Degassing and purging LNG tanks:
safe and efficient handling of LNG
Liquefied natural gas (LNG) is becoming increasingly important world-wide as an alternative to conventional fossil fuels – whether for public energy supply, as an energy source in industrial processes or as a fuel in shipping. When LNG tanks are to be refilled, serviced or decommissioned, the tanks must be completely free of gases to create a safe working environment. This not only protects staff, but also the environment from potential hazards caused by the uncontrolled release of gas. When changing products, in preparation of maintenance and repair or during LNG cool down, degassing and purging the LNG fuel tanks is therefore of great importance for safety and environmental protection.
What are LNG tanks?
LNG tanks and LNG fuel tanks are special containers for storing liquefied natural gas that is cooled at around -162 °C. The specially insulated storage tanks ensure optimum cooling so that the liquefied gas stays in its liquid state. The great advantage of LNG: in liquid form, natural gas is much more compact and can be transported efficiently – by ship or truck instead of in pipelines. LNG can also be used as an alternative fuel, for example for ships, as it is less polluting than conventional heavy fuel oil. To ensure that the liquefied natural gas can be stored safely, LNG tanks are very heavily insulated, usually double-walled and made of materials that can withstand extreme temperature fluctuations. They come in various sizes and designs – from small mobile containers to large tank installations on ships or in petrol stations.
What is LNG cool down?
A special feature of the operation of LNG systems is the so-called cool down process. It is necessary before an empty tank can be refilled with LNG. As LNG is stored at such extremely low temperatures, the entire pipework system and the tank itself must be cooled down to a similar temperature. This avoids sudden temperature shocks and large temperature differences, which can lead to stresses or damages to the material. The LNG cool down process can take up to twelve hours.
Why is the degassing and purging of LNG tanks necessary?
The particular challenge with LNG fuel tanks is dealing with the boil-off gas. As LNG is stored close to the vaporization temperature, even a small amount of heat can lead to slight vaporization of the cargo. Despite the insulation of the tanks, it is not possible to completely prevent heat from entering the LNG tank from outside. When some of the LNG changes from a liquid to a gaseous state, so-called boil-off gases are produced – partly during storage and especially during unloading. Residual gases therefore remain after LNG tanks have been emptied. They must be controlled and safely removed before product changes, maintenance work or decommissioning of the tanks. Mobile gas treatment technologies such as degassing and purging are ideal for this task.
Boil-off gases are also produced during LNG cool-down: as a small amount of LNG is deliberately sprayed into the tank, some of the gas vaporizes. The resulting boil-off gas must also be treated in a controlled manner. Otherwise, material stresses, cracks or malfunctions could occur – especially in the sensitive cryogenic components. In addition, oxygen must be removed from LNG fuel tanks before each loading, maintenance work or inspection. To ensure a safe working environment, the oxygen content inside the tank must be reduced. LNG fuel tanks can be purged with nitrogen for this purpose – as an inert gas, nitrogen does not cause any undesirable reactions such as oxidation and displaces the oxygen.
Mobile solution for the degassing and purging of LNG fuel tanks for maximum flexibility and efficiency
The degassing and inertisation of LNG fuel tanks are technically demanding processes and place the highest demands on safety, efficiency and environmental compatibility. To fulfil these requirements, mobile systems and technologies can be used flexibly and directly on-site.
Mobile vapor combustion units can be used for the controlled removal of boil-off and residual gases in LNG tanks. The systems destroy gases, gas mixtures and vapors of the explosion groups IIA, IIB and IIC almost completely and without an open flame at a combustion efficiency of over 99.99%. As the combustion units are mobile and easy to transport as trailers or containers, the systems can be used flexibly at various locations such as filling stations or harbors – without major logistical effort or additional infrastructure. Our technology portfolio also includes mobile vaporizers with nitrogen tanks for the inertisation and purging of LNG fuel tanks. The mobile combustion chambers and nitrogen vaporizers from ETS Degassing can be combined with each other, but can of course also be used individually.
Application insights in the degassing and purging of LNG tanks
In total, we have successfully completed over 3,000 applications to date, including numerous degassings and purgings of LNG tanks. The following applications are examples of our work with LNG tanks and LNG fuel tanks:
- Degassing and purging of LNG fuel tanks: In the Netherlands, we have degassed and purged the LNG fuel tanks of an LNG tanker. A mobile vapor combustion unit with a combustion capacity of 10 MW and a mobile nitrogen vaporizer were used for this application.
- Cool down process of a large cruise ship: In the Port of Rotterdam, the cool down of a large cruise ship had to be carried out on short notice. We were notified on a Friday evening, our operators were already on-site on Saturday morning. The process started around midday and was completed by Sunday night. In addition to a mobile vapor combustion unit and a mobile nitrogen vaporizer, we have also provided additional equipment for degassing and purging during the LNG cool down.
- Degassing a tanker loaded with LNG: In the Port of Amsterdam, an LNG tanker had to be degassed and made gas-free in order to enable the repairs that were to be carried out in the shipyard as a result of storm damage. We have provided two mobile vapor combustion units for this purpose. Following the repairs, we have degassed and purged the ship again to enable the cool-down for refilling the tanks with LNG.
- Degassing of LNG fuel tanks: In another application in the Netherlands, we have carried out the degassing of the LNG fuel tanks on a ship. The mobile nitrogen vaporizer was also used to pre-cool the tank so that it could hold the LNG.
- Decommissioning of an LNG fuel station: In France, we have supported the decommissioning of an LNG fueling station. A 5 MW vapor combustion unit and the mobile nitrogen vaporizer were used in this project to degas and purge the LNG tanks.
Safety and efficiency in handling LNG: key processes for safe operation
LNG as an alternative energy source has many advantages, but also places special demands on technology, safety and operational processes. For example, the cool-down, degassing and purging of LNG fuel tanks are essential for safe and reliable operations. These complex tasks can be carried out flexibly, safely and efficiently applying mobile technologies. Degassing, purging and cool down of LNG tanks are therefore an indispensable part of modern LNG infrastructures and make an important contribution to the safe utilization of an increasingly relevant energy source.