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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a device for dosing a liquid or a slurry
or for pumping it in a controlled manner.
Swedish Patent Specification No. 373,206 discloses a device for dosing
liquids, a device in which a piston in a cylinder is reciprocated by the
force of the pressure of the liquid. A separate valve mechanism controls
the valves at the ends of the cylinder in such a manner that the pressure
of the liquid to be fed moves the piston alternately in different
directions and presses the liquid ahead of it to the dosing point. The
arrival of the piston at one end of the cylinder is indicated by means of,
for example, an inductive switch. At this time the valve mechanism shifts
the pressurized feed flow to the other end of the cylinder, and the piston
starts off in the opposite direction, simultaneously forcing the liquid
ahead of it through the valve mechanism to the feeding point.
In the above device the liquid feed rate depends on the feed pressure of
the liquid and on the viscosity of the liquid. When the feed pressure
increases or the viscosity decreases, for example owing to a change in the
temperature, the liquid flow rate in the pipe system increases and,
likewise, the dosing rate increases. In addition, the dosing rate must be
adjusted by adjusting the feed pressure, a procedure which requires
complicated equipment.
Swedish Patent Specification No. 380,347 discloses a device of
corresponding type for dosing liquids. Compared with the previous patent,
this device has, additionally, separate clockwise mechanisms in series
with the switches which indicate the position of the piston. Thus, when
the piston arrives at one end of the cylinder, the clockwise mechanism
delays the shift of the state of the valve mechanism by a predetermined
length of time. Thus the piston waits at the end of the cylinder for a
predetermined length of time before a new movement. The average dosing
rate of the device can be adjusted by adjusting the waiting time.
In the device described above, changes in the feed pressure and viscosity
of the liquid produce a change in the dosing rate. The device regulates
only the waiting time of the piston at the ends of the cylinder. When the
feed pressure or the liquid viscosity changes, the velocity of the piston
changes, and thus there is a change in the length of one cycle of the
device, i.e. the sum of the time consumed for the movement of the piston
from one end of the cylinder to the other and the waiting time. This
further changes the average feed rate of the device.
Finnish Patent Specification No. 31 300 discloses a device of a
corresponding type for dosing liquid, a device in which a signal for the
reciprocating movement of the piston is given by means of an external
impulse. When the piston reaches the end of the cylinder, the pressure of
the liquid shifts the valve mechanism in such a way that the piston moves
back to the initial position, in which the piston remains waiting for the
new feed impulse.
The device described above does not indicate at all how completely the
piston has time to make a full feeding movement during one cycle. In the
event that, owing to a decrease in the liquid feed pressure or an increase
in the liquid viscosity, the liquid flow is slowed down and the piston
does not have the time to complete the movement before a new feed impulse,
a change in the average feed rate is produced without the device itself
being capable of detecting or indicating it.
The object of the present invention is to provide a completely automatic,
precise and rapid-acting device, operating by means of the pressure of the
liquid or slurry for dosing or for pumping in a controlled manner a liquid
or slurry. The present invention thus relates to a device of the
above-mentioned type, having a piston which reciprocates freely in the
cylinder; members connected to the cylinder on the opposite sides of the
path of the piston movement, for feeding the liquid or slurry, under
pressure, into the cylinder in order to move the piston, and for
discharging, when the piston returns, the liquid or slurry from the
cylinder by means of the pressure of the liquid or slurry directed into
the cylinder to the opposite side of the piston; valve members in the
liquid or slurry feed and discharge members, for their alternate opening
and closing; switches sensing the piston position, fitted at the ends of
the cylinder; and a control unit for adjusting the piston stroke frequency
and thereby the volume flow of the liquid or slurry and for transmitting
the signals given by the switches to the valve members in order to open
and close them at different positions of the piston.
SUMMARY OF THE INVENTION
Contrary to the above-mentioned devices, in the device according to the
invention the control unit has been fitted to control the time interval
between the starts of the piston movement. The control unit is preferably
a clockwork mechanism or an external impulse, for example a time relay.
Simultaneously with the starting of the movement of the piston, the state
of action of the valve members is determined by signals received from the
switches and the control unit. According to one embodiment of the
invention, the control unit gives an error signal if the piston has not
within the predetermined time interval moved from one end of the cylinder
to the other.
In the device according to the invention, the piston stroke frequency, and
thereby the volume flow of the liquid or slurry, remains constant in spite
of variation in the feed pressure and viscosity of the liquid or slurry.
Consequently, it can be used for dosing liquids or slurries more
precisely, more rapidly and more reliably than previously. Furthermore,
the device according to the invention is of simple construction and easy
to service.
The switches sensing the piston position are preferably inductive switches,
in which case the piston is at least partly of some inductively detectable
material, such as refined iron. The valve members, for their part, can be
three-way valves fitted in the feed and discharge members or one common
four-way valve.
In practice the same liquid often has to be dosed to several different
feeding points. When the device according to the invention is used, only
one liquid pump is required for feeding the liquid. The dosing does not
require an additional source of power. When the pressure of the feed line
is above 100 kPa, a piston having a diameter of 20-30 mm already operates
well, and the velocity of its movement in the cylinder is several
centimeters per second. In this case the feed rate reaches several liters
per minute.
In the practical embodiments of the invention, the cylinder can be detached
manually, without auxiliary equipment. Thus the cylinder can, for example
in case of damage, be replaced rapidly, or it can be replaced with a
cylinder of a different volume when the required amount of chemicals
changes substantially.
The device can thus operate within a wide operating range without a
detrimental effect on its precision.
The device according to the invention can easily be applied in such a
manner that the control from the time relay is replaced with, for example,
control from a computer. In such a case the movement is also monitored
continuously at the ends of the cylinder, and so any operational
disturbances can be detected by means of alarm circuits.
DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cross sectional schematic side elevation of a
preferred embodiment of the invention, and
FIG. 2 illustrates a cross sectional side elevation of an alternative
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one embodiment of the invention, depicted in FIG. 1, the device consists
of a cylinder 1 and a piston 2 reciprocating inside it. Switches 4, which
sense the piston, are installed in the ends of the cylinder. The switches
4 can, for example, operate according to the induction principle, in which
case at least part of the piston 2 is made of a material such as refined
iron which enables the piston 2 to be sensed inductively. The valve
mechanism 5 is connected to the ends of the cylinder by means of pipes 6
and 7. The valve mechanism consists of a four-way valve, and the valves
can be, for example, electromagnetic valves. The liquid to be fed
determines the materials of the device.
The position of the valves 5 is determined by means of signals from the
switches 4 in the ends 3 of the cylinder 1 and, for example, from a time
relay 10. The cylinder 1 is dimensioned on the basis of the maximum feed
rate required. The feed rate is adjusted by changing the stroke frequency
of the piston 2 by means of the time relay 10. The invention is not,
however, limited to the piston moving in the cylinder 1 from end to end,
but, when desired, the stroke length of the piston 2 can be limited.
Liquid flows by its own pressure along the pipe 8 into the valve mechanism
5 and further, under pressure, along the pipe 6 into the cylinder 1. The
liquid pressure moves the piston 2 to the opposite end of the cylinder 1,
whereby the liquid ahead of the piston 2 is pushed along the pipe 7 into
the valve mechanism 5 and further along the discharge pipe 9 to the liquid
feeding point. When the piston 2 reaches the end of the cylinder 1, the
flow stops. The piston waits at the end of the cylinder until a
predetermined time has elapsed from the previous start of the piston, and
the time relay 10 gives a signal for a new movement. At this time the
valve mechanism 5 selects, with the aid of the signals from the switches 4
in the ends 3 of the cylinder, the position in which the liquid flows by
its own pressure from the inlet pipe 8 along the pipe 7 into the cylinder
1. The piston 2 pushes ahead of it liquid into the pipe 6. At this stage
of the operation the valve mechanism 5 directs the liquid from the pipe 6
along the pipe 9 to the feeding point of the liquid. The time relay 10
determines the time interval between the starting times of the strokes of
the piston 2, i.e. the number of strokes per time unit, and thereby the
liquid volume flow per time unit. If, at the time of a signal from the
time relay a signal has not been received by means of switches 4 at the
ends of the cylinder that the piston has moved to the opposite end of the
cylinder after the previous signal, the time relay 10 gives an error
signal.
The device shown in FIG. 2 differs from the device shown in FIG. 1 in that
two three-way valves 14 are used instead of one common four-way valve 5.
In this case the liquid or slurry flowing under pressure in the feed pipe
13 flows through one of the three-way valves 14 into pipe 6 or 7 and
further into the cylinder 1 to one side of the piston 2 in it. At this
time, the other valve 14, which is closed to the liquid or slurry flowing
in the feed pipe 13 is in such a position that the liquid or slurry on the
opposite side of the piston 2 in the cylinder 1 can flow along the pipe 9
or 8 into the discharge pipe 11.
In the embodiment depicted in FIG. 2, mechanical switches 12 are used
instead of inductive switches; their length is preferably adjustable, and
on the basis of the data given by them the time relay or data center 10
closes and opens the three-way valves 14.
It is evident that it is possible to use four one-way valves instead of two
three-way valves. It is also evident that the movement of the piston 2
moving in the cylinder 1 can be used for driving the valve mechanism by
means of mechanical switches fitted at both ends of the cylinder. In such
a case, no external source of energy is required for driving the device
according to the invention, and the device is operated solely by means of
the pressure of the liquid or slurry.
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