Valves are key components in industrial piping systems, and their sealing performance is crucial. However, after long-term use, the sealing surfaces of valves often wear out, reducing their tightness. Repairing the sealing surfaces is not only labor-intensive but also highly demanding in terms of technical skills. This article will provide a detailed introduction to the repair and grinding techniques for valve sealing surfaces, helping readers gain a comprehensive understanding of this important process.
During long-term use, valves are subject to frequent opening and closing operations, erosion by the medium, corrosion, and wear by impurities. These factors gradually cause wear on the sealing surfaces of the valve disc and seat. This wear can lead to a decrease in the valve's sealing performance, resulting in leakage. Such leakage not only affects production efficiency but can also trigger safety accidents. Therefore, timely repair of the sealing surfaces is essential to ensure the normal operation of valves.
There are mainly two methods for repairing sealing surfaces: grinding and welding. For lightly worn surfaces, grinding is an effective repair method. For severely worn surfaces, welding must be performed first, followed by turning and then grinding. Grinding is the core process in the repair of valve sealing surfaces, as it can restore the surfaces to a smooth and flat condition, thereby ensuring the valve's sealing performance.
Valve sealing surface grinding is a key step in the repair process and can be divided into three main stages: cleaning and inspection, grinding operations, and final inspection. Each stage is crucial to ensure that the sealing surface is restored to its optimal condition.
Before grinding, the sealing surfaces must be cleaned and inspected. The purpose of cleaning is to remove oil, impurities, and rust from the surfaces to clearly observe the extent of damage. Typically, the sealing surfaces are immersed in an oil pan and cleaned with specialized cleaning agents. During cleaning, the surfaces should be inspected for obvious scratches, pits, or cracks. For fine cracks that are not visible to the naked eye, dye penetrant inspection can be used.
After cleaning, the fit between the valve disc or gate and the seat sealing surfaces should be checked. A common method is to use red lead for testing. Red lead is evenly applied to the sealing surface, and then the valve disc is fitted with the seat. The distribution of the red lead marks indicates the condition of the fit. If the marks are uniform and continuous, the fit is good; if they are uneven or interrupted, there are defects in the sealing surfaces. Another method is to draw several concentric circles on the sealing surfaces with a pencil, then fit and rotate the valve disc with the seat to check the erasure of the pencil lines. If the lines are completely erased, the fit is good; if parts remain, there are defects.
If the fit is found to be poor, a standard flat plate can be used to inspect the valve disc or gate sealing surfaces and the valve body sealing surfaces separately to determine the specific areas that need grinding.
Grinding is the core process in the repair of valve sealing surfaces and consists of three stages: coarse grinding, medium grinding, and fine grinding.
The purpose of coarse grinding is to eliminate defects such as scratches, indentations, and erosion points on the sealing surfaces, achieving a high degree of flatness and a certain level of smoothness to lay the foundation for medium grinding. Coarse grinding typically uses coarse sandpaper or coarse grinding paste with a grit range of 80#-280#. Since the grit is coarse, the cutting amount is large and the efficiency is high, but the cutting marks are deep and the surface is relatively rough. The main task of coarse grinding is to evenly remove the pits on the valve head or seat.
During coarse grinding, grinding heads or seats can be used as tools. The tools must be tightly fitted to the sealing surfaces and moved uniformly during grinding. After coarse grinding, the defects on the sealing surfaces should be basically eliminated, and the surface flatness should be significantly improved.
The purpose of medium grinding is to eliminate the coarse marks on the sealing surfaces, further improving their flatness and smoothness. Medium grinding uses fine sandpaper or fine grinding paste with a grit range of 280#-W5. Since the grit is finer, the cutting amount is smaller, which helps reduce the surface roughness. The corresponding grinding tools should be replaced and kept clean.
After medium grinding, the contact surfaces of the valve should be bright. Pencil lines drawn on the valve head or seat should be completely erased when the valve head or seat is lightly rotated. This indicates that the flatness and smoothness of the sealing surfaces have met the requirements of medium grinding.
Fine grinding is the final step in valve grinding, primarily aimed at improving the smoothness of the sealing surfaces. Fine grinding can be performed using micro-powders finer than W5, diluted with engine oil or kerosene, and the valve head is ground against the valve seat. No grinding tools are used during fine grinding, which is more conducive to the fit of the sealing surfaces.
After fine grinding, the sealing surfaces should have a high degree of smoothness, with no obvious grinding marks. A very fine line should be visible on the valve head and seat, with a black and shiny appearance. Finally, the sealing surfaces should be wiped clean with a clean cloth.
After grinding, the sealing surfaces must be strictly inspected to ensure that the grinding quality meets technical requirements. The main inspection methods are red lead testing or pencil line drawing to check the fit of the sealing surfaces. If the fit is good, the grinding quality is qualified; if defects remain, re-grinding is necessary.
Grinding is a finishing process that can achieve high dimensional accuracy, geometric shape accuracy, and surface roughness for valve sealing surfaces. The basic principles of grinding include the grinding process, grinding motion, grinding speed, grinding pressure, and grinding allowance.
During grinding, the grinding tool is tightly fitted to the sealing surface, and the tool moves in a complex grinding motion along the fitted surface. Grinding agent is placed between the grinding tool and the sealing surface. When the tool and the surface move relatively, the abrasive particles in the grinding agent slide or roll between them, removing a very thin layer of metal from the surface. The protrusions on the surface are ground off first, gradually achieving the required geometric shape.
Grinding is not only a mechanical process but also involves chemical action. The oil in the grinding agent forms an oxide film on the surface being processed, accelerating the grinding process.
When the grinding tool moves relatively to the sealing surface, the relative sliding path of each point on the surface to the tool should be the same, and the direction of relative motion should constantly change. The continuous change in the direction of motion ensures that each abrasive particle does not repeat its own trajectory on the surface, preventing noticeable grinding marks and increased surface roughness. Additionally, the changing direction helps distribute the grinding agent more evenly, allowing for more uniform removal of metal from the surface.
Despite its complexity, the grinding motion always follows the fitted surface between the tool and the sealing surface. Whether manual or mechanical grinding is used, the geometric shape accuracy of the sealing surface is mainly affected by the geometric shape accuracy of the grinding tool and the grinding motion.
The faster the grinding speed, the higher the grinding efficiency. With a higher speed, more abrasive particles pass over the workpiece surface in a given time, removing more metal. The grinding speed typically ranges from 10 to 240 m/min. For workpieces requiring high grinding accuracy, the speed generally does not exceed 30 m/min. The grinding speed for valve sealing surfaces depends on the material of the surface. For copper and cast iron sealing surfaces, the speed is 10-45 m/min; for hardened steel and hard alloy sealing surfaces, it is 25-80 m/min; and for austenitic stainless steel sealing surfaces, it is 10-25 m/min.
Grinding efficiency increases with higher grinding pressure, but the pressure should not be too high, generally ranging from 0.01 to 0.4 MPa. For sealing surfaces made of cast iron, copper, and austenitic stainless steel, the grinding pressure is 0.1-0.3 MPa; for hardened steel and hard alloy sealing surfaces, it is 0.15-0.4 MPa. Higher values are used for rough grinding, and lower values for fine grinding.
The size of the grinding allowance depends on the machining accuracy and surface roughness of the previous process. The allowance should be as small as possible while ensuring the removal of machining marks from the previous process and correction of geometric shape errors of the sealing surface. The sealing surface should generally be finish-ground before grinding. The minimum grinding allowance for a finish-ground surface is: a diameter allowance of 0.008-0.020 mm; a flatness allowance of 0.006-0.015 mm. Smaller values are used for manual grinding or harder materials, and larger values for mechanical grinding or softer materials. For valve body sealing surfaces that are difficult to grind, precision turning can be used. The surface must be rough-ground before fine grinding, with a flatness allowance of 0.012-0.050 mm.
The specific operating methods for valve sealing surface grinding are key steps to ensure the quality of valve sealing surface repair. Here is the detailed operating procedure.
Manual grinding is a commonly used method in valve grinding. During grinding, the valve is generally rotated clockwise by about 60-100°, then counterclockwise by about 40-90°, and ground gently for a short period before checking. When the surface becomes bright and a very fine line appears on the valve head and seat with a black and shiny color, the valve should be ground gently with engine oil a few more times and then wiped clean with a clean cloth.
Whether for rough or fine grinding, manual grinding always involves a combination of lifting and placing, rotating and reciprocating, and tapping and reversing operations. The purpose is to avoid repeated abrasive particle trajectories, ensuring even grinding of the tool and sealing surface, and improving the flatness and smoothness of the sealing surface.
Throughout the grinding process, inspection stages are continuously integrated to keep track of the grinding situation, ensuring that the grinding quality meets technical requirements. It should be noted that different valves should use grinding tools suitable for various sealing surface forms to improve grinding efficiency and ensure grinding quality.
Mechanical grinding is a more efficient and precise grinding method. It uses the mechanical motion of a grinding machine to achieve the grinding process, ensuring the uniformity and consistency of the grinding motion. Mechanical grinding is typically used for mass production or when high grinding accuracy is required.
During mechanical grinding, the grinding speed, pressure, and allowance should be reasonably selected based on the material of the sealing surface and the grinding requirements. The grinding tool of the machine must be tightly fitted to the sealing surface, and the grinding agent should be evenly distributed.
The repair and grinding of valve sealing surfaces is a highly technical task that directly affects the sealing performance and service life of valves. Through the introduction in this article, we have learned that the grinding process includes four main stages: cleaning, inspection, grinding, and inspection. Each stage has its specific operating methods and precautions. The basic principles of grinding and the selection of grinding agents are also key factors in ensuring grinding quality. In actual operations, grinding personnel need to continuously accumulate experience, improve operational skills, and strictly follow technical requirements to ensure the quality of valve sealing surface repair. Only in this way can valves play their due role in industrial production, ensuring the safety and stable operation of production processes.
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