Discussion on internal leakage issues in methanol synthesis towers

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Discussion on the internal leakage problem in methanol synthesis towers – Abstract: The causes of leakage defects that occur during the operation of methanol synthesis towers are analyzed, and solutions to address these defects are studied, in order to provide guidance for the manufacturing and use of such towers. Keywords: parameters, defect detection, cracks, stress, causes, treatment, measures 1 Introduction For chemical manufacturing enterprises, the stability, reliability, and efficiency of the operation of their key equipment are crucial for their survival. In methanol production plants, the methanol synthesis tower serves as the core and key equipment; any failure in this tower will affect the operation of the entire system. Moreover, this type of equipment is difficult to manufacture, requires a long production time, and is expensive. The methanol synthesis towers in a certain methanol plant are three towers combined together; one of them was put into operation in June 2000. This synthesis gas reactor is of the shell-and-tube type, with copper-based catalysts installed in the tube side. The synthesis gas comes into contact with these copper-based catalysts in the tube side, where a reaction takes place to produce methanol. The shell side contains boiler water, which is used to remove the heat of reaction from within the tube side, while generating steam as a by-product. During operation of the tower, CO gas was detected in samples taken from the online monitoring of the non-condensable gases in the steam drum; multiple sampling analyses led to the conclusion that an internal leak had occurred in the tower. During maintenance, multiple crack defects were found in the corner welds of the tube sheet and the base layer of this tower; eddy current testing of the tubes revealed external wall defects of varying degrees in 12 of the tubes. The parameters and details of the 2 methanol synthesis towers are shown in Table 1. The tube material of the methanol synthesis tower is SAF2205 duplex stainless steel, with a total of 2,292 tubes ； The length of the tube bundle is 7m, and the tube specifications are Ф44×2mm. The thickness of the base layer of the tube sheet is 70 mm, with a material composition of 20MnMoNi55. A 2-mm thick overlay is applied using E309L welding wire in the transition layer, while a 5-mm thick overlay is used with E308MoL in the surface layer. 3 Defect detection: After an internal leak was detected in the synthesis tower, actions were taken to remove the catalyst in order to locate the leak. During the hydrostatic test of the shell side of the synthesis tower at 2.8 MPa, two leakage points were found in the upper tube sheet: one was a crack in the corner weld of the tubes, and the other was a sand hole in the surfacing layer of the tube sheet. The non-destructive testing methods used for the tube sheet, tube sheet fillet welds, and tubes in shell-and-tube equipment include surface inspection, magnetic particle testing, radiographic testing, and eddy current testing. Given the special structure and material properties of this synthesis tower, in order to identify equipment defects more comprehensively, a 100% PT test was first conducted on the tube sheet; the results revealed 1 crack and 12 suspected pores (micro-defects may not require any treatment). For the tube bundles, eddy current testing combined with internal visual inspection of the surface was used for inspection; the test results are shown in Table 2 and Figure 1. For the two defects identified through hydrostatic testing as leaks, and the one crack defect detected by PT testing, manual grinding with a grinding wheel revealed that the cracks in the tube sheets were caused by cracks in the corner welds of the tubes; after grinding to a depth of about 3 mm, the cracks disappeared. After surface grinding of the tube sheet sand holes, multiple hidden cracks were found; and after grinding away 7 mm of the surface weld layer, it was discovered that there were still numerous crack defects in the underlying layer of the tube sheet. 4. Cause analysis: A comprehensive evaluation and analysis were conducted on the aforementioned defects in conjunction with the operating condition of the equipment; the reasons for the multiple defects in the synthesis tower are as follows. (1) Fatigue cracking caused by long-term operation. Fatigue cracking usually occurs at locations prone to stress concentration, such as areas with geometric discontinuities. This synthesis tower has been in use for nearly 20 years. The fillet welds at the tube sheet are constantly subjected to repeated external loads such as pressure fluctuations, temperature changes, and the impact of boiling boiler water. In particular, due to production fluctuations and equipment maintenance, the unit was started and stopped dozens of times; temperatures and pressures changed significantly during these start-up and shutdown processes. Additionally, since the fillet welds at the tube sheet are in areas with high stress concentration, fatigue cracking is likely to occur there. This is also one of the reasons for cracks in the girth welds of the synthesis tower. (2) Effect of temperature difference stress. When loading the catalyst into the methanol synthesis tower, in order to maximize reaction efficiency, thereby achieving maximum production capacity and reduced consumption, catalyst is also installed at a certain height above the upper tube sheet. This is done to compensate for the reduction in the volume of the catalyst within the tubes that occurs after the new catalyst is heated and reduced, thus preventing gaps from forming in that catalyst. However, due to uneven loading or differences in volume changes during the heating and reduction process, the catalyst is distributed unevenly among the tubes on the tube sheet. During production, the catalyst reacts with syngas, releasing large amounts of heat; the heat generated by this reaction cannot be removed promptly from the catalyst remaining on the tube sheet, resulting in localized high-temperature areas. The temperature differences give rise to distinct temperature layers between the tube sheet and the tubes, thereby causing significant thermal stress. This temperature difference stress creates the conditions necessary for the formation of cracks in the tube sheet fillet welds. (3) Hydrogen embrittlement tendency of high-strength steel at the base level. The conditions for hydrogen embrittlement are mainly hydrogen partial pressure, temperature, exposure time, and the chemical composition of the steel. According to the well-known Nelson curve and practical data, for carbon steel, hydrogen corrosion does not occur unless the hydrogen partial pressure is below 1 MPa or the temperature is below 200°C. The base material of the tube sheet is 20MnMoNi55, a high-strength heat-resistant steel; this material exhibits reduced plasticity and toughness as well as increased brittleness, and it has a strong tendency to suffer from hydrogen embrittlement. Hydrogen constitutes the majority of the medium in the methanol synthesis tower; under conditions of high hydrogen partial pressure and high temperature over a long period of time, this leads to hydrogen corrosion of the equipment materials, posing a significant risk. (4) Tube sheet structure. The tube sheet of this equipment has a diameter of 2800 mm and a thickness of 77 mm. During the production process, uneven heating on both sides of the tube sheet results in a certain temperature difference; this difference increases as the thickness grows, leading to significant thermal stresses. Furthermore, this methanol synthesis tower is of the fixed tube-sheet type; the cylinder is made of 20MnMoNi55, while the tubes are made of SAF2205. The two materials have different coefficients of thermal expansion, and the stress resulting from thermal expansion and contraction under various operating conditions can be quite severe. The stress resulting from temperature changes mentioned above is also one of the causes of cracks in the tube bundle fillet welds. (5) Structure of the equipment itself. The heat exchange tubes in the methanol synthesis tower have a diameter of Ф44×2mm, and it features a fixed tube sheet design. There are 3 support plates in total, with plate spacing of 1770mm, 1720mm, and 1720mm respectively. The eddy current testing results revealed defects on the outer wall of the tube bundles, with over 50% of these defects located near the support plates. Therefore, during long-term operation, vibration causes friction between the tubes and the support plates, resulting in localized wear and thinning of the outer walls of the tubes. Over time, this poses a significant safety risk to the operation of the equipment. 5 Defect Treatment and Preventive Measures: Based on the defects identified through various inspection methods in the synthesis tower, and in order to ensure its continued safe operation, the existing defects are repaired in accordance with the following plan. (1) Crack defect treatment. The first step is to determine the location and extent of the crack defect using PT testing ； In the second step, first drill stop holes at both ends of the crack using a drill, then grind the crack with a manual grinder, and confirm through PT testing that the crack has been completely eliminated ； In the third step, the heat exchange tubes are sealed with asbestos blankets, and preheating is carried out prior to welding to ensure that the preheating temperature is above 150°C. Manual TIG welding is used for repair, followed by hydrogen removal treatment. (2) Tube thinning defects. The first step is to identify the tubes that need to be thinned, carry out pre-welding cleaning, and use special stainless steel grinding wheels to remove any dirt within 20 mm of the tube ends, thereby revealing the metallic shine ； In the second step, the upper and lower pipe ends are used for straight grinding to clean the inner wall of the tubes, revealing a metallic shine ； In the third step, use a drill to create small holes of Ф8mm in the inner wall of the tubes that need to be sealed, so that the conditions inside the tubes are identical to those in the shell side during operation. This prevents the sealed tubes from being subjected to pressure during subsequent operation, while also ensuring temperature balance between the inside and outside ； In the fourth step, plugs are inserted at both ends of the pipe, and corner welds are welded using manual TIG welding, with the interpass temperature to be controlled at no more than 150°C. After all defects have been resolved, a 100% PT inspection is carried out, along with a hydrostatic test, to ensure that the repairs are satisfactory. Based on the operation and maintenance of methanol synthesis towers, as well as defect detection and root cause analysis, to effectively avoid various defects in equipment manufacturing, selection, operation, and maintenance, it is recommended to take action from the following aspects. (1) From the perspective of equipment material selection, materials with good resistance to hydrogen corrosion should be chosen within the safe limits of material use. As can be seen from the Nelson curve, the higher the content of elements such as chromium, molybdenum, and vanadium in steel, the better its resistance to high-temperature hydrogen corrosion. Since these elements are stabilizers for carbon, adding them to steel allows the formation of stable alloy carbides with carbon, thereby reducing the tendency of the steel to methaneate. At the same time, it is essential to strictly avoid the presence of dispersed non-metallic impurities in the steel, as hydrogen or methane can easily accumulate in such locations, leading to brittle fracture of the metal. Furthermore, the heat treatment condition of steel also has a significant impact on high-temperature hydrogen corrosion. During the equipment manufacturing process, the quality of normalizing the steel plates and performing post-weld heat treatment is also an important factor affecting the occurrence of hydrogen-induced cracks in the equipment. (2) The connection between the tube bundle and the tube sheet in this synthesis tower uses a combination of strength welding and expansion fitting; this design places the fillet welds of the tubes in high-temperature areas, making them susceptible to thermal shock and cracking. Deep hole welding is recommended; this design hides the weld seam in the low-temperature area, reducing the impact of thermal shock. (3) Reasonable operational control under working conditions. Maintain stable operation during use, avoid significant fluctuations in gas volume, temperature, and pressure, and reduce the pressure difference across the shell side and tube side. During the start-up and shutdown of the device, the rate of temperature change inside the equipment is strictly controlled to prevent thermal stress caused by too rapid temperature changes. (4) Adopt more advanced and rational new structural technologies for methanol synthesis towers. Currently, there are mainly two structural types of methanol synthesis towers in use. The first type of low-pressure methanol synthesis tower uses a tubular synthesis tower, with catalysts filled inside the tubes; boiling water is present between the tubes, and the heat of reaction is transferred to this boiling water through the tube walls, thereby generating steam. The temperature conditions of the entire synthesis tower system are controlled by steam pressure, thereby ensuring that the catalyst layer maintains a roughly isothermal profile. However, this type of synthesis tower structure inevitably suffers from discrepancies in temperature control, which leads to temperature differential stresses that affect its operational performance. The second type of isothermal methanol synthesis tower is a single-stage isothermal tower cooled by boiling water inside coils. Inside the tower, spiral coiled tubes are placed within the catalyst layer; boiler water is fed in from the lower part of the tubes, while medium-pressure steam and circulating water are discharged from the upper part. Maintaining a stable system temperature through a water circulation system not only enhances the efficiency of the synthesis reaction but also helps to reduce thermal stress caused by temperature differences in specific areas of the synthesis tower through isothermal control. 6 Conclusion As a core piece of equipment, the safe and stable operation of the methanol synthesis tower is of obvious importance. On the one hand, methanol production facilities need to adopt reasonable process routes, as well as advanced equipment manufacturing techniques and structural designs ； On the other hand, it is necessary to ensure proper control and optimization of the equipment parameters during the production process, so as to keep the equipment in a good and stable operating condition ； Furthermore, as the production department, special protection is provided for critical equipment in terms of equipment management, with various measures taken to ensure the safe, stable operation of these devices over extended periods of time.