This invention relates to the pumping of a fluid medium, for example liquid or suspension, containing gas. The invention, more precisely, relates to a method and a device for removing the gas from the medium in a pump.

When pumping a medium containing gas, for example pulp suspensions containing air or another gas, by means of centrifugal pumps, the pumping is rendered more difficult in that gas is collected centrally in the pump in front of the impeller. A gas bubble formed thereby in¬ creases in size until the pump ceases to operate.

In order to eliminate this problem, the centrifugal pumps have been designed with impellers, which are provided with openings for evacu¬ ating the gas flow from the central portion of the pump and further through evacuation channels out of the pump. According to known tech¬ nique, usually vacuum pumps are used for evacuating the gas. The gas thus to be evacuated is requested to be practically free of impurities so as to limit the wear on the vacuum pumps.

Especially at the pumping of pulp suspensions problems arise by pulp following along with the gas. This applies particularly to the pumping of pulp at low concentration. The pulp concentration normally is in the range 1-20%. A further problem is that the pulp suspension flowing through cannot be pumped back to the container, to which the pulp pump is connected. A normal vacuum pump cannot manage high temperatures, either.

The present invention has the object to solve the aforesaid problems by using an ejector for evacuating the gas from the medium in the pump.

The characterizing features of the invention are apparent from the attached claims.

By diverting a partial flow of the pumped medium after the pump, i.e. in the pressure line of the pump, and causing this partial flow to drive an ejector, the suction side of which is coupled to the evacu¬ ation channels of the pump, the gas can effectively be removed from the medium in the pump. The partial flow can thereafter be recycled to the container from which the medium is pumped.

The gas-containing medium can consist, for example, of pulp suspens¬ ions or liquor from the pulp manufacturing process. Liquor pumps are employed in digester houses, screen rooms, washer rooms, bleach plants etc.

The invention is described in greater detail in the following, with reference to the accompanying drawings illustrating some embodiments of the invention.

Figs. 1-4 show different systems for degassing pulp suspensions. Figs. 5-6 show different systems for degassing liquor.

At the embodiment according to Figs. 1-4 a chest 1 is provided for a pulp suspension. The pulp is supplied to the chest through a feed line 2. At the top of the chest 1 a degassing valve 3 is located. The chest further comprises an agitator 4. An outlet line 5 for the pulp is connected to the lower portion of the chest 1. The pulp is pumped by a centrifugal pump 6 from the chest through the outlet line 5 and further through the pressure line 7 of the pump to subsequent proc¬ essing stages, which in this case consist of a press 8 for dewatering and in combination with washing. An ejector 9 is coupled with its suction side 10 to the pump 6 via a suction line. 11. The ejector effect is obtained in known manner in that a drive medium passes through the ejector from an inlet 12 to an outlet 13.

According to Fig. 1, a branch line 14 is provided from the pressure line 7 to the inlet 12 of the ejector 9, and a return line 15 from the outlet 13 of the ejector back to the chest 1. The return line 15 is

connected to the chest 1 directly in front of the outlet line 5 and agitator 4. Said agitator 4 is formed with a propeller 16, which effects a flow along the opposed wall.which is provided with an inclined portion 17. This implies, that the ejector 9 is driven by a partial flow of the medium, so that this partial flow via the suction line 11 sucks with it gas from the pump 6. The gas-enriched partial flow is returned to a portion of the chest 1 where the pulp flows upward due to the agitator effect. At the top of the chest a substantial part of the gas evacuates through the valve 3.

According to Fig. 2, the return line 15 from the ejector 9 is conn¬ ected to the chest 1 above the outlet line 5 and agitator 4. In- this case the rotation direction of the propeller 16 of the agitator 4 is reverse, so that the upward flowing pulp flow in the chest passes past the connection of the return line 15. According to this embodim¬ ent, the line laying is more advantageous, because the ejector 9 is located directly above the pump 6 and agitator 4.

According to Fig. 3, the chest 1 is provided with an extra agitator 18, which can be necessary at large chests. In other respects, the arrangement corresponds to the embodiment shown in Fig. 2.

At high pulp temperatures it may be necessary to partially or entirely replace the partial flow of the pulp suspension by another liquid with lower temperature, preferably liquor used for diluting the pulp.

According to Fig. 4, a container 19 for liquor is used for diluting the pulp. The liquor is pumped by a pump 40 through a line 22 to the chest 1 at the agitator 4 and to the outlet line 5. In this case the ejector 9 is driven ba a partial flow of liquor via a branch line 22a from the dilution liquor line 22. In other respects, the arrange¬ ment corresponds to the embodiment shown in Fig. 2.

According to Fig. 5, a container 19 with liquor is used for diluting the pulp. The liquor arrives via a line 20 from the press 8 where

the liquor is pressed out of the pulp. The liquor used for diluting the pulp is pumped ba a degassable centrifugal pump 21. Dilution liquor is there directed through a line 22 to the chest 1 at the agitator 4 and to the outlet line 5. In this case an ejector 23 is driven by a partial flow of liquor via a branch line 24 from the dilution liquor line 22. Degassing of the liquor in the pump 21 takes place by the ejectόrrvia a suction line 25. A return line 26 for the partial liquor flow through the ejector is connected to the liquor container 19 above the..'maximum liquor level. The liquor container is at the top provided with a degassing valve 27.

According to Fig. 6, a glass gauge 28 for filtrate from the washing press 8 is located below the press. A degassable liquor pump 29 can there be connected to the glass gauge 28 with an accessory ejector 30 for degassing via a suction line 31. The liquor pump 29 pumps the liquor through a line 32 to the liquor container 19. This implies, that necessary filtrate flow from the press 8 can be obtained without any great difference in level.

In this case the ejector 30 is driven by a partial flow of the liquor via a branch line 33 from the line 32. A return line 34 for the partial flow through the ejector 30 is connected to a line 35 for degassing the glass gauge 28. This line 35 is connected to the liquor container 19 above the maximum liquor level. This arrangement further implies that the size of the liquor container 19 cen be reduced substantially.

The invention, of course, is not restricted to the embodiments described above, but can be varied within, the scope of the invention idea.