Method for cleaning and protecting steam turbine condenser tubes from carbonate deposits

Abstract

FIELD: thermal power engineering.SUBSTANCE: maintenance of thermal power plants (TPP), used for cleaning and protecting heat exchange surfaces from carbonate deposits and dezincification, in particular the condenser tubes of steam turbines cooled by mineralized water. A method for cleaning the condenser tubes of a steam turbine from carbonate deposits, which includes the use of an inhibited acidic cleaning solution based on formic acid (cleaning solution), circulation of the cleaning solution through the condenser tubes, removal of the cleaning solution with dissolution products and subsequent hydraulic flushing. According to the invention, preliminary hydrodynamic flushing of the condenser tubes is carried out under a pressure of 100–250 kgf / cm2 before circulation of the cleaning solution. After removal of the cleaning solution, intermediate hydraulic flushing and thermal drying of the tubes at a temperature of 30–80 °C are conducted. After thermal drying, a final hydraulic flush is carried out, and then a protective coating is formed on the surface of the tubes using an aqueous emulsion of a surfactant. A cleaning solution is used, which is a 3–7 wt.% solution of inhibited formic acid, which is heated to a temperature of 40 to 60 °C. The protective coating is formed using an aqueous emulsion of a film-forming amine, which is maintained at a temperature of 65 to 80 °C until the equilibrium concentration of amine is established on the metal surface, after which the aqueous emulsion of film-forming amine is removed.EFFECT: increase in the cleaning efficiency and an increase in the service life between cleanings of the condenser tubes.3 cl, 8 dwg

Description

[0001] Technical field

[0002] The invention relates to thermal power engineering, namely to the maintenance of thermal power plants (TPP), and can be used for cleaning and protecting heat exchange surfaces from carbonate deposits and dezincification, in particular the tubes of steam turbine condensers cooled by mineralized water.

[0003] Technology Level

[0004] A hydrodynamic cleaning system for the inner surface of heat exchanger tubes is known (RU 2771538, IPC F28G 1 / 16). The hydrodynamic cleaning system for the inner surface of a heat exchanger tube, comprising a high-pressure hose and a pumping station, further comprises an electromechanical cleaning robot with a digital software control unit, including a retractable hollow rod, which is made of an elastic material and is connected at the input through a high-pressure hose to the output of the pumping station, mechanisms for two-coordinate longitudinal and transverse reciprocating movement of the rod from the drives of the corresponding electric motors with the possibility of its positioning, as well as at least one differential sensor for the location of the rod and the speed of its movement, connected via a signal output to a signal input of the electromechanical cleaning robot, the control outputs of which are connected to the control inputs of the corresponding electric motors.

[0005] A disadvantage of this method is the inability to completely remove mixed carbonate and corrosion deposits from the inner surface without causing unacceptable damage to the tube walls. The method does not remove corrosion products (Zn and Cu oxides) tightly adhering to the tube surface. This limits its application for completely removing dense deposits without damaging the tube walls, making it unsuitable for this task.

[0006] A method for ball cleaning of steam turbine condensers is known (RU 2058008, IPC F28G 1 / 12), which consists of using elastic balls made of porous rubber circulating in a closed circuit through condenser tubes, preventing and removing deposits on the tube walls.

[0007] A disadvantage of the ball cleaning method is its preventative nature, making it unsuitable for removing established carbonate corrosion deposits. Its key drawbacks include the inability to break up dense deposits with high surface adhesion, the risk of ball jamming in defective tubes, and the potential for increased dezincification due to the abrasive effect on the protective brass film. Furthermore, operating a ball cleaning system involves significant ongoing costs, including the cost of refilling the balls.

[0008] Methods for mechanical cleaning of condenser tubes are known (RU 2300068, IPC F28G 1 / 02, RU 2006121329, IPC F28G 3 / 00, RU 181461, IPC F28G 3 / 08, RU 208825, IPC F28G 3 / 10). Mechanical cleaning of condensers is performed with bristle brushes, roller cutters, drills, drills fixed on long cleaning rods and driven manually or by a motor, as well as rubber cylinders or pistons pushed through the tube with cleaning rods, water or air under pressure using special guns, which requires significant manual labor and time.

[0009] Using abrasive or drilling tools creates a direct risk of mechanical damage, rupture, or deformation of the walls. This risk is especially critical for long tubes with deflections and for tubes already weakened by the dezincification process. These methods do not guarantee a completely clean surface. Mechanical action destroys the natural protective oxide film, which subsequently intensifies corrosion processes and re-fouling.

[0010] A method for cleaning turbo-generator condenser tubes using thermal drying is known (RU 2181470, IPC F28G 1 / 16). This method is designed to combat biological deposits. It involves drying the inner surfaces of the tubes at a temperature of 40-70°C. The deposits dry, crack, and flake off the tube surface. The dried, flake-off deposits are removed with water.

[0011] The main drawback of this method is its narrow focus—it is ineffective for removing carbonate-corrosion deposits tightly adhered to the surface of the pipes. This makes it unsuitable for the task at hand.

[0012] A known method for cleaning and preventing deposits on capacitor surfaces uses surfactants (SUF) (RU 2602653 IPC F01K 13 / 00, priority dated May 6, 2015). The method involves forming a polymolecular adsorption anticorrosive surfactant film on the inner surface of the condenser tubes. The adsorbed layer provides a positive shift in the surface potential and smooths the microrelief by filling microcavities, which hinders the attachment and growth of crystallization nuclei. These effects help slow the rate of accumulation of new deposits during operation. In the presence of an existing deposit layer, the disjoining pressure created by the adsorbed surfactant film can lead to its partial loosening and delamination without chemical interaction with the base metal.

[0013] The disadvantages of this method include low cleaning speed and efficiency, as the loosening process is extremely slow to break up dense, multilayered, and carbonate-corrosion deposits due to diffusion limitations of surfactant penetration and insufficient wedging pressure to break up monolithic layers. This method does not ensure complete removal of loosened deposits, necessitating the use of additional hydraulic flushing.

[0014] A known method for chemically cleaning condenser tubes from mineral deposits is using an inhibited hydrochloric acid solution (RD 34.22.501-87 "Guidelines for Preventing the Formation of Mineral and Organic Deposits in Turbine Condensers and for Cleaning Them"). The method involves determining the amount of deposits, calculating the required volume of acid, preparing a cleaning composition based on 2-5% technical hydrochloric acid with the addition of corrosion inhibitors and an antifoam agent, setting up a circulation loop, filling it with a cleaning solution, circulating the cleaning solution in a closed loop, followed by neutralization and hydraulic flushing of the condenser.

[0015] A disadvantage of this method is the high corrosiveness of the cleaning solution toward the tube material. Even in the presence of inhibitors, the acidic environment intensifies corrosion of the brass alloy, especially in areas where dezincification has already begun. During rinsing, the corrosion rate of brass can exceed permissible limits by two or more times. Given the uneven distribution of deposits across the inner surface, this leads to preferential corrosion of the least contaminated areas. These factors make the use of hydrochloric acid-based cleaning solutions undesirable for cleaning capacitors due to the unacceptable risk of damaging the base metal.

[0016] The closest in technical essence to the proposed invention is a method for chemically cleaning condenser tubes from carbonate deposits using an inhibited 5-7% formic acid solution (RD 34.22.501-87 "Methodological guidelines for preventing the formation of mineral and organic deposits in turbine condensers and their cleaning"). The method includes determining the amount of deposits, calculating the required volume of acid, preparing a cleaning composition based on formic acid and corrosion inhibitors, organizing a circulation circuit, filling it with a cleaning solution, circulating the cleaning solution in a closed circuit, followed by neutralization and water rinsing of the condenser.

[0017] The disadvantages of this technical solution are the high consumption of the cleaning composition, due to the need to dissolve not only carbonates, but also associated alluvial and organic deposits, the low rate of dissolution of dense carbonate deposits, the inability to remove corrosion products tightly adhering to the surface (zinc and copper oxides), which is explained by the inhibitory effect of the components of the composition, as well as the absence of a passivation stage of the cleaned surface, which does not allow suppressing subsequent processes of deposit accumulation and selective corrosion.

[0018] Disclosure of the essence of the invention

[0019] Thus, the problem that the claimed invention is aimed at solving is the creation of an effective, safe for equipment and economically feasible method for cleaning condenser tubes from carbonate deposits, ensuring not only their complete removal, but also the formation of a protective coating on the cleaned surface that slows down the processes of deposit accumulation and corrosion during further operation.

[0020] The invention is aimed at solving the associated problem of selective corrosion (dezincification) of brass tubes.

[0021] The technical result of using the invention consists in increasing the cleaning efficiency and increasing the service life of the condenser tubes between cleanings.

[0022] The technical result is achieved by using a method for cleaning the tubes of a steam turbine condenser from carbonate deposits, which includes the use of an inhibited acidic cleaning solution based on formic acid (cleaning solution), circulation of the cleaning solution through the tubes of the condenser, removal of the cleaning solution with dissolution products and subsequent hydraulic flushing, according to the invention, before circulating the cleaning solution, preliminary hydrodynamic flushing of the condenser tubes is carried out under a pressure of 100-250 kgf / cm 2 After removing the cleaning solution, an intermediate hydraulic flush and thermal drying of the tubes is carried out at a temperature of 30-80°C; after thermal drying, a final hydraulic flush is carried out, and then a protective coating is formed on the surface of the tubes using an aqueous emulsion of a surfactant.

[0023] The washing solution used is a 3-7% by weight solution of inhibited formic acid, which is heated to a temperature of 40 to 60°C.

[0024] The protective coating is formed using an aqueous emulsion of a film-forming amine, which is maintained at a temperature of 65 to 80°C until an equilibrium concentration of the amine is established on the metal surface, after which the aqueous emulsion of the film-forming amine is removed.

[0025] Brief description of drawings

[0026] Fig. 1 shows a typical appearance of deposits on the inner surface of the samples before hydrodynamic cleaning.

[0027] Fig. 2 shows a typical appearance of deposits on the inner surface of samples after hydrodynamic cleaning at a pressure of 100 kgf / cm 2 .

[0028] Fig. 3 shows a typical appearance of deposits on the inner surface of samples after hydrodynamic cleaning at a pressure of 180 kgf / cm 2 .

[0029] Fig. 4 shows a typical appearance of deposits on the inner surface of samples after hydrodynamic cleaning at a pressure of 250 kgf / cm 2 .

[0030] The images of Fig. 1-4 were obtained using a scanning electron microscope.

[0031] Fig. 5 shows a micrograph of the surface of the condenser tubes, illustrating the original state (before cleaning).

[0032] Fig. 6 shows a micrograph of the surface of the condenser tubes, illustrating the state after hydrodynamic cleaning.

[0033] Fig. 7 shows a micrograph of the surface of the condenser tubes, illustrating the state after chemical cleaning and thermal drying.

[0034] Fig. 8 shows a micrograph of the surface of the condenser tubes, illustrating the state after final hydraulic washing (clean surface).

[0035] Implementation of the invention

[0036] The claimed method for cleaning and protecting steam turbine condenser tubes from carbonate deposits is implemented in the following sequence of steps.

[0037] The first stage involves preliminary hydrodynamic cleaning. For this, after opening the condenser's water chambers, high-pressure water is injected into each tube sequentially. The treatment is carried out at a pressure of 100 to 250 kgf / cm. 2 (from 10 to 25 MPa). The choice of this range is due to the fact that at pressures below 100 kgf / cm 2 crack formation in dense carbonate deposits does not occur, and the pressure exceeds 250 kgf / cm 2 This step leads to the risk of damaging the tube walls. This step removes the loose top layer of deposits and creates a network of microcracks in the remaining dense layer, reducing the overall volume of contaminants and preparing the deposit morphology for subsequent chemical treatment.

[0038] The second stage involves chemical flushing. This involves creating a circulation loop comprising the condenser tube space, a circulation pump, solution preparation tanks, and a sump. An inhibited acidic cleaning solution based on formic acid is circulated through the condenser tubes. The solution is used at a concentration of 3 to 7% by weight of inhibited formic acid and heated to a temperature of 40 to 60°C. These parameters are optimal: at a concentration of less than 3%, flushing time exceeds the technologically permissible limit, and at a concentration of more than 7%, unnecessary reagent consumption occurs without accelerating cleaning. The temperature regime is optimized to intensify the dissolution of carbonate deposits (calcium and magnesium salts).At temperatures below 40°C, the rate of carbonate deposit dissolution slows, and at temperatures above 60°C, the corrosion inhibitors contained in the cleaning solutions cease to be effective. After circulation, the cleaning solution, along with the dissolution products, is removed by displacement with process water. An intermediate hydraulic flush is then performed to remove any remaining cleaning solution from the tube surfaces.

[0039] The third stage involves thermal drying. With the condenser covers open, the tubes are heated to a temperature of 30 to 80°C. These parameters are optimal: at temperatures below 30°C, the drying time exceeds the technologically permissible limit, while the upper limit is limited by the operating instructions for steam turbine condensers to prevent thermal deformation or damage. Heating can be accomplished, for example, by filling the steam path with hot water or by introducing steam. The drying process continues until residual contaminants are visibly removed. The goal of this stage is dehydration and subsequent cracking of the wall-bound deposit layer, which is enriched with poorly soluble zinc compounds that were not removed in the previous stages.

[0040] At the fourth stage, a final hydraulic flush is carried out to remove all exfoliated cleaning products from the cavity of the tubes.

[0041] In the fifth and final stage, the protective coating is formed. An aqueous emulsion of a surfactant, such as an aqueous emulsion of a film-forming amine such as octadecylamine, is injected into the cleaned and rinsed tubes. The emulsion is maintained at a temperature of 65 to 80°C until the amine concentration on the metal surface reaches equilibrium, after which it is removed. Maintaining the temperature within this range is critical: the lower limit (65°C) is required to create a high-quality emulsion, as the melting point of higher aliphatic amines is 48-62°C. The upper limit (80°C) is limited by the operating instructions for steam turbine condensers to prevent thermal deformation or damage.

[0042] Thanks to the described sequence of stages, including the removal of loose and weakly adhering deposits and the creation of microcracks in dense deposits by hydrodynamic flushing, chemical dissolution of carbonates, thermal destruction of the corrosion layer and final passivation, a synergistic effect is achieved: complete removal of all types of contaminants with minimal consumption of reagents and the formation of long-term protection against corrosion (in particular, against dezincification) and re-formation of deposits.

[0043] The effectiveness of the claimed method is confirmed by the results of experimental studies.

[0044] The studies were conducted on experimental specimens made of brass tubes (L68 alloy) with operational carbonate deposits (CaO > 40%, loss on ignition > 30%, (CuO + ZnO) > 7%). Analysis of the initial state and after various cleaning stages was performed using a scanning electron microscope with an energy-dispersive X-ray spectrometer.

[0045] An analysis of the effectiveness of hydrodynamic cleaning using a high-pressure system revealed its inadequacy. Metallographic analysis showed that standard hydrodynamic cleaning primarily removes the upper loose layer, reducing deposit thickness by 35-50%. However, the remaining layer exhibits an increased number of cracks and delaminations. Complete removal of the dense wall layer is impossible with this method without risking damage to the pipe wall. Increasing the operating pressure above 250 kgf / cm 2 leads to damage to the condenser tubes. At operating pressures below 100 kgf / cm2 crack formation does not occur in dense carbonate deposits (Fig. 1-4).

[0046] A comparative analysis of cleaning compositions for chemical cleaning was conducted. Three compositions (MC) were studied:

[0047] MK-A and MK-B are commercial formulations based on inhibited formic acid.

[0048] MK-V is a laboratory composition based on 2-5% inhibited hydrochloric acid, prepared according to the method described in RD 34.22.501-87 “Methodological guidelines for preventing the formation of mineral and organic deposits in turbine condensers and their cleaning.”

[0049] Tube samples 6 cm long, with a protected outer surface, were immersed in 400 cm 3 solution and kept for 7 hours at 50-60°C.

[0050] MK-V showed an unacceptably high corrosion rate of brass - 10.2 g / (m 2 ⋅h), which exceeds the norm of 5 g / (m 2 ⋅h). MK-A and MK-B showed acceptable and comparable activity: 2.7 and 2.6 g / (m3) 2 ⋅h) respectively.

[0051] The cleaning efficiency (by the mass of deposits removed) was:

[0052] - on the original tubes: MK-A - 39.2%, MK-B - 36.1%;

[0053] - on tubes after preliminary hydrodynamic cleaning: MK-A - 94.8%, MK-B - 93.9%.

[0054] Thus, preliminary hydrodynamic cleaning increases the efficiency of subsequent chemical cleaning by 2.4-2.6 times.

[0055] Despite the high effectiveness of the combined treatment, it was not possible to completely clean the surface. Energy-dispersive X-ray analysis revealed the cause: the wall layer of deposits (the area immediately adjacent to the metal) is characterized by an abnormally high zinc content, which migrated from the tube material. Inhibited acidic compositions are unable to effectively dissolve zinc compounds within a technologically acceptable time.

[0056] To remove the residual zinc-rich layer, which becomes loose after chemical treatment, a thermal drying step was proposed. Laboratory tests confirmed that holding hydrodynamically and chemically cleaned samples at ~60°C for 2 hours, followed by a water rinse, resulted in complete removal of the residual deposits (Figs. 5-8).

[0057] Using the claimed method results in increased cleaning efficiency and an extended service life of condenser tubes between cleanings. This technical result is achieved through the synergistic effect of the stated sequence of steps, which manifests itself as follows:

[0058] - ensures complete removal of persistent carbonate-corrosion deposits, including the zinc-rich wall layer;

[0059] - the consumption of the cleaning composition is reduced due to preliminary hydrodynamic cleaning, which removes loose and weakly adhering deposits, and also creates cracks in the remaining tightly adhering deposits;

[0060] - forms a protective coating on the cleaned surface, which, on the one hand, slows down the rate of re-accumulation of deposits, and on the other hand, suppresses corrosion processes, in particular, dezincification of brass tubes, which together ensures an extension of the period between cleanings.

Claims

1. A method for cleaning steam turbine condenser tubes from carbonate deposits, including the use of an inhibited acidic cleaning solution based on formic acid - a cleaning solution, circulation of the cleaning solution through the condenser tubes, removal of the cleaning solution with dissolution products and subsequent hydraulic flushing, characterized in that before circulation of the cleaning solution, preliminary hydrodynamic flushing of the condenser tubes is carried out under a pressure of 100-250 kgf / cm 2 After removing the cleaning solution, an intermediate hydraulic flush and thermal drying of the tubes is carried out at a temperature of 30-80°C; after thermal drying, a final hydraulic flush is carried out, and then a protective coating is formed on the surface of the tubes using an aqueous emulsion of a surfactant.

2. The method according to claim 1, characterized in that a cleaning solution is used, which is a 3-7% by weight solution of inhibited formic acid, which is heated to a temperature of 40 to 60°C.

3. The method according to claim 1, characterized in that the protective coating is formed using an aqueous emulsion of a film-forming amine, which is maintained at a temperature of 65 to 80°C until an equilibrium concentration of the amine is established on the surface of the metal, after which the aqueous emulsion of the film-forming amine is removed.

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