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Why initial cooling of cargo tanks required prior loading LNG cargo ?

Liquefied natural gas (LNG) compresses to a small fraction of its original volume (approximately 1/600) under liquefaction. With the amount of flammable material that LNG contains, it has the potential to be an extremely dangerous chemical, if handled improperly.Preparation for loading LNG cargo ,initial Cool down of cargo tanks need extreme caution

Cargo systems are designed to withstand a certain service temperature; if this is below ambient temperature the system has to be cooled down to the temperature of the cargo before cargo transfer. For LNG and ethylene the stress and thermal shock caused by an over-rapid cooldown of the system could cause brittle fracture. Cooldown operations should be carried out carefully in accordance with instructions.

Initial cool down of cargo tanks

Cool down is an operation to pre-cool cargo tanks and lines required before taking on cryogenic LNG. Cargo tank cool down is carried out by spraying LNG through the spray nozzles of each cargo tank, using LNG received from the shore terminal. The cool down operation from an ambient temperature (from a condition after gassing up) to a planned temperature, is called ‘initial cool down’ and is to be differentiated from an ordinary cool down operation carried out on ballast voyage.

Before LNG can be introduced into the cargo system of an LNG vessel, the system, and in particular the cargo tanks, have to be cooled down to a temperature close to that of the LNG which is to be loaded. The reasons for this are as follows:

Vapor generation

If LNG is introduced directly into warm tanks, the LNG will almost immediately turn into vapour. LNG has a liquid to gas expansion ratio 1: 600. Therefore, to enable the liquid to be loaded into the tank at a reasonable loading rate, necessity of large compressors would be required to remove the vapour generated in the process.

By reducing the cargo tank temperature, the amount of heat that is available to transfer into and heat the LNG is minimized. Consequently the amount of vapour generated can be maintained within reasonable limits.

LNG ship at sea
Fig:LNG carrier underway

Cargo tank Material

Most cargo tanks are constructed of stainless steel which is a material, that retains its flexibility and strength characteristics over the temperature range being considered (-180ºC - 50ºC). However problems could occur if the material is subjected to very local and rapid cooling such as when a small droplet of LNG comes into contact with a warm tank wall. Because of the transfer of the heat from the wall into the liquid, the temperature at the particular point will decrease rapidly causing large thermal stresses to arise between the point and the surrounding material. This could lead to stress cracking.

Pipe tower construction

The tower which supports the pipe-work within the tank is constructed of stainless steel bars. If subjected to rapid cooling thermal stress within the material can be excessive, leading to the material cracking.

All three reasons are of equal importance as each, if not carefully controlled, can have a significant impact on the tank structure and overall safety of the vessel.

Lng tank cooling down with liquid from shore

After the cargo system has been gassed up the headers and tanks must be cooled down before loading can commence. The cool down operation follows immediately after the completion of gassing up using the LNG supplied from the terminal.

The rate of cool down is limited for the following reasons, note that although many of the reasons remain similar between Membrane and Moss vessels the Moss vessels have additional requirements that must be complied with.

To avoid excessive pump tower stresses. Vapour generated during the cool down of the tank must remain within the capabilities of the HD compressors, to maintain a tank pressure safely below that release pressure of the safety valves.

On Membrane vessels to remain within the capacity of the Nitrogen system, to maintain the primary and secondary insulation spaces at the required pressure. To stay within the vertical thermal gradients and equatorial cool down rate as specified by the tank manufacturers. This is particularly important on Moss vessels.

LNG is supplied from the terminal to the manifold cool down line and from there directly to the spray header. The various spray valves are operated in order to produce a temperature profile in line with, but not exceeding, the manufacturers’ instructions. Some tanks may require a minimum equatorial temperature before bulk loading may commence; this temperature is to be strictly adhered to.

During the cool down Nitrogen flow to the primary and secondary barriers (particularly on Membrane vessels) will significantly increase. It is essential that the rate of cool down is controlled in order to maintain the pressure in the primary and secondary barriers at the pressure determined by the manufacturer.

Vapour is returned to the terminal via the HD compressors or, if requested in writing by the terminal, may be consumed in the vessels boilers.

Lng tank cooled down with liquid from shore

Fig:Lng tank cooled down with liquid from shore

Related Information:

  1. Drying of Cargo Tanks and preparation for loading LNG cargo

  2. LNG spill risk during marine transportation and hazards associated

  3. How to tackle fire on board LNG ship

  4. Fire fighting plan for LNG cargo

  5. Increased Cargo Capacity for LNG ships & Advantages of the dual fuel diesel electric propulsion

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