the pressure-reducing device and the outlet header of the generator. This throttling device may be in the form of a nozzle. As a modification, a check valve may be installed in the tube between the pressure-reducing device and the outlet header.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself however and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing in which Fig. 1 is a diagrammatic illustration of an arrangement of steam generating and superheating tubular heating surfaces in a steam generator according to the invention;

Fig. 2 is a diagrammatic showing of a modified arrangement of the steam generator according to the invention;

Fig. 3 is a diagrammatic illustration of a further modiflcation of a steam generator according to the invention;

Fig. 4 is a diagrammatic showing of another modification of a steam generator according to the invention;

Fig. 5 is a diagram indicating the pressures in a steam generator according to the invention during a temporary pressure reduction;

Fig. 6 is a diagrammatic vertical section through a steam generator according to the invention;

Fig. 7 is a diagrammatic showing of an arrangement of the steam generating part of a system according to the invention.

The same numerals designate the same parts in all figures.

As seen in the drawing, the steam generating tubes are so arranged that there is no connection between the individual tube lines which form the heating surface between the generator inlet and the zone in which the liquid is completely transformed into vapor, i. e. the operating fluid is conducted through the steam generator in a plurality of separate streams, each stream constituting, in fact, an individual steam generator.

Referring more particularly to Fig. 1 of the drawing, the operating fluid is pumped by means of a feed pump 1 into an inlet header 2 and distributed thereby into a plurality of tube lines 3 of a forced circulation steam generator. The individual tube lines are separated from each other until they reach the zone in which the vapor is superheated. The operating fluid flows in the direction of the arrows, the tube lines passing through a preheating zone 4 and thereafter through a zone 5 in which the liquid is transformed into vapor. Thereupon, the operating fluid is superheated in a zone 6 and collected in a header '7. From the header 7, the fluid enters a superheater 8 which, however, has no bearing on the present invention.

For washing out the tubes, the pressure in each tube is reduced after a certain period of time, the pressure reduction in the individual tubes being effected in one tube after the other, and not simultaneously in a plurality of tubes. The pressure in the tube line to be washed out is reduced to below the critical pressure, so that wet steam is produced in the zone where salt has been deposited, and relatively cold feed water is caused to flow to the parts of the tube system where salts are deposited. The flushing action of the wet steam dissolves the crusts I of salts, so that the latter may be removed in a simple manner from the tube system. The pressure reduction is effected by means of a pressure-reducing device 9 one of which is connected with each tube line downstream the transformation zone 5. It is obvious that irregularities are less noticeable in a steam generating system having long and separately arranged tube lines than in a system in which the tube lines are interconnected several times by means of headers or collectors. An important advantage is obtained by arranging relatively great lengths of tubes without cross-connections because the specific heat reception in long tubes is more equalized than in short tubes. Sedimentation will be distributed equally over long tubes. If one compares two boilers having equally stable operating characteristics, however one having a system of uninterrupted long tubes, and the other having a system of tubes which are frequently interrupted by collectors, the former is superior to the latter. This is also evidenced by the fact that a boiler whose tubes are arranged according to the invention in parallel relation over a great length, is less sensitive to diflerences in the construction of individual boiler parts which may affect the pressure drop in the boiler, such as length of tube, diameter of tube, bends, and the like, so that pressure drops and other undesirable conditions can be overcome without difficulty and without disturbing the balanced operation of the boiler. This is in part due to the fact that existing or developing differences, due, for example, to tube diameter and sedimentation, are better equalized in long, separately arranged tube lines than in a plurality of tube systems individually consisting of short tubes. The advantages produced by the present invention are not only noticeable at normal boiler operation, but are of particular importance when removing sludge from the individual tube lines. In a boiler according to the invention, the salts which are separated at the end of the transition zone from liquid to vapor are deposited within a relatively short zone which has about the same position in all tube lines because the thermal and flow conditions are similar in all tube lines. A displacement of the sludging zone due to a displacement of the slagging zone is without influence on the efficiency of the sludge removal operation. In tube lines which are repeatedly interrupted by collectors, the washing-out operation is performed in relatively short tubes between two headers, while salt deposits in the tubes preceding or following the headers are unaflected by the sludge removal operation. If, for example, the salts are deposited because of slagging conditions, in a tube portion other than the portion of a tube which has been predetermined for sludge removal, there is no sludge removal in the predetermined tube portion.

A further advantage is due to the fact that in a tube system constructed according to the invention, the sludge is removed by means of a washing fluid which is taken directly from the inlet header and which has the heat content of the boiler feed water. Washing out by means of an inherently liquid and colder fluid, which is converted into wet steam while passing through the tube system, produces more favorable conditions for sludge removal and is more eflective than washing out by means of a very hot operating fluid which must first be converted into wet steam.

In order to prevent that greater amounts of steam flow back through the outlet header 7 during desludging of a tube line, a reverse flow impedance producing means, for example in the form of a tube 10 is preferably provided following the connection of the pressure-reducing device 9, which tube is so long that the pressure drop in the steam flowing back through the tube from the header .7 at the end of the tube 10 sufficiently limits the return flow of steam (Fig. 2).

The return flow of steam can also be reduced or completely prevented by special devices. For example, a

' nozzle or venturi tube 11 may be. arranged the tube 4 after the connection of the pressure-reducing device 9, which nozzle offers only little resistance to the flow of the operating fluid in normal direction, but offers great resistance when the operating fluid flows in the opposite direction (Fig. 3). The return flow can also be avoided by means of a check valve 12, as indicated in Fig. 4.

The pressure drop in a heating surface constructed according to the invention is shown in the diagram Fig. 5 The pressure of the operating fluid is indicated by the ordinate p, and the length of the tube is indicated by the abscissa l. The curves plotted in the diagram Fig. 5 correspond to the operating condition which occurs in steam generators according to Figs. 1 to 4 and 6 and 7 during pressure reduction of the operating fluid. The curve p indicates the steam pressure during the sludge removal operation in the tubes which are not desludged. The pressure in the tube which is desludgcd corresponds to the curve 11 this pressure falls below the dotted line p to the desired expansion pressure which is lower than the critical pressure of the operating fluid. The curve [1 indicating the pressure of the steam flowing back from the outlet header 7 is considerably steeper due to the arrangements made after the pressure-reducing device 9.

Fig. 6 is a diagrammatic illustration of a forced circulation steam generator according to the invention. The feed water enters through the inlet header 2, from which it is divided into three streams flowing through three tube lines 3, which are separated from each other and are disposed in a preheating zone 4, and in a zone 5, in which the water is converted into steam, and in a subsequent superheating zone 6, an outlet header 7 being provided at the ends of the tubes 3. The high pressure steam is subsequently conducted through a superheater 8 which is made of four parallel tubes. It is important that the zone in which the water is converted into steam is arranged in a moderately heated part of the boiler. It may be exposed to radiant heat, but in a part of the combustion chamber where the heat is moderate, or it may be arranged in the convection part of the boiler. A pressure-reducing device 9 is connected with each tube line 3 at a point where dry or slightly superheated steam flows through the tubes. As has been previously explained, the salts deposited in the individual tubes 3 in the zone where the water is converted into steam, are washed out by means of wet steam produced in the individual tubes by temporary actuation of the pressure-reducing devices. Nozzles 11 which are arranged in the individual tubes 3 after the points where the pressure-reducing devices are connected, prevent the return flow of steam from the tubes which are not washed out through the outlet header 7 to the tube which is being washed.

The number of tube lines arranged in the individual parts of the steam generator may differ from the number shown in the drawing. The individual tube lines may be divided, as shown in Fig. 7. For example, each of the two tube lines 3 shown in Fig. 7 may be split into a pair of tubes 3' and 3" which are connected with the inlet header 2. The tubes 3 and 3 are united into the tubes 3 approximately at the middle of the boiler, transition of water into steam occurring in the united lines 3, and a pressure-reducing device 9 being connected with each tube 3 and each of the latter being provided with a nozzle 11 before the tube is connected with an outlet header, not shown.

While a specific embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that various changes, modifications,'substitutions, additions and omissions may be made therein without departing from the spirit and scope of the invention as set forth in the appended claim.

I claim:

A method for individually washing out selected tubular conduits of a forced flow supercritical pressure vapor generator including the steps of continuously supplying operating fluid, and combustible and oxidizing media to the generator at relative rates producing a substantially constant temperature of the generated supercritical pressure vapor at a normal location in its path, of temporarily reducing the pressure in the selected conduit to be washed out to below the critical pressure of the fluid for producing wet vapor in the selected conduit upstream of the normal location of the vaporizing zone, and of blowing off contaminated wet vapor from the selected conduit which is to be washed out, while leaving undisturbed at least the critical pressure of the fluid in all the other conduits.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Heat Power Engineering, part1, third edition; Barnard, Ellenwood and Hirshfield, pp. 259, 260, 272-275; John Willey and Sons.

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