NOVELTY - A device for a plant producing biogas from biological material by anaerobic fermentation, comprises a container of a fermenting mass; a cover (4) detachably connected in a fluid-sealing manner to side walls (12) of the container to form a closed first space (43) above the mass; and a fluid-communicating conduit forming a connection between space and surroundings. The conduit portion is provided with a first valve to limit gas pressure above fermenting mass to gas pressure. The first valve is formed by conduit portion that is submerged in a vessel (3) filled with liquid.    USE - For a plant producing biogas, that is gas containing great proportion of methane gas, typically about 60%, from biological material e.g. manure from domestic animals and abattoir waste; for producing methane useful as energy source, as fuel in a plant to produce heat e.g. for heating rooms and/or water.    ADVANTAGE - In the device, gaseous fermentation products liberated from the fermenting mass during the fermentation process are retained in a space above the fermentation mass. The device is reasonable in use, also under different climate conditions in which forest and snow place great demands on biogas plant. In the device, the raw gas is carried to an elevated area within the second space in order thereby to be distributed evenly throughout the space. Whenever there is a need to remove deposited sulfur, the covers and the cloth can easily be removed for cleaning. In order to prevent the liquid in the vessel from freezing, portions of the fluid lines connected to the devices for the supply, circulation and removal of the biological material forming the raw material for the biogas production, can be passed in a submerged state through the entire or parts of the vessel in order thereby to keep the liquid temperature sufficiently high, when heat is transferred from the biological material which exhibits, regardless of the stage of the fermentation process, a relatively high temperature. The covers are kept suspended without the need of using supporting structures to give the container lid sufficient strength to carry its own weight and any additional load.    DETAILED DESCRIPTION - A device for a plant producing biogas from biological material by anaerobic fermentation, comprises a container containing a fermenting mass; a first cover (4) detachably connected in a fluid-sealing manner to side wall(s) (12) of the container to form a closed first space (43) above the fermenting mass; and a fluid-communicating conduit forming a connection between the first space and surroundings. The fluid-communicating conduit portion is provided with a first valve arranged to limit the gas pressure above the fermenting mass to a first gas pressure. The first valve is formed by the conduit portion that is submerged in a vessel (3) filled with liquid. The device has a second cover (5) that is detachably connected in a fluid-sealing manner to the side wall(s), and forms a closed second space (53) above the first space.    DESCRIPTION OF DRAWING(S) - The figures show a sectional view of the container/device for plant producing biogas, and a sectional view of the container in which a conduit has been extended from a first space to the highest portion of a second space.    Vessel (3)    First cover (4)    Second cover (5)    Side walls of container (12)    First space (43)    Second space (53)