• How to ensure a gas-tight seal of a valve after a long time of use?

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How to ensure a gas-tight seal of a valve after a long time of use?

  • Mar 05, 2019
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After a valve is used for a long time, the sealing surface of the valve disc and the valve seat will wear and the tightness will be affected. Repairing the sealing surface is a large and important task. The main method of repair is grinding. For the severely worn sealing surface, the first step is surfacing welding, and then turning machining which is followed by grinding. Grinding of the valve includes cleaning and checking processes, grinding processes, and inspection processes.

1. Cleaning and checking process
Clean the sealing surface in the oil pan, use a professional cleaning agent, and check the damage of the sealing surface while cleaning. Blind cracks which are difficult to determine with the naked eye can be treated with a coloring flaw detection method. After cleaning, check the tightness of the sealing surfaces between the valve flap or valve gate and the valve seat. A pencil and red lead are used during checking. Use red lead to check the sealing surface and determine the tightness of the sealing surface; or use a pencil to draw a few concentric circles on the valve flap and seat sealing surface, then rotate the valve flap and the valve seat tightly to check the pencil circle condition and determine the tightness of the sealing surface. If the tightness is not good, the sealing surface of the valve disc or the valve flap and the sealing surface of the valve body can be separately inspected by a standard plate to determine the grinding position.

2. Grinding process
The grinding process is essentially a cutting process without a lathe. And the pitting or small hole depth on the valve head or the valve seat is generally within 0.5 mm, and the grinding method can be used for maintenance. The grinding process is divided into rough grinding, moderate grinding and fine grinding.

Rough grinding is to eliminate the defects such as scratches, indentations and corrosion spots on the sealing surface, so that the sealing surface has a higher flatness and a certain degree of smoothness, which lays a foundation for the moderate grinding of the sealing surface. The rough grinding uses the grinding head or the grinding seat tool, and adopts coarse grain sandpaper or coarse grain grinding paste, the particle size of which is 80#-280#. With coarse grain particle, the cutting amount is large and the efficiency is high, but the cutting lines are deep, and the sealing surface is rough. Therefore, the rough grinding can be performed by simply removing the pitting of the valve head or the valve seat.

Moderate grinding is to eliminate the rough lines on the sealing surface and further improve the flatness and smoothness of the sealing surface. Fine-grained sandpaper or fine-grained abrasive paste with a particle size of 280#-W5 is used, with fine particle size and small cutting amount, which is beneficial to reduce roughness. At the same time, a corresponding grinding tool is used, and the grinding tool should be clean. After the moderate grinding, the contact plane of the valve should be bright. If you use a pencil to draw a few circles on the valve head or seat, and turn the valve head or seat against one turn, the pencil line should be erased.

Fine grinding is the last step in valve grinding, mainly to improve the smoothness of the sealing surface. During the fine grinding, W5 or finer particles are diluted with engine oil, kerosene, etc., and the valve head of the valve is used to grind the valve seat of the valve without using grinding tool, which is more conducive to the sealing performance of the sealing surface. Generally, it should be rotated clockwise by 60-100°, and then rotated 40-90° in the opposite direction. After gently grinding for a while, it must be checked once until the sealing surface of the seat is polished shiny and a thin line can be seen on the valve head and valve seat. When the color is black, grind it gently with oil and clean it with clean gauze. After the grinding, the other defects are eliminated, that is, they should be assembled as soon as possible, so as not to damage a polished valve head.

Manual grinding, regardless of rough grinding or fine grinding, is aimed at avoiding the repetition of the abrasive grain track, so that the grinding tool and the sealing surface are uniformly ground, and the flatness and smoothness of the sealing surface are improved.

3. Inspection process
In the grinding process, the inspection is always carried out. The purpose is to grasp the grinding situation at any time, so that the quality of the grinding can meet the technical requirements. It should be noted that different valves should be ground with abrasive tools suitable for various sealing surfaces to improve grinding efficiency and ensure grinding quality.