Dry Gas Seals tame extreme cold

EagleBurgmann, a leading global manufacturer of mechanical seals for industry, has developed a class of dry gas seals for the extreme cold operating conditions of liquid natural gas (LNG) production and shipping. These seals, designed for long-life reliability down to -170 °C operating temperatures, where liquefaction of natural gas occurs, have passed rigorous testing and been approved by the major manufacturers of LNG compressors.

The global abundance of natural gas juxtaposed against the high cost of liquefying and regasifying it for shipping to overseas markets is driving producers towards ever-larger economies of scale in LNG production. Manufacturers of LNG production equipment are scaling up their products accordingly. EagleBurgmann’s LNG family includes seals for ‘super sized’ compressors being developed for the largest LNG plants. The biggest seal, with a nominal diameter of 390 millimeters, is the largest dry gas seal in the market, validated for temperatures as low as -170°C. This seal employs a similar design to that of a 390-mm seal EagleBurgmann developed for a steel-mill cogeneration plant in China that is still the world’s largest dry gas seal and rated for temperatures up to 250°C which is in successful operation since end of 2009. The unique EagleBurgmann seal design allows wide ranges of operating temperatures and provides the industry the reliability required. Creating ‘super sized’ seals play a critical role in the scaling up of compressor capacity. This Dry Gas Seal technology ensures a reliable sealing of a compressor even with extreme operating conditions. LNG operators can expect a maximum in safety, environmental safety and compressor uptime by using this type of sealing solutions provided by EagleBurgmann.

EagleBurgmann’s participation in developing seals for LNG production goes back a decade to designing seals for boil-off gas re-liquefaction companders on LNG vessels. Seals installed on LNG vessels need to cover the extreme operation in geared compressors in addition to the extreme temperature of -170°C. Full-scale development of dry gas seals with a nominal size of 390 mm started in 2005 as part of a qualification program for a major manufacturer’s compressors. Designing and manufacturing these very large LNG seals presented particular challenges. They had to be a special design; scaling up a smaller, standard seal wouldn’t work. The design, as well as the selection of materials, had to take into consideration the distortions under these operating conditions, to ensure even under extreme temperature ranges a reliable sealing function.

To assure zero leakage, EagleBurgmann engineers selected a tandem concept with intermediate labyrinth. PTFE sealing elements were used as secondary sealing elements and ensure together with the proven EagleBurgmann design of the dynamic sealing element the high operational safety even with high temperature fluctuations. The sliding faces consist of silicon carbide together with a DLC (diamond like carbon) coating. The selection of silicon carbide against silicon carbide for sliding faces is essential for a reliable operation with dry nitrogen. Alternative materials such as carbon tend to extreme wear under these operational conditions and a lower reliability as a consequence. With their high rotating mass, these huge compressors take a long time to come to rest so each seal has to be robust enough to withstand a significant period of slow-speed operation when the normally separated seal face elements begin touching. EagleBurgmann’s extensive design experience provided the basis for a robust design that was fine-tuned and verified through extensive testing to stimulate operating conditions in the LNG environment. EagleBurgmann manufactures seals for the full range of LNG process machinery: larger diameters for compressors and smaller diameters for equipment used in production and on-board LNG tankers. The company is looking forward to building even larger seals tomorrow, since the downward pressure on production costs is likely to increase as countries with surplus gas ramp up competition for export sales.