Valve sealing rating and selection

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Valves are important components in the piping systems of petrochemical plants; there are many types of them and a large number of such valves, making them one of the main sources of leakage in these plants. Therefore, strict requirements regarding valve leakage are essential. The sealing performance of a valve refers to the ability of its various sealing parts to prevent the leakage of the medium. The main sealing areas of a valve are: the mating surface between the operating element and the valve seat, the fit between the packing and the valve stem as well as the packing box, and the junction between the valve body and the valve cover. The first type of leakage is called internal leakage; it directly affects the valve’s ability to cut off the medium and the normal operation of the equipment. The leaks in the latter two locations are referred to as external leaks, meaning that the medium leaks from inside the valve to outside it. This directly affects safe production, leads to losses of working medium as well as financial losses for the enterprise and environmental pollution; in severe cases, it can result in production accidents. Especially for media that are high-temperature and high-pressure, flammable, explosive, toxic, or corrosive, external leakage from valves is absolutely not permissible, as the consequences of such leakage are more severe than those of internal leakage. Therefore, valves must have reliable sealing properties to meet the requirements regarding leakage levels imposed by their operating conditions. 1 Classification standards for valve sealing grades in China: At present, the two main classification standards for valve sealing grades that are commonly used in China are as follows. 1.1 Classification of valve sealing grades according to Chinese standards – Standard GB/T 13927 \"Pressure testing of industrial valves\". 1.2 Classification of valve sealing grades according to Chinese machinery industry standards The machinery 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 the degree of sealing of valves and their intended uses, valves were classified according to their sealing performance. 2.2 Classification of valve sealing grades by the International Organization for Standardization The 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). API Standard API 598–2004, \"Inspection and Testing of Valves\". 2.4 Classification of valve sealing grades by the Mechanical Services Society (MSS) of the United States manufacturers in the valve and pipe fitting industry. The MSS standard \"Pressure Testing of Steel Valves\" MSS SP61 specifies the following requirements regarding allowable valve leakage: (1) When one of the sealing surfaces in the valve seal assembly is made of plastic or rubber, there shall be no visible leakage during the duration of the sealing test. (2) The maximum allowable leakage per side when closed shall 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 seal ratings according to American ** standards/ANSI/FCI standards: The American ** standard, ANSI/FCI 70-2 (ASME B16.104), “Leakage at Control Valve Seats”. 2.6 Classification of valve sealing classes according to EU standards European standard EN 12266—1 \"Testing of industrial 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/T13927 (General Pressure Testing for Valves). Compared with GB/T13927, 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 purchase contract, non-metallic elastic-sealed valves shall meet Class A requirements, while metal-sealed valve assemblies shall meet Class D requirements⋯ ”Generally, Class D is suitable for ordinary valves; for more critical valves, a leakage class above Class D should be selected. (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 API 598--1996. Applicable to 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 seals, elastic seals, and non-metallic seals (such as ceramic). Currently, GB/T 9092 is 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. 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 classification of sealing classes in the European standard EN 12266-1 is in accordance with the provisions of the International Organization for Standardization standard ISO 5208; however, three classes—AA, CC, and EE—are missing. 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 classes with those in the API 598 and EN 12266 standards. ④. A comparison of sealing ratings for other nominal sizes can be made by calculating the leakage volume based on the diameter. (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. American standard valves are usually not tested according to MSS SP61. API 598 is applicable to the sealing performance testing of valves manufactured in accordance with the following API standards: flanged, lug-type, wafer-type, and butt-welded check valves; API 594 – metal plug valves with flanged, threaded, and butt-welded connections; API 599 – steel gate valves, globe valves, and check valves for use in the oil and gas industry, with sizes up to DN100; API 602 – corrosion-resistant plug-valve bonnet gate valves with flanged and butt-welded connections; API 603 – metal ball valves with flanged, threaded, and butt-welded connections; API 608 – double-flanged, lug-type, and wafer-type butterfly valves. Note for engineering design: Compared to the 1996 version, API 598–2004 eliminates the 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 carried out 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 grade classification system of ISO 5208. The API 600 standard, implemented on September 11, 2009, corrected this inconsistency 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 must not exceed Class A of ISO 5208 (no visible leakage allowed); the leakage rate of valves with metal seats must 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, higher leakage requirements than the standard are specified in engineering design and should be provided 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 gas as the testing medium, will replace the liquid-on-seal test and the liquid sealing test. 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 rate of qualified valves. When selecting a high-pressure sealing test, it should be noted that elastic-sealed valves may experience a reduction in their sealing performance under low-pressure conditions after undergoing such a test. The valve sealing test requirements should be selected appropriately based on the actual operating conditions of the medium, which can effectively reduce the production costs of valves. (4) The American **Standard Institute for Instrumentation standard ANSI/FCI 70-2 (ASME B16.104) applies to the specifications for the sealing classes of control valves. In engineering design, a decision should be made between metal elastic sealing and metal sealing based on factors such as the properties of the medium and the frequency of valve operation. The sealing class of metal-sealed control valves 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 chosen in engineering design. Generally, for ordinary metal-sealed control valves, grade IV is the minimum required, while for more critical control valves, grades V or VI are used. For the control valve design of the flare system in an ethylene plant, metal-sealed Class IV requirements were adopted, resulting in good operational performance. (5) Additionally, it should be noted in engineering design that API 6D specifies that the chloride content in the water used for sealing tests of austenitic stainless steel valves must not exceed 30 ug/g, while both ISO 5208 and API 598 stipulate that this chloride content must not exceed 100 ug/g. Due to the different requirements of various standards, it is recommended that the valve purchase contract specify the chloride content of the water used in the sealing test. 4 Classification standards for the sealing performance of low-leakage valves. Low-leakage valves are those that have an extremely low actual leakage rate; conventional water or air pressure sealing tests are insufficient to determine their sealing quality, and more advanced methods and instruments are required to detect such minimal leaks. This slight leakage of the valve to the external environment is referred to as low leakage. Currently, the three most commonly used international standards for detecting low leakage in valves are as follows: (1) U.S. Environmental Protection Agency EPA ****od 21 \"Detection of Volatile Organic Compound Leaks\". (2) International Organization for Standardization ISO 15848 (Industrial valves – Procedures for measurement, testing, and qualification of 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 in the United States specifies only the testing methods without defining any leakage levels; it is a local standard and is therefore used less frequently. Both standards, ISO 15848 from the International Organization for Standardization and SHELL MESC SPE 77/312 from Shell Oil Company, evaluate the performance of valves based on three aspects: tightness grade, durability grade, and temperature grade. The tightness grades, which relate to leaks at the seal between the valve stem and the valve body, are divided into three levels: A, B, and C. According to the ISO 15848 standard, the leakage at the valve body seal is required to be ≤50 em3/m3, while the leakage at the valve stem is calculated based on the diameter of the valve stem in both standards. 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 rating of low-leakage valves is below grade B, while bellows-sealed valves have a sealing rating lower than grade 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 applications with special requirements regarding valve leakage levels, bellows-sealed valves were generally chosen. However, due to the high difficulty in manufacturing bellows-sealed valves and the stringent technical requirements involved, as well as the fact that the materials used for these bellows cannot yet be fully produced domestically, their cost is too high, which limits their widespread use 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 enterprises can meet low-leakage standards, this will undoubtedly **reduce the release of such toxic, flammable, and explosive substances within the facilities, thereby preventing accidents that could lead to 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 precision for manufacturing 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 content. 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 increases 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 media, leaks of the medium between the valve sealing surfaces can cause personal injury, economic losses, and even accidents. Therefore, for valves used to transport flammable, explosive, and toxic media, the sealing requirements should be established reasonably 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 leakage of the fluid is completely acceptable, as such leakage is not sufficient to affect the proper functioning of the valve. 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 leakage rate. Valves with low leakage requirements impose stricter standards 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 folding, 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 stuffing 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, and the gaps between the packing and the valve stem, as well as between the packing and the stuffing box, must be strictly controlled. 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 valves whose sealing level adheres to the principles of safety, rationality, and cost-effectiveness.