METHOD FOR INHIBITING CORROSION FATIGUE IN AN EVAPORATION TUBE IN A BOILER

Abstract

METHOD FOR REDUCING CORROSION FATIGUE IN EVAPORATION TUBE IN BOILERS. The present invention relates to the effective reduction of corrosion fatigue in an evaporation tube in a boiler that occurs in association with a corrosive environment or repeated application of stress due to the presence of scale. The invention also relates to a method for reducing corrosion fatigue in an evaporation tube in a boiler, in which each chloride ion concentration and the sulfate ion concentration in the boiler water is managed to 10 mg / l or less. It is preferable to manage each chloride ion concentration and the sulfate ion concentration in the boiler water by subjecting the boiler feedwater to a desalination treatment with an ion exchange device, a reverse osmosis membrane device, or an electrodeionization device, or by increasing the boiler condensate collection rate.

Description

1 / 16 Descriptive Report of the Invention Patent for METHOD FOR INHIBITING CORROSION FATIGUE IN AN EVAPORATION TUBE IN A BOILER. TECHNICAL FIELD

[001] The present invention relates to a method for inhibiting corrosion fatigue of an evaporation tube in a boiler. In particular, the present invention relates to a method for effectively inhibiting corrosion fatigue of an evaporation tube in a boiler associated with a corrosive environment and repeated stress due to the presence of scale. BACKGROUND OF THE TECHNIQUE

[002] In recent years, many boilers on the low-pressure boiler market have a heat transfer area of ​​10 to 40 m2, and in particular, a compact multi-tube special circulation boiler, consisting only of a header, a steam separator, and an evaporation tube (hereinafter referred to as a small-type single-pass boiler) has the overwhelming share. The small-type single-pass boiler is frequently operated with a multi-vessel installation, in which a plurality of boilers are installed and operated with repeated starts and stops at many times, depending on the amount of steam demand. Water is fed particularly to the bottom of the evaporation tube with a circulation rate of 2 or less. Thus, the proximity to the bottom presents a low pH and a high level of dissolved oxygen, making corrosion likely.In many cases, corrosion fatigue resulting from this corrosive environment, and the repeated stresses generated by temperature changes due to boiler start-ups and shutdowns, cause cracks in the evaporator tube, leading to water leaks.

[003] It is difficult to completely avoid this corrosion fatigue. Petition 870240064615, dated 07 / 31 / 2024, page 6 / 45 2 / 16 only by conventional treatment of the boiler feedwater (hereinafter also referred to as boiler feedwater). Consequently, cracks may occur in the evaporator tube to cause water leakage in approximately three years at a minimum.

[004] With insufficient management of boiler feedwater quality, a fouling component often causes contamination of the boiler feedwater, generating scale that adheres to the boiler. In particular, scale adhesion to a boiler heat transfer surface inhibits heat transfer, decreasing the boiler's thermal efficiency and leading to increased fuel costs. Thus, preventing scale adhesion, in addition to preventing corrosion fatigue, is necessary for the boiler.

[005] The technique for inhibiting corrosion and the technique for inhibiting scale adhesion are conventionally reported separately.

[006] For example, measures to inhibit corrosion include the following: (1) to (3). (1) The pH of the water in a boiler steam generator (hereinafter, boiler water) is maintained within an upper limit of a standard control value. (2) The concentration of an oxygen scavenger is kept elevated within a standard control value. (3) The bottom of the vessel is not blown out immediately after the end of the operation, but immediately before the start of the operation, to inhibit contamination by high concentrations of oxygen, and to inhibit a reduction in pH.

[007] Measures to inhibit fouling include the following: (1) a Petition 870240064615, dated 07 / 31 / 2024, page 7 / 45 3 / 16 (3) . (1) Reverse osmosis membrane treatment or boiler feedwater softening treatment is carried out. (2) In order to inhibit the generation of scale, a scale dispersant or a boiler compound (e.g., PTL 1 and 2) is added. (3) Water blow-off control in the boiler is carried out to eliminate the scale component. LIST OF QUOTES PATENT LITERATURE

[008] PTL 1: JP 2017-74550A

[009] PTL 2: JP 2017-12991A SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0010] Corrosion fatigue of the evaporation tube in the boiler cannot be sufficiently inhibited by conventional corrosion inhibition methods in some cases.

[0011] The present inventors investigated a cause of corrosion fatigue and, as a result, discovered that, in many cases, scale adhering to a portion with a high heat transfer load, such as the evaporation tube, increases the temperature of the evaporation tube wall, causing the tube to expand and contract with the boiler's start-up and shutdown operation. Consequently, increased stress and corrosion fatigue break the evaporation tube, causing water leakage. From this discovery, the inventors determined that corrosion fatigue of the evaporation tube in the boiler is significantly associated not only with the corrosive environment but also with the presence of scale. Petition 870240064615, dated 07 / 31 / 2024, page 8 / 45 4 / 16

[0012] One objective of the present invention is to provide a method for effectively inhibiting corrosion fatigue of the evaporator tube in the boiler associated with the corrosive environment and repeated stress due to the presence of scale. SOLUTION TO THE PROBLEM

[0013] The present inventors have discovered that by controlling the concentration of chloride ions and the concentration of sulfate ions in the boiler water to be low, corrosion can be sufficiently inhibited and corrosion fatigue of the evaporator tube can be effectively inhibited. Furthermore, the present inventors have discovered that adding a scale dispersant to the boiler water to inhibit scale adhesion can further inhibit corrosion fatigue of the evaporator tube.

[0014] The present invention is summarized as follows.

[0015] [1] A method for inhibiting corrosion fatigue of an evaporation tube in a boiler, comprising controlling each of a chloride ion concentration and a sulfate ion concentration of the water in the boiler, to 10 mg / L or less.

[0016] [2] The method for inhibiting corrosion fatigue of an evaporation tube in a boiler according to [1], wherein the chloride ion concentration and the sulfate ion concentration of the water in the boiler are controlled by treating the raw water used for boiler feedwater with an ion exchanger, a reverse osmosis membrane device or an electric deionizer, or by increasing the boiler condensate recovery rate.

[0017] [3] The method for inhibiting corrosion fatigue of an evaporation tube in a boiler according to [1] or [2], comprising adding a scale dispersant to the water in the boiler.

[0018] [4] The method for inhibiting corrosion fatigue in a pipe Petition 870240064615, dated 07 / 31 / 2024, page 9 / 45 5 / 16 evaporation in a boiler according to [3], wherein the scale dispersant is at least one poly(meth)acrylic acid compound selected from the group consisting of polyacrylic acid having an average molecular weight of 20000 to 170000 and a salt thereof, and polymethacrylic acid having an average molecular weight of 1000 to 100000 and a salt thereof.

[0019] [5] The method for inhibiting corrosion fatigue of an evaporation tube in a boiler according to [3] or [4], wherein a concentration of the scale dispersant in the boiler water is 1 to 1000 mg / L.

[0020] [6] The method for inhibiting corrosion fatigue of an evaporation tube in a boiler according to any one of [1] to [5], wherein the boiler is a small type single-pass boiler. ADVANTAGEOUS EFFECT OF THE INVENTION

[0021] According to the present invention, corrosion fatigue of the evaporator tube in the boiler, caused by the corrosive environment and repeated stress due to the presence of scale, is effectively inhibited in order to extend the service life of the boiler. BRIEF DESCRIPTION OF THE DRAWING

[0022] Figure 1 is a schematic view illustrating a water system flow in a boiler from a target test in Test Example I. DESCRIPTION OF MODALITIES

[0023] Next, embodiments of the present invention will be described in detail.

[0024] A method for inhibiting corrosion fatigue of an evaporation tube in a boiler according to the present embodiment is characterized by controlling each of a chloride ion concentration and a sulfate ion concentration of the water in the boiler, i.e., water in a steam generator in a boiler, at 10 mg / L or Petition 870240064615, dated 07 / 31 / 2024, page 10 / 45 6 / 16 less.

[0025] According to the present embodiment, each of the concentrations of the corrosive chloride ion and sulfate ion (hereinafter, these may be referred to as corrosive ions) is controlled to 10 mg / L or less, to inhibit corrosion of the evaporator tube that triggers corrosion fatigue. This configuration can inhibit corrosion fatigue.

[0026] In the present embodiment, a lower concentration of corrosive ions in the boiler water is preferable, and it is preferable to control each of the concentrations to 1.0 mg / L or less.

[0027] To adjust the concentration of corrosive ions in the boiler water to the upper or lower limit above, usable methods include: desalination treatment of raw water used for boiler feedwater with an ion exchanger (such as a multi-bed column type or mixed-bed column type ion exchanger), a reverse osmosis membrane device, or an electric deionizer; increasing the boiler condensate recovery rate; or similar methods. Two or more of these methods may be used in combination.

[0028] When the quality of the raw water varies, the quality of the treated drinking water will likely vary depending on the method used with the reverse osmosis membrane device. Therefore, it is preferable to use an electric deionizer in combination with it.

[0029] The concentration of corrosive ions in the boiler water is controlled, for example, by regulating the concentration of corrosive ions in the feedwater to the boiler to be equal to or lower than an upper limit of the concentration of corrosive ions in the boiler water using the method above.

[0030] In the present embodiment, as well as the concentration of corrosive ions in the boiler water, a silica concentration is Petition 870240064615, dated 07 / 31 / 2024, page 11 / 45 7 / 16 preferably controlled to equal or lower than the concentration of corrosive ions in the boiler water (e.g., 10 mg / L or less) from the point of view of inhibiting scale formation, in order to more safely inhibit corrosion fatigue of the evaporator tube.

[0031] The silica concentration can also be reduced by treatment with an ion exchanger, a reverse osmosis membrane device or an electric deionizer; or by regulating the recovery rate of condensed water, similar to the concentration of corrosive ions.

[0032] In the present embodiment, corrosion fatigue can be inhibited even more effectively by reducing the concentration of corrosive ions in the boiler water as described above, and also by adding a scale dispersant to the boiler water to inhibit scale adhesion in the evaporator tube.

[0033] The scale dispersant inhibits the scaling of a hardness component contained in the water system, or rejects scale that has adhered. Examples of scale dispersants include: phosphate salts, such as trisodium phosphate and sodium tripolyphosphate; and polymers such as polyacrylic acid and / or a salt thereof, polymethacrylic acid and / or a salt thereof, a copolymer of acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid and / or a salt thereof. The scale dispersant is preferably a poly(meth)acrylic acid compound, such as polyacrylic acid with an average molecular weight of 20,000 to 170,000 and / or a salt thereof, or polymethacrylic acid with an average molecular weight of 1,000 to 100,000 and / or a salt thereof. The average molecular weight of polyacrylic acid and / or a salt thereof is more preferably greater than 20,000 and less than 170,000, and even more preferably greater than 50,000 and less than 120,000.The average molecular weight of polymethacrylic acid or a salt thereof is more preferably greater than 1000 and 100000 or less. Petition 870240064615, dated 07 / 31 / 2024, p. 12 / 45 8 / 16 and even more preferably above 5000 and 80000 or less.

[0034] When using scale dispersant, the location where scale dispersant is added is not particularly limited, provided the scale dispersant is contained in the boiler water. Scale dispersant is added to at least one location in a boiler water system, including a feedwater system (water feed line), water supply system (water supply line), boiler water, and boiler condensate system (water condensate line). Scale dispersant may be added to any boiler feedwater piping, a feedwater tank, water supply piping, a water supply tank, water condensate piping, and a water condensate tank, and may be added to two or more of these locations.

[0035] When scale dispersant is added to the boiler water system, the amount added is not particularly limited and varies depending on the quality of the boiler feedwater (dissolved oxygen concentration and corrosive salt concentrations) and the scale tendency derived from the boiler operating conditions. From the point of view of reducing the cost of chemicals and obtaining a sufficient scale inhibition effect, for example, with polymer-type scale dispersant, the scale dispersant is preferably added in such an amount that the concentration in the boiler feedwater becomes 1 to 1000 mg / L, particularly 10 to 500 mg / L.

[0036] When ion exchange water is used for boiler water, the electrical conductivity of the ion exchange water is preferably 1 mS / m or less, particularly 0.5 mS / m or less, and especially 0.1 mS / m or less. With the water exchange Petition 870240064615, dated 07 / 31 / 2024, page 13 / 45 9 / 16 ionic in which salts are rejected so that the electrical conductivity is equal to or less than the upper limit above, the corrosion of the evaporation tube can be inhibited more safely to improve the corrosion fatigue inhibition effect.

[0037] In the boiler water system, soft water, in which the raw water is subjected to a softening treatment with a softener, is used for boiler feedwater to inhibit scale formation in some cases. However, the softening treatment alone only rejects a cationic component, and cannot reject corrosive salts, leading to an insufficient effect in inhibiting corrosion fatigue in the evaporator tube.

[0038] The boiler to which the present invention is applied is not particularly limited, but is preferably a boiler with a steam pressure of 4 MPa or less, and more preferably a low-pressure boiler with a steam pressure of 2 MPa or less, from the point of view that corrosion fatigue of the evaporation tube is likely to occur. Examples of representative low-pressure boilers include a small-type single-pass boiler. The steam pressure of the small-type single-pass boiler is, for example, 1 MPa or less.

[0039] Small type single-pass boiler types include a round vessel body and a rectangular vessel body. The round vessel body is a vessel body that has a combustion chamber in the central part, and has a burner at the top. The rectangular vessel body is a vessel body that does not have a combustion chamber, and has a gas burner adjacent to a side face of a water tube.

[0040] Corrosion fatigue of the evaporator tube is more likely to occur in the rectangular vessel body than in the round vessel body. According to the present invention, the control of Petition 870240064615, dated 07 / 31 / 2024, page 14 / 45 10 / 16 concentration of chloride ions and the concentration of sulfate ions in the boiler water, and, in addition, the use of a scale dispersant, allow sufficiently to inhibit corrosion fatigue of the evaporation tube, even in a small rectangular single-pass boiler.

[0041] Corrosion fatigue is more likely to occur in the rectangular vessel body, presumably due to the thermal load becoming non-uniform in the rectangular vessel body compared to the round vessel body, due to its structure and thus the evaporator tube will likely expand and contract to cause corrosion fatigue triggered by corrosion. EXAMPLES

[0042] Experimental Examples will be described next, Examples and Comparative Examples.

[0043] Hereafter, concentrations of chloride ions, sulfate ions, and silica ions were analyzed in advance according to JIS B8224 Boiler feedwater and boiler water - Test methods. EXPERIMENTAL EXAMPLE 1

[0044] The effect of salts on corrosion fatigue was evaluated by a corrosion fatigue test. The quality of a test solution was defined to contain chloride ion, sulfate ion and silica, with the expectation of soft water.

[0045] First, the following preparation was carried out to determine a rupture condition of a sample used.

[0046] A sample made of STB340 material was processed for a circular arc fatigue test. Specifically, the sample was processed into a shape with a parallel part and an R part (circular arc part), and the parallel part and the R part were polished to #800 grit.

[0047] This sample was used to perform a tensile test on Petition 870240064615, dated 07 / 31 / 2024, page 15 / 45 11 / 16 atmosphere, and a stress corresponding to a maximum load in an elastic region was determined to be 220 MPa. This value was defined as the maximum stress in the corrosion fatigue test.

[0048] Next, the above sample was placed on a shaft of a corrosion fatigue tester, and 1 l of a test solution was poured into a fixed beaker, so that the sample was completely immersed.

[0049] The test solution was adjusted to have the following composition and pH of the test solution using ion-exchange water, sodium chloride and sodium sulfate which were special grade reagents, 48% sodium hydroxide and sodium silicate No. 3. The temperature of the test solution was regulated to 85°C.

[0050] Next, oxygen gas was continuously supplied to the beaker, so that a dissolved oxygen concentration in the beaker became a value shown in Table 1, and the amount of gas supplied was controlled by monitoring a dissolved oxygen meter, supplied in a circulation line so that the dissolved oxygen concentration became constant.

[0051] When the dissolved oxygen concentration became constant, the maximum stress was set at 220 MPa aiming for a test time of 96 hours to perform a test with the corrosion fatigue tester.

[0052] The test conditions are as follows. Corrosion fatigue test conditions Sample: Circular arc fatigue sample made of STB340 (considering ASTM E466-96)

[0053] Sample pretreatment: Parallel part and R part were finished at #800. Control: load control Waveform: sine wave Petition 870240064615, dated 07 / 31 / 2024, page 16 / 45 12 / 16 Maximum stress: 220 MPa Frequency: 4.0 Hz Test duration: 96 hours maximum Test solution temperature: 85°C

[0054] Test solution composition: The concentrations of chloride ion, sulfate ion and silica were as shown in Table 1.

[0055] In No. 1, the concentration was 100 mg / L each with the expectation of a concentration 10 times higher in the steam generator when the concentration of each substance was 10 mg / L in the soft water feed.

[0056] In No. 2, the water quality was as expected with soft water feed and a condensate recovery rate of 90% or more.

[0057] In No. 3, the water quality was as expected with ion-exchange feedwater and a concentration 50 times or more. Since silica is more likely to leach in a small amount than salts in ion-exchange water, only the silica concentration was 10 mg / L. pH of the test solution: 11.0 Dissolved oxygen concentration in the test solution: 8.0 mg / L Quantity of test solution: 1 L Results

[0058] Table 1 shows the results. Table 1 Test No. Chloride Ion Concentration (mg / L) Sulfate Ion Concentration (mg / L) Silica Concentration (mg / L) Number of tensioning tests (ten thousand) Presence / absence of sample rupture Observation 1 100 100 100 68 Presence Example Comparison 2 10 10 10 80 Presence Example Petition 870240064615, dated 07 / 31 / 2024, page 17 / 45 13 / 16 Test No. Chloride ion concentration (mg / L) Sulfate ion concentration (mg / L) Silica concentration (mg / L) Number of tensioning tests (ten thousand) Presence / absence of sample rupture Observation 3 1 1 10 141 Lack of inventiveness Consideration

[0059] Table 1 demonstrates the following.

[0060] It has been confirmed that the lower the concentration of corrosive ions, the greater the number of stresses that lead to sample rupture, and with each corrosive ion concentration of 10 mg / L or less, sample rupture is unlikely. In particular, it has been confirmed that no rupture occurs with each corrosive ion concentration of 1 mg / L or less.

[0061] Presumably, this occurred because the lower concentration of corrosive ions would likely not generate corrosion on the sample surface or inhibit corrosion growth in fine cracks. TEST EXAMPLE I Examples I-1 to 5 and Comparative Examples I-1 to 3

[0062] In a water system in a boiler illustrated in Figure 1. A relationship was investigated between: the concentration of corrosive ions in the boiler feedwater and the presence / absence of the addition of a scale dispersant; and a state of corrosion fatigue in the evaporator tube.

[0063] In this boiler water system, the raw water treated with an ion exchanger or softener 1 passes through a feedwater tank 2 to be fed into a boiler 4, and the condensate generated in boiler 4 is returned to feedwater tank 2. The dissolved oxygen concentration was measured immediately after an outlet from feedwater tank 2. The scale dispersant and a regulator of Petition 870240064615, dated 07 / 31 / 2024, page 18 / 45 14 / 16 pH type hydroxide was injected into a water feed line, which was upstream of a boiler 3.

[0064] With this boiler water system, when the concentration of corrosive ions was not controlled, cracks in the evaporator tube due to corrosion fatigue occurred, causing a leak accident within eight years of the start of operation.

[0065] The target boiler specifications are as follows. Target boiler: small type single-pass boiler. Operating pressure: 1.0 MPa or less.

[0066] Feed water type: Any soft water, soft water + condensate, ion-exchange water, and ion-exchange water + condensate. Boiler water quality: pH = 11.0 to 11.8 Electrical conductivity = 50 to 400 mS / m Dissolved oxygen concentration = 8.0 mg / L

[0067] The concentrations of chloride ion, sulfate ion and silica are as shown in Table 2.

[0068] Scale dispersant: the following was used: sodium polyacrylate (PANa), sodium polymethacrylate (PMNa), or an orthophosphate salt. The amount of polymer added was 30 mg / L as a pure component, and the amount of orthophosphate salt added was 30 mg / L as a phosphate ion, from the boiler water. PANa (4000): Sodium polyacrylate with a weight-average molecular weight of 4000 PANa (60000): Sodium polyacrylate with a weight-average molecular weight of 60000 PMNa (10000): Sodium polymethacrylate with a weight-average molecular weight of 10000 Orthophosphate salt: sodium salt of orthophosphoric acid. Petition 870240064615, dated 07 / 31 / 2024, page 19 / 45 15 / 16 Number of boiler vessels targeted for investigation: 559 Results

[0069] The operation period was eight years. The number of crack occurrences in the evaporation tube and the crack occurrence rate over the eight-year operation period were investigated. Table 2 shows the results. Table 2 Concentrations in boiler feedwater (mg / L) Presence / absence of scale dispersant addition Number of vessels with cracks Number of vessels without cracks Crack occurrence rate (%) Chloride ion Sulfate ion Silica Example Comparative I -1 80-100 80-100 80-100 Absence 75 192 28.1 Example Comparative I -2 80-100 80-100 80-100 PANa (4000) added 15 53 22.1 Example Comparative I -3 more than 10 and 50 or less more than 10 and 50 or less more than 10 and 50 or less PMNa (10000) added 10 48 17.2 Example I -1 1 1 1 Absence 4 33 10.8 Example I -2 1 1 1 Orthophosphate salt added 2 22 8.3 Example I -3 1 1 1 PANa (4000) added 2 26 7.1 Example I -4 1 1 1 PMNa (10000) added 1 28 3.4 Example I -5 1 1 1 PANa (60000) added 1 47 2.1 Consideration

[0070] Table 2 demonstrates the following.

[0071] With a high concentration of corrosive ions and without the scale dispersant (Comparative Example I-1), fatigue due to Petition 870240064615, dated 07 / 31 / 2024, page 20 / 45 16 / 16 corrosion was more likely to occur.

[0072] When no scale dispersant was added, in Example I-1 where each corrosive ion concentration was as low as 1 mg / L, corrosion was presumed to be inhibited, tensile stress was unlikely to increase, and the occurrence of corrosion fatigue was inhibited compared to Comparative Example I-1 where each corrosive ion concentration was as high as 80 to 100 mg / L. In Examples I-2 to 5 where each corrosive ion concentration was 1 mg / L and the scale dispersant was used, almost no scale adhered and few cracks occurred. Presumably, this occurred because the tensile stress due to scale adhesion was reduced and the low salt concentrations produced an environment where fatigue and its growth due to corrosion were less likely to occur.

[0073] From the above, it has been confirmed that, in order to inhibit corrosion fatigue, not only inhibiting corrosion of the evaporation tube, but also inhibiting the adhesion of scale, exhibits an even greater effect.

[0074] The present invention has been described in detail using the specific aspect, but it is obvious to those skilled in the art that various modifications can be made without departing from the object and scope of the present invention.

[0075] The present application is based on the Patent Application Japanese Patent No. 2020-04329 5, filed on March 12, 2020, is incorporated herein by reference in its entirety. LIST OF REFERENCE NUMBERS ION EXCHANGER OR SOFTENER WATER FEED TANK BOILER Petition 870240064615, dated 07 / 31 / 2024, page 21 / 45

Claims

1 / 1 CLAIMS 1. A method for inhibiting corrosion fatigue of an evaporation tube in a boiler, characterized in that it comprises: controlling each of a chloride ion concentration and a sulfate ion concentration of water in a boiler steam generator to 1.0 mg / L or less;and adding a scale dispersant to the boiler water, wherein the scale dispersant is at least one poly(meth)acrylic acid compound selected from the group consisting of polyacrylic acid with an average molecular weight of more than 20,000 to 170,000 or less and a salt thereof, and polymethacrylic acid with an average molecular weight of more than 1,000 to 100,000 or less and a salt thereof, wherein the chloride ion concentration and the sulfate ion concentration of the boiler water are controlled by desalination raw water treatment used for boiler feedwater with an ion exchanger, a reverse osmosis membrane device, or an electric deionizer, or by increasing a boiler condensate recovery rate.

2. A method for inhibiting corrosion fatigue in an evaporation tube in a boiler, according to claim 1, characterized in that the concentration of the scale dispersant in the boiler water is 1 to 1000 mg / L.

3. Method for inhibiting corrosion fatigue of an evaporation tube in a boiler, according to claim 1 or 2, characterized in that the boiler is a small type single-pass boiler. Petition 870240064615, dated 07 / 31 / 2024, page 22 / 45

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