Heat exchangers are important equipment in the petrochemical industry, accounting for approximately 20% of the plant investment costs and 40% of the total weight of process equipment. During the production and operation process, due to harsh and harsh usage conditions, the heat exchanger will suffer from crevice corrosion, perforation, and leakage under the action of corrosion and erosion of high-temperature liquid phase, gas phase, or multi-phase corrosive media, and the heat exchanger will be damaged. Life span is shortened, causing huge direct and indirect economic losses . The anti-corrosion methods of heat exchangers mainly use corrosion-resistant materials, organic coating protection, electrochemical protection, high-temperature aluminizing, zincizing and other methods , among which organic coating protection is the most economical and effective method. There are many varieties of protective coatings used for heat exchangers, and most of them are thermosetting coatings that require high-temperature curing. There are few varieties that are cost-effective, simple in construction technology, and cured at room temperature with excellent anti-corrosion properties. This time, the anti-corrosion coating on the inner wall of the tube side of a set of atmospheric and vacuum pressure constant-top oil and gas-crude oil heat exchangers at Sinopec Zhenhai Refinery used a newly introduced American product – polyurethane anti-corrosion paint. Polyurethane anti-corrosion paint relies on Its excellent comprehensive performance has been applied for the first time in the coating of the inner wall of heat exchangers.
2 Advantages of polyurethane anti-corrosion coating
Compared with the currently used anti-corrosion coatings, the polyurethane anti-corrosion coating has the following characteristics:
(1) Super adhesion: The adhesion between polyurethane anticorrosive paint and steel substrate can reach 24.9Mpa (see Table 1 for some test data), while other anticorrosive coatings generally do not exceed 10Mpa. It contains a unique adhesive component that has a strong penetration and wrapping effect on the substrate. It relies on the reaction of polar groups and interface metal atoms to form extremely strong chemical bonds, which enhances the adhesion between the coating and the substrate and improves the coating quality. The overall corrosion resistance of the layer.
(2) Non-porous water-permeable coating film: The advanced technological feature of polyurethane anti-corrosion paint that distinguishes it from other anti-corrosion coatings is that there are no pores after the moisture-cured polyurethane paint film is formed, thus completely eliminating water vapor, oxygen, various Penetration of chemically corrosive media, the coating has good acid and alkali resistance, protecting metals from corrosion.
(3) High temperature resistance: It can withstand high temperatures up to 315°C, providing excellent protection for the heat exchanger operating in harsh high temperature environments.
(4) Ultra-high elongation of 79%, and its flexibility allows it to withstand peeling and cyclic stress.
(5) After curing, the coating film becomes hard, flat and smooth, which can effectively prevent scaling and coking of the heat exchanger tube wall and ensure the good operation of the heat exchanger.
(6) Good thermal conductivity, overall improving heat transfer effect.
(7) Good coating performance, high surface tolerance, and excellent edge coverage performance.
3 Polyurethane anticorrosive paint pipe inner wall alumina ceramic coating process
(1) Technical parameters of a set of atmospheric and vacuum constant top oil and gas-crude oil heat exchanger
①Tube side medium: normal top oil and gas (including H2SHClH2O); shell side medium: crude oil; temperature: 110℃
②The heat exchanger has a diameter of 1.4m, a length of 6m, more than 2,000 tubes, and a tube diameter of 12mm.
③Coating parts: inner wall of pipe side, oil and gas side; material: carbon steel
(2) The process flow of anti-corrosion on the inner wall of the pipe  is shown in Figure 1. A brief description of the main processes:
① Inspection before construction: Focus on the problems that affect the coating quality such as deformation and crowding of heat exchanger tubes, clogging of mechanical impurities in the tubes, severe wear and corrosion, etc.
②Sandblasting and rust removal in the tube side: Place the heat exchanger flat on the rotating table, sandblast and remove rust on the inner wall of the tube side, and the rust removal quality reaches Sa2.5 level.
③Purge inside the pipeline to clean the remaining steel sand and floating dust on the inner surface. It is recommended to vacuum with negative pressure.
④ Filling: The paint pump fills and rotates, and the empty paint flows out at a large angle until the liquid paint flows out from the paint discharge end in a drop shape, and the dripping is removed by brushing.
⑤ Accelerate surface drying: Use a hot air blower to enhance ventilation, accelerate solvent evaporation, and speed up the curing time of the coating film. It takes about 12 hours to finger dry.
⑥ Refers to the subsequent coating of the paint film after it dries. There are three layers in total. The total dry film thickness is about 150µm. It takes more than 4 days for the coating film to fully cure at room temperature. In order to exert the best performance, it needs to be cured for more than 20 days before being put into use. . During this period, after the coating film has cured for 8 days, if conditions permit, the equipment can be heated to 70~80°C for about 30 minutes to release the residual solvent, improve the rigidity of the coating film, and improve performance.
2 Coating quality control and inspection
(1) Polyurethane anticorrosive paint is a moisture-curing paint that is sensitive to changes in temperature and humidity in the air. When the temperature and humidity are too high, the curing reaction will be too fast and bubbles will easily occur; similarly, a coating film cannot be too thick, as the gas generated by the curing reaction will be difficult to release, causing coating film defects. The following technical keys that affect quality should be noted:
① Before sandblasting and rust removal, it is necessary to ensure that the inner wall of the pipe and the abrasive are clean and dry.
②Stop sandblasting and painting when the ambient humidity reaches 85% and the steel surface temperature is more than 3°C lower than the atmospheric dew point.
③Avoid construction in high temperature and high humidity environments, and a single coating film cannot be too thick.
④ Ensure the uniformity of coating thickness. Before formal painting, the coating thickness should be adjusted through process testing, and appropriate process parameters should be found out before formal painting is carried out.
(2) Quality control and testing
① Rust removal and purging quality control Use a comparative rust removal standard plate and a roughness detector to detect the surface quality of the rust removal to ensure that the roughness reaches 40~50µm, the rust removal level reaches Sa2.5 level, and the inner wall surface is clean and dry .
②Coating quality control
a Visual inspection or endoscope check the appearance quality of the anti-corrosion layer tube by tube: whether the surface is flat, smooth, and whether there is any leakage or bleeding.
b Coating thickness detection: After the coating is dry, use a point thickness gauge to measure the thickness. The coating thickness should be greater than 160µm.
③ Check the continuous integrity of the coating: Use a high-voltage spark leak detector to conduct spark leak detection on the anti-corrosion pipe process.
④ Carry out strict quality management: follow up at any time and solve problems in time.
3 Summary of polyurethane anti-corrosion paint coating process In the anti-corrosion construction of heat exchangers, compared to thermosetting paints cured at high temperatures, polyurethane anti-corrosion paint is a room temperature curing paint. The construction is relatively simple, but it also encounters some problems. , mainly have the following two points:
(1) The interval between the first pouring and the second pouring is too short, the pipe length is too long, and the pipe diameter is too thin. The polyurethane anticorrosive paint cannot be fully cured. It is recommended to ventilate and extend the curing time.
(2) Because the pipe length is too thin, the viscosity of the polyurethane anticorrosive paint is slightly high, which is not conducive to the construction of pouring coating. It is recommended to dilute it with a special polyurethane solvent, and the dilution amount does not exceed 5%.
In the past few months since the heat exchanger was put into use, practice has proved that polyurethane anti-corrosion paint is a new anti-corrosion product with leading technology and excellent performance; polyurethane anti-corrosion paint is used to anti-corrosion coat the inner wall of the heat exchanger tube. Installation is an anti-corrosion method with simple construction, high efficiency and stable and reliable coating quality. Through the introduction of this article, I hope that this product can be applied in more heat exchanger anti-corrosion projects
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