Zhang Hui Summary: The causes of high temperature and high pressure hydrogen heat exchanger corrosion were analyzed. The method of on-site repair of the equipment was comprehensively introduced, including key processes such as corrosion surface processing, preheating, welding operation and safety protection, post-weld heat treatment and inspection. ER309LSi stainless steel is used as the surfacing layer to mitigate the corrosion in the high temperature hydrogen-hydrogen sulfide environment through a reasonable welding process.
(Panjin Vocational and Technical College, Panjin, Liaoning 124010)
Key words: high temperature and high pressure; hydrogen equipment; corrosion analysis; repair
A petrochemical company hydrocracking unit reaction feed / reaction effluent heat exchanger EA101A / B is a vertical U-tube heat exchanger, designed and manufactured by the Japanese steel manufacturing company according to BBS specifications. Since the heat exchanger is adjacent to the reactor, the corrosion environment on the side of the reaction product is substantially the same as that of the reactor, characterized by high operating pressure, high temperature, and the medium is in a hydrogen sulfide corrosion environment. The equipment was in normal operation at the initial stage of operation. After two years of operation, it was found that the shell side of the EA101A/B tube plate and the surrounding shell were corroded. The corrosion of EA101B was slight and the corrosion of EA101A was serious. The maximum corrosion depth is 12mm. For this reason, the cause of corrosion was analyzed, and on-site repair welding was performed. After the repair, the equipment was running normally.
1. Equipment introduction and corrosion characteristics
The hydrogenation reaction is carried out under high temperature and high pressure catalytic conditions, and EA 1 0 1 A/B is used to recover the heat of reaction and heat the raw material gas. The pyrolysis gas having a pressure of 14 MPa and a temperature of 295 to 300 ° C is heat-exchanged through a heat exchange shell side and a feed gas having a tube feed of 280 to 285 ° C. The heat exchanger tube and shell side are all under high temperature and hydrogen sulfide hydrogen corrosion conditions. The pipe box is a forging material with a base material of 2.25Cr1Mo, and is lined with two layers of stainless steel surfacing layer, the bottom layer material is 308L, and the surface layer is 309L. The housing and lower head are made of 2.25Cr1Mo and have no stainless steel overlay.
In actual operation, the initial operation of the device is operated under design conditions, and the corrosion rate of the casing is about 0.25 mm/a, which satisfies the requirement of the maximum corrosion rate of the casing of about 3.5 mm/a under the design conditions. After a period of operation, the catalyst gradually fails and the same yield is maintained by increasing the temperature. During the annual inspection, it was found that EA101A/B was corroded, and EA101A was seriously corroded. Corrosion mainly occurs in the bottom area of ​​the gasket (as shown in Figure 1), and extends to the lower part of the housing for about 500mm, wherein the local corrosion under the gasket base is the most serious, the depth is up to 12mm, and the thickness of the corrosion layer increases with the extension distance. Weakened. According to this calculation, the average corrosion rate is greater than 3 mm/a. The calculation results show that when the corrosion thickness reaches 16.5mm, the shell at the joint with the tube sheet will collapse, and the consequences are unimaginable.
2. Analysis of corrosion causes
The reason for such a high corrosion rate is not fully understood, and preliminary analysis is considered to be caused by H2S corrosion in a high temperature H2-H2S environment.
In the hydrogen-rich environment, the corrosion characteristics of high-temperature sulfides are complicated. Under the promotion of hydrogen, hydrogen sulfide can accelerate the corrosion of metals. This is because in a hydrogen-rich atmosphere, hydrogen as a gap-type proton can continuously invade the sulfide corrosion layer, causing the scale layer to be loose and porous, destroying the protective effect of the hydrogen sulfide corrosion film, and allowing the metal atom and the hydrogen sulfide medium to diffuse each other. Infiltration, causing corrosion of hydrogen sulfide to continue. The main factors affecting the corrosion of high-temperature hydrogen sulfide are temperature and H2S concentration. The corrosion rate generally increases with the increase of temperature. The dry hydrogen sulfide gas has little corrosion to steel below 200-250 °C. Studies have shown that in the H2-H2S system, when the partial pressure of H2S is between 3.43 and 343 kPa, and the operating temperature is greater than 316 °C, the corrosion of hydrogen sulfide is intensified, and the general chromium-molybdenum steel cannot meet the requirements. The corrosion zone of EA101A/B heat exchanger is gas retention zone, the heat transfer effect is poor, and the concentration of H2S is easy to increase, and the inlet temperature fluctuates greatly during operation, sometimes reaching more than 300 °C, which accelerates H2S corrosion.
3. Patching options
Single-layer overlay welding is used in the corroded area below the gasket base, and the repair material is AWS ER309LSi stainless steel. Due to the low hydrogen diffusion of the austenitic stainless steel coating layer, the hydrogen activity is hindered and the hydrogen partial pressure in the base low alloy is lowered. Although the austenitic stainless steel coating absorbs a certain amount of hydrogen during operation, it does not become brittle itself, and the sensitivity of cracking does not increase, but the dehydrogenation rate is lowered, thereby protecting the base layer. AWS ER309LSi stainless steel electrode contains more than 12% Cr, and has better resistance to hydrogen embrittlement and hydrogen sulfide corrosion. The welding preheating temperature is 200 ° C to prevent martensite cold cracking and embrittlement.
The heat exchanger is placed upright and cannot be moved during the repair process. The inner diameter of the housing is only 1200mm, and the operation space is limited. Considering the low efficiency of manual arc welding, especially the difficulty in ensuring the installation tolerance of the heat exchange tube bundle, semi-automatic pulse gas shielded welding is adopted. Strict welding process and operating procedures were established before welding, and tests were carried out on carbon steel and 1.25Cr steel. The thickness of one surfacing was about 5mm, and the 309LSi was up to 3mm. After the welding process is successful, it is applied to the actual repair operation. During the pulse gas shielded welding, a small amount of dilution is produced.
4. Repair welding repair
4.1 Surface treatment
The heat exchanger housing is machined after forging, which has certain manufacturing errors and cannot guarantee the ellipticity required for semi-automatic welding. At the same time, due to the corrosion product iron sulfide, a particle layer having a depth of 1 to 2 mm appears on the surface of the casing. To prevent cracks and pores from occurring on such surfaces. Machining on site, removing about 3mm surface layer, to ensure that the surface is clean, dry, no lubricant, etc., to ensure the quality of repair welding.
4.2 preheating
The area under the gasket, the wall thickness is 300mm, and the material is 2.25Cr1Mo. Repair welding is carried out by means of high temperature preheating. The preheating temperature is 200 °C. After the welding is completed, the post-weld heat treatment is performed before the temperature of the casing is lowered to the ambient temperature. The preheating is wound around the outside of the casing using a resistance heating belt. In order to ensure the safety of the operator, the heating is manually controlled, and a thermocouple thermometer is placed in the housing to continuously monitor the temperature inside the container. When the preheating temperature is reached, the heating is stopped, and the preheating temperature is maintained by the welding heat; when the temperature is lower than 200 ° C, the power is turned on after the operator leaves the container, the preheating temperature reaches the requirement to turn off the power, and the operator enters the container again. Welding.
4.3 welding operation and protection
Welded zone operation, the weld zone height is 500mm and the length is 1250mm. The welder is mounted on a special circumferential automatic welding device. When the weld gun returns to the starting position after completing a weld bead, the welding torch is lifted and a weld bead is automatically welded. After completing a welding work area, the insulation box is rotated by 120°. Repeat the above process. A smooth transition at the junction of the two welding work areas. After the entire circumferential band is repaired, the insulation box rises to the top of the repairing zone, and the corrosion is more serious in the 200mm area below the gasket. The second layer is repaired and the third layer is repaired under the gasket. Smooth transition between.
Repair welding is carried out in a container with a preheating temperature of 200 ° C and there is a risk of electric shock. In order to ensure the safety of the operator, an insulated work box is specially made, and the insulated work box can be lifted outside the container at any time by means of a lift provided at the top of the container. The operator has enough free space in the work box. The outside of the work box is insulated, and a circulation pipe vent at the bottom of the work box is continuously blown into the cold air to cool down. Working hours are limited to 45 minutes. During operation, the temperature and oxygen in the operating box are closely monitored outside the container by the monitoring system.
4.4 Post-weld heat treatment
The normal heat treatment temperature was 695 °C. The heating rate was 20 ° C / h. However, since the wall thickness of the casing varies from 300 mm to 50 mm, the temperature difference is allowed to fluctuate within 20 °C. The temperature measured by the thermocouple during the actual heat treatment was 680 ° C - 710 ° C, which achieved the intended purpose. The holding time was 6 hours, then cooling, the cooling rate was limited to 20 ° C / h, and after cooling to 400 ° C, it was naturally cooled. The heating source is mainly outside the container, and a heater is built in the thickest part of the wall thickness of 300 mm below the gasket to make the entire casing heat evenly. 18 thermocouple thermometers were placed on the repair layer inside the container, and the temperature between 300 mm and 50 mm was measured. The temperature recorder recorded the temperature of the whole process.
4.5 Post-weld inspection
The welding process is a semi-automatic process, the process parameters are set by the test procedure, the possibility of welding defects between the layers is small, and since the preheating temperature of 200 °C avoids the transformation of martensite, the possibility of cracking is exclude. Therefore, the main type of defect is that the pinhole or surface crack extends to the entire repair layer thickness. It is difficult to identify the weld surface or a defect of about 1.5 mm or less due to ultrasonic flaw detection. Therefore, the coloring penetration method is used for inspection, and the surface of the casing of the repair welding area is cleaned before inspection. No defects were found in the inspection and the welding quality was qualified.
5. Conclusion
Practice has proved that the repairing program is advanced and reasonable, and the safety protection measures are proper. During the regular maintenance after one year of repair operation, the shell-side repair layer of the heat exchanger is slightly corroded, and the corrosion rate is within the design allowable range. This equipment is currently working well. This time, the welding repair of the high temperature and high pressure hydrogen heat exchanger was successful, and accumulated experience in the field repair of petrochemical equipment under special conditions.
references
[1]Mi Jie. Design Analysis of High Temperature, High Pressure and Hydrogen Equipment[J].化工设备与管é“,2006,(6):31-32.
[2]Wang Ronggui, Wang Wei. Corrosion and protection of high temperature hydrogen and hydrogen sulfide mixed gas equipment[J]. Fertilizer Design, 2005, (1): 22-26.
[3] China Society of Mechanical Engineering Welding Society. Welding Manual (Third Edition) [M]. Beijing: Mechanical Industry Press, 2008.
[4] International Welding Society. The WRC-1992 Diagram [J]. Svetsaren, 1993, 47(2)