【Haichuan Chemical Valve Management】Series Posts--What to do if the valve flange sealing surface is damaged? Is online repair more reliable or offline repair?

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Proper management of valves is the foundation for stable operation. Everyone is welcome to engage in discussions and share insights on this series of posts: 【HaiChuan Chemical Valve Management】 series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ---------------------------------------------------------------- Disclaimer: The content contained in this article is intended solely for technical exchange and reference purposes only; it does not constitute any form of professional engineering advice, design basis, or operational guidance. ----------------------------------------------------------------------------------- What to do if the sealing surface of the valve flange is damaged? Is online repair more reliable or offline repair? The flange sealing surface is a critical part of the connection between valves and pipes, and it is also a location that is prone to damage. Dents during transportation, scratches during installation, and corrosion after long-term operation can all damage the sealing surface. If the flange surface is uneven, the gasket cannot be pressed tightly, and fluid will leak. Several common types of damage to sealing surfaces: Scratches: caused by tools coming into contact during installation, or by scraping that occurs while aligning the flanges. Minor scratches can be polished with an emery stone, but not deep ones. Corrosion pits: caused by medium corrosion, resulting in an uneven surface. Corrosion pits are difficult to repair because material needs to be added. Deformation: The bolt was tightened too much, or there is a problem with the quality of the flange itself, causing the flange surface to warp. Deformation can only be remanufactured. Rust: Carbon steel flanges exposed to humid environments for extended periods develop rust on their surfaces. Mild rust can be removed by grinding, but severe rust means the item is unusable. Online repair: Can it be fixed without shutting down the system? When the production line cannot be stopped, online repair can be considered. But note that online repair is only suitable for minor damage. Manual grinding method: Manually grind the sealing surface using an oil stone or sandpaper to remove scratches and rust. The advantage is that it’s simple and requires no special equipment ； The downside is that it can only handle minor damage, and it requires an experienced operator; uneven grinding only makes things worse. Online turning method: A specialized portable flange turning machine is used to machine the sealing surface while the valve is in place. The advantage is high precision, allowing the flatness of the sealing surface to be restored ； The disadvantages are high costs, complex operation, and the need for a professional team. Online repair is suitable for cases where the damage is not severe and the production line cannot be stopped. However, if there are deep corrosion pits or extensive deformation on the sealing surface, online repair is practically impossible. Offline repair: The most thorough solution is to remove the valve and send it to a workshop for repair; this is the most reliable method. Lathe machining: Fix the valve on the lathe and machine the flange sealing surface. It offers the highest precision and can be restored to its factory settings. But the valve needs to be able to fit on the lathe; large valves may not fit. Post-welding machining: If there are deep corrosion pits on the sealing surface, a layer of corrosion-resistant material (such as stainless steel or Stellite alloy) is first welded on, and then the surface is machined to the required dimensions. This is the most thorough repair method, but it is costly and requires high technical skills. Replace the flange: If the flange is already severely deformed or damaged, replacing it is better than trying to repair it. Cut off the old flange and weld a new one on. This requires welding qualifications and heat treatment. The process for offline repair: Remove the valve and make proper markings. Clean the flange sealing surface to remove oil and impurities. Inspect the damage to determine the repair plan. Perform repair work. Use red lead powder to check the flatness of the sealing surface. Install the gasket and conduct a pressure test. Reinstall after it passes the test. How to check whether it is qualified after repair? After the sealing surface is repaired, it should be inspected using the \"staining method\" – red lead powder is applied, and the surface is rubbed against the matching flange to observe the distribution of the contact spots. The contact spots should be continuous and uniform, with no interruptions. The imprint lines of gate valves and globe valves should be continuous, with a width of not less than 4 mm, and the imprints should be even. The indented surface of the ball valve should be continuous, with a width not less than the outer diameter of the valve body’s sealing ring. The roughness of the sealed surface after repair shall be no less than Ra1.6. When should it be replaced rather than repaired? The depth of corrosion pits on the sealing surface exceeds 0.5 mm, and they are distributed densely. The flange surface warping deformation exceeds 0.3 mm. The same flange has been repaired more than twice. The repair cost exceeds 50% of the price of a new valve. A small tip: Prevention of flange sealing surface issues is more important than repair. Protect the flange surface during transportation and installation to avoid damage. Clean it thoroughly before installation; there should be no foreign objects. The bolts should be tightened to the standard torque; do not over-tighten them. Conduct regular inspections to address minor issues early; don’t wait until they become serious before fixing them.