Leakage testing of LNG ultra-low temperature ball valves and analysis of leakage causes

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【Abstract】Leakages in LNG ultra-low temperature ball valves are of two types: external leakage and internal leakage. In most cases, the consequences of external leaks are often more severe than those of internal leaks. In industrial production, leaks from valves not only result in waste of raw materials and energy but also cause direct environmental pollution; they can even lead to serious accidents that threaten life safety, such as fires, explosions, and poisonings, causing severe losses to ** and enterprises. This article provides detailed requirements and analysis regarding the selection criteria, testing methods, acceptance standards, and causes of leakage for LNG ultra-low temperature ball valves. It also discusses preventive and corrective measures for different causes of leakage, in the hope of offering some guidance for identifying and addressing valve leakage issues during the procurement, installation, and use of ultra-low temperature valves. 【Keywords】Micro-leakage ； Extended valve cover ； leakage ； valve seat ； filler ； 【Overview】With the rapid economic development of our country, the demand for and consumption of energy have been increasing steadily, making our country the world’s largest energy consumer. However, the environmental problems caused by energy consumption are attracting increasing attention, especially fine particulate matter in the atmosphere, namely PM2.5, which remains a topic of ongoing concern. Therefore, finding ways to address these environmental issues and the problem of excessive PM2.5 levels has become an issue that must be taken into consideration in development efforts. As a clean and efficient energy source, LNG is gaining increasing popularity; many **choose LNG as their preferred fuel. With a growth rate of around 12% per year, LNG is one of the fastest-growing energy sectors in the world. Ultra-low temperature ball valves are widely used in LNG production plants, LNG receiving stations, LNG filling stations, and LNG transport ships. These valves feature a full-bore, top-mounted design; when fully open, there are no obstructions, and their internal diameter meets the requirements specified by API6D. Although the valve body has a monolithic structure, which reduces the likelihood of leakage compared to two-piece or three-piece valves, leakage issues have still been observed during their use, posing a significant threat to safe production in enterprises. All valve leakage problems must be given serious attention by enterprise managers and technical personnel. 1. Main contents of the leak test for ultra-low temperature ball valves: The leak test for ultra-low temperature ball valves is divided into normal temperature tests and low temperature tests. Before leaving the factory, ball valves are tested at room temperature, primarily to assess the strength of the valve body, the sealing performance of the body, and the internal leakage at the valve seats on the upstream and downstream sides. Low-temperature testing is divided into internal leakage and micro-leakage tests. The low-temperature internal leakage test involves sealing tests of the valve seat at the upstream and downstream ends of the ball valve, while the low-temperature micro-leakage test focuses on sealing tests of the valve body, valve stem, and intermediate seals. 2. Standards for leak testing and how to apply them: At present, there are no national or industry standards in China that specify the requirements for testing ultra-low temperature valves. In countries that are major producers or users of LNG, such as Japan, the United States, and various European countries, there are their own standards for leak testing ultra-low temperature ball valves, and these standards vary considerably from one another. For those who are new to the procurement and inspection of ultra-low temperature valves, it can be difficult to determine the appropriate criteria for acceptable valve leakage levels as well as the correct testing procedures. This document summarizes the various testing standards for LNG ultra-low temperature ball valves used abroad. (1) Test execution standards: EN12266-1:2003 – General tests for industrial valves; BS6363 – Low-temperature valves; MESC SEP77/200 – Valves for low-temperature media; MESC SEP77/312 – Micro-leakage test.
(2) Proper use of standards: EN12266-1:2003 was developed by CEN/TC96, the European Committee for Standardization/Technical Committee on Industrial Valves, with the aim of standardizing test procedures and acceptance criteria for industrial valves. It can also be used for type tests and acceptance tests. This standard specifies the mandatory requirements for valve testing, the testing procedures, and the acceptance criteria for testing industrial valve products. The normal-temperature testing procedures and acceptance requirements for ultra-low temperature valves shall be carried out in accordance with this standard. BS6363: This British standard was developed under the guidance of the Committee for Standards in Mechanical Engineering, and it is an earlier **grade standard. This standard specifies the design, manufacturing, and testing requirements for ultra-low temperature valves. Chapter 3 and Appendix A of the standard specify the procedures, methods, and requirements for low-temperature testing of ultra-low temperature valves, and serve as the fundamental basis for such testing. MESC SEP77/200: This standard is a specification for low-temperature and ultra-low-temperature valves developed by Shell, and it is applicable to the testing and acceptance of low-temperature valves with extended valve covers operating at minimum temperatures ranging from -30°C to -196°C. These specifications detail the specific length of extended valve covers, as well as the conditions to be verified prior to testing, valve selection, testing instruments, qualifications of test personnel, testing media, testing temperature, and testing procedures. The maximum allowable leakage rate for internal leakage in valves is specified based on different valve types and seat types, as shown in Table 1. Therefore, it is recommended that the procedures for verifying pre-testing conditions, valve selection, testing instruments, qualifications of test personnel, testing media, testing temperature, and testing procedures for internal leakage (seat leakage) in ultra-low temperature valves be followed, with the testing procedures and acceptance criteria being established in accordance with these standards. MESC SEP77/312: This standard is a revision and supplement to ISO15848-2 by Shell, specifying the requirements for tests on products with micro-leakage (this specification does not apply to internal micro-leakage type tests, which shall be carried out in accordance with MESC SEP77/300). This specification revises aspects such as the sampling ratio and sampling methods for product micro-leakage in ISO15848-2, the acceptance criteria for leakage in valve stems and valve bodies, and painting requirements. It also adds provisions regarding the pressure for micro-leakage testing, stabilization time, testing equipment, qualifications of test personnel, and acceptance criteria, while standardizing the units used to express leakage rates. With these revisions and additions, this standard sets more specific and stringent requirements for the testing of low-temperature external leakage (micro-leakage) in ultra-low temperature ball valves; therefore, it is recommended that such testing be carried out in accordance with this standard. Through the analysis and explanation of the four standards above, it has been clarified what guidelines should be followed for the various testing items of ultra-low temperature valves. 3. Testing Requirements and Acceptance Criteria 3.1. Testing at Normal Temperature (1) Valve Body Strength Test The main purpose of the valve body strength test is to determine whether the valve body can withstand internal pressure. The testing process is relatively simple, so it will not be described here; instead, the test pressure and minimum duration will be specified below.  Testing medium: A liquid should be used. When water is chosen as the testing medium, it is necessary to strictly control the chloride ion content in the water. How can the chloride ion content be controlled? According to Article 89 of the TSG D0001-2009 Regulations on Safety Inspection of Pressure Piping – Industrial Piping, \"When conducting a hydraulic test on austenitic stainless steel piping, or on piping connected to austenitic stainless steel piping or equipment, the chloride content in the water shall not exceed 0.005%\"” ； Article 8.6.4 of the Code for Construction of Industrial Metal Piping Projects (GB 50235-2010) stipulates that the hydraulic test shall comply with the following requirements: “Clean water shall be used for the hydraulic test; when testing stainless steel, nickel, or nickel alloy pipes, or pipes connected to such pipes or equipment, the chloride content in the water shall not exceed 25×10￣6 (25 ppm).”  Test pressure: 1.5 times the allowable pressure at room temperature or 1.5 times the design pressure.  Minimum duration: See Table 1. Minimum duration for shell tests. Table 1. Minimum duration for shell tests. Valve nominal size – Minimum duration: Product testing and acceptance testing (s); Type testing (min): ≤DN50: 15; 10; DN65–DN200: 60; 10; ≥DN250: 180; 10.  Acceptance criterion: No visible external leakage is allowed on the outer surface of the valve shell. (2) Valve housing seal test. The main purpose of the housing seal test is to ensure that the seals at the housing and the operating mechanism remain effective under internal pressure; liquid or gas is used as the testing medium.  Testing medium: Liquid or gas shall be used.  Testing pressure: Refer to the testing pressure for valve body strength.  Pressure duration: Refer to the minimum duration specified for valve body tests.  Acceptance criteria: When the testing medium is a liquid, refer to the acceptance criteria for valve body strength testing ； When the testing medium is gas, no visible bubbles are formed on the water surface when the valve is immersed in water, and no continuous bubbles are produced when a detection agent is applied to the exterior of the valve. (3) Upstream and downstream valve seat sealing test: The main purpose of the upstream and downstream valve seat sealing test is to determine whether the sealing capability of the valve seats at normal temperature meets the specified leakage rate.  Testing medium: Nitrogen is recommended.  Maximum testing pressure: The maximum testing pressure shall be 1.1 times the allowable pressure at room temperature or 1.1 times the design pressure.  Pressure holding time: Refer to Table 2 for the minimum holding time for seat sealing tests. Table 2. Minimum Holding Time for Seat Sealing Tests. Nominal valve size – Minimum holding time (nitrogen medium), Product testing and acceptance testing (s), Type testing (min): Metal-sealed seats, Soft-sealed seats. All valves: ≤DN50 – 15, 15, 10; DN65–DN200 – 15, 15, 10; DN250–DN450 – 30, 30, 10; ≥DN250 – 30, 60, 10.  Testing steps: Step 1: Open the valve partially, introduce nitrogen at a pressure of 0.2 MPa, close the valve, and measure the torque. Maintain this pressure for the minimum time specified in Table 2. After measuring the leakage rate, test the torque when the valve is in a partially open position as well as fully open ； Step 2: Partially open the valve, fill it with nitrogen at 35% of the maximum test pressure, close the valve, maintain the pressure (as required in Step 1), and measure the leakage rate ； Step 3: Open the valve partially, fill it with nitrogen at 70% of the maximum test pressure, close the valve, and maintain that pressure (as required in Step 1), then measure the leakage rate ； Step 4: Open the valve partially, fill it with nitrogen at the maximum test pressure, close the valve and measure the torque; maintain this pressure (as required in Step 1), measure the leakage rate, and then test the torque when the valve is partially open and fully open ； Step 5: For two-way valves, repeat the above steps on the other side.  Acceptance criteria: The leakage rate measured within the specified testing time must be within the prescribed limit; the prescribed leakage rates are shown in Table 3. Maximum allowable leakage amounts for different leakage rate categories (mm3/s) – Table 3. Maximum allowable leakage amounts for different leakage rate categories (mm3/s). Medium, Leakage Rate A, Leakage Rate B, Leakage Rate C, Leakage Rate D…… Nitrogen: No visible leakage throughout the duration of the test; 0.3×DN, 3.0×DN, 30×DN……  Measurement tools: Measuring cylinder or measuring cup. 3.2. Low-temperature testing (1) Sealing test of upstream and downstream valve seats. The purpose of testing the sealing performance of the upstream and downstream valve seats at low temperatures is to determine whether their sealing ability at ultra-low temperatures (-196°C) meets the specified leakage requirements; this is a crucial step in evaluating the performance of the valve.  Testing medium: The medium is helium, with liquid nitrogen used as the refrigerant.  Maximum testing pressure: Refer to the sealing test pressure for the upstream and downstream valve seats at room temperature.  Leak detection tool: Measuring cylinder or measuring cup.  Pressure duration: See the testing procedure for details.  Testing steps: Figure 1 shows the schematic of the low-temperature testing setup. First, connect the valve to the testing device (see Figure 1 for the schematic of the low-temperature testing setup). Then place the valve in the designated refrigerant test tank, ensuring that the connection flanges of the valve body and the valve cover are fully submerged in the tank, and making sure that the seal area of the valve stem in the packing box is not affected by the cold evaporated gas. During the cooling process, the helium pressure inside the valve chamber should be maintained at 0.5 MPa to prevent moisture from entering the chamber, with the valve remaining in an fully open state. Step 1: Fill the interior of the valve with helium at 0.5 MPa, and control the temperature to reach the testing temperature for the valve by adjusting the rate of liquid nitrogen spraying ； Step 2: After reaching the test temperature, keep the valve insulated for 60 minutes to stabilize the temperature across all its parts ； Step 1: Open the valve partially, introduce helium at 0.2 MPa pressure; after 5 cycles, close the valve and measure its closing torque. Maintain the pressure for 5 minutes, then measure the leakage rate. Finally, test the opening and partial-opening torques ； Step 3: Repeat the process 5 times with the valve partially open; helium should be introduced in stages to reach the pressure values specified in the data table, with each increment being held for 10 minutes (the pressure increments are listed in Table 4, Pressure Increment Table). At each pressure increment, measure the valve seat leakage and record it. Step 4: When the pressure specified in the data sheet is reached, close the valve and measure the torque. Maintain this pressure for 10 minutes; after measuring the leakage rate, open the valve to test the torque at a partially open position ； Step 5: After completing the high-pressure seat test, impact the valve seat 5 times with the pressure value specified in the data sheet (to open the valve at the specified pressure), and measure and record the torque during opening and closing. Step 6: For two-way valves, repeat the above steps on the other side.  Acceptance criteria: Refer to Table 3. Maximum allowable leakage rates for different leakage grades, and Table 4. Pressure increase table. Nominal pressure (PN) / Increase (bar): 20 / 3.5; 50 / 7.5; 64 / 10.0; 100 / 20. (2) Low-temperature micro-leakage test: The low-temperature micro-leakage test involves testing the seals at the valve stem and other sealing areas. Its main purpose is to determine whether the external leakage rate of the valve meets the specified requirements under low-temperature conditions. The micro-leakage testing steps are relatively simple and will not be described. Testing medium: Helium gas. Testing pressure: See Table 5. Optional testing pressures for valves of micro-leakage class B. Table 5: Optional testing pressures for valves of micro-leakage class B. Optional testing pressures for valves of micro-leakage class B; ASME pressure class, Testing pressure in bar (psi), Acceptance criteria: 150, 6 (87), In accordance with leakage class A (HS) in Tables 1 and 2; 300, 18 (261); 600, 33 (479); 900, 50 (725); 1500, 63 (914); 2500, 130 (1885). Testing instrument: Helium mass spectrometer. Acceptance criteria: The allowable value for the valve stem seal leakage rate is given in Table 6. Maximum valve stem seal leakage rate ； The maximum allowable value for the valve body seal leakage rate is shown in Table 7. The maximum leakage rate for the middle-stage seal is also specified in Table 7. Table 6. Maximum stem seal leakage rate Micro-leakage level Maximum stem seal leakage rate atm•cm³/s Pa•m³/s A(HS) 1.76×10⁻⁷ 1.78×10⁻⁸ B 1.76×10⁻⁶ 1.78×10⁻⁷ Table 7. Maximum body seal leakage rate Micro-leakage level Maximum body seal leakage rate atm•cm³/s Pa•m³/s A(HS) 1.76×10⁻⁸ 1.78×10⁻⁹ B 1.76×10⁻⁷ 1.78×10⁻⁸ 4. Analysis of causes and countermeasures for external leakage in cryogenic ball valves The main locations of external leakage in cryogenic ball valves are leakage from the valve body itself and leakage at the sealing areas between the valve body and the stem, bonnet, etc. (i.e., the stem and intermediate sealing areas). For the cast valve body, casting defects such as sand holes can easily form; these sand holes in the valve body can lead to leakage of the medium, which usually manifests as seepage. Such defects can be detected using methods such as non-destructive testing and hydrostatic testing. Castings with sand holes are not used any longer, and the causes of leakage in the valve body are relatively easy to identify and resolve. The following mainly analyzes the causes of leakage at the joints such as the extended valve cover, middle flange, and bottom cover, as well as the measures taken to address them. 4.1. Causes of leakage The leakage at joints such as extended valve covers, intermediate flanges, and bottom covers is mainly caused by design flaws and improper valve assembly; factors such as valve transportation and construction installation have little impact. This paper primarily analyzes the reasons related to valve design and assembly. 1) Due to changes in the physical properties of non-metallic gaskets at low temperatures—such as a reduction in their volume or resilience—the sealing specific pressure between the gasket and the mating surface decreases; in some cases, gaps even form, leading to seal failure ； 2) During assembly at normal temperature, the initial sealing specific pressure acting on the gasket is low, and the structural design does not take into account mechanisms to compensate for or maintain an adequate sealing specific pressure of the gasket at low temperatures ； 3) Under ultra-low temperature conditions, scratches on the valve stem surface, whose surface hardness was not increased by the PCTFE sealing material, led to leaks. 4.2. Measures to be taken: 1) Add a pair of anti-loosening disc springs under the nut to compensate for deformations caused by various factors ； 2) The tightening torque of the nut should be controlled within the upper part of its allowable torque range ； 3) The valve stem surface is treated with chromium plating, nitriding, or nickel-phosphorus plating to improve surface hardness ； The thermal spraying process can also be used to coat the surface with wear-resistant and corrosion-resistant alloys such as molybdenum and STL ； To prevent the nut from seizing with the bolt, nuts are generally made of Mo steel or Ni steel, and molybdenum disulfide is applied to the thread surface. 5. Analysis of the causes of internal leakage in ultra-low temperature ball valves and corrective measures. Leakage that occurs due to improper sealing of the valve is considered internal leakage, and it usually takes place at the valve seat sealing surface. 5.1. Causes of leakage The reasons for internal leakage in valves are quite complex; it can be caused by various factors such as the valve’s design structure, the choice of sealing materials, assembly processes, transportation, installation, and cleaning and pressure testing procedures. 1) Excessive roundness during sphere machining prevents proper sealing; or if the surface finish of the sphere is substandard, it can damage the contact surface of the valve seat when the valve is opened or closed, resulting in internal leakage ； 2) Due to changes in the physical properties of the valve seat at low temperatures (such as reduced volume, increased hardness and brittleness, as well as diminished material elasticity), the valve seat cannot fit perfectly with the ball at these low temperatures, which results in excessive leakage ； 3) The spring force providing the initial seal is too low, resulting in leakage beyond acceptable limits ； 4) During transportation of the valve, the valve body is subjected to impacts, which cause damage or deformation of the sealing surface and result in leakage at that surface ； 5) During installation, excessive debris enters the pipelines; this debris accumulates at valves during purging, resulting in improper valve closure or damage to the sealing surfaces, thereby causing leaks ； 6) After the hydrostatic test of the pipeline, water remained trapped in the valve cavity. During the pre-cooling process of the pipeline, the valve cooled down and froze, resulting in improper closure and leakage. 5.2. Measures to be taken 1) The roughness of all surfaces subject to sealing requirements must be improved by 2 to 3 grades compared to the standard requirements for ordinary valves. The measures taken include enhancing machining precision, polishing, grinding, etc ； 2) To minimize the deformation of the valve seat in low-temperature environments, its design should involve embedding it as much as possible within a metal support ring; any protruding parts and sealing surfaces should be reduced to a minimum, with transitions to the support ring made via curved or inclined surfaces ； 3) Under the premise of ensuring that the mutually contacting components remain undamaged, the width of the valve seat sealing surface should be reduced, so as to increase the sealing specific pressure on the sealing surface and minimize leakage ； 4) Develop packaging and transportation measures for valves, utilizing specialized delivery vehicles for transport ； 5) During pipeline installation, construction workers must inspect the interior cleanliness of each pipe. After installation, any impurities inside the pipelines should be promptly removed, and the pipe ends must be protected to prevent external contaminants from entering ； 6) For pipeline pressure testing, a gaseous medium should be used whenever possible. In cases where it is unavoidable to use water as the medium, clean freshwater with a chloride ion content of no more than 25 PPM must be utilized first. During the pipeline purging phase, open the drain ports of each valve; wrap each valve body with electric trace heating tape (the temperature is generally maintained at 105°C; this temperature may vary slightly depending on the characteristics of the sealing materials used by different manufacturers). This process evaporates any liquid water within the valve chambers. Additionally, the dew point temperature of each valve must be measured (it is required that this temperature be below -40°C). 6. Conclusion: Given that LNG has a low molecular weight, low viscosity, and high penetrability, it is extremely prone to leakage. Additionally, it is flammable and explosive. Therefore, stricter requirements must be set during the valve design process; specifically, the allowable leakage rate for ultra-low temperature LNG valves should be at least at Level B or higher. To address the leakage issue of LNG ultra-low temperature ball valves, it is necessary to implement controls at every stage, including design, material selection, processing, assembly, testing, packaging and transportation, on-site installation, overall pipeline pressure testing, and purging. Strengthened process supervision and inspection are required, with particular attention and specific regulations needed in areas such as re-inspection of raw materials, weld solution treatment, cryogenic treatment of valve components, and the selection of PCTFE and lip seals. References: MESC SEP77/312-2007, Micro-leakage testing: Netherlands, Shell, 2007; MESC SEP77/200-2007, Valves for low-temperature media: Netherlands, Shell, 2007