Digestion Cover: Gas Holder
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Gas holder covers provide for gas storage by riding on a constant pressure, variable volume gas bubble. Normally this type of cover is used with a constant sludge level and this is the most economical design, and would be applied on a second stage digester.
As a two-stage digestion system, it is desired that all digestion occur in the first tank while the second tank is used for supernatant separation and gas storage. Gas storage is a necessity since most gas burners will operate on an on-off cycle and in this manner the gas holder serves as a "surge" tank.
The rim plate depth on a gas holder cover is determined by the required gas storage volume or travel. Generally, a 5' to 7' deep rim plate is sufficient for most applications. This cover also has one steel membrane over its structural framework. Rollers and long columns are provided for cover travel and to maintain stability throughout its travel range. Note that the ballast is located at the bottom of the skirt to lower its metacenter and increase its stability.
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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A gas dome located at the center of the cover provides a gas reservoir from which gas can be withdrawn without also removing entrained liquid, scum, foam, etc.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 These wells have quick-opening hatches, with seal pipe extending below liquid, so that samples can be withdrawn without losing gas.Pressure relief and vacuum-breaker valve, mounted atop the gas dome for the purpose of relieving gas upon over-pressurization, and allowing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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It is less costly than any cover other than the Fixed cover, and that it stores gas for usage by the boiler or other energy producer. The disadvantage is that it has little insulating value.