Facilities for Liquification of Natural Gas – LNG
1 The value chain
Natural gas is typically fed in through pipelines and compressors transporting the natural gas with high pressure, through thousands of kilometers. In areas where natural gas production is not connected directly to pipeline networks, specially designed ships for liquefied natural gas (“LNG”), is the only distribution channel.
Critical stages of the LNG value chain include:
- Treatment and exploration of natural gas
- The process of liquefaction of natural gas into LNG
- Transporting LNG
- Storing LNG
- Re-gasification process of LNG into natural gas
- Supply
1.1 Liquefaction of natural gas
In order to ship, natural gas must be liquefied. The process in a liquefaction facility involves gas flows running through several heat exchangers, then being cooled in several stages until it obtains its liquid form, at -259°F/-161°C. In the process of liquefying the gas, its volume is reduced by a factor of 600.
During the process, prior liquefaction components of the natural gas are removed; while the amount of these components in the natural gas are negligible, they can still cause damage to the facilities, an example is where they freeze up and break the facilities. Specific components include hydrocarbons, water, carbon dioxide, nitrogen, oxygen and especially sulfur compounds.
1.2 Transporting LNG
For LNG transport double-hulled ships (a tank within a tank) are specifically designed to handle the low temperature of LNG under atmospheric pressure. An inner hull prevents LNG from heating up, expanding and overpressure, while the outer hull prevents the LNG from spreading out into ambient air. At the same time, during loading and transportation, a small amount of LNG will evaporate; this so called “boil-off-gas” can be used for supplemental fuel for the carriers or gets re-liquefied and sent back to the tanks.
There are two main carrier types which differ by their tank design: spherical tanks (A) LNG ships and membrane LNG ships (B). While LNG spherical tank ships have been the most common in the past, the membrane ships have become more and more in demand because their cargo capacity is higher despite the same size.
1.3 Terminal
When LNG ships arrive at the terminals the LNG is loaded or unloaded. If arriving with loaded tanks at the LNG terminals, unloading arms on the dock will be connected to the ship, and the ship’s pump transfers LNG through pipelines to storage tanks.
1.3.1 LNG storage tank
Specifically designed for LNG, storage tanks are placed next to liquefaction, receiving and re-gasification facilities. Once liquified, LNG is stored in the tanks, awaiting shipment on specified LNG ships.
Once reaching their final destinations, the LNG ship is first moored, the unloading arms on the dock connected, at which time the ship’s pumps transfer LNG into onshore LNG storage tanks.
LNG can be stored in two different types of storage tanks: above ground and below ground.
Being economical in construction easy to maintain, above ground tanks are most common worldwide. More expensive and requiring complex maintenance are below ground tanks, which are further comprised of in-ground tanks, Underground tanks and In-pit tanks are variants of below ground tanks. While the roofs of in-ground tanks are above the ground, underground tanks are completely buried under the earth. Underground in-pit tanks have a double metal shell with an inner and outer tank. The inner tank is made of metal with high resistance to low temperature. Additional insulation of thermal insulating materials and dry nitrogen gas fills the space between the inner and outer tanks. LNG is cryogenic which means that it is not necessary to store LNG under pressure.
1.3.2 Re-gasification of LNG into natural gas
Before it is fed in the local pipeline network LNG is re-heated back to its gaseous phase by heat exchangers in re-gasification facilities. LNG is pumped from the storage tanks to the re-gasification facilities, in regasification, different media is used.
In a process called “Open Rack Vaporizer“, sea water is used as a heating source for vaporizing LNG: seawater flows down the outside surface of either an aluminum or stainless steel heat exchanger panel, and vaporizes LNG inside the panel.
In the “Submerged Combustion Vaporizer“ process, a submerged combustion burner uses natural gas to heat water, used as a heat source for vaporizing.
Additional methods for vaporizing LNG include a “Double Tube Vaporizer” (DTV) process, “Plate Fin Vaporizer” (PFV) process, and “Air Fin Vaporizer” (HAV) process.
2 Challenge
Specific regulations, guidelines and standards dictate safety arrangements regarding LNG facilities, some of the standards below address the gas detector systems.
- 49CFR Part 193 (Liquefied Natural Gas Facilities: Federal Safety Standards)
- NFPA 59A (Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG)
- EN 1473 (Installation and equipment for liquefied natural gas – Design of onshore installations)
- ICG Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk)
The greatest potential hazard is when LNG spills and forms a methane-ambient air – vapor cloud, which happens when leakages in facilities and pipes occur. Every facility is vulnerable to this potential danger, when the methane concentration amounts to between 5 Vol.-% and 15 Vol.-% in this mixture, it is flammable. Potential leakage sources include supply lines, flanges and valves, so in order to warn workers onboard as well on land at or in the facilities within time to take action and prevent harm, gas and fire detection systems are necessary.
2.1.1 On board
Gas and fire detection systems are installed in compressor rooms, control rooms and around tanks.
2.1.2 On land
At nearly all facilities, besides smoke detectors, low temperature detectors also gas detectors especially IR sensors for methane detection, open path systems and flame detectors, are deployed. These include
Open Path Systems used at the unloading berth, at the storage tanks and liquefaction facilities; Infra-red point gas detectors used at the air inlet to fire equipment and at building heating, ventilation, and air-conditioning (HVAC) inlets, at storage tanks and in the liquefaction and re-gasification facilities.
For water treatment, electrochemical sensors might be used for O3 and NH3 detection; H2S sensors can be used for detecting H2S releases during the gas treatment before liquefaction, because untreated natural gas contains considerable amounts of H2S. O2 Sensors might be also installed in control rooms and areas where workers stay. Unloading berths are covered by flame detectors.
Evacuation and extinguishing systems will be activated by foam and or water for firefighting and to limit vapor cloud dispersion, in case of leakage or flame detection, so that workers will be warned, as an emergency shut down is activated (ESD). It is important that gas and fire detection systems work properly and reliable, as false alarms lead to enormous costs.
Flame detection
Combustible Gas detection
Controllers