Method and apparatus for pumping high consistency pulp

Abstract: The present invention relates to a method and apparatus for pumping high consistency pulp, which invention is especially suitable for pumping high consistency fiber suspensions in the pulp and paper industry. One of the problems in the prior art techniques has been the difficulty of bringing high consistency pulp to the impeller of the pump or like, because high consistency pulp tends to form strong fiber network, which stops the pulp flow. A method has been developed as a solution to the problem, in which liquid is discharged from pulp during the pumping process and the liquid is fed back to the pulp to be pumped upstream of the pumping. This method is enabled by an apparatus, in which at least part of the surfaces of the pump (1) are arranged with filter surfaces (6, 7, 8, 9, 10), through which the liquid separated from pulp is returned either directly in front of the suction opening (11) of the pump (1) or to some other suitable place in the mass tower.

Description: The present invention relates to a method and apparatus for pumping high consistency pulp respectively according to the preamble of claim 1 and 6. The method and apparatus in accordance with the invention are especially suitable for treating high consistency fiber suspension in the pulp and paper industry. From DE-B-11 25 281 a centrifugal pump is known having a rotor, an inlet and outlet respectively for the accept and reject, and a spiral housing, the first 180° of which is a closed wall and the subsequent portion a screen surface or sieve. A number of these pumps are arranged together to wash a starch-fibre suspension, obtained by grinding swollen corn such that the suspension is treated in stages. The water or washing liquid flow is directed in the opposite direction to the flow of the substance to be treated. The pumps applied are only suitable for treating a low consistency fibre suspension. There are several known methods and apparatuses for pumping high consistency pulp. Previously only displacement pumps, such as screw pumps or like, were used to pump high consistency pulp. Nowadays the tendency has been to replace the displacement pumps because of their deficiencies and the drawbacks brought about by them. The intention has been to develop a centrifugal pump, normally used for pumping water and corresponding material, for pumping high consistency pulp. One of the first problems met when trying to pump pulp with the consistency of more than 8 % is that the pulp does not independently flow to the impeller of the pump in the suction opening. It has been suggested as a solution to this problem that a special screw feeder be used to feed the pulp to the suction opening of the pump or that a so called inducer be arranged in the suction opening of the pump to convey the pulp in the suction opening towards the pump impeller. However, it has not been possible in practice to pump any pulp having a consistency which approaches 15 %. A third example of possible embodiments is a so called fluidizing centrifugal pump used for pumping high consistency pulp, in which pump the fluidizing rotor extends to the suction opening of the pump or in some cases through it as far as to the mass tower. By using this kind of fluidizing rotor it has been possible to reach the above-mentioned consistency of 15 %, which does not, however, satisfy all requirements for pulp conveyancing in the pulp and paper industry. One possibility is, of course, the dilution of the pulp prior to the pump and the thickening of the pulp subsequent to the pump, as was done with different pulp treating devices according to the prior art techniques. This described technique, however, also has disadvantages such as the increased need of technical devices, because both feeding and mixing apparatuses are needed in the chamber prior to the pump and thickening apparatuses are needed subsequent to the pump to bring the pulp back to its original consistency. A further disadvantage resides in the increase in consumption of energy, which is required in thickening of pumped pulp. The method and apparatus according to the present invention make it possible to eliminate or minimize the hitherto deficiencies and problems and thus enable the easy pumping of pulp with consistency more than 15 % without separate diluting apparatuses and thickeners. According to the invention, liquid is separated through filter surfaces from the pulp being pumped, which separated liquid is used for dilution of the pulp entering the pump. The method according to the invention is characterized in that all of the liquid separated from the pulp being pumped in the pumping process returned back to the pumping process to dilute the pulp to be pumped. The apparatus according to the invention is characterized in that at least part of the filter surface or surfaces is or are connected with one or more ducting means which return all of the liquid separated from the pulp back to the pump to dilute the pulp to be pumped. Other features characteristic of the method and apparatus in accordance with the invention can be seen in the accompanying claims. The invention is discussed in detail, by way of example, below with reference to the accompanying drawings, in which: Fig. 1 is a schematic cross-sectional view illustrating an apparatus according to one preferred embodiment of the invention, Figs. 2a and 2b are schematic illustrations of an apparatus according to another preferred embodiments of the invention, Fig. 3 is an advantageous application according to the invention, Fig. 4 is a second advantageous application according to the invention, Fig. 5 is a third advantageous application according to the invention, and Fig. 6 is a fourth advantageous application according to the invention. An apparatus in accordance with the invention according to Fig. 1 comprises a pump, such as a centrifugal pump 1 conventionally including a shaft 2 and an impeller 3. It is possible to arrange a fluidizing rotor 4 or some other apparatus in communication with the impeller 3 to feed pulp to the impeller. According to Fig. 1 filter surfaces 6, 7, 8, 9 and 10 are arranged in the housing 5 of the pump 1, which surfaces may be located in shown fashion in the housing 5. Filter surface 6 is located on the rim of the suction opening 11 of the pump either as a uniform cylinder or as separate surfaces. Respectively, filter surface 7 located in the wall of the housing of the pump 1 closest to the front side of the impeller 3, may either be annular or be formed of separate surfaces. Furthermore, the filter surface 8 on the outer rim of the pump housing is similar to the previous surfaces, as well as filter surface 9 at least partly locating behind the impeller on the rear wall of the housing of the pump and filter surface 10 arranged in communication with an outlet opening 12 of the housing of the pump. A chamber is arranged behind each of filter surfaces 6, 7, 8, 9 and 10, which recovers the liquid filtered through the filter surfaces and from which chambers the liquid is guided further on. This kind of pump 1 can be mounted from the flange surrounding the suction opening either to a mass tower, drop leg, suction pipe or like. Fig. 2a & 2b discloses an alternative embodiment of the apparatus according to the invention, which includes a centrifugal pump 20 mounted directly on a wall 22 of a mass tower 21 in such a way that no actual suction duct is needed. In this case (Fig. 2a) a pipe formed from a filter surfaces 23 is arranged inside a mass tower 21, inside of which pipe an apparatus, such as a fluidizing rotor 24, facilitating feed of pulp is advantageously located. In this case the liquid filtered through the filter surfaces flows directly to mass tower 21 and dilutes pulp outside pipe 23, which thus more easily flows to pipe 23. According to Fig. 2a the fluidizing rotor 24 may extend to the outside of pipe 23 and thus facilitate the flow of pulp to pipe 23. In the modified embodiment of Fig. 2b there is arranged, for example, a front wall 26 of the housing of conventional centrifugal pump 25 to operate as a filter surface, whereby the impeller causes the liquid to flow through filter surface 26 directly to the pulp to be pumped. Pulp flowing to the mass tower, drop leg, suction pipe or like is thus diluted by the dilution liquid gained from the filter surfaces, whereby the consistency of the pulp flowing to pipe 23 or to the suction opening is lower than the average consistency of the pulp in the above-mentioned space, which again is lower than the consistency of the pumped pulp corresponding to the consistency of the pulp fed into the mass tower. It is, of course, possible that the pump according to this embodiment includes one or more filter surfaces, by means of which the consistency of pulp is also raised in the pump or more accurately restored to its original value. Fig. 3 shows an advantageous application in accordance with the invention, which comprises a mass tower, drop leg or like 30 and pump equipment 31 connected to it. The pump equipment 31 may comprise pumps according to Figs. 1 and/or 2, the liquid chambers of the filter surfaces 6 - 10 of which pumps are connected with a pipe 32 leading to the mass tower 30. A valve 33 arranged in pipe 32 is used to control the amount of dilution liquid. It is, of course, possible to arrange a separate pump to pump liquid from the spaces behind the filter surfaces to the mass tower. Fig. 4 illustrates a situation otherwise similar to that of Fig. 3 except that a pressure accumulator installation 34 with valves 35 is connected with the return pipe 32 of dilution liquid, which valves can provide a pulsating feed of dilution liquid to the mass tower. This method may be utilized in the modification of Fig. 5, e.g. by feeding dilution liquid with nozzles 42 to the boundary surface 41 of pulp and tower 40 or to a pulp and suction pipe, whereby the liquid pulses reduce the friction and further facilitate the intensity of dilution. Thus the consistency of the surface layer of pulp is lower and the pulp flows more easily, for example down the mass tower wall. The dilution liquid may also be fed, if desired, onto the pulp in the mass tower as a pulsating flow via nozzles 36, as shown in Fig. 4. Respectively it is possible to feed dilution liquid also to the bottom of the mass tower as a pulsating or even, continuous flow, whereby the consistency of the pulp in the bottom part of the mass tower reduces and the pulp flows more easily to the pump. If dilution liquid is fed as a pulsating flow to the bottom of the mass tower, it may be possible to create a slight fluidized-bed phenomenon, which further facilitates the flow of the pulp to the suction opening of the pump. Similarly dilution liquid can be fed also in the embodiment of Fig. 6, in which pump 50 is arranged in the pipe installation with suction and discharge pipe 51 and 52. In this case dilution liquid is taken through filter surfaces shown in Fig. 1 and returned via pipe 53 back to the pulp in suction pipe 51. In this embodiment pulse feed shown in Figs. 4 and 5 can also be advantageously applied and thus reduce the friction between pulp and suction pipe 51. In some cases it is advantageous also to feed dilution water to behind the impeller of the pump, for example in some pumps, in the area of the back vanes used due to degasification in order to dilute pulp flowing between the vanes so as to create an improved pulp flow from behind the impeller to the main flow of pulp and not to clog the gap between the impeller and the back wall of the pump. Thus, according to the invention, it is possible to feed filtered liquid to dilute pulp flocks which may occasionally form inside the pump and which are harmful to the pumping process. It is also possible to duct liquid discharged from the pulp to a container to be temporarily stored and to be fed, when desired, to dilute occasional consistency peaks or pulp flocks in some part of the pump or close to it, which flocks may without dilution endanger the trouble-free pumping operation. It is to be appreciated that it is not necessary to lead the liquid separated from all the filter surfaces to the same dilution point, but it is possible to duct liquid at the same time to many different points. None of the above figures disclose a return device for the internal dilution liquid filtered from the pulp, because no return device (in other words a pump) is needed if the filter surfaces are located on the discharge side in a pressurized chamber, since the liquid flows on its own due to the pressure caused by the height of the pulp in the mass tower and the pump. On the other hand, if the dilution liquid is taken from the filter surface on the suction side, a pump may be necessary to convey the liquid to the pulp chamber. Similarly, a pump may be necessary when feeding dilution liquid to the pressure accumulator, if such is used, since only in extremely advantageous cases can the pressure accumator be charged without a separate hydraulic pump, in other words when liquid is discharged from the discharge side, whereby the liquid is discharged almost in the pressure developed by the pump. It is also possible to arrange a heat exchanger in the return pipe of the dilution liquid, either to recover heat from the filtered liquid or to heat the filtered liquid. Especially when the pumping unit is located in communication with a washer or like, the outlet pulp of which is extremely hot, it is advantageous to recover heat. It may be advantageous to heat the pulp in certain circumstances. As it can be seen from the above description, a totally new type of centrifugal pump has been developed with which it is possible to pump thicker pulp than before without a risk of arching of the pulp in proximity of the suction opening of the pump or that the pulp does not otherwise reach the impeller of the pump. However, the embodiments described above have been given by way of example for explanation only, and no unnecessary limitations should be understood. Thus it is by no means necessary for a centrifugal pump in operation to be a so called fluidizing pump, but also other solutions are possible. It is quite possible to apply the invention to a conventional centrifugal pump or to a pump, in which a screw feeder, such as a so called inducer or the like is used to feed pulp to the pump. Neither is it necessary for the filter surfaces filtering dilution water to be located exactly where they are located in the shown embodiments, in other words in Figs. 1 and 2. Similarly, the term pumping region used in the claims includes all the operations connected to the pumping, in other words from the moment on when pulp begins to move towards the pump until the moment when it is discharged onwards from the discharge opening of the pump. Also the degasifying system means the whole part of the pump or additional devices in communication with it, which are used for degasifying in the pump.

Claims: A method of pumping high consistency pulp, wherein liquid is separated during the pumping process from the pulp-being pumped, characterized in that all of the liquid separated from the pulp being pumped in the pumping process is returned back to the pumping process to dilute the pulp to be pumped. A method according to claim 1, characterized in that separated liquid is fed to pulp in the region of back vanes located on an impeller in the pumping region of the pumping process or to a degasifying system arranged in the pumping region of the pumping process to dilute pulp flocks harmful to the pumping process. A method according to claim 1, characterized in that the separated liquid is fed to the pulp to be pumped flowing to the pump so that the consistency of the pulp to be pumped is lower than that of the pulp subsequent to the pumping process. A method according to claim 1, characterized in that the separated liquid is returned to the pulp to be pumped as a pulsating or even, continuous liquid flow in such a manner that the separated liquid is used to reduce friction between the pulp and the walls of a mass tower or like vessel including the pulp and at the same time to dilute the pulp. A method according to claim 1, characterized in that the separated liquid is returned to dilute pulp to be pumped either immediately before reaching the range of suction of the pumping process or prior to the whole pumping process. An apparatus for pumping high consistency pulp, which apparatus comprises a pump with a shaft and pulp pumping members, wherein one or more filter surface is or are arranged in communication with the pump and by means of which liquid is separated from the pulp being pumped, characterized in that at least part of the filter surface or surfaces respectively is or are connected with one or more ducting means which return all of the liquid separated from the pump back to the pump to dilute the pulp to be pumped. Apparatus according to claim 6, characterized in that the ducting means are provided as conduits which lead to a mass tower or like vessel arranged upstream of the pump or to a desired point inside the pump. Apparatus according to claims 6 and 7, characterized in that the pump is a centrifugal pump, one or more filter surfaces being in communication with the housing thereof. Apparatus according to claim 8, characterized in that a filter surface is provided protruding from the suction opening of the pump, and that the pump is provided with a rotor which extends to the inside of the filter surface to cause the discharge of liquid separated from the pulp to flow directly to the pump through the filter surface, and wherein part of the rotor extending to the inside of the filter surface is formed, for example, by fluidizing vanes or a screw-like feeding member. Apparatus according to claim 8, characterized in that the pump is a centrifugal pump which is connected with a suction tower or like vessel in such a manner that the separated liquid is discharged directly to the pump flowing to the pump through the front wall of the housing of the pump.

Inventors: TIMPERI JUKKA, VESALA REIJO, VIKMAN VESA, TIMPERI, JUKKA, VESALA, REIJO, VIKMAN, VESA
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1988
Main classification: F04D7
IPC codes: D21C9, F04D7, F04D29