Views:0 Author:Site Editor Publish Time: 2020-05-26 Origin:Site
The scale and heat transfer sheet of the plate heat exchanger are firm in structure and difficult to be cleaned by physical methods, so it replaces the acid cleaning method in chemical cleaning to remove scale. In this method, a chemical solution is circulated through the heat exchanger to dissolve and discharge the dirt on the surface of the plate. This method does not require disassembly of the heat exchanger, which simplifies the cleaning process and reduces the degree of cleaning. Since the corrugation of the plate can promote the flow of the cleaning liquid and is beneficial to the dissolution of the scale layer, the chemical cleaning method is an ideal method.
The choice of cleaning agent is currently pickling, which includes organic acids and inorganic acids. Organic acids mainly include oxalic acid and nitric acid; inorganic acids mainly include hydrochloric acid and nitric acid. According to the fouling and process of the heat exchanger, the analysis of the materials and scale components leads to:
(1) The heat exchanger has a small circulation area and complex internal structure, and it is not easy to discharge the cleaning fluid if it is precipitated.
(2) The use of hydrochloric acid as the cleaning liquid is easy to cause strong corrosion to the plate, and shorten the insertion of the heat exchanger.
Therefore, it is best to choose cation as the cleaning solution. By adding buffer and surfactant to the cationic cleaning solution, the cleaning effect is better, and the corrosion of the cleaning solution to the plates is gradually reduced.
(1) Dissolution: The acid solution easily reacts with calcium, magnesium and sodium carbonate scales to form soluble compounds and dissolve the scale.
(2) Stripping and removing effect: The acid solution can dissolve the oxide on the surface of the metal and destroy the combination with scale, so that the scale attached to the surface of the metal oxide is replaced and reduced.
(3) Gas tilting effect: After the acid solution reacts with calcium, magnesium and calcium phosphate scales, a large amount of carbon dioxide is generated. During the overflow process, the carbon dioxide gas has a certain tilting power for the insoluble or slow-dissolving scale layer, so that the scale can The displacement of the heated surface of the heat exchanger drops.
(4) Loosening effect: For mixed scales containing silicate and sulfate, as the oxides of calcium, magnesium, carbonate and iron are dissolved in the acid solution, the remaining scale will become loose and easily washed down by the flowing acid solution .
3. Process requirements for cleaning scale
(1) Pickling degree: Increasing the pickling temperature is beneficial to improve the descaling effect. If the temperature is too high, the corrosion of the pickling liquid to the heat exchanger plates will be aggravated, and the temperature of the pickling liquid should be controlled at 60 degrees.
(2) Concentration of pickling solution: The pickling solution should be prepared according to the concentration of 81% formic acid, 17% water, 1.2% buffer, and 0.8% surfactant. The cleaning effect is better.
(3) Pickling method and time: The pickling method should be combined with static immersion and dynamic circulation. The pickling time is to soak for 2 hours today and then dynamically cycle for 3 to 4 hours. During the pickling process, the acid pickling concentration should be frequently sampled and tested. When the concentration of the next two pickling tests is less than 0.2%, the acid pickling reaction can be considered completed.
(4) Dunhua treatment: After the pickling is completed, most of the scale and metal oxides on the surface of the board are dissolved and peeled off, exposing brand new metals, which are easy to corrode.
4. Specific steps for cleaning scale
(1) Flushing: Before acid pickling, the heat exchanger is firstly opened to make the heat exchanger free of mud, scale and other impurities, which can not only improve the effect of pickling, but also reduce the acid consumption of pickling.
(2) Pour the cleaning fluid into the cleaning equipment and then pour it into the heat exchanger.
(3) Pickling: Soak the heat exchanger filled with acid solution for 2 hours, and then continuously and dynamically circulate for 3 to 4 hours, during which the positive and negative cleaning is performed alternately every 0.5 hours. After the pickling is completed, the pH value of the weak acid solution is greater than 2, and the acid solution can be reused. Otherwise, the acid solution should be diluted and neutralized and then discharged.
(4) Alkaline washing: After the acid washing is finished, it is prepared with sodium hydroxide, sodium, polonium and demineralized water according to a certain proportion, and the heat exchanger is alkali washed by dynamic circulation to achieve acid and alkali neutralization and make the heat exchanger The plates no longer corrode.
(5) Water washing: After the alkali washing, rinse the heat exchanger repeatedly with clean demineralized water for 0.5 hours to rinse the residues in the heat exchanger thoroughly.
(6) Record: During the cleaning process, the time of each step should be strictly recorded to check the cleaning effect.
5. Auxiliary work of cleaning system
In order to obtain the ideal cleaning effect, it is necessary to equip temporary pipe joints, flanges, valves, pipes, and pumps must be correctly assembled to make their operation safe and convenient. In order to ensure that the heat exchanger is not corroded, hang test pieces of the same material in the temporary pipeline, and observe the surface condition at any time during the cleaning process. After cleaning, the corrosion rate is determined based on the change in quality.
After the pickling is completed, the organic acid with a concentration of 0.1% is poured into the heat exchanger for 30 minutes, and then poured into the passivation solution for 5 hours to form a dense protective film on the washed metal surface. After the cleaning is completed, check the inlet and outlet of the heat exchanger and other parts, all the scales are all out, and no residue is deposited.
6. Measures to prevent fouling of plate heat exchanger
(1) Strictly control the water quality during operation. The softened water in the drinking water softening tank in the system must be strictly tested for water quality, and only after passing the test can it be injected into the pipe network.
(2) When the new system is put into operation, the heat exchanger should be separated from the system. After a period of circulation, the heat exchanger should be incorporated into the system to avoid impurities in the pipe network from entering the heat exchanger
(3) In the entire system, in addition to the irregular cleaning of the dirt filter and the filter, the pipe network should also be kept clean to prevent the heat exchanger from being blocked.