Abstract: A tank cover, especially a cover on a sewage treatment tank, is provided with an access opening with a liftable hatch cover. Along with the hatch cover is a safety barrier or grate, either connected directly to the hatch cover or to structure surrounding the hatch opening. When the hatch is opened the barrier is deployed to prevent a worker from accidentally falling through the opening. In one form the barrier is pivotally attached to the bottom side of the hatch cover, and can be swung out as the hatch is opened to form an “A” shaped or roof shaped barrier, with the hatch cover and the barrier both inclined and essentially forming a roof over the opening. Braces or struts hold these components in the erected position and provide barriers from the sides. For entry of a person through the opening, the barrier or grate can be retained against the bottom of the hatch cover as the cover is swung fully open, through 180°.

Abstract: A hinged hatch in a gas containment cover particularly for a sewage treatment tank has a slide bolt contained within a shaped channel of an extrusion of the hatch, positioned to engage the bolt at a mid-level position of the adjacent fixed beam or other member of the tank cover. The position of the bolt enables the hinged hatch to fully seal against a rubber-covered lip of the adjacent structural member, so that the bolt latch does not compromise integrity of the gas seal. As the bolt is slid into locking position it forces the hatch cover down more tightly against the gas seal.

Abstract: A sheet containing the apertures to be filled is coated with a layer of crystalline material. A localized heat source heats small portions of the layer at a time. The crystalline material in each portion is melted and allowed to flow into the apertures. The localized heat source is then moved to heat a different portion of the crystalline layer. In this fashion the layer of crystalline material is scanned with the localized heat source so that the crystalline material melts, flows into the apertures to be filled and then recrystallizes.