The base flange of sightglass unit is welded into or onto the vessel wall; the glass window with gaskets on both sides is firmly held in place by bolted cover flange.
Lumiglas Intrinsically Safe Rectangular Light Series REL-01

The base flange of sightglass unit is welded into or onto the vessel wall; the glass window with gaskets on both sides is firmly held in place by bolted cover flange.