Scraper device for a continuous digester

The present invention relates to a scraper device, preferably a bottom scraper arranged at the bottom of a continuous digester for pulp production, which scraper comprises at least one scraper arm (12), preferably arranged with shovel members (13), a shaft (6) and drive device (15) for driving the scraper arm (12), an axially extending bore (8) inside the shaft (6) for supplying liquid, and a sealing arrangement (3, 4, 5, 14) for sealing between the atmosphere and the medium in which the scraper arm (12) is intended to rotate, the bore (8) extending along only a limited part of the shaft (6) at that end of the shaft (6) on which the scraper arm (12) is arranged.

TECHNICAL FIELD 
The present invention relates to a scraper device, preferably a bottom 
scraper arranged at the bottom of a continuous digester for pulp 
production, the purpose of the scraper being to ensure an even feeding-out 
from a container. 
In the text which follows reference is made only to a bottom scraper 
arranged at the bottom of a continuous digester for pulp production. 
However, the person skilled in the art will realise that the invention is 
not limited to this illustrative use. 
PRIOR ART AND PROBLEMS 
It has long been known to use a scraper device at the bottom of a 
continuous digester in order to ensure a continuous feeding-out of pulp 
from the digester. The scraper device consists of a number of scraper arms 
which are arranged on the shaft disposed vertically for the operation. The 
movement of the arms in the pulp suspension counteracts bridge formation 
and channelling and other undesirable effects, as a result of which a 
continuous feeding-out of pulp having the desired properties is ensured. 
From an early stage, the above mentioned shaft for operating the said 
scraper arms has been used for adding liquid in the lower part of the 
digester. Liquid has in this case been added by making the whole shaft 
hollow and introducing the liquid by this route. Liquid was added in the 
first place in order to wash the pulp. Later, this bore has also been used 
to add liquid in order to dilute the pulp for the purpose of ensuring the 
feeding-out, which is sometimes found to be necessary in conjunction with 
exceptionally large digesters. 
This known solution, with a continuous bore in the drive shaft, results in 
a number of disadvantages. Because the liquid is supplied to the bore at 
the lower end, it has not been possible to drive the shaft directly, and, 
instead, an intermediate transmission, normally a gear transmission, has 
been necessary. Such a transmission is not only expensive to purchase and 
expensive to install, but also requires relatively extensive maintenance. 
In addition, in the case of a shaft with a continuous bore, bearing 
problems have been experienced when the liquid which is added through the 
pipe exceeds a certain temperature since this has led to carbonization and 
the like which adversely affects the useful life of the bearing. 
Perhaps the most serious problem is, however, the sealing problem, 
associated with sealing between the medium inside the vessel, usually 
pulp, and the atmospheric pressure at which the lower part of the shaft is 
situated. A stuffing box is normally used for this sealing. On account of 
the position of this stuffing box, the entire digester must be emptied in 
order to be able to replace the box. To empty a digester is both costly 
and difficult. This sealing is thus an extremely critical component in a 
conventionally designed digester. Due to the fact that at the one end it 
is sealed directly against a pulp suspension, its operating conditions are 
in addition exceptionally arduous. 
SOLUTION AND ADVANTAGES 
An object of the present invention is to propose a scraper device which 
remedies at least some of the above mentioned problems. This object is 
achieved by means of a scraper device according to the invention, 
preferably a bottom scraper arranged, at a continuous digester for pulp 
production, which scraper comprises at least one scraper arm (12), 
preferably arranged with shovel members (13), which scraper arm (12) is 
arranged at a first end of a rotatable shaft (6), a drive device (15) for 
driving the said shaft (6), an axially extending bore (8) inside the said 
shaft (6) for supply of liquid, and a sealing means (3, 4, 5, 14) for 
sealing between the atmosphere and the medium in which the said scraper 
arm (12) is intended to rotate, the said bore (8) extends along only a 
limited part of the said shaft (6) in the said first end of the shaft (6) 
at which the said scraper arm (12) is arranged, and the said bore (8) 
communicates with at least one liquid supply channel (1), which opens out 
into a preferably annular space (16). 
The above features are known from U.S. Pat. No. 4,056,429, from which the 
invention is distinguished by the following, i.e. the invention is 
characterized in that said supply channel (1), which, without going 
through the shaft (6), opens out into an area in, or in the vicinity of, 
the said sealing means (3, 4, 5, 14) and that said space (16) is delimited 
towards the outside by a plate (2) or the like and a housing (3, 4) fixed 
on the said plate (2) or the like, and is delimited towards the inside by 
the said shaft (6), and that there is arranged, between the plate (2) or 
the like and the said shaft (6), a seal (14), preferably a stuffing box, 
which is accessible from the side with atmospheric pressure, so that a 
good possibility of inspection and maintenance is obtained, and that there 
is arranged, on the said shaft (6), a co-rotating sealing part (5), 
preferably of the labyrinth seal type, which provides a seal against the 
liquid in the said space (16) so that the said medium, which is usually 
pressurized, is prevented from flowing into the said space (16). 
In this way it is possible to provide an advantageous solution concerning 
the sealing of the shaft and it is also possible for the shaft to be 
driven directly at the lower end of the shaft. In addition, such a 
construction offers the possibility of mounting the shaft in such a way 
that the bearings do not surround any liquid-conducting bore in the shaft. 
In addition, by providing a seal, preferably a stuffing box, accessible 
from the side with atmospheric pressure, between the said plate or the 
like and the said shaft, it is possible to replace the said seal simply, 
without needing to empty the digester. 
Furthermore, it is possible to arrange a diaphragm seal, for example a 
labyrinth seal, in an advantageous manner between the said space and a 
part co-rotating with the shaft, so that the pulp suspension is prevented 
from entering the said space, as a result of which the said stuffing box 
acquires optimum operating conditions.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows parts of a digester shell 18 which at the bottom is provided 
with a bottom seat 19. In such a digester, chippings are fed in at the top 
and cooked pulp is fed out via the feed-out opening 20 in the bottom seat 
19. A plate 2 is fitted at the end of the bottom seat 19. In this plate 2 
there is a central hole through which a shaft 6 extends. Arranged at the 
upper end of the shaft 6 are two scraper arms 12. The shaft 6 is driven at 
its lower end by means of a direct drive device 15 (shown 
diagrammatically). 
The shaft 6 is arranged with an axially extending bore 8. At the upper end 
of the shaft 6 this bore 8 communicates with a sleeve 9, which in turn is 
in open communication with two branch lines 10. Each branch line 10 then 
connects the bore 8 to liquid channels 11 which have been arranged on each 
scraper arm 12, on that side which is situated to the rear with respect to 
the direction of rotation. The lower end of the bore 8 is in open 
communication with a radially extending passage 7 through the shaft 6. 
During rotation, the openings of the said passage 7 at all times open into 
an annular space 16. This annular space is delimited towards the outside 
by a rotationally symmetrical housing 3, 4 and by parts of the said plate 
2. Two liquid supply channels 1 extend through the plate 2 and open into 
the annular space 16. The reason for using two liquid supply channels is 
to keep the dimensions small. At the top, towards the pulp suspension in 
the digester 18, the annular cavity 16 is sealed off by means of a 
diaphragm seal 4, 5. This diaphragm seal is preferably formed in a manner 
known per se by means of a labyrinth seal, in which overlapping sealing 
parts project towards each other on the one hand from the fixed part of 
the rotationally symmetrical housing 4 and on the other hand from the 
upper sealing section 5 co-rotating with the shaft. This sealing section 
is preferably arranged on a further shovel device 19. 
At the bottom, the annular space 16 is sealed off by means of a stuffing 
box 14, the gland of which is therefore accessible from the outside. At 
least one bearing 23, such as a ball bearing or a roller bearing, is 
provided for supporting the shaft 6. FIG. 2 shows, from above, a scraper 
device according to a preferred embodiment of the invention. It can be 
clearly seen here that the liquid channels 11, which have been arranged on 
the scraper arms 12, are fixed on that side of the scraper arm which does 
not meet the pulp when the scraper arms 12 rotate. It is also shown that 
the scraper device is arranged centrally with a ring 17 for the purpose of 
forming a support for a conical plate 21 which distributes the pulp 
outwards towards the periphery in order to avoid channelling, etc. 
The function of a scraper device according to the invention will be 
explained in greater detail herein below. The scraper arms 12 are rotated 
by means of the drive device 15 rotating the vertical shaft 6 in a 
clockwise direction in the vertically arranged digester vessel 18. 
Dilution liquid is supplied through the outer liquid supply channels 1 and 
empties out into an annular cavity 16, after which it penetrates through a 
radial passage 7 into the shaft 6 and thereafter passes through the bore 8 
in the shaft 6 to the sleeve 9 arranged at the end of the shaft. The 
liquid is then guided further from the sleeve 9 through lines 10, 11 which 
are arranged on each scraper arm 12, and it finally empties out into the 
pulp suspension, which is in this way diluted. The diluted liquid is fed 
by means of the shovel arms 13 into and down towards the bottom seat 19 
and then out through the feed-out opening 20. A further slight dilution of 
the suspension may take place at the level of the feed-out opening 20 
because the sealing arrangement 4, 5 between the annular cavity 16 and the 
pulp suspension around it is not completely tight, as the dilution liquid 
is added at a higher pressure then the pressure existing in the pulp 
suspension. By means of the shovel members 22 arranged at the bottom, even 
this possible dilution liquid is efficiently mixed, and the pulp is 
guaranteed an even feeding-out through the feed-out opening 20. 
A further advantage of the invention is, as has already been indicated, 
that the bearing can be arranged at a distance from the dilution liquid 
supply point, as a result of which the bearing is not adversely affected 
in the event of extremely hot liquid being added. In addition, there is of 
course the considerable advantage that the shaft 6 can be operated 
directly. Finally, there is the considerable advantage that the stuffing 
box 14 functions against a medium without a fibre content or another 
undesirable content, which can, for example, be abrasive. Perhaps the 
greatest advantage of the invention is that the said stuffing box 14 can 
be replaced without the digester 18 having to be emptied of its contents, 
since the diaphragm seal 4, 5 always provides a certain sealing between 
the pulp suspension in the digester 18 and the annular cavity 16 with 
which the stuffing box 14 is associated. 
The invention is not limited to the embodiment shown above, but instead can 
be varied within the scope of the patent claims which follow; thus, for 
example, it is obvious to the person skilled in the art that it is 
possible to use only one supply channel 1 for the dilution liquid, or more 
than two. Other obvious modifications are the number and the design of the 
scraper arms 12 etc., etc.