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How to repair the sealing surface of a valve that has been used for a long time

Update:26-08-2020
Summary:

After the valve is used for a long time, the sealing su […]

After the valve is used for a long time, the sealing surface of the valve disc and the valve seat will be worn and the tightness will be reduced. Repairing the sealing surface is a large and very important task. The main method of repair is grinding. For the severely worn sealing surface, it is surfacing welding, turning, and then grinding.

The grinding of the valve includes:

Cleaning and inspection process;

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. The fine cracks that are difficult to identify with the naked eye can be carried out by staining flaw detection.

After cleaning, check the tightness of the disc or gate valve and the sealing surface of the valve seat. Use red and pencil when checking. Use red lead to test the red, check the seal of the sealing surface to determine the tightness of the sealing surface; or use a pencil to draw several concentric circles on the sealing surface of the valve disc and the valve seat, and then rotate the valve disc and the valve seat tightly, and check the pencil circle Wipe off the condition to confirm the tightness of the sealing surface.

If the tightness is not good, a standard flat plate can be used to test the sealing surface of the disc or gate and the sealing surface of the valve body respectively to determine the grinding position.

2. Grinding process

The grinding process is essentially a cutting process without a lathe. The depth of the pits or small holes on the valve head or valve seat is generally within 0.5mm, and the grinding method can be used for maintenance. The grinding process is divided into coarse grinding, middle grinding and fine grinding.

Rough grinding is to eliminate defects such as scratches, indentations, and corrosion points on the sealing surface, so that the sealing surface can obtain a higher flatness and a certain degree of smoothness, and lay the foundation for the middle grinding of the sealing surface.

Coarse grinding uses grinding head or grinding seat tools, using coarse-grained sandpaper or coarse-grained grinding paste, with a particle size of 80#-280#, coarse particle size, large cutting volume, high efficiency, but deep cutting lines and rough sealing surface surface. Therefore, rough grinding only needs to smoothly remove the pitting of the valve head or valve seat.

Middle grinding is to eliminate rough lines on the sealing surface and further improve the flatness and smoothness of the sealing surface. Use fine-grained sandpaper or fine-grained abrasive paste, the particle size is 280#-W5, the particle size is fine, the cutting amount is small, which is beneficial to reduce the roughness; at the same time, the corresponding grinding tool should be replaced, and the grinding tool should be clean.

After the middle grinding, the contact surface of the valve should be bright. If you draw a few strokes on the valve head or valve seat with a pencil, turn the valve head or valve seat lightly, and erase the pencil line.

 

 

Fine grinding is the last process of valve grinding, mainly to improve the smoothness of the sealing surface. For fine grinding, it can be diluted with engine oil, kerosene, etc. with W5 or finer fractions, and then use the valve head to grind the valve seat instead of drama, which is more conducive to the tightness of the sealing surface.

When grinding, generally turn it clockwise about 60-100°, and then turn it about 40-90° in the opposite direction. Gently grind for a while. It must be checked once until it is polished and can be seen on the valve head and valve seat. When there is a very thin line and the color is black and bright, lightly rub it with engine oil several times and wipe it with clean gauze.

After grinding, other defects should be eliminated, that is, assembling as soon as possible, so as not to damage a ground valve head.

Manual grinding, regardless of rough grinding or fine grinding, always runs through the grinding process of lifting, lowering, rotating, reciprocating, tapping, and reversing operations. Its purpose is to avoid the repetition of the abrasive grain track, so that the grinding tool and the sealing surface can be uniformly ground, and the flatness and smoothness of the sealing surface can be improved.

3. Inspection stage

In the grinding process, the inspection stage runs through the whole process. The purpose is to keep abreast of the grinding situation at any time so that the grinding quality can meet the technical requirements. It should be noted that when grinding different valves, grinding tools suitable for various sealing surface forms should be used to improve grinding efficiency and ensure grinding quality.

Valve grinding is a very meticulous work, which requires constant experience, exploration, and improvement in practice. Sometimes the grinding is very good, but after installation, it still leaks steam and water. This is because there is an imagination of grinding deviation during the grinding process. The grinding rod is not vertical, skewed, or the angle of the grind tool is different.

Because the abrasive is a mixture of abrasive and grinding fluid, the grinding fluid is only general kerosene and engine oil. Therefore, the most critical part of the correct selection of abrasives is the correct selection of abrasives.

4. How to choose valve abrasives correctly?

Alumina (AL2O3) Alumina, also known as corundum, has high hardness and is widely used. Generally used to grind workpieces made of cast iron, copper, steel and stainless steel.

Silicon carbide (SiC) Silicon carbide is available in green and black, and its hardness is higher than alumina. Green silicon carbide is suitable for grinding hard alloys; black silicon carbide is used for grinding brittle and soft materials, such as cast iron and brass.

Boron carbide (B4C) has hardness second only to diamond powder and harder than silicon carbide. It is mainly used to replace diamond powder to grind hard alloys and grind hard chrome-plated surfaces.

Chromium oxide (Cr2O3) Chromium oxide is a kind of high hardness and extremely fine abrasive. Chromium oxide is often used in the fine grinding of hardened steel, and it is generally used for polishing.

Iron oxide (Fe2O3) Iron oxide is also a very 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 is Crystal C, it is the hardest abrasive with good cutting performance, especially suitable for grinding cemented carbide.

5. Selection of grinding tools

Due to the different degree of damage to the sealing surface of the valve clack and the valve seat, they cannot be directly researched. Instead, a certain number and specifications of fake valve discs (that is, grinding heads) and fake valve seats (that is, grinding seats) specially made in advance are used to respectively check the valve. Grind the seat and disc.

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 valve seat placed on the valve.

If grinding is done manually, various grinding rods are required. Grinding rods and grinding tools must be assembled properly and not skewed. In order to reduce labor intensity and speed up the grinding speed, electric grinders or vibration grinders are often used for grinding.

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