First, let's look the differences between LNG and LPG.

1. Names have different meanings

LPG(Liquefied Petroleum Gas) is a colorless, volatile gas that is formed from refinery gases or natural gas (including associated gas from oil fields), which is pressurized, cooled, and liquefied.

LNG(Liquefied Natural Gas) is a low-temperature liquid fuel that can be stored and transported at atmospheric pressure.

CNG(Compressed Natural Gas) usually converts LNG into compressed natural gas after vaporization and compression.

2. Different ingredients

LPG is a mixture of propane and butane, usually with a small amount of propylene and butane.

The components of LNG and CNG are composed of methane, and the properties of the two are the same, and the methane content is the same.

3. Different purity

LPG fuel contains a small amount of pentane, pentene and trace sulfur compounds impurity, emission of gas has a small amount of pollution, emission of the main components are basically the same as the gas emitted by the internal combustion engine.

LNG and CNG are clean fuels with very little pollution, and are preferred as new energy gases by many developed countries.

4. Different applications

LPG was used in the modification of some taxis in the early years, but due to lack of power, it has been gradually phased out. Now there are a lot of households and LPG is the liquid gas used in our daily life.

LNG and CNG will eventually be reduced to low-pressure gaseous states, which are widely used in metallurgy, food, porcelain, various industries, catering, civil, automobile and other fields.

The liquefaction temperature of LPG is generally -40℃, mainly propane and butane. The liquefaction temperature of LNG is generally -162℃, dominated by methane.

How to distinguish LPG and LNG storage tank trailers?

Classification of LPG tanks:

(1) Normal temperature and high pressure storage tank trailer: the tank is usually spherical with high pressure. Storage tank capacity generally does not exceed 10,000m³.

(2) Low temperature and atmospheric pressure storage tank trailer: internal low temperature and atmospheric pressure storage tank is generally in the form of vault tank. There is no great limit on the storage capacity, but it is generally not more than 100,000m³. The low-temperature storage tank trailer is generally designed as a double layer, with the inner layer of low-temperature steel, the outer layer of ordinary carbon steel, and the heat insulation material added in the middle. A low temperature tank capacity is large and low shipping cost.

LNG storage tanks are all low temperature storage tanks, which are also divided into two types:

(1) High-pressure tank trailer: the inner layer is stainless steel, the outer layer is carbon steel, and the middle is vacuum pumped. The maximum capacity of a single tank is 150m³, which is basically adopted by small LNG gasification stations.

(2) Atmospheric tank trailer: It is similar to LPG tank in form. Generally, it is a large storage tank with double-layer mechanism and heat insulation material filled in the middle. Both internal and external tanks are installed and manufactured on site with a long production cycle. The inner layer adopts 9Ni steel. Generally, the volume of a single tank at room temperature is more than 50,000m³. The temperature in the tank should be lowered below -160℃. Submerged arc welding is used for girth and horizontal welds, while manual welding is used for most other welds.

The technological difference between LNG and LPG tanks.

LPG tank welding

Tank materials: The main applications are 09MnNiDR, 15MnV, 15MnVR, CF-62 (07MnNiCrMoVDR), 16MnR (WH510), 07MnNiMoVDR. Gas welding methods include manual arc welding, submerged arc welding, tungsten argon arc welding and gas shielded arc welding. The longitudinal weld of storage tank is welded by hand, and the girth weld is basically submerged arc welding. These are the two main welding methods.

LNG tank welding

Small tanks are usually made of 304 stainless steel, and the welding materials are mainly manual electrode and TIG wire of 308L type.

The large tank is made of 9Ni steel. Submerged arc welding is used for general horizontal and horizontal welding positions to improve efficiency, while manual welding is used for other positions. Welding materials are usually ni-cr-mo welding materials of Ni > 60, such as nicrmo-4 and nicrmo-6. 9Ni steel has a high magnetic sensitivity, so pay attention to magnetic bias blowing during welding. Ac welding equipment is recommended.

The impact test of LNG welded materials shall be conducted at -196℃.

LNG storage tank design is based on the standard specification, LNG storage tank is basically a two-wall full capacity tank structure, which is mainly different from general petrochemical storage tanks in several aspects:

1. It is composed of an inner tank of 9%Ni low temperature steel plate and an outer tank of reinforced concrete cap, prestressed concrete wall and reinforced concrete roof. The design temperature of LNG storage tank is -165℃.

2. The inner tank is provided with rigid foam glass insulation layer, the suspended ceiling is covered with glass fiber insulation layer, and the inner and outer tank walls are filled with pearlite and elastic blanket insulation layer. The elastic blanket defines the extrusion pressure of the outer insulation against the inner tank.

3. The upper part of the wall and bottom plate adopts independent 9%Ni steel liner to form a protection system so that this part will not be affected by low temperature under leakage. This part of the structure constitutes the thermal angle protection system (TCP).

How to choose LNG and LPG tanks trailer?

Cylindrical tank trailers, usually are atmospheric type that used to hold atmospheric liquids. The spherical ones are usually pressure tanks.

For LPG, the boiling point is higher and the pressure is about 2MPa,it can be liquid at room temperature. Therefore, the pressure spherical tank can increase the pressure to 2MPa. There is no need for cooling facilities. It can be stored directly all the time without volatilizing. If the storage pressure is atmospheric (such as 1 atmosphere), the boiling point is -30~-40℃. Then the environment heating gasification, the gas produced must be cooled down with a cooler, otherwise it will be wasted. And it's better keep cool, otherwise the gas generate too much, you have to select a larger cold machine.

As for LNG, it does not liquefied at -162℃ boiling point and 10MPa pressure. It is impossible to make such a high pressure spherical tank trailer, and even if the high-pressure spherical tank is used to storage natural gas, it exists only in gaseous form and has very limited storage volume. With the same storage capacity, atmospheric cylindrical tanks have a great advantage in LNG storage. Like saving money.