Analysis of plate corrosion and rubber mat aging in plate heat exchanger
Plate heat exchanger as a compact and efficient heat exchange equipment, although its development has been nearly a hundred years old, and in some sectors (such as food, pharmaceutical, chlor-alkali, etc. Industry) has a wide range of applications, but due to its lack of temperature, pressure, corrosion resistance and other capabilities, it limits its application. In the evaporation section of the ion membrane workshop of a factory in our company, two MB100 plate heat exchangers are used to cool the lye in parallel. The hot fluid inlet medium of the heat exchanger is an alkali solution (containing a small amount of salt) with a temperature above 85 °C, and is exported. The temperature is about 50 °C. The plate heat exchanger is made of 304 stainless steel plate, which suppresses the adult-shaped 0.6mm thick corrugated plate, and is composed of a labyrinth-shaped fluid passage with a reversed left and right herringbone groove. After two years of use, the plate heat exchanger leaked, and the plate heat exchanger was prevented from leaking after one month. The following is a brief analysis of the reasons for the failure of the plate heat exchanger.
Sealing failure
Seam gasket material
The detachable plate heat exchanger leaks when used, and the failure of the gasket material is an important factor in addition to sheet corrosion. There are mainly the following ways: 1 When the rubber gasket material is in contact with the fluid, when a fluid component penetrates into the rubber, sometimes the soluble substance can be extracted, resulting in swelling of the rubber gasket; 2 the presence of halogen and ozone The effect is to soften the gasket or cause stress corrosion cracking. 3 Under certain conditions, especially at high temperatures, the elastic material will decompose and release chemicals, which themselves will corrode the sheet material, such as FPM rubber will release oxygen. After disassembling the plate heat exchanger, it was found that the cold fluid end surface of the plate was bright, a small amount of black scale was locally, the number of corrosion pits was small and did not penetrate, and there was black deposit in the corrosion pit, so it can be inferred as a gasket. Piece of corrosion exists.
Fluid pressure
Removable plate heat exchangers leak when used within the rated working pressure, except for the quality factors in the manufacturing assembly, mainly in the system The water hammer and the air hammer are related to the abnormal impact load, which is a phenomenon that is not easily observed during use. The instantaneous pressure peak caused by the impact is often 1-3 times higher than the normal working pressure, causing the rubber gasket installed in the plate heat exchanger to shift, resulting in seal failure. Since the heat transfer element of the device is made of a stainless steel sheet (thickness of 0.5-1.0 mm), the sealing rigidity is relatively poor and the sealing perimeter is long, so the impact pressure resistance is much lower than that of the shell-and-tube heat exchanger. The abnormal impact of the plate heat exchanger can be judged according to the equipment for providing the conveying medium. When the alkali pump for transporting the alkali liquid to the plate heat exchanger is repaired, it is found that there is a small amount of honeycomb in the pump casing, indicating that the pump is running. There is cavitation; at the same time, due to the leakage around the previous plate, it can be inferred that the change of fluid pressure is also a possible factor of leakage.
Temperature
A sharp change in temperature can also cause seal failure. When the temperature changes too fast, the coefficient of linear expansion of the rubber gasket does not match the elastic deformation amount and the sealing pre-tightening force, so that the sealing pre-tightening force is lowered, and the pressure bearing capacity of the device is lower than the rated design pressure.
From the working conditions of the plate heat exchanger, the temperature has little effect on it, so it can be excluded.
Corrosion
The corrosion of plate heat exchangers mainly refers to the corrosion of the plates. Plate heat exchangers are mostly made of stainless steel sheets. Although stainless steel has good corrosion resistance, it will cause uniform or partial corrosion of the stainless steel sheets after the passivation film is destroyed. The corrosion phenomenon exhibited by the plate heat exchanger is mostly stress corrosion caused by Cl-, which often occurs at the bottom of the sealing groove of the plate and the bottom of the scale after the formation of dirt. The main reason is that: 1 stainless steel plate is mechanically cold pressed. Inevitably, a certain amount of surface residual stress remains inevitably. For a stainless steel sheet containing no molybdenum element, the elimination of surface residual stress is difficult. After the two sheets are assembled, a multi-slit structure is formed, and the corrugated top ends of the adjacent sheets cross each other to form a large number of contacts. There will be gaps in the cross-contacts, and the medium in the gaps will not flow smoothly, so that there is a difference in the concentration of oxygen inside and outside the gap, causing crevice corrosion. 3 When the dirt on the surface of the plate is serious, the corrosion elements (Cl, S, etc.) in the medium may adhere to the dirt in a large amount and be enriched in the gap of the bottom. 4 The harmful elements in the bottom of the sealing groove are often the precipitation of Cl in the adhesive due to temperature rise. For example, the neoprene series of adhesives and compressed asbestos mats (containing CaCl2) tend to form ClCl and H+ in the water and steam conditions, resulting in severe stress corrosion at the bottom of the groove. Cracking. When the unpacked sheets were inspected, it was found that most of the sheets at the inlet of the hot fluid side had cracks, and there was a thick layer of dark brown deposit before the cleaning; on the hot-fluid inlet side of the sheet, the top of the herringbone corrugated, adjacent The contacts on the top of the corrugated corrugations have point-like surface damage and corrosion spots, surrounded by dark brown deposits. Perforation corrosion occurs in 1 or 2 places on each plate, and is located at the cross-over contacts of the corrugated tip near the hot fluid inlet side, and the local area is corroded pitting and pit. The crack on the side of the hot fluid in the perforated corrosion pit is obvious, and most of the corrosion is in the shape of a slit, and most of it has penetrated. Therefore, it can be judged that the corrosion condition of the plate heat exchanger is a comprehensive reaction of various factors.
Analysis and Discussion
According to the above analysis and test results, the heat exchanger plate failure is mainly corrosion failure,
The heat exchanger uses EPDM as a gasket. Due to the high inlet temperature and the pressing force, the S element in the pad is in contact with the plate. Produces a certain sulfide that promotes the occurrence and expansion of corrosion.
The heat exchanger is composed of a plurality of plates alternately arranged and pressed together, and the corrugated top ends of adjacent plates cross each other to form a large number of contacts. There will be gaps on the cross-contacts, and the medium in the gaps will not flow or flow poorly, resulting in a difference in the concentration of oxygen inside and outside the gap, causing crevice corrosion, causing certain areas in the gap to preferentially corrode and dissolve, which is in contact The main cause of perforation corrosion at the point.
contains a small amount of sodium chloride in the lye, so that the plate contains a higher concentration of Cl- ions in the gap, due to its strong penetrating ability, damage It even penetrates the surface passivation film, so that the base metal directly contacts the medium, causing corrosion to dissolve; the presence of the S element may generate some sulfide, which promotes the occurrence and expansion of corrosion.
The heat exchanger herringbone corrugated sheet is stamped and formed into a large amount of martensite, and the crystal is dispersed into fine carbides. Stainless steel has reduced corrosion resistance. The residual stress after deformation, together with the tightening force applied during the assembly of the sheet, causes large stresses at the top of the corrugation, promotes intergranular corrosion, and causes cracks to propagate along the crystal.
In the operating system, when the heat exchanger is subjected to an abnormal impact exceeding the rated load, the displacement of the gasket is liable to occur and leakage is caused.
The quality and service life of plate heat exchangers depends to a large extent on the quality and service life of the gasket. For large logarithmic heat exchangers, the plates are generally not damaged, and the gaskets are prone to aging and cause leakage, so the quality of the gasket determines the quality and service life of the heat exchanger.
