Regarding the internal leakage issue of check valves

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During the water test of check valves, when testing from 1/2” to 21/2”, internal leakage is minimal, which basically meets the requirements specified at the time of manufacture. Our testing process is also very simple: the product is filled with water, and after some time we check to see if there is any leakage from the plate area; afterwards, an air pressure test is conducted to verify whether there is any external leakage. . . . Small and medium sizes can meet this requirement. . . Here comes the problem. For check valves larger than 21/2”, the leakage at the plate area increases significantly. Don’t know how to control it or how to improve it effectively? Does anyone have any experience or insights in this area? ? ? ? For the flatness of the plate, after machining it on a lathe, we also perform manual grinding to smooth it out. . . . . However, the results are still not satisfactory. . . . I hope everyone can offer as many suggestions as possible. Thank you~~~