1 Overview
The high-pressure hydrogenation process is an important means of deep processing of petroleum. It can not only improve the recovery rate of light oil per unit of crude oil. And it can improve the quality of fuel oil, thereby improving the overall efficiency of the refinery. The high-pressure hydrogenation device can also provide high-quality raw materials for petrochemical devices and is also an ideal device for oil desulfurization. China began the construction of high-pressure hydrogenation units in the 1990s. It is expected that in the next few years, as the amount of foreign oil refining in our country increases year by year, the construction of high-pressure hydrogenation units will continue to be a hot spot in the petrochemical industry. Major petrochemical companies Companies also want to take this opportunity to improve their oil refining levels and capabilities to meet the challenges of joining the WTO.
2. Working conditions
The medium environment of the high-pressure hydrogenation unit has two outstanding characteristics, namely high pressure and operation near hydrogen (and accompanied by hydrogen sulfide). High-pressure operation is not only because of its high operating pressure (generally 14-20 MPa), but also because its medium is flammable and explosive high-pressure gas (hydrogen or oil + hydrogen). High-pressure gas stores greater pressure energy. Once its storage and transportation equipment (including pipeline valves) is damaged, the accident will be catastrophic. Operating with hydrogen and hydrogen sulfide. It shows the strict requirements for materials of storage and transportation equipment. Hydrogen is a medium that can penetrate into the interior of metal materials and cause material degeneration (deterioration) at room temperature or high temperature. It can cause embrittlement and deformation of metal materials at room temperature, and it can cause internal and external decarburization of metal materials at high temperatures. The corrosion of metal materials by hydrogen sulfide is also a very thorny problem. It can cause stress corrosion cracking of many metal materials at room temperature, and it can cause rapid and uniform corrosion of metal materials at high temperatures. All these characteristics place strict requirements on the materials, structural design and strength design of high-pressure hydrogenation valves.
3. Demand situation
The valves in the high-pressure hydrogenation unit have both hydrogen-facing and non-hydrogen-facing conditions, and high-pressure and non-hydrogen-facing conditions. This article only discusses valves used under high-pressure hydrogen conditions. Although the number of such valves is small, their prices account for a large proportion. Such valves include gate valves, globe valves, check valves, ball valves and plug valves. The pressure grade is ASME CL900~2500, the temperature is normal temperature to 400℃, and the main materials are ASTMA105, A182-F11/F22/F321, A216- WCB, A217-WC6/WC9, A351-CF8C, valve diameter is DN15-400 mm. The functions of these valves are the same as ordinary valves, but China currently does not have the ability to supply complete sets. The main reason is that the valve manufacturer has insufficient communication with users (especially the engineering design department), does not understand the use conditions and requirements of the valve, and lacks a There is no set of technical documents, molds and drawings that are suitable for the medium conditions, and there is a lack of experience in industrial mass production.
Developing valves in high-pressure hydrogenation units can not only reduce valve prices. It can also reduce the supply cycle, facilitate replenishment of orders, and at the same time promote the development of the valve industry. In fact, some valve manufacturers in my country currently have the ability and conditions to produce high-pressure hydrogenation valves, and they have been successfully used in hydrofining devices (operating pressure 8~10 MPa). A valve with good performance is better in all aspects and links such as design, manufacturing, inspection and testing, and quality control. This article discusses several of the main issues (including internal leakage, external gurgling, strength design, materials, inspection tests and quality control of the valve) combined with the medium conditions of the high-pressure radon adding device.
4. Control of internal leakage and external leakage
The quality of valve internal and external leakage control is an important parameter that reflects the quality of the valve. Manufacturers take various measures to minimize internal and external leakage of valves. External and internal leakage of oil and hydrogen will not only pollute the environment, but also easily cause fire or explosion. The internal leakage of the valve mainly occurs at the gate plate, and the external leakage mainly occurs at the valve stem packing and valve cover gasket. Therefore, to obtain good sealing performance, we must start from these three places.
4.1 Packing seal leakage
For gate valves and globe valves with DN ≥ 50mm, it is a common practice for foreign remanufacturers to use flexible graphite rings and flexible graphite braided packing at the valve stem seal. The upper and lower rings of the packing are made of flexible graphite braided rings to prevent graphite from being squeezed into the valve cavity or outside by taking advantage of its higher strength (compared to molded graphite rings). A molded graphite ring is used in the middle (sometimes Also add a braided loop at an appropriate position in the middle). Because flexible graphite has good fluidity and anti-friction properties, and graphite also has good physical and chemical stability, using it as a filler can not only achieve good sealing, but also reduce wear on the valve stem. For ball valves, plug valves, small-diameter gate valves and globe valves with smaller stuffing boxes, only flexible graphite braided packing can be used. Pre-compression of the filler during the filling process is also one of the measures to ensure good sealing (especially long-term good sealing). Most domestic valves are limited to compacting the packing during the packing filling process, so their sealing performance deteriorates as the application time increases. Some foreign Yanmen products use a pre-pressure of 28 MPa during the filling process and design special pre-compression tools, so they can maintain good sealing for a long time. Of course, if the pre-pressure of the packing is too large, it will increase the wear on the valve stem and increase the opening force of the valve. Therefore, improvements should be made in packing selection and valve stem surface treatment at this time. The use of packing mainly composed of flexible graphite rings can reduce the wear of the valve stem and also reduce the opening torque of the valve. There are also some valves that use a “live load” structure at the packing gland. That is, a disc spring is added to the bolts of the packing gland so that the packing gland always exerts greater pressure on the packing, thus preventing leakage caused by loosening of the gland bolts or loose packing.
4.2 Gasket seal leakage
For gate valves, globe valves and check valves with DN ≥ 50mm, pressure sealing valve covers can be used. For ball valves, plug valves (mostly lower bonnets) and gate widths with DN ≤ 40mm, stop valves and check valves, flange-connected bonnets are generally used. There are two seals in the pressure sealing wide cover, one is the contact point between the threshold cover and the ring gasket (Figure 1), and the other is the contact point between the ring gasket and the valve body. Some foreign valve products design the contact surface between the ring gasket and the valve cover as a variable-angle arc transition surface, so that the seal between it and the valve cover is a line seal, thereby significantly improving its tightness, but it increases the environmental impact. The processing difficulty of the gasket. The surface of the ring gasket and the corresponding sealing surface of the valve body are also plated with a layer of silver. Because silver has good plasticity and can easily fill the microscopic pores on the sealing surface, the sealing performance of the valve cover gasket is greatly improved. For the flange connection valve cover structure, most manufacturers use spiral wound gaskets because spiral wound gaskets have good elasticity and high rebound rate, making it easier to ensure sealing while ensuring stability. The valve cover seal matching the wound gasket adopts a box structure, that is, the gasket is placed in a special groove connecting the valve cover and the valve body, thereby ensuring the stability of the spiral wound gasket (that is, it will not be due to the larger The sealing specific pressure causes the gasket to become unstable) and at the same time increases the resistance to medium leakage. Facts have proved that this sealing structure is better.
4.3 Internal leakage
① Gate valve
There is not much difference in the gate structure design of domestic and foreign manufacturers. Generally, gate valves with DN ≤ 40mm use wedge-shaped integral gates, while gate valves with DN ≥ 50mm use flexible wedge-shaped gates. As long as the processing accuracy of the sealing surface meets the requirements, this structure can meet the initial sealing requirements. But users hope that the valve can maintain a good seal for a long time. Under normal circumstances, initial sealing is easy to achieve, but it is not easy to achieve good long-term sealing. Here are 2 improvements for reference. One is to strive to improve the accuracy of the gate guide rail so that it maintains a small matching gap with the gate plate to reduce uneven wear of the gate plate and valve seat, thereby ensuring a longer sealing life. The guide rail is generally cast integrally with the valve body and is not easy to machine. Therefore, the matching gap with the gate plate is large. It is recommended that the manufacturer pay attention to this aspect. The second is to consider the corrosion of the gate plate and the lower part of the valve seat. Because when the valve is in the half-open and half-closed position, a high-speed medium flow will be formed, which will cause serious erosion of the area where the medium flows. Moreover, due to the low adhesion between hydrogen sulfide and metal corrosion products, it is easily blocked by the high-speed medium flow. The medium is washed away, thereby accelerating the corrosion of the metal, that is, forming erosion corrosion. Measures to prevent erosion corrosion can start with selecting good materials, that is, surfacing appropriate carbide materials on the corresponding parts. In addition to overlaying hard alloy on the sealing surface, some manufacturers also overlay welding hard alloy on the lower part of the gate and valve seat within a certain range to prevent erosion and corrosion of the non-sealing surface. Tightness doesn’t matter much, but it’s good for the valve as a whole.
② Stop valve
First, ensure the machining accuracy of the valve disc and valve seat to obtain a good initial seal. In terms of anti-erosion corrosion, a foreign manufacturer adopts a stepped valve disc structure (Figure 2). That is, during the process of opening or closing the valve, the non-sealing steps on the valve disc first form a smaller flow channel to withstand erosion. Corrosion occurs, and the sealing surfaces of the valve disc and valve seat are no longer directly exposed to erosion corrosion, or the degree of erosion corrosion is alleviated.
③Ball valve
Eccentric or other frictionless spherical structures and metal-to-metal sealing forms should be used. This is because the structure has high sealing reliability, good fire safety, and no friction, which extends the effective life of the valve.
④Plug valve
In foreign countries, the inverted cone balanced valve core (ie cock) structure is generally used (Figure 3). Generally, the CL ≤ 300 grade adopts a mechanical balanced structure, and the CL ≥ 600 grade adopts a pressure balanced structure, which can not only ensure good sealing, but also reduce the wear of the metal on the sealing surface. On this basis, a foreign manufacturer sprayed the cock with plastic, thus saving the maintenance workload of regular injection of sealant.
5. Valve strength design
The pressure-bearing parts of the valve should undergo strength analysis and strength design. These parts mainly include the valve body, valve cover, gate plate and valve cover bolts. For stop valves, sometimes (depending on the structure) it is necessary to perform strength verification on the valve seat part. As a non-pressure-bearing component, the valve stem is also an important component that must be designed for strength. The so-called strength design should include two parts: strength and stiffness.
Some major foreign valve specifications such as ANSIB16.34 and API600 provide the minimum wall thickness (diameter) of the main parts of the valve. Some domestic manufacturers directly use (or slightly exceed) these specification values ​​as design dimensions, and do not Then carry out strength design. This is not strict because ① the internal parts of the valve have different structures and the structural dimensions of the upper cavity of the valve are different, so the calculated wall thickness is also different. The appearance structure of the valve, especially the processing of sudden changes in the shape, is also different, and its calculated stress values, especially the level of stress concentration, are also different, which may ultimately lead to different calculated wall thicknesses. ②Most valve bodies are castings, and valve factories with poor smelting conditions and complex sources of raw materials have large differences in the performance of their casting materials. The properties of casting materials are related to their own casting defects (such as segregation, dendrite structure, inclusions, pores, porosity and cracks, etc.), and fluctuate greatly, resulting in relatively large differences in the basic data for strength calculation (such as application). big. ③ Different application environments have different corrosion conditions and different corrosion margins that should be considered. Due to the influence of these factors, it is very necessary to calculate the strength and/or stiffness of the relevant parts of the high-pressure hydrogen valve, because a slight error may have serious consequences. At present, most domestic valve factories use mathematical analysis methods to calculate the strength and/or stiffness of valves. This method is laborious and time-consuming, and the calculation accuracy is relatively poor. Especially for parts with sudden changes in shape, it cannot accurately determine the stress level. Most foreign valve factories have adopted the finite element analysis method, which is both fast and accurate. For high-pressure and demanding valves, strength and/or stiffness analysis of components using the finite element method is necessary.
6. Materials
The operating conditions of the high-pressure hydrogenation device not only have high requirements on the reliability of the materials, but also the medium (such as hydrogen and hydrogen sulfide) itself has high requirements on the material properties, that is, the medium is sensitive to defects in the material itself. If there are non-metallic inclusions, slag inclusions, pores, cracks and other discontinuous defects in the material, it will easily lead to the accumulation of hydrogen. The local high pressure formed at room temperature will cause hydrogen deformation and even induce micro-cracks, and will also make the material brittle. Deterioration (hydrogen embrittlement). At high temperatures, these defects are more conducive to hydrogen-induced internal decarburization, thereby accelerating the process of hydrogen corrosion cracking of the material. Hydrogen sulfide medium is more sensitive to external discontinuous defects in materials. Especially in wet hydrogen sulfide environments, external discontinuous defects often become the cause of stress corrosion cracking. Therefore, reducing or limiting defects in the pressure-bearing parts of valves is one of the key factors to ensure their reliability and extend their service life.
​There are two manufacturing methods for valve pressure-bearing parts: casting and forging. Forged parts do not have defects such as pores, porosity, large-sized circular inclusions, columnar structures, and dendritic structures. The metal is dense, has good overall mechanical properties, and has high reliability. Therefore, forging is an ideal method for manufacturing high-pressure hydrogenated Amen pressure-bearing parts. . However, considering that the shapes of most pressure-bearing parts are relatively complex, and many of them exceed the size of general die forging, most valve factories at home and abroad still use castings for the main pressure-bearing parts of DN>50 mm valves. In order to ensure the quality of castings, control should be carried out from three main aspects: smelting, casting process and welding repair. The impact of smelting on material quality is the most basic influencing factor. Different smelting methods have relatively large differences in the quality of the materials obtained. At present, domestic valve factories generally use electric furnace smelting, while most foreign valve factories use VOD or AOD smelting methods. Compared with electric furnace smelting, VOD/AOD has less burning loss of beneficial alloy elements, the material composition is more likely to be close to the ideal state, and it has good degassing properties and fewer harmful impurity elements, so the quality of the material obtained is relatively high. The casting process is a key factor affecting material performance. It involves the selection of casting film materials, the involvement of wooden mold external models, casting temperature control and casting method selection. In short, casting processes that are conducive to improving the quality of castings, such as precision casting, pressure casting and vacuum casting, should be the future development direction of valve manufacturers. Welding is a remedial measure to deal with casting defects. Most castings require welding repairs. If the defects exceed the standard, they will be scrapped, which will increase the production cost of the valve. However, the number of welds, the welding area and the number of welding times for each valve should be limited, because the metal in the welding area is different from the cast metal. The greater the number of weldings, the larger the welding area, resulting in uneven cast metal. The more serious the problem, the lower the overall performance of the material will be. Each welding repair is equivalent to heating the casting once, and heating the casting multiple times will bring a series of adverse effects to it, so the number of welding repairs for the valve should also be limited. The ASTM specification puts forward certain requirements for welding repair of casting materials, but the requirements are on the low side. The control level of casting welding repair of most foreign valve manufacturers is stricter than that specified by ASTM. In fact, the control of casting welding repair also reflects the balance between the quality of casting materials and production costs. Therefore, the key is to improve the casting quality of castings and minimize casting defects.
7. Inspection and testing
The strength (hydraulic pressure) test, sealing test and necessary non-destructive testing of the valve are necessary conditions to measure whether the valve is qualified, but they are not sufficient conditions to measure the quality of a valve. At least for high-pressure hydrogenation valves, they are not yet complete. Reflects the overall quality of a valve. Take the sealing of valves as an example. Under normal circumstances, as long as the machining accuracy of the seal is ensured, it will easily pass the factory sealing test in the cold state, but it does not mean that the valve will be used for a long time (especially under high temperature and high pressure working conditions). ) conditions can maintain good sealing. Once valve components have problems such as wear, corrosion, stress relaxation, deformation and material degeneration, etc. It will affect the sealing performance of the valve, and in severe cases, the seal will fail. The stability and uniformity of the metal structure of the component and the local stress level caused by the component structure will all affect the corrosion, stress relaxation, deformation and degeneration of the material to some extent. These problems of materials cannot be checked by non-destructive testing and pressure testing in cold state. For medium and low-pressure valves used in general media, the above problems may not exist or are not prominent. However, for valves used under high-pressure hydrogen conditions, these problems cannot be ignored. Therefore, valves are used for high-pressure hydrogen. A comprehensive inspection and test should be carried out on it. The material quality of valve pressure-bearing parts (especially casting materials) has a great impact on the service life of the valve. Therefore, the inspection and testing of pressure-bearing castings are further discussed.
The quality of casting materials mainly depends on its production process (such as smelting process, manufacturing process, etc.), and various inspection tests on its products are only a means of quality assessment, and it cannot change the quality of the material. Therefore, it is particularly important to conduct systematic and scientific process evaluation of valve parts before starting industrial production. The quality positioning in process assessment directly determines the quality of materials (products). Based on the medium characteristics of the high-pressure hydrogenation unit, here we propose the process evaluation items that should be carried out before industrial production of its main pressure-bearing components.
①Appearance inspection The items listed in MSSSP-55 should be inspected item by item.
②Chemical composition analysis includes furnace front analysis and product analysis. Particular attention should be paid to the analysis of harmful impurity elements (such as sulfur, phosphorus, arsenic, antimony, hydrogen and ammonia, etc.). Chemical composition analysis should be performed in accordance with the appropriate ASTM standards.
③ Mechanical performance tests include tensile tests (simultaneously testing tensile strength, yield strength, elongation and section shrinkage, and performing fracture analysis), bending tests, impact tests and hardness tests, etc. Mechanical property testing should be performed in accordance with the appropriate ASTM standards.
④ Macrostructure inspection includes inspection of defects such as pores, cracks, looseness, central slag, large-sized non-metallic inclusions, sulfur and phosphorus segregation. Inspection is carried out according to standards such as ASTME381.
⑤Microstructure inspection includes inspection of defects such as segregation, strip (gui) structure, dendrite structure, grain size, small-sized non-metallic inclusions, etc. For austenitic materials, the determination of ferrite content should also be included. Inspection is carried out according to ASTME38l/E45/E112 and other standards.
⑥Non-destructive inspection includes RT, UT, MT, PT and other inspections. And give appropriate qualification indicators and qualification rate control indicators. Non-destructive inspection should be carried out in accordance with MSSSP-54, ASTM A388, ASTM A275, ASTM B165 (Practice B) standards respectively.
⑦Intergranular corrosion test This requirement is only applicable to austenitic stainless steel materials. Try to perform according to ASTME262 (Practice E) standard.
⑧Welding performance testing includes tensile, bending, impact, hardness and non-destructive testing.
⑨Valve body burst test The liquid burst test is conducted according to ANSTB31.2 standard. Only when certain control indicators are met, the process parameters determined through process evaluation can be used as production parameters, and they must be strictly implemented in industrial production to obtain high-quality valves that meet the high-pressure hydrogen application conditions.
8 Quality Control
The quality problems of domestic valves mostly arise in management. Because many manufacturers in China do not have a specific, detailed and practical operating procedure for each process of product manufacturing. Even if there is such a procedure, it cannot guarantee that every staff member will perform the work to the letter. Acceptance procedures. Because of this, the quality of many domestic products is inferior to foreign products. At present, most domestic valve factories have obtained ISO9000 quality certification, which to some extent strengthens the quality management of the valve production process. However, we must also pay attention to the establishment of technical documents, because the advancement of products mainly depends on the advanced nature of technical documents.
9 Conclusion
There are many factors that affect the quality of valves. To produce high-quality valves, all aspects must be done well. In the information age, valve manufacturers should also go out and communicate more with the outside world and with users (including engineering design departments). Understand the product needs and the working medium conditions of the valve. Only in this way can we produce suitable products and win the market. Communicate with foreign valve manufacturers. Can learn about advanced technologies and new products and use them to improve our products.