A diesel engine exhaust valve failure analysis 1, exhaust valve working conditions Marine diesel exhaust valve operating conditions are very poor, the valve bottom and high temperature combustion products in direct contact with the valve is also open during the high temperature ( 900 ~ 1000 ° C) and corrosive gases (up to 600m / s), the center temperature of the valve is as high as 700 ~ 800 ° C, the middle of the transition arc between valve disc and valve stem is 600 ~ 700 ° C . Excessive temperature will reduce the mechanical properties of metal materials, thermal deformation of the material. When the valve surface seal is not strict, it will cause high temperature gas on the valve surface burning. When the valve is seated, the inertia force of the valve and the valve seat and the spring force exert a considerable impact on the alternating load, the valve beating or valve clearance increases, the load will be obvious increase. Valve and valve seat impact, easy to form the sealing surface deformation and severe wear. Due to the vast majority of marine diesel engines for the supercharged diesel engine, due to the fresh air pressure within the intake blocked from the valve guide to obtain the possibility of oil, so prone to dry friction between metals. However, in the general diesel valve and the exhaust valve seat of the supercharged diesel engine alloy surface will always be a layer of lubricating oil or grease and other lubricants. In addition, there is wear between the stem and the conduit, and the top of the stem is struck and wobbled by the rocker arm. 2. Influences of Additional Factors Due to rising fuel prices and fierce competition in the shipping market, shipowners use low-priced and inferior fuel for the purpose of lowering their costs and achieving more profits. These fuels have high viscosities, long shelf life, and high levels of vanadium, sodium and sulfur. When this fuel is burned in a diesel engine, only a portion of the emissions (fuel ash) contained in the residue leaves the machine together with the exhaust gas and the remainder remains in the engine at some elevated temperature (497-797 ° C) On the parts. For example, the exhaust vent and piston crown form deposits that cause so-called "high temperature corrosion." So far, there is no economically rational process to remove the corrosive elements from the residual oil, even the advanced alloy steel and the surfacing exhaust valve steel are also corroded by the fuel. In the diesel engine running violates the provisions of car maintenance, low temperature start the diesel engine, low temperature forced loading, diesel cylinder combustion temperature abrupt changes in the diesel engine load condition, sharp change handle position, the combustion engine cylinder deterioration, a large number of poor atomized heavy heavy oil Particles into the cylinder, resulting in serious afterburning and incomplete combustion, serious coke valve surface to the exhaust valve is also contaminated by coke, and even lead to the difficulty of starting the host, which became the next host to drive shortly after Oil head and exhaust valve fault hidden dangers, so these bad habits of diesel engine maintenance, maintenance, diesel engine valve is also caused by failure of the factors. Second, the exhaust valve common fault analysis 1, exhaust valve burning Exhaust valve burning is the most common fault exhaust valve. The main reason is the tightness of the exhaust valve, resulting in high temperature gas leaks, causing severe overheating and melting of metal materials. The main reasons for the poor sealing of the exhaust valve are as follows: (1) As the shape and thickness of the different parts of the valve disc are different, the temperature distribution on the valve disc circumference is not uniform due to the different conditions of heating and cooling. The center temperature is higher than the surrounding temperature, Valve radial temperature difference between the valve plate, too much temperature difference will result in valve disc deformation resulting in the generation of gas leakage. ⑵ marine fuel containing impurities in the combustion chamber after a variety of complex thermal process in the exhaust valve disc and valve seat sealing cone deposited as a layer of carbon mixed glassy hard brittle material mixed with Sodium sulfate, calcium sulfate, iron oxide and other substances. When this layer of glassy sediment deposition thickness is too large, the valve will be cracked under the impact of the valve closing, repeated impact and then developed into flaking, forming a high-temperature gas injection channel burning valve. ⑶ ordinary exhaust valve sealing cone hardness at work temperature is not very high, the deposition of hard combustion products in the valve closing the impact of the valve, the sealing surface can be pitted to form a leak. 2, the exhaust valve temperature corrosion Currently in the shipping market commonly used in poor quality fuel contains a lot of vanadium, sodium and sulfur and other elements. In the process of burning. Sulfur, vanadium and sodium and other elements form sulfur oxides, vanadium pentoxide and sodium oxide (the chemical composition of these oxides depends on excess oxygen and combustion temperature). Oxide to react between, but also with the oil in the reaction of calcium, the formation of low melting point salts, sodium sulfate, calcium sulfate and different components of sodium vanadate and so on. The melting point of these salts mixture is generally about 535 ° C, at the same time has a strong corrosive. When the part temperature is above 550 ° C, it is enough for the vanadium and sodium compounds to be melted and adhere to the part surface. When the exhaust valve is in operation, it is deposited as a liquid on the transition surfaces of the valve disc and valve seat, as well as the stem and valve surface, due to venting (valve temperature up to 650-800 ° C). At this moment, even a very hard alloy steel that is resistant to corrosion will be corroded, and as a result, pitting and pitting will be formed on the sealing conical surface. Pits connected may cause air leaks. Since the above corrosion is generated under high temperature, it is called "high temperature corrosion". In the above high-temperature corrosion of harmful elements to the greatest harm of vanadium. 3, valve sealing cone wear too fast In the combustion chamber under the pressure of the valve seat and the valve plate are under the action of elastic deformation, the valve seat impact can also cause valve seat and valve disc elastic deformation, it will make the valve disc cone Repeated wedge surface, the sealing cone relative movement, resulting in sealing cone wear. Valve clearance is too large, valve disc and seat stiffness is not enough, valve and valve seat material performance does not meet the requirements or mismatch, heavy oil contains more vanadium, sodium, sulfur and other harmful elements, high load operation or combustion deterioration , Poor cooling, valve stem and catheter gap is too large, valve mechanism vibration so that the valve seat speed is too large, can make the wear rate increases. 4, the valve plate and valve stem in the transition between the valve disc and the corner of the valve stem and the installation of the Ministry of the groove block, due to the stress concentration of these parts easily, when the stress concentration to a certain extent, there will be fatigue fracture damage. Causes of fracture are: the stem and the catheter gap is too large; valve plate and valve seat deformation so that the local stress is too large; valve clearance is too large, severe fatigue damage; valve mechanism vibration. Stem installation card block groove is the weakest part of the valve, if the groove processing bad or heavy valve closing impact fatigue failure will occur. 5, the valve stuck dead valve is mainly due to the gas valve stem and the gap between the catheter is too small, when the gap after the expansion of the gap between the two card jamming phenomenon. On the other hand, when the valve stem is bent and deformed, the valve stem can also be stuck in the catheter. 6, the valve spring rupture valve spring itself unreasonable structure, internal defects, improper processing or use of the distortion or fatigue limit in the work will lead to fracture. Valve spring rupture directly destroy the diesel engine normal work, serious, the valve may fall into the cylinder. Third, the diesel engine valve repair 1, the valve and valve seat repair before the major technical maintenance, measuring the bottom surface of the valve and the cylinder head cover the distance is greater than the provisions. If the difference between this value is too large, it will affect the compression ratio of the diesel engine and the valve and piston impact, it is necessary to replace the valve seat pad. Overhaul, the valve after long-term work should be carried out the following measurements: After the stem wear circularity error and cylindrical error, the error can be measured with an outer diameter micrometer to check. When these two errors exceed the limit of 50% of the catheter with the gap should be scrapped replacement; inspire the cone on the outer surface of the stem radial runout should be less than 0.003mm. Otherwise it should be scrapped replacement. Check the valve will be placed on the top of the lathe spindle and tail thimble between the top of the two thimble valve in the thimble hole in the hole, the dial indicator seat block, and the contact of the table and the gas Valve cone contact. Valve rotation of a week, the maximum reading of the dial indicator is the radial runout. At this point the valve holding state as shown above; on the valve crack check, pits and pits and other checks. The outer surface of the stem allows 10 and less than 20mm length of the hairline, the cone and the valve stem lock clamp should be carried out magnetic inspection and testing degaussing. Cracked persons should be scrapped. (1) Valve and valve seat welding process Relieved from the host of the exhaust valve and the valve seat has a lot of grease, pitted pits, ablation or burn through, you must clean. Cleaning work can be machined in the lathe surface dirt and the original spray layer (the thickness of the weld layer can be sprayed from the basement process and the base metal of different colors, or car scrap iron is flaky and coil Judge it). Spray-welding layer is hard, the iron chips for the small, while the parent metal mostly filamentous. Parts of the serious erosion of the local grinding wheel with dirt, stain-free flaw detection, the welding can be repaired. The electrode can be made of high-chromium nickel austenitic stainless steel electrode, the effect will be better with molybdenum, generally 200 ° C before welding, preheating for 1.5 hours, applying a small current, multi-layer multi-track welding, Between the temperature of about 200 ° C. For the part of the substrate above the grinding wheel worn off, no flaw detection after the flaw detection weld spray. (2) valve and valve seat grinding process Disc and valve seat grinding angle must be strictly adjusted and measured in accordance with instructions. In particular, the valve stem sealing surface grinding angle must be carefully adjusted before grinding
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