Cleaning method of civil scaling and process blockage of plate heat exchanger 2.1. the choice of cleaning agent is pickling, which includes organic acid and inorganic acid. Organic acids are mainly oxalic acid, formic acid, etc. Inorganic acids are mainly hydrochloric acid, nitric acid, etc. According to the analysis of fouling and process, material and scale composition of the heat exchanger, it is concluded that: 1) the flow area of the heat exchanger is small, the internal structure is complex, and the cleaning solution is not easy to discharge if the precipitation is produced. 2) The heat exchanger is made of Ni Ti alloy, and hydrochloric acid is used as cleaning solution, which is easy to produce strong corrosion on plate and shorten the service life of heat exchanger. Through repeated experiments, it was found that the best effect was formic acid as cleaning solution. Adding buffer and surfactant into formic acid cleaning solution can improve the cleaning effect and reduce the corrosion of the cleaning solution. The chemical test of scale samples shows that formic acid can effectively remove the scale. The results show that formic acid can effectively remove the scale attached to the plate, and its corrosion effect on the plate of heat exchanger is also very small. 2.2. basic principle of removing scale 1) dissolution: acid solution is easy to react with calcium and magnesium carbonate scale, and form soluble compounds to dissolve scale. 2) Stripping effect: acid solution can dissolve oxide on metal surface, destroy the combination with scale, so that scale attached to metal oxide surface can be stripped and peeled off. 3) Gas lift: acid solution reacts with calcium, magnesium and carbonate salt water scale, and produces a large amount of carbon dioxide. In the process of carbon dioxide spillage, the scale layer which is difficult to dissolve or dissolve slowly has certain lifting power, which makes the scale fall off the heating surface of the heat exchanger. 4) Loose action: for the scale mixed with silicate and sulfate, the remaining scale will become loose and easily washed down by flowing acid solution due to the dissolution of oxides of calcium, magnesium, carbonate and iron in acid solution. 2.1. process requirements for cleaning scale 1) pickling temperature: it is beneficial to improve the descaling effect by increasing the pickling temperature. If the temperature is too high, the corrosion of pickling solution to the plate of heat exchanger will be intensified. Through repeated tests, it is found that it is advisable to control the pickling temperature at 60 ℃. 2) Acid pickling solution concentration: according to repeated test, the pickling solution should be prepared according to the concentration of formic acid 81.0%, water 17.0%, buffer 1.2%, surfactant 0.8%, and the cleaning effect is excellent. 3) Pickling method and time: pickling method shall be carried out by combining static immersion and dynamic cycle. The pickling time was first soaked for 2 hours, then it was circulated dynamically for 3 ~ 4H. The pickling concentration should be sampled and tested frequently during the pickling process. When the difference between the two adjacent tests is less than 0.2%, the acid washing reaction is considered to be over. 4) Passivation treatment: after pickling, most of the scale and metal oxide on the surface of plate heat exchanger are dissolved and shed, exposing new metals, which are very easy to corrode. Therefore, after pickling, passivation treatment is carried out for the plate of heat exchanger. 2.4. specific steps for cleaning scale 1) flushing: before pickling, the heat exchanger shall be washed open to make sure that there is no mud, scale and other impurities in the heat exchanger. This can not only improve the effect of pickling, but also reduce acid consumption of pickling. 2) Pour the cleaning fluid into the cleaning equipment and then into the heat exchanger. 3) Pickling: soak the heat exchanger filled with acid solution for 2 hours, and then circulate continuously for 3 ~ 4h, and wash it in positive and negative alternately every 0.5h. After the acid cleaning, if the pH value of acid solution is greater than 2, the acid solution can be reused. Otherwise, the acid washing solution shall be diluted and then dropped. 4) Alkali washing: after the acid cleaning, NaOH, Na3PO4 and softened water are prepared in a certain proportion. The heat exchanger is alkaline washed by dynamic circulation to reach acid-base neutralization so that the plate of heat exchanger will not be corroded. 5) Water washing: after the alkali washing, wash the heat exchanger with clean softened water for 0.5h repeatedly, and thoroughly wash the residue in the heat exchanger. 6) Record: during cleaning, the time of each step shall be strictly recorded to check the cleaning effect. In industrial systems such as power plants and large quantities of sewage treatment, because of the large heat exchanger and dirty water quality, the heat exchanger will appear frequent blockage and scaling. At this time, it is very difficult to disassemble the heat exchanger. The solution is mainly the system backwash and built-in filter. The conventional reverse flushing system is to install the reverse flushing valve on the inlet and outlet pipes of the heat exchanger. The diameter of the flushing valve shall match the system pipeline. When the system operation resistance is more than twice the design resistance, it can be judged that the heat exchanger has blocked. At this time, the operation of the heat exchanger shall be stopped, the inlet and outlet valve of the heat exchanger shall be closed, and the outlet flushing valve of the heat exchanger shall be connected with at least 0.2MPa pressure water, Open the outlet and inlet cleaning valve of the heat exchanger, and the clean water flows out of the inlet cleaning valve of the heat exchanger. When the water flowing out becomes clear from turbidity, it can be re connected with the cleaning agent for scale cleaning, and the heat exchanger can be descaled. Another way is to install a built-in filter screen with the same size as the size of the heat exchanger in the inlet diameter of the heat exchanger on the side where the water quality of the heat exchanger is dirty, such as the open circulating water side. Before the open water enters the heat exchanger, it will pass the internal filter screen before the heat exchange. After a period of operation, the blind flange can be opened from the back plate blind flange of the heat exchanger, the internal filter screen can be pulled out, washed or replaced. This process can be cleaned without disassembling the heat exchanger, saving time and resources and will not affect the process production. However, it is to be noted that whether it is reverse flushing or internal filter device, it only delays the blockage and scaling of the heat exchanger, but can not solve the blockage and scaling of the heat exchanger. To extend the plugging time of the heat exchanger, the most important thing is to choose a more reasonable plate type which is not suitable for plugging in the initial design of the heat exchanger. To remove the scale completely, the heat exchanger should be removed for pickling and alkali washing. Finally, after the heat exchanger is disassembled and cleaned, the heat exchanger shall be pressure tested and can be used only after it is qualified.