Reasons and Measures Affecting Service Life of HK-40 Reformer Tube

1 The steam reformer is the critical equipment of the ammonia plant, it is the most important component in the reformer.  After desulfurizedation, the raw material are mixed with steam, and the Convection section of the reforming furnace is preheated to 500 cyaenide, hydrazine, fur, beam and high temperature conditions for conversion reaction. The furnace tube is at a wall temperature of up to 900 degree. In the case of continuous operation, at the same time, it is affected by the corrosion of the medium and the high temperature flue gas outside the tube, and the working conditions are extremely bad. The furnace tube is made of high-chromium-nickel alloy steel 2520, and the investment resources are lacking. Therefore, the service life pf furnace tube is extremely important.

The structure of the synthetic reforming furnace is a double-row tube double-sided radiation type side burning reforming furnace. The upper part of the reforming furnace tube is hung on the top hanging lug of the furnace, and the upper and lower sides are connected to the collecting tube by a pigtail tube. The specification of the furnace tube is 152 15mm. The length is 11m, the external surface of the Centrifugal casting tube is not machined.

Causes of reduced service life of 2 40 furnace tubes 2.1 Creep damage When the metal temperature rises to a certain point, although the stress is constant, the initial strain is also a small elastic strain. However, as time goes by, the deterioration of the material structure gradually produces creep voids and creep cracks, and the metal will slowly deform continuously, and it is irreversible plastic deformation, which eventually leads to fracture. The creep damage characteristics of the 40-centrifugal cast tube are similar to those of the cast heat-resistant steel at high temperature creep. At the boundary between the interfacial carbide and the matrix at the boundary of the dendritic grain boundary, the boundary between the matrix and the matrix and the boundary between the precipitate and the precipitate are preferentially formed, and then the pores are expanded along the boundary between the precipitation boundary and the substrate or at the grain boundary to form microcracks. Then develop into a crack. It turns out that the main factors affecting creep are temperature and stress.

When the furnace tube is partially overheated, it will accelerate the creep acceleration and the sharp decrease of the permanent strength. The diameter of the furnace tube becomes larger and the wall thickness is thinner, which makes the creeping speed faster, which causes the tube to become red or partial. The erythema causes the tube to rupture; the other is the influence of stress. There is a temperature difference inside the reformer, which generates thermal stress, and the furnace tube is subjected to the working stress and self-weight stress caused by the internal pressure. As the stress increases, the creep strain accelerates. .

2.2 Local overheating of the furnace tube Since 1975, the temperature of the conversion tube wall has been measured and monitored, and the measurement length is long. When the wall temperature is maintained high for a long time, the service life is significantly reduced. In the past, most of the damaged parts of the furnace tube were within 2 m from the top of the furnace, and the rupture ports were all axially split, and all of them were facing the fire nozzle, that is, the rupture occurred at a high temperature. The Reformer tube is used at a high temperature, and as the use time increases, the finely dispersed carbide of the austenite structure gradually grows and aggregates, and the diameter of the carbide becomes shorter as the temperature increases. The growth of these carbides increases the creep strength of the steel, and these carbides are hard and brittle, which is the origin of cracks or cracks. Under such high temperature conditions, the void nucleation of the grain boundary grows, thereby developing a crack and causing the tube to be destroyed. It has been proved that when the furnace tube is in normal production, when the wall temperature reaches 880, the inner wall of the furnace tube is obviously oxidized and carbonized. The inner wall is found to have different degrees of change in the loose layer, and the middle and lower parts of the furnace tube are aggregated and grown. And rods and horny carbides appear, the inner layer crystal surface has small particles of secondary carbide precipitation, the grain boundary carbides grow and grow coarsely, the old phase precipitation temperature range is 600900, the sensitive temperature is 750800, just the furnace inner wall temperature Correspondence 1. Therefore, the secondary carbide in the tube can be easily converted into a phase, so that the room temperature high temperature impact strength strength elongation and the area shrinkage rate are drastically decreased, the material creeps and the life is greatly shortened.

The reasons for the overheating of the furnace tube caused by Heilongjiang Petrochemical are as follows. 1 Prior to 1975, the most important source of gas contained sulfur, which poisoned the catalyst.

The catalyst carbonizes, causing a decrease in catalyst activity, or even loss of activity, resulting in overheating of the furnace tube. (2) Catalyst loading is unreasonable, resulting in bridging; in addition, steam containing salt is high in silicon, easy to make catalyst salt, improper operation, water and carbon ratio is out of balance, causing catalyst coking and powdering, gas resistance increases, and raw material gas distribution is uneven When the amount of ventilation is small, the amount of heat absorbed is reduced, and the wall temperature of the furnace tube is increased to cause overheating. 3 The misalignment or damage of the Burner, the liquid hydrocarbon water entrained in the fuel and the change of the gas composition will cause the burner to ignite the red flame or the nozzle to block the carbon. This causes uneven furnace temperature, corrosion and contamination of the tube surface, especially when the large flame directly hits the furnace tube and causes local overheating. 4 When the raw material gas enters the furnace tube, the resistance is uneven due to the piping, and the throttle plate is not reasonably used, and the uneven distribution of the air flow causes local overheating of the furnace tube.

2.3 The stress internal pressure caused by the internal pressure of stress 1 varies from the top to the bottom of the furnace tube, roughly in the range of 3.173.555, and there is a directional stress on the pipe wall. Among them, the hoop stress is the largest, and the axial stress and the radial stress are only about half of the hoop stress. 2 The temperature difference between the inner and outer walls of each part of the stress furnace tube caused by the temperature difference between the inner and outer walls is 28.4101. The enthalpy, enthalpy, second, and difference are 50 ft. The instantaneous maximum thermal stress can reach 7485. The axial stress is slightly smaller than the hoop stress. The stress is tensile in the inner half of the wall and the outer half is the pressure. 3 The relaxation of thermal stress is constant due to the internal pressure. The thermal stress caused by the temperature difference can be relaxed with the temperature time. The higher the temperature, the faster the stress relaxation. The 4 open stop affects the inner side of the tube at the moment of driving. Pressure; the opposite when parking. This causes a kind of tensile and compressive fatigue on the material, and due to the sudden rise and fall temperature, a large temperature difference generated in a short time, the material will generate stress concentration in a certain part. 5 Long-term internal and external temperature difference caused by the difference in material properties Although the materials are the same, but the temperature difference of 50 弋 or more, the material's permanent strength can differ by nearly an order of magnitude, which results in the difference in actual material strength. There are also residual stresses in the 6 furnace tube welding and casting. 7 Additional stress caused by long-term use and deformation or expansion is not free.

In summary of the above, if the stress is simplified to the main stress direction, the hoop stress is the maximum. Therefore, the failure of the furnace tube should be dominated by axial cracks, which is consistent with the situation on site. Due to the combined action of various stresses, the stress is greatest at a wall thickness of about 13 from the inner surface of the furnace tube. More cracks are generated. As a result, not only the top 27, 1 of the furnace, but also the first damage in this area.

2.4 The outer surface of the inner and outer corrosion furnace tubes is mainly oxidized and decarburized, and the inner surface is mainly affected by the medium to form carburization and oxidation. After carburizing, chromium-based carbides are formed, and then oxidized, while carbon continues to carbonize inward, but This corrosion has little effect on the base metal. At the weld, the corrosion layer can enter the effective wall thickness of the furnace tube, but from the field, the service life of the furnace tube life tube has not been reached.

3 Method to extend the service life of the furnace tube 3.1 Control and prevent overheating of the furnace tube In order to prevent damage to the furnace tube caused by overheating, the following measures are taken for various reasons. 1 Strict management of scientific filling during catalyst loading.

The catalyst is first placed in a long bag, and the bag is slowly fed to the bottom of the tube along the wall of the tube, and the bag is withdrawn from the tube, thus preventing bridging and chalking of the catalyst. Before each filling, the pressure difference of each tube is measured, so that the pressure drop of each tube is kept within ±5 of the average pressure drop of all tubes to ensure uniform fluid distribution. 2 Control the appropriate stable steam-carbon ratio, maintain the upper limit as much as possible, not less than 3. The high steam-to-carbon ratio can inhibit the carbon deposition reaction of the catalyst and prevent the carbon deposition of methane and light hydrocarbons in the tube. 3 Maintain the activity of the catalyst. This not only directly affects the conversion rate, but also directly relates to the life of the conversion tube. To a large extent, wall temperature is the most direct and reliable gauge for measuring catalyst conditions. In the case of poor catalyst activity, replace it as soon as possible, and avoid using the method of increasing the furnace temperature to make up for the shortage of the catalyst. (4) Strengthen the management of burners.

Control the temperature difference of the single pipe within 30, and do the work of desulfurization, deoiling and dehydration before the gas is put into the furnace, and reduce the corrosion of the gas on the furnace tube. The burner is made flat and short, and the fire is resolutely put out. 5 In the piping design of the conversion pipe, it should be reasonably configured according to the specific conditions.

3.2 Control process indicators Strictly control various process indicators, make stable production, minimize the number of parking stops, and prevent thermal fatigue.

3.3 Do a good job in the inspection of the quality of the purchased furnace tube when purchasing the furnace tube. It is recommended to purchase tubes that are not machined on the outer wall to ensure excellent cast structure. At the same time, the entire wall thickness of the pipe should be dense layer structure, the inner wall should be carefully processed, and casting defects such as inclusion shrinkage holes should not be left to prevent carburization.

3.4 The replacement of the furnace tube is to compete for high temperature and also for the stress zone. Therefore, most of the furnace tubes improve the high temperature resistance of the furnace tubes by changing the chemical composition of the furnace tubes. 19942015ChmaAcademicJoumal and creep resistance. At present, 40140 has been developed to wait for new modified alloys, and its performance is much better than 40 furnace tubes.