principle of a rapidly rotating impeller located in the liquid flow. They provide high delivery
rates and are very robust, i.e. the internals are exposed to little mechanical stress. They do,
however, require a free-flowing intake arrangement, because they are not self-priming
(regenerative).
Data of pumps
Practically all centrifugal pump characteristics are geared to water. They show the delivery
rates for various heads, the achievable efficiency levels, and the power requirement for the
pump motor. Consequently, such data cannot be directly applied to biogas systems, since
the overall performance and efficiency level of a pump for re-circulating slurry may suffer a
serious drop-off as compared to its standard "water" rating (roughly 5-10%).
Substrate
Sometimes, namely when the substrate is excessively viscous, a centrifugal pump will no
longer do the job, because the condition of the substrate surpasses the pump’s physical
delivery capacity. In such cases, one must turn to a so-called positive-displacement or
reciprocating type of pump in the form of a piston pump, gear pump or eccentric spiral pump,
all of which operate on the principle of displacing action to provide positive delivery via one or
more enclosed chambers.
Positive displacement pumps
Positive displacement pumps offer multiple advantages. Even for highly viscous substrate,
they provide high delivery and high efficiency at a relatively low rate of power consumption.
Their characteristics - once again for water - demonstrate how little the delivery rate depends
on the delivery head. Consequently, most of the characteristics show the delivery rate as a
function of pump speed.
The main disadvantage as compared to a centrifugal pump is the greater amount of wear
and tear on the internal occasioned by the necessity of providing an effective seal between
each two adjacent chambers.
Pump delivery lines
Pump delivery lines can be made of steel, PVC (rigid) or PE (rigid or flexible), as well as
appropriate flexible pressure tubing made of reinforced plastic or rubber. Solid substrate, e.g.
dung, can also be handled via conveyor belt, worm conveyor or sliding-bar system, though
none of these could be used for liquid manure. When liquid manure is conducted through an
open gutter, small weirs or barrages should be installed at intervals of 20-30 m as a means
of breaking up the scum layer.
Each such barrier should cause the scum to fall at least 20-30 cm on the downstream side.
All changes of direction should be executed at right angles (90°). Depending on the overall
length, the cross gutter should be laid some 30-50 cm deeper than the main gutter.
Transitions between a rectangular channel and a round pipe must be gradual. An inclination
of about 14% yields optimum flow conditions. The channel bottom must be laid level, since
any slope in the direction of flow would only cause the liquid manure to run off prematurely.
All wall surfaces should be as smooth as possible.
Weak ring
Position of the weak ring
The weak/strong ring improves the gas-tightness of fixed-dome plants. It was first introduced
in Tanzania and showed promising results. The weak ring separates the lower part of the
hemispherical digester, (filled with digesting substrate), from the upper part (where the gas is
stored). Vertical cracks, moving upwards from the bottom of the digester, are diverted in this
ring of lean mortar into horizontal cracks. These cracks remain in the slurry area where they
are of no harm to the gas-tightness. The strong ring is a reinforcement of the bottom of the
gas-holder, it could also be seen as a foundation of the gas-holder. It is an additional device
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