【Haichuan Chemical Valve Management】Series Posts -- Detailed Introduction to Valve Sealing Surface Grinding Technology

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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. ----------------------------------------------------------------------------------- The valve sealing surface is the key component that determines the sealing performance and service life of a valve, and the grinding process is the crucial step for repairing defects in the sealing surface and restoring its sealing precision. Based on field practices and industry standards, these specifications establish a set of practical technical requirements that cover everything from preparation and grading of grinding to quality inspection and safety management. They are suitable for the repair and precision machining of sealing surfaces on industrial valves such as gate valves, globe valves, ball valves, and butterfly valves, taking into account both manual and mechanical grinding methods to ensure consistent grinding quality and reliable sealing. Comprehensive preparation is required before grinding: first, disassemble the valve, thoroughly clean the sealing surfaces of any oil, rust, or impurities by soaking and wiping them with kerosene or a specialized cleaning agent, and then dry them with compressed air ； Subsequently, the defects on the sealing surface are inspected; imperfections such as pitting, scratches, corrosion pits, and indentations are examined visually. The flatness is checked using a ruler or a flat surface, and colored penetrant testing is employed when necessary to detect minor cracks. Defects with a depth of ≤0.4 mm can be ground directly, while those exceeding 0.4 mm must first be leveled using a lathe or grinder before proceeding with the grinding process. The tools and materials to be used must be matched to the material and dimensions of the sealing surface; manual grinding requires standard flat plates, grinding rods, grinding heads, red lead powder, pencils, and roughness reference blocks ； For mechanical grinding, a portable grinder, grinding discs of the appropriate specifications, and universal couplings are used to ensure that the tool is coaxial and perpendicular to the sealing surface, thereby preventing misalignment during grinding. Abrasives are selected according to the specific processing steps: 80#-280# brown corundum or silicon carbide grinding paste/paper is used for rough grinding, 280#-W5 fine grinding paste is used for medium grinding, and W3.5-W1.5 micro-powder grinding paste or chromium oxide polishing agent is used for fine grinding. Clean engine oil is used as a diluent in all cases, to ensure that the abrasive is applied evenly without any clumping. The grinding process strictly follows a four-stage sequence of rough grinding, medium grinding, fine grinding, and precision grinding. At each stage, it is necessary to thoroughly remove any remaining grinding agents to prevent coarse particles from scratching the sealing surfaces. Coarse grinding aims to remove defects and restore basic flatness. During manual grinding, the sealing surface is pressed against a standard flat plate; a uniform application of coarse grinding agent is used, and a \"8\" shape motion is employed, with alternating movements at 60°-100° in the forward direction and 40°-90° in the reverse direction. The pressure is kept between 0.1-0.3 MPa, with this range being lower for cast iron and stainless steel materials, and higher for cemented carbide ； The mechanical grinding speed should be 10–30 rpm; checks should be carried out every 3–5 minutes. Grinding should cease once any significant defects are eliminated and the surface becomes smooth, in order to avoid over-grinding that could thin out the sealing surface. During medium grinding, the goal is to eliminate the scratches left by rough grinding and improve surface smoothness; medium-grit abrasives are used, the manual pressure is reduced to 0.05–0.15 MPa, and the mechanical speed is set at 5–20 rpm. Checks are conducted every 5 minutes until no visible scratches remain on the surface and it achieves a uniform matte finish. Fine grinding further optimizes the surface roughness; a W5-W3.5 fine powder grinding paste is used, with gentle manual pressure and slow, alternating grinding at a mechanical speed of 3-10 r/min and a pressure of 0.02-0.08 MPa, until the surface texture becomes smooth with no traces of rough machining. Fine grinding is the final polishing step, in which W1.5 or finer powder or chromium oxide polishing agents are used; the surface is polished manually with very light pressure and in small ‘8’ shaped motions, at a mechanical rotation speed of ≤5 r/min and a pressure of ≤0.05 MPa, until the sealing surface achieves a uniform mirror-like finish without any visible scratches. During the grinding process, it is necessary to control the key parameters to prevent quality defects: the grinding pressure should be reduced step by step as the process progresses; the maximum pressure during rough grinding shall not exceed 0.4 MPa, while the minimum pressure during fine grinding shall not be lower than 0.02 MPa ； The grinding speed should be matched to the size and material of the sealing surface: for small diameters (DN≤50mm), the manual grinding speed is 80-100 r/min, while it drops to 50 r/min for larger diameters (DN＞100mm). For mechanical grinding, the speed is 15-50 m/min for cast iron and copper, and 20-90 m/min for hardened steel and cemented carbide ； The grinding path should avoid a single direction; an “8” pattern or alternating rotations are preferred to ensure even stress distribution and consistent grinding on the sealing surface ； Keep the abrasive moist throughout the process, and replenish the diluent promptly to prevent high temperatures and scratches caused by dry grinding ； After each stage of the process is completed, the sealing surfaces and grinding tools must be thoroughly cleaned with kerosene, and dried before moving on to the next stage; it is strictly prohibited to proceed to the next stage of grinding without completing the current one. Quality inspection is carried out in three steps to ensure that the sealing surfaces meet the standards. First is the visual inspection: the sealing surface is free of pitting, scratches, erosion pits, or cracks; its surface has a uniform gloss, with no signs of localized over-grinding or uneven wear. Next is precision testing: flatness is checked by comparing it with a standard flat plate; after printing with red lead powder, the impressions on the sealed surface are continuous and evenly distributed, with no discontinuous areas ； Surface roughness is measured using a roughness meter: for rigid sealing surfaces, Ra ≤ 0.4μm; for ordinary sealing surfaces, Ra ≤ 1.6μm; for conical and lens-shaped gasket sealing surfaces, Ra ≤ 3.2μm ； It can also be determined by comparing with roughness test blocks. Finally, there is the seal integrity test: the valve disc is brought into contact with the sealing surface of the valve seat, and concentric circles are drawn on the sealing surface using a pencil. After rotation, it is checked whether the lines drawn by the pencil are erased evenly; alternatively, red lead can be used for testing. A satisfactory result is achieved when the continuous width of the seal ring is ≥90%, meeting the requirements of the API 598 or GB/T 13927 seal testing standards. Safety and management requirements are applied throughout the entire process; the work site must be kept well-ventilated and clean, away from sources of fire and dust, with grinding agents and cleaning agents stored separately ； When using manual grinding, wear protective gloves and goggles; when using mechanical grinding, check the insulation and grounding of the equipment. It is strictly prohibited to operate the equipment when it is faulty. Always shut down the machine and disconnect the power before replacing the grinding agent or sandpaper ； The waste oil and waste residues generated during grinding are collected together and treated in accordance with environmental protection requirements; random discharge is prohibited. After grinding is completed, quality records should be kept, including the valve model, material of the sealing surface, any defects present, the grinding procedures followed, and the inspection results. These records are to be filed for easy tracking.