Detailed Introduction to Valve Sealing Surface Grinding Technology

Thread Content

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 accuracy. Based on practical field operations and industry standards, this specification establishes a set of readily implementable technical requirements covering preparation, graded grinding, quality inspection, and safety management. It is applicable to the repair and precision finishing of sealing surfaces on industrial valves such as gate valves, globe valves, ball valves, and butterfly valves. The specification caters to both manual and mechanical grinding scenarios, ensuring consistent grinding quality and reliable sealing performance. Comprehensive preparation is required before grinding: first, disassemble the valve and thoroughly clean the sealing surfaces of any oil, rust, or impurities; soak and wipe them with kerosene or a specialized cleaning agent, and then dry them with compressed air ； Next, inspect the sealing surface for defects: visually check for pitting, scratches, corrosion pits, indentations, etc. Use a straightedge or flat plate to test the flatness. If necessary, employ dye penetrant testing to detect any minor cracks. Defects with a depth of ≤0.4 mm can be directly ground; those exceeding 0.4 mm must first be leveled using a lathe or grinder before proceeding to 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 a universal coupling 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 processing stage: 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 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 surface. 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 conducted every 3–5 minutes. Grinding should be stopped once any significant defects are eliminated and the surface becomes smooth, in order to avoid over-grinding which could thin out the sealing surface. Medium grinding aims to eliminate coarse grinding scratches and improve surface flatness. Switch to a medium-grit abrasive; manually apply pressure at 0.05–0.15 MPa, with the machine rotating at 5–20 r/min. Check the surface every 5 minutes until no visible scratches remain and the surface exhibits 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. The mechanical rotation speed is 3-10 r/min, and the pressure is 0.02-0.08 MPa, until the surface texture becomes smooth with no traces of rough machining remaining. Fine grinding is the final polishing step, in which W1.5 or finer abrasive powders or chromium oxide polishing agents are used; the surface is polished by hand with very light pressure and in small ‘8’ shaped movements, 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, with a maximum value of no more than 0.4 MPa for rough grinding, and a minimum value of not less than 0.02 MPa for fine grinding ； 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 materials, 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 on the sealing surface and consistent grinding ； 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 with grinding across stages. 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, corrosion 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 fit inspection: the valve disc is brought into contact with the seal surface of the valve seat, and concentric circles are drawn on the seal surface using a pencil. After rotation, it is checked whether the lines drawn by the pencil are erased evenly, or red lead powder can be used for testing. A seal ring width of ≥90% is considered acceptable, meeting the seal test requirements of API 598 or GB/T 13927. 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.