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1 Classification standards for valve sealing grades in China. At present, the two main classification standards for valve sealing grades commonly used in China are as follows. 1.1 Classification of valve sealing classes in Chinese **standards**: Standard GB/T 13927 “Pressure testing of industrial valves”. 1.2 Classification of valve sealing grades according to Chinese mechanical industry standards The mechanical industry standard JB/T 9092 \"Inspection and testing of valves\". 2 International classification standards for valve sealing grades Currently, the five most commonly used international classification standards for valve sealing grades are as follows. 2.1 Classification of valve sealing grades in the former Soviet Union To select products based on their sealing performance and intended applications, valves were classified according to their degree of sealing. 2.2 International Organization for Standardization’s classification of valve sealing grades: International Organization for Standardization standard ISO 5208, \"Pressure testing of industrial valves – Metal valves\". 2.3 Classification of valve sealing classes by the American Petroleum Institute (API). American Petroleum Institute standard API 598–2004 “Inspection and testing of valves”. 2.4 Classification of valve sealing grades by the Manufacturers Standardization Society of the Valve and Fittings Industry, Inc. (MSS) in the United States. The MSS standard “Pressure Testing of Steel Valves,” MSS SP61, specifies the allowable valve leakage amounts as follows: (1) When one of the sealing surfaces in the valve’s sealing pair is made of plastic or rubber, there should be no visible leakage throughout the duration of the seal test. (2) The maximum allowable leakage per side when closed should be: 0.4 milliliters per hour per millimeter of nominal size (DN) for liquids ； The gas flow rate is 120 milliliters per hour per millimeter of nominal diameter (DN). (3) The allowable leakage amount of the check valve can be increased by 4 times. 2.5 Classification of control valve sealing ratings according to American ** standards/ANSI/FCI standards. The American ** standard is ANSI/FCI 70-2 (ASME B16.104), \"Leakage of Control Valve Seats\". 2.6 Classification of valve sealing classes according to EU standards: European Standard EN 12266-1 “Industrial valves – Testing of valves”, Part 1. Pressure testing, test methods, and acceptance criteria – mandatory requirements. 3 Selection of Valve Sealing Grades 3.1 Selection of Valve Sealing Grades in China (1) The standard GB/T 13927 «Pressure Testing of Industrial Valves», which came into effect on July 1, 2009, was formulated with reference to the European standard ISO 5208. Suitable for the inspection and pressure testing of industrial metal valves, including gate valves, globe valves, check valves, plug valves, ball valves, and butterfly valves. The classification of seal tests and the maximum allowable leakage amount are the same as those specified in ISO 5208. This standard is a revision of GB/T 13927 (General Pressure Testing for Valves). Compared with GB/T 13927, six new grades, namely AA, CC, E, EE, F, and G, have been added. The new standard stipulates that “the selection of the leakage level should be the strictest one specified in the relevant valve product standards or required in the purchase contract.” Unless otherwise specified in the product standards or order contracts, non-metallic elastic-sealed valves shall meet Class A requirements, while metal-sealed valve assemblies shall meet Class D requirements…. ”Generally, grade D is suitable for ordinary valves, while valves that are more critical should use a leakage rating higher than grade D. (2) The mechanical industry standard JB/T 9092 “Inspection and testing of valves” is a revision of ZB J16006. The maximum allowable leakage amount for the seal test is established with reference to the American Petroleum Institute standard API598--1996. Suitable for the inspection and pressure testing of valves used in the petroleum industry, including gate valves, globe valves, plug valves, ball valves, check valves, and butterfly valves with metal sealing pairs, elastic sealing pairs, and non-metallic sealing pairs (such as ceramics). GB/T 9092 is currently under revision. (3) In engineering design, it should be noted that: **Standard GB/T 19672 (Technical Requirements for Pipeline Valves) was formulated with reference to the European standard ISO 14313 and the American Petroleum Institute standard API 6D. **The standard GB/T 20173 \"Pipes and valves for pipeline transportation systems in the oil and gas industry\" was formulated with reference to the European standard ISO 14313. Both standards GB/T 19672 and GB/T 20173 specify acceptance criteria for valve leakage rates that are in line with the requirements of ISO 5208 grades A and D. Therefore, when higher leakage requirements than the standard are specified in the engineering design, they should be stated in the purchase contract. ④A comparison of sealing ratings for other nominal sizes can be made by calculating the leakage rate based on the diameter. 3.2 Selection of valve sealing grades abroad (1) The classification of valve sealing grades used in the former Soviet Union was primarily applied in the 1950s; with the dissolution of the former Soviet Union, most countries no longer use this classification system for sealing grades, opting instead for the classification systems established by European and American standards. The sealing grade classification of the European standard EN 12266—1 complies with the provisions of the International Organization for Standardization standard ISO 5208, but it lacks the grades AA, CC, and EE. Compared to the 1999 version, ISO 5208 adds six new grades: AA, CC, E, EE, F, and G. The ISO 5208 standard provides a comparison of several sealing grades with those in the API 598 and EN 12266 standards. (2) The American Petroleum Institute standard API 598 is the most commonly used inspection and pressure testing standard for American-standard valves. Manufacturer standard MSSSP61 is commonly used for the inspection of steel valves in the \"fully open\" and \"fully closed\" positions, but it is not applicable to control valves. US standard valves generally do not undergo MSS SP61 testing. API 598 applies to the testing of the sealing performance of valves manufactured in accordance with the following API standards: flanged, lug-type, wafer-type, and butt-welded check valves. API 594 covers metal plug valves with flanged, threaded, and butt-welded connections. API 599 pertains to steel gate valves, globe valves, and check valves with a nominal diameter of DN100 or less for use in the oil and gas industry. API 602 relates to corrosion-resistant bolted bonnet gate valves with flanged and butt-welded connections. API 603 covers metal ball valves with flanged, threaded, and butt-welded connections. API 608 applies to double-flanged, lug-type, and wafer-type butterfly valves. In terms of engineering design, it should be noted that compared to the 1996 version, API 598–2004 no longer includes inspection and pressure testing requirements for API 600 (“Steel Gate Valves with Bolted Bonnets for Use in the Oil and Gas Industry”). Standard API 600/2001 (ISO 10434–1998) specifies that the sealing performance tests for valves shall be conducted in accordance with ISO 5208; however, the leakage rates specified in Tables 17 and 18 of this standard follow the requirements of API 598–1996, rather than using the sealing class classification system of ISO 5208. The API 600 standard, implemented on September 11, 2009, resolved this contradiction in the 2001 version by specifying that the sealing performance tests for valves should be conducted in accordance with API 598, without specifying a version, which in turn conflicts with API 598–2004. Therefore, when selecting API 600 and its sealing performance testing standard API 598 in engineering design, it is essential to specify the version of the standards to ensure consistency in their provisions. (3) The American Petroleum Institute standard API 6D (ISO 14313), \"Petroleum and Natural Gas Industries – Pipeline Systems – Pipeline Valves\", specifies the acceptance criteria for valve leakage as follows: The leakage rate of soft-sealed valves and oil-sealed plug valves shall not exceed Class A of ISO 5208 (no visible leakage allowed); the leakage rate of valves with metal seats shall not exceed Class D of ISO 5208 (1993). However, according to the sealing test described in B.4, their leakage rate shall not be more than twice that of Class D of ISO 5208 (1993), unless otherwise specified. ”The standard states: “Special applications may require a leakage rate lower than Class D of ISO 5208(1993).” ”Therefore, any leakage requirements in engineering design that exceed the standard should be specified in the purchase contract. Appendix B of API 6D–2008, which outlines additional test requirements, specifies the additional test requirements for valves that the manufacturer must carry out when required by the purchaser. Sealing tests are divided into low-pressure and high-pressure gas sealing tests; the high-pressure sealing test, which uses inert gases as the testing medium, will replace the liquid-based sealing tests. The sealing test should be selected based on the type, diameter, and pressure class of the valve; the provisions of ISO 5208 standard can be referred to. For valves on long-distance pipelines GAl and industrial pipelines GCl, it is recommended to use a low-pressure sealing test, which can increase the pass rate of these valves. When conducting high-pressure seal tests, it should be noted that after such tests, the sealing performance of elastomeric-sealed valves under low-pressure conditions may be reduced. The requirements for valve seal testing should be reasonably selected based on the actual operating conditions of the medium; this can effectively reduce the production costs of valves. (4) The American **standard shadowing the provisions of the Standard ANSI/FCI 70-2 (ASME B16.104) issued by the American Instrument Society regarding the sealing classes of control valves.** In engineering design, considerations should be given to selecting either metal elastic seals or metal seals based on factors such as the characteristics of the medium and the frequency of valve operation. The sealing grade of the metal-sealed control valve shall be specified in the purchase contract. Based on experience, for metal-sealed control valves, the requirements for grades I, II, and III are relatively low, and they are seldom selected in engineering design. Generally, for ordinary metal-sealed control valves, grade IV is the minimum required level, while for more critical control valves, grades V or VI are used. For the control valves in the flare system of a certain ethylene plant, Class IV metal sealing requirements were selected, and they have been operating well. (5) Additionally, it should be noted in engineering design that API 6D specifies that the chloride content in the water used for the sealing test of austenitic stainless steel valves must not exceed 30 ug/g, while both ISO 5208 and API 598 require that the chloride content in the water used for such tests must not exceed 100 ug/g. Due to the varying requirements of different standards, it is recommended that the chloride ion content of the water used in the seal test be clearly specified in the valve purchase contract. 4 Classification criteria for seal ratings of low-leakage valves. Low-leakage valves are those with a very small actual leakage rate; such minimal external leakage cannot be detected through conventional hydraulic or pneumatic seal tests, and thus requires the use of more advanced methods and instruments for detection. This slight leakage of the valve to the external environment is referred to as low leakage. Currently, there are mainly three internationally recognized standards for detecting low leaks in valves: (1) EPA Method 21 by the U.S. Environmental Protection Agency, “Leak Detection of Volatile Organic Compounds”. (2) International Organization for Standardization ISO 15848 (Industrial valves: Measurement, testing and qualification procedures for low leakage). (3) SHELL MESC SPE 77/312 by Shell Petroleum Company, \"Industrial Valves: Low Leakage Measurement, Classification Systems, Qualification Procedures, and Type Approval and Product Testing for On/Off and Control Valves.\" The EPA Method 21 standard of the U.S. Environmental Protection Agency only specifies the testing method; it does not define any classifications for leak levels. It is a local standard/regulation and is rarely applied. Both standards—ISO 15848 by the International Organization for Standardization and SHELL MESC SPE 77/312 by Shell Petroleum Company—evaluate valve performance based on three aspects: tightness level, durability level, and temperature level. The tightness classes, which pertain to leaks at the stem and body seals of valves, are divided into three categories: A, B, and C. According to the ISO 15848 standard, the allowable leakage rate at the valve body seal must be ≤50 cm³/m³. For leaks at the valve stem, both standards calculate the permissible leakage rate based on the diameter of the stem. Grade A of ISO 15848 has the highest sealing rating, while grades B and C correspond to the SHELLMESC SPE 77/312 standard. Typically, the sealing grade of low-leakage valves is below class B, while bellows-sealed valves have a sealing grade lower than class A due to the use of metal bellows for sealing at the valve stem area. 5 Selection of low-leakage valves: Bellows-sealed valves are one type of low-leakage valve. In the past, for operating conditions with special requirements regarding the external leakage level of valves, bellows-sealed valves were typically chosen. However, due to the high manufacturing difficulty and stringent technical requirements associated with bellows-sealed valves, their bellows materials cannot yet be fully produced domestically; this results in excessively high costs, thereby hindering their widespread adoption in the petrochemical industry. Currently, as people’s awareness of safety and environmental protection continues to grow, along with increased technical cooperation abroad and the improvement of the technical capabilities of domestic valve manufacturers, domestic technicians’ understanding of low-leakage valves is also advancing, which is expanding their range of applications. If the valves used for flammable, explosive, and toxic substances in petrochemical plants can meet low-leakage standards, this will undoubtedly **reduce the release of such toxic, flammable, and explosive substances within the plants, thereby preventing accidents that could cause fires, explosions, poisoning, and other threats to human life due to valve leaks. Low-leakage valves that meet the ISO15848 and SHELL MESC SPE 77/31 standards have a simpler structure and are easier to manufacture compared to bellows valves; their cost is about 10% to 20% higher than that of conventional valves. Based on the previous analysis and comparison of these two standard sealing grades, valves with a tightness grade of B generally meet the low-leakage requirements in certain special operating conditions. The required machining precision is relatively easy to achieve, and the manufacturing cost increases only slightly; as such, they can replace some bellows valves. Currently, low-leakage valves are more practical for the purification systems of oil and gas fields with high hydrogen sulfide levels. Since hydrogen sulfide is a highly toxic and flammable gas that is heavier than air and can accumulate in low-lying areas, inhaling certain concentrations of it can harm the body or even cause death; therefore, stricter requirements are imposed on leaks from such natural gas purification facilities. 6 Conclusion When selecting the sealing grade and the specified allowable leakage amount, it should be noted that leakage of the medium between the sealing surfaces in high-pressure valves can cause surface erosion. If a corrosive medium leaks, the metal at the leakage site will be corroded. As the leakage gap widens, the amount of leakage will increase rapidly, eventually leading to the destruction of the valve. Therefore, for valves operating in high-pressure or corrosive media environments, higher requirements must be placed on ensuring sealing performance. In pipelines transporting flammable, explosive, and toxic substances, leaks of the medium between the valve sealing surfaces can cause harm to people, financial losses, and even accidents. Therefore, for valves used to transport flammable, explosive, and toxic media, the requirements regarding sealing performance should be established appropriately based on the hazard level of the media. Any seal sometimes allows for a slight amount of leakage; if this amount of leakage has no practical effect, it can be considered a good seal. The technical standards for valve manufacturing generally stipulate that for metal-to-metal seals, a certain level of leakage is allowed when testing the sealing performance in the closed state. To ensure high sealing performance of the valve, it is necessary to grind the sealing surfaces with great care in order to increase the specific pressure on those surfaces; however, this pressure must remain below the allowable specific pressure for the material of the sealing surfaces. At the same time, it is important to enhance the stiffness of the structure. Experience with the use of valves shows that in many cases it is unnecessary to impose overly high requirements on their sealing performance, as there are operating conditions where a slight amount of leakage from the valve is completely acceptable, as such leakage is not sufficient to affect the valve’s functionality. On the contrary, improving the sealing performance of these valves complicates the manufacturing process, increases costs, and leads to unnecessary waste. The structural design and manufacturing process of the valve itself have the most significant impact on its external leakage. Valves with low leakage requirements impose stricter demands on the design and manufacturing of key components such as the valve body, valve stem, and packing box. For example: (1) the quality of the valve body and valve cover, with care being taken to avoid defects such as folds, inclusions, pores, poor microstructure, hidden cracks, as well as uneven composition during forging or casting. (2) The machining quality of components at the connection between the valve stem and the valve body, particularly the roughness of the valve stem and the packing box, the straightness of the valve stem, the perpendicularity of the holes in the valve cover packing box, and the precision of machining. (3) For the selection of the valve packing box structure, since the sealing at the valve stem is a dynamic seal, the packing tends to wear out as the valve stem rotates or slides; therefore, special low-leakage packing seals and combinations of such seals must be used, along with strict control over the gap between the packing and the valve stem, as well as the gap between the packing and the packing box. In summary, when selecting a valve type, in addition to meeting the process requirements and standard specifications, various operating conditions must also be taken into full consideration. In engineering design, efforts should be made to choose valve sealing grades that adhere to the principles of safety, rationality, and cost-effectiveness.