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Common failure analysis of exhaust valve.

Date:2018-03-20 Author: Click:

1. Exhaust valve is burned.

Exhaust valve burning loss is the most common failure of exhaust valve.
        The main reason is that the exhaust valve is not tightly sealed, which causes the high temperature gas leakage, which causes the place to overheat, and even melts the metal material.

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

(1) due to the different parts of the valve plate in the shape and thickness is different, different heat, cooling condition, valve plate of uneven distribution of the temperature on the circumference of a circle, the center temperature is higher than the surrounding temperature, the cause of valve disc on the radial temperature difference, big temperature difference will cause a deformation resulting in leakage of valve plate.

Diction of Marine fuel oil contains impurities after all sorts of complex thermal processes of the combustion chamber in the exhaust valve valve plate and valve seat seal cone deposit into a mixed with carbon particle layer of vitreous hard brittle material, its mixed with sodium sulfate, calcium sulfate, ferric oxide, etc.
When the vitreous sediment deposition thickness is too big, at the time of closing valve occurs under the impact force of crack, flake and then develop into, after repeated impact to form high temperature gas jet channel valve burning.

The sealing cone of the ordinary exhaust valve is not very high in the working temperature, and the hard combustion product particles deposited in the valve can cause a hole in the sealing surface of the sealing surface, thus forming a leak.

2. High temperature corrosion of exhaust valve.

        There is a large amount of vanadium, sodium and sulfur in the low-quality fuel used in the shipping market.
        In the combustion process.
        Elements such as sulphur, vanadium, and sodium form sulphur dioxide, vanadium oxide and sodium oxide (the chemical composition of these oxides depends on excess oxygen and combustion temperature).
        There is a reaction between the oxides, and it also reacts with the calcium in the oil, forming a low-melting salt, sodium sulfate, calcium sulfate, and sodium vanadate.
        The melting point of these salt mixtures is usually about 535 degrees C, with strong corrosivity.
        When the temperature of the parts is above 550 C, it is enough to make the vanadium and sodium-table objects melt and attach to the surface of the parts.
        When the exhaust valve is in work, due to the exhaust reason (the air valve temperature can reach 650?
        Above 800 ° C), making it in liquid form deposits on the valve plate and valve seat and valve stem and valve of the transition surface.
        At this time even the very corrosion resistant hard alloy steel will be corroded, the corrosion results in the sealed cone formed pits, pits.
        A hole in the pit may cause a leak.
        Because the above corrosion is produced under high temperature conditions, it is called "high temperature corrosion".
        Vanadium is the most harmful element in the high temperature corrosion.

        3. The valve seal cone is worn too fast.

In the combustion chamber under the action of the explosion pressure of valve seat and valve disc are elastic deformation occurs, valve seat impact also causes the elastic deformation of valve seat and valve disc,                which can make the valve plate cone repeatedly wedge is fashionable, seal cone of relative movement, cause the seal cone wear.
        Valve clearance is too large, the valve plate and valve seat stiffness is insufficient, air valve and valve seat material performance can not meet the requirements or do not match, the heavy oil contains many harmful elements such as vanadium, sodium, sulfur, high load operation or combustion deterioration, cooling, the valve stem and duct clearance is too large, the valve seat valve body vibration velocity is too large, can make the wear rate increases.

4. Disc and stem break.

In the groove between the valve disc and the stem and the valve stem, the stress is easy to concentrate, and when the stress is concentrated to a certain extent, the fatigue fracture will occur.
        The causes of fracture are: the gap between the stem and the pipe is too large;
        Deformation of disc and seat causes local force to be too large;
        The air valve clearance is too large, and the percussion is serious fatigue damage;

Vibration of the valve mechanism.
        The groove of the valve stem is the weakest part of the air valve, and fatigue fracture can occur if the groove processing technology is bad or the impact of the closed valve is heavier.

5. Valve card dead.

The valve card dies mainly because the gap between the valve stem and the pipe is too small.

On the other hand, when the stem is bent, the stem card will die in the pipe.
        6. The valve spring breaks.

        The valve spring itself is not reasonable in structure, internal defects, improper processing or use of distortion or fatigue limit in the work will lead to fracture.
        The valve spring break directly damages the normal operation of the diesel engine, and the valve may fall into the cylinder when serious.


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