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May 14,2022

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Analysis of Common Failures of Exhaust Valve

Ⅰ. Burnout of exhaust valve

The exhaust valve burnout is the most common failure of the exhaust valve. The main reason is that the exhaust valve is not tightly sealed, causing high-temperature gas leakage, causing serious overheating in the area, and even melting through metal materials. The main reasons for the poor sealing of the exhaust valve are as follows.

1. Due to the different shapes and thicknesses of different parts of the valve disc, the heating and heat dissipation conditions are different, the temperature distribution on the circumference of the valve disc is uneven, and the center temperature is higher than the surrounding temperature, resulting in excessive temperature difference in the radial direction of the valve disc. The temperature difference will cause the deformation of the valve disc and lead to air leakage.

2. The impurities contained in marine fuel oil are deposited on the exhaust valve disc and the sealing cone of the valve seat to form a layer of glassy harder and brittle material mixed with carbon particles after various complex thermal processes in the combustion chamber. Mixed with sodium sulfate, calcium sulfate, iron oxide and other substances. When the thickness of this layer of glassy deposits is too large, cracks will occur under the impact force when the valve is closed, and will develop into peeling after repeated impacts, thereby forming a high-temperature gas ejection channel to burn the valve.

3. The hardness of the sealing cone of the ordinary exhaust valve is not very high at working temperature, and the deposited hard combustion product particles can cause pits on the sealing surface under the impact of the closed valve, thereby forming air leakage.

Ⅱ. High temperature corrosion of exhaust valve

In the combustion process, sulfur, vanadium, sodium and other elements form sulfur oxide, vanadium pentoxide and sodium oxide. The chemical composition of these oxides depends on excess oxygen and combustion temperature. The oxides have to react with each other, and also react with the calcium in the lubricating oil to form low-melting-point salts, such as sodium sulfate, calcium sulfate and sodium vanadate of different components. These salt mixtures generally have a melting point of about 535°C and are highly corrosive. When the temperature of the part is above 550°C, it is enough to make the vanadium and sodium compounds in a molten state and adhere to the surface of the part. When the exhaust valve is in operation, because the temperature of the valve can reach above 650-800°C, it is deposited as a liquid on the valve disc and seat, and the transition surface between the valve stem and the valve surface. At this time, even the very corrosion-resistant hard alloy steel will be corroded. As a result of the corrosion, pits and pits will be formed on the sealing cone surface, which may cause air leakage if the pits are connected. Because the above corrosion is produced under high temperature conditions, it is called high temperature corrosion. Among the harmful elements of high temperature corrosion, vanadium is the most harmful element.

Ⅲ. The sealing cone of the exhaust valve wears too fast

Under the action of the explosive pressure in the combustion chamber, both the valve seat and the valve disc are elastically deformed. The impact of the valve seat will also cause the elastic deformation of the valve seat and the valve disc. This will cause the sealing cone to be produced when the valve disc cone is repeatedly wedged. Relative movement causes wear of the sealing cone. The valve clearance is too large, the rigidity of the valve disc and the valve seat is insufficient, the material performance of the valve and the valve seat does not meet the requirements or does not match, the heavy oil contains more harmful elements such as vanadium, sodium, sulfur. Many problems will aggravate wear and tear like high load operation or deterioration of combustion, poor cooling, excessive clearance between the valve stem and the conduit, and excessive valve seat speed caused by the vibration of the valve mechanism and so on.

Ⅵ. The exhaust valve disc and stem are broken

At the transition fillet of the valve disc and the valve stem and the groove of the valve stem where the clamp block is installed, the stress is easy to concentrate in these parts. When the stress is concentrated to a certain degree, fatigue fracture will occur. The reasons for the fracture are: the gap between the valve stem and the pipe is too large. The deformation of the valve disc and the valve seat causes the local force to be too large. The valve gap is too large, and the knock is severely fatigued. The vibration of the exhaust valve mechanism. The groove of the valve stem where the clamp block is installed is the weakest part of the exhaust valve. If the groove processing technology is poor or the valve closing impact is heavy, fatigue fracture will occur.

Ⅴ. The exhaust valve is stuck

The air valve jam is mainly because the gap between the valve stem and the pipe is too small. When the gap between the two is too large after thermal expansion, the jam will occur. On the other hand, when the valve stem is bent and deformed, the valve stem will also be stuck in the catheter.

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