After a long-term use of the valve, the sealing surface of the valve disc and valve seat will be worn and the tightness will be reduced. Repairing the sealing surface is a large and very important work. The main method of repairing is grinding. For the seriously worn sealing surface, the surfacing welding is carried out first and then grinding after turning.


Grinding of valves includes:


Cleaning and inspection process;


Grinding process;


Inspection process.


1 cleaning and inspection process


Clean the sealing surface in the oil pan, use professional cleaning agent, and check the damage of the sealing surface while washing. The fine cracks that are difficult to be determined by the naked eye can be detected by dye detection.


After cleaning, check the sealing condition between the disc or gate valve and the sealing surface of the valve seat, and use red and pencil for inspection. Use red lead to test the red color, check the seal surface print, and determine the sealing surface sealing condition; or use pencil to draw several concentric circles on the sealing surface of valve clack and valve seat, and then rotate the valve clack and valve seat sealing, check the pencil circle wiping condition, and determine the sealing surface sealing condition.


If the sealing is not good, the standard plate can be used to inspect the sealing surface of the valve disc or ram 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 lathe. The depth of pits or holes on the valve head or valve seat is generally within 0.5mm. The grinding method can be used for maintenance. The grinding process is divided into rough grinding, medium grinding and fine grinding.


Rough grinding is to eliminate the defects such as scratch, indentation and corrosion point on the sealing surface, so that the sealing surface can get a higher flatness and a certain degree of finish, which lays a foundation for the middle grinding of the sealing surface.


Coarse grinding uses grinding head or grinding seat tool, and uses coarse abrasive paper or paste. The particle size is 80 × 3-280 × 3, the particle size is coarse, the cutting amount is large, and the efficiency is high, but the cutting pattern is deep, and the sealing surface is rough. Therefore, rough grinding only needs to remove the pitting of valve head or valve seat smoothly.


Medium grinding is to eliminate the coarse lines on the sealing surface and further improve the flatness and finish of the sealing surface. Use fine-grained sandpaper or fine-grained grinding paste, the particle size is 280 × 5-w5, the particle size is fine, the cutting amount is small, which is conducive to reducing the roughness; at the same time, replace the corresponding lapping tool, and the lapping tool shall be clean.


After medium grinding, the contact surface of the valve shall be bright. If you use a pencil to make several strokes on the valve head or valve seat, turn the valve head or valve seat slightly for one circle, and wipe off the pencil line.



Fine grinding is the last process of valve grinding, mainly to improve the finish of sealing surface. During fine grinding, W5 or finer micro parts can be used to dilute with oil, kerosene, etc., and then the valve head of the valve is used to grind the valve seat instead of acting, which is more conducive to sealing surface sealing.


When grinding, generally turn 60-100 ° clockwise, and then turn 40-90 ° in the opposite direction, and gently grind for a while. It must be checked once. When it is polished to be bright and shiny, and a circle of very thin lines can be seen on the valve head and valve seat. When the color reaches black and bright, gently grind it several times with engine oil, and then wipe it with clean gauze.


After grinding, other defects shall be eliminated, i.e. assembly shall be carried out as soon as possible, so as to avoid damaging the well ground valve head.


Manual grinding, no matter rough grinding or fine grinding, is always through the grinding process of lifting, dropping, rotating, reciprocating, tapping, reversing and other operations. Its purpose is to avoid the repetition of abrasive tracks, make the grinding tool and sealing surface get even grinding, and improve the flatness and finish of sealing surface.


3 inspection stage


In the process of grinding, it is always in the inspection stage. Its purpose is to grasp the grinding situation at any time, know it well and make the grinding quality meet the technical requirements. It should be noted that when grinding different valves, grinding tools suitable for various types of sealing surfaces should be used to improve grinding efficiency and ensure grinding quality.


Valve grinding is a very meticulous work, which needs constant experience, exploration and improvement in practice. Sometimes it is well ground, but after installation, it still leaks steam and water. This is because of the imagination of grinding deviation in the process of grinding, the grinding rod is not vertical, skewed, or the dimension angle of the grinding tool is deviated.


Because abrasive is a mixture of abrasive and abrasive fluid, and abrasive fluid is only general kerosene and engine oil. Therefore, the key to choose abrasive correctly is to choose abrasive correctly.


How to select valve abrasive correctly?
Alumina (Al2O3) alumina, also known as corundum, is widely used because of its high hardness. Generally used for grinding cast iron, copper, steel and stainless steel and other materials.
Silicon carbide (SIC) there are two kinds of silicon carbide, green and black, its hardness is higher than alumina. Green silicon carbide is suitable for grinding hard alloy; black silicon carbide is used for grinding brittle materials and soft materials, such as cast iron, brass, etc.
The hardness of boron carbide (B4C) is next to that of diamond powder but harder than silicon carbide. It is mainly used to grind hard alloy and hard chromium plated surface instead of diamond powder.

Chromium oxide (Cr2O3) chromium oxide is a kind of abrasive with high hardness and very fine. It is often used to polish hardened steel when finishing.
Iron oxide (Fe2O3) iron oxide is also a very fine valve abrasive, but its hardness and grinding effect are worse than that of chromium oxide, and its use is the same as that of chromium oxide.

Diamond powder is crystal powder C, which is the most hard abrasive with good cutting performance, especially suitable for grinding cemented carbide.
See table for classification and application scope of common abrasives (click to enlarge)