Labyrinth-piston compressors (Laby) manufactured by Burckhardt Compression AG are successfully employed for the recovery of boil-off gaswhich occurs during transport and storage of liquefied natural gas.
The materials and construction of the machines utilised for the compression of these gases must be suitable for temperatures of down to minus 160¢ªC.
During the transport and storage of liquid natural gaspart of the liquid evaporates whichin the interests of safetyhas to be reliquefied to maintain constant pressure in the storage tank. This is accomplished economically with oil-free piston compressors. The technical characteristics of such installations – not only with regard to their dimensions but also the standard of technical development – are unusual.
A compressor for the gaseous evaporation products of liquefied natural gas has to cope with extreme conditions; such as:
- Lowest temperatures: At atmospheric pressureliquefied natural gas has a boiling point of -160°Cwhich is considerably below the limit at which material may become unserviceable.
- No moisture: The boil-off gas is free of water vapourbecause its dew point is -160°C. Without moisture and lubricationthe wear increases significantly. In the case of ordinary piston compressorsfor which dry-running and self-lubricating materials are in usethe mechanical and thermal limitations have to be considered.
When the labyrinth-piston sealing principle is appliedhoweverthere are no tribological restrictionsand there is more freedom in the choice of materials to optimise the compressor design.
The first labyrinth-piston compressor designed to cope with these severe conditions was installed in a terminal in the Middle East in 1985. It is still running to the full satisfaction of the operating company and confirms the excellent design of this compressor.
On the basis of this experiencea further three identical Labys were installed in a terminal in Taiwan in 1988. As a result of the successful operation of these unitsa contract was awarded for an additional compressor when the terminal was extended. Another two compressors for the boil-off of liquid natural gas were commissioned in a Korean terminal. TodayLNG terminals in SpainPortugalGermanyUKNorwayMexicoUSACanada and China are or will be equipped with labyrinth-piston compressors.
Up to nowthe machine installed in the Middle East has been operating for well over 160000 hourswhich is more than 16 years’ actual operating time. With the scheduled maintenance interval between two and three years it was only necessary to replace parts that are subject to wear such as piston rod packingscrankshaft sealspiston rod guide bearings and valves. The above mentioned seven compressors with a total of 28 pistons have up to now accumulated close to 500000 running hours without a change of a piston.
Low gas temperatures are a challenge for gas compressor design in two different ways:
- Contact with cold gas which impacts the material properties.
- Absence of moisture with apparent influence on the tribological behaviour in the case of non-lubricated systems.
The labyrinth seals in piston compressors offer a logical answer to these problems. Labyrinth-piston compressors have proven successfully at temperatures down to the boiling point of natural gasie -160 °Cunder industrial conditions. The machines can be built in such a way that no leakage gas is emitted to the environment. In operationthey are characterised by their minimum maintenance requirement and outstanding reliability.
Burckhardt Compression AG is based WinterthurSwitzerland. www.burckhardtcompression.com
"
