How To Repair The Sealing Surface And Improve The Air Tightness After Using The Valve For A Long Time?

- Nov 06, 2019-

After the 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 reduced. 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 surfacing welding is followed by grinding and then grinding.

Grinding of valves includes:

1, cleaning and inspection process

2. Grinding process;

3. 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. Fine cracks which are difficult to determine with the naked eye can be carried out by a coloring flaw detection method.

After cleaning, check the tightness of the valve flap or gate valve to the sealing surface of the seat. Use red and pencil for inspection. Use red lead to test red, check the sealing surface to determine the sealing surface tightness; 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 Wipe off the condition and determine the tightness of the sealing surface.

If the adhesion is not good, the valve plate or the sealing surface of the valve plate and the sealing surface of the valve body can be separately inspected by a standard plate to determine the grinding position.

stainless steel Check valve

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 inspection. The grinding process is divided into coarse grinding, medium grinding and fine grinding.

Rough grinding is to eliminate the defects such as scratches, indentations and eclipses 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 medium 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#, the particle size is coarse, the cutting amount is large, the efficiency is high, but the cutting grain is deep, and the surface of the sealing surface is rough. Therefore, the rough grinding can simply remove the pitting of the valve head or the valve seat.

Medium 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, fine particle size and small cutting amount, which is beneficial to reduce roughness; at the same time, the corresponding lap is replaced, and the lap should be clean.

After the middle grinding, the contact plane of the valve should be bright. If you use a pencil to make a few strokes 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. When fine grinding, it can be diluted with W5 or finer parts with oil, kerosene, etc., and the valve head of the valve is used to grind the valve seat of the valve without using drama, which is more conducive to the sealing of the sealing surface.

Generally, it should be rotated clockwise by 60-100°, and then rotated 40-90° in the opposite direction. Gently grind for a while, it must be inspected once, to be polished and shiny, and can be seen on the valve head and valve seat. When you get a thin line and the color is black and black, grind it gently with oil and clean it with clean gauze.

After grinding, other defects are eliminated, that is, they should be assembled as soon as possible to avoid damaging a polished valve head.

Manual grinding, regardless of rough grinding or fine grinding is always carried out by lifting and lowering; rotating, reciprocating; tapping, reversing and other operations. The purpose is to avoid the repetition of the abrasive grain trajectory, to evenly grind the lap and the sealing surface, and to improve the flatness and smoothness of the sealing surface.

3 inspection stage

In the grinding process, the inspection stage is always running. 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.

When the valve is ground, a very detailed work needs to be constantly experienced, explored and improved in practice. Sometimes it is well grounded, but it is still leaking after the installation. This is because there is a grinding in the grinding process. The hand holding the grinding rod is not vertical, skewed, or the angle of the lap is deviated.

Since the abrasive is a mixture of abrasive and abrasive, the slurry is only normal kerosene and engine oil. Therefore, the most critical part of the correct choice of abrasive is the correct choice of abrasive.

Butterfly valve

4 How to choose the valve abrasive correctly?

• Alumina (AL2O3) Alumina, also known as corundum, has a high hardness and is very common. Which generally used to grind work pieces,  such as cast iron, copper, steel and stainless steel.

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

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

• Chromium Oxide (Cr2O3) Chromium Oxide is a high hardness and very fine abrasive. Quenched steel is often used for hardened steel. It is also commonly used for polishing.

• Iron oxide (Fe2O3) Iron oxide is also a very fine valve abrasive, but the hardness and grinding effect are worse than chrome oxide, the use is the same as chrome oxide.

• Diamond powder is crystalline C, which is the hardest abrasive and has good cutting properties, especially suitable for grinding hard alloys.

5 classification and application range of commonly used abrasives

In addition, the thickness of the abrasive grain size (particle size of the abrasive) has a significant effect on the polishing efficiency and the surface roughness after the grinding. When rough grinding, the surface roughness of the valve workpiece is not high. In order to improve the grinding efficiency, the coarse-grained abrasive should be used. When the grinding is small, the surface roughness of the workpiece is high, and fine-grained abrasive can be used.

When the sealing surface is rough, the particle size of the abrasive is generally 120#~240#; the fine grinding is W40~14.

The valve modulates the abrasive, usually by adding kerosene and engine oil directly to the abrasive. The abrasive prepared by mixing 1/3 kerosene plus 2/3 engine oil and abrasive is suitable for rough grinding; the abrasive prepared by mixing 2/3 kerosene plus 1/3 oil and abrasive can be used for fine grinding.

When grinding a workpiece having a high hardness, the effect of using the above abrasive is not ideal. At this time, three pieces of abrasive and a piece of heated lard can be used to mix and form a paste after cooling, and then add some kerosene or gasoline to mix thoroughly.

6 choice of grinding tools

Due to the different degree of damage, the valve flap and the seat sealing surface cannot be directly researched. Instead, the fake disc (ie, the grinding head) and the fake valve seat (ie the grinding seat) are respectively used for the valve. Seat and disc are ground.

The grinding head and grinding head 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 the grinding is performed manually, various grinding rods must be provided. Grinding rods and laps should be properly assembled and not skewed. In order to reduce the labor intensity and speed up the grinding, it is often ground by an electric grinder or a vibrating mill.