[introduction] at present, because of the heat exchanger leakage caused by various reasons, causing material waste and environmental pollution, affects the normal operation of the factory production equipment and economic benefits, has become a noticeable problem in production. Understanding of the heat exchanger may produce leakage of the site and the causes of leakage, and from the design, manufacture and use of the three aspects of the control, you can greatly reduce the leakage rate of heat exchangers.
Defects in manufacturing process
The welding quality is the key to the heat exchanger manufacture. Heat affected zone is formed on both sides of the weld, which is the weak part of the welded joint, and the residual deformation and residual stress are easy to form, which is the basic condition for forming the stress corrosion. If the impact of corrosion environment, such as in OH-, H2S and other environments (austenitic stainless steel in Cl-, OH-, etc.), the stress corrosion cracking will occur, resulting in heat exchanger pipe joints leak. The gap between the tube and the tube sheet is not flowing liquid, and the gap is caused by the concentration difference cell, which causes the crevice corrosion, and also causes the leakage of the heat exchanging pipe joint. Tube and tube plate welding structure is characterized with closely spaced small circular single seam, the tube plate is thick, if improper welding process, it is easy to cause the root of weld slag, poor fusion, welding defects such as cracks and pores. In the course of the operation, the influence of alternating stress is extended, which leads to the enlargement of the leakage channel, which leads to leakage. This has become a common cause of the failure of the heat exchanger.
Two, tube bundle vibration
The main source of fluid induced vibration is the transverse flow of the medium perpendicular to the tube bundle. If the flow of gas is flowing into the shell, an acoustic standing wave is formed in the direction perpendicular to the flow direction and the tube axis. When the frequency of the acoustic standing wave is in agreement with the Kaman vortex frequency and the main vibration frequency of the turbulence, the acoustic standing wave can be excited. During the operation, tube vibration will cause the pores between the tube plate and tube in weld metal and other defects to expand or extend the development to a certain extent, these defects will be punctured or lead to fatigue cracking. The vibration is related to the natural frequency of the tube of the heat exchanger, and the natural frequency is related to the structure and size of the tube bundle, so the design defect of the heat exchanger is the cause of the vibration of the tube bundle.
In the expansion joint, the tube plate and the heat exchange tube should have proper hardness difference, otherwise, the spring back is larger than that of the tube sheet, and the expansion joint is not tight. In addition, when the gap between the outer diameter of the pipe and the pipe orifice is larger, the pipe must have a larger deformation quantity to achieve the purpose of expanding. At the same time, the hardness increases with the increase of the deformation of the tube, so that the hardness of the heat exchange tube is greater than that of the tube sheet.
Three, thermal stress and thermal fatigue
Heat exchanger is widely used in petrochemical industry, steam generator, re boiler, boiler feed water preheater and so on are typical heat exchange equipment. Most of the heat exchanger in the running process of the tube between the shell fluid have a certain temperature, the heat pipe heat exchanger process also exists between the large temperature difference in multipass, the temperature difference of both sides of the tube sheet and heat pipe between the thermal stress. When the temperature difference stress reaches a certain value, the metal will produce plastic deformation and creep. If the operating temperature of the heat exchanger is periodically changed, or the operating conditions are repeated pressure, temperature and pressure relief, cooling process, then repeated changes in thermal stress will cause thermal fatigue. The survey found that many of the refinery's steam generators are due to this reason and leakage failure.
Four, operating factors
The heat exchanger in the operation process, the production process itself or due to the process operation of non-standard lead medium pressure instability, thermal shock stress caused by temperature change. This operating pressure and temperature fluctuations will lead to the pressure of the gasket on the flange sealing surface change, repeated cycle, resulting in flange bolt loose, sealing failure. Heat shock is applied in the form of great speed and impact, resulting in a greater temperature gradient of the specific heat, which can cause the material to lose its ductility and brittle fracture.
Causes of heat exchanger leakage may occur in the design, manufacture and use of the three links in a link, but also may be the result of two or three aspects of the common role.
Process equipment network finishing release, reproduced please indicate the source.