US2524757A - Cleaning scaled vessels - Google Patents

Description

950 M. E. BRINES El AL. ,5

CLEANING SCALED VESSELS Filed May 17, 1945 INVENTORS.

Me/w'n 5. Br/nes y Theodore W Sarge ATTOQNE Y5 Patented Oct. 10, 1950 2,524,151 CLEANING scam-1n vEssEns Melvin E. Brines and Theodore W. Sarge, Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application May 17, 1945, Serial No. 594,310

The invention relates to methods of treatment of vessels, particularly the interior surfaces thereof, such as those of evaporators, steam generators, heat exchangers, and the like, so as to remove incrustants, such as scale, rust, or the like.

In many industrial operations vessels of one kind or another are used the surfaces of which become encrusted with deposits of oxide or rust, or scale containing more or less insoluble silicate, phosphate, sulfate, or carbonate or mix tures of such as other scale-forming matter. In many instances, the deposits contain organic matter such as oil, grease, and carbon. In the generation of steam from water the surfaces of the system exposed to the water usually, sooner or later, become incrusted with scale from scaleforming constituents which enterthe system with the feed water. Other heat-exchange surfaces, such as condenser tubes, feed water heaters, engine jackets, radiators, piping systems, and similar fluid-containing systems, likewise, are usually subject to scaling or rusting or both during use to an extent requiring at least occasional and usually regularly scheduled cleaning.

One of the methods of removing such deposits from fluid systems, particularly of the types aforementioned, involves the introduction into the system of a liquid deincrustant, usually an acid solution. In this method the active liquid is brought into contact with the incrustation to be removed. the removal being based upon either the chemical or physical dissolving action of the liquid. After the incrustant removing liquid has done its work the spent liquid containing the decomposed, dissolved, or disintegrated incrustations is drained from the system. The system is them rinsed by repeated filling and draining after which it may be returned to service.

In applying the foregoing procedure to a wide variety of fluid systems subject to scaling, such as steam generators (boilers), evaporators, and other pressure vessels, heat exchangers, condensers, and the like, particularly with aqueous acid solutions, e. g. 5-25 per cent solutions of hydrochloric acid, we have discovered many difliculties arise which militate against the usefulness of the method.

One of these is that an inordinately large amount of rinsing is required to remove sufficiently the active ingredients of the treating solutions and dislodged incrustants or deposits from the equipment to permit it to be returned to service and the method is thus wasteful of rinsing liquid. This factor also leads to an added disadvantage of excessively prolonging the down time of the equipment as the rinsing operation.

is time-consuming. Another disadvantage accruing to conventional rinsing involved in such cleaning operations is that cosiderable corro ion.

2 Claims. (Cl. 134--3) occurs of the metal while it is exposed to air after of rust. In addition, the oily, greasy, or tarry scum which usually collects on the surface of the treating solution as it acts upon and removes the scale deposits is redeposited upon the metal when the vessel being treated is drained of solution before therinsing step. Scum so deposited cannot be removed subsequently bymere rinsing. In addition to the aforementioned difiiculties, when an acid-descaling solution is used, there is an explosion hazard existing between the steps of draining the treated vessel and applying the rinse due to accumulation of hydrogen in the vessel, the hydrogen being formed as a result of the corrosive action of the acid-descaling solution on the metal of the vessel. Some of the hydrogen thus produced is absorbed in the spent acid and is released into the vessel as it is being drained of the spent solution, and, together with the hydrogen accumulated during treatment, forms an explosive mixture with air drawn into the vessel at the same, time. To guard against serious explosions, great care must be exercised to avoid igniting such mixtures.

We have now found that the foregoing and other disadvantages inherent in the conventional method of descaling fluid-containing systems are overcome and an enhanced cleaning effect is obtained by an improved method which constitutes the subject of the present invention.

According to the method contemplated by our invention, the vessel or system, to be freed from deposits removable by a liquid deincrustant or descaling agent, is filled with'the liquid deincrustant, the composition of which is selected in accordance with the character of the deposits to be removed. Most of the deposits of rust and scale found in steam generating equipment are removable with hydrochloric acid solutions, e. g. 5-25 per cent aqueous hydrochloric, and this solution is, therefore, preferably used. In some cases, it is advantageous to add a fluoride to the acid solution as described in U. S. Patent No. 2,094,479. To minimize the corrosive action on metals, a suitable inhibitor, such as a soluble-arsenic compound, a mercaptan, or an organic nitrogen base, is usually added, as is well known in the art, e. g. U. S. Patent No. 1,877,504. In some cases, as for example, when the scale is nearly all calcium sulfate, a strong aqueous solution of caustic soda may be used as the deincrustant, e. g. 10-50 per cent of NaOH by weight.

The vessel to be treated is then charged with descaling liquid or deincrustant, the amount used bein usually suflicient to fill the vessel. The actual depth of fill is regulated in accordance with the location of the deposits to be removed. In some cases, for example in steam boilers, these re filled to, but not usually beyond, the normal water line maintained during steam generation.

In thus charging the vessel, it is preferable to introduce the descaling liquid at the lowermost point in the system and to provide a gas vent at the uppermost point so that during the introduction of the liquid the gases in the system will be displaced toward, and expelled from, the vent.

After filling the vessel to the desired depth, or completely filling it if necessary or desirable, the descaling solution is left in contact with the deposits to be removed, the duration of this contact depending on the rate at which the solution attacks and removes the deposits. In many instances, the rateand effectiveness of the action of the deincrustant solution can be greatly enhanced by raising the temperature of the solution and metal surface in contact with it above normal atmospheric temperatures. The usual temperatures to employ are from about 60 F. to as much as 125 F. to 180 F., when inhibited hydrochloric acid is used. Higher temperatures may be employed for short intervals without excessive corrosion but the added advantage of faster action at higher temperatures is generally off-set by increased corrosive action of the solution on the equipment being treated. Even though the production of some hydrogen is inevitable when acid solutions are used, even with the best available inhibitors, the amount of the acid attack on the metal is insignificant and the amount of metal so removed is usually less than when mechanical scrapers are used for removing the scale.

The length of time of exposure of the deposits to the removal action of the descaling solution to effect a removal of the deposits may be as little as ten minutes, with easily attacked deposits, to as much as four to six hours or more under the most unfavorable conditions and with the most refractory deposits. It is advantageous to test the activity of the descaling solution during its descaling action as by making a titration for its acidity (when an acid solution is used) or to determine the concentration of dissolved solids or to make other suitable analytical determinations so as to ascertain when the descaling action has ceased or become too slow to warrant its further use.

After the descaling action has proceeded to the desired extent or when the descaling solution has become spent, the equipment or vessel is then drained from the lowermost point and simultaneously charged with a rinsing liquid introduced at an upper point in the system, the rinsing liquid being introduced at a rate such that it fully occupies the space vacated by the spent descaling solution, thereby preventing not only air from entering the system but also deposition on the treated surface of the vessel of the scum otherwise formed on the surface of the spent solution and deposited upon the treated surface of the vessel. At the same time, since by this method, air is excluded from the system, not only is there no explosion hazard, but, also, oxidation of clean metal is prevented.

In carrying out the displacement of the used descaling solution a rinse liquid is employed having a lower specific gravity than the spent descaling solution. In most cases water is used as it will usually dissolve the constituents which have been brought into solution by the aqueous descaling solution and it is of lower specific gravity. In order to prevent the descaled surfaces from rusting while in contact with rinse water it should be deaerated before use as by boiling such water preferably under reduced pressure or by adding a deoxidant, such as sodium sulflte, in amount stoichiometrically equivalent to the oxygen in the water.

Because the rinse liquid is of lower specific 5 gravity than the descaling solution, the two liquids remain essentially unmixed and stratified while displacement of one by the other occurs during removal of the spent descaling solution from the vessel. As a consequence very much less rinse liquid is needed to purge the vessel of descaling liquid. The amount needed does not materially exceed the volume of the vessel or system and at most need not exceed about 1.5 times the volume of the vessel. To attain a similar reduction in concentration of dissolved constituents in the rinse by conventional methods requires an amount of liquid upwards of three times the volume of the system and at least two or three times as long to perform the operation.

The following example is illustrative of the practice of the invention as applied to a conventional steam generator, reference being made to the accompanying drawing in which the single figure is a schematic view largely in vertical section of a conventional three drum steam generator equipped for carrying out the invention.

Referring more particularly to the drawing, the steam generator shown comprises the upper drum I provided with a feed water distributing trough 2, valved feed water inlet 3, and valved vent line 4. Vertical banks 5 of tubes connect the upper drum with the lower or mud drum 6 provided with a valved blow-down line I. Upper front drum 8 is connected to the drum l by banks of water tubes 9 and steam tubes l0, and to drum 6 by banks of water tubes I l. Steam leaves from the upper drum through pipe l2 into the saturated steam header I3 from which it is distributed through the bank of superheater tubes i4 connected to the superheated steam header l5. An external'downcomer I6 leads from the lower drum 6 to the header I? from which extend the tubes I 8 of the water wall tube bank to drum 6. The header I1 is provided with the valved blow-down line It. The furnace setting and its various accessories are not shown as they are not involved in the practice of the method. The normal water line during operation is approximately at the mid-point in depth of the drum I and correspondingly slightly over the mid-point in drum 8.

Details of example A steam generator of the foregoing type having a volume of 23,000 gallons to the normal water line, a heating surface of about 34,000 square feet, and normally operated at 400 pounds per square inch using chemically softened deaerated feedwater had become incrusted with scale during use. An average sampling of the 6 scale deposits from various parts of the system gave the following approximate analysis:

Per cent Caro(OH)z(PO4)e 40 Silicate 40 Iron oxides 20 The generator prior to treatment was cooled to about 180 F. and drained, all outlets being closed except the vent 4 and lowermost drain l9. While at 180 F. and immediately after draining the generator was charged with 23,000 gallons of a 9 per cent hydrochloric acid solution heated to about 180 F. through the blow-down line or drain IS, the lowermost point in the system. To the acid solution had been added 0.4 per cent by weight of a commercial preparation designated Reilly No. 22, manufactured by the Reilly Tar & Chemical Corp., and comprising organic nitrogen bases largely of the pyridine group as a corrosion inhibitor. This charging operation required about three hours during which the vent line 4 was left open and gases in the system were displaced through it while the descaling solution filled the generator to the mid-point of drum 1. The acid solution was left in the steam generator and maintained in the temperature range of 160 to 180 F. for about six hours, during which 736 cubic feet of hydrogen, measured at 355 C. and 1 atmosphere, was discharged from the vent.

Following the six hour period of detention of the acid solution in the generator, deaerated rinse water was introduced through the feed water inlet 3 displacing the gases in the space above the water line in the upper drums l and 8 through the vent 4. Rinse water also passed from drum 1 into the saturated steam headerl3 and tubes 14, thereby washing them. The lowermost drain l9 was then opened and the introduction of rinse water continued under a pressure of about 50 pounds per square inch, thereby maintaining the generator filled with liquid while displacing the spent acid solution through the drain. The liquid discharged from the drain during the displacement of the spent acid solution was sampled pe riodically and the samples tested for acidity. When the acidity (concentration of HCl) thus observed became less than 0.5 per cent the introduction of rinse water was stopped and the drain closed. The amount of rinse water thus used was 30,000 gallons. The level of the rinse water in the generator was lowered to the normal water level by opening drain l9 for a short time and about 500 pounds of -soda ash dissolved in water was introduced into the water in drum I to form a neutralizing solution for the residual acidity in the rinse water. The temperature of the generator was then raised bringing about circulation and boiling of the resulting alkaline solution, vent 4 being closed and the pressure allowed to remain at about 25 pounds per square inch. The alkaline solution thus used served to remove the last traces of the acid solution by neutralization and to' assist in liberating any hydrogen absorbed in the steel of the generator. After boiling the alkaline solution for about two hours the system was drained through I9 and examined. Examination showed the boiler drums and tubes to be free from ofly. greasy or tarry deposits, substantially free from incrustants, no corrosion was evident, and the generator passed all requirements of boiler insurance inspection.

Although the foregoing example is illustrative of the application. of the invention to cleaning steam generators using a hydrochloric acid solution, it is to be understood that the invention is not limited thereto but may be applied to other forms of liquid or fluid systems subject to incrustation and used-with other descaling liquids.

Among the advantages of the invention are: only a relatively small amount of rinse water is needed to accomplish athorough rinsing; incrustants suspended in the descaling liquid during the descaling operation are more easily and completely purged from the system; deposition of oily. greasy or tarry matter on the descaled surfaces is prevented; reoxidation of the metal surface during cleaning and rinsing is prevented; there is a large saving of time. being as much as 75 per cent or more with the usual steam generator descaling operation; and danger of hydrogen explosions when acid solutions are used on metal systems is obviated.

What is claimed is: i

1. In a method of treating a fluid-containing system subject to accumulation of deposits of incrustants on the working surfaces thereof requiring periodic cleaning so as to effect removal of such deposits, the steps which consist in introducing into the system an aqueous solution containing from 5 to 25 per cent of HCl and a corrosion inhibitor, the volume of the HCl solution being sutlicient to fill the system to the level of the uppermost incrustations while venting the system of gases at the uppermost point, maintaining the HCl solution in the system at a temperature between about 60 F. and 180 F., thereby permitting the deincrustant to remove the incrustations, thereafter displacing the spent HCl solution with water introduced into the system near the uppermost point while draining the spent HCl solution from the lowermost point so as to maintain the system filled with liquid, the rinse liquid being stratified above the spent HCl solution, said introduction of water being continued until the concentration of HCl in the displaced HCl solution is less than about 0.5 per cent, introducing sodium carbonate into the water in the uppermost portion of the fluid-containing system,

' bringing the water in the system to a boil so as to mix the alkali with the water'and neutralize the HCl, and then draining the system from the lowermost point while admitting air,

2. In a method of treating a fluid-containing system subject to accumulation of deposits of incrustants on the internal surfaces thereof requiring periodic cleaning so as to effect removal of such deposits, the steps which consist in introducing into the system an'aqueous solution containing from 51:0 25 per cent of HCl and a corrosion inhibitor, the volume of the H Ci solution being sufllcient to fill the system to the level of the up most incrustations while venting the system of gases at the uppermost point, maintaining the HCl solution in the system at a temperature between about 60 and F., thereby permitting the deincrustant to remove the incrustations, thereafter displacing the spent HCl solution with water introduced into the system near the uppermost point while draining the spent HCl solution' from the lowermost point so as to maintain the system filled with liquid, the rinsing water being stratified above the spent HCl solution. said introduction of water being continued until the concentration of HCl in the displaced solution is less than about 0.5 per cent.

- MELVIN E. BRINES.

THEODORE W. BAR/GE. L

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number

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