Properties of LNG and knowledge on its storage and transportation
Thread Content
This post was last edited by caoguanggui on 2009-5-22 14:08. Uses of natural gas: I. Chemical fuel, fuel for household use, vehicle fuel, combined power generation, heat pumps, fuel cells, etc. nqP 2 Liquid natural gas: The main component of natural gas is methane, whose critical temperature is 190.58 K. LNG is stored in low-temperature tanks at a temperature of 112 K (–161°C) and a pressure of around 0.1 MPa; its density is more than 600 times that of methane under standard conditions, and its volumetric energy density is 72% that of gasoline. ;Fd, the 3 LNG plants can be mainly divided into two categories: base-load type and peak-shaving type. The liquefaction process is primarily based on the APCI (Air Products and Chemicals) process. (Propane pre-cooling mixed refrigerant liquefaction process) H (4) Natural gas accounts for only 2.6% of China’s total energy consumption; by 2010, this proportion is expected to reach 7%–8%. )@Dt3 5 China’s LNG plants: In the late 1990s, with the early development and utilization of natural gas in the East China Sea, a natural gas peak-shaving facility with a processing capacity of 100,000 cubic meters per day was built in Shanghai. In 2001, the Central Plains Petroleum Exploration Bureau built the first commercial liquefied natural gas plant with a daily processing capacity of 150,000 cubic meters. In 2002, Xinjiang Guanghui Group began building an LNG plant with a processing capacity of 1.5 million cubic meters, featuring storage tanks with a designed capacity of 30,000 cubic meters. . 6 LNG receiving terminals: Dapeng Bay in Shenzhen, Meizhou Bay in Fujian, Zhejiang, Shanghai and other locations. ] 7 Pretreatment of natural gas: Removal of impurities such as hydrogen sulfide, carbon dioxide, moisture, heavy hydrocarbons, and mercury from natural gas, to prevent these impurities from corroding equipment and causing blockages in equipment and pipelines due to freezing at low temperatures. !,_&K= 8 Dehydration: If natural gas contains moisture, in the liquefaction unit, this water will freeze into ice or frost on the surfaces of the heat exchangers and in the working parts of the throttling valves at temperatures below zero. In addition, natural gas and water can form natural gas hydrates, which are semi-stable solid compounds that can form at temperatures above zero; these hydrates can not only cause blockages in pipelines but also clog nozzles and separation equipment. M@xZhT 15 Natural gas liquefaction processes: cascade liquefaction process, mixed refrigerant liquefaction process, liquefaction process with an expander. J-$16 natural gas liquefaction plants come in basic load types and peak shaving types. Basic load natural gas liquefaction plants are large-scale facilities that produce gas for local use or for export; their liquefaction units typically employ cascade liquefaction processes or mixed refrigerant liquefaction processes. A peak-shaving liquefaction plant refers to a natural gas liquefaction facility designed to handle peak loads or to supplement fuel supply during winter; it typically liquefies and stores the excess natural gas during off-peak periods for use in gaseous form during peak times or in emergency situations. Its liquefaction units commonly use a liquefaction process with an expander and a mixed-refrigerant liquefaction process. At present, in over 80% of the natural gas liquefaction plants designed for basic load applications around the world, a propane-precooled mixed-refrigerant liquefaction process is used. The process consists of three parts: a mixed refrigerant cycle, a propane pre-cooling cycle, and a natural gas liquefaction circuit. In this liquefaction process, the propane pre-cooling cycle is used to pre-cool the mixed refrigerant and natural gas, while the mixed refrigerant cycle is used for deep cooling and liquefying natural gas. |The Rt 18 integrated cascade liquefaction process (CII process) developed by GDF Suez represents the development trend in natural gas liquefaction technology. The CII liquefaction process built in Shanghai is the process used in China’s first peak-shaving natural gas liquefaction plant. njJX3 19 Liquefaction process with an expander: A Claude cycle refrigeration process for natural gas liquefaction, which utilizes high-pressure refrigerant to achieve adiabatic expansion through a turbine expander. As the gas expands and cools in the expander, it can generate work, which can be used to drive the compressors in the process. The investment is moderate, making it suitable for natural gas liquefaction plants of smaller capacity used for peak-shaving purposes. ~}( The specific power consumption for 20 typical cascade liquefaction processes is 0.33 KW•h/kg. The propane pre-cooling single-stage mixed refrigerant liquefaction process is 1.15 times of it. 47$Ao0 21 The natural gas liquefaction plant consists of a natural gas pretreatment process, a liquefaction process, a storage system, a control system, and a fire protection system. {z 22 The floating liquefied natural gas production, storage, and unloading unit is a new type of facility for liquefying natural gas from marginal gas fields and offshore gas fields, offering advantages such as lower investment costs, shorter construction time, and ease of relocation. The J 23 LNG project is mostly developed through joint ventures between large multinational oil companies such as SHELL and Total, and the countries that own the resources. \ 24 Currently, LNG accounts for 5.6% of the global gas market and 25.7% of total gas exports. In a typical LNG industry chain, the proportion of investment costs for each major stage is as follows: 10% for upstream gas field development, 40% for LNG plants, 30% for LNG transportation, and 20% for receiving terminals. a\:=Z 26 Terminal facilities that receive LNG by sea are called LNG receiving terminals. It receives LNG shipped by ship from basic-load natural gas liquefaction plants, stores and regasifies it, and then distributes it to users. The receiving terminal has a high regasification capacity, and the storage tank also has a large capacity. It mainly consists of dedicated docks, unloading equipment (LNG unloading arms), LNG pipelines, LNG storage tank regasification units and gas delivery systems, gas metering and pressure control stations, evaporated gas recovery systems, control and safety protection systems, and maintenance systems. There are two process flows for the K+ 27 LNG receiving terminal: one is the direct output type ； ； Another type is the recondensation type. In the direct output process, the evaporated gas is pressurized by a compressor and then sent to low-pressure users with stable pressure requirements; under unloading conditions, these low-pressure users should be able to receive large amounts of evaporated gas. In the recondensation process, the vapor is compressed and then sent to the recondenser, where it exchanges heat with the LNG pumped from the storage tank; this causes the vapor to cool and liquefy. After being pressurized by an external pump, it is sent to the vaporizer and delivered to the end users. ) 28 The first LNG receiving terminal under construction in our country: cl) 1 LNG unloading system: The unloading system consists of a unloading arm, unloading pipelines, vapor return arms, an LNG sampler, vapor return pipelines, and LNG circulation refrigeration pipelines. After the O684 LNG carrier arrives at the dock, the LNG outlet pipeline on board is connected to the onshore unloading pipeline using a unloading arm at the dock, and the transfer pump (submersible pump) located in the ship’s storage tanks continuously pumps the LNG into the storage tanks at the terminal. As LNG is continuously exported, the vapor pressure in the ship’s storage tanks gradually decreases. To maintain a constant vapor pressure, some of the evaporated gas from the onshore storage tanks is pressurized and then sent to the ship’s storage tanks via return pipelines and return arms. The YLTM LNG unloading pipeline generally adopts a dual-mast design. During unloading, the two main pipes operate simultaneously, each handling 50% of the flow rate. When one main pipe fails, the other can still operate, preventing disruptions in unloading. During periods when unloading does not take place, the dual pipes allow the unloading pipeline to form a loop, facilitating cyclic cooling of the main pipe and thus helping to maintain low temperatures and reducing the amount of LNG that evaporates due to heat loss from the pipelines. Typically, a portion of LNG is diverted from the outlet of the pump in the onshore storage tank to cool the pipelines that require cooling, and then returns to the tank via the circulation cooling pipeline. Before each unloading, the unloading arm and other components must be pre-cooled using the LNG on board; once pre-cooling is complete, the unloading rate is gradually increased to the normal level. The unloading pipeline is equipped with a sampler, which takes samples before each unloading operation to analyze the composition, density, and calorific value of the LNG. Ix 2 LNG storage system: The LNG storage system consists of a cryogenic storage tank, associated pipelines, and control instruments. During storage of liquids in low-temperature containers, despite the container’s good insulation, some heat still manages to enter the container through various means. Due to the influx of heat, a portion of the low-temperature liquid will vaporize, causing the pressure inside the container to rise. The daily evaporation rate of the storage tank is approximately 0.06%–0.08%. During unloading, the amount of evaporated gas can increase several times due to heat dissipation from the transfer pumps inside the ship’s storage tanks, the pressure difference between the ship’s storage tanks and the terminal storage tanks, heat leakage from the unloading arms, and the displacement of LNG liquid vapor. To minimize the amount of vapor evaporated during unloading, the pressure in the storage tank at this time should be increased as much as possible. Generally, the receiving terminal should have at least 2 storage tanks of equal volume. { 3 LNG regasification/export system: The LNG regasification/export system includes transfer pumps (submersible pumps) inside the LNG storage tank, low/high pressure export pumps outside the tank, open-type water-cooled evaporators, submerged combustion evaporators, and metering equipment. $.B
Training materials for LNG and RLNG emergency gas supply stations, Guangzhou Nansheng Gas Technology Development Co., Ltd., July 2007. With the vigorous progress of China’s \"West-to-East Gas Transmission\" project, a nationwide enthusiasm for the use of natural gas has emerged. As the best energy source in the world today, natural gas has received significant attention in the selection of urban energy sources in our country; promoting its use has become part of our country’s energy policy. However, due to the large scale of projects involved in long-distance pipeline transportation of natural gas, as well as the high costs and long construction periods, it is difficult for such pipelines to reach most cities in a short time. By using high pressure to compress natural gas by about 250 times to enable transportation in the form of CNG, and then reducing the pressure, the natural gas supply issue in some cities has been addressed. Compared with the CNG method, the LNG supply approach, which uses ultra-low temperature freezing technology to turn natural gas into a liquid state (reducing its volume by about 600 times), employs ultra-low temperature insulated tanks for long-distance transportation of natural gas via vehicles, trains, ships, etc., stores it in such insulated tanks, and then regasifies it, offers higher transportation efficiency and greater safety and reliability, enabling a better solution to the issue of urban natural gas supply. Chapter 1: Basics of LNG I. What is LNG? II. Composition and properties of LNG III. Characteristics of LNG IV. Wide-ranging applications of LNG V. The LNG value chain VI. LNG projects VII. Safety and environmental aspects of LNG VIII. Conclusion I. What is liquefied natural gas? LNG (Liquefied Natural Gas) • LNG is the abbreviation for Liquefied Natural Gas, that is, liquefied natural gas. •It is a product of natural gas (methane CH4) that has been purified and cooled to ultra-low temperatures (–162°C, 1 atmosphere) to become liquid. Liquefied natural gas has a **reduced volume**; at 0°C and 1 atmosphere of pressure, its volume is about 1/600 of that of gaseous natural gas. In other words, 1 cubic meter of LNG can produce 600 cubic meters of natural gas when it is vaporized. •It is colorless and odorless; its main component is methane, with very few other impurities, making it a highly clean source of energy. Its liquid density is approximately 426 kg/m3, while the gas density at this temperature is about 1.5 kg/m3. The explosion limit is 5%-15% (by volume%), and the ignition point is around 450°C. •The natural gas produced from oil/gas fields is formed through liquid removal, acid removal, drying, distillation, and low-temperature condensation, resulting in its volume being reduced to 1/600 of the original amount. II. Composition and Properties of LNG Taking the LNG imported from the Zhongyuan Oilfield, Xinjiang Guanghui, and Shenzhen Dapengwan as examples, their composition and properties are shown in the table below. III. Characteristics of Liquefied Natural Gas 1. Low temperature, large gas-liquid expansion ratio, high energy efficiency, and ease of transportation and storage. • 1 cubic meter of natural gas contains approximately 9,300 kilocalories of heat energy. • 1 ton of LNG can produce 1,350 cubic meters of natural gas, which is sufficient to generate 8,300 units of electricity. 2. Clean energy — LNG is considered the cleanest fossil fuel on Earth! •LNG has an extremely low sulfur content; if all 2.6 million tons per year of LNG were used for power generation, it would result in a reduction of about 450,000 tons of SO2 emissions compared to using coal (lignite), which is roughly equivalent to twice Fujian’s annual SO2 emissions, thereby preventing the further spread of acid rain. •The NOX and CO2 emissions from natural gas-powered power plants are only 20% and 50% of those from coal-fired power plants, respectively. High safety performance—thanks to the excellent physical and chemical properties of LNG! After vaporization, it is lighter than air, colorless, odorless, and non-toxic. •High ignition point: the auto-ignition temperature is approximately 450℃ ； Narrow combustion range: 5%–15% ； Lighter than air and easy to disperse! As an energy source, LNG has the following characteristics: 1. LNG produces virtually no pollution when burned. 2. The reliability of LNG supply is ensured by the contracts and operations across the entire chain. 3. The safety of LNG is fully ensured by strictly adhering to a series of international standards throughout the design, construction, and production processes. LNG has been in operation for 30 years without any serious accidents. 4. Using LNG as an energy source in power plants helps with peak load management in the power grid, ensures its safe operation and optimization, and improves the structure of the power supply. 5. As an urban energy source, LNG can **improve the stability, safety, and cost-effectiveness of gas supply.** IV. Wide-ranging uses of LNG As a clean fuel, LNG is bound to become one of the main energy sources in the new century. In summary, its uses mainly include: (1) serving as a source to meet peak loads and handle emergencies in urban gas distribution systems; (2) acting as the primary gas supply for pipeline gas delivery in large and medium-sized cities; (3) being used as a fuel for LNG vaporization in residential areas; (4) serving as fuel for vehicle refueling; (5) being used as aircraft fuel; (6) utilizing the cold energy from LNG; (7) being part of distributed energy systems. V. The LNG “chain”: From extraction from gas fields to its use by consumers, natural gas goes through a series of interconnected production and supply chains that are interdependent with one another. Each link in the supply and demand chain is closely connected and mutually restrictive, guaranteed by strict contracts. VI. LNG Projects LNG projects generally should include the following production stages: 1. Natural gas production, which involves the exploration, extraction, and transportation of gas from upstream fields, as well as the removal of water and hydrocarbons from the gas, followed by its transport to liquefaction plants. 2. Natural gas liquefaction, during which the gas is purified, liquefied, and stored in liquefaction plants. The liquefaction and freezing process typically employs a compression cycle freezing method using ethane, propane, and mixed refrigerants as the circulating medium. 3. LNG Transportation: At present, LNG transportation in China is carried out using road tankers. The maximum LNG water volume per tank truck is 37 m3, with a transport capacity of 22,000 Nm3 of gaseous natural gas. The design pressure of the tank truck is 0.8 MPa, while the operating pressure is 0.3 MPa. The normal average driving speed is 60 km/h. The entire transportation process is safe and stable. Based on on-site measurements while following the vehicle, the pressure inside the LNG tank truck remained essentially constant during operation; it increased by about 0.02 MPa during short stops, and there was no leakage from the safety valve, resulting in almost no loss of LNG. 4. LNG Stations 1) LNG Storage Tanks: LNG storage tanks (cryogenic storage tanks) are devices used for storing LNG. Special characteristics of LNG storage tanks: Large-capacity LNG storage tanks operate at ultra-low temperatures (-162°C), which gives them distinct properties compared to other storage tanks used in the petrochemical industry. Meanwhile, during operation, since the stored LNG is in a boiling state, the intrusion of external heat, or impacts during filling and changes in atmospheric pressure, will cause the stored LNG to continue vaporizing into gas. Therefore, it is necessary during operation to consider the control of pressure inside the storage tank, the extraction, treatment of the vaporized gas, as well as cooling and insulation. In addition, accessories such as the safety valves, level gauges, thermometers, and expansion joints of the inlet and outlet pipes in LNG storage tanks must also be resistant to low temperatures. The safety devices of the storage tank must also be able to operate reliably at low temperatures and low pressures. LNG storage tanks are key equipment in gasification stations, and their thermal insulation and sealing performance directly affect the evaporation and leakage rates of LNG, that is, the rate of LNG loss and its utilization efficiency. The performance parameters of storage tanks mainly include vacuum level, leakage rate, and static evaporation rate. As a low-temperature vessel, LNG storage tanks must meet the relevant technical requirements specified in ** and industry standards. The vacuum sealing degree of a storage tank reflects its vacuum level, but the vacuum strength decreases over time ； The leakage rate of the storage tank affects its vacuum life, that is, the rate at which the vacuum level within the tank changes ； The static evaporation rate can provide a more direct indication of the heat-insulation performance of the storage tank during use. Taking a 50 m3 storage tank as an example: (1) The leakage rate is 1x10-9 Pa·m3/s. (2) Static evaporation rate: 0.3%/d. When a 50m3 LNG tank is filled with LNG, it takes nearly a year for the LNG to completely evaporate if left unused. The static evaporation rate can be measured experimentally, or it can be calculated through the analysis of data from actual operations. 2) Gasification of LNG: The gasification of LNG is an endothermic process, and depending on the heat source used, there are various methods of gasification such as seawater, air temperature, and water bath. Currently, domestic LNG vaporization stations all use a two-stage vaporization method that combines air-cooled and water-bath types. Air-to-air heat exchangers utilize natural air for heat exchange without the need for additional energy. Their vaporization capacity is primarily determined by the heat exchange surface, which is why fins are usually used. The specifications of the heat exchanger are primarily determined by the hourly gas flow rate of the emergency gas supply; the gasification capacity of a single unit can reach up to 3,200 cubic meters per hour (with an actual maximum capacity of 3,840 cubic meters per hour). Equipment in the gasification station such as storage tank boosters, BOG heaters, and EAG heaters also utilize air-to-air heat exchangers. 3) Pre-cooling and liquid feeding of LNG: After the construction of the equipment and pipelines at the LNG vaporization station is completed, due to the extremely low temperatures and the special requirements associated with LNG, it is necessary to use an intermediate medium for pre-cooling before the facility can start operating. LNG can only be received after the pre-cooling process has been tested and found to be satisfactory; this process also serves as a means of testing the equipment and the entire installation. Liquid nitrogen is commonly used as the pre-cooling medium. The main equipment in the gasification station includes LNG storage tanks, BOG (vaporized gas) tanks, gasifiers, compressors, BOG heaters, EAG (venting exhaust gas) heaters, as well as related process pipelines and fittings. The pre-cooling of the LNG storage tanks is a key aspect of the pre-cooling process in the gasification station. Several technical parameters for pre-cooling: Inlet liquid temperature: below -80°C; Storage tank pressure: 0.3–0.55 MPa; Inlet flow rate: 3 meters cubed per minute; Pre-cooling time: approximately 4–5 hours per tank (depending on the current operating conditions of the storage tank); Liquid nitrogen consumption: about 10 cubic meters (depending on the current operating conditions of the storage tank). VII. Safety and environmental aspects of LNG: Gaseous natural gas is less dense than air, so it spreads easily in case of a leak, whereas liquefied petroleum gas behaves differently ； The explosive limit of natural gas is 5–15%, with a lower limit that is higher than 1% for liquefied petroleum gas; in other words, a larger amount of gas leakage is required to cause an explosion, so the risk is somewhat lower ； Furthermore, LNG is safer when stored at low temperatures. To date, there have been no reports of explosions caused by the combustion of LNG worldwide. Inside the gasification station, the LNG storage tanks are equipped with three layers of protection measures: self-pressurization reduction, manual venting via pressure alarms, and safety valve activation. Additionally, emergency shut-off devices are installed at the liquid-phase inlets and outlets of the tanks as well as on the main outlet pipeline, ensuring safety within the station. During the liquefaction process, natural gas has its impurities such as H2O, heavy hydrocarbons, and H2S removed, making it purer than ordinary natural gas; it burns more completely and is one of the cleanest forms of energy. China Gas LNG Training VIII. Conclusion: Since LNG effectively solves the problem of cities not being able to use natural gas via pipelines, and although it is still a relatively new technology in our country, its advantages are evident, its development is rapid, and its prospects are bright. LNG is a low-temperature flammable substance, and it goes through processes such as liquefaction, transportation, and gasification; as a result, its production process is relatively complex. Therefore, theoretical and practical training for managers and operators is extremely urgent! Chapter 2: Operating Techniques and Operational Management of LNG Supply Stations I. Pre-commissioning preparations II. Operating techniques for LNG supply stations III. Operational safety management I. Pre-commissioning preparations Before a LNG supply station is put into formal operation after passing the completion inspection, the entire process system must undergo drying → pre-cooling → simulated trial operation → inerting → replacement of nitrogen with natural gas (i.e., passivation). 1. Drying and pre-cooling: Drying involves using nitrogen to displace moisture from the equipment and pipelines in the supply station’s process system, thereby preventing the moisture from freezing at low temperatures and causing blockages or damage to the equipment, pipelines, and valves. Before drying, the process system should have completed purging, pressure testing, instrument calibration, the installation and verification of safety valves, as well as the completion of insulation for equipment and pipelines. China Gas LNG Training 2: System Simulation Testing, Inerting and Purging. The purpose of simulation testing is to use liquid nitrogen at -196°C to adjust the process parameters of the LNG process system and evaluate its operational performance. Since liquid nitrogen has a temperature as low as -196°C, the area where white mist appears in the system is the leakage point and requires treatment. Simulating trial operations using liquid nitrogen can **improve the operational reliability of the system under LNG operating conditions. II. Operation Techniques for LNG Supply Stations 1. Gasification process flow at supply stations (the process flow for urban LNG supply stations is shown in the diagram). Diagram of the LNG gasification station process flow. 2. LNG unloading process: LNG is transported from the source location to the LNG supply station in the city using road tankers or container trucks. An air-cooled pressure-raising vaporizer installed on the tanker is used to raise the pressure in its storage tank to 0.6 MPa (or a dedicated unloading pressure-raising vaporizer at the station is used to raise the pressure in the container truck); at the same time, the pressure in the storage tank is reduced to around 0.4 MPa, creating a pressure difference of about 0.2 MPa between the tanker and the LNG storage tank. This pressure difference is utilized to transfer the LNG from the tanker into the storage tank of the supply station. At the end of unloading, the gaseous natural gas in the tank truck is recovered through the gas pipeline at the unloading station. 3. Automatic pressurization of the LNG tank: Driven by pressure, LNG flows from the tank to the air-cooled vaporizer, where it is vaporized and then supplied to users. As the LNG inside the tank flows out, the pressure within the tank continues to drop, and the rate at which LNG exits the tank gradually slows down until it stops. Therefore, during normal operation, gas must be continuously supplied to the storage tank to maintain the pressure inside it within a certain range, so that the LNG vaporization process can continue. The pressurization of the storage tank is achieved by an automatic pressurization system composed of a self-acting pressure control valve and a small air-temperature gasifier. During operation, improper adjustment of the operating pressure often results in the newly installed pressure booster valve becoming ineffective, forcing operators to open the bypass pipe to provide manual pressure boost. When manually pressurizing, the liquid-phase inlet valve of the pressure vaporizer should be opened slowly to prevent accidents caused by an excessive opening of the valve. There was an incident in which, during manual pressurization, the opening degree of the liquid-phase inlet valve of the pressure vaporizer was too large; as a result, a large amount of LNG flowed into the pressure vaporizer and could not be fully vaporized. It entered the outlet pipeline of the pressure vaporizer in a gas-liquid mixture state and rushed out through the safety valve, spraying onto the outer wall of the LNG storage tank and causing the outer tank to crack over a length of 350 mm. Therefore, self-acting pressure boost valves should be used as much as possible for automatic pressure boosting. China Gas LNG Training 4: Pressure Control of LNG Tanks. The normal operating pressure of the tanks is determined and controlled by the set pressure value (back pressure) of the self-acting pressure boosting control valve. The maximum allowable operating pressure of the storage tank is determined and automatically controlled by the set pressure value (pre-pressure) of the self-acting pressure reducing control valve installed on the vapor outlet pipeline of the tank. When the normal operating pressure of the storage tank is below the opening pressure of the pressure booster valve, the latter opens to increase the pressure automatically; when the maximum allowable operating pressure of the storage tank reaches the setting opening value of the pressure relief valve, the latter opens automatically to release pressure and thus protect the safety of the storage tank. To ensure that the booster valve and the pressure relief valve do not interfere with each other during operation, the closing pressure of the booster valve must not overlap with the opening pressure of the pressure relief valve; a pressure difference of at least 0.05 MPa should be maintained. For example, if the closing pressure of the pressure-boosting control valve is set at 0.693 MPa, then the opening pressure of the pressure-reducing control valve is 0.76 MPa. Taking into account the actual manufacturing precision of the valves, the appropriate pressure difference should be carefully adjusted and determined during the on-site installation of the valves. 5. Overpressure protection for LNG storage tanks: During storage, LNG evaporates slowly due to \"environmental heat leakage\" from the tanks, which causes the pressure inside the tanks to increase gradually, ultimately posing a risk to the safety of those tanks. Therefore, the design incorporates three levels of safety protection measures to ensure overpressure protection for the storage tank: installation of a self-acting pressure reducing control valve on the tank, manual venting for pressure alarms, and a safety valve that activates in case of overpressure. The vacuum pressure storage tanks commonly used in China have a maximum operating pressure of 0.80 MPa and a nominal volume of 100 m3 per tank. The maximum opening pressure of the pressure relief valve is set at 0.76 MPa, while the alarm pressure is set at 0.78 MPa. The opening pressure and discharge pressure of the tank’s safety valve are set at 0.8 MPa and 0.88 MPa respectively. The sequence of protection is as follows: when the pressure in the tank rises to the set value of the pressure relief valve, the valve opens automatically to release pressure; if the pressure relief valve fails and the pressure inside the tank rises to the pressure alarm threshold of 0.78 MPa, an alarm is triggered and manual release is required to lower the pressure; if the pressure relief valve fails and manual release is not possible, and the pressure inside the tank reaches 0.80 MPa, the tank’s safety valve activates, and it continues to discharge pressure until it reaches 0.88 MPa. This ensures the safety of the storage tank while making full use of its strength capacity (the design pressure of the tank is 0.84 MPa). As the safety valve discharges pressure, when the operating pressure inside the tank drops to 85% of the discharge pressure set by the safety valve, it automatically closes to seal the tank. Under normal operation, the safety valve should not open frequently. 6. Overfilling and low liquid level protection for LNG storage tanks: The amount of LNG filled into a tank is primarily controlled by the liquid level inside the tank. The storage tank is equipped with two independent level systems: a fullness indicator and a differential pressure level gauge, which are used to indicate and measure the liquid level in the tank. In addition, it is also equipped with a high liquid level alarm (at 85% filling level), an emergency shut-off system (at 95% filling level), and a low level alarm (when 10% LNG remains). The 95% high liquid level (maximum tank capacity) is determined based on the density of the saturated liquid under operating pressure; in practice, it must be adjusted downward for different gas sources. China 7: Rolling of LNG and Its Prevention. As a mixture of different components, LNG can stratify during storage, which leads to rolling. This results in significant evaporation of LNG and overpressure in the storage tank; if pressure is not relieved in a timely manner, it can pose a serious threat to the safety of the tank. Numerous studies have shown that rolling occurs due to the stratification of LNG for the following reasons: ① The LNG filled into the tank at different times comes from different sources and thus has different compositions, leading to varying densities; ② The LNG filled at different times has different temperatures, which results in different densities; ③ The LNG filled first has a different density due to the evaporation of the light component methane compared to the LNG filled later. Common measures to prevent stratification of LNG in tanks are as follows: (1) Store LNG from different sources separately. (2) To prevent a density difference from arising between the LNG injected into the tank at different times, the following filling methods are adopted: ① When the density of the LNG in the truck is similar to that of the LNG already in the tank, filling is carried out through the lower inlet of the tank; ② When light LNG from a truck is being filled into a tank containing heavy LNG, filling is done through the lower inlet of the tank; ③ When heavy LNG from a truck is being filled into a tank containing light LNG, filling is carried out through the upper inlet of the tank. (3) The inlet pipes in the storage tank use mixed nozzles and porous tubes, which enable the newly filled LNG to mix thoroughly with the existing LNG. (4) For LNG stored for long periods, regular tank transfer is carried out to prevent stratification due to stagnation. China Gas LNG Training III. Operational Safety Management The basic requirements for operational safety management of LNG supply stations are as follows: ① Prevent leaks of LNG and NG from forming flammable explosive mixtures with air; ② Install combustible gas concentration monitoring and alarm devices in areas where natural gas leaks may occur, such as storage tanks, vaporization areas, and unloading platforms; ③ Eliminate the conditions that could lead to combustion or explosions, and provide fire protection for LNG processing systems and equipment in accordance with regulatory requirements; ④ Prevent overpressure in LNG equipment and any release of gas due to excessive pressure; ⑤ Prevent the harmful effects of LNG’s low temperature and large temperature differences on the processing systems, as well as cold burns to operators. Strict safety measures must be established, safety operating procedures must be strictly followed, and potential dangers as well as fire hazards must be eliminated. The head of the gas supply station should regularly inspect the production equipment and fire protection facilities at the station on a daily basis. 1. Potential dangers of LNG: Although LNG is stored and vaporized at low temperatures, it is used in its gaseous state at normal temperatures, just like pipeline natural gas. This gives rise to the following potential dangers of LNG: (1) Dangers associated with low temperatures; (2) Dangers related to BOG; (3) Fire hazards; (4) Roll-over risks.
2. Rationality and comprehensiveness of the process design for LNG stations: 1) LNG stations should have a well-structured process flow; 2) They should have adequate operational mechanisms; 3) Reliable safety measures are necessary; 4) The storage, vaporization, and transportation equipment used in LNG stations must be of high quality in terms of manufacturing and installation.
3. Safety technology management for LNG stations: Given the inherent characteristics and potential dangers of LNG, it is essential to carry out rational process design, safety planning, and high-quality equipment manufacturing for LNG stations. This will lay a solid foundation for effective safety technology management in such stations. 1) Organization and staffing of LNG stations: There should be a dedicated organization responsible for the safety and technical foundations of LNG stations ； At the same time, professional technical management personnel should be assigned ； It is necessary to define the various production positions and assign appropriate operators to them. Both management personnel and operational staff must receive professional technical training and pass the assessment before they can take up their posts. 2) Technical management: (1) Establish and maintain technical archives for LNG stations, including preliminary research documents, initial design documents, construction drawings, complete set of construction materials, approval procedures and documents from relevant departments, etc. (2) Formulate operating procedures for various positions, including procedures for LNG unloading, procedures for pressurizing LNG storage tanks, procedures for transferring liquid between LNG storage tanks, procedures for operating LNG air bath (water bath) vaporizers, procedures for operating BOG storage tanks, procedures for operating fire pumps, procedures for switching central dispatch control systems, procedures for weighing and measuring LNG at entry/exit points, and procedures for odorizing natural gas. China Gas LNG Training 3) Production Safety Management (1) Provide proper safety and technical training for personnel on duty. This includes training on the process flow of LNG stations, the structure and working principles of equipment, operational procedures, daily maintenance of equipment, as well as the use and maintenance of fire-fighting equipment; all of these aspects should be covered to ensure that employees know what they need to know and can do what is required. (2) Establish safety production responsibility systems for various positions, as well as a system for regular inspections of equipment; these are also prerequisites for standardizing safe behaviors. For LNG that has been stored for an extended period, regular tank transfer operations should be carried out as part of a systematic approach to prevent the occurrence of \"rolling\" phenomena. (3) Establish and maintain various types of original records that meet the process requirements: including unloading records, LNG storage tank storage records, central control system operation records, inspection records, etc., and ensure their proper implementation. (4) Establish an emergency response and rescue plan. The plan should clearly specify the organization for rapid rescue, the division of tasks, procedures for reporting incidents, and the methods for dealing with various types of accidents (such as minor or major LNG leaks, as well as fires). And conduct regular drills in this area to establish a system. (5) Strengthen the management of fire protection facilities. Special attention should be paid to regular inspection (testing) of fire water tanks (tanks), fire pumps, sprinkler systems for LNG storage tanks, dry powder fire extinguishing systems, and combustible gas alarm systems, to ensure their proper functioning. (6) Strengthen daily safety inspections and assessments. Through these inspections and assessments, operational practices can be standardized, violations of regulations can be prevented, and complacency can be overcome. The unloading of LNG, for example, deserves to be regulated; there should be a set of comprehensive procedures covering everything from the arrival of the tank truck at the site, through measurement and weighing, positioning of the tank truck, pressurization of the tank truck, connection of hoses, connection of static grounding wires, purging of the LNG pipelines, recovery of any remaining gas after unloading, removal of the tank truck from the site, as well as inspections during the unloading process. Additionally, there are requirements regarding the connection between the unloading station and the liquid storage tanks. 4. Equipment management: Since the production equipment at LNG stations (storage tanks, gasification equipment, etc.) is all of domestic manufacture, and due to the lack of standardized guidelines, it is necessary to strengthen the management of the production facilities within these stations. 1) Establish and maintain proper records and cards for production equipment, with dedicated personnel in charge to ensure that the records, cards, and actual equipment are in agreement. Pressure vessels such as LNG storage tanks must obtain a \"Pressure Vessel Use Certificate\" ； The equipment’s user manual, certificate of conformity, quality certificate, technical structure diagram, maintenance records, etc. should be kept in good condition and archived. 2) Establish a comprehensive equipment management system, maintenance system, and standards for optimal condition. Specific production equipment should have dedicated personnel in charge of its regular maintenance. 3) Strengthen the daily maintenance and routine inspections of equipment (1) LNG storage tanks: Check whether the exterior is clean ； Is there any corrosion? ； Is there any frosting or sweating? ； Are the safety accessories in good condition? ； Whether the foundation is solid, etc. (2)LNG vaporizer: Is the exterior clean? ； Is the frosting (during vaporization) uniform? ； Are there any cracks or leaks at the weld seam? ； Is the switching (automatic) between groups working properly? ； Are the safety accessories in good condition? (3)LNG process pipelines: Are the loading/unloading pipelines and the insulation layers of the LNG storage tank outlet pipelines in good condition? ； Check whether the expansion and contraction of the process pipelines during loading/unloading and the liquefied gas vaporization process are normal, and whether there are any leaks at the welds ； Are there any leaks in the valves on the process pipelines (especially the low-temperature valves)? ； Is there any leakage at the flange connection? ； Check whether the safety accessories are in good condition. (4) No issue identified during the regular inspection of equipment should be taken lightly; efforts should be made to resolve it promptly. 4) Ensure regular inspections of equipment: (1) LNG storage tanks: Overall condition of the tanks (frequency: once a year) ； Measurement of the vacuum level in the interlayer of vacuum powder insulated storage tanks (frequency: once a year) ； Determination of the daily evaporation rate of storage tanks (can be determined through the amount of BOG emitted). The measurement period can be long or short; however, when a sudden increase or decrease in the daily evaporation rate is detected, the cause must be identified and addressed immediately ； Robust storage tank foundation, condition changes (frequency: every three months) ； If necessary, the welds of the storage tank can be re-inspected. At the same time, the original operation records of the storage tank should be checked. (2)LNG vaporizer: Overall appearance condition ； Check whether the fins are deformed and whether there are cracks in the welds ； Is the equipment foundation solid? ； Non-destructive testing of the weld joint can be performed if necessary. Inspection cycle: once a year. (3)LNG process pipelines: Based on daily routine inspection records, check the overall operating condition of the process pipelines; welds can be inspected if necessary ； The insulation layer can also be removed to check its condition ； Inspect the carburization of the stainless steel bare tube. (Inspection cycle: one year) (4) Safety accessories: Inspect safety valves, pressure gauges, thermometers, level gauges, pressure transmitters, differential pressure transmitters, temperature transmitters, and interlock devices installed on various equipment and process pipelines. Inspection cycle: one year. It should be noted that the inspection of the aforementioned safety accessories must be carried out by units with the appropriate inspection qualifications. (5) Others: The inspection of lightning protection and anti-static facilities is carried out twice a year. Other equipment and facilities should also be inspected regularly and in a timely manner. China Gas LNG Training Chapter 3: An Overview of Pre-cooling Technology in LNG Regasification Stations I. LNG Regasification Station Process II. The Necessity of Pre-cooling III. Objectives of Pre-cooling IV. Pipeline Purging Before Pre-cooling V. Materials Required for Pre-cooling VI. Preparatory Work Before Pre-cooling VII. Principles of Pre-cooling VIII. Main Steps of Pre-cooling IX. Safety Precautions During Pre-cooling X. Inspection Items During Pre-cooling I. LNG Regasification Station Process An LNG regasification station is the primary setup used in downstream natural gas applications; its main functions are to store, regasify, and transport LNG. It mainly includes a unloading platform, low-temperature storage tanks, a pressurization system, a gasification system, as well as pressure regulation, metering, and odorization systems. Process flow diagram of LNG vaporization station II. The necessity of pre-cooling: Before low-temperature liquids enter the LNG vaporization station, the low-temperature pipelines and storage tanks must first be thoroughly cooled, which is the pre-cooling process. LNG storage tanks and pipelines are typically made of austenitic stainless steel. Austenitic stainless steels exhibit excellent low-temperature performance, but have a high linear expansion coefficient. At LNG temperatures, the shrinkage rate of stainless steel is approximately 0.3 percent; for 304L piping, at an operating temperature of -162°C, a 100m length of pipe will shrink by about 300mm. Therefore, measures must be taken during design to prevent damage caused by cold shrinkage. The contraction and compensation of LNG pipelines is an important issue that requires careful consideration. Between two fixed points, the stress resulting from cold contraction can far exceed the material’s yield point. Especially, the requirements for pipelines inside LNG storage tanks are even stricter; any problems that arise can lead to serious consequences. Therefore, effective measures must be considered during pipeline design to compensate for this. In LNG equipment and pipelines, elbows and expansion joints are generally used to compensate for thermal contraction. Although compensation for cold shrinkage is taken into account during design, when the rate of temperature change is high, there is still a risk of excessive temperature changes and excessive thermal stress, which can cause damage to the material or the joints. This requires pre-cooling the low-temperature pipelines and equipment before introducing the low-temperature liquid, in order to ensure safe operation. III. Purposes of pre-cooling: To examine and test the low-temperature performance of low-temperature equipment and pipelines, including: (1) Checking whether the quality of low-temperature materials is satisfactory; (2) Assessing the quality of welding; (3) Examining the degree of cold contraction of pipelines and changes in support provided by pipe supports; (4) Verifying the sealing performance of low-temperature valves; (5) Bringing the storage tank into operational condition and testing its vacuum performance. IV. Pipeline purging before pre-cooling: 1. Importance of pipeline purging before pre-cooling: The pipelines must be thoroughly cleaned before pre-cooling. If the purging is not thorough, it will cause the valve to freeze. Since most of the valves in low-temperature pipelines are welded with few flanges, it is not conducive to pipeline purging. Therefore, measures must be taken to strictly control purging. 2. Purging qualification standard: The airflow is directed at a pad equipped with a semi-wet white towel placed near the pipe opening; it is considered qualified if there is no dust or impurities on the towel. 3. Principles for pipeline purging: (1) Purging should be carried out in sections during construction, with each section separated by welded valves. It is important to seal the pipelines promptly after construction to prevent debris and rainwater from entering. (2) To prevent rust and slag from carbon steel pipes from entering the low-temperature pipes, carbon steel pipes cannot be used for purging the low-temperature pipes. (3) Blowing cannot be performed into the storage tank; it must be done from inside the tank outward. (4) No instrument equipment may be purged. (5) Due to the need for purging, temporary gaskets should be used during installation, and the proper gaskets should be replaced before achieving airtightness. (6) Tap the pipe surface and weld areas during purging. (7) Develop specific purging plans based on the process flow of each station. V. Materials required for pre-cooling: (1) Liquid nitrogen; (2) Portable thermometers and portable combustible gas detectors; (3) Copper-based fastening tools along with quick-connect fittings for connecting to the unloading ports of liquid nitrogen tankers; (4) Work clothes, work shoes, and frost-resistant gloves required by the personnel responsible for pre-cooling; (5) Watches and record sheets needed for monitoring, with records to be taken every 15 minutes. VI. Preparatory work before pre-cooling: (1) Check the valves to ensure that all of them are in the closed position. (2) Confirm that all blind flanges in the vent system have been removed and that the vent system is unobstructed. (3) Open all root valves of the safety valves, and open the upstream and downstream valves of the two pressure reduction control valves. Open the root valve for the gas phase vent of the storage tank. (4) The automatic protection system tests passed and is fully operational. The nitrogen system was put into operation, and all emergency shut-off valves were opened. (5) All the root valves of the pressure gauge are opened. The root valve and gas-liquid equilibrium valve of the tank level gauge are opened. (6) Replace the air in the pipeline with dry nitrogen to prevent condensation water at the valves during pre-cooling from freezing them. VII. Pre-cooling principles: During pre-cooling, the temperature of storage tanks and pipelines should be reduced gradually to avoid sudden cooling, thereby preventing damage to equipment and fittings caused by rapid temperature drops. Based on relevant operational experience, a cooling rate of 50°C/h is considered safe. VIII. Main steps for pre-cooling 1. First, pre-cool using low-temperature nitrogen. (1) Check that the unloading hose is in good condition, with no rainwater, debris, or other impurities inside it. The hose is connected to the tank truck, and it is checked to ensure the connection is secure. (2) Increase the pressure in the tank truck, open the vapor valve of the tank truck, and check for any leaks at the hose connections. (3) Slowly inject low-temperature nitrogen into the storage tank; once the pressure in the tank rises to 0.2 MPa, close the liquid discharge valve on the unloading platform. After insulating the tank for 15 minutes, open the manual vent valve for the gas phase of the tank to release the nitrogen. Rising and falling pressures occur repeatedly. (4) Determine the internal temperature of the storage tank by releasing gas through the fill valve and measuring it with a thermometer; once the desired temperature is reached, the gas pre-cooling process is complete. 2. Liquid nitrogen pre-cooling: (1) Release the pressure in the storage tank to a slight positive pressure, and close the lower inlet valve. Close the level gauge balance valve and activate the level gauge. (2) Slowly open the liquid-phase valve of the tank truck to a small opening, and slowly close the gas-phase valve of the tank truck, allowing a small amount of liquid nitrogen to enter from the upper part of the storage tank. Control the opening degree of the unloading platform valve; keep it slightly open at a low degree to maintain the pressure at 0.3 MPa. When the pressure in the storage tank rises to 0.2 MPa–0.3 MPa, the valve at the unloading station should be closed promptly, and the manual vent for the gas phase of the storage tank should be opened to relieve pressure. Repeat this operation. (3) Release gas through the fill valve; once the temperature reaches a certain level, or when the level gauge indicates a certain liquid level, slowly open the inlet emergency shut-off valves at the bottom of the storage tank, allowing liquid to flow in from both the top and the bottom. During the liquid inlet process, closely monitor and record the pressure in the storage tank to prevent it from rising. When the pressure increases, the lower inlet valve should be closed promptly. Feel the temperature of the tank’s outer surface with your hand to confirm that there are no issues with the tank. (4) The liquid level gauge of the storage tank reaches a certain value, and the liquid feeding stops. (5) After the liquid storage task is completed, close the liquid-phase valve of the tank truck and open the gas-phase valve of the tank truck to purge the liquid discharge pipeline into the storage tank. (6) Close the tank truck valves and the liquid discharge valves on the unloading platform, remove the hoses, handling them with care, and ensure that people stay clear. After closing the unloading platform valve, the liquid between this valve and the check valve should be drained. (7) Close the manual vent valve for the gas phase in the storage tank, as well as the manual valve before the emergency shut-off valve for liquid inlet at the bottom of the storage tank. The inlet valve at the top of the storage tank should be closed only after the LNG pipeline for unloading returns to normal temperature. (8) Use the liquid nitrogen in the storage tank to pre-cool the booster, air-bath vaporizer, and their cryogenic pipelines. 3. Utilization of vented low-temperature nitrogen: When pre-cooling with liquid nitrogen, it is necessary to vent low-temperature nitrogen through gas pipes. This low-temperature nitrogen can be used to pre-cool other storage tanks via gas pipes connected to those tanks, thereby saving liquid nitrogen. IX. Safety precautions during pre-cooling: (1) In a confined space, liquid nitrogen absorbing external heat can cause a sharp rise in pressure; therefore, attention must be paid to the sequence in which valves are closed, and it is strictly forbidden for low-temperature liquids to become trapped. (2) Be sure to check for leaks at the hose connections, and people should stay away from this area. (3) Pay close attention to the increase in pressure in the pipelines and storage tanks. (4) Make sure to check for frost buildup behind the safety valve. X. Inspection items during pre-cooling: (1) Check whether the low-temperature materials exhibit low-temperature cracking. (2) Check the welded areas of low-temperature pipelines for cracks, especially at the flange welds. (3) Check the pipe cold shrinkage and changes in bracket support. (4) Check the sealing performance and flexibility of the low-temperature valve, and verify whether it has frozen. (5) Check whether there is any leakage at the flange connection, and whether the bolt tension has decreased due to cold contraction. (6) The liquid nitrogen is stored in the tank for 2 to 3 days. Observe the changes in liquid level and pressure rise. It also measures the changes in the vacuum level of the storage tank before and after pre-cooling, in order to evaluate the tank’s performance.
2. Rationality and comprehensiveness of the process design for LNG stations: 1) LNG stations should have a well-structured process flow; 2) They should have adequate operational mechanisms; 3) Reliable safety measures are necessary; 4) The storage, vaporization, and transportation equipment used in LNG stations must be of high quality in terms of manufacturing and installation.
3. Safety technology management for LNG stations: Given the inherent characteristics and potential dangers of LNG, it is essential to carry out rational process design, safety planning, and high-quality equipment manufacturing for LNG stations. This will lay a solid foundation for effective safety technology management in such stations. 1) Organization and staffing of LNG stations: There should be a dedicated organization responsible for the safety and technical foundations of LNG stations ； At the same time, professional technical management personnel should be assigned ； It is necessary to define the various production positions and assign appropriate operators to them. Both management personnel and operational staff must receive professional technical training and pass the assessment before they can take up their posts. 2) Technical management: (1) Establish and maintain technical archives for LNG stations, including preliminary research documents, initial design documents, construction drawings, complete set of construction materials, approval procedures and documents from relevant departments, etc. (2) Formulate operating procedures for various positions, including procedures for LNG unloading, procedures for pressurizing LNG storage tanks, procedures for transferring liquid between LNG storage tanks, procedures for operating LNG air bath (water bath) vaporizers, procedures for operating BOG storage tanks, procedures for operating fire pumps, procedures for switching central dispatch control systems, procedures for weighing and measuring LNG at entry/exit points, and procedures for odorizing natural gas. China Gas LNG Training 3) Production Safety Management (1) Provide proper safety and technical training for personnel on duty. This includes training on the process flow of LNG stations, the structure and working principles of equipment, operational procedures, daily maintenance of equipment, as well as the use and maintenance of fire-fighting equipment; all of these aspects should be covered to ensure that employees know what they need to know and can do what is required. (2) Establish safety production responsibility systems for various positions, as well as a system for regular inspections of equipment; these are also prerequisites for standardizing safe behaviors. For LNG that has been stored for an extended period, regular tank transfer operations should be carried out as part of a systematic approach to prevent the occurrence of \"rolling\" phenomena. (3) Establish and maintain various types of original records that meet the process requirements: including unloading records, LNG storage tank storage records, central control system operation records, inspection records, etc., and ensure their proper implementation. (4) Establish an emergency response and rescue plan. The plan should clearly specify the organization for rapid rescue, the division of tasks, procedures for reporting incidents, and the methods for dealing with various types of accidents (such as minor or major LNG leaks, as well as fires). And conduct regular drills in this area to establish a system. (5) Strengthen the management of fire protection facilities. Special attention should be paid to regular inspection (testing) of fire water tanks (tanks), fire pumps, sprinkler systems for LNG storage tanks, dry powder fire extinguishing systems, and combustible gas alarm systems, to ensure their proper functioning. (6) Strengthen daily safety inspections and assessments. Through these inspections and assessments, operational practices can be standardized, violations of regulations can be prevented, and complacency can be overcome. The unloading of LNG, for example, deserves to be regulated; there should be a set of comprehensive procedures covering everything from the arrival of the tank truck at the site, through measurement and weighing, positioning of the tank truck, pressurization of the tank truck, connection of hoses, connection of static grounding wires, purging of the LNG pipelines, recovery of any remaining gas after unloading, removal of the tank truck from the site, as well as inspections during the unloading process. Additionally, there are requirements regarding the connection between the unloading station and the liquid storage tanks. 4. Equipment management: Since the production equipment at LNG stations (storage tanks, gasification equipment, etc.) is all of domestic manufacture, and due to the lack of standardized guidelines, it is necessary to strengthen the management of the production facilities within these stations. 1) Establish and maintain proper records and cards for production equipment, with dedicated personnel in charge to ensure that the records, cards, and actual equipment are in agreement. Pressure vessels such as LNG storage tanks must obtain a \"Pressure Vessel Use Certificate\" ； The equipment’s user manual, certificate of conformity, quality certificate, technical structure diagram, maintenance records, etc. should be kept in good condition and archived. 2) Establish a comprehensive equipment management system, maintenance system, and standards for optimal condition. Specific production equipment should have dedicated personnel in charge of its regular maintenance. 3) Strengthen the daily maintenance and routine inspections of equipment (1) LNG storage tanks: Check whether the exterior is clean ； Is there any corrosion? ； Is there any frosting or sweating? ； Are the safety accessories in good condition? ； Whether the foundation is solid, etc. (2)LNG vaporizer: Is the exterior clean? ； Is the frosting (during vaporization) uniform? ； Are there any cracks or leaks at the weld seam? ； Is the switching (automatic) between groups working properly? ； Are the safety accessories in good condition? (3)LNG process pipelines: Are the loading/unloading pipelines and the insulation layers of the LNG storage tank outlet pipelines in good condition? ； Check whether the expansion and contraction of the process pipelines during loading/unloading and the liquefied gas vaporization process are normal, and whether there are any leaks at the welds ； Are there any leaks in the valves on the process pipelines (especially the low-temperature valves)? ； Is there any leakage at the flange connection? ； Check whether the safety accessories are in good condition. (4) No issue identified during the regular inspection of equipment should be taken lightly; efforts should be made to resolve it promptly. 4) Ensure regular inspections of equipment: (1) LNG storage tanks: Overall condition of the tanks (frequency: once a year) ； Measurement of the vacuum level in the interlayer of vacuum powder insulated storage tanks (frequency: once a year) ； Determination of the daily evaporation rate of storage tanks (can be determined through the amount of BOG emitted). The measurement period can be long or short; however, when a sudden increase or decrease in the daily evaporation rate is detected, the cause must be identified and addressed immediately ； Robust storage tank foundation, condition changes (frequency: every three months) ； If necessary, the welds of the storage tank can be re-inspected. At the same time, the original operation records of the storage tank should be checked. (2)LNG vaporizer: Overall appearance condition ； Check whether the fins are deformed and whether there are cracks in the welds ； Is the equipment foundation solid? ； Non-destructive testing of the weld joint can be performed if necessary. Inspection cycle: once a year. (3)LNG process pipelines: Based on daily routine inspection records, check the overall operating condition of the process pipelines; welds can be inspected if necessary ； The insulation layer can also be removed to check its condition ； Inspect the carburization of the stainless steel bare tube. (Inspection cycle: one year) (4) Safety accessories: Inspect safety valves, pressure gauges, thermometers, level gauges, pressure transmitters, differential pressure transmitters, temperature transmitters, and interlock devices installed on various equipment and process pipelines. Inspection cycle: one year. It should be noted that the inspection of the aforementioned safety accessories must be carried out by units with the appropriate inspection qualifications. (5) Others: The inspection of lightning protection and anti-static facilities is carried out twice a year. Other equipment and facilities should also be inspected regularly and in a timely manner. China Gas LNG Training Chapter 3: An Overview of Pre-cooling Technology in LNG Regasification Stations I. LNG Regasification Station Process II. The Necessity of Pre-cooling III. Objectives of Pre-cooling IV. Pipeline Purging Before Pre-cooling V. Materials Required for Pre-cooling VI. Preparatory Work Before Pre-cooling VII. Principles of Pre-cooling VIII. Main Steps of Pre-cooling IX. Safety Precautions During Pre-cooling X. Inspection Items During Pre-cooling I. LNG Regasification Station Process An LNG regasification station is the primary setup used in downstream natural gas applications; its main functions are to store, regasify, and transport LNG. It mainly includes a unloading platform, low-temperature storage tanks, a pressurization system, a gasification system, as well as pressure regulation, metering, and odorization systems. Process flow diagram of LNG vaporization station II. The necessity of pre-cooling: Before low-temperature liquids enter the LNG vaporization station, the low-temperature pipelines and storage tanks must first be thoroughly cooled, which is the pre-cooling process. LNG storage tanks and pipelines are typically made of austenitic stainless steel. Austenitic stainless steels exhibit excellent low-temperature performance, but have a high linear expansion coefficient. At LNG temperatures, the shrinkage rate of stainless steel is approximately 0.3 percent; for 304L piping, at an operating temperature of -162°C, a 100m length of pipe will shrink by about 300mm. Therefore, measures must be taken during design to prevent damage caused by cold shrinkage. The contraction and compensation of LNG pipelines is an important issue that requires careful consideration. Between two fixed points, the stress resulting from cold contraction can far exceed the material’s yield point. Especially, the requirements for pipelines inside LNG storage tanks are even stricter; any problems that arise can lead to serious consequences. Therefore, effective measures must be considered during pipeline design to compensate for this. In LNG equipment and pipelines, elbows and expansion joints are generally used to compensate for thermal contraction. Although compensation for cold shrinkage is taken into account during design, when the rate of temperature change is high, there is still a risk of excessive temperature changes and excessive thermal stress, which can cause damage to the material or the joints. This requires pre-cooling the low-temperature pipelines and equipment before introducing the low-temperature liquid, in order to ensure safe operation. III. Purposes of pre-cooling: To examine and test the low-temperature performance of low-temperature equipment and pipelines, including: (1) Checking whether the quality of low-temperature materials is satisfactory; (2) Assessing the quality of welding; (3) Examining the degree of cold contraction of pipelines and changes in support provided by pipe supports; (4) Verifying the sealing performance of low-temperature valves; (5) Bringing the storage tank into operational condition and testing its vacuum performance. IV. Pipeline purging before pre-cooling: 1. Importance of pipeline purging before pre-cooling: The pipelines must be thoroughly cleaned before pre-cooling. If the purging is not thorough, it will cause the valve to freeze. Since most of the valves in low-temperature pipelines are welded with few flanges, it is not conducive to pipeline purging. Therefore, measures must be taken to strictly control purging. 2. Purging qualification standard: The airflow is directed at a pad equipped with a semi-wet white towel placed near the pipe opening; it is considered qualified if there is no dust or impurities on the towel. 3. Principles for pipeline purging: (1) Purging should be carried out in sections during construction, with each section separated by welded valves. It is important to seal the pipelines promptly after construction to prevent debris and rainwater from entering. (2) To prevent rust and slag from carbon steel pipes from entering the low-temperature pipes, carbon steel pipes cannot be used for purging the low-temperature pipes. (3) Blowing cannot be performed into the storage tank; it must be done from inside the tank outward. (4) No instrument equipment may be purged. (5) Due to the need for purging, temporary gaskets should be used during installation, and the proper gaskets should be replaced before achieving airtightness. (6) Tap the pipe surface and weld areas during purging. (7) Develop specific purging plans based on the process flow of each station. V. Materials required for pre-cooling: (1) Liquid nitrogen; (2) Portable thermometers and portable combustible gas detectors; (3) Copper-based fastening tools along with quick-connect fittings for connecting to the unloading ports of liquid nitrogen tankers; (4) Work clothes, work shoes, and frost-resistant gloves required by the personnel responsible for pre-cooling; (5) Watches and record sheets needed for monitoring, with records to be taken every 15 minutes. VI. Preparatory work before pre-cooling: (1) Check the valves to ensure that all of them are in the closed position. (2) Confirm that all blind flanges in the vent system have been removed and that the vent system is unobstructed. (3) Open all root valves of the safety valves, and open the upstream and downstream valves of the two pressure reduction control valves. Open the root valve for the gas phase vent of the storage tank. (4) The automatic protection system tests passed and is fully operational. The nitrogen system was put into operation, and all emergency shut-off valves were opened. (5) All the root valves of the pressure gauge are opened. The root valve and gas-liquid equilibrium valve of the tank level gauge are opened. (6) Replace the air in the pipeline with dry nitrogen to prevent condensation water at the valves during pre-cooling from freezing them. VII. Pre-cooling principles: During pre-cooling, the temperature of storage tanks and pipelines should be reduced gradually to avoid sudden cooling, thereby preventing damage to equipment and fittings caused by rapid temperature drops. Based on relevant operational experience, a cooling rate of 50°C/h is considered safe. VIII. Main steps for pre-cooling 1. First, pre-cool using low-temperature nitrogen. (1) Check that the unloading hose is in good condition, with no rainwater, debris, or other impurities inside it. The hose is connected to the tank truck, and it is checked to ensure the connection is secure. (2) Increase the pressure in the tank truck, open the vapor valve of the tank truck, and check for any leaks at the hose connections. (3) Slowly inject low-temperature nitrogen into the storage tank; once the pressure in the tank rises to 0.2 MPa, close the liquid discharge valve on the unloading platform. After insulating the tank for 15 minutes, open the manual vent valve for the gas phase of the tank to release the nitrogen. Rising and falling pressures occur repeatedly. (4) Determine the internal temperature of the storage tank by releasing gas through the fill valve and measuring it with a thermometer; once the desired temperature is reached, the gas pre-cooling process is complete. 2. Liquid nitrogen pre-cooling: (1) Release the pressure in the storage tank to a slight positive pressure, and close the lower inlet valve. Close the level gauge balance valve and activate the level gauge. (2) Slowly open the liquid-phase valve of the tank truck to a small opening, and slowly close the gas-phase valve of the tank truck, allowing a small amount of liquid nitrogen to enter from the upper part of the storage tank. Control the opening degree of the unloading platform valve; keep it slightly open at a low degree to maintain the pressure at 0.3 MPa. When the pressure in the storage tank rises to 0.2 MPa–0.3 MPa, the valve at the unloading station should be closed promptly, and the manual vent for the gas phase of the storage tank should be opened to relieve pressure. Repeat this operation. (3) Release gas through the fill valve; once the temperature reaches a certain level, or when the level gauge indicates a certain liquid level, slowly open the inlet emergency shut-off valves at the bottom of the storage tank, allowing liquid to flow in from both the top and the bottom. During the liquid inlet process, closely monitor and record the pressure in the storage tank to prevent it from rising. When the pressure increases, the lower inlet valve should be closed promptly. Feel the temperature of the tank’s outer surface with your hand to confirm that there are no issues with the tank. (4) The liquid level gauge of the storage tank reaches a certain value, and the liquid feeding stops. (5) After the liquid storage task is completed, close the liquid-phase valve of the tank truck and open the gas-phase valve of the tank truck to purge the liquid discharge pipeline into the storage tank. (6) Close the tank truck valves and the liquid discharge valves on the unloading platform, remove the hoses, handling them with care, and ensure that people stay clear. After closing the unloading platform valve, the liquid between this valve and the check valve should be drained. (7) Close the manual vent valve for the gas phase in the storage tank, as well as the manual valve before the emergency shut-off valve for liquid inlet at the bottom of the storage tank. The inlet valve at the top of the storage tank should be closed only after the LNG pipeline for unloading returns to normal temperature. (8) Use the liquid nitrogen in the storage tank to pre-cool the booster, air-bath vaporizer, and their cryogenic pipelines. 3. Utilization of vented low-temperature nitrogen: When pre-cooling with liquid nitrogen, it is necessary to vent low-temperature nitrogen through gas pipes. This low-temperature nitrogen can be used to pre-cool other storage tanks via gas pipes connected to those tanks, thereby saving liquid nitrogen. IX. Safety precautions during pre-cooling: (1) In a confined space, liquid nitrogen absorbing external heat can cause a sharp rise in pressure; therefore, attention must be paid to the sequence in which valves are closed, and it is strictly forbidden for low-temperature liquids to become trapped. (2) Be sure to check for leaks at the hose connections, and people should stay away from this area. (3) Pay close attention to the increase in pressure in the pipelines and storage tanks. (4) Make sure to check for frost buildup behind the safety valve. X. Inspection items during pre-cooling: (1) Check whether the low-temperature materials exhibit low-temperature cracking. (2) Check the welded areas of low-temperature pipelines for cracks, especially at the flange welds. (3) Check the pipe cold shrinkage and changes in bracket support. (4) Check the sealing performance and flexibility of the low-temperature valve, and verify whether it has frozen. (5) Check whether there is any leakage at the flange connection, and whether the bolt tension has decreased due to cold contraction. (6) The liquid nitrogen is stored in the tank for 2 to 3 days. Observe the changes in liquid level and pressure rise. It also measures the changes in the vacuum level of the storage tank before and after pre-cooling, in order to evaluate the tank’s performance.