1. Reasonable process design
When designing, place steam on the side of the tube to avoid high-speed gas flow through the shell side. When the shell side has a large flow medium, multiple shell inlets can be designed to buffer the pressure. In addition, an anti-shock plate should be provided to reduce the erosion corrosion caused by high-speed fluid to the equipment.
In order to avoid the retention of residual liquid and sediment, double-sided butt welding and continuous welding are used as much as possible to avoid lap welding and spot welding. In the welding process, the stress causing stress corrosion cracking should be based on actual experience. The residual stress is mainly composed of internal stress caused by cold working and welding.
Heat treatment of cold-worked and welded parts helps to eliminate residual stresses and thus helps prevent stress corrosion. Stress annealing heat treatment is often used to eliminate residual stress or other methods to eliminate residual stress, such as hydraulic testing, vibration aging and hammering.
In addition, the tube bundle must be hoisted with a nylon belt to ensure that the metal surface is flat, without scratches, and can be smoothly inserted into the shell.
2. Corrosion resistant materials
Corrosion-resistant materials (such as binocular stainless steel, Hastelloy, titanium, titanium alloy, copper, etc.), these materials are highly resistant to corrosion and can improve the service life of the cooler. However, these highly corrosion-resistant materials are expensive, the manufacturing cost is high, and the cost of one-time investment is large, and enterprises are generally difficult to accept and difficult to promote.
3. Electrochemical protection method
The electrochemical protection method can not only prevent stress corrosion cracking, but also stop expanding even if cracks are generated under the condition that the protection parameters are properly selected. A method of sacrificing anion/anode protection or surface spraying of a corrosion resistant metal may be employed.
Cathodic protection method:
The use of an external DC power source protects the anode on the metal surface from becoming a cathode.
This method consumes a lot of power, is expensive, and uses very little.
Anode protection method:
The protected device is connected to the anode of the power supply to form a passivation film on the metal surface to achieve protection. Carbon steel coolers are low in cost but poor in corrosion resistance.
Sacrificial anode protection technology can increase the life of the cooler. However, the protection of this technology is limited to a limited length at the inlet of the tube, and it is difficult to achieve cathodic protection in the depth of the tube. Therefore, the application of the sacrificial anode protection method on the cooler is greatly limited.
4. Add corrosion inhibitor method
In corrosive media, a small amount of certain substances are added, which can cause the corrosion of the metal to be greatly reduced or even stopped. Such substances are called corrosion inhibitors. Figure 6 is a comparison before and after the use of corrosion inhibitors. The addition of corrosion inhibitors should be based on the principle of not affecting the production process and product quality.
Stress corrosion can be controlled by removing dissolved oxygen and oxidant from the medium. Reducing the mass concentration of CL in the medium and strictly controlling the mass concentration of sulfur in the medium are also effective measures to control stress corrosion.
5. Anti-corrosion coating method
On the metal surface, a corrosion-resistant coating protection layer is applied by a certain coating method to avoid direct contact of the metal surface with the corrosive medium. This technique is the most cost-effective and is initially used to prevent corrosion of gaseous media. Most of the coatings used are organic polymer mixture solutions. Nowadays, people are gradually developing towards oil-proof and anti-solvent coatings, high-temperature coatings, heavy-duty coatings and special environmental coatings.
6. Cooler operation
When the cooler is in operation, the cold fluid is first filled in the container, the inlet is closed, and the hot fluid is slowly injected, so as to minimize the difference in thermal expansion between the tube and the casing formed by introducing the fluid.
After parking, use dry compressed air to remove all fluid from the cooler, which minimizes stress and avoids stress corrosion. During operation, the upper and lower water valves remain fully open to avoid slowing of the flow rate, and impurities in the medium precipitate on the tubular surface causing corrosion after scaling.