Frist, Maintenance of plate heat exchanger
The precautions for the removal of the plate heat exchanger are as follows:
a. Before the plate heat exchanger is disassembled, the compaction length of the bundle should be measured and recorded (resize according to the reload).
b. If the gasket is stuck in the groove between the two plates, use a screwdriver to carefully separate it. The screwdriver should be inserted from the easily peelable part and then along it. Separate the perimeter and do not damage the heat exchanger plates and gaskets.
c. When replacing a new gasket, use a solvent such as acetone or other ketone organic solvent to wipe the gasket groove. The synthetic resin adhesive is evenly applied to the grooves by a brush.
d. Check the heat exchanger plate for perforation, generally with a magnifying glass of 5 times, sometimes with a light or kerosene infiltration method.
e. If the medium inlet and outlet short tubes and channels are found to have debris accumulation, the filter is invalid and should be cleaned in time.
f. There are three ways to clean the plate, namely, the recoil method (do not open the cleaning), the manual cleaning method (open cleaning), and the chemical cleaning method (do not open the cleaning).
(1) manual cleaning method. When the scale of the heat exchanger sheet is very thin and insoluble in water, it can be disassembled, and the sheet is treated with pressurized water (0.1 to 0.2 MPa) or with low pressure steam with water. It is difficult to wash with water. The sediment can be washed with a soft fiber brush and a bristle brush.
(2) chemical cleaning method. The surface of the heat exchanger sheet, especially the dead corner of the medium flow, has hard deposits (oxide or carbide), which is difficult to solve by manual cleaning. It can be different according to the material of the heat exchanger sheet. Chemical solvent to clean.
g. When the heat exchanger plate is crusted, avoid brushing with a wire brush or a wire brush, especially a stainless steel plate to prevent corrosion of the plate. If there is stain or rust on the plate, remove it with a decontamination powder.
h. Clean water must be free of salt, sulfur and other ingredients.
i. After the heat exchanger plates are cleaned by chemical means, they must be cleaned with water, then cleaned with a fine gauze and placed in a clean place for later use.
j. When disassembling the sheet of titanium, it is strictly prohibited to contact the open flame to prevent oxidation.
k. Check the gasket for aging, deterioration, cracks, etc., and disable hard objects on the surface.
l. It is strictly forbidden to store solid particles such as sand and iron slag on the surface of the gasket and the heat exchanger plate (in the groove).
m. Check the heat exchanger plate for local deformation. If it exceeds the allowable value, it should be trimmed or replaced.
 
Second, plate heat exchanger assembly reloading
Notes on reloading of plate heat exchanger components are as follows:
a. Before reassembling the components, the parts of the qualified heat exchanger plates, gaskets, heads (heads), clamping bolts and nuts must be cleaned.
b. Before the gasket is bonded to the groove, the residual glue in the groove must be melted with acetone or other similar organic solvent, and then wiped with a fine gauze.
c. Apply the synthetic resin adhesive to the groove of the plate with the same width as the gasket groove, then press it into the gasket and flatten it with a flat steel plate. , put it for 48h.
d. Dissolve and remove the residue that has been squeezed out of the groove with an organic solvent such as acetone.
e. When replacing a new gasket, carefully check the four corners of the new gasket for the same position as the old gasket.
f. When one plate of the plate heat exchanger is damaged and there is no spare part, the plate and the adjacent plate can be removed at the same time, and then the clamping bolts are tightened.
g. When the umbrella plate heat exchanger is reassembled, the plates should be rotated 180° alternately; no misplacement is allowed. Repair the replacement plate and add the bundle as required. If the plate is damaged and there are no spare parts, the adjacent plates can be removed at the same time. If the difference between the plates is wrong, it should be judged from the center triangle mark of the plate, as long as the center triangle mark of the plate is reversed (▽) positive (△), the stacking is wrong.
h. Clamping bolts should be even, symmetrical, and cross-tightened. The clamping bolts are tightened until the length of the bundle reaches the calculated size.
i. To prevent the gasket from sticking to the sheet, apply a layer of the seal to the gasket. The ratio of oil, alcohol, and talc used in the mixture is 1:1:2 by weight.
  
三  Cleaning of the plate heat exchanger after scaling
Board heat exchanger scaling method
1, choice of cleaning agent
The choice of cleaning agent, currently used is pickling, which includes organic and inorganic acids. The main organic acids are: oxalic acid, formic acid and the like. The main inorganic acids are: hydrochloric acid, nitric acid and the like. According to heat exchanger fouling and process, material and scale composition analysis:
1) The heat exchanger has a small flow area, complex internal structure, and it is difficult to discharge if the cleaning liquid is precipitated.
2) The heat exchanger is made of nickel-titanium alloy, using hydrochloric acid as the cleaning solution. It is easy to produce strong corrosion on the plate and shorten the service life of the heat exchanger.
Through trial and error, it is found that the choice of formic acid as a cleaning solution is effective. The buffering agent and the surfactant are added to the formic acid cleaning solution, the cleaning effect is good, and the corrosion of the cleaning liquid on the plate can be reduced. Chemical tests on scale samples have shown that formic acid is effective in removing scale. Through the acid immersion test, it was found that formic acid can effectively remove the scale attached to the sheet, and at the same time it has little corrosion effect on the heat exchanger sheet.
2, the basic principle of removing scale
1) Dissolution: The acid solution easily reacts with calcium, magnesium and carbonate scales to form soluble compounds, which dissolve the scale.
2) Peeling effect: acid solution can dissolve the oxide on the metal surface. Destruction and scale bonding. Thereby, the scale adhering to the surface of the metal oxide is peeled off. And fell off.
3) Gas 掀 effect: After the acid solution reacts with calcium, magnesium and carbonate scale, it produces a lot of carbon dioxide. Carbon dioxide gas is in the process of overflow. For the scale layer which is difficult to dissolve or dissolve slowly, it has a certain enthalpy power, so that the scale falls off from the heated surface of the heat exchanger.
4) Loose effect: For the silicate and sulfate mixed scale, the residual scale will become loose due to the dissolution of calcium, magnesium, carbonate and iron oxides in the acid solution. It is easily washed away by the flowing acid solution.
3, process requirements for cleaning scale
1) Pickling temperature: increasing the pickling temperature is beneficial to improve the descaling effect. If the temperature is too high, it will aggravate the corrosion of the acid heat treatment plate by the acid washing liquid. It is found through repeated tests that the pickling temperature is controlled at 60~E.
2) pickling solution concentration: According to repeated tests, the pickling solution should be based on formic acid 81. O%, water 17. O%, buffer 1.2%, surfactant 0.8% concentration, the cleaning effect is good.
3) pickling method and time: pickling method should be combined with static soaking and dynamic cycling. The pickling time is first static soaking for 2 h, then dynamic cycling for 3 to 4 h. The pickling concentration should be sampled frequently during the pickling process. When the difference between the two adjacent assays is less than 0.2%, the pickling reaction is considered to be complete.
4) passivation treatment: after the end of pickling, the scale and metal oxide on the surface of the plate heat exchanger are mostly dissolved and detached, exposing new metal, extremely corrosive, so in acid After washing, the heat exchanger sheets are passivated.
4, the specific steps to clean scale
1) Punching: Before pickling, first open the heat exchanger, so that there is no mud, dirt and other impurities inside the heat exchanger, which can improve the pickling effect, but also Reduce the acid consumption of pickling.
2) Pour the cleaning solution into the cleaning equipment and then inject it into the heat exchanger.
3) pickling: statically soak the heat exchanger filled with acid solution for 2h. Then continuously cycle for 3 to 4 hours. The positive and negative alternate cleaning is performed every 0.5h. After the acid washing is finished, if the pH of the acid solution is greater than 2, the acid solution can be reused. Otherwise, the acid washing solution should be diluted and neutralized and drained.
4) Alkali washing: After the acid washing is finished, use NaOH, Na, PO, demineralized water to prepare according to a certain ratio, and use the dynamic circulation method to wash the heat exchanger to achieve acid and alkali. Neutralization, so that the heat exchanger plates no longer corrode.
5) Washing: After the alkali washing, use clean softened water. The heat exchanger was repeatedly rinsed for 0.5 h to thoroughly rinse the residue in the heat exchanger.
6) Record: During the cleaning process, the time of each step should be strictly recorded to check the cleaning effect. In short, after the cleaning is completed, the heat exchanger is subjected to a compression test. It can be used after passing the test.
5, measures to prevent scaling of plate heat exchangers
1) Strict water quality in the operation, the water in the system and the softened water in the softening tank must be subjected to strict water quality testing before being qualified to inject into the pipe network.
2) When the new system is put into operation, the heat exchanger should be separated from the system for a period of time before the heat exchanger is incorporated into the system. To avoid impurities entering the heat exchanger in the pipe network.
3) In the entire system, the decontamination device and filter should be cleaned occasionally, and the pipe network should be kept clean to prevent the heat exchanger from being blocked.
Overall. Strictly follow the cleaning method of the plate heat exchanger, which is an important guarantee for the normal operation of the production.