Glass Sealing

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Sealing glass to metal creates a hermetic seal between two dissimilar materials. Choosing the proper sealing method is critical for designing sight glasses and viewports. Some of the main sealing methods are:

  • – Fusing glass to metal
  • – Bonding sapphire or quartz to metal with adhesives
  • – Brazing sapphire or quartz to metal
  • – O-ring seals
  • – Metal seals, typically C-seals

Borosilicate glass is fused to a stainless steel housing

Fused sight glass. Notice no other materials are involved

It seems logical for the glass to flow out from the metal housing at temperature and pressure as the metal expands when hot. This is a very reasonable engineering observation. However, this does not happen because the housing is compressing the glass at its maximum yield stress. This is a very strong compression. In other words, the metal housing is under constant hoop stress and when the housing expands at temperature, there is no dimensional expansion, only relaxation of the hoop stress. Dimensionally the housing maintains the same internal diameter as when it’s cold. Only at relatively high temperature the housing completely relaxes the hoop stress and then starts to increase its I.D., and O.D.During sight glass fabrication the glass and the metal housing, both, are heated above 950 degrees C. The glass melts inside the housing and fuses to the metal wall. The glass and the metal become inseparable, this is the effect of molecular tension of liquid glass and the roughened surface of the inner metal wall. On the cooling down cycle, the glass anneals and becomes solid, now permanently fused to the housing. At this point the temperature is still above 700 degrees C. As cooling continues, the metal now collapses onto the solid glass and develops a permanent hoop stress.

The final product thus becomes as strong as the metal. Fused sight glasses are the strongest of any kind. Assemblies with O-rings, gaskets, thick glass, are an order of magnitude weaker to rupture than fused sight glasses. These have been tested at temperate and pressure applied at the same time. The proof pressure is approximately 4X of the rated pressure at the maximum rated temperature. At room temperature these have been tested to 9,000 psi with no damage. At over 9,000 psi the glass develops cracks, but still stays in place, does not fly out. There is no sudden glass failure as with the assembled types.

Bonding glass to metal

Glass bonded to a metal housing with an adhesive

Bonding glass to metal is a popular method for applications requiring low cost alternatives to brazing. Another advantage of organic sealing is relative elasticity. Organic bonding materials and can create a stronger seal than brazing because of forgiving CTE mismatch between crystalline window and the metal housing. Bonding materials are available meeting the FDA requirements for food applications, 3-A Standards, or high-strength requirements for high pressure and high temperature applications in downhole cameras.

Brazing sapphire or quartz into metal

Sapphire or a Quartz window is brazed to a Kovar sleeve, which is welded to the metal flange.

Brazing sapphire or quartz into metal or other crystalline housings is designed for high temperature applications. Also, brazing quartz and sapphire into housings is used for vacuum application where clean He-leak-tight seals are required. For operating temperatures are up to 1000°C quartz or sapphire can be brazed directly into a housing made of alumina, or yttria stabilized zirconia, (YAG-stabilized zirconia). The most common sight glasses with brazed windows have a metal housing, not a crystalline housing. Typically, quartz or sapphire is metallized with Titanium and then subsequently brazed with a silver-based material into a Kovar sleeve. Then the sleeve with the brazed window is welded into a metal housing, typically Inconel 718, which retains its strength up to 700°C. An intermediate Kovar sleeve is required to accommodate a CTE mismatch between the housing and the crystalline windows materials.

Sapphire is brazed to Niobium metal.

Sapphire window is brazed to Niobium

Brazing process of sapphire into Niobium is similar to brazing quartz into Kovar. Because Niobium closely matches CTE of sapphire, the sleeve can be made thicker allowing higher operating pressures of the sight glass assembly.

O-rings sealing glass

Window is sealed and cushioned into the flange with O-rings.

O-rings sealing glass is a simple method widely used in sight glass designs. O-rings sandwich a glass disk and create a seal. For semiconductor or pharmaceutical applications, Viton or Kalrez materials are commonly selected materials for elastomer O-rings for temperatures up to 327°C (Kalrez) and pressures up to 2,000 psi. In plasma chambers elastomer O-rings cannot be used as these materials eventually harden in presence of strong UV light, and therefore lose their sealing ability.

C-seals are a good replacement for elastomer O-rings.

Metal C-Seal seals the glass to a flange.

Crossection of a “C” metal seal is in the shape of the letter “C”, therefore the name. C-seals come is a external pressure or internal pressure variety, where the C facing toward the pressure is the internal pressure seal. C facing away from the center of the seal is for external pressures. For high temperatures, C-seals made of Waspalloy is the preferred choice.

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