Apparatus for the discharge of a mass tower

An apparatus for the discharge of a mass tower containing medium to high consistency pulp and operable without the need for further dilution includes a vessel for storing the pulp; a suction duct extending from a zone of moving pulp to a region outside of the tower; and a pump attached to the end of the suction duct for discharging the pulp.

FIELD OF THE INVENTION 
The present invention relates to an apparatus for the discharge of pulp 
from a mass tower in the pulp and paper industry, wherein a suction duct 
extends from a zone of moving pump at one end to an area outside of the 
tower at the other end where it is attached to a pump. 
BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to an apparatus for the discharge of a mass 
tower filled with medium or high consistency pulp of the type used in the 
pulp and paper industry. 
It is known in the art, to dilute medium and high consistency pulp down to 
a maximum consistency of about 6% before discharging it from the bottom 
portion of a mass tower by means of a conventional centrifugal pump. 
During the last decade, the use of centrifugal pumps for high consistency 
pulp has gained ground, and therefore, it is not surprising to find that 
the discharge of a mass tower has been carried out through the use of a so 
called MC.RTM.-pump. In the known art, an MC.RTM.-pump may be positioned 
at the bottom of the mass tower, and if required equipped as illustrated 
in European patent application 87117593 and publication 0323749 so as to 
ensure the flow of the pulp to the pump. However, it has been shown that 
it is more advantageous, in accordance with a preferred embodiment Of the 
invention, to arrange an MC.RTM.-pump to communicate with the wall of the 
mass tower, so as to pump diluted pulp therefrom. By using this kind of 
arrangement, pulp with at least two different consistencies can be 
discharged from the tower. 
The above referenced pump, when arranged in communication with the mass 
tower, is in most cases attached to the conic wall portion thereof. 
However, when an MC.RTM.-pump is arranged in the above described manner, 
significant problems can develop. As time progresses, the output of the 
MC.RTM.-pump gradually decreases while the energy consumption increases. 
The conclusion that is reached is that the pump for some reason does not 
receive a sufficient amount of pulp, despite the addition of dilution 
water, which only offers a temporary solution to the problem. 
As such, an object of the present invention and as proven during tests 
performed, is to eliminate the above described problems. To achieve the 
above objective, one end of a suction duct is positioned in a zone of 
moving pulp located in the interior of the tower while the other end is 
located in an area outside of the vessel where it is connected to a pump. 
The end of the duct within the vessel is disposed so as to be positioned 
beyond the pulp layer which thickens about the periphery of the vessel. By 
positioning the orifice of the suction duct in a region free from the 
thickened pulp layer and in a region of moving pulp, the inherent problems 
associated with the movement and pumping of medium to high consistency 
pulp from a mass tower can be obviated. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages, and specific object attained by its use, reference 
should be had to the drawing and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the known art, FIG. 1 illustrates a modern discharge 
configuration of a high consistency mass tower or vessel. A high 
consistency mass tower 10 as shown in FIG. 1, consists of two major 
portions: first, an upper part comprising, in this embodiment, a 
cylindrical wall portion 12 and a conical wall portion 14; and, second, in 
this embodiment, a cylindrical bottom part 16. A conventional centrifugal 
pump 20 designed for pumping low consistency pulp is attached to the wall 
of the cylindrical bottom part 16 of the tower. A feed conduit (not shown) 
for a dilution liquid and a mixer 22 for mixing the liquid into the pulp 
so as to improve the pumpability of the pulp have been positioned in front 
of pump 20 or, more broadly stated, in the bottom part 16 of the tower 
which may also be called as the diluting zone. The wall in the conical 
upper part 14 of the tower 10 has been provided with an MC.RTM.-pump 24, 
wherein the pulp is pumped without dilution and at its original 
consistency. Moreover, an MC.RTM.-pump may, either additionally or 
alternatively, also be attached to the cylindrical upper part 12 of the 
tower, as illustrated by an upper MC.RTM.-pump 26 in FIG. 1. 
Naturally it has to be noted that the shape of the tower may be of whatever 
applicable shape, the drawings only show the most frequently used tower 
shape i.e. the one having a substantially narrow bottom portion for 
diluting the pulp and a wider upper part of the tower for, for instance, 
storing the pulp. 
FIG. 2 illustrates the theory of why the pumping of high consistency pulp 
at least from the vicinity of the conical upper portion of the tower does 
not succeed with conventional MC.RTM.-pumping means in the long run. The 
broken lines A and B in FIG. 2 show the variations of the pulp surface. It 
must be noted that whenever the surface of the pulp decreases, the pulp 
surface A in the center part of the tower is relatively lower than that 
portion of the surface found closer to the interior wall surface of the 
tower. The difference in level is due to the friction between the pulp and 
the wall of the tower. Moreover, the conical part of the tower forces the 
pulp to move towards the center of the tower, so as to contribute to the 
deceleration of the downward movement of the pulp. In addition, it has 
been found that the pulp thickens adjacent to the tower wall. The reason 
being that the liquid in the pulp moves more readily than the pulp and 
flows in a downward direction. Accordingly, liquid adjacent or in 
proximity to the wall is more readily filtrated. The greater the disparity 
in level between the pulp surface A in the center of the tower and the 
pulp adjacent to the tower wall, the greater the filtering capability and 
the greater is the increase in the consistency of the pulp at the 
periphery of the tower. When the lower is filled (broken line B), the 
effects are reversed. The pulp accumulating at the center of the tower 
follows the profile of curves B, but when the surface again decreases, the 
pulp adjacent to the wall quickly regains its original consistency. When 
the pulp is sufficiently thickened in close proximity of the walls thereof 
it does not move downwards in the tower, but forms a thickened pulp layer 
at least in the conical part 14, said pulp layer continually thickening as 
time progresses. During experiments, it has been noted that the 
consistency of such a layer rises above 20%. If under these circumstances, 
an MC.RTM.-pump 24 attached to the conical part of the tower is used for 
pumping the pulp, the pump operates well at the onset, but as time 
progresses, the pump's pumping capability decreases. The decrease in 
pumping capability is due to a build up of a pulp collar surrounding the 
suction opening of the pump 24, at the edge of the influence range of the 
fluidizing rotor. The less, i.e., the shorter the distance, the rotor 
extends into the vessel, the sooner the pump encounters the difficulties 
referenced above in properly receiving pulp. 
FIG. 3 illustrates a solution to the above mentioned problem. A vertical 
suction duct 32 is disposed through a wall 30, preferably through the 
conical part 14 (if such exists) of the mass tower 10, so as to reach far 
enough inside of vessel 10, and extend through the thickened pulp layer 
and into the zone of moving pulp. Moreover, the suction opening of the 
duct is preferably open against the flow direction of the pulp during 
discharge, so that there is no need to doubt that the pulp will flow down 
to the bottom of said suction duct 32, and wherein said bottom of said 
suction duct 32 is provided with a separate pump 34, preferably also an 
MC.RTM.-pump. It must be further noted that the realization of the 
arrangement in accordance with the present invention is characterized in 
that the bottom part of the tower is provided with means for discharging a 
first portion of pulp from the vessel. Usually said means is a pump (not 
shown), by which pulp is pumped substantially continuously from the vessel 
10, and which causes the continuous flow of the pulp past the orifice or 
the so-called suction opening of the suction duct 32. However, said means 
may also be a mere opening and a discharge conduit attached thereto if the 
pulp is diluted to a sufficiently low consistency whereby the pulp is able 
to flow out of the vessel due to mere gravity. The extension of the 
suction duct 32 into the inside of the vessel, i.e., the distance of the 
suction duct opening from the inner surface of the tower wall is 
preferably, according to the results of experimentation, at least about 
25% of the diameter or width of the suction duct depending on its 
cross-sectional shape. Of course, if the extension of the duct is greater 
than about 25%, of the diameter, the discharge of the pulp will not be 
hindered. However, if the extension or length of the duct falls below 
approximately 25%, problems may be encountered in the case of easily 
filtered pulp types. 
FIG. 4 additionally illustrates an alternative embodiment, wherein a 
suction duct 36 is disposed upright with respect to the conical wall 
portion 30. An additional advantage for both arrangements is that the 
fluidizing MC.RTM.-pump 34 or the device disclosed in FIG. 7 of European 
patent application 88312336 can be positioned on a conventional base, i.e. 
horizontally, instead of hanging perpendicularly with respect to the wall 
of the mass tower, as is characteristic of the previous technique. 
In addition, the embodiments as shown in FIGS. 3 and 4 incorporate the use 
of a collar plate 42 mounted either on the suction duct (32, 36) or on the 
tower wall so as to cover a pocket generated between the suction duct and 
the tower wall and wherein pulp would otherwise tend to accumulate. The 
collar plate 42 is preferably formed so as to deflect pulp to the sides of 
the suction duct, thereby preventing the generation of a pulp layer on the 
collar plate. 
In yet another embodiment, FIG. 5 illustrates how an open screw 40 can be 
arranged in front of, or above, the opening of an upright suction duct 38 
so as to prevent the generation of a thickened pulp layer in front of the 
suction duct 38 and to transfer pulp towards the orifice, of the suction 
duct 38. By means of forcing the pulp to flow against the wall of the pulp 
tower the generation of the pulp layer on the vessel wall is prevented 
whereby the pulp is able to flow downwardly into the opening of the 
suction duct. 
FIG. 6 illustrates a similar arrangement disposed in front of, or above, 
the orifice of the discharge duct for pulp located upright with respect to 
the conical part of the wall. The purpose of the screw 40 is to feed pulp 
towards the wall of the mass tower 10 and thus prevent the generation of 
pulp layer tending to accumulate on the wall. The screw does not actually 
operate as a feeding apparatus for the suction duct, but prevents the 
thickening of the pulp on the surface of the mass tower by feeding the 
pulp against the wall of the mass tower. 
FIGS. 7a and 7b illustrate a rotating scraper which may also be used for a 
similar purpose, wherein a scraper mechanically loosens the pulp layer 
which accumulates on the wall. 
FIG. 8 illustrates yet another embodiment of the invention. The suction 
duct 46 is, in accordance with the invention, arranged to extend through 
the bottom of the pulp tower to the upper part of the pulp tower i.e. well 
above the dilution zone so that the pulp may be discharged from the tower 
in its original consistency. The advantage of this embodiment is that both 
the MC.RTM.-pump and the pump pumping the diluted pump may be arranged on 
the same base plate underneath the pulp tower. The embodiment is 
especially designed for cases where there is sufficient space beneath the 
pulp tower for placing the pumps. 
FIG. 9 is a schematic view of the present invention wherein a wall portion 
of the pulp containing vessel 10 is formed by a cylindrical wall and 
wherein the suction duct 32 extends through the vessel wall into the 
vessel 10. 
FIG. 10 shows a pulp containing vessel 10 having a conical wall portion and 
above the cylindrical wall portion a wall portion which is at least 
partially formed of a cylindrical wall and wherein the suction duct 32 
extends through the cylindrical portion of the vessel. 
FIG. 11 shows a pulp containing vessel 10 in which a conduit 50 for 
discharging a portion of the pulp is attached to the lower cylindrical 
wall portion of the vessel. Alternatively, the conduit may also be 
attached to the bottom of the pulp vessel as shown in dotted lines by the 
numeral 50'. 
As is readily apparent from the above description, a new apparatus has been 
developed for the reliable and safe discharge of pulp from mass towers 
without the need to dilute the pulp. It must be recognized however, that 
the above description discloses only a few embodiments of the invention. 
In reality the apparatuses may deviate considerably from the above 
described embodiments and may still be within the scope of the appended 
patent claims. Thus, it is quite possible that the suction duct in 
accordance with the present invention is arranged in an inclined position 
with respect to the cylindrical part of the mass tower, or that the screw 
shown in the drawings is positioned in some position other than a 
completely horizontal position or that the vessel is completely 
cylindrical or shaped in some other form appropriate for the purpose, or 
that the suction duct of the vessel has been extended deep into the 
vessel, for example in the way shown in FIG. 8, just to mention a few of 
the variations of the invention. 
Thus, it should be understood that the preferred embodiments and examples 
described are for illustration purposes only and are not to be construed 
as limiting the scope of the present invention which is properly 
delineated only in the appended claims. 
The invention is not limited by the embodiments described above which are 
presented as examples only but can be modified in various ways within the 
scope of protection defined by the appended patent claims.