Digestion Cover: Fixed
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Fixed covers are used in digesters with no variation in total volume, and are often used on the primary digester in a two-stage system. The sludge volume can be varied, but only by varying the gas volume at the same time so as to keep the total volume constant. If gas is to be pressurized for use, it must also have a companion floating cover connected by gas lines.
Fixed covers are anchored to the top of the digester tank, and normally there is a static or constant sludge level. Thus the rim plate only need be as deep as the tank freeboard plus the tank operating pressure and an appropriate seal depth to contain the gas. A typical rim plate depth might be approximately 2'6" with 1'0" freeboard. A fixed cover has only one steel membrane over its structural framework. -
The following features are used with this type of cover:
A gas dome located at the center of the cover provides a gas reservoir from which gas can be withdrawn without also removing entrained liquid, scum, foam, etc.Sampling wells, usually one near the center, one at about 1/2 radius. These wells have quick-opening hatches, with seal pipe extending below liquid, so that samples can be withdrawn without losing gas.
Pressure relief and vacuum-breaker valve, mounted atop the gas dome for the purpose of relieving gas upon over-pressurization, and allowing atmospheric air to enter if vacuum exists when cover on corbels. Basically, this is a safety device, protecting the structure.
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The advantage to this type of cover is lowest cost. The disadvantages are that it will not pressurize and store gas by itself, will not depress scum, and provides little insulating value.