Cleaning and maintenance of plate heat exchangers

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1 Overview The plate heat exchanger has a compact structure, light weight, small footprint, and a large heat exchange area per unit volume. Its heat transfer coefficient is 3 to 5 times higher than that of ordinary steel shell and tube heat exchangers, and it is easy to change the heat exchange area. Plate heat exchangers need to be cleaned regularly, and the cleaning frequency is not low under certain process conditions. The main cleaning methods include overall chemical cleaning and disassembly cleaning. During the production process, the chemical cleaning of the lean liquid cooler of the decarbonization system of CNOOC Petrochemical Co., Ltd. has achieved good results. This is used as an example to discuss the chemical cleaning and maintenance of the plate heat exchanger. Common faults of plate heat exchangers are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0702251435113069.jpg 2. Cleaning of plate heat exchangers. Taking the plate heat exchanger as a water cooler as an example, the cleaning process is shown in Figure 1. The dotted line is the temporary takeover (101.6mm). The circulating cooling water is isolated, and the chemical cleaning liquid enters from S4 and flows out from S2. The cleaning procedure is divided into leak testing, flushing, chemical cleaning, rinsing and other steps. Before cleaning, perform a leak test on the plate heat exchanger to check whether there is internal leakage (the medium leaks from the high-pressure side to the low-pressure side): Drain the medium on both sides of the plate heat exchanger, fill the side to be cleaned with nitrogen to the operating pressure and then maintain the pressure, leaving an open shower or exhaust hole on the other side. If the pressure does not drop, it means that the sealing performance is very good. Or check various places where leakage may occur in the plate heat exchanger. If internal leakage is found, the plate heat exchanger plates need to be repaired or replaced. As shown in Figure 1, flush the plate heat exchanger with hot water (about 50°C), control the outlet pressure and flow rate of the cleaning pump to operate under the conditions allowed by the plate heat exchanger, and flush out the scale attached to the plates as much as possible until the water at the outlet of the plate heat exchanger is no longer obviously turbid. Rinse with hot water not only helps reduce the amount of chemicals used, but also shortens the time of chemical cleaning. http://www.nmtech.com.cn/jishuwang/upload/0702251435446242.jpg 1) Pickling liquid formula. The dosage of nitric acid is 3% to 5%, the dosage of LX9-001 is 0.1%, and the temperature is normal temperature. 2) Pickling process conditions and operations. Add circulating cooling water and LX9-001 corrosion inhibitor to the acid tank for dissolution. Start the pumping cycle, and add nitric acid in batches after 20 minutes to make about 3% to 5% pickling liquid. At the same time, three stainless steel hanging pieces were hung in the acid tank to measure the corrosion rate. The pickling time is preferably 2 to 3 hours, generally no more than 3 hours. To judge the end point, if the scale is dominated by calcium carbonate, the foam will disappear, the liquid level in the tank will drop, the turbidity will no longer rise, and the acid concentration will be stable to determine the cleaning end point. At the end of cleaning, the coupons should be taken out in time to observe the corrosion situation and calculate the average corrosion rate. Table 2 shows the analysis data at the end of cooler cleaning. http://www.nmtech.com.cn/jishuwang/upload/0702251436402037.jpg After the pickling is completed, drain the residual acid liquid of the system to a temporary storage tank for neutralization and discharge, add a large amount of clean water to the tank for cyclic cleaning, and replenish water while draining until the pH value becomes neutral, then stop replenishing water and discharging. After pickling, dry the equipment with nitrogen or compressed air. Rinse is to further replace the residual acid in the plate heat exchanger. According to the cleaning flow chart, the one-pass rinsing method is more efficient and effective than the circulation method. If there is corrosion under scale on the plates of the plate heat exchanger, a system assessment is required before a cleaning plan or other measures are determined. 3) Analysis items and frequency during chemical cleaning process. total iron: Analyze once before pickling and twice afterward ； Acid concentration: 2 times/h ； Turbidity: 2 times/h. 3 Maintenance of plate heat exchangers The important components and functions of plate heat exchangers are shown in Table 3. Among them, the vulnerable parts are the sealing gaskets of plate heat exchangers. Generally, EPDM rubber gaskets are used, which are acid-, alkali-, and salt-resistant. They are suitable for situations with severe corrosion such as chlorides and organic solvents. The operating temperature is generally between -20 and 150°C. Practice has proved that aMDEA solution with defoaming agent will cause rubber pad swelling. Based on the current operating conditions, the service life of the rubber pad is approximately 1 to 2 years. Correct operation has a direct impact on the service life of the plate heat exchanger. Therefore, when the heat exchanger is running, improper operations such as water hammer that affect its performance should be avoided, and a suitable filter should be added to the media inlet. http://www.nmtech.com.cn/jishuwang/upload/0702251437147269.jpg For standby equipment, if it is not used for a long time, the tightening bolts should be loosened to the specified size to ensure the service life of the gasket and heat exchanger plate, and then clamped as required when in use. When the equipment is running continuously, analysis should be carried out when the medium flows out of the signal hole. If the bolts are loose or elongated due to long-term heat exchange, you should be very careful when clamping them, because the main reason why the plate heat exchanger has a high heat transfer coefficient is that its plate corrugations can cause turbulence in the fluid at a small flow rate. Non-professionals are likely to cause irreparable damage when clamping. It is recommended not to clamp when the device is in use. If the sealing gasket is found to be aging, it should be replaced. Chemical cleaning of plate heat exchangers is relatively reliable. For mechanical cleaning, please consult the manufacturer. During routine maintenance and startup and shutdown, prevent water hammer and pressure sudden changes from impacting the equipment. The medium of the plate heat exchanger should be clean and the designed flow rate should be maintained. A suitable filter should be added to the medium inlet of the heat exchanger, and the filter should be cleaned regularly.