Pumping device

Abstract

A device for pumping a pulp suspension and simultaneous abduction of gas from the suspension. The device comprises at least one pulp pump (1) with a housing (2) with inlet (3) and, respectively. outlet (4) for the suspension and an outlet (5) for gas. The gas outlet (5) is connected via a pipe (7) to at least one external vacuum pump (8). The pipe (7) is formed with a substantially vertical portion, the highest point of which is located on a level above the pulp pump (1). A means (10) for the supply of flushing water to the pipe (7) is connected upwardlly in the vertical portion of the pipe (7).

Description

[0001] This invention relates to a device for pumping a pulp suspension and simultaneously abducting gas from the suspension. The invention, more precisely, refers to a pulp pump, which is connected to a vacuum pump for evacuating gas, such as air, from the pulp in the pump.

[0002] Gas following along with the pulp causes problems at pumping by pulp pumps, especially in the form of centrifugal pumps. The gas in the pulp cumulates to bubbles, which accumulate in front of the impeller and thereby deteriorates the capacity of the pump. This problem can be removed by evacuating the gas from the space in front of the impeller of the centrifugal pump to a sealing space behind the impeller and further out of the housing of the pulp pump, for example by means of vacuum pumps of the liquid ring type. These vacuum pumps often are separate pumps, the suction pipe of which is connected to the sealing space in the housing of the pulp pump. Alternatively, they can be designed as a separate vacuum impeller in the form of a liquid ring impeller located on the shaft of the pulp pump, so that common operation for both impellers is obtained. In the last mentioned case, however, the capacity of the vacuum pump is restricted for space reasons.

[0003] A system with external vacuum pump comprises a pulp pump, which by a pipe is coupled to a vacuum pump for evacuation of gas. Such a pulp pump is disclosed, for example, in the patent specification SE 467 466. The pipe usually includes a stop valve and a vacuum control means. Normally flushing water is supplied to the pulp pump. The flushing water has the object to prevent clogging of the pipe to the vacuum pump and to dilute the pulp in the space behind the pulp impeller. The stop valve is controlled by a control valve for the pulp pump, so that the stop valve is opened at a predetermined setting of the control valve of the pulp pump. The vacuum control means is set so that at normal process conditions stable conditions are achieved.

[0004] Such a system has the disadvantage that a definite vacuum must be set in spite of that the process conditions can vary. This implies, that an optimum vacuum cannot be maintained in each situation. As a consequence thereof, the system in certain cases can permit pulp fibers to follow along with the gas through the pipe to the vacuum pump, which results both in fiber losses and in possible damage of the vacuum pump. A vacuum pump is constructed to pump gas, and fiber transfer constitutes a weakness, which reduces the reliability of the entire pulp pumping system. In other cases, the pulp pump can operate with a much too great gas amount in the pump housing, which reduces the efficiency of the pump and causes so much gas to remain in the pulp that disturbances arise in subsequent process steps.

[0005] The present invention implies that the aforesaid problems can be eliminated in that the pulp pump is connected to an external vacuum pump via a pipe as defined in the claims. The pipe, thus, is formed with a substantially vertical portion, the highest point of which is located on a level above the pulp pump, and upwardly in the vertical portion of the pipe a means for supply of flushing water is connected.

[0006] The height of the substantially vertical portion of the pipe should be at least the height (measured as water column), which corresponds to the atmosphere pressure, normally about 10 m, plus the height, which corresponds to the pressure in the inlet of the pulp pump. The supplied flushing water flows down through the pipe to the pulp pump. The pipe shall be partially filled with water, so that the gas is permitted to pass upward, while the fibers follow along with the water back to the pulp pump. The water level in the pipe depends on the pressure in the pulp pump, the design of the pulp pump and the setting of the vacuum pump. The water level will vary automatically depending on the process conditions.

[0007] The invention implies that the fiber losses can be avoided or be reduced essentially. An optimum vacuum can be obtained at different process conditions without the use of special control means. The stop valve for the vacuum pump as well as the vacuum control valve can be eliminated. The demand of flushing water can be reduced by up to 80% due to the fact that no fibers are transferred to the driving liquid of the vacuum pump in the liquid ring. Used driving liquid can thereby be recycled as new driving liquid.

[0008] If for installational or other reasons it is not possible or desirable to achieve sufficient height of the pipe, see above, it can be necessary to control the vacuum system. The maximum vacuum must then be limited so that fibers do not follow along the vacuum pump. This can be achieved by vacuum control in a suitable way which, for example, can give a signal when maximum vacuum is exceeded.

[0009] According to one embodiment of the invention, a pulp pump of the type centrifugal pump is coupled via a pipe to a vacuum pump located on a higher level. Alternatively, the vacuum pump can be placed on an optional level in relation to the pulp pump, if the pipe from the pulp pump first extends upward to the required level and from there to the vacuum pump.

[0010] According to another embodiment, two or more pulp pumps are coupled together to two or more vacuum pumps. All pulp pumps are placed on a lower level than the vacuum pumps. The vacuum pumps are connected to a common pipe of coarser dimension. This coarser pipe is located above the pulp pump, and from it extends a smaller substantially vertical pipe down to each of the pulp pumps for de-airing according to the invention. A means for supply of flushing water is connected to each of the smaller pipes. According to this embodiment, a stop valve can be placed between each vacuum pump and the coarser pipe in order to permit the switching-on and switching-off of the vacuum pumps after demand, both at operation and in connection with maintenance or reparation without disturbing the operation and function of the pulp pumps. It applies also here that the vacuum pumps can be placed on optional level in relation to the pulp pumps, if the pipes from the pulp pumps first extend upward to the required level and from there to the vacuum pumps via the common coarser pipe.

[0011] The invention is described in greater detail in the following with reference to the FIGS. 1 and 2 illustrating different embodiments of the invention.

[0012] FIG. 1 shows a pulp pump 1 in the form of a centrifugal pump, which is located on a first level. The pulp pump comprises a housing 2 with inlet 3, and respectively, outlet 4 for the pulp suspension and an outlet 5 for gas. The pump is driven by a motor 6. The housing 2 of the pulp pump is connected via a pipe 7 to a vacuum pump 8 in the form of a liquid ring pump with associated motor 9. The vacuum pump is located on a second level, which is higher than the first level. The pipe 7 has a substantially vertical portion, and a means 10 for the supply of flushing water to the pipe is connected above the vertical portion of the pipe.

[0013] FIG. 2 shows an installation with several pulp pumps and vacuum pumps of the type shown in FIG. 1. The same reference numerals as in FIG. 1 have been used. The pulp pumps 1 are located on a first level, and the vacuum pumps (8) on higher levels. The vacuum pumps 8 are connected to a common coarser pipe 11, which extends above the pulp pumps 1. A smaller substantially vertical pipe 7 extends from the coarser pipe 11 down to each pulp pump 1 for de-airing. A means 10 for the supply of flushing water is coupled to each pipe 7. Between each vacuum pump 8 and the coarser pipe 11 a stop valve 12 is inserted. Thereby a vacuum pump can be disconnected for service, reparation, exchange or the like without requiring a pulp pump to be cut off. It is also possible to reduce the number of vacuum pumps, because every pulp pump does not need a vacuum pump of its own.

[0014] Upon starting the installation, one vacuum pump 8 suitably can be taken into operation, while the other vacuum pumps are idling, i.e. the associated stop valves 12 are closed, With increasing gas amount the pressure increases, i.e. the capacity of the vacuum pump is not sufficient. When a predetermined pressure has been achieved, additionally one vacuum pump is coupled-in by opening the associated stop valve 12, and so forth.

[0015] The invention is not restricted to the embodiments described and shown, but can be varied within the scope of the claims with reference to the description and Figures.

Claims

1. A device for pumping a pulp suspension and simultaneous abduction of gas from the suspension, comprising at least one pulp pump (1) with a housing (2) with inlet (3) and, respectively, outlet (4) for the suspension and an outlet (5) for gas, where the gas outlet (5) is connected via a pipe (7) to at least one external vacuum pump (8), characterized in that the pipe (7) is formed with a substantially vertical portion, the highest point of which is located on a level above the pulp pump (1), and that a means (16) for the supply of flushing water to the pipe (7) is connected upwardly in the vertical portion of the pipe (7).

2. A device as defined in claim 1, characterized in that each pulp pump (1) via the pipe (7) for gas abduction is connected to a vacuum pump (8).

3. A device as defined in claim 1, characterized in that several pulp pumps (1) each via its pipe (7) are connected to a common coarser pipe (11), to which several vacuum pumps (8) are connected.

4. A device as defined in any one of the claims 1-3, characterized in that the height of the substantially vertical portion of the pipe (7) is at least the height (measured as water column) corresponding to the atmospheric pressure plus the height (measured as water column) corresponding to the pressure in the inlet of the pulp pump.

5. A device as defined in any one of the claims 1-3, characterized in that vacuum control is used for limiting the maximum vacuum in the system.

6. A device as defined in claim 3, characterized in that a stop valve (12) is placed between each vacuum pump (8) and the coarser pipe (11) to permit switching-on and switching-off the vacuum pumps.