Causes and Countermeasures of cracks in the hottes

2022-08-21
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Causes and Countermeasures of cracks in comprehensive tower group

comprehensive tower is one of the key equipment in the production of medium nitrogen fertilizer. The state of its equipment directly affects the safe operation of production. In recent years, penetrating cracks from inside to outside have been found in the comprehensive tower of a chemical fertilizer plant from the middle annular tower wall section. The number has increased from a few to more than a dozen, seriously affecting the normal progress of production. Taking advantage of the opportunity of system shutdown, through the analysis of the causes of cracks in the tower, a series of treatments have been carried out. After one year of use, the effect is good, and no new cracks have been found so far. This paper analyzes the causes of group cracks in the comprehensive tower and puts forward corresponding solutions

the integrated tower is composed of two sections, the middle part is a large gas cap. 4. The power source (electromechanical) of the experimental machine is also called the motor. The lower part of the gas cap is the auxiliary tower section, and the tower body above the gas cap is the recovery and cleaning section. It mainly uses the soft water from the top of the tower to absorb the ammonia in the bottom-up gas through the uniformly distributed strip tray from top to bottom, forming dilute ammonia (%), and the dilute ammonia forms a certain liquid level on the cap tray, Then from the tower wall beside the liquid seal plate φ eighty-nine × The connecting pipe of 4 is discharged to the liquid seal tank of the auxiliary equipment of the integrated tower. After the tower was put into operation for more than five years, penetrating cracks began to appear in the annular area of the tower wall outside the gas cap. Most of these cracks gathered on the base metal welded with the tower wall by internals such as the liquid seal plate, and on the circumferential T-shaped welded joints and longitudinal and circumferential seams. From the process of grinding cracks, it is found that these cracks are penetrating cracks from inside to outside, and they are dendritic bifurcation, which are typical stress corrosion cracks

stress corrosion cracking is the most common form of corrosion in chemical plants. It is characterized by a certain lag, which generally begins to occur after the equipment is put into use for a period of time. At present, the research believes that stress corrosion cracking is a special fracture mode produced by the joint action of sensitive metal, alloy under a certain stress (external force and residual stress in the manufacturing process) and a certain corrosive medium environment (sometimes even in water). Although there are various theories about its development mechanism at present, there is no definite conclusion, but it has been proved in practical use, The main feature of stress corrosion is generally that the crack source is generated from the inner wall of the container, and then extends to the outer wall of the container, that is, it is a penetrating crack from inside to outside, and the location of the crack is mainly concentrated in the welded joint between the inner part and the tower wall. Longitudinal and circumferential seams of the shell, T-joints, welds between the cylinder and the head and their heat affected zones. These facts further prove that the cracks in the comprehensive tower group are caused by stress corrosion

the three elements of stress corrosion are stress, medium and material. Then, in the whole tower, why are the cracks only concentrated on the gas cap belt? What is the relationship between this and the mechanism of stress corrosion? It is analyzed from the following aspects

II. Cause analysis

1. Material and medium

the comprehensive tower is made of 16MnR with a thickness of 16mm. A large number of experiments and facts show that certain materials will produce stress corrosion in certain media. For example, austenitic stainless steel in water containing CI -, when the CI concentration is greater than 25mg/l, the tendency of stress corrosion will occur. 16MnR material is concentrated ammonia water, nitrate solution, The tendency of stress corrosion is greater

the medium in this section is that the rising carbonated gas is absorbed by the soft water in the process of falling, forming dilute ammonia solution (6-7%) and carbonated gas tail gas. The medium itself is not highly corrosive. However, a certain amount of H2S gas (mg/l) contained in the carbon water tail gas is also absorbed, forming a wet H2S environment. A large number of experiments and facts have proved that in a wet H2S environment, even if its content is very low, For pressure vessels with actual structure that may have stress concentration, especially during manufacturing and installation, many small cold cracks or regenerative hot cracks are left. As long as H2S is continuously supplemented to the vessels with stress gradient, stress corrosion can occur after a long enough time, generally (3-6 years). Therefore, the cracks of this tower are mainly caused by wet H2S

2. 5. Record the water consumption stress

manufacturing stress: the manufacturing time of the container is from January to February, which is in winter. 16MnR material is notch sensitive material. During the welding process, especially in the midwinter season, 16MnR welding produces cold cracks due to hardening tendency and fast cooling speed. It can be said that it is a key wrinkle. Generally, preheating before welding is adopted, During welding, the interlayer temperature and post welding heat treatment are controlled to ensure the welding quality. The diameter of the integrated tower is φ 3000, with a tower height of 17000mm, such a huge equipment is difficult to meet the environmental temperature requirements for welding in winter, and it is also difficult to ensure that the above welding measures are in place. According to the equipment archives, the welding quality of the tower was poor, which was mainly reflected in poor weld formation and high repair rate. There were more than 40 repair places, Щ There are 6 stages, and the annular area of the lift cap is the most complex part of the internals of the tower. There are many welding points, narrow places and poor welding conditions, which are also the places where the welding quality is the most difficult to guarantee. From the above situation, it can be seen that during the manufacturing of the tower, due to the poor welding quality, the welding stress is large, and the binding force of internals is large, which has buried hidden dangers for stress corrosion in the future

structural stress: from the perspective of structure, the gas rising from bottom to top in this section of the tower must pass through the central pipe of the gas cap φ 950, then change the direction through the cap and rush down to the liquid level of the tray, forming dilute ammonia with the soft water from top to bottom. In this process, the gas from top to bottom changes from the original φ 3000 diameter uniformly distributed, rising through the gas cap and then becoming through φ 1400/φ The narrow annular channel of 950 rushes to the tray, forming an annular local impact force on the liquid level of the tray, which worsens the stress condition of the tray, and the impact also causes a great scouring force on the tower wall, so that this section of the tower body is affected not only by the welding residual stress and the binding force between internals, but also by the impact force of gas on the tray

it can be seen that stress corrosion is inevitable under the joint action of stress, material and medium

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