Stirrer for use in liquid storage tanks

A device with vertical buoyancy tanks fitted to its upper part is provided with rotors which generate horizontal and vertical propulsion, and which stirs the liquid uniformly as it moves freely through a liquid storage tank, thereby preventing the accumulation of sludge. The vertical buoyancy tanks stabilize the stirrer and provide a restoring force if the stirrer is upset. The stirrer is fitted with an ultrasonic ultrasonic wave emitter, and ultrasonic receptors are installed at several positions in the liquid tank. An ultrasonic pulse is emitted after a certain interval when the stirrer is at rest on the bottom of the tank. The position of the stirrer is computed for each pulse emission, and the average is taken to give the actual position of the stirrer.

BACKGROUND OF THE INVENTION 
This invention relates to a device for stirring liquid in a storage tank 
while moving around in the tank, thus redissolving sludge which has 
separated out from the liquid, and preventing the accumulation of sludge 
on the bottom of the tank. It also relates to a method for determining the 
position of the stirrer in the tank so that the motion of the stirrer may 
be controlled. 
In large tanks used for storage of liquid, (for instance, crude oil, 
residual oil, etc.) sludge may gradually separate out from the liquid over 
long periods of time. If this sludge is allowed to build up on the bottom 
of the tank, the quantity of liquid which can be stored in the tank is 
effectively reduced, and the tank must therefore be cleaned regularly to 
remove accumulated sludge. In order to clean the tank, it is necessary to 
drain off all the liquid. A device was therefore developed to obviate this 
necessity, consisting of a stirrer rotor inside the tank rotated by a 
motor outside the tank, which stirred the liquid and prevented sludge from 
separating out. The position of these stirrer rotors was however fixed, 
which meant that similar device had to be installed at a large number of 
points around the circumference of the tank. This solution is not 
practical and does not permit efficient stirring in the center area of the 
tank. 
In order to solve this problem, the Applicant, in Japanese Patent 
Publication No. 59-46658 and Japanese Patent Provisional Publication No. 
58-214383, proposed a device which stirred the liquid while moving around 
in the tank. This stirrer has rotors which permit it to move up and down, 
and rotors which permit it to move in a horizontal direction. By 
controlling these rotors from outside, the stirrer can be made to move 
around freely inside the tank. The liquid in the tank can therefore be 
stirred uniformly, sludge can be redissolved, and sludge can be prevented 
from accumulating anywhere on the bottom of the tank. 
In order to lighten the weight of the stirrer and enable it to move 
smoothly through the liquid, it was suspended by a rope from a float to 
give it buoyancy. However, as the stirrer was suspended from the float, 
the stirrer sometimes lost its balance when it was moved horizontally or 
rotated, and sometimes collided with the tank inner wall. 
Further, in order to move the stirrer to a desired location somewhere in a 
large tank by means of an external operation, it is necessary to determine 
the actual position of the stirrer at any time. As liquid storage tanks 
are usually completely enclosed, however, the position of the stirrer 
cannot be confirmed from outside. 
The Applicant, in Japanese Patent Provisional Publication No. 58-213210, 
proposed a device for determining the position of the stirrer wherein a 
wire wound around a drum was suspended from the roof of the storage tank 
and connected to the upper part of the stirrer, the direction and length 
of wire released, as measured by a potentiometer or other means, being 
used to determine the position of the stirrer. 
The structure of this device is however complex, and if the direction and 
length of wire released are not measured with a high degree of precision, 
it is difficult to determine the position of the stirrer with accuracy. 
Further, as the wire is paid out over a long distance, it sometimes goes 
slack or becomes entangled with the electric cable of the stirrer. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide a stirrer 
which can move smoothly in both horizontal and vertical directions in the 
liquid in the tank, and which is stable to change of direction. 
A further object of this invention is to provide a method for determining 
the precise position of the stirrer. 
For these purposes, vertical buoyancy tanks are fitted to the upper part of 
the frame which constitutes the skeleton of the stirrer, and a 
perpendicular flow plate is provided on the lower part of the frame. A 
pair of motor-driven horizontal rotors are arranged on either side of the 
flow plate in order to provide propulsion in a horizontal direction. In 
addition, an elliptical housing for enclosing a pair of vertical rotors, 
also motor-driven, is fitted to the lower part of the flow plate which 
provide propulsion in a vertical direction. The rotation of the motors is 
controlled by a control unit connected to the stirrer by a cable. The flow 
set up in a horizontal direction by the horizontal rotors causes the 
stirrer to move horizontally in the opposite direction to the flow, while 
the flow set up in a vertical direction by the vertical rotors causes the 
stirrer to rise and fall in the opposite direction to the flow. At the 
same time, a flow of liquid is set up in the tank, and this stirring 
action causes sludge to redissolve in the liquid. Due to the vertical 
buoyancy tanks fitted to the upper part of the frame in a one-piece 
construction, there is a strong vibration damping action and a restoring 
action which prevent the stirrer from being upset and maintain it in its 
correct orientation. 
Due to the floats, the weight of the stirrer in the liquid is practically 
zero. The driving force required to lift the stirrer is thus reduced and, 
by decreasing the speed of the stirrer when it is falling, a collision 
with the bottom of the tank is prevented. 
The flow plate is parallel to both the horizontal and vertical directions 
of the rotors, which stabilizes the orientation of the stirrer when it is 
moving. 
In this invention, in order to determine the position of the stirrer in the 
liquid storage tank, an ultrasonic wave emitter is fitted to the stirrer, 
and several receptors are installed at different points inside the tank to 
receive the ultrasonic wave so emitted. A computing device is provided to 
compute the position of the stirrer, which functions by determining the 
time taken by each receptor to receive this ultrasonic pulse. The time 
taken to receive the pulse from the ultrasonic wave emitter is directly 
proportional to the distance to each receptor, and so the position of the 
stirrer can be found from these times. To reduce errors due to noise when 
the measurements are being carried out, all motors are stopped, and the 
stirrer allowed to settle on the tank bottom before emitting an ultrasonic 
pulse from the emitter. This emission is repeated several times. The 
period between emission of pulses is chosen to be greater than a time 
interval equivalent to the time taken to traverse the greatest distance 
inside the tank plus the time taken for attenuation of echos due to 
reflections from the tank inner wall. As measuring errors arise if the 
echos of the ultrasonic pulses are used, distances are measured using the 
time taken for the first pulse to be received in each emission cycle. The 
position of the stirrer is computed for pulses in several cycles, and the 
average value obtained is taken to be the actual position. 
The above object, other objects and advantages of this invention will be 
made clearer by the following description and reference to the attached 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As can be seen from FIGS. 1-3, this device consists of a vertical frame 1 
and an elliptical housing 2 fixed to the lower part of the frame, the 
housing 2 being provided with several windows 2a. These elements together 
form frame 4 which is the skeletal structure of the stirrer 3. 
On the upper part of frame 1, a motor 5 is provided which drives the 
vertical rotors 6a, 6b arranged symmetrically on either side of elliptical 
housing 2. Rotation is transmitted to rotors 6a, 6b by means of gearboxes 
5a, 5b connected to the shaft of motor 5. Rotors 6a, 6b consist of 
impellers with several blades, and their rotation sets up a practically 
perpendicular fluid flow. On either side of the motor 5, there are a pair 
of motors 7a, 7b, and gearboxes 8a, 8b installed in vertical frame 1. On 
either side of frame 1, there are also horizontal rotors 9a, 9b oriented 
so as to set up a practically horizontal fluid flow. These rotors 9a, 9b 
are driven by motors 7a, 7b. 
Rotating brushes 11a, 11b are arranged via bearing boxes 10a, 10b in the 
lower part of elliptical housing 2, and are connected to the shafts of 
rotors 6a, 6b respectively such that they rotate together with them. 
On the upper part of frame 4, a pair of primary buoyancy tanks 12a, 12b, 
which have a semicylindrical form and are hollow, are arranged so as to 
enclose between them the upper part of frame 1 and the motors 5, 7a, 7b. 
The lower parts of the tanks are fixed to frame 1, and the upper parts are 
fixed in a one piece construction to the joint box 13 provided at the 
upper end. 
In addition a secondary buoyancy tank 14, which has an elliptic cylindrical 
form, is vertical and is of large volume, is positioned on the upper part 
of joint box 13 to which it is fixed as one piece. 
The under surfaces of primary buoyancy tanks, 12a, 12b slant away from 
frame 1 towards the exterior so as not to offer any resistance to the 
vertical flow of fluid set up by said rotors 6a, 6b. 
Between primary buoyancy tanks 12a, 12b, frame 1 and elliptical housing 2, 
a flow plate 15 is arranged at right angles to frame 1, that is, parallel 
to the fluid flow set up by both the rotors 6a, 6b and the rotors 9a, 9b. 
This flow plate 15 is fixed to tanks 12a, 12b, frame 1 and elliptical 
housing 2, and also serves to provide greater strength. 
Several obstacle detectors 16 are installed on the outer wall of elliptical 
housing 2. These obstacle detectors 16 use tape switches, limit switches 
or pressure sensitive switches, or any other common device which emits a 
signal when it comes into contact with an obstacle. 
A depth finder 17 is fitted to joint box 13, and may for example consist of 
a strain gauge, potentiometric gauge or Bourden tube pressure gauge. It is 
so designed as to output a depth signal due to the pressure detected when 
the stirrer 3 is immersed in the liquid in the tank. 
A position finding device is provided, as shown in FIGS. 4 and 5, for 
determining the position of the stirrer in the liquid in the tank. 
This position finding device involves the use of an ultrasonic wave system. 
An ultrasonic emitter 19, for example of the cylindrical 
piezo-electromagnetic type, is fitted to the center of the upper part of 
secondary buoyancy tank 14. Three receptors 20a, 20b, 20c, which are 
intended to receive the ultrasonic wave pulse and are suspended from the 
roof 27 of tank 22, are then placed in their respective positions. 
As can be seen from FIGS. 5 and 6, when an ultrasonic pulse is emitted at 
fixed time intervals from the emitter 19, the time (t) required for this 
pulse to reach the receptors 20a, 20b and 20c (t.sub.1, t.sub.2, t.sub.3), 
is directly proportional to the distance from the emitter to the receptor. 
The distance from emitter 19 to each of the receptors 20a, 20b and 20c, 
that is l.sub.1, l.sub.2, l.sub.3, may then be measured by multiplying the 
respective times t.sub.1, t.sub.2, t.sub.3 recorded by measuring 
instrument 32, by the velocity of sound. A computer 33 is provided to 
compute the position of stirrer 3 in the liquid storage tank from the 
distances l.sub.1, l.sub.2, l.sub.3. 
As shown in FIG. 5, if the position of the ultrasonic emitter 19 in 
three-dimensional coordinates is (x, y, z), the positions of the 
ultrasonic receptors 20a, 20b, 20c in the same coordinate system are 
(x.sub.1, y.sub.1, z.sub.1), (x.sub.2, y.sub.2, z.sub.2), x.sub.3, 
y.sub.3, z.sub.3), and the distances l.sub.1, l.sub.2, l.sub.3 are 
determined, the point (x, y, z) may be found from the following equations 
(1)-(3): 
EQU (x-x.sub.1).sup.2 +(y-y.sub.1).sup.2 +(z-z.sub.1).sup.2 =l.sub.1.sup.2 (1) 
EQU (x-x.sub.2).sup.2 +(y-y.sub.2).sup.2 +(z-z.sub.2).sup.2 =l.sub.2.sup.2 (2) 
EQU (x-x.sub.3).sup.2 +(y-y.sub.3).sup.2 +(z-z.sub.3).sup.2 =l.sub.3.sup.2 (3) 
If the receptors 20a, 20b, 20c are arranged at the same height (z.sub.1 
=z.sub.2 =z.sub.3), the computation is simplified. 
The ultrasonic emitter 19 is made to emit several pulses separated by a 
given time interval, and the computer 33 performs the computation 
described above for each pulse. 
By repeating this computation several times and taking the average, it is 
possible to obtain an accurate measurement for the position of the stirrer 
with very little error. If, however, motors 5, 7a and 7b are running when 
the ultrasonic pulse is emitted, errors of measurement can easily arise 
due to noise. All motors are therefore first stopped, and the pulse is 
emitted after a certain time has elapsed, that is, after the stirrer has 
settled on the bottom of tank 22. In carrying out the measurement, the 
interval of ultrasonic emission is determined by the maximum value of the 
distance to be measured (equivalent to the diameter of the tank). If 
however there are considerable echos due to collision of the ultrasonic 
wave with and reflection from the tank inner wall, the interval is 
determined as shown in FIG. 7 by time consisting of the time S.sub.1 
corresponding to the greatest distance of traverse, plus the time S.sub.2 
required for the echos to absorb completely. 
Even in this case, however, the ultrasonic receptors 20a, 20b, 20c may 
receive echos after they have received the first pulse until the next 
pulse emission begins. The measuring instrument 32 therefore treats the 
first pulse received as correct information, and ignores the subsequent 
echos. This is because the shortest distance between the emitter 19 and 
the receptors 20a, 20b, 20c is the straight line distance connecting them, 
and echos which do not take this path but are reflected must necessarily 
be received after the regular pulse has arrived. 
The motor drive and signal cable 21 attached to the stirrer 3 is wired 
along the secondary buoyancy tank 14. This cable 21, as shown in FIG. 4, 
is supported by floats 24, 25 positioned in the liquid 23 contained in 
tank 22, and is connected to the control unit 26 outside tank 22. 
If the roof 27 of the liquid tank 22 is of the floating type, the ends of 
the floats 25, as shown in FIG. 8, come into contact with roof 27. The 
length l of the floats 25 is greater than the length l.sub.1 of the 
columns 28, so that cable 21 does not become tangled around columns 28 
which support floating roof 27 when it reaches the bottom of the tank. If 
a cable duct 29 is provided for cable 21 to the floating roof 27, the 
length l of the floats 25 is of course chosen to be greater than the 
length l.sub.2 of the duct. 
The control unit 26 determines the position of stirrer 3 when the above 
position finding device is operated. It also drives motors 5, 7a, 7b and 
controls the motion of the stirrer based on the position found, the 
contact signals received from said obstacle detectors 16, and the depth 
signal from depth finder 17. 
Alarm indicators are provided on the control unit to show the status of 
stirrer position, depth and contact with obstacles. A manual operation 
switch and auto operation switch are provided so that the control 
operations can be performed either manually or automatically as desired. 
As shown in FIG. 4, the stirrer is introduced into tank 22 through a 
manhole 30 in the roof. When the stirrer is introduced, it is adjusted by 
means of the primary and secondary buoyancy tanks 12a, 12b and 14, such 
that its weight in the liquid is practically zero. 
The motors 5, 7a, 7b are started from control unit 26 and, by rotating the 
vertical rotors 6a, 6b and the horizontal rotors 9a, 9b in a given 
direction or in the reverse direction, the stirrer is made to rise, fall, 
move forwards or backwards horizontally, or change its direction. 
If the vertical rotors 6a, 6b are rotated clockwise, liquid is pushed 
vertically downwards by the impeller blades, and stirrer 3 therefore 
rises. If rotors 6a, 6b are rotated in the reverse direction, liquid is 
pushed vertically upwards and stirrer 3 falls. 
If the horizontal rotors 9a, 9b are rotated clockwise, liquid 23 is pushed 
back horizontally, and the stirrer moves forward. If rotors 9a, 9b are 
rotated counter-clockwise, liquid 23 is pushed forwards, and stirrer 3 
moves backward. Further, if rotors 9a, 9b are rotated in mutually opposite 
directions, the direction of the stirrer changes. 
Thus, as the stirrer moves almost uniformly through liquid storage tank 22, 
the rotors 6a, 6b or rotors 9a, 9b stir the liquid at the same time. This 
causes the sludge which tends to separate out from the liquid and 
accumulate on the bottom of storage tank 22 to be stirred, lifted up by 
the flow, dispersed, and prevented from accumulating on the bottom. 
Further, when the stirrer is on the bottom of tank 22, the rotating 
brushes 11a, 11b are driven to mechanically pulverize the sludge and shear 
through it. The sludge thus becomes finer, floats up and is carried on the 
upward flow produced by rotors 6a, 6b, tending to promote its 
redissolution in the liquid. 
As has already been mentioned, vertical buoyancy tanks 12a, 12b and 14 are 
fitted to frame 4 in a one piece construction. As a result, when the 
stirrer 3 is moving forwards or backwards and loses its balance or 
oscillates, there is a strong vibration damping effect and restoring force 
tending to maintain its orientation stable. 
Due to buoyancy tanks 12a, 12b and 14, the weight of stirrer 3 in the 
liquid is practically zero. It does not therefore require such large 
driving force to make the stirrer rise. Similarly, there are no sudden 
falls when the stirrer is falling, and damage due to collision of the 
stirrer with the bottom of storage tank 22 is avoided. 
The stirrer 3 is also provided with a flow plate 15. When the stirrer is 
moving forwards or backwards, therefore, its direction is constrained by 
this flow plate 15, so that motion is executed in a perfectly straight 
line. 
This sequence of operations can be performed manually while watching 
various indicators on control unit 26, and automatically by means of a 
control program set up in control unit 26. 
In the case of automatic operation, control unit 26 first lifts the stirrer 
from a resting position on the bottom of the tank, moves it horizontally 
by a given distance, allows its to settle on the bottom again, and stirs 
the liquid. This normal control sequence is effected as follows: 
Motors 5, 7a and 7b are driven so as to rotate vertical rotors 6a, 6b 
clockwise, and horizontal rotors 9a, 9b both clockwise. These rotations 
cause the stirrer to move forwards while rising. After rotors 6a, 6b have 
been rotated clockwise for a time T.sub.1, they are rotated reverse 
direction for a time T.sub.2 (T.sub.1 &lt;T.sub.2) , and rotors 9a, 9b are 
stopped after a time T.sub.3. 
These operations cause the stirrer 3 to move forward a given distance and 
to fall to the bottom of the tank 22. The rotating brushes 11a, 11b are 
then rotated so as to remove sludge 31 adhering to the bottom of the tank 
and clean the bottom. After the rotors 6a, 6b have rotated in the reverse 
direction for time T.sub.2, control unit 26 again rotates rotors 6a, 6b 
clockwise for a time T.sub.1, and rotates rotors 9a, 9b for a time 
T.sub.3. By repeating these operations over and over again, the stirrer 3 
is made to rise, fall and move forward, stirring and cleaning liquid 
storage tank 22 as it does so. 
If, during the above operations, a signal is received from the obstacle 
detectors 16 indicating that the stirrer has come into contact with an 
obstacle or with the inner wall of tank 22, evasive action is taken. 
This evasive action consists of lifting the stirrer from its position and 
rotating it, the direction of this rotation and the magnitude of the angle 
of rotation being completely random. If this action were given some 
regularity, the stirrer's motion would fall into a steady pattern (such 
as, for example, executing a back and forth motion between opposite 
walls), which it is intended to avoid. This control sequence is effected 
as follows. 
If rotors 6a, 6b are rotating counter-clockwise, control unit 26 drives 
motor 5 so as to rotate them clockwise; while if rotors 6a, 6b are 
rotating clockwise, motor 5 is driven so as to continue this clockwise 
rotation. At the same time, motors 7a, 7b are driven so as to rotate 
rotors 9a, 9b in the opposite direction to one another. Due to the 
clockwise rotation of rotors 6a, 6b, and the rotation of rotors 9a, 9b in 
mutually opposite directions, stirrer 3 rises while rotating about its own 
axis. 
The stirrer is made to rise for a time T.sub.4, following which rotors 6a, 
6b are rotated counter-clockwise for a time T.sub.5 (T.sub.4 &lt;T.sub.5), 
and rotors 9a, 9b are stopped after a time T.sub.6 (T.sub.6 &lt;2T.sub.4). 
Due to these operations, the stirrer body rotates, and then falls and 
settles on the bottom of the tank. The direction of this rotation is 
determined by the mutually opposed directions of rotation of rotors 9a, 
9b, and the angle of rotation is determined by T.sub.6. The choice of 
direction and the magnitude of T.sub.6 depend on the signals from the 
random signal generator incorporated in control unit 26. 
If control unit 26 continues to receive signals from the obstacle detectors 
16 even after the end of period T.sub.6, the above control operations are 
repeated until no further signals are received. When no further signals 
are received, the stirrer is operated normally as described previously. 
Control unit 26, apart from carrying out normal control and obstacle 
evasion control, also determines the depth and position of the stirrer by 
receiving signals from depth finder 17 and the position finding device. 
The range of movement of the stirrer can be set depending on its depth and 
position. 
The position of the stirrer is determined as described previously. In 
carrying out the determination, control unit 26 operates the position 
finding device when the stirrer is at rest, as shown by FIG. 9. As shown 
in FIG. 9, the stirrer body 3 rises when rotors 6a, 6b are rotating 
clockwise. When rotation is stopped, however, the stirrer body 3 falls 
under its own weight, and settles on the bottom of the tank after a time 
T.sub.0. After a further time T'.sub.0 , an ultrasonic wave pulse is 
emitted from ultrasonic emitter 19 in the position finding device, and the 
position of the stirrer is determined. This determination is carried out 
with the stirrer stationary on the bottom of the tank in order to 
eliminate the effect of noise due to motors 5, 7a and 7b, and to give a 
stable reading for the position. 
When the stirrer reaches the limit of the set range of movement, depending 
on the position found, control unit 26 rotates rotors 6a, 6b and rotors 
9a, 9b, and changes the direction of stirrer 3 as in the obstacle evasion 
sequence. If stirrer 3 was rising, it is made to fall. By setting the 
range of movement beforehand, therefore, the stirrer moves only over the 
set range, and stirs the liquid in the tank efficiently.