The real double seal

Pumps that transport LNG require the highest security. When submerged in the LNG carrier, they must be sealed hermetically. The gas pressure and a temperature of -162°C place high demands on their electrical penetrations. Modern LNG carriers can carry up to 264 000 m³ of LNG inside their storage vessels. In order to unload the gas, each of the vessels has powerful electric pumps. Eliminating the need for shaft seals that require frequent maintenance, the pumps and their motors are mostly integrated into a chassis and work completely immersed in liquefied gas. Terminal header assemblies serve as the hermetic feedthroughs for the three-phase electrical power, as well as the control and instrumentation signals.

Small component – Big impact: in order to unload liquefied gas, the storage tanks on board a LNG tanker contain powerful electric pumps. The pumps and their motors are mostly integrated into a chassis and work completely immersed in liquefied gas. Schott Eternaloc^® terminal header assemblies serve as the hermetic feedthroughs for the three-phase electrical power, as well as the control and instrumentation signals. Source: Schott.

Sealing for safety

Terminal headers need to perform two main functions: safely provide electricity to highly susceptible appliances potentially exposed to explosive atmospheres, and maintain the pressure boundary integrity of the containment structure. Epoxy, ceramics and glass are used as insulating materials. However, epoxy seals contain organic substances that age naturally, particularly when exposed to severe temperature fluctuations, and thus require frequent maintenance. An inorganic alternative is ceramic. However, this material cannot be sealed directly to metals such as steel or copper. Brazing or welding is required that often tends to corrode in harsh environments, starting with surface imperfections that can develop into fissures. Due to the lack of compression sealing, ceramic isolators are also more likely to develop surface cracks over time. Such developments can impair the isolator's dielectric function and lower the resistance of the seal following an accident. In addition, loss of tightness may occur.

Conductors in confinement

Glass, in contrast, is a non-ageing inorganic material that can be melted directly to metal. Schott has matched the physical properties of the special glass sealant to the metal housing and conductors in a way that allows for creating a pressure-proof, natural compression seal. The glass-to-metal sealing technology has performed well in the most demanding fields, including LNG applications, hydrogen-cooled generators and nuclear power plants. The preassembled component is heated up to a temperature at which the glass melts to the metal. During the cooling process, the metal housing contracts to a greater extent than the glass, creating a robust, compression-sealed unit that provides excellent pressure-proof hermeticity. Schott Eternaloc^® feedthroughs are resilient to thermo-cycling, thermo-shock, and temperature ranges from -196 to +100°C.

Double-sealed Eternaloc^® terminal header assembly from Schott: the conductors are insulated with two separate glass-to-metal sealed flange modules that are hermetically connected and form a compact double barrier that offers superior safety features on par with electrical penetrations designed for nuclear power applications. Source: Schott.

 

Two in one

Most cryogenic pumps require feedthroughs with a redundant safety barrier. Conventionally, a pipe with two integrated single seal feedthroughs that are connected by cables is used to maintain the pressure integrity in the event of a failure of one of the seals. In close cooperation with its customers, Schott has developed another solution. Two equivalent but completely separated single seals are hermetically connected, achieving a ‘double seal’ (also known as a double barrier feedthrough). This double sealed terminal header assembly has fewer parts, less joints to be sealed and reduced weight and size.

Schott delivers customised single and double-sealed power and C&I terminal headers. All Eternaloc^® products can be certified according to explosion-proof standards (IECEx and local standards such as ATEX, CU TR or KOSHA). Source: Schott.

Compact designs with high safety

The glass is melted to the conductor and the flange over the entire length of the two seals. Glass is not protruding the flange, which makes the compact design possible. In addition, this double sealed terminal header assembly offers the possibility of nitrogen purging for periodic or permanent leakage monitoring and inertisation of both pressure barriers. If not required, the inner chamber can also be closed and certified as a flameproof enclosure. Schott subjects each single and double barrier feedthrough to one and a half times the maximum design pressure, up to 390 bar. It is thoroughly checked for leak tightness with helium mass spectrometers. Testing for electric strength and insulation provides reliable performance. While Eternaloc^® power penetrations are designed for up to 13 800 V and current ratings of up to 1500 amperes, the control and instrumentation penetrations are designed for up to 28 V and current ratings of up to 4 amperes. The glass-to-metal seals also comply with the European ATEX directive and the international IECEx standard, as well as specific local regulations such as those from KOSHA (Korea Occupational Safety and Health Agency) for South Korea or CU TR (Customs Union Technical Regulation) for Russia.

Conclusion

Terminal header assemblies need to maintain the pressure boundary integrity of the containment structure while safely supplying LNG pumps and expanders inside cryogenic vessels with electricity. Schott’s glass-to-metal sealing technology thereby combines the advantages of other sealing materials while eliminating relevant downsides. Glass is inorganic and therefore non-ageing. It does not protrude from the flange and melts directly to the metal to create a lasting hermetic, compression-sealed pressure barrier. This allows compact single and double sealed terminal header assemblies to be designed, which have provided maintenance-free durability and reliability in more than 6000 on and offshore LNG applications since 1985, including the Prelude facility off the coast of Western Australia and the LNG plant in the Russian Arctic.

Eternaloc^® terminal headers for LNG applications are engineered and produced at the Schott location in Landshut, Germany. Source: Schott.

Read the article online at: https://www.lngindustry.com/special-reports/14102015/the-real-double-seal-1451/