The core basis for selecting and testing the leakage rate of valves

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The leakage rate of valves is a key indicator for assessing the sealing performance of industrial valves; it has a direct impact on the safety and stability of process systems, as well as on energy consumption control and environmental compliance. Whether it is for routine equipment maintenance or for selection and procurement, it is essential to understand the relevant standards and key aspects related to this parameter. Based on domestic and international standards such as GB/T 13927-2022, API 598-2023, ANSI/FCI 70-2, and ISO 5208, this article simplifies and summarizes the definition, classification, testing methods for valve leakage rates, as well as the key points for selection and acceptance. The so-called valve leakage rate refers to the flow rate or volume of fluid that passes through the sealing surfaces (including the valve seat and the operating element, the valve body and the valve cover, the valve stem and the packing) of a valve under specified test pressures, test media, and over a certain period of time. It is generally divided into visible leakage and minor leakage; the former can be observed directly with the naked eye, such as liquid dripping or continuous bubbling of gas, while the latter requires the use of specialized instruments to measure accurately. A unified standard system has been established in the industry; GB/T 13927, API 598, and ANSI/FCI 70-2 are specifically designed for control valves, while ISO 5208 is the internationally recognized standard for pressure testing metal valves. These standards are mutually compatible and hierarchically corresponding; they serve as the core basis for selection and testing in daily work. The leakage rate of valves is classified into six grades from I to VI, based on their sealing capacity; each grade corresponds to a specific allowable leakage level, allowing them to meet different operational requirements. Grade I represents the lowest level of sealing; there are no requirements regarding quantitative leakage, and only the basic sealing function needs to be ensured. It is suitable for non-critical applications such as ordinary cooling water and atmospheric air transportation, where no specialized leakage testing is required ； Grade II permits a small amount of leakage: the liquid leakage should not exceed 0.1×DN (drops per minute), and the gas leakage should not exceed 30×DN (cubic millimeters per second); it is suitable for ordinary gate valves and globe valves under normal operating conditions ； Grade III corresponds to moderate sealing requirements; its leakage rate is lower than that of Grade II, and it can be used in applications such as low-pressure regulation in the chemical industry and conventional water treatment ； Grade IV is a commonly used grade for metal hard-sealed valves; its leakage rate is merely 0.01% of the rated flow rate. It is suitable for harsh operating conditions such as high-temperature and high-pressure steam, as well as oil and gas pipelines ； Grade V seals offer higher precision, with a leakage rate that is only one-tenth that of Grade IV; they are suitable for transporting hazardous media such as flammable and toxic substances ； Grade VI, being the highest sealing grade, is typically reserved for soft-sealed valves; it requires no visible leaks. Valves with small diameters (DN≤50mm) must have zero bubbles, while valves with larger diameters allow for a small amount of bubbles. This grade is suitable for applications in the food and pharmaceutical industries, as well as for high-purity gases and LNG, where extremely high sealing standards are required. To accurately determine the leakage performance of a valve, it is necessary to strictly follow the standard test requirements. The leakage test must be conducted after the shell strength test has been passed. For the test medium, clean water free of impurities is preferred for liquid tests, while air or nitrogen is used for gas tests. In special cases, appropriate media can be selected, with proper documentation kept. Regarding the test pressures, the high-pressure sealing test is conducted at 1.1 times the valve’s nominal pressure, while the low-pressure sealing test is carried out at 0.4–0.7 MPa; these tests are primarily used to detect minor leaks. Valves equipped with an upper sealing mechanism also require an upper sealing test, with the pressure being 1.1 times the nominal pressure. The test duration must be adjusted according to the valve diameter: the test should last at least 60 seconds for liquid, and at least 30 seconds for gas. For valves with a large diameter (DN > 100 mm), the test duration should be at least 120 seconds. Readings should be taken only after the pressure remains stable. There are also clear requirements regarding the detection methods: for liquid leaks, a measuring cup is used to count the number of drops or measure the volume; for gas leaks, the valve can be immersed in water to count the bubbles, or a flow meter can be used for direct measurement. For Class VI seals, the bubble test must be employed for evaluation. During routine selection and acceptance, it is necessary to choose the appropriate leakage level based on specific operating conditions. For ordinary industrial pipelines, grade III or grade IV is generally selected, with grade IV given priority ； Grade IV is also preferred for metal hard-sealed valves. For flammable, toxic, highly corrosive media or high-purity systems, Class V or even Class VI valves must be used; 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 measured leakage rate does not exceed the allowable limit for the corresponding grade; the absence of visible dripping of liquid is a basic prerequisite, while for gases, measurement by instruments or bubble counting is used as the standard. For defective valves, the sealing surfaces should be disassembled and inspected; after repairing or replacing the seals, they should be tested again. If repair is not possible, they should be discarded. Mastering these key points not only ensures the sealing performance of valves and prevents leakage risks, but also avoids over-design, thereby enabling reasonable control of procurement and maintenance costs and supporting the long-term stable operation of the system.