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How to descale plate heat exchanger?

Views:1     Author:Site Editor     Publish Time: 2020-03-26      Origin:Site

Preparation of plate heat exchanger before cleaning


Fouling analysis of plate heat exchangers


  Plate heat exchangers can be generally divided into two modes: water-water exchange and steam-water exchange. The water-water exchange method exchanges water with cold and hot media, and the temperature difference between cold and heat is not large, about 70 ~ 90 ° C. The scaling on both sides is basically the same; the steam-water exchange method is water vapor, which is generally not easy to scale. The cold medium is water, and the temperature is about 90 ° C, which is easy to scale. Its scale can be roughly divided into scale and dirt, especially scale. The scale is mainly that various salts dissolved in water are reduced in thermal decomposition solubility and crystals are deposited on the heat transfer sheet, usually carbonates, phosphates, sulfates and silicates. Crystals on a certain scale are dense, relatively hard and difficult to remove; Dirt is generally composed of mud with fine particles, dust, insoluble salts, colloidal hydroxide, debris, corrosion products, oil, especially the bacterial and algae corpses and their viscous replicas. The volume shrinks, the thickness is loose and soft, and it is easier to remove.


Selection of descaling cleaning method and cleaning process for plate heat exchanger


  The scale sample of the plate heat exchanger is mainly scale, which is relatively hard, and it is firmly combined with the heat transfer sheet, which is difficult to be removed by physical methods. Therefore, the acid cleaning method in chemical cleaning is selected for descaling. According to the fouling of the plate heat exchanger, the degree of intensification and the requirements of the user, the chemical cleaning of the plate heat exchanger can be divided into two methods: separation cleaning and non-disassembly cleaning. Disassembly, cleaning, and descaling are relatively thorough, and the effect is good, but the workload is large, the processing is complicated, and it is easy to cause adverse effects such as leakage of the heat exchanger and damage to parts when the plate heat exchanger is replaced; The labor is small, the processing is simple, and it is not easy to cause adverse effects such as leakage of heat exchangers and damage to spare parts. The fouling situation is serious, the heat exchange efficiency is low, and even when it is suspended, it must be disassembled and cleaned; when the plate heat exchanger is lightly fouled or severely aged, it can be cleaned without disassembly.

  In chemical cleaning, a cleaning process combining cycle cleaning and immersion cleaning can be adopted. Circulation cleaning is a closed circulation system consisting of a circulation pump, a cleaning tank, a plastic pipe, and a cleaning object.     The circulation system is added with an appropriate amount of cleaning agent, and the circulation pump is used for cleaning to ensure the concentration of the cleaning agent. During the circulation process, every 1 h To detect the concentration of the cleaning agent in the cleaning tank once, so that the concentration of the cleaning agent reaches a certain level, turn off the circulating pump to soak. The concentration is always kept within the safe and effective range of 0 · 10 ~ 0 · 15mol / L, and a cleaning agent needs to be added if necessary. At noon or at night, you can take the cleaning agent after soaking.


Choice of cleaning agent


  The heat transfer sheet material of the plate heat exchanger is generally austenitic stainless steel, and the models are roughly AISI 304, 304L, AISI 316, 316L, 316T, and i stainless steel materials need to be cleaned with nitric acid-based cleaning agents. (NBR) 130-140, butyl rubber (RCB) 140, ethylene propylene rubber (EPDM) 150, these materials are resistant to acid, alkali, ketone, alcohol and other solutions. Based on this, we use the special cleaning agent for stainless steel products developed by the Energy Research Institute of the Henan Academy of Sciences (the cleaning agent has passed the technical identification organized by the Henan Provincial Department of Science and Technology), mainly nitric acid, glycolic acid, corrosion inhibitors, penetrants, and disinfectants Composed of chemical agents such as foaming agents, this cleaning agent has the characteristics of high descaling rate, low corrosion rate, and fast descaling rate for stainless steel products. The optimal cleaning concentration is in the range of 0 · 10 ~ 0 · 15mol / L. The performance indicators of the cleaning agent have been tested in accordance with HG / T 2387-2007 "Quality Standards for Chemical Cleaning of Industrial Equipment".


Disassembly and cleaning of plate heat exchanger


Disassembly


  First close the inlet and outlet valves of the heat medium of the plate heat exchanger, then close the inlet and outlet valves of the cold medium, drain the medium (usually water or steam), and use a steel ruler to measure the net size of the heat exchanger's compressed heat exchanger. (The distance between the inner surfaces of the two pressing plates) and record. Remove the clamping bolts of the heat exchanger evenly and symmetrically, gently push the movable compression plate along the track, and remove the heat transfer sheet one by one (be careful not to tear or tear off the sealing rubber pad), and follow the order Put it right. Since some heat exchangers use heat transfer fins with multiple lines, it is important to keep in mind the number of fins and the combination order of multiple lines.


Disassembly chemical cleaning


Connection of chemical cleaning circuit

  Use a cleaning pump, a cleaning tank, and a chemical cleaning cycle system composed of plastic pipes, and place the removed heat transfer sheets in the cleaning tank in order (the heat transfer sheets should be parallel to the direction of water flow). After the water is injected, check whether the system is normal.

Chemical cleaning process implementation

  Add an appropriate amount of cleaning agent and start cycle cleaning. According to the chemical cleaning process, the concentration of the cleaning agent in the cleaning tank is checked every 1h during the cycle, so that the concentration of the cleaning agent is always kept within a safe and effective range of 0 · 10 ~ 0 · 15mol / L, and according to the measurement Add cleaning agent to data. In the early stage of cleaning, in comparison with the reaction, a large amount of foam appeared, and the defoaming agent could be sprayed to defoam. At the same time, when the concentration of the cleaning solution was not changed for 2 consecutive hours or the change was very small, and the scale on the surface of the heat transfer sheet was significantly reduced. It can be removed with a light brush with a plastic brush. At this time, it can be regarded as the end of the cleaning.

Descaling

  After the chemical cleaning is completed, remove the heat transfer sheet one by one, and use a washing machine to remove the residual scale on both sides of the heat transfer sheet. If necessary, use a plastic brush to assist in scrubbing. During the cleaning process, the pressure of the washing machine should not be too high, and 490 ~ 588kPa is appropriate. The water column should not be perpendicular to the heat transfer sheet, and there should be a certain angle to avoid damage to the heat transfer sheet. At the same time, the order of the heat transfer plates and the safety of the gaskets on the heat transfer plates should be paid attention to.

Assembly

  After the descaling is completed, start from the fixed pressing plate in the order of superposition of the heat transfer sheet when disassembling, and then stack the heat transfer sheets along the track in turn (note that the sealing rubber pad on the heat transfer sheet should not be replaced, if there is a replacement sealant available) . After all the heat transfer plates are installed, carefully press the movable compaction plate along the track, and place the compaction bolts in the wire grooves in turn, and evenly, slowly, and symmetrically (otherwise it is easy to crush or crush the heat transfer plate). Deflection, causing the plate heat exchanger to leak water or air) until the compressed net size of the heat transfer fin reaches or is slightly smaller than the original size.

Hydraulic test

  After the plate heat exchanger is assembled, the single-sided hydraulic test should be performed separately according to the requirements of GB 16409-1996 "Plate Heat Exchanger". During the test, the pressure should be slowly increased. After the specified pressure is reached, the holding time is 10 ~ 30min. Redesign the holding pressure under pressure, check all sealing surfaces to see if there is leakage, and hold the pressure for more than 30min.

Restore

  After the hydraulic test is safe, remove the connecting pipe, close each sewage valve, first open the inlet and outlet valves of the cold medium, and then open the inlet and outlet valves of the hot medium.


Non-disassembly cleaning of plate heat exchanger


Non-decomposing chemical cleaning


Connection of chemical cleaning circuit

  First close the inlet and outlet valves of the heat medium of the plate heat exchanger, and then close the inlet and outlet valves of the cold medium to drain the medium (usually water or steam). The inlet and outlet of the hot and cold medium are used as the cleaning liquid inlet, and the outlet and outlet of the hot and cold medium are used as the cleaning liquid outlet (if there is no outlet, you need to add and add a valve). The sequence of the medium outlet, as well as the cleaning pump, the cleaning tank, and the plastic pipe form a chemical cleaning cycle system.

Chemical cleaning process implementation

  According to the chemical cleaning process, the concentration of the cleaning liquid in the cleaning tank should be detected every 1 h during the cycle to keep the concentration of the cleaning liquid constant. Keep it within the safe and effective range of 0 · 10 ~ 0 · 15mol / L, and add cleaning agent based on the measured data. Chemical cleaning without disassembling may take a long time, and can be combined with cyclic cleaning and osmotic cleaning. When the concentration of the cleaning agent does not change for 2 hours or the change is small, the cycle can be stopped.

Rinse

  After chemical cleaning, due to the small internal flow of the plate heat exchanger, there is always scale and slag (especially dirt) on the heat transfer sheet, so the cycle and scum need to be repeatedly washed away. Drain the waste liquid in the cleaning tank, fill it with fresh water, rinse it with a circulating pump, and drain the waste water. Repeat the flushing, and observe the water continuously until no more slag is discharged from the plate heat exchanger.

Neutralization

  Neutralization is to neutralize the residual acid solution in the equipment after cleaning without corroding the equipment; passivation is to form an electronic conductor film on the metal surface that can inhibit the dissolution process of the metal. This film itself has a very slow dissolution rate in the medium. , So that it can keep the metal's anode dissolution rate at a small value. Since the chemical cleaning requires several sections of pipes connected to the plate heat exchanger, the heat transfer sheet is made of stainless steel and does not need to be neutralized and turbidized, but the pipes are made of steel. The scale and rust on the surface after cleaning are all cleaned. Remove and expose the nature of steel. It is in a very active and activated state and is susceptible to rust, so it needs to be neutralized and transparentized to prevent secondary rust.

Neutralization

  The neutralization treatment can be supplemented with a neutralization aid such as sodium hydroxide, sodium carbonate, etc., which is used at a dosage of 0.5%, that is, 1t of water plus 5kg of neutralization agent to neutralize the acid cleaning agent remaining in the circulation system. The treatment was stopped until the pH reached 7 cycles.

Darkening

  After the neutralization treatment, passivation treatment is performed. An appropriate amount of passivation pre-filming agent is added to the circulation system. After the circulation is uniform, the pH is controlled between 8 and 9. The amount of transparent pre-filming agent is calculated based on the circulating water of the cleaning agent. , Water plus 10kg of passivation pre-filming agent.

Restore

  After passivation, remove the connection pipe, close each sewage valve, open the inlet and outlet valves of the cold medium, and then open the inlet and outlet valves of the hot medium.

  The chemical cleaning of the plate heat exchanger can be divided into separation cleaning and non-separation cleaning. Each has its advantages and disadvantages. It can be selected according to the specific conditions of the equipment and the requirements of the user, providing customers with a variety of choices. After repeated practice cleaning, the two chemical cleaning methods have mature technological processes, short cycles, thorough cleaning, economical, fast, and satisfying customers. They provide a reliable guarantee for the normal and safe operation of the plate heat exchanger.


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