Quality requirements for valves in high-pressure hydrogenation units

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1 Overview The high-pressure hydrogenation process is an important method in the deep processing of petroleum; it not only increases the yield of light oils per unit of crude oil. It can also improve the quality of fuel oil, thereby enhancing the overall efficiency of refineries. High-pressure hydrogenation units can also provide high-quality raw materials for petrochemical plants, and they are ideal devices for desulfurizing petroleum products. China began constructing high-pressure hydrogenation units in the 1990s. It is expected that in the coming years, as China’s crude oil refining volume continues to rise annually, the construction of such units will remain a major focus in the petrochemical industry. Major petrochemical enterprises also aim to enhance their oil refining standards and capabilities through this initiative, so as to meet the challenges posed by China’s accession to the WTO. 2 Operating Conditions The medium environment in high-pressure hydrogenation units has two prominent characteristics, namely high pressure and operation in the presence of hydrogen (along with hydrogen sulfide). High-pressure operation is characterized not only by its high operating pressure (typically 14–20 MPa), but also by the fact that the medium used is a flammable and explosive high-pressure gas (hydrogen or oil/gas + hydrogen). High-pressure gas stores a great amount of pressure energy. Should its storage and transportation equipment (including pipelines and valves) be damaged, the resulting accidents would be catastrophic. Operated in the presence of hydrogen along with hydrogen sulfide. This indicates stringent requirements for the materials used in storage and transportation equipment. Hydrogen is a medium that can penetrate into the interior of metal materials and cause their deterioration at room temperature or elevated temperatures. At room temperature, it can lead to embrittlement and deformation of metal materials; at high temperatures, it can cause decarburization both internally and externally in metal materials. The corrosion of metal materials by hydrogen sulfide is also a very challenging issue; at room temperature it can cause stress corrosion cracking in many metal materials, while at high temperatures it leads to rapid and uniform corrosion of these metals. All these characteristics impose strict requirements on the materials, structural design, and strength design of high-pressure hydrogenation valves. 3 Requirements: The valves used in high-pressure hydrogenation units are subjected to both hydrogen-containing and non-hydrogen-containing conditions, as well as both high-pressure and non-high-pressure conditions. This article discusses only valves used under high-pressure hydrogen conditions. Although there are not many of such valves, they account for a large proportion of the total cost. Such valves include gate valves, globe valves, check valves, ball valves, and plug valves. The pressure rating ranges from ASME CL900 to 2500, while the operating temperature ranges from room temperature up to 400°C. The material used for the valve bodies is ASTMA105, A182—F11/F22/F321, A216—WCB, A217—WC6/WC9, and A351—CF8C. The valve diameter ranges from DN15 to 400 mm. These valves have the same functions as ordinary valves, but China currently lacks the capability to supply them in complete sets. The main reason for this is insufficient communication between valve manufacturers and users (especially engineering design departments); these parties are not aware of the conditions and requirements for using the valves. There is also a lack of technical documents, molds, drawings, etc. that are suitable for the characteristics of the media being handled, as well as experience in carrying out industrial-scale mass production. Developing valves for high-pressure hydrogenation units can not only reduce the cost of these valves. It can also shorten the supply cycle, facilitate reordering, and at the same time promote the development of the valve industry. In fact, some valve manufacturers in our country currently possess the capability and conditions to produce high-pressure hydrogenation valves, which have already been successfully applied in hydrogenation refining units (with operating pressures of 8–10 MPa). Valves with good performance have excellent design, manufacturing, inspection and testing, as well as quality control in all aspects and at every stage. This article discusses several of the main issues involved (including internal and external leakage of valves, strength design, materials, inspection tests, and quality control), in conjunction with the medium conditions of high-pressure hydrogenation units. 4 Control of internal and external leakage: The degree of control over internal and external leakage of valves is an important parameter reflecting their quality. Manufacturers take various measures to minimize both internal and external leakage. Leaks of oil and hydrogen, both external and internal, not only pollute the environment but can also cause fires or explosions. Internal leakage of valves occurs mainly at the gate, while external leakage takes place primarily at the valve stem packing and the valve cover gasket; therefore, to achieve good sealing performance, attention must be paid to these three areas. 4.1 External leakage due to packing sealing: For gate valves and globe valves with a DN of ≥ 50 mm, the use of flexible graphite rings and flexible graphite woven packing at the valve stem seal is a common practice among foreign remanufacturing factories. Among them, the topmost and bottommost rings of the packing consist of flexible graphite braided rings, which are used to prevent graphite from being forced into or out of the valve cavity by taking advantage of their relatively high strength (compared to molded graphite rings). In between, molded graphite rings are used (and sometimes braided rings are also added at appropriate positions in the middle). Due to the good flowability and anti-friction properties of flexible graphite, as well as its excellent physical and chemical stability, using it as a packing material can ensure effective sealing while reducing wear on the valve stem. For ball valves, plug valves, and small-diameter gate and globe valves with relatively small stuffing box sizes, flexible graphite braided packing alone is sufficient. Pre-compressing the packing during the packing process is also one of the measures to ensure good sealing (especially long-term good sealing). Domestic valves, during the packing process, mostly only involve compacting the packing; as a result, their sealing performance deteriorates over time as they are used. Some foreign valve products use a pre-pressure of 28 MPa during the packing process, and specialized pre-compression tools are designed for this purpose; as a result, they can maintain an excellent seal for a long time. Of course, too high a pre-pressure on the packing will increase wear on the valve stem and raise the force required to open the valve; therefore, improvements should be made in the selection of the packing and in the surface treatment of the valve stem. The use of packing primarily composed of flexible graphite rings can reduce wear on the valve stem, as well as lower the opening torque of the valve. There are also some valves that use a \"live load\" design at the packing gland. In other words, disc springs are added to the bolts of the packing gland, so that a higher pressure is always exerted on the packing by the gland, thereby preventing leaks caused by loose gland bolts or loose packing. 4.2 Leakage due to gasket sealing: For gate valves, globe valves, and check valves with a DN ≥ 50 mm, pressure-sealing valve covers can be used. For ball valves, plug valves (mostly with lower valve covers), and gate valves, stop valves, and check valves with DN ≤ 40 mm, flanged valve covers are generally used. There are two sealing points in the pressure-sealed bonnet: one is at the contact between the bonnet and the ring gasket (Figure 1), and the other is at the contact between the ring gasket and the valve body. In some foreign valve products, the contact surface between the ring gasket and the valve cover is designed as a variable-angle arc transition surface, resulting in a line-type seal with the valve cover; this significantly improves its tightness. However, it also increases the processing difficulty of the ring gasket. Additionally, a layer of silver is plated on the surface of the ring gasket as well as on the corresponding sealing surface of the valve body; since silver has good plasticity and can easily fill the microscopic pores in the sealing surface, this **improves the sealing performance at the valve cover gasket area. For flanged valve cover structures, most manufacturers use wound gaskets, as they possess good elasticity and a high rebound rate, which makes it easier to ensure sealing while maintaining stability. The valve cover seal designed for wound gaskets adopts a box-type structure, in which the gasket is placed in a special groove at the junction of the valve cover and the valve body. This ensures the stability of the wound gasket (preventing it from becoming unstable due to high sealing pressures), while also increasing the resistance to fluid leakage. It turns out that such a sealing structure is better. 4.3 Internal leakage ① Gate valves: There is not much difference in the design of the gate structures among manufacturers both domestically and internationally. Generally, gate valves with a DN ≤ 40 mm use integral wedge-shaped gates, while those with a DN ≥ 50 mm employ flexible wedge-shaped gates. As long as the machining precision of the sealing surface meets the requirements, this structure can satisfy the initial sealing needs. But users want the valve to maintain good sealing for a long time. Generally, achieving initial sealing is easy, but maintaining good sealing over the long term is not so simple. Here are 2 improvements provided for children's reference. One approach is to strive to improve the precision of the gate guide rails, so as to maintain a small clearance between them and the gate, thereby reducing uneven wear between the gate and the valve seat and ensuring a longer sealing life. The guide rails are generally cast integrally with the valve body and are difficult to machine; therefore, the clearance between them and the gate is relatively large. It is recommended that manufacturers pay attention to this aspect. The second is to consider the corrosion at the bottom of the gate and valve seat. This is because when the valve is in a partially open position, a high-speed flow of the medium is generated, which causes severe erosion in the areas through which the medium flows. Moreover, since the adhesion between hydrogen sulfide and the corrosion products formed on metals is very low, these products are easily carried away by the high-speed flow, thereby accelerating metal corrosion – a phenomenon known as erosion corrosion. Measures to prevent erosion-corrosion can start with selecting suitable materials; that is, surfacing appropriate hard alloy materials on the relevant parts. In addition to hardfacing the sealing surfaces, some manufacturers also hardface a certain area beneath the gate and valve seat to prevent erosion-corrosion of non-sealing surfaces. Although this has little effect on the valve’s sealing performance, it is beneficial for the valve as a whole. ②For stop valves, it is essential to ensure the machining accuracy of the valve disc and seat in order to achieve good initial sealing. To prevent erosion corrosion, a foreign manufacturer has adopted a stepped valve disc design (Figure 2); during the opening or closing of the valve, the non-sealing steps on the valve disc create smaller flow channels that help to withstand erosion corrosion, while the sealing surfaces between the valve disc and the valve seat are not exposed to erosion corrosion directly, or the degree of such corrosion is reduced. Ball valves should use an eccentric or other friction-free ball design, along with a metal-to-metal sealing mechanism. This is because the structure offers high sealing reliability, good fire safety, no friction, and extends the effective lifespan of the valve. ④For plug valves, an inverted conical balanced valve core (i.e., plug) structure is generally used abroad (Figure 3). Generally, models with a CL rating of ≤ 300 use a mechanically balanced design, while those with a CL rating of ≥ 600 employ a pressure-balanced design; this approach ensures good sealing while also reducing wear on the metal surfaces of the sealing elements. A manufacturing plant abroad applied powder coating to the valves on this basis, thereby reducing the need for regular maintenance involving the application of sealants. 5 Valve strength design: The pressure-bearing components of valves must undergo strength analysis and design; these components mainly include the valve body, valve cover, gate, and valve cover bolts. For globe valves, sometimes (depending on the design) it is also necessary to reinforce the valve seat locally in terms of strength. The valve stem, as a non-pressure-bearing component, is also an important part that requires strength design. The so-called strength design should include two aspects: strength and stiffness. Some major international valve standards, such as ANSI B16.34 and API 600, specify the minimum wall thickness (diameter) for the key components of valves. Domestic manufacturers generally adopt these standard values (or slightly higher ones) as the design dimensions, without conducting additional strength calculations. Doing this is not strict, because ① the internal components of the valves vary, and the structural dimensions of the upper chambers of the valves differ, which results in different calculated wall thicknesses. Depending on the external structure of the valve, particularly the way in which sudden changes in shape are handled, the calculated stress values, especially the level of stress concentration, also vary, which can ultimately lead to different calculated wall thicknesses. ②Most valve bodies are castings, and in valve manufacturers where the smelting conditions are poor and the sources of raw materials are complex, there is a significant variation in the properties of the casting materials. The properties of casting materials are also related to their inherent casting defects (such as segregation, dendritic structure, inclusions, pores, porosity, and cracks), and these properties vary considerably, resulting in large differences in the basic data used for strength calculations (such as allowable stresses). ③In different application environments, the degree of corrosion varies, and thus the corrosion margin that needs to be taken into account also differs. Due to the influence of these factors, it is essential to perform strength and/or stiffness calculations on the relevant components of high-pressure hydrogen service valves, as even minor errors can lead to serious consequences. At present, most valve manufacturers in China use mathematical analysis methods to calculate the strength and/or stiffness of valves. This method is both laborious and time-consuming, and its computational accuracy is relatively low; especially at points where the shape of the components changes abruptly, it is unable to determine the stress levels with precision. Most valve manufacturers abroad use the finite element analysis method, which is both fast and accurate. For high-pressure and demanding valves, it is necessary to use the finite element method to analyze the strength and/or stiffness of the components. 6 Materials: The operating conditions in high-pressure hydrogenation units impose high requirements not only on the reliability of materials, but also on their properties due to the media involved (such as hydrogen and hydrogen sulfide); in other words, these media are quite sensitive to any existing defects in the materials. If the material contains discontinuous defects such as non-metallic inclusions, slag inclusions, pores, and cracks, it is prone to hydrogen accumulation. At room temperature, the local high pressure resulting from this accumulation can cause hydrogen-induced deformation, and even lead to the formation of microcracks; it also exacerbates the brittleness of the material (hydrogen embrittlement). At high temperatures, these defects facilitate hydrogen-induced internal decarburization, thereby accelerating the process of hydrogen corrosion and cracking of the material. On the other hand, hydrogen sulfide environments are sensitive to external discontinuities in the material; especially in wet hydrogen sulfide conditions, such external discontinuities often serve as triggers for stress corrosion cracking. Therefore, reducing or limiting defects in the pressure-bearing components of the valve is one of the key factors in ensuring its reliability and extending its service life. There are two methods for manufacturing the pressure-bearing components of valves: casting and forging. Forged parts are free from defects such as pores, porosity, large-sized circular inclusions, columnar structure, and dendritic structure; moreover, the metal is dense, offering good comprehensive mechanical properties and high reliability. Therefore, forging is an ideal method for manufacturing pressure-bearing components for high-pressure hydrogenation reactors. However, given that the shapes of most pressure-bearing components are relatively complex, and many of them exceed the dimensions typical for conventional die forging, most valve manufacturers around the world still use castings for the main pressure-bearing components of valves with a DN > 50 mm. To ensure the quality of castings, control should be exercised from three main aspects: smelting, casting processes, and welding repairs. The impact of smelting on material quality is the most fundamental factor. Different smelting methods result in significant variations in the quality of the materials produced. Currently, domestic valve manufacturers generally use electric furnaces for smelting, while most foreign valve manufacturers employ VOD or AOD smelting methods. Compared to electric arc furnace smelting, VOD/AOD results in less loss of beneficial alloying elements, allowing the material composition to be brought closer to an ideal state. It also features good degassing properties and fewer harmful impurity elements, thereby yielding materials of higher quality. The casting process is a key factor affecting the properties of the material; it involves aspects such as the selection of casting film materials, the use of wooden molds, control of casting temperature, and the choice of casting methods. In summary, casting processes that help improve the quality of castings, such as precision casting, die casting, and vacuum casting, should be adopted as the future development direction for valve manufacturers. Welding repair is a remedial measure for dealing with casting defects. Most castings require welding repairs; if the defects exceed acceptable limits, they are discarded, which increases the production costs of valves. However, the number of welds, the welding area, and the number of repair welds for each valve should be limited, as the metal in the welded areas is different from the cast metal. The more welds there are and the larger the welding area, the greater the unevenness in the cast metal, which in turn leads to a decline in the overall performance of the material. Each welding repair is equivalent to heating the casting once, and heating the casting multiple times has a series of adverse effects on it; therefore, the number of welding repairs on valves should also be limited. The ASTM standards set certain requirements for welding repairs on cast materials, but these requirements are relatively low; the control standards for welding repairs of castings applied by most valve manufacturers abroad are stricter than those specified by ASTM. In fact, the control of welding repairs on castings also reflects the balance between the quality of the casting material and production costs; therefore, it is crucial to improve the casting quality and minimize casting defects as much as possible. 7 Inspection and Testing: The strength (hydrostatic) test, sealing test, and necessary non-destructive testing of valves are essential conditions for determining whether a valve is qualified; however, they are not sufficient conditions to assess the quality of a valve. At least for high-pressure hydrogenation valves, these tests do not fully reflect the overall quality of such valves. Take the sealing of valves as an example. Under normal circumstances, as long as the machining accuracy of the seals is ensured, it is easy to pass the factory seal test at room temperature. However, this does not guarantee that the valve will maintain good sealing performance over long-term use—especially under high-temperature and high-pressure operating conditions. Once problems such as wear, corrosion, stress relaxation, deformation, and material degradation occur in the valve components. It will affect the sealing performance of the valve, and in severe cases, it can cause the seal to fail. The stability and uniformity of the metal microstructure of components, as well as the local stress levels caused by their structure, all affect to some extent issues such as material corrosion, stress relaxation, deformation, and transformation. These issues with the material cannot be detected by non-destructive testing or pressure tests at room temperature. For medium- and low-pressure valves used in general media, the aforementioned problems may not exist or may not be significant. However, for valves operating under high-pressure hydrogen service, these problems cannot be ignored. Therefore, for valves used in high-pressure hydrogen service. Comprehensive inspection and testing should be carried out on it. Since the material quality of pressure-bearing components of valves (especially casting materials) has a significant impact on their service life. Therefore, further discussion is held on the inspection and testing of pressure-bearing castings. The quality of casting materials depends primarily on their production processes (such as smelting and manufacturing techniques), while various inspection and testing procedures carried out on the products are merely means to assess their quality; they do not change the quality of the materials themselves. Therefore, it is particularly important to conduct systematic and scientific process evaluations of valve components before starting industrial production. Furthermore, the quality positioning in process evaluation directly determines the quality of the material (product). Considering the characteristics of the media in high-pressure hydrogenation units, the process evaluation items that should be carried out on their main pressure-bearing components prior to industrial production are proposed here. ①Visual inspection shall be carried out item by item in accordance with the items listed in MSSSP-55. ②Chemical composition analysis includes analysis prior to smelting and product analysis; special attention should be paid to the analysis of harmful impurity elements (such as sulfur, phosphorus, arsenic, antimony, hydrogen, ammonia, etc.). Chemical composition analysis shall be conducted in accordance with the corresponding ASTM standards. ③Mechanical property tests include tensile testing (which simultaneously determines tensile strength, yield strength, elongation, and reduction of area, as well as performs fracture analysis), bending testing, impact testing, and hardness testing. Mechanical property tests shall be conducted in accordance with the corresponding ASTM standards. ④Macroscopic structure inspection includes the detection of defects such as pores, cracks, porosity, slag inclusions, large-sized non-metallic inclusions, and sulfur/phosphorus segregation. Testing is carried out in accordance with standards such as ASTM E381. ⑤Microstructural examination includes the inspection of defects such as segregation, strip (flake) structure, dendritic structure, grain size, and small-sized non-metallic inclusions. For austenitic materials, the determination of ferrite content should also be included. Inspections are carried out in accordance with standards such as ASTM E381/E45/E112. ⑥Non-destructive testing includes inspections such as RT, UT, MT, PT, etc. And appropriate compliance criteria and pass rate control indicators are provided. Non-destructive testing should be carried out in accordance with the standards MSSSP—54, ASTMA388, ASTMAA275, and ASTMB165 (Practice B), respectively. ⑦Crystal corrosion test is required only for austenitic stainless steel materials. Try to follow the ASTM E262 (Practice E) standard. ⑧Welding performance tests include tensile, bending, impact, hardness, and non-destructive testing. ⑨Valve body burst test: The liquid burst test is carried out in accordance with ANSTB31.2 standard. Only the process parameters determined through process evaluation, under conditions that meet certain control criteria, can be used as production parameters and strictly implemented in industrial production to obtain high-quality valves suitable for high-pressure hydrogen service. 8 Quality Control: Quality issues with domestically produced valves are more often related to management problems. This is because many manufacturers in China do not have specific, detailed, and practical operating procedures for each step of the product manufacturing process; and even if such procedures exist, it is not guaranteed that every worker will follow them strictly. It is for this very reason that the quality of many domestic products is inferior to that of foreign products. At present, most valve manufacturers in China have obtained ISO9000 quality certification, which enhances the quality management of the valve production process to a certain extent. However, attention should also be paid to the creation of technical documents, as the advancement of a product depends primarily on the sophistication of its technical documents. 9 Conclusion There are many factors that affect valve quality; to produce high-quality valves, it is necessary to ensure excellence in all aspects. In the information age, valve manufacturers should also go out and engage in more exchanges with the outside world, as well as with users (including engineering design departments). It is necessary to understand the product requirements as well as the operating conditions of the medium used in the valves; only by doing so can suitable products be manufactured and the market secured. Communicate with foreign valve manufacturers. We can learn about advanced technologies and new products, and use them to improve our products.