Whether you are in business or needed to transport such things as Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG), you will definitely get a solution for that.
A lot has gone into research, design and the manufacturing of tank trailers used for the transportation of these products. This chapter discusses liquefied natural gas (LNG) tank trailers in detail.
The specific lessons that you will learn about these trailers in the chapter include:
- The description of LNG tank trailers
- The structure of an LNG tank trailer
- Different forms of thermal insulation
Next, there will be discussions about the Liquefied Petroleum Gas (LPG) tank trailers. Some of the aspects of the trailers covered include:
- The applications of LPG tank trailers
- The structure of LPG tanks
Here are the different kinds of tank trailers guide:
- Fuel Tank Trailer Guide – Diesel, Petrol, Oil Tanker Trailers Designs & Specs
- What is a bulk cement / powder pneumatic tank trailer?
- CNG Tank Trailer Guide – What is CNG Tanker?
- Asphalt / Bitumen Tank Trailer Ultimate Guide (Insulated Tank with Heating Device)
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Table of Contents
1. What is an LNG tank trailer?
LNG tank trailers are designed to transport liquified natural gas. The tank trailers arer widely across globe because they are cost-effective, strong and flexible. Presently, there are two main models of LNG tank trailers in the market.
The first model consists of an LNG tank that is installed on the trailer:
The second model consists of an LNG tank container installed on a trailer
The LNG storage tank (low temperature storage tank) is one of the LGN carriers. This type of LNG carrier works in very specific conditions. The large capacity of LNG storage tanks works under the ulta-low temperaure (-162°C).
The trailer is more particular than other types of LNG tank trailers. When tank trailers are transporting LNG, the liquified gas boils and continues to gasify. Several conditions contribute to the continous boiling and gasification.
The conditions include external heat invasion, the impact or filling, and the change in the atmospheric pressure. Any of these conditions or a combination of the conditions will cause the LNG to gasify.
Hence, manufacturers must consider the pressure control in the tank, gasific gas extraction, and refrigeration cooling when designing LNG tanks.
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2. LNG Tank Structure
An LNG tank has eight supporting points between the inner vessel and the outer jacket. The designs of the rear four supporting points and the front four supportings points are different.
The different designs preven the rear tubing breakaway due to the expansion and contraction of the inner vessel. The rear four supporting points are fixed while the front four supporting points have a sliding design.
the connecting support between inner vessel and outer jacket shall be reliable and bear the inertial force load impact below:
- Movement direction, two times of the maximum weight
- The horizontal direction which perpendicular to the movement direction, the maximum weight
- perpendicular direction up, maximum weight
- perpendicular direction down, two times of maximum weight
3. Different forms of thermal insulation
At present, four forms of thermal insulation are used in LNG tank trailer. The four types include:
- Packed heat insulation
- High vacuum heat insulation
- Powder fiber insulation (vacuum)
- High vacuum multi-layer insulation (vacuum)
In this section, we shall introduce you to the highest form of insulation, which is the high vacuum multi-layer insulation. The insulation has an alternate structure that is made of highly reflective and low thermal conductivity material.
The high vacuum multi-layer insulation requires a vacuum to eliminate the gas heat transfer and thermal conductivity.
The multilayer thermal insulation material is composed of two kinds of material. The first kind is the highly reflective and lowly emissive materials such as aluminum foil, or aluminum polyester film.
The second type consists of materials with low thermal conductivity and low deflation rate to function as the spacer material. Examples of such materials include glass fiber cloth, fiber paper, nylon mesh and mesh wire.
Due to the low thermal conductivity in the second type of materials, there is a middle layer between each reflection screen materials and reflective screen materials.
Hence, it can reduce the thermal conductivity of solids effectively. The main advantages of the high vacuum multi-layer insulation include:
- The insulation is super adiabatic
- Low thermal conductivity
- The thermal insulation space required is little
- The effective mass is light
- The insulation is suitable for transporting liquids with a low boiling point
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4. What is an LPG tank trailer?
An LPG tank trailer has a different structure and applications from a LNG trailer. LPG tank trailers are used to transport a variety of liquefied gases including liquid ammonia, propane, propylene, dimethyl ether, isobutat, and butadiene.
Other liquefied gases that can be transported in an LPG tank trailer are methylamine and acetaldehyde. Ideally, the transport maximum working pressure for this transporter is greater than or equal to 0.1MPa.
Additionally, the design temperature is not greater than +50 ℃ liquefied gas, and for the steel tank as well.
5. LPG Trailer Design and Construction
Liquefied gas transport vehicles made from the tank and the chassis of a combination of two parts. The main pressure components are mainly the tank body, head, manhole cover, flanges, bolts and the nominal diameter of 250mm or more take over.
Still for this category is a Low-temperature tanker designed to transport liquid oxygen, liquid nitrogen, liquid hydrogen, liquid argon, liquid carbon dioxide and other media. The tank is made of steel and its external insulation layer and pressure jacket tanker. Such models need to be equipped with safety accessories such as rupture disk device, emergency shut-off device, electrostatic device, safety valves, pressure gauge, liquid level meter and a thermometer unit. The tank can be bare, with insulation or an insulation type. Most importantly, the tanker design, manufacture, use, transport, inspection, repair, renovation must comply with industry standards and traffic management regulations.
The design of an LPG tank puts much into weight considerations. Ideally, the LPG cylinder tanks are heavy in nature. When in full capacity, the weight can be twice as much as the weight of the LPG contents. Considerations are there made based on the following scenarios:
- LPG Tank Trailer
When the design is an LPG tank trailer, manufacturers usually design to ensure:
- Weight balance should enhance stable towing
- The trailer stays within the maximum recommended weight by the manufacturers. Typically, this refers to the gross vehicle weight also commonly known as the permissible maximum weight.
- Again, the nose weight should not surpass the set towing bracket by the manufacturer
- The trailer should not surpass the set towing weight of the vehicle towing it
- The combined weight of the trailer and the towing vehicle should not surpass the recommendations of the manufacturer of the towing vehicle
- Tank on a Converted Vehicle
When the cylinder tank is in a converted vehicle, the following considerations are to be made: The weight should not exceed the Maximum Allowable Mass of the vehicle. This should be interpreted as the gross vehicle weight or the permissible maximum weight.
Positioning of Cylinder Tanks
LPG cylinder tanks are carried either on a converted vehicle or on a trailer. In either scenario, they should be placed in a position that has minimal risk of damage following an accident while in transportation.
Where pipes seem to exit the partition, the annular space must be fire stopped. Again, the it is required that the gas box be made from a fire proof material that allows a fire resistance of 30 minutes.
A warning sign is normally set on the access door showing that an LPG that is highly flammable is carried within.
6. Liquefied Gas Tanker Category
Liquefied gas tankers can be categorized based on the specific use. According to different loading materials, they can be divided into: propane transporter, propylene transporter, dimethyl ether transporter, liquid ammonia transporter, methylamine transporter, acetal transporter and so on.
The LPG tank has a cylindrical shape for two main reasons. The first reason is the high pressure of the liquid cargo. The second is that the manufacturing process of cylindrical tanks is simple.
While all LPG tanks are in cylindrical shapes, there will be noticeable differences between them. Sometimes, the LPG tanks are designed in a cylindrical shape but different diameter. The purpose of this design is to lower the center of gravity for the tanks hence enhancing their stability.
The cylidner is made of cold roll forming while the heads are standard elliptical. All joints on the tanks must go through a full penetratiton type of double-sided welding.
In addition, they must go through a 100% flaw detection. The processes are necessary because the tanks are used to store high-pressure gas hence there should not be any construction defects.
|Material||Yield strength (Mpa)||Permissible stress (Mpa)||Comparison|
|Q235-C||225||125||The permissible stress of these two materials is small. Thus, the wall thickness of the tank is thicker and the empty weight is larger and hence the load capacity is less.|
|16MnR||345||170||The wall thickness is moderate, that is, 10-14mm. It is the commonly used materials in the current manufacture of LPG tanks. The welding technology is mature and the price is suitable.|
|15MnVR||390||177||Performance is similar to 16MnR. However, price of the material is high.|
Four materials are usually used to make LPG tanks. The materials include Q235-C, 20R, 16MnR, and 15MnR. The table below shows the comparison of the four materials, their yield strength, and permissible stress.
From this table, it is clear that 16MnR is the most appropriate material to use. Under the -40° C impact test, the Akv of the 16MnR is greater than 21J.
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7. Principle of loading an LPG Tank Trailer
The loading process of a LPG tank trailer is a transfer process that takes place between the LPG storage tank and the LPG transport truck. The process involves the use of a sliding vane pump to unload the storage tank while at the same time filling in the transport truck. Here is a detailed process on how this happens:
- Before starting off the process, equalization of pressure between the two tanks is done to facilitate the transfer of the liquefied gas.
- The sliding vane pump is then started and works by pulling the liquefied petroleum gas from the bottom of the storage tank through a delivery pipe and delivers the gas through the bottom of the transport tank
- As the transporter tank gets filled with liquefied gas, accumulated vapor is passively transferred from the top of the transporter tank to the top of the storage tank. This ensures that pressure is kept in balance during the transfer process.
- The liquefied petroleum gas transfer process is complete when the transport tank is filled to desired volume
The choice of a good LNG and/or LPG tank trailer is pegged upon the buyer’s knowledge of what goes into the making of such transportation equipment.
A lot of the design work is entirely dependent on the nature of the product to be transported. This is a very delicate transportation process and therefore, extra care should be employed when working around the different aspects of the tank trailer.
Their shape, which is cylindrical in nature should be maintained throughout for a safe and secure transport procedure. When buying your LNG or LPG tank trailer get into the detailed design by asking for drawings from the manufacturer to ensure the safety nature of your equipment.
Make sure that all the safety apparatus work to facilitate the transfer process of your liquefied gas from the storage tank to the transporter tank.
Detailed here is a simple guideline to help you understand about the basics of an LPG and LNG tank. Make use of this information to identify the best tank trailer for transporting liquefied gases from one point to another.