A Guide to Valve Sealing Performance and Standards

In industrial production, piping systems are responsible for transporting various media, including raw materials, finished products, energy, and waste. Valves play a vital role in this process by controlling the flow of media and ensuring the smooth and safe operation of the production process. Each opening and closing of a valve determines the direction and volume of the media flow, impacting the efficiency and stability of the entire production system.

However, the performance of a valve is not solely dependent on its ability to open and close properly; the sealing performance of the valve is also a critical factor. The quality of a valve's sealing performance directly affects whether it can fulfill its crucial role and can even impact the safety and efficiency of the entire production system. Today, let's delve into the intricacies of valve sealing performance and how to select the appropriate sealing grade standards.

Valve sealing performance, in simple terms, refers to the ability of the various sealing parts of a valve to prevent the leakage of media. This may seem like a minor issue, but in reality, it has a profound impact on the entire industrial production process. There are three key locations for valve sealing: First is the mating surface between the closure member and the valve seat, which is the core sealing area of the valve. If leakage occurs here, it is called internal leakage. Internal leakage directly affects the valve's ability to cut off the media, thereby disrupting the normal operation of the equipment. Imagine if a valve cannot effectively cut off the media, the entire production process could be thrown into chaos, and production efficiency could plummet. The other two sealing locations are the packing and valve stem, as well as the packing box, and the connection between the valve body and the valve cover. Leakage at these locations is referred to as external leakage. External leakage is a more serious issue because it means that the media is leaking from inside the valve to the outside. This not only results in the loss of the working media, causing economic losses to the enterprise, but it can also lead to environmental pollution. What's more alarming is that if the leaking media has hazardous characteristics such as flammability, explosiveness, toxicity, or corrosiveness, external leakage could directly lead to production accidents, endangering the lives of personnel. Therefore, for such high-risk media, external leakage of valves is absolutely not allowed, and the consequences are much more severe than internal leakage.

Since valve sealing performance is so important, how do we measure the sealing performance of a valve? This involves the selection of sealing grades.

In the 1950s, the former Soviet Union had its own valve sealing grade classification system. However, with the dissolution of the Soviet Union, this standard gradually fell out of use in most countries. It was replaced by the European and American standards for sealing grade classification, which are more scientific, rigorous, and in line with international requirements. The European Standard EN 12266-1:2003 is a typical example. It complies with the International Organization for Standardization Standard ISO 5208:2008, except for the omission of grades AA, CC, and EE. The ISO 5208:2008 standard itself, compared to the 1999 version, has added six grades: AA, CC, E, EE, F, and G, allowing for a more detailed classification of valve sealing performance. In addition, the ISO 5208:2008 standard provides comparisons with several sealing grades of the API 598 and EN 12266 standards, facilitating the conversion and comparison between different standards. Comparisons of other nominal size sealing grades can be derived by calculating the leakage rate based on the valve size.

The American Petroleum Institute Standard API 598 is the most commonly used inspection and pressure testing standard for American Standard valves. This standard is widely applied to various types of valves, including flanged, lugged, wafer, and butt-welded check valves (API 594), metal plug valves (API 599), steel gate valves, globe valves, and check valves (API 602), corrosion-resistant bolted bonnet gate valves (API 603), metal ball valves (API 608), and butterfly valves (API 609). It is worth noting that compared to the 1996 version, API 598:2004 has eliminated the inspection and pressure testing for API 600 (steel gate valves with bolted bonnets for the petroleum and natural gas industries). The API 600:2001 (ISO 10434:1998) standard stipulates that the valve sealing performance test should refer to ISO 5208, but its Tables 17 and 18 have leakage rates consistent with the API 598:1996 standard, rather than adopting the sealing grade classification of ISO 5208. This contradiction was rectified in the API 600:2009 standard, which was implemented on September 11, 2009, specifying that the valve sealing performance test should be conducted according to API 598, without specifying the version, thus creating a new contradiction with API 598:2004. Therefore, when selecting API 600 and its sealing performance test API 598 standards in engineering design, it is essential to clarify the version of the standard to ensure the consistency of the standard content.

The American Petroleum Institute Standard API 6D:2008 (ISO 14313:2007) “Petroleum and Natural Gas Industries - Pipeline Transportation Systems - Pipeline Valves” has clear regulations on the acceptance criteria for valve leakage. For soft-seated valves and oil-sealed plug valves, the leakage rate must not exceed ISO 5208 Grade A (no visible leakage); for metal-seated valves, the leakage rate must not exceed ISO 5208 (1993) Grade D. However, according to the sealing test described in B.4, the leakage rate must not exceed twice the ISO 5208 (1993) Grade D level, unless otherwise specified. This means that in special application scenarios, if higher requirements for leakage rate are needed, they should be clearly stated in the purchase contract. Appendix B of API 6D:2008 also stipulates the additional valve test requirements that the manufacturer must conduct if requested by the purchaser. The sealing test is divided into low-pressure and high-pressure gas sealing tests, with high-pressure sealing tests using inert gas as the test medium gradually replacing liquid upper sealing tests and liquid sealing tests. When selecting the sealing test based on the type, size, and pressure rating of the valve, the provisions of the ISO 5208 standard can be referred to. For valves on long-distance pipelines GAl and industrial pipelines GCl, it is recommended to use low-pressure sealing tests to increase the pass rate of valves. However, when selecting high-pressure sealing tests, it should be noted that elastic-sealed valves may experience a reduction in sealing performance under low-pressure conditions after high-pressure sealing tests. Therefore, the valve sealing test requirements should be reasonably selected based on the actual conditions of the media usage, which can not only ensure the sealing performance of the valve but also effectively reduce the production cost of the valve.

The American National Standard American Instrument Society Standard ANSI/FCI 70-2 (ASME B16.104) - 2006 applies to the regulations for control valve sealing grades. In engineering design, the selection of metal elastic sealing or metal sealing should be considered based on the characteristics of the media and the frequency of valve opening, among other factors. The sealing grade of metal-sealed control valves should be clearly specified in the purchase contract. According to experience, for metal-sealed control valves, Grades I, II, and III have lower requirements and are less commonly selected in engineering design. Under normal circumstances, the minimum grade for general metal-sealed control valves is Grade IV, while more critical control valves are selected with Grades V or VI. For example, in the design of control valves for a torch system in an ethylene plant, Grade IV metal-sealed requirements were selected, and the operation has been satisfactory.

There is another detail in engineering design that is easily overlooked. API 6D stipulates that the chloride content in the water used for the sealing test of austenitic stainless steel valves must not exceed 30ug/g. ISO 5208 and API 598, on the other hand, specify that the chloride content in the water used for the sealing test of austenitic stainless steel valves must not exceed 100ug/g. Since the requirements for chloride content vary among different standards, it is recommended to clearly specify the chloride content of the water used in the sealing test in the valve purchase contract to avoid problems arising from inconsistent standards.

With the increasing demands for environmental protection and safety in industrial production, the need for low-leakage valves is also growing. Low-leakage valves refer to valves with very small actual leakage rates that cannot be determined by conventional hydraulic or pneumatic sealing tests and require more advanced means and instruments for detection of minor external leakage. Currently, the following three standards are widely used internationally for detecting low-leakage valves:

The United States Environmental Protection Agency EPA Method 21 “Detection of Volatile Organic Compound Leaks” is a detection standard specifically targeting the leakage of volatile organic compounds. This standard only specifies the detection method and does not classify leakage levels. Since it is a local standard regulation, its application scope is relatively narrow.

The International Organization for Standardization ISO 15848 “Industrial Valves: Low-Leakage Measurement, Testing, and Qualification Procedures” is a more comprehensive standard. It evaluates the performance of valves from three aspects: tightness grade, durability grade, and temperature grade. The tightness grade targets leakage at the valve stem and valve body sealing areas, divided into grades A, B, and C. For the ISO 15848 standard, the leakage at the valve body sealing area should be ≤50 cm³/m³, while the leakage at the valve stem is calculated based on the diameter of the valve stem.

The Shell Oil Company SHELL MESC SPE 77/312 “Industrial Valves: Low-Leakage Measurement, Grading System, Qualification Procedures, and Type Approval and Product Testing for On-Off Valves and Control Valves” is also an important standard. It similarly evaluates the performance of valves from three aspects: tightness grade, durability grade, and temperature grade. In terms of tightness grade, it differs from the ISO 15848 standard. The highest sealing grade in ISO 15848 is Grade A, while Grades B and C are equivalent to the SHELL MESC SPE 77/312 standard. Under normal circumstances, the sealing grade of low-leakage valves is below Grade B, and bellows-sealed valves, which use metal bellows for sealing at the valve stem, can achieve a sealing grade below Grade A.

From the introduction above, we can see that valve sealing performance is a crucial link in industrial production. It not only affects production efficiency and economic benefits but also directly involves the safety of personnel and the protection of the environment. When selecting valves, it is essential to choose the appropriate sealing grade standards based on specific working conditions. At the same time, the detection of low-leakage valves also needs to be based on scientific and rigorous standards. Only in this way can we ensure that valves perform optimally in practical applications and safeguard industrial production.