Digestion - Combination Cover
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The combination cover is the least understood of all, and is probably used for the wrong reason in most instances. In reality, this cover combines features of the floating cover and the gasholder. This can be visualized in two manners. The combination gasholder/cover is a floating cover with a longer skirt, columns, and with the ballast moved from the attic to the skirt bottom, and it is a gasholder with a ceiling plate added to the bottom of the truss.
Its use may be the most misunderstood part. Many specifiers will say they desire a gasholder that will suppress scum, therefore, they desire a submerged ceiling plate. But, when gas is stored (which is the majority of the time), the ceiling plate is not submerged and therefore not "submerging scum".
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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A gas dome located at the center of the cover provides a gas reservoir from which gas can be withdrawn without also removing entrained liquid, scum, foam, etc.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 These wells have quick-opening hatches, with seal pipe extending below liquid, so that samples can be withdrawn without losing gas.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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The advantages are that it has better insulating qualities than a gasholder due to existence of an attic, and the trusses are not exposed to the corrosive interior of digester. The disadvantage is that it is the most costly of all covers.