What are LNG & LPG Tank Trailer? – Learn Tankers Specs & Design

Whether you are engaged in commercial activities or need to transport liquefied natural gas (LNG) and liquefied petroleum gas (LPG), you are sure to find a solution at BENCHI Trailer.

A great deal of effort has gone into the research, design and manufacture of the tank trailers used to transport these products. This chapter discusses LNG and LPG tank trailers in detail.

Specifics about these trailers in this chapter include:

• Description of LNG tank trailers
• Structure of LNG tank trailers
• Different forms of insulation

Liquefied petroleum gas (LPG) tank trailers are discussed next.
Some of the contents of the trailer include:

• Applications of LPG tank trailers
• Structure of LPG tanks

Catalogue

1. What is an LNG tank trailer?
2. Liquefied natural gas tank structures
3. Different forms of heat insulation
4. What is an LPG tank trailer?
5. Design and structure of LPG trailers
6. LPG tank trailer categories
7. LPG tank trailer loading principle

LNG tank trailers are designed for transporting LNG. These tank trailers are widely used across the globe as they are cost-effective, robust and flexible. Currently, there are two main models of LNG tank trailers available in the market.

The first model consists of LNG tanks mounted on the trailer:

The second model consists of LNG tank containers mounted on trailers:

Liquefied Natural Gas (LNG) storage tanks (cryogenic tanks) are one of the LGN carriers. These LNG carriers operate under very special conditions. Large capacity LNG tanks operate at very low temperatures (-162°C).

This trailer is more special than other types of LNG tank trailers. When the trailer is transporting LNG, the LNG boils and continues to gasify. There are a variety of conditions that cause continued boiling and gasification.

These conditions include external heat intrusion, impingement or filling, and changes in atmospheric pressure. Any one of these conditions or a combination of conditions can cause LNG to gasify.

Therefore, manufacturers must consider pressure control, gasification gas extraction and refrigeration cooling within the tank when designing LNG tanks.

LNG tanks have eight support points between the inner vessel and the outer jacket. The rear four support points and the front four support points have different designs.

The different designs prevent the rear pipework from breaking due to expansion and contraction of the inner vessel. The rear four support points are fixed while the front four support points have a sliding design.

The connecting support between the inner tank and the outer protective jacket must be reliable and able to withstand the impact of the following inertial force loads:

• In the direction of movement: twice the maximum weight.
• Horizontal direction perpendicular to the direction of movement: maximum weight.
• Vertical upwards: maximum weight.
• Vertically downwards: twice the maximum weight.

At present, four forms of thermal insulation are used in LNG tank trailer. The four types include:

1. Packed heat insulation
2. High vacuum heat insulation
3. Powder fiber insulation (vacuum)
4. High vacuum multi-layer insulation (vacuum)

In this section, we will introduce you to the highest level of heat insulation, the high vacuum multilayer insulation. This type of heat insulation has an alternative structure and is made of highly reflective, low thermal conductivity materials.

a. High vacuum multilayer insulation requires a vacuum to eliminate gas heat transfer and heat conduction.
Multilayer heat insulation consists of two materials. The first is a highly reflective, low thermal conductivity material such as aluminium foil or aluminium polyester film.

b. The second type consists of materials with low thermal conductivity and low outgassing, which are used as spacer materials. Such materials include glass fibre cloth, fibre paper, nylon mesh and mesh wire.
Due to the low thermal conductivity of the second type of material, there is an intermediate layer between each piece of reflective screen material and the reflective screen material.

Therefore, it can effectively reduce the thermal conductivity of the solid. The main advantages of high vacuum multilayer heat insulation materials include:

• Super adiabatic heat insulation properties.
• Low thermal conductivity.
• Small space required for heat insulation.
• Low effective mass.
• Heat insulation is suitable for transporting liquids with low boiling points.

LPG tank trailers are different in construction and use from LNG trailers. LPG tank trailers are used to transport a variety of liquefied gases, including liquid ammonia, propane, propylene, dimethyl ether, isobutane and butadiene.

Other liquefied gases that can be transported in LPG tank trailers are methylamine and acetaldehyde. Ideally, this type of trailer is designed to transport a maximum working pressure greater than or equal to 0.1 MPa.

In addition, liquefied gases designed for temperatures not higher than +50 °C, steel tanks as well.

The liquefied gas transport vehicle consists of two parts: the tank and the chassis. The main pressurised parts are tank, head, manhole cover, flange, bolts and receiver with nominal diameter above 250mm.

There are also cryogenic tankers in this category, designed for transporting liquid oxygen, liquid nitrogen, liquid hydrogen, liquid argon, liquid carbon dioxide and other media. The tanks are made of steel with external heat insulation and pressure jackets. Such models are required to be equipped with safety accessories such as rupture disc devices, emergency shut-off devices, electrostatic devices, safety valves, pressure gauges, liquid level gauges and thermometer devices. Tanks can be bare, with heat insulation or heat insulated. Most importantly, tanker trucks must be designed, manufactured, used, transported, inspected, maintained and refurbished in accordance with industry standards and traffic management regulations.

The main consideration in the design of LPG tanks is the weight factor. Ideally, LPG cylinder tanks are heavy. At full capacity, its weight may be twice the weight of the LPG inside. Considerations are based on the following:

1. LPG tank trailer

When designing a trailer for LPG tanks, the manufacturer usually ensures the following:

• The weight balance should contribute to stable towing of the trailer.
• The weight of the trailer should be within the manufacturer’s recommended maximum weight range, usually referred to as the gross vehicle weight, also known as the maximum permissible weight.
• The front weight of the trailer should not exceed the manufacturer’s set weight capacity of the towing bracket.
• The trailer should not exceed the towing weight set by the towing vehicle.
• The combined weight of the trailer and towing vehicle shall not exceed the manufacturer’s recommendation for the towing vehicle.

2. Tanks on converted vehicles

When cylindrical tanks are mounted on converted vehicles, the following shall be considered: their weight shall not exceed the maximum permissible mass of the vehicle, which shall be interpreted as the gross vehicle weight or the maximum permissible weight.

LPG cylindrical tanks may be mounted on a converted vehicle or on a trailer. In either case they shall be placed in a position that minimises the risk in the event of an accident.

Where pipework appears to pass through a bulkhead, the annular space shall be fire blocked. In addition, the gas tank box must be made of fire-resistant material that provides at least 30 minutes of fire resistance.

A warning sign is usually provided on the inlet door indicating the presence of highly flammable LPG.

LPG tanker trucks can be classified according to specific purposes. According to the different materials loaded, they can be divided into: propane transport trailer, propylene transport trailer, dimethyl ether transport trailer, liquid ammonia transport trailer, methylamine transport trailer, acetal transport trailer and so on.

There are two main reasons why LPG tanks are cylindrical. The first reason is the high pressure of the liquid cargo. The second reason is the simplicity of the manufacturing process for cylindrical tanks.

Although all LPG tanks are cylindrical, there can be significant differences between them. Sometimes LPG tanks are designed to be cylindrical but with different diameters. The purpose of this design is to lower the centre of gravity of the tank, thereby increasing its stability.

The cylinders are cold roll-formed, while the heads are standard oval shaped. All tank seams must be fully welded on both sides. In addition, a thorough defect inspection had to be carried out. These processes are necessary because the tanks are used to store high pressure gases and therefore must not contain any structural defects.

Four materials are commonly used in the manufacture of LPG tanks. These are Q235-C, 20R, 16MnR and 15MnR. The table below shows a comparison of these four materials, their yield strengths and allowable stresses.

The table shows that 16MnR is the most suitable material. The Akv of 16MnR is more than 21J in impact test at -40 degree Celsius.

The LPG tank trailer loading process is a transfer process between the LPG storage tank and the LPG transport vehicle. The process uses a slide pump to unload the storage tank while filling the transport vehicle. A detailed description of the process is given below:

1. Before the work begins, a pressure equalisation between the two tanks is required to facilitate the transfer of liquefied gas.
2. Activate the slide pump, which draws the LPG from the bottom of the storage tank through a transfer tube and transfers the gas through the bottom of the transport tank.
3. As the transport tank is filled with liquefied gas, the accumulated vapour is passively transferred from the top of the transport tank to the top of the storage tank. This ensures that a pressure balance is maintained during the transfer process.
4. When the transport tank is filled to the required volume, the LPG transfer process is complete.

The choice of a high-quality liquefied natural gas (LNG) and/or liquefied petroleum gas (LPG) tank trailer depends on the buyer’s knowledge of the manufacturing process of such transport equipment. A great deal of the design work is entirely dependent on the characteristics of the product being transported. This is a very delicate transport process and therefore extra care should be taken when dealing with all aspects of the tank trailer.

Their shape should always be cylindrical to ensure a safe and stable transport procedure. When purchasing an LNG or LPG tank trailer, one should go through its design in detail and ensure the safety features of the equipment by requesting the design drawings from the manufacturer.

Ensure that all safety equipment is working properly to facilitate the process of transferring liquefied gases from the storage tank to the transport tank.

This article provides a simple guide to help you understand the basics of LPG and LNG tanks. Use this information to identify the tank trailer that is best suited to transporting liquefied gases from one location to another.