After long-term use of the valve, the sealing surfaces of the valve disc and valve seat will wear and reduce their tightness. Repairing the sealing surface is a large and very important task, and the main method of repair is grinding. For severely worn sealing surfaces, it is first welded, turned, and then ground.
Grinding of valves includes cleaning and inspection processes; Grinding process; Inspection process.
1. Cleaning and inspection process
Clean the sealing surface in the oil pan, use a professional cleaning agent, and check the damage of the sealing surface while washing. Tiny cracks that are difficult to determine with the naked eye can be detected using dye penetrant testing.
After cleaning, the sealing surface between the valve disc or gate valve and the valve seat should be checked for tightness, using red and pencil. Test the sealing surface with red lead, check the imprint on the sealing surface, and determine the sealing surface's sealing condition; Alternatively, use a pencil to draw several concentric circles on the sealing surface of the valve disc and valve seat, then rotate the valve disc and valve seat tightly, check the wiping off of the pencil circle, and confirm the sealing surface is tight.
If the sealing is not good, a standard flat plate can be used to inspect the sealing surface of the valve disc or gate and the sealing surface of the valve body separately to determine the grinding position.
2. Grinding process
The grinding process is essentially a cutting process without the use of a lathe. The depth of pits or small holes on the valve head or valve seat is generally within 0.5mm, and grinding can be used for maintenance. The grinding process is divided into coarse grinding, medium grinding, and fine grinding.
Coarse grinding is to eliminate scratches, indentations, corrosion spots and other defects on the sealing surface, so as to achieve a higher flatness and a certain degree of smoothness on the sealing surface, laying the foundation for medium grinding of the sealing surface.
Coarse grinding uses a grinding head or grinding seat tool, using coarse sand paper or coarse grinding paste with a particle size of 80 # -280 #. The particle size is coarse, with a large cutting amount and high efficiency, but the cutting pattern is deep and the sealing surface is rough. Therefore, rough grinding only needs to remove the pits on the valve head or valve seat smoothly.
The purpose of medium grinding is to eliminate rough lines on the sealing surface and further improve the flatness and smoothness of the sealing surface. Using fine-grained sandpaper or fine-grained grinding paste with a particle size of 280 # - W5, with fine particle size and small cutting amount, is beneficial for reducing roughness; At the same time, the corresponding grinding tools should be replaced and cleaned.
After intermediate grinding, the contact surface of the valve should reach brightness. If you use a pencil to draw a few lines on the valve head or valve seat, gently rotate the valve head or valve seat in one circle and erase the pencil line.
Fine grinding is the next step in valve grinding, mainly aimed at improving the smoothness of the sealing surface. During fine grinding, W5 or finer fractions can be diluted with engine oil, kerosene, etc. Then, the valve head of the valve can be used to grind the valve seat instead of acting, which is more conducive to the sealing surface sealing.
When grinding, it is usually done by turning clockwise at around 60-100 ° and then counterclockwise at around 40-90 °. Gently grind for a while, and it must be checked once. When the grinding is shiny and shiny, a circle of fine lines can be seen on the valve head and valve seat. When the color reaches black and shiny, gently grind with engine oil several times and wipe clean with clean gauze.
After grinding, other defects should be eliminated, that is, they should be assembled as soon as possible to avoid damaging the polished valve head.
Manual grinding, whether rough or fine, always runs through lifting and lowering; Rotating and reciprocating; A grinding process that combines tapping, reversing, and other operations. The purpose is to avoid repeated abrasive trajectory, achieve uniform grinding of the grinding tool and sealing surface, and improve the flatness and smoothness of the sealing surface.
3. Inspection stage
Throughout the grinding process, the inspection stage is always present, with the aim of keeping track of the grinding situation and ensuring that the grinding quality meets technical requirements. It should be noted that grinding tools suitable for various sealing surface forms should be used when grinding different valves to improve grinding efficiency and ensure grinding quality.
Valve grinding is a very meticulous task that requires continuous experience, exploration, and improvement in practice. Sometimes, the grinding is very good, but after installation, there is still steam and water leakage. This is because there is an imagination of grinding deviation during the grinding process, holding the grinding rod not vertical or skewed, or there is a deviation in the size and angle of the grinding tool.
Due to the fact that abrasive is a mixture of abrasive and abrasive fluid, and the abrasive fluid is only ordinary kerosene and engine oil. Therefore, the key to selecting abrasives correctly is to choose abrasives correctly.
4. How to select valve abrasives correctly?
Aluminum oxide (AL2O3), also known as corundum, has a high hardness and is widely used. Generally used for grinding workpieces made of materials such as cast iron, copper, steel, and stainless steel.
Silicon carbide (SiC) There are two types of silicon carbide: green and black, with a higher hardness than alumina. Green silicon carbide is suitable for grinding hard alloys; Black silicon carbide is used for grinding brittle materials and soft material workpieces, such as cast iron, brass, etc.
Boron carbide (B4C) has a hardness second only to diamond powder and harder than silicon carbide. It is mainly used to replace diamond powder in grinding hard alloys and grinding hard chromium plated surfaces.
Chromium oxide (Cr2O3) Chromium oxide is a type of abrasive with high hardness and extremely fine particles. Chromium oxide is often used for precision grinding of hardened steel and is generally used for polishing.
Iron oxide (Fe2O3) Iron oxide is also an extremely fine valve abrasive, but its hardness and grinding effect are worse than chromium oxide, and its use is the same as chromium oxide.
Diamond powder, also known as crystalline C, is a hard abrasive with good cutting performance and is particularly suitable for grinding hard alloys.
In addition, the particle size of the abrasive (the particle size of the abrasive) has a significant impact on the grinding efficiency and surface roughness after grinding. During rough grinding, the surface roughness of the valve workpiece is not required to be high. To improve grinding efficiency, coarse-grained abrasives should be selected; During precision grinding, the grinding allowance is small, and the requirement for surface roughness of the workpiece is high. Fine grained abrasives can be used.
When rough grinding the sealing surface, the particle size of the abrasive is generally 120 #~240 #; Precision grinding is W40-14.
Valve modulation abrasive, usually by directly adding kerosene and engine oil to the abrasive. A grinding agent composed of 1/3 kerosene and 2/3 engine oil mixed with abrasive is suitable for rough grinding; A grinding agent composed of 2/3 kerosene and 1/3 engine oil mixed with abrasive can be used for precision grinding.
When grinding workpieces with higher hardness, the effect of using the above grinding agents is not ideal. At this point, three parts of abrasive and one part of heated lard can be used to mix and cool to form a paste. When using, some kerosene or gasoline can be added to mix well.
5. Selection of grinding tools
Due to different degrees of damage, the sealing surfaces of the valve disc and valve seat cannot be directly ground. Instead, a certain number and specifications of pre made fake valve discs (i.e. grinding heads) and fake valve seats (i.e. grinding seats) are used to grind the valve seat and valve disc separately.
The grinding head and grinding seat are made of ordinary carbon steel or cast iron, and the size and angle should be equal to the valve disc and seat placed on the valve.
If grinding is done manually, various grinding rods need to be equipped. The grinding rod and grinding tool should be properly assembled and not tilted. To reduce human labor intensity and accelerate grinding speed, electric grinding machines or vibration grinding machines are often used for grinding.