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Each electrical penetration for liquefied natural gas application will be tested also for electrical strength and insulation resistance before it is shipped to the customer. Photo: SCHOTT/J. Küsters
Oliver Hahr

Seals for Gas made from Glass

Glass-to-metal feedthroughs from SCHOTT Electronic Packaging seal off the pumps of liquefied gas tankers.

The demand for natural gas is on the rise – and with it the need for appropriate means of transportation. However, because pipelines are expensive, often cross through crisis ­regions and put suppliers in a position to cut off the supply rather easily, the manufacturing and consuming countries are increasingly relying on ships to transport the energy carrier, usually inside four or five huge tanks, after it has been liquefied by reducing its temperature. Around 270 of such tankers are currently sailing on the world’s oceans and more than 130 have already been ordered and should be launched by the year 2011. 45 of these are for Qatar alone, the world’s largest producer of liquefied gas.

In order to be shipped, the gas is liquefied by cooling it down to minus 165 degrees Celsius and then pumped into the tanks on board the ship. This means that 600 times as much gas can be stored in comparison to ambient temperature. While it is being pumped, liquefied gas is subjected to high pressure of up to 150 bar. The housings and internal electric motors of the pumps that are a permanent component of the tank wall are immersed in liquefied gas. For this reason, they must be sealed perfectly, especially where the electrical connections from the deck of the ship lead into the pump.
Countries of origin and consumption are relying more and more on liquefied gas tankers.
Photo: SCHOTT/A. Schneiderwind
Here, SCHOTT relies on a technique called compression seal. ”For this purpose, both the glass isolator and the copper conductor are assembled in a stainless steel housing and then heated up so that all of the elements fuse together,” explains Dr. Oliver Fritz, Technical Manager for Large Scale Feedthroughs at SCHOTT Electronic Packaging in Landshut, Germany. ”As the assembly cools down the glass solidifies and the stainless steel housing contracts to a greater degree than the glass. Due to the differences in the coefficients of thermal expansion of the materials used, the glass isolators are subjected to compression and a hermetic joint is created,” he adds.

Later, when the cold liquefied gas flows through the pump, the stainless steel housing of the feedthrough contracts further and exerts even more pressure on the glass and, thus, helps ensure that it remains hermetic. Unlike the epoxy grouting used by some competitors, the pressure barrier in the compression glass seals that SCHOTT relies on does not contain any organic compounds that age rapidly in response to the severe temperature shifts and, thus, lose their hermeticity. In comparison with hermetic pressure barriers that feature a ceramic-to-metal bond, glass feedthroughs are less likely to experience breakage as ­exposed.

Because feeding electrical conductors through to the submerged pump is a rather sensitive task, SCHOTT adheres to the highest possible safety standards during manufacturing. ”We are capable of supplying all types of products together with ­certification in accordance with the European ATEX standard and the international IEC standard for electrical safety,” explains Ulrich Dirr, Sales Manager for Large Scale Feedthroughs at SCHOTT Electronic Packaging in Landshut. ”Before it is shipped, each and every liquefied gas feedthrough is subjected to one and a half times the maximum required design pressure and then tested for hermeticity by means of helium mass spectrometers. Testing dielectric strength and insulation resistance guarantees reliable performance at voltages of up to 6,600 volts and currents of 600 amperes,” he adds. Due to the technology of compression glass-to-metal sealing that SCHOTT has brought to perfection by optimizing material combinations and manufacturing processes for these extreme applications, the company has captured the lead position in this market. SCHOTT started manufacturing the first glass-to-metal sealed feedthroughs for liquefied gas applications some 25 years ago. These products are still doing their jobs today, without requiring any maintenance or causing any problems.
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