Heat Exchanger Leak Causes investigation
Read the number：(3796) Time：2014/3/9 17:00:09
Heat Exchanger Leak Causes investigation
http://china.toocle.com 2009 年 09 月 17 日 09:02 Chinese heat exchanger network
Social business Sept. 17 hearing
Lu Yi ** Zhangsuo Long (Jiangsu Polytechnic University)
Foreword
And tube heat exchanger is heat exchange equipment widely used in the petrochemical industry, its quality directly affects the safety of petrochemical enterprises and economic benefits. Currently, due to various reasons exchanger leak, resulting in material waste and environmental pollution, affecting the normal operation of the plant's production facilities and economic benefits, has become the production can not be ignored. Understanding heat exchanger may leak site and produce a general cause of the leak, and to control the design, manufacture, use three links working, you can greatly reduce heat leakage rate. The author of a number of different types of heat exchanger leak leak site and analyze the causes and propose appropriate measures to provide reference for heat exchanger design, manufacture and use. A heat exchanger leakage mechanism mainly thermal stress and thermal fatigue 1.1
In the heat exchanger is widely used in the petrochemical industry, steam generators, reboilers, boiler feed water preheater, etc. are typical heat transfer equipment. Most heat during operation, both between the shell and tube side fluid is a certain temperature, there is a big difference in temperature between the multi-tube heat exchanger tubes of each process, the temperature difference between the two led to the tube sheet thermal stress generated between the side and the tubes. When the thermal 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 operating conditions of repeated pressing, heating and pressure relief, the process of cooling, the thermal stress changes repeatedly cause the device to generate heat fatigue. The survey found that many refineries are due to this reason the steam generator leak failure. 1.2 bundle vibration
Medium perpendicular to the lateral flow tubes are a major source of fluid-induced vibration, if they fall into the shell of gas in the tube will flow direction and the direction perpendicular to the axis of both acoustic standing wave formation. When the frequency of the acoustic standing wave Karman vortex and turbulence buffeting the frequency and consistency when the main frequency, but also stimulate the acoustic standing wave vibration. In operation, the vibrations will cause the tube bundle pores and other defects in the deposited metal tube between the tube plate and the presence of enlarged or extended, to a certain extent when these defects will lead to fatigue cracking or breakdown. The natural frequency of vibration of the heat exchanger tube, whereas the natural frequency of the tube bundle structure, size, and therefore the design of the heat exchanger itself is the cause of the defect bundle vibration. The survey analyzes multiple reboiler [2,3], which go in the shell, high-pressure steam tube go liquids are produced in the run shortly after the leak, repair, and then every three months regularity then drain. Initially considered a thermal stress, but re-designed and manufactured reboiler plus expansion Festival, the situation has not improved. The Yangzi Petrochemical Factory and Shanghai Petrochemical Aromatics Aromatics plant similar device reboiler also take the tube to go, the high-pressure steam, liquid shell go, but over the years there have been no leaks put this description, high-pressure steam pressure , velocity, induced vibration caused by defects in the design of the heat exchanger is the main reason for the leak. Defects in the manufacturing process 1.3
Welding quality is the key to the heat exchanger manufacture. When the heat exchanger tube and tube sheet welding [4], formed on both sides of the weld heat-affected zone, which is the weakest part of welded joints, prone to residual deformation and residual stress, which is easy to form the basic conditions of stress corrosion. If the impact of a corrosive environment encountered, for example, H2S, OH-like environment (austenitic stainless steel in Cl-, OH-like conditions), the stress corrosion cracking will occur, causing leakage of the heat transfer tube joints. Presence crevice between the tube and tube plate is not flowing liquid, and the liquid outside the crevice formed concentration cell, causing crevice corrosion, heat exchange tube joints can also cause leaks. Features tube and tube plate welded structure is tightly packed with small round single pass welds, tube sheets thicker, if improper welding process, it could easily lead to the root of the weld slag, poor fusion, cracks, holes and other welding defects. In operation these defects affect the alternating stress will be extended, so that the leak path to expand, resulting in leakage. This has become a common cause of heat exchanger failure.
Expansion of the use of the intensity of the heat exchanger when expanding, tube plate and tubes should be appropriate hardness difference, otherwise the tube is greater than the resilient tube plate, resulting in expansion joint is not tight. Further, when the gap between the outer diameter of the tube between the tube sheet hole, to achieve the purpose of swelling, the tube must have a larger amount of deformation. Expansion deformation of the pipe at the same time increase its hardness also increases, so that the tubes more hardness greater than the hardness of the tube sheet, resulting in a more expanded joint is not tight]. 1.4 operational factors
Heat during operation, due to the characteristics of the production process itself or due process operation is not standardized media pressure led to instability, sudden changes in temperature caused by the impact of thermal stress. This momentary fluctuations in operating pressure and temperature will cause compaction force pipe flange gasket sealing surface changes, repeated cycles, resulting in loose flange bolts, seal failure. Great thermal shock is applied in the form of speed and impact, resulting in greater than a temperature gradient thermal fatigue, loss of ductility of the material can, brittle fracture [6]. Where and why the two heat exchanger leak analysis
To sum up, resulting in a heat exchanger may be due to a leak in the design, manufacture and use of the three links in a link, it may be the result of two or three aspects of the role. Through the analysis of a large number of heat exchanger, now generated by the leak location and the causes are summarized as shown in Table 1.
3 Conclusion
For leaks in different locations due to different causes, different approaches should be used to solve them. Failure design factors, often need to re-design and renovation; welding defects generated during the manufacturing improvements should be repaired; improper operation is relatively easy to improve.
3.1 design should pay attention to the problem
(1) perpendicular to the tube bundle cross-flow media is the main source of fluid-induced vibration, and the key factors affecting the cross-flow velocity is the inlet end of the first piece of shell baffle and baffle spacing distance plate and tube plate. Permitted pressure drop in the fluid condition shorten baffle spacing, increasing the diameter of the heat transfer tube can increase the natural frequency of the tube bundle cross-flow velocity is reduced, to prevent vibration bundle object. Replaced with a rod baffle baffle support, you can solve the problem of flow-induced vibrations.
(2) In addition to considering the selection pressure, outside temperature, medium factors, for a temperature difference of the occasion, equipment manufacturing timber should also pay attention to choose the linear expansion coefficient, thermal conductivity of large, plastic good material. In the structural design using temperature compensation, try to eliminate and reduce stress concentration, so that cross-sectional smooth transition, while taking good insulation measures to reduce the temperature difference between the inside and outside walls, reduce heat stress, avoid excessive thermal stress generated leaving the vessel plastic deformation and creep.
(3) a reasonable choice baffle spacing, baffle spacing is too large, easy to produce induced vibration, and baffle spacing is too small, rigid tubes would be too large, low distortion and coordination, to generate thermal stress. Therefore, by design calculations, the premise of guaranteeing the vibration does not occur choose a larger spacing. 3.2 should be noted that manufacturing problems
(1) improve the welding process, welding strict clean-up site, ensure the welding quality. Strengthen the weld strength, elongation longer tubes protruding tube plate to increase the fillet height, raise tubes to withstand the pull-off force capabilities. Mn and low low low current multi-channel Si wire welding, to reduce the chance of weld defects. Residual stress and avoid stress concentration.
(2) The strength of welded affixed inflation. Inflation is not directly attached to the role of stress can eliminate the gap between the tube and tube plate, the vibration generated in the weld defects to avoid the impact of vibration on the bundle between the tube and pipe weld metal plates that exist, while preventing the gap corrosion, greatly increasing the tensile strength of the junction off. In addition, the strength of welded affixed inflation also has a dual role of insurance, if the weld is the breakdown, there is a sticker swelling backing, will not leak immediately.
(3) strict control of bolt preload, give sufficient preload shim pressure and compression ratio. Avoid bolt preload is not enough, or transportation, bolts loosening caused by vibration operation to ensure the quality of the seal pipe flange sealing surface. 3.3 Operation in question should be noted
To smooth the process operation, to avoid pressure, temperature suddenly raised and lowered, to avoid the tube to produce a strong vibration.
Reference links:
China HVAC Network: http://www.cnhvacrnet.com/detail/4793702.html
Chinese heat exchanger network: http://www.31hrq.com/detail/4793702.html
