Many different methods and apparatus for separating gas from liquids are known. Most literature on such gas/liquid separation relates to the separation of vapor from gasoline. Very often such publications relate to gas/vapor separation in aircraft fuels. Most devices utilize the known characteristic of centrifugal pumps to separate gaseous medium from the pumped liquid which may otherwise result in a poor performance and uneven operation of the pump. Devices have been developed to cooperate with such kind of pumps for guiding the gas from a location in front of the impeller to a gas discharge without causing any disturbance in the operation of the pump. Relevant examples of this kind of apparatus are, e.g. SE Patent 202 830 and U.S. Pat. Nos. 3,271,933; 3,276,187; 3,290,864 and 4,201,555. These publications relate to a single-stage gas separation, whereby gas is separated from a liquid at least to such an extent that the residual gas will not disturb the pumping process. In some cases the main purpose of the process in addition to pumping is gas discharge, but the centrifugal pump is not able to separate, in one stage, all of the gas contained in the suspension being pumped.
In the pulp industry, however, the presence of air or other gas in the fiber suspension disturbs the proper treatment of suspension, for example, in washing and bleaching processes. At least prior to the paper machine the suspension flow is therefore guided to a large vessel, which is subjected to a reduced pressure for separating the gases in the suspension by permitting the gas to emerge as bubbles at the surface of the suspension in the vessel. Such a vessel is disclosed, for example, in U.S. Pat. No. 2,642,950. On the other hand, oversized storage vessels are also known, for collecting, for example, the filtrate from washers and paper machines. These filtrates will settle in the vessel, and permit the entrained gases to separate and to emerge as bubbles at the surface thereof. Chemicals are also used to prevent foaming as well as any harmful effects of the gas in the liquid.
All these gas discharge methods are not free from problems. Large vessels, both storage vessels and gas discharge vessels require a large amount of space and due to the large size, are very expensive. On the other hand, the emissions from these vessels are harmful, since the gases separated from the filtrate or suspension are rarely clean or pure air, but mostly contain large amounts of chemical vapors. Furthermore, different chemicals, which are used for minimizing the foaming problem, cause environmental damage, because they are entrained in the waste water and easily flow into the sewer system therewith.