Industrial Valve Sealing Surface Wear and Repair

 
Over time, industrial valve disc and seat sealing surfaces naturally wear out due to frequent opening, closing, and exposure to the working medium. This wear reduces the sealing performance, which can lead to leaks and affect system operation, potentially causing safety risks. Therefore, repairing the sealing surface is essential to maintain industrial valve performance. The primary method for restoring sealing is grinding. For valves with mild wear, grinding can restore their sealing ability. However, for severely worn valves, additional steps like overlay welding, turning, and then grinding are required. Below is a detailed overview of the three main steps for valve sealing surface repair: cleaning and inspection, the grinding process, and reassembly and testing.

Cleaning and Inspection

 
Before starting the grinding process, it's important to clean and inspect the valve sealing surface thoroughly. The purpose of cleaning is to remove oil, rust, dust, and other contaminants, so that the damage can be assessed more clearly.

1. Cleaning the Sealing Surface

 
Place the sealing surface in a cleaning pan and use a specialized cleaning agent to clean it. During cleaning, inspect the sealing surface visually to assess the extent of the damage. For micro-cracks that are hard to detect with the naked eye, a dye penetrant test can be used, which is more accurate in identifying small cracks and imperfections.

2. Seal Fit Check

 
After cleaning, check how well the valve disc or gate fits against the valve seat. Common methods include:
Red Lead Test: Apply red lead pigment to the valve disc or seat and lightly close the valve. After opening, observe the imprint on the sealing surface to check for evenness and how well the surfaces fit.
Pencil Concentric Circles Method: Draw concentric circles on the valve disc or seat, then rotate the valve disc. Check if the pencil lines are evenly erased. If they are, the sealing surfaces are in good contact; if not, there's a fitting issue.
If the fit is poor, use a standard flat tool to further inspect the flatness of the valve disc, gate, and body sealing surfaces and identify specific areas for grinding.

Grinding Process

 
Grinding is the key part of valve repair. Essentially, it involves using abrasives to finely cut the sealing surface to remove defects and restore smoothness and flatness. Valve grinding typically consists of three stages: rough grinding, intermediate grinding, and fine grinding.

1. Rough Grinding

 
The goal of rough grinding is to remove major defects like scratches, dents, and corrosion spots, improving surface flatness. Coarse sandpaper or grinding paste is used in this stage. As coarse abrasives are more aggressive, they remove material quickly but leave deep marks on the surface. Rough grinding is complete when most of the pits on the valve head or seat surface are removed, but high polish is not necessary at this stage.

2. Intermediate Grinding

 
Intermediate grinding removes the rough marks left from rough grinding and improves the flatness and smoothness of the sealing surface. Finer sandpaper or grinding paste is used, and the material removal is slower to create a smoother surface. At this stage, tools and equipment should be kept clean to ensure an even grinding effect. After intermediate grinding, the surface should appear relatively bright.
To check the progress, draw a few lines on the surface with a pencil and gently rotate the valve head or seat. If the lines are evenly erased, the intermediate grinding is successful.

3. Fine Grinding

 
Fine grinding is the final stage, aiming to achieve the highest surface smoothness. Very fine abrasive mixed with oil or kerosene is used as the grinding agent. At this point, manual grinding can be done by rotating the valve head against the seat in alternating clockwise and counterclockwise directions, with rotation angles between 60° and 100°. This ensures even grinding across the surface.
Fine grinding is complete when the surface shines like a mirror, with a dark luster. At the end, a light oil polish and clean cloth wipe finish the process.

Reassembly and Testing

 
Once grinding is complete, the valve should be reassembled promptly to prevent contamination or damage to the sealing surfaces. During reassembly, all parts must be kept clean to avoid introducing debris.
Testing should occur throughout the grinding process to ensure the seal meets technical requirements. After reassembly, perform a seal test to check for leaks under working pressure. This confirms the sealing integrity has been fully restored.
For different valve types, use tools suited to the specific sealing surface to improve efficiency and ensure quality. Improper grinding may result in uneven surface wear, leading to leaks even after reassembly. Therefore, it’s crucial to keep grinding tools perpendicular to the sealing surface to avoid angle deviations.

Conclusion

 
Valve sealing surface repair is a meticulous and highly technical process involving several steps, from cleaning and inspection to grinding and final testing. Each phase requires precise operation to ensure the flatness and smoothness of the sealing surface meet the necessary standards. By selecting the appropriate tools and methods based on the valve type and extent of wear, and by gaining experience over time, technicians can enhance their grinding skills. Proper grinding and testing are essential to prolong valve lifespan and ensure the system operates smoothly.