[Ocean Chemical Valve Management] series of stickers - valve leakage standards

Thread Content

Good management of valves is the basis for stable operation. Welcome everyone to communicate, discuss and share [Ocean Chemical Valve Management] series of posts. https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ----------------------------------------------------------------Disclaimer: The content contained in this article is for technical communication and reference purposes only and does not constitute any form of professional engineering advice, design basis or operating guidance. ----------------------------------------------------------------------------------- Valve leakage is the core indicator for measuring the sealing performance of industrial valves. It is directly related to the safety and stability of the process system, energy consumption control and environmental compliance. Whether it is daily equipment maintenance or selection and procurement, it is crucial to master its relevant standards and core points. This article combines mainstream domestic and foreign standards such as GB/T 13927-2022, API 598-2023, ANSI/FCI 70-2, and ISO 5208 to streamline and sort out the definition, classification, test methods, and selection and acceptance points of valve leakage. The so-called valve leakage refers to the flow or volume of the medium passing through the sealing pair (including the valve seat and opening and closing parts, valve body and valve cover, valve stem and packing, etc.) under the specified test pressure, test medium and duration. It is usually divided into visible leakage and trace leakage. The former can be directly observed through visual inspection, such as liquid dripping and continuous gas bubbling, while the latter requires the help of professional instruments to accurately measure. At present, a unified standard system has been formed in the industry. GB/T 13927, API 598, and ANSI/FCI 70-2 are specifically aimed at regulating valves. ISO 5208 is an internationally accepted pressure test standard for metal valves. These standards are compatible with each other and correspond to different levels, which are the core basis for selection and testing in daily work. Valve leakage is divided into six levels from low to high according to sealing capacity, from low to high. Different levels correspond to different allowable leakages and adapt to different working conditions. Level I is the lowest sealing level. There is no quantitative leakage requirement. It only needs to ensure basic sealing function. It is suitable for non-critical scenarios, such as ordinary cooling water and normal pressure air transportation. No special leakage test is required. ； Level II allows a small amount of leakage. The liquid leakage does not exceed 0.1×DN (drops/minute) and the gas leakage does not exceed 30×DN (cubic millimeters/second). It is suitable for the general working conditions of ordinary gate valves and globe valves. ； Level III has medium sealing requirements, and the leakage volume is lower than level II. It can be used in scenarios such as low-pressure regulation in the chemical industry and conventional water treatment. ； Level IV is a common level for metal hard-sealed valves. The leakage is only 0.01% of the rated flow. It is suitable for harsh working conditions such as high-temperature and high-pressure steam and oil and gas pipelines. ； Level V sealing accuracy is higher, the leakage volume is only one-tenth of level IV, and is suitable for the transportation of flammable, toxic and other dangerous media. ； Level VI, as the highest sealing level, is mostly exclusive to soft-sealed valves and requires no visible leakage. Small-diameter (DN≤50mm) valves must achieve zero bubbles, while large-diameter valves allow a small amount of bubbles. They are suitable for scenes with extremely high sealing requirements such as food and medicine, high-purity gases, and LNG. In order to accurately determine the leakage performance of the valve, it is necessary to strictly follow the standard test requirements. The leakage test must be conducted after the shell strength test is passed. The test medium is preferably clean and impurity-free water (liquid test) or air or nitrogen (gas test). Corresponding media can be used for special working conditions and recorded. In terms of test pressure, the high-pressure sealing test is 1.1 times the nominal pressure of the valve, and the low-pressure sealing test is 0.4-0.7MPa. They are mainly used to check for micro leaks. For valves with an upper sealing structure, an upper seal test is also required, and the pressure is 1.1 times the nominal pressure. The test duration needs to be adjusted according to the valve diameter. The liquid test should be no less than 60 seconds, the gas test should be no less than 30 seconds, and the large diameter (DN>100mm) valve should be no less than 120 seconds. The reading should be taken after the pressure is stable. There are also clear requirements for the detection method. For liquid leakage, use a measuring cup to measure the number of drops or volume. For gas leakage, the valve can be immersed in water to count bubbles, or directly measured with a flow meter. Level VI sealing must be determined by the bubble method. During daily selection and acceptance, the appropriate leakage level needs to be selected based on specific working conditions. Ordinary industrial pipelines generally use Grade III or Grade IV, with Grade IV being given priority. ； Metal hard-sealed valves are also preferred to be grade IV. For flammable, toxic, highly corrosive media or high-purity systems, Class V or even Class VI must be selected. However, it should be noted that although soft-sealed valves can easily reach Class VI, their temperature resistance has certain limitations. The core criterion for acceptance is that the actual measured leakage does not exceed the allowable limit of the corresponding grade. The basic prerequisite is that there is no visible dripping of liquids. For gases, instrument measurement or bubble counting shall prevail. For unqualified valves, the sealing surface should be disassembled and inspected, the seal should be repaired or replaced and retested. If it cannot be repaired, it should be scrapped. Mastering these core points can not only effectively ensure the sealing performance of the valve, avoid leakage risks, but also avoid over-design, thereby reasonably controlling procurement and maintenance costs, and helping the long-term stable operation of the system.