Materials regarded as suitable alternatives to standard grades of steel are galvanized sheet
metal, plastics (glass-fiber reinforced plastic (GRP), plastic sheeting) and ferro-cement with a
gas-tight lining. The gas-holders of water-jacket plants have a longer average service life,
particularly when a film of used oil is poured on the water seal to provide impregnation.
Fixed-dome gasholders
A fixed-dome gas-holder can be either the upper part of a hemispherical digester
(CAMARTEC design) or a conical top of a cylindrical digester (e.g. Chinese fixed-dome
plant). In a fixed-dome plant the gas collecting in the upper part of the dome displaces a
corresponding volume of digested slurry. The following aspects must be considered with
regard to design and operation:
• An overflow into and out of the compensation tank must be provided to avoid over-
filling of the plant.
• The gas outlet must be located about 10 cm higher than the overflow level to avoid
plugging up of the gas pipe.
• A gas pressure of 1 m WC or more can develop inside the gas space. Consequently,
the plant must be covered sufficiently with soil to provide an adequate counter-
pressure.
• Special care must be taken to properly close the man hole, which may require to
weigh down the lid with 100 kg or more. The safest method is to secure the lid with
clamps.
The following structural measures are recommended to avoid cracks in the gas-holder:
• The foot of the dome (gas-holder) should be stabilized by letting the foundation slab
project out enough to allow for an outer ring of mortar.
• A rated break/pivot ring should be provided at a point located between 1/2 and 2/3 of
the minimum slurry level. This in order to limit the occurrence or propagation of
cracks in the vicinity of the dome foot and to displace forces through its
stiffening/articulating effect such that tensile forces are reduced around the gas
space. Alternatively, the lowest point of the gas-holder should be reinforced by a steel
ring or the whole gas-holder be reinforced with chicken mesh wire.
Normally, masonry, mortar and concrete are not gas-tight, with or without mortar additives.
Gas-tightness can only be achieved through good, careful workmanship and special
coatings. The main precondition is that masonry and plaster are strong and free of cracks.
Cracked and sandy rendering must be removed. In most cases, a plant with cracked
masonry must be dismantled, because not even the best seal coating can render cracks
permanently gas-tight.
Some tried and proven seal coats and plasters:
• multi-layer bitumen, applied cold (hot application poses the danger of injury by
burns and smoke-poisoning; solvents cause dangerous/explosive vapors). Two to
four thick coats required;
• bitumen with aluminum foil, thin sheets of overlapping aluminum foil applied to the
still-sticky bitumen, followed by the next coat of bitumen;
• plastics, e.g. epoxy resin or acrylic paint; very good but expensive;
• paraffin, diluted with 2-5% kerosene, heated up to 100°C and applied to the
preheated masonry, thus providing an effective (deep) seal. Use kerosene/gas torch
to heat masonry.
• multi-layer cement plaster with water-proof elements
In any case, a pressure test must be carried out before the plant is put in service.
Plastic gas-holders
Gas-holders made of plastic sheeting serve as integrated gas-holders, as separate
balloon/bag-type gas-holders and as integrated gas-transport/storage elements. For plastic
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