Due to the existence of large reservoirs of natural gas, many national economies use natural gas as an alternative to crude oil, which is becoming steadily more expensive and increasingly scarce. A large percentage of natural gas is transported in liquefied form as liquefied natural gas (LNG). Converting natural gas into a liquid form involves cooling it to below -161C and requires special plant and equipment.
To effectively shut off pipes when handling LNG, valves have to be reliable and their design requires them to be capable of coping with the special physical and chemical properties of this medium. Pump and valve manufacturer KSB has been producing butterfly valves for the entire LNG process chain for several decades and recently, working in co-operation with the companies Technip and Eurodim, KSB has developed an offshore loading system known as Connectis.
LNG properties
The temperature of LNG is extremely low and at these exceptionally cold temperatures most metals become brittle and have little strength. Materials that are especially cold-resistant are therefore essential for manufacturing the mechanical parts and seals. Very few metals fulfil the high standards that guarantee operating reliability, a long service life and functional safety. As every valve undergoes a test with liquid nitrogen (see Figure 1), the valves are required to work safely and properly at temperatures as low as -196C. Accidental leakages of LNG constitute a high risk to the environment due to the large volume of cold liquid and the risk of explosions and fire.
Different requirements in the logistics chain
As well as being able to withstand low temperatures, all components involved in the process must also be able to cope with substantial heat expansions. These occur during the transitional phases of the process of being cooled and then warmed up again to ambient temperature. Another factor which places strain on the valves is the operating pressure and the resulting differential pressure. The pressure classes applied in such systems are ANSI Class 150, 300 and 600, which correspond to 20, 50 and 100 bar. The current trend among valves for these applications is towards compliance with ANSI Class 900 (150 bar) and developments are accordingly already underway. Depending on where they are used, the valves are either flanged or welded into the pipe.
After the natural gas has been purified, it is turned into a liquid state via a complex liquefying train. At the present time, these liquefying trains are located on land. However, the construction of offshore plants is envisaged on floating production storage and offloading (FPSO) tankers for natural gas fields that are located far out at sea (see Figure 2). These offshore plants are capable of handling all pressure classes and a wide range of diameters. At the ports, the LNG is loaded onto tankers via flexible loading arms fitted with valves. Only valves to ANSI pressure class 150 with diameters from 6 to 36 inches (DN 150 to DN 900) are used for this operation and typically each liquefied gas tanker is equipped with 60 to 80 valves.
On arrival at the terminals, the liquid natural gas is fed into huge LNG tanks by means of flexible loading arms. These terminals are currently located on shore and have their own port facilities. In order to save space, but also for safety reasons and to avoid proximity to cities, plans are underway to construct floating storage re-gasification units (FSRU) or offshore terminals several kilometres out to sea. The demands placed on valves for these terminals will be the same as those for liquefying trains.
Double offset and triple offset valves
The design of cryogenic valves is determined by three factors: low temperatures (affecting the materials used), operating pressure (directing the body design) and differential pressure (requiring tight shut-off).
In the case of liquefied gas and the associated temperatures, tight shut-off can only be ensured by a metal seat. This requires three design features which clearly distinguish these butterfly valves from softseated, centred variants. Firstly, the stem is offset relative to its passages through the valve body ie. it is not in the centreline of the seat axis. It is equipped with a graphite joint ring and O-rings. In the event of a fire, this solution ensures that the valve provides tight shut-off for a limited amount of time. Operators are therefore offered a reliably sealing, maintenance-free valve. Secondly, a further offset refers to the stem which is laterally offset relative to the pipe axis. It helps reduce the angle at which the disc is in contact with the seat during closing and opening; this angle is only 35 degrees compared with 70 degrees in the case of a single offset butterfly valve. The contact pressure and wear are thus reduced while the service life is prolonged. Thirdly, to achieve shut-off at even higher pressures, triple offset is required. It refers to the disc’s geometry and reduces the angle at which the seal element is in contact with the seat/disc interface to as little as 5 degrees. Despite higher pressures, this helps increase the valve’s service life. This so-called ‘conical offset’ is employed worldwide by the leading valve manufacturers (see Figure 3). All of these valves are built and tested at KSB’s La Roche-Chalais plant in France, where each valve undergoes a test in a liquid nitrogen bath to check whether it is absolutely leak-tight.
Liquefaction on ships
Natural gas essentially contains methane CH4. In gas form, it is transported in pipelines over thousands of kilometres. The geological (deep sea) and political conditions make it more difficult to construct pipelines, particularly over land where it depends on the goodwill of the countries through which the pipeline crosses. This, therefore, leaves tankers as a transport solution. However, tankers can only be used in an economically viable manner if the volume of gas is reduced through liquefaction by cooling to a temperature of -161C. This reduces the volume by approximately 600 times and the density of the liquid is 423 kilograms per cubic metre – less than half that of water. LNG contains small quantities of other light hydrocarbons (5-10 percent ethane, a little propane and butane). On the tankers, the LNG is subjected to a pressure of 10 bar.
The transport costs can be broken down to represent 60 percent for the liquefaction process, 20 percent for transportation on the tankers and 20 percent for re-gasification and interim storage before the natural gas is fed into the distribution network. A liquefaction plant consumes around 12 percent of the natural gas for the liquefaction process itself.
