Analysis of Common Faults of Exhaust Valve
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2022-02-12 10:37:29

1. Exhaust valve burnt

Exhaust valve burning is a common fault of exhaust valve Z. The main reason is that the exhaust valve is not tightly sealed, resulting in the leakage of high-temperature gas, causing serious overheating and even melting through metal materials.

The main reasons for poor sealing of exhaust valve are as follows:

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

(2) The impurities contained in the marine fuel oil are deposited on the sealing cone of the exhaust valve disc and valve seat after various complex thermal processes in the combustion chamber into a layer of glass-like hard and brittle substances mixed with carbon particles, which are mixed with sodium sulfate, calcium sulfate, iron oxide and other substances. When the thickness of this layer of glassy deposit is too large, cracks will occur under the impact force when the valve is closed, which will develop into flaking after the impact, thus forming a high-temperature gas injection channel and burning the gas valve.

(3) The hardness of the sealing cone of the common exhaust valve is not very high at the working temperature. The hard combustion product particles deposited under the impact of the closed valve can make the sealing surface appear pits, thus forming air leakage.

2. High temperature corrosion of exhaust valve

At present, the inferior fuel widely used in the shipping market contains a large amount of vanadium, sodium, sulfur and other elements. During combustion, elements such as sulfur, vanadium and sodium form sulfur oxide, vanadium pentoxide and sodium oxide (the chemical composition of these oxides depends on excess oxygen and combustion temperature). The oxides will react with each other and with the calcium in the lubricating oil to form salts with low melting point, including sodium sulfate, calcium sulfate and sodium vanadate with 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 carbide table in the melting state and attach to the part surface. When the exhaust valve is in operation, it is deposited on the valve disc and valve seat as well as the transition surface between the valve stem and the valve face in liquid form due to the exhaust reason (the temperature of the air valve can reach 650? 800 ° C or above). At this time, even the very corrosion-resistant cemented carbide steel will be corroded, and the corrosion results in pitting and pits on the sealing cone. The concatenation of pits may cause air leakage. Because the above corrosion is generated under high temperature conditions, it is called "high temperature corrosion". Among the above harmful elements of high-temperature corrosion, vanadium is the most harmful.

3. The sealing cone of the gas valve is worn too fast

Under the explosive pressure in the combustion chamber, both the valve seat and the valve disc will undergo elastic deformation. The impact of the gas valve seating will also cause the elastic deformation of the valve seat and the valve disc. This will cause the relative movement of the sealing cone when the valve disc cone is wedged, causing the wear of the sealing cone. Excessive clearance of air valve, insufficient rigidity of valve disc and valve seat, unsatisfactory or mismatched performance of air valve and valve seat materials, heavy oil containing more harmful elements such as vanadium, sodium and sulfur, high load operation or combustion deterioration, poor cooling, excessive clearance between valve rod and guide pipe, vibration of air valve mechanism and structure causing excessive seating speed of air valve, all can increase the wear rate.

4. The valve disc and valve rod are broken

At the transition fillet between the valve disc and the valve rod and at the groove where the valve rod is installed with the clamp block, because the stress in these parts is easy to be concentrated, fatigue and fracture will occur when the stress is concentrated to a certain extent. The reasons for the fracture are: the clearance between the valve rod and the guide pipe is too large; The deformation of valve disc and valve seat causes excessive local stress; The air valve clearance is too large, and the knocking is seriously tired and damaged; Vibration of air valve mechanism and structure. The groove where the valve rod is installed with the clamp block is the weak part of the gas valve. If the groove processing technology is poor or the valve closing impact is heavy, fatigue and fracture will also occur.

5. The air valve is stuck

The gas valve is stuck mainly because the gap between the valve rod and the guide tube is too small. When the valve is heated and expanded, the gap between the two is jammed. On the other hand, when the valve rod is bent and deformed, the valve rod will also be stuck in the guide tube.

6. Air valve spring is broken

The gas valve spring itself has unreasonable structure, internal defects, improper processing, or distortion in use or reaching in operation, which will lead to fracture. The rupture of the valve spring directly damages the normal operation of the diesel engine. In serious cases, the valve may fall into the cylinder.

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