How to Ensure the Safe Use of LNG Storage Tanks?

In today's energy sector, liquefied natural gas (LNG) is increasingly widely used as an efficient and clean energy source. However, LNG storage tanks store flammable and explosive hazardous gases, which require extremely high safety standards. Despite the good development of LNG storage tanks, potential dangers still exist. Today, we will delve into how to reduce the risks of LNG storage tanks and ensure their safe use.

Safety Design of LNG Storage Tanks

The safety design of LNG storage tanks is crucial for ensuring their safe operation during storage and transportation. Since LNG is a flammable and explosive hazardous gas, the design of the storage tanks must meet extremely high safety standards. The following are the main aspects of the safety design of LNG storage tanks.

1. Robust Outer Shell

The outer shell of LNG storage tanks is constructed from stainless steel for several reasons. The stainless steel outer shell can temporarily contain any leaked LNG liquid, preventing it from spilling out. Moreover, it has a higher fire resistance than ordinary carbon steel, stronger impact and collision resistance, and better corrosion resistance. These characteristics can effectively extend the service life of the tank and provide the first line of defense for the safe use of LNG storage tanks.

2. Protective Role of the Cold Box

Key components of the LNG storage tank, such as joints, valves, and vaporizers, should be located within a highly airtight cold box. The purpose of the cold box is to contain any leaked LNG and prevent the diffusion of flammable gases, thereby reducing the hazardous area. This design can effectively lower the probability of leakage accidents, and even if a leak occurs, it can minimize the potential harm.

3. Advantages of Integrated Design

Adopting an integrated design approach, which involves the overall design and manufacturing of LNG storage tanks, cold boxes, heat exchangers, machine bases, and water curtain protection pipelines, forms a complete "gas supply unit." This design not only facilitates overall risk control but also simplifies product approval. Through integrated design, the coordinated operation between various components is more efficient, further enhancing the safety of the entire system.

Daily Usage Precautions for LNG Storage Tanks

The safe use of LNG storage tanks relies not only on the quality of their design and manufacturing but also on the management and maintenance during daily use. Proper operation and regular inspections can effectively reduce the risk of accidents and ensure the safe operation of the tanks. The following are some key daily usage precautions.

1. Management of Liquid Oxygen

LNG storage tanks are primarily used for gas storage. To avoid problems during use, special attention must be paid to the management of liquid oxygen. First, the acetylene concentration in liquid oxygen should be regularly analyzed and kept below 0.1×10⁻⁶. If the concentration exceeds this limit, the liquid oxygen must be immediately discharged. Second, when using LNG storage tanks for the closed storage of liquid oxygen, pressure must be monitored to prevent overpressure. Additionally, liquid oxygen should not splash onto unprotected skin to avoid severe frostbite. When the tank has been emptied of all liquid but cannot be immediately heated, all valves must be closed immediately. This is because the temperature inside the tank is very low, and moist air can enter through connected pipes, causing ice blockages in the pipes.

2. Evaporation and Replenishment

During storage, LNG will evaporate due to the ingress of external heat. Low-boiling-point components evaporate before high-boiling-point components, so the composition of LNG in the low-temperature storage tank will change slightly over time, with an increase in the content of heavier hydrocarbons. Generally, the rate of LNG evaporation caused by external heat ingress is very low, and the evaporated natural gas can be re-liquefied or fed into the urban natural gas pipeline network.

When the LNG in the tank is almost depleted, it needs to be replenished in a timely manner. Since the density, vapor pressure, and other parameters of LNG are temperature-dependent, when the density of the newly injected LNG differs from that of the remaining LNG in the tank, a temporary stratification phenomenon will occur. Due to heat and mass transfer between layers, the layers gradually mix, and the evaporation rate of LNG also changes during this process. Under certain specific conditions, the evaporation rate can increase dramatically, causing a rollover phenomenon that leads to the pressure inside the LNG storage tank exceeding the design requirements.

3. Prevention of Rolling Phenomenon

LNG rolling is a violent evaporation process. Stratification in the tank can suppress heat transfer to lower layers, causing the lower LNG to become supersaturated. If the upper layer is denser than the lower, the lower layer may rise suddenly, leading to rapid evaporation. Without preventive measures, this can cause serious accidents.

For example, if an LNG tank contains remaining LNG with a different composition than LNG delivered by ship, slight temperature differences during side-bottom filling may prevent mixing. Stratification forms with low-density, cooler LNG on top and higher-density, warmer LNG on the bottom. Convection and heat transfer gradually equalize the temperatures, while methane in the upper layer continues to evaporate. When the upper layer becomes denser than the lower, it sinks, causing supersaturated lower LNG to rise and methane to evaporate rapidly. If the newly added LNG volume is large, methane may not vent quickly enough, raising pressure beyond the tank’s safety limits.

Preventive measures include:

Light LNG fed from the tank bottom or heavy LNG from the tank top, or a combination;

Installing automatic density meters to monitor different layers;

Using internal pumps to circulate liquid from bottom to top;

Maintaining nitrogen content below 1% and closely monitoring vaporization rates.

Response Measures for LNG Storage Tank Leaks and Spills

Although various safety measures have been taken in the design and use of LNG storage tanks, the risk of leaks and spills still exists. Therefore, it is crucial to understand how to respond to leak and spill accidents, which can not only protect personnel safety but also minimize damage to the environment and equipment.

1. Prevention of Leaks

When planning LNG storage tanks, the potential hazards of LNG leaks or spills during storage and handling to staff and major equipment should be fully considered. If an LNG spill or leak occurs, the flow, diffusion, or entry of LNG into sewers should be strictly controlled to prevent it from rapidly vaporizing and mixing with air to form flammable and explosive mixtures.

2. Control of Leaks

In the event of an LNG storage tank leak, facilities should be set up around the tank and in the loading and unloading area to prevent the spread of LNG in all directions, thereby minimizing the harm caused by LNG leaks. For small LNG storage tanks, common methods include setting up barriers around the tank, such as dikes, retaining walls, or liquid collection pits. For valves and joints that may leak, baffles should be installed to prevent LNG from spraying; a liquid collection pan should be placed below to collect the leaked LNG and direct it into the liquid collection pit through a liquid discharge pipe. When LNG overflows or leaks, it can be contained within dikes, retaining walls, or liquid collection pits to prevent it from spreading everywhere.

For large LNG storage tanks, it is best to build the tank underground so that the highest liquid level inside the tank is below ground level. The liquid collection capacity of dikes, retaining walls, or liquid collection pits should primarily be considered based on the maximum capacity of the tank, while also fully taking into account the impact of winter snow accumulation or other factors. Therefore, they should have a certain margin of liquid collection capacity, and their capacity should be greater than the total volume of LNG when the tank is fully loaded.

Conclusion

The safe use of LNG storage tanks is a systematic project involving design, use, and emergency response. By adopting measures such as stainless steel shells, cold box protection, and integrated design, we can improve the safety of LNG storage tanks from the source. In daily use, strictly controlling the concentration of liquid oxygen, monitoring pressure, preventing liquid oxygen from splashing onto the skin, and promptly closing valves can effectively prevent accidents. At the same time, timely replenishment of LNG, prevention of stratification, and prevention of rollover can ensure the stability of LNG storage tanks during use. In the face of leaks or spills, taking effective control measures such as setting up barriers and liquid collection pans can minimize the harm. Only in this way can we ensure the safe use of LNG storage tanks and provide a reliable guarantee for the development of the energy sector.