US 3000767 A
Description (OCR text may contain errors)
3000767 METHOD OF cLEANrN INTERNAL FERROUS METAL SURFACES OF ST'E Q NT AM GENERATING The invention relates to methods of cleaning the internal ferrous metal surfaces of steam generating equipment.
The principal object of the invention it the provision of an improved method of removing certain incrustations from the internal ferrous metal surfaces of high pressure steam generating equipment which is operated in .CODIICCUOII with a condenser whose condensing surface is of copper alloy.
In the operation of high pressure steam generating equipment (over 600 pounds per square inch steam pressure) in which the feed water is largely returned condensate from a copper alloy condenser, incrustations are usually produced upon the steam generating surfaces of the steam generator despite the fact that the feed water 1s substantially pure. These incrustations often-times contaln copper, both in metallic form and combined with oxygen, corroded from the copper alloy condenser by the action of the condensed steam which carries the copper to the steam generator.
Attempts to remove such incrustations, as by the use of conventional acidizing procedures, are not successful. Tests have shown that by acidizing incrusted steam generating surfaces of the usual high pressure steam generator, having copper in the incrustation, some of the copper is removed from the incrustation and redeposited elsewhere on the surfaces of the steam generating equipment during the acidization so that only a partial removal of copper from the incrusted surface results.
With the exception of the method herein disclosed, I believe there is no method commercially available for treating the internal ferrous metal surfaces of high pressure steam generating equipment subject to the deposition of copper containing incrustations so as to free the surfaces of the incrustations and the copper. Those skilled in the art will recognize that the redeposition of the copper occurs by reason of the acid used; an aqueous hydrochloric acid, forming an electrolite which plates the copper on the metal surfaces of the steam generating equipment, etc.
In order to remove the incrustations and the copper from such steam generating equipment completely I have determined that it is necessary to remove most of the copper as a first step and secondly remove the incrustations and the remaining copper in a manner so that no replating of the metal surfaces is possible. In order to do so the two treatment steps must be applied to the ferrous metal surface from whch the copper containing incrustation is to be removed. These steps are as follows:
(a) The incrusted surface is first treated with an aqueous oxidizing solution, such as one of the following:
Type A.Chlorate type containing sodium chlorate, ammonia and ammonium sulphate.
Type B.-Chlorate with added bromate type containing sodium chlorate, slight added amounts of potassium bromate, ammonia and ammonium nitrate.
Type C.Persulphate type containing ammonium persulphate, ammonia and sodium hydroxide.
Type D.Bromate with added chlorate type containing potassium bromate, slight added amounts of sodium chlorate ammonia, and ammonium carbonate.
The solvents of type A and type B should be used at temperatures in excess of 175 F. to rapidly oxidize and ted States Patent Patented Sept. 19, 1961 dissolve copper. In the type B solvents containing chlorate with slight added amounts of bromate operational temperatures should be between F. and 190 F. as the bromate acts first and limited amounts of chlorate react depending on the temperatures maintained. The type C solvent may be used at relatively low temperatures, for example, as low as 70 F. The type D solvent is used at temperatures of 140 F. to F.
Each of the solvents is used by filling the steam generator with a sufficient volume of the solution to cover the incrustations. The solution is preferably circulated through the equipment for approximately three hours, tests being made at regular intervals to determine the percentage of copper in the solution. The following examples are illustrative of solutions usable in this step. The proportions are by gallons and pounds:
to the reaction of the oxidizing agent on metallic copper to form cupric oxide which in turn is dissolved in the ammoniacal solution to form a soluble cupric-ammonium complex ion. It is necessary that excess oxidizing material be available in the solvent throughout the treatment period. The ammonia present must be maintained at a uniformly high level of concentration, and the cupricammonium ion concentration must be uniform throughout the solvent. I I I If the amount of copper to be dissolved from the boiler is underestimated and the bromate available for oxidation is depleted, additional bromate may be conveniently added to the recirculating line to correct this condition while the treatment is still in progress. An excess of oxidant is assured throughout the treatment since the chlorate present in the solvent is practically nonreactant at the indicated temperature conditions. The use of ammonium carbonate in place of ammonium nitrate or ammonium sulphate in the solvent results in a noticeable decrease in the amount of iron oxide formed.
Data from actual boiler cleaning operations with the type D solvent indicates that 0.7 of a pound of copper is oxidized and dissolved for each pound of bromate con sumed. This overall treatment efficiency in terms of oxidant used of about 70% is much higher than the efficiency obtained with any of the other types of solvents.
When indicated copper pick-up becomes negligible during two successive hourly readings, with ample bromate present for oxidation, the boiler should be drained. Two rinses with water will pick up most of the free ammonia remaining after the solvent has been drained.
The second step B comprises treating the incrusted surface with an aqueous hydrochloric acid solution having an HCl content of 5% to 25% as by filling the steam generator with a suflicient volume of the acid solution to cover the incrustation. The acid solution is modified by the addition of a suflicient thiourea (H NCS-NH to form a sequestering agent with respect to copper remaining in the incrustation. The use of the sequestering agent prevents such copper as is freed from the incrustations during the action of the aqueous hydrochloric acid from plating out on the metal surfaces in the steam generating equipment as would otherwise be the case. Those skilled in the art will observe that if desired a corrosion inhibitor may be added to the acid solution to reduce the rate of attack upon the ferrous metal underlying the incrustation. A suitable operating temperature is about 150 F. and the acid solution is allowed to remain in contact with the incrustation preferably until the dissolving action substantially ceases. The spent solution is then drained and the equipment flushed with water to which soda ash or a similar chemical has been added to act as a neutralizing agent.
The foregoing two step method will be found to completely remove copper containing incrustations formed upon the steam generating surfaces in high pressure steam generating equipment using the condensed steam from copper alloy condensers as the feed Water. The foregoing two step method completely eliminates the undesirable plating out of the copper in the equipment which has occurred when previously known methods of treating such equipment with acid solutions were employed; the present method, by first removing a majority of the copper and secondly preventing the remaining copper from plating out during the acid treatment achieves the objects of the invention and having thus described my invention, what I claim is:
1. The method of removing copper-containing incrustations deposited upon the ferrous metal surfaces of high pressure steam generating equipment which comprises the steps of first inundating the incrusted surfaces With an aqueous ammonia solution containing ammonium hydroxide, potassium bromate and ammonium nitrate as an oxidizing agent and allowing the solution to remain in contact with the incrustation until the solution no longer increases in copper content, removing the spent aqueous ammonia solution of the oxidizing agent from the treated ferrous metal surface and rinsing the same, subsequently inundating the ferrous metal surfaces with an aqueous solution of hydrochloric acid containing 5% to 25% of HCl and containing thiourea (H NCS-NH as a sequestering agent and allowing the acid to act upon the incrustation until the dissolving action has substantially ceased, removing the resulting spent acid and 4 sequestered copper from the treated surface and rinsing the same.
2. The method of removing copper containing incrustations deposited upon the ferrous metal surfaces of high pressure steam generating equipment which comprises the steps of first inundating the incrusted surfaces with an aqueous ammoniacal solution containing ammonium hydroxide, potassium hromate, ammonium nitrate and ammonium bifluoride as an oxidizing agent and allowing the solution to remain in contact with the incrustation until the solution no longer increases in copper content, removing the spent aqueous ammoniacal solution of the oxidizing agent from the treated ferrous metal surface and rinsing the same, subsequently inundating the ferrous metal surfaces with an aqueous solution of hydrochloric acid containing 5% to 25% of HCl and containing thiourea (H NCS-NH as a sequestering agent and allowing the acid to act upon the incrustation until the dissolving action has substantially ceased, removing the resulting spent acid and sequestered copper from the treated surface and rinsing the same.
3. The method of removing copper-containing incrustations deposited upon the ferrous metal surfaces of high pressure steam generating equipment which comprises the steps of first inundating the incrusted surfaces with an aqueous ammoniacal solution containing aqua ammonia, potassium bromate and ammonium carbonate as an oxidizing agent and allowing the solution to remain in con- References Cited in the file of this patent UNITED STATES PATENTS 2,396,938 Bersworth Mar. 19, 1946 2,461,228 Miles Feb. 8, 1951 2,567,835 Alquist et al. Sept. 11, 1951 2,628,199 Lowenheim Feb. 10, 1953 2,674,523 McDonald Aug. 6, 1954 2,710,792 McDonald et al. June 14, 1955