Application status and key design requirements of cryogenic valve in LNG field
What is liquefied natural gas?
Liquefied natural gas or LNG, is mainly composed of methane and is recognized as the cleanest fossil energy on earth. It is colorless, tasteless, nontoxic and noncorrosive. Its volume is about 1/625 of that of the same amount of gas, and the quality of liquefied natural gas is only about 45% of that of the same amount of water.
The manufacturing process is to purify the natural gas produced in the gas field and transport it by LNG ship after a series of ultra-low temperature liquefaction. Liquefied natural gas (LNG) is a kind of advanced energy because of its small air pollution and large heat emission.
Liquefied natural gas is a kind of natural gas which is compressed and cooled to its boiling point (- 161.5 ℃) and then becomes liquid. Generally, liquefied natural gas is stored in a low temperature storage tank of – 161.5 ℃ and 0.1MPa. Its main component is methane, which is transported by special ship or tank car and gasified again when in use. Since the 1970s, the world’s LNG production and trade volume have increased rapidly. In 2005, the international LNG trade volume reached 188.81 billion cubic meters, with Indonesia as the largest exporter, 31.46 billion cubic meters for export, and Japan as the largest importer, 76.32 billion cubic meters.
At atmospheric pressure, the density of LNG is about 430-470kg / (slightly different due to different components), the ignition point is about 650 ℃, the calorific value is 52mmbtu (1mmbtu = 2.52 × 108cal), and the explosion limit (volume) in air is 5% – 15%. Liquefied natural gas storage is one of the natural gas storage methods. The LNG storage tank is usually a double-layer metal tank. The inner material of the LNG contact is 9%Ni low temperature steel, the outer material is carbon steel, the middle insulation layer is expanded perlite, and the insulation layer of the tank bottom is foam glass.
In the rapid development of LNG industry, the demand for low temperature valve is increasing day by day. Because LNG has the obvious characteristics of inflammable, explosive and ultra-low temperature, and LNG plant is usually located in the sea salt fog environment, so higher requirements are put forward for the selection and design of low temperature valve. At present, the cryogenic valves for LNG in most domestic stations still need to be imported, and some cryogenic valves for severe working conditions are still monopolized by foreign countries, such as J-T valve, breathing valve of large LNG storage tank, large caliber and high-weight valve, etc. Although the domestic material design meets the requirements of low temperature, the pressure bearing and sealing performance of some parts still need to be improved.
Type of cryogenic valve for LNG
In the small-scale natural gas liquefaction plant, the low-temperature valves are mainly concentrated in the liquefaction unit and LNG storage unit. The pressure level is not high and the inner diameter is small. The more severe working condition is that the mixed refrigeration cycle process also uses the low-temperature mixed refrigerant throttle valve and the LNG throttle valve, which is a multi-stage pressure reducing control valve. According to rough statistics, the number of low temperature valves used in general liquefaction plant accounts for about 30% of the total number of valves in the whole plant. In the LNG receiving station (mainly refers to the large-scale station with a receiving capacity of more than 2 million tons / year), the number of low-temperature valves is about 2000, including about 700 small-size valves, which account for more than 90% of all valves. Some valves have high pressure rating and large caliber.
The low and ultra-low temperature operation valves in LNG field mainly include single seat through valve (including angle valve), ball valve, butterfly valve, stop valve, check valve and safety valve.
Key points of selection of valves for LNG severe working conditions
J-T valve is one of the worst control valves in LNG liquefaction plant. At present, the most widely used liquefaction process is the mixed refrigerant circulation. The J-T valve and LNG J-T valve are respectively used to throttle and depressurize the mixed refrigerant and natural gas. The two J-T valves used in the mixed refrigeration cycle are in the ultra-low temperature environment below – 150 ℃. Under the condition of normal opening of the valve, the pressure on both ends of the J-T valve is usually between 30 and 80 bar. Because J-T valve is in the environment of high pressure difference and ultra-low temperature, it is easy to produce gas corrosion, and the surface of valve core, valve seat and valve body is easy to be damaged. If the valve core is damaged and not closed tightly, the valve will leak and frost at the fully closed position. Moreover, the technical requirements for the performance of J-T valve are high. In order to achieve the refrigeration effect, it is necessary to ensure that the pressure difference at both ends of the valve and the outlet temperature are stable during the use process, and the J-T valve needs to be precisely adjusted when the device changes load. Therefore, in the selection of J-T valve, in addition to the general rules of low-temperature valve design, the special requirements for the selection and design of valve internals under high-pressure differential conditions must be considered.
Special requirements for selection of J-T valve
Comparison and selection of structure and performance of valve internals
The pressure difference ratio of J-T valve and LNG J-T valve for general refrigerant is more than 0.9, and multi-stage pressure reduction method is needed. At present, the basic idea of valve trim design is zigzag channel, multi-stage decompression, expanding flow area and porous design. The valve internals with this structure can make the pressure of high-speed fluid at each point when passing through the valve core and valve seat higher than the saturated steam pressure at this temperature. There are mainly two types of structure, porous sleeve type regulating valve and labyrinth type regulating valve.
The common feature of products from famous foreign suppliers is to use multi-stage sleeve to realize step-by-step pressure reduction. The difference lies in the selection of labyrinth and porous sleeve structures. CCI, Masoneilan, copes Vulcan, severnglocon and other major producers of labyrinth valves, Fisher and flowsever (valtek) mainly produce many
Bore sleeve valve. Labyrinth control valve is better than porous sleeve valve. When selecting the series of sleeve valves, labyrinth valve should be selected if the flow rate of porous sleeve valve does not meet the requirements. If labyrinth valve core is adopted, the service life of J-T valve shall be at least two years, 5-10 times longer than that of sleeve valve. The sleeve drilling valve is suitable for the multi-stage pressure reducing condition with small size and the labyrinth valve with large size. Labyrinth valve has a wide range, but its price is higher than porous sleeve valve.
Control requirements for outlet flow rate
The high pressure difference produces cavitation, and the high velocity of the medium also directly affects the service life of the valve body and the valve internals. The multi-channel design method is needed to carry out step-by-step pressure reduction, so that the pressure velocity curve of the fluid in the regulating valve becomes smooth transition. Therefore, when selecting the flow structure of the valve internals, we should focus on controlling the flow velocity of the medium, and try to make the high-speed fluid impact, resulting in the consumption of kinetic energy, so as to reduce the abrasion of the valve caused by cavitation and vibration. It shall meet the requirements of speed and dynamic pressure energy of medium outflow valve internals given in ISA standard.
Anti blocking design
In the liquefaction plant, the main causes of blockage are water vapor and CO2 in the medium leading to ice blockage; unclean residual welding slag, copper scraps and other impurities caused by pipeline purging. Therefore, it is necessary to consider the anti blocking function of multistage pressure reducing sleeve design. The smooth and stable flow path shall be selected as far as possible. If necessary, the valve seat diameter shall be reduced to improve the throttling speed to improve the “self-cleaning” performance, and the actuator with enough rigidity and thrust (torque) can also be configured.
Key technology of LNG cryogenic valve selection
General low temperature technical requirements
Low temperature or ultra-low temperature valves involved in LNG field shall meet the following basic requirements:
- ① The valve cover of low temperature valve shall be extended, and the valve cover shall be of integral bolt connection structure;
- ② The extended stem shall be equipped to ensure that the packing is at the ambient temperature, and all stems are blow out proof structures.
- ③ The valve trim shall be easy to disassemble, especially the valve seat can be maintained online and replaced easily, the trim and seal can be disassembled from the top of the valve quickly and can be maintained online.
- ④ Valves with soft seal structure shall be of fire-proof design.
- ⑤ All ball valves shall be equipped with pressure relief devices.
- ⑥ Ball valve and globe valve with external sealing structure shall have external sealing performance.
- ⑦ The valve body, internals, bonnet, stem, actuator, etc. shall be designed with anti-static structure.
Precautions for selection of low temperature valve
Material selection of pressure parts
Material quality is an important index to measure the strength reliability and service life of valves. At present, austenitic stainless steel is recommended for low temperature valve for LNG. The common brands of valve body are A351 GR.CF3M, A182 GR.F316L and A182 GR.F304L. The common brands of valve core are A351 GR.CF8M, A182 GR.F316 and A182 GR.F304.
Type of sealing surface and selection of its material
The cryogenic valve for LNG is usually of hard sealing structure. The material of valve core (or valve plate) shall not be less than 316 stainless steel, and hard stainless steel or stellite alloy shall be used in the case of high pressure difference or cavitation or flash, and STL alloy shall be overlaid on the valve core (or valve plate) and valve seat. Metal seal + embedded soft seal are recommended for the valve seat seal of LNG cryogenic ball valve, and ptfce can be used as the soft seal material. The structure of “lip seal + graphite” is generally recommended for stem packing.
Reasonable determination of extended length of valve stem
For LNG ultra-low temperature medium, the extension of valve cover is the key design index. 80% of the reasons for the leakage of LNG project valves on site are due to the manufacturer’s failure to allocate sufficient extension valve covers to save costs. Bs6364 and shell MESC SPE77 / 200 have different requirements for this length. Shell has strict requirements on extension degree, but it is lack of site basis to ensure that the packing does not freeze only by increasing the extension amount. It is recommended to select the extension amount based on the requirements of non cold box valve in bs6364, and comprehensively consider the material selection, local environmental dew point value, project experience of various manufacturers and other factors, through thermal analysis and actual test conclusion.
Design consideration of valve piping accessories
In order to prevent damage to the soft sealing structure of the valve, both ends of the hardfacing ball valve shall be equipped with short welded pipes. Ball valve with short pipe can be welded online. For valves with DN greater than or equal to 80mm, the length of the short pipe shall not be less than 150 mm. It is recommended that the material of the short pipe be consistent with that of the pipe. In addition, all valves with extended bonnet shall be provided with drip tray for cold insulation to ensure that the stuffing box is at ambient temperature. The drip pan shall be fully welded with the valve cover, and the material shall be consistent with the valve body.
Core test items of low temperature valve
Low temperature valve materials shall be inspected, especially for domestic products. Generally, the material standard is relatively low. Relevant test items shall be well done, mainly including radiographic test, liquid penetrant and other non-destructive test for pressure parts and welding joints. The parts must be treated with cryogenic treatment at low temperature in the process of processing, so that the phase transformation of materials is sufficient. On the premise that the structural design and material inspection meet the requirements, the valve shell hydraulic test, low-pressure air tightness test, upper seal leakage test (upper seal structure) and low-temperature leakage test are the test items that must be completed. Before mass production, the products need to be tested in five categories: low temperature impact test, normal temperature performance test, low temperature performance test, non-destructive testing and low temperature life test.
Application suggestion of domestic low temperature valve
At present, domestic LNG industry has applied domestic low-temperature valves in varying degrees. Both PetroChina and Sinopec have set up localization working groups to carry out joint research and development work with Suzhou newway, Dalian Dagao, CNR and other manufacturers designated by the Energy Bureau. CNOOC also uses domestic ball valves in low temperature ball valves below 2 “. In the next step, it is suggested to track and promote the localization of low temperature valves from the following aspects.
- 1) A complete and strict valve design and application assessment procedure shall be followed for localization implementation. Starting from the technical requirements, a set of localization implementation procedures covering product design, design calculation, raw material procurement, qualified after raw material inspection and NDT, qualified after machining and inspection, qualified after heat treatment and inspection, qualified after re machining and inspection, qualified after cleaning and inspection, qualified after assembly, qualified after inspection and test, expert identification, field test, etc. is proposed, and each of them is equipped with Evaluation criteria for the step.
- 2) The localization risk of the first application product shall be fully considered. For ball valves above 3 “, large-diameter butterfly valves, check valves, stop valves and other domestic products, in view of the existing performance is less, it is recommended to focus on the application occasions for the first time, to avoid too scattered products, once leakage occurs, it will be easy to monitor and find. Therefore, it is suggested that the liquefaction units of small-scale liquefaction plants and micro mobile liquefaction units should be selected for application and promotion.
- 3) It is necessary to establish the mechanism of user field investigation and product post evaluation. At present, the domestic LNG industry has the most application achievements in small-size domestic low-temperature ball valves. The use of a large number of application valves should be examined through the maintenance rate during the overhaul period of the project, especially for the application occasions of bulk procurement rather than test nature. For the newly developed valves with a small number of applications, an assessment period (about 8 months) can be set up to record the use performance of the products and compare with the use of the world-class products.
Based on the long-term development prospect of LNG industry, this paper combs the use of common low-temperature valves in two key links of LNG industry chain, natural gas liquefaction plant and LNG terminal. Aiming at the multi-stage pressure reducing valve with slightly complicated structure design and difficult localization, the process requirements and the key points of multi-stage pressure reducing structure selection are analyzed in detail. The key points for attention in the design and test of low temperature valves in existing stations are summarized. The next work suggestions for the promotion and application of domestic valves are put forward: the localization assessment process, risk control, application performance collection and other work shall be done well in combination with the actual application, so as to continuously improve the manufacturing level of core supporting equipment in China’s LNG industry.
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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