Cause analysis and preventive measures of industrial gas pipeline leakage
The stable operation of blast furnace, coke oven and converter gas pipeline networks is of great significance to meeting the energy requirements of steel and metallurgical enterprises and maintaining normal and safe production. Pipeline transportation is the main method of gas transmission and distribution. Gas is an extremely dangerous gas. Improper management or improper use, once leaked, the gas quickly leaks out and expands, releasing a large amount of energy, which will cause explosions and fire accidents, which will cause serious consequences and huge losses to the surrounding environment, personnel and enterprise safety production. As an example of industrial gas pipeline, the operating conditions of the operating gas pipeline were investigated, the cause of leakage in the severely corroded and failed pipe section was analyzed, and preventive measures were proposed.
Analysis of the causes of gas leakage
Table of Contents
- 1 Analysis of the causes of gas leakage
- 2 Analysis of the causes of pipeline corrosion
- 3 Countermeasures
- 4 Conclusion
Figure.1 shows the morphology of the outer surface of gas pipeline, which shows severe corrosion and multiple corrosion penetrations. Figure.2 shows the morphology of the inner surface of gas pipeline, which shows that the corrosion is also quite serious. There is a thick layer of loose corrosion products at the bottom of gas pipeline, which are brown. And light yellow, there are corrosion holes at the bottom. From the on-site corrosion of gas pipeline, the corrosion basically occurs in the lower half of gas pipeline, mainly internal corrosion.
Fig.1 Corrosion pattern of pipeline
Fig.2 Corrosion products in pipeline
Analysis of corrosion products in gas pipeline
The mixed gas composition (mass%) of a steel plant is: H2 25.45, N2 34.47, CH 49.11, CO 11.17, CO 212.72. When the gas is transported, it is greatly affected by the ambient temperature. When the ambient temperature is lower than the gas dew point temperature, it will cause Condensation is formed on the wall of the gas pipe. The PH of the condensed water is 6.31, and the Cl- content of the condensed water is 61 mg/L. The main components of the sediment in gas pipeline are chemically qualitatively analyzed. SO3 and Fe2O3. The content of SO3 (mass %, the same below) is as high as 37%~47%, and its corrosiveness is extremely strong, which can form sulfuric acid medium corrosion.
Figure.3 is a metallographic photo of the severely corroded part of gas pipeline. The corrosion morphology of gas pipeline edge shows that the corrosion is serious. The metal surface has been uneven and becomes local pit corrosion, but no intergranular corrosion cracks are seen. Therefore, pipeline corrosion is both common. The basic feature of “general corrosion” has more serious local accelerated corrosion, that is, the feature morphology of “local pit corrosion”.
Fig.3 Corrosion rusty scale pattern
SEM energy spectrum analysis results (Figure.4) show that the corrosion products on the inner surface of gas pipeline contain corrosive elements S and oxygen, the S content reaches 14.94% to 18.95%, and the oxygen content reaches 20.08% to 31.70%.
Fig.4 Composition analysis of Energy spectrum
The XRD analysis results of the inner and outer surfaces of gas pipeline show that the corrosion products on the inner surface of gas pipeline are mainly FeCO3, FeS, Fe3S4 and SiO2, and the corrosion products on the outer surface of gas pipeline are mainly composed of S, FeS and FeS2.
Corrosion is the most common hidden danger in the operation of gas pipelines. The main cause of corrosion is the damage caused by the chemical or electrochemical effects of gas pipeline contacting gas or the atmosphere. From the corrosion situation of gas pipeline on-site, the corrosion basically occurs under gas pipeline. The half part is mainly internal corrosion, which is mainly manifested as overall thinning, deposits in gas pipeline, and air leakage after local corrosion.
The main factor of gas pipeline corrosion is the impurities in the gas. The composition of gas is very complicated. In addition to CO, CO2, H2, CH4, N2, O2, gas usually contains impurities such as ammonia, H2S, benzene, tar, etc. According to the corrosion mechanism, The corrosion of gas pipelines caused by these impurities in coal gas can be divided into acid chemical corrosion and electrochemical corrosion. The former refers to the corrosion caused by the chemical action of the inner surface of the gas pipeline in contact with acid gas or electrolyte solution. The acidic substances in the gas When it encounters condensed water, it dissolves in water and ionizes, ionizes hydrogen ions, so that the condensed water is acidic. The latter refers to the formation of a galvanic cell in the presence of an electrolyte solution due to the potential difference between the metal and the impurities Metal corrosion. When the impurity electrode potential is higher than the Fe electrode potential, a corrosion reaction occurs.
The difference between electrochemical corrosion and chemical corrosion is that when electrochemical corrosion occurs, there are cathodes and anodes on the metal surface, and there is a small current between the two electrodes; pure chemical corrosion does not form micro currents. The internal corrosion of gas pipelines is The result of the combination of the two.
External corrosion of pipeline
The external corrosion of gas pipeline is mainly atmospheric corrosion. The atmospheric corrosion is the electrochemical corrosion that occurs when a water film exists on the metal surface. It is mainly the process of reducing the cathode by oxygen. The main factors affecting the corrosion of gas pipeline in the atmosphere are relative humidity, temperature, atmospheric composition, etc. . At a certain temperature, the corrosion rate of dry air to steel is very low. Only when the relative humidity of the air increases to a certain range, the corrosion rate suddenly increases. When the surface of the pipe is in air that is higher than its own temperature In the metal surface, condensation may occur on the metal surface. Whether condensation can be caused is related to temperature changes. The atmospheric corrosion rate is higher in areas with high average temperature. The effect of temperature is mainly manifested in the case of temperature differences, that is, periodically on the metal surface Corrosion is more serious during condensation.
Industrial atmosphere contains SO2, H2S, CO2, ash layer and other pollutants. Among them, SO2 is the most harmful pollutant, and its corrosion mechanism can be explained by the effect of “acid regeneration cycle”: SO2 is first adsorbed on the surface of gas pipeline Together with oxygen, FeSO4 is formed, and FeSO4 is hydrolyzed to form free H2SO4. H2SO4 accelerates the corrosion of steel, and the newly formed FeSO4 is hydrolyzed to form free acid. Repeated cycles like this accelerate the corrosion of steel. The solid dust particles fall on the surface of the pipe and become adsorbed water. The condensing center of molecules.
Corrosion inside gas pipeline
When discussing the causes of internal corrosion in gas pipelines, another important factor cannot be ignored, the sediment in the gas pipeline. There is a layer of solid impurities at the bottom of the gas pipeline that has been operating all year round, mainly containing S impurities, the SEM-EDS analysis and XRD of the corrosion products The analysis results also prove this.
The main reason for the internal corrosion of gas pipelines is electrochemical corrosion caused by harmful impurities in the gas, which mostly occurs in the lower part of the gas pipeline. During the gas transportation process, some impurities gradually deposit to the bottom of gas pipeline. Because the gas contains water, when When the gas temperature is lower than the gas dew point, condensed water is precipitated from the gas, and the liquid and solid impurities accumulated in the lower part of the gas pipeline become the cause of pipeline corrosion. The gas contains corrosive gases such as CO, CO2, and H2. The gas pipeline produces acid chemical corrosion again, forming porous and loose corrosion deposits on the inner surface of gas pipeline. The corrosion product has no protective effect on the pipe wall, but accelerates the corrosion of gas pipeline. The aqueous ion analysis shows the presence of SO42– and Cl-, Because they are strong acid radicals, after more than ten years and continuous corrosion reactions, their cumulative corrosion effect cannot be ignored. Chemical qualitative analysis shows that the SO3 content is as high as 37% to 47%. SO3 is extremely corrosive. The formation of sulfuric acid Corrosion of the medium can easily cause pitting corrosion of gas pipeline system, which is extremely harmful.
According to the information obtained from various sources, after the blast furnace gas dust removal is changed to dry dust removal, the corrosion degree of the gas has generally increased, but the sensitivity of the steel companies to its corrosion degree is different. The main reason for the increased corrosion degree is: wet dust removal can The corrosive components in the gas (such as SO2, SO3, Cl2 and H2S) are removed by dissolution or reaction through a venturi or scrubber, and enter the water treatment system. The dry dust removal and corrosive components can only enter with the flow of gas The next process can enter the condensate with the cooling of the gas and the precipitation of moisture, forming highly corrosive acid. The dry dust removal technology of blast furnace gas has energy saving, water saving, small footprint, environmental protection and lower production costs, etc. Multiple benefits, but blast furnace gas is more corrosive.
In the blast furnace smelting process, in addition to circulating in the furnace, part of S and Cl enters the main pipe with the gas in the form of SO2, SO3 and H2S, and reacts with the H2O in the gas to form H2SO4 and H2SO3.
Dry blast furnace gas dust removal is a relatively mature environmental protection and energy saving technology, but the corrosive problems generated after the use of this technology should be paid enough attention. How to eliminate or reduce its hazards needs to be further explored and studied. Suggestions and countermeasures are as follows:
- (1). Reduce the S and Cl content of raw fuels and additives into the furnace;
- (2). When designing gas pipelines and auxiliary facilities, they must be appropriately thickened on the basis of national standards;
- (3). When ordering gas system materials and equipment, the corrosive characteristics of gas must be emphasized;
- (4). Gas pipes and auxiliary facilities are made of acid-resistant materials. This is a simple and effective measure.
The main reason for the corrosion of industrial gas pipelines is chemical or electrochemical corrosion caused by the contact of the steel plate of gas pipeline with acid gas or acid in the gas. The inner wall of the gas pipeline has the most serious corrosion. The corrosion of the gas pipeline has both uniform corrosion and serious local depression. Pit corrosion. The former uniform corrosion is dominant, and the corrosion distribution is relatively uniform, which is a less dangerous corrosion, while the latter is the most harmful.
Author: Guo Xiaohong, Qu Gang, Liangkuai Yun, Zhang Qiang, Liu Zhiwei, Liufeng Lian, Zhong Bin
Source: China Gas Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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