General corrosion of heat exchanger tubes General corrosion is often due to improper material selection during design, or process conditions that cannot meet the requirements of design conditions, or long-term use due to erosion and electrochemical corrosion that causes the overall wall thickness of heat exchange tubes to decrease. The data can be measured when the heat exchanger tubes are overhauled, and the safe use period can be estimated according to the corrosion rate of the heat exchanger tubes. It is also possible to prolong the service life by adjusting the process parameters and taking some corrosion inhibition measures. When heat exchanger tubes are partially corroded, such as pitting corrosion, stress corrosion, etc., it is unpredictable. The length of about 50cm from the inlet of the heat exchanger tube is the most prone to corrosion. The machined convex and concave parts in the heat exchanger tubes and the outer bends of the U-shaped tubes are prone to corrosion. Because the radius of curvature of the tubes in the center of the tube bundle of the U-shaped tube heat exchanger is too small, the outer side is often where stress corrosion occurs, and most of the tubes are blocked with plugs in the center.
The parts of the heat exchanger shell that are prone to corrosion:
(1) The heat exchanger shell and its accessories are completely welded, so the weld seam and heat-affected zone of the heat exchanger are prone to corrosion cracks, especially the fluid medium of the heat exchanger shell is a corrosive medium. Poor welding and heat treatment quality of the device is more prone to leakage.
(2) When the temperature and concentration of the fluid in the heat exchanger shell are high, the corrosiveness increases suddenly, and chemical corrosion and stress corrosion often form in the heat exchanger weld and heat-affected zone. Reduce the concentration and temperature of the solution and add a corrosion inhibitor.
The most fundamental way to prevent heat exchanger corrosion is to use metal and non-metal materials that are resistant to medium corrosion, start with controlling the parameters of the medium, add corrosion inhibitors, or add activated carbon filters to eliminate highly corrosive impurities.
On the surface of the heat exchanger in contact with the corrosive medium, it is an economical and effective method to cover a layer of corrosion-resistant paint protection layer through a certain coating method to avoid direct contact between carbon steel and the corrosive medium.
The main reasons for the internal leakage of the shell and tube heat exchanger are:
(1) Wall thickness reduction and perforation caused by corrosion and wear of the tubes of the tube-and-tube heat exchanger.
(2) Packaging, corrosion, and vibration of the tube-and-tube heat exchanger cause the expansion part to loosen.
(3) The tubes of the tube-and-tube heat exchanger are in contact with the baffle to cause wear and perforation.
The main reasons for the emergency shutdown of the plate-fin heat exchanger during operation are:
(1) The structure of the plate-fin heat exchanger and the external iron box is seriously deformed.
(2) Damage to the plate-fin heat exchanger and pipes may cause a fire or cannot be dealt with urgently.
(3) Abnormal phenomena occur in the system during the start and stop of the plate-fin heat exchanger, which endangers the safety of the equipment.
(4) Other emergencies that endanger personal and equipment safety.
The cold box with the plate-fin heat exchanger in normal operation, after stopping due to a fault, generally needs to be thawed. The treatment methods mainly include:
(1) Isolate the cold box of the plate-fin heat exchanger and bypass the process gas.
(2) Into the cold box of the plate-fin heat exchanger, pass heated N2 with a dew point lower than -50°C or process gas dried by molecular sieves. Note that the temperature of the hot gas should not be higher than 65°C, and the heating rate should be controlled at 15~20°C. °C/h, must not exceed 30 °C/h.
(3) By adjusting the valves on the thawing line of the plate-fin heat exchanger, the cold box can be heated and purged faster, reasonably and evenly.
(4) Analyze the moisture at each discharge point of the plate-fin heat exchanger. When the oil dew point is lower than -60°C and the oil content is less than 2×10-6, it is considered that the purge is qualified, and the cold box is closed for start-up.
(5) If it is blocked by dust, after the plate-fin heat exchanger heats up, blasting should be carried out to purge the unremoved dust. After purging, the plate-fin heat exchanger equipment should be reset immediately, and replaced with qualified gas for standby.
(6) Take corresponding measures according to the reasons for the emergency shutdown of the plate-fin heat exchanger.
General corrosion of heat exchanger tubes General corrosion is often due to improper material selection during design, or process conditions that cannot meet the requirements of design conditions, or long-term use due to erosion and electrochemical corrosion that causes the overall wall thickness of heat exchange tubes to decrease. The data can be measured when the heat exchanger tubes are overhauled, and the safe use period can be estimated according to the corrosion rate of the heat exchanger tubes. It is also possible to prolong the service life by adjusting the process parameters and taking some corrosion inhibition measures. When heat exchanger tubes are partially corroded, such as pitting corrosion, stress corrosion, etc., it is unpredictable. The length of about 50cm from the inlet of the heat exchanger tube is the most prone to corrosion. The machined convex and concave parts in the heat exchanger tubes and the outer bends of the U-shaped tubes are prone to corrosion. Because the radius of curvature of the tubes in the center of the tube bundle of the U-shaped tube heat exchanger is too small, the outer side is often where stress corrosion occurs, and most of the tubes are blocked with plugs in the center.
The parts of the heat exchanger shell that are prone to corrosion:
(1) The heat exchanger shell and its accessories are completely welded, so the weld seam and heat-affected zone of the heat exchanger are prone to corrosion cracks, especially the fluid medium of the heat exchanger shell is a corrosive medium. Poor welding and heat treatment quality of the device is more prone to leakage.
(2) When the temperature and concentration of the fluid in the heat exchanger shell are high, the corrosiveness increases suddenly, and chemical corrosion and stress corrosion often form in the heat exchanger weld and heat-affected zone. Reduce the concentration and temperature of the solution and add a corrosion inhibitor.
The most fundamental way to prevent heat exchanger corrosion is to use metal and non-metal materials that are resistant to medium corrosion, start with controlling the parameters of the medium, add corrosion inhibitors, or add activated carbon filters to eliminate highly corrosive impurities.
On the surface of the heat exchanger in contact with the corrosive medium, it is an economical and effective method to cover a layer of corrosion-resistant paint protection layer through a certain coating method to avoid direct contact between carbon steel and the corrosive medium.
The main reasons for the internal leakage of the shell and tube heat exchanger are:
(1) Wall thickness reduction and perforation caused by corrosion and wear of the tubes of the tube-and-tube heat exchanger.
(2) Packaging, corrosion, and vibration of the tube-and-tube heat exchanger cause the expansion part to loosen.
(3) The tubes of the tube-and-tube heat exchanger are in contact with the baffle to cause wear and perforation.
The main reasons for the emergency shutdown of the plate-fin heat exchanger during operation are:
(1) The structure of the plate-fin heat exchanger and the external iron box is seriously deformed.
(2) Damage to the plate-fin heat exchanger and pipes may cause a fire or cannot be dealt with urgently.
(3) Abnormal phenomena occur in the system during the start and stop of the plate-fin heat exchanger, which endangers the safety of the equipment.
(4) Other emergencies that endanger personal and equipment safety.
The cold box with the plate-fin heat exchanger in normal operation, after stopping due to a fault, generally needs to be thawed. The treatment methods mainly include:
(1) Isolate the cold box of the plate-fin heat exchanger and bypass the process gas.
(2) Into the cold box of the plate-fin heat exchanger, pass heated N2 with a dew point lower than -50°C or process gas dried by molecular sieves. Note that the temperature of the hot gas should not be higher than 65°C, and the heating rate should be controlled at 15~20°C. °C/h, must not exceed 30 °C/h.
(3) By adjusting the valves on the thawing line of the plate-fin heat exchanger, the cold box can be heated and purged faster, reasonably and evenly.
(4) Analyze the moisture at each discharge point of the plate-fin heat exchanger. When the oil dew point is lower than -60°C and the oil content is less than 2×10-6, it is considered that the purge is qualified, and the cold box is closed for start-up.
(5) If it is blocked by dust, after the plate-fin heat exchanger heats up, blasting should be carried out to purge the unremoved dust. After purging, the plate-fin heat exchanger equipment should be reset immediately, and replaced with qualified gas for standby.
(6) Take corresponding measures according to the reasons for the emergency shutdown of the plate-fin heat exchanger.