What are the effects of corrosion on brazed plate heat exchangers
Stainless steel sheet is mechanically cold stamped, can not prevent a certain amount of residual residual stress remaining, for the non-molybdenum stainless steel sheet, the elimination of residual stress is very difficult.
After the panel is assembled, a multi-slit structure is formed, 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 smoothly, so that there is a difference in the concentration of oxygen inside and outside the gap, which causes crevice corrosion.
When the surface of the sheet is heavily fouled, the corrosive elements (ClS, etc.) in the medium may adhere to the dirt in a large amount and be enriched in the gap at the bottom of the scale.
The harmful elements in the bottom of the sealing groove are often the precipitation of Cl in the adhesive due to the increase in temperature, such as the neoprene series of adhesives, compressed asbestos mats (containing CaCl2 often in Under the conditions of water and steam, the precipitation of Cl and H+ formed HCl, causing severe stress corrosion cracking at the bottom of the groove. When the unpacked plate was inspected, it was found that most of the plates at the inlet of the hot fluid had cracks. And there is a thick layer of dark brown deposit before it is cleaned;
There are 12 perforated corrosion on each plate, which are on 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.
Based on the above analysis and test results. The heat exchanger plate failure is mainly caused by corrosion failure.
Heat exchanger Herringbone corrugated sheet is a large amount of martensite formed by stamping deformation. Reduce the corrosion resistance of stainless steel. Residual stress after deformation, and the crystals are dispersed into fine carbides which are precipitated. In addition, the tightening force applied during the assembly of the plate causes a large stress at the top of the corrugation, promotes intergranular corrosion, and causes cracks to propagate along the crystal.
Contains a small amount of sodium chloride in the lye. Due to its strong penetrating power, the plate gap contains a relatively high concentration of Cl- ions. Destruction or even penetration of the external passivation film, so that the base metal directly contacts the medium, causing corrosion and dissolution; the presence of S element may cause some sulfides, which promote the occurrence and expansion of corrosion.
The heat exchanger uses EPDM as a gasket. The S element in the mat is brought into contact with the sheet to form a certain sulfide, which is due to the high inlet temperature and the pressing force. Promotes the occurrence and expansion of corrosion.
The heat exchanger is composed of a plurality of plates alternately arranged and compacted. The medium in the gap does not flow or the flow is not smooth, and the corrugated top ends of the adjacent plates cross each other to form a large number of contacts. There will be a gap on the cross contact. The concentration difference of oxygen exists inside and outside the gap, which causes crevice corrosion, and some areas in the gap preferentially corrode and dissolve, which is the main reason for perforation corrosion at the contact.