Plate corrosion and rubber pad aging analysis of plate heat exchanger
The plate heat exchanger as a compact and efficient heat transfer equipment, although its development has a history of nearly 100 years, and in some sectors (such as food, pharmaceutical, chlor alkali industry) has been widely used, but because of the lack of temperature, pressure and corrosion ability and restrict the scope of application. My company a factory workshop of ionic membrane evaporation, using two BR0.2 parallel plate type heat exchanger for cooling of the lye, heat medium fluid entrance temperature is over 85 DEG lye (containing a small amount of salt), the outlet temperature is about 50 degrees celsius. The plate heat exchanger is made of 304 stainless steel plates, which is pressed by an adult shaped 0.6m m thickness corrugated plate, and is composed of a labyrinth type fluid passage with a reverse herringbone groove. After two years of use, plate heat exchanger leakage, take plate heat exchanger leak prevention measures after a month to overhaul the overall replacement. The following briefly analyzes the reasons for the failure of the plate heat exchanger.
Seal failure
Gasket material
When the plate type heat exchanger is used, the reason is that the failure of gasket material is an important factor in addition to plate corrosion. The following procedure: 1 rubber sealing gasket material in contact with the fluid, when the fluid components into the rubber, sometimes dissolved substance extraction which leads to swelling rubber gasket; in the presence of halogen and ozone in the role of the gasket become soft or the occurrence of stress corrosion cracking. Under certain conditions, especially at high temperatures, the elastic material will break down and release chemical substances, and these substances themselves will corrode the sheet material, such as FPM rubber will release oxygen. Remove the plate heat exchanger, found the plate cold fluid end surface light, part of a small amount of black furring, corrosion pits number did not penetrate, and corrosion pits in black sediment, so it can be inferred as gasket corrosion.
fluid pressure
Detachable heat exchanger leakage in use within the rated working pressure, in addition to the quality factor in equipment manufacturing and assembly of the main and the water hammer, the hammer with non normal impact load, which is not easily observed in the process of using. Shock caused by the instantaneous peak pressure is usually higher than the normal operating pressure of 1-3 times, so that the plate installed in the rubber seal gasket shift, resulting in seal failure [2]. Due to the heat transfer element of the equipment, the manufacturing stainless steel sheet (0.5-1.0m thickness m), its relatively poor sealing and sealing rigid surrounding is very long, so the impact pressure is far lower than that of shell and tube heat exchanger. The heat exchanger of non normal impact can provide basis for the transmission medium equipment to judge, found for the heat exchanger conveying lye in the maintenance of the pump, the pump shell there is a small amount of honeycomb, shows the pump cavitation during operation; at the same time because around pre plate leakage situation, a possible factor changes it can be inferred that the fluid pressure and leakage.
temperature
Rapid changes in temperature can also cause seal failure. When the temperature change is too fast, the linear expansion coefficient of the rubber gasket is not matched with the elastic deformation quantity and the seal pretightening force, which makes the sealing pre tightening force decrease, and the bearing capacity of the equipment is lower than the rated design pressure.
From the working conditions of the plate heat exchanger, the temperature has little effect on it, it can be ruled out.
The corrosion of plate heat exchanger is the main corrosion fingerboard tablets. Plate heat exchangers are mostly made of stainless steel sheet, stainless steel has excellent corrosion resistance, but the passivation film will be destroyed after the stainless steel sheet to produce uniform or localized corrosion. The corrosion phenomenon of plate heat exchanger showed mostly stress corrosion caused by Cl, in the plate sealing groove bottom and dirt after the formation of scale bottom often happens, the main cause of stainless steel plate by stamping machine and surface residual, inevitably a certain amount of residual stress for stainless steel sheet without molybdenum elements, to eliminate the residual stress on the surface is very difficult. After the plate is assembled, a plurality of gap structures are formed, and the corrugated plates of the adjacent plates are crossed with each other to form a plurality of contact points. There will be a gap in the cross contact, poor flow of media in the gap, so that the gap between the concentration of oxygen inside and outside the gap, resulting in crevice corrosion. When the dirt on the surface of the plate is serious, the corrosive elements (Cl, S, etc.) in the medium may be attached to the dirt and accumulate in the bottom of the scale. The harmful elements in the bottom of the sealing groove are often the Cl of the adhesive is unstable due to the increase of temperature. Such as adhesive, chloroprene rubber series compressed asbestos pad (containing CaCl2), often in the water and steam conditions, precipitation and enrichment of Cl and H + H to form Cl, the bottom of the gap of serious stress corrosion cracking. When the board opened inspection found that hot fluid inlet side most plate crack, and no pre cleaning has a thick layer of dark brown sediments; the contact plate hot fluid inlet side of herringbone corrugated plate, the top adjacent chevron bellows on the top, with punctate surface damage and corrosion spots, black brown sediment around. There are 1 ~ 2 perforation corrosion in each plate sheet, overlapping contacts are in corrugated near the top hot fluid inlet side of the local area, a pit, pitting corrosion. One side of the hot water side of the corrosion pit is more obvious, most of which are in the shape of cracks, and most of them have penetrated. Therefore, it can be concluded that the corrosion of the plate heat exchanger is a comprehensive reaction of various factors.
Analysis and discussion
According to the above points
