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(11) | EP 1 270 077 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Pressure discharge flotation device. |
| (57) The invention relates to a pressure discharge flotation device comprising a flotation
tank fitted with an aerating system, which aerating system comprises a device for
pressurising a flotation liquid and dissolving a gas therein, and at least one pressure
discharge device for reducing the pressure of the pressurised flotation liquid containing
dissolved gas, which pressure discharge device comprises at least one valve arranged
in a conduit line of the flotation liquid, which valve can be controlled by means
of a fluid under a working pressure, whilst pressure control means are present for
controlling the working pressure of said fluid. The object of the invention is to provide a pressure discharge device, which is less susceptible to wear and which will not become obstructed, which does not comprise any mechanical parts and which furthermore allows precise adjustment within a wide working range and which, once it has been adjusted in such a manner that gas bubbles having the correct diameter are obtained, will automatically maintain said adjustment independently of changes in the flotation liquid being supplied. According to the invention, this objective is accomplished in that the valve comprises a flexible hose member, which hose member forms at least part of the wall of the conduit line at the location of the valve, and wherein the passage of said hose member is adjustable through omnilateral deformation thereof by the fluid under a working pressure. |
[0001] The invention relates to a pressure discharge flotation device comprising a flotation tank fitted with an aerating system, which aerating system comprises a device for pressurising a flotation liquid and dissolving a gas therein, and at least one pressure discharge device for reducing the pressure of the pressurised flotation liquid containing dissolved gas, which pressure discharge device comprises at least one valve arranged in a conduit line of the flotation liquid, which valve can be controlled by means of a fluid under a working pressure, whilst pressure control means are present for controlling the working pressure of said fluid.
[0002] Such a pressure discharge flotation device is known from US Patent No. 5,199,684. The pressure discharge device disclosed therein comprises pneumatically or hydraulically operated valves, in which a pneumatically or hydraulically controlled fixed body closes the fixed opening to a greater or smaller extent.
[0003] Such valves, in which a fixed body moves with respect to a fixed opening so as to regulate the dimension of the opening available for flow, wear out quite rapidly and adjustment thereof to a position such that the correct pressure difference across the valve is obtained takes a great deal of effort and experience each time (for example after cleaning). The correct pressure difference is necessary in order to ensure that the gas bubbles resulting from the pressure discharge have the correct diameter, viz. a diameter ranging between 20 and 70 µm.
[0004] Another embodiment of a pressure discharge flotation device according to the introduction is disclosed in International Patent Application No. WO 95/26930. Said patent specification describes the use of a valve provided with a flat membrane which can be adjusted under the influence of a fluid under a working pressure. The valve is of complex construction and comprises several parts which are susceptible to wear and which consequently require maintenance. Furthermore, the fluid, which is under a working pressure, acts on one side of the membrane, and the flow through the membrane can be adjusted to a limited extent as a result of the presence of the valve in the system of conduits. As a result, an accurate adjustment is not possible. Owing to the limited adjustability and the winding flow therethrough, the valve will very easily become obstructed with impurities.
[0005] In addition to that, US Patent No. 3,542,675 discloses a valve, in which a valve disc attached to a valve stem is movable with respect to a valve seat under the influence of a fluid under a working pressure (pressurised air), which acts on a membrane, for adjusting the medium flowing through the valve. Also this valve comprises several parts which are susceptible to wear. As a result of the small clearance or passage between the valve disc and the valve seat, the working range of the valve is only adjustable to a limited extent, so that a precise adjustment of the valve is not possible very well. Similarly to WO 95/26930, the valve is easily obstructed by impurities in the liquid flowing through the valve as a result of the small passage thereof.
[0006] The object of the invention is to provide a pressure discharge device, which is less susceptible to wear and which will not become obstructed, which does not comprise any mechanical parts and which furthermore allows precise adjustment within a wide working range and which, once it has been adjusted in such a manner that gas bubbles having the correct diameter are obtained, will automatically maintain said adjustment independently of changes in the flotation liquid being supplied.
[0007] According to the invention, this objective is accomplished in that the valve comprises a flexible hose member, which hose member forms at least part of the wall of the conduit line at the location of the valve, and wherein the passage of said hose member is adjustable through omnilateral deformation thereof by the fluid under a working pressure.
[0008] As a result of the omnilateral exertion of the force on the hose from the outside, the area of the passage of the hose can be adjusted very precisely to a value which depends on the pressure of the fluid and the pressure of the flotation liquid. In addition to that, this construction does not comprises any parts that are susceptible to wear and the risk of obstruction during operation is virtually non-existent.
[0009] One preferred embodiment of a pressure discharge flotation device according to the invention is characterized in that the passage of the flexible hose member is substantially identical to the passage of the conduit line if the working pressure of the fluid equals zero. Not only does this enable adjustment of the valve over a large range, but in addition to that any obstruction problems can be remedied very quickly and effectively.
[0010] In a specific embodiment, the pressure control means for controlling the working pressure of the fluid comprise a pressure chamber which is cylindrically oriented round the hose member.
[0011] Yet another preferred embodiment of a pressure discharge flotation device according to the invention is characterized in that the pressure control means comprise sensor means for sensing the flow of the pressurised flotation liquid, which pressure control means are arranged for reducing the working pressure of the fluid if the through-flow falls below a previously adjusted value.
[0013] In the unhoped-for event of a complete or partial blockage of a valve, this will manifest itself in a reduced through-flow of the flotation liquid in comparison with the through-flow of the flotation liquid if no such blockage occurs. The amount of flotation liquid that should flow through per unit time with a given working pressure and an unobstructed valve is known per se. If the measurement by the sensor indicates that less flotation liquid is flowing through than expected with a particular working pressure, this indicates a possible obstruction of a valve. Said obstruction can easily be remedied in that case by reducing the working pressure of the fluid, as a result of which a larger amount of liquid can flow through the enlarged passage of the valve and said passage can become so large that any obstructing element that may be present can flow through the enlarged opening yet.
[0014] The invention also relates to a method for unblocking a conduit line for flotation liquid in a pressure discharge flotation device, in which a valve whose passage can be adjusted is incorporated in the conduit line, which method is according to the invention characterized in that the valve comprises a flexible hose member, whose passage can be adjusted through omnilateral deformation of the hose member, and wherein said passage is so adjusted in the case of an at least partial blockage of the conduit line that the internal diameter of the hose member is at least as large as that of the conduit line.
[0015] The invention will now be explained in more detail with reference to the accompanying figures, in which:
Figure 1 is a schematic representation of a pressure discharge flotation device according to the invention; and
Figure 2 is a longitudinal sectional view of such a valve.
[0016] In Figure 1, numeral 1 schematically indicates a pressure discharge flotation device. The device 1 comprises a flotation tank 2, a recirculation pump 3, a saturation vessel 4 and pressure discharge valves 5. The flotation tank 2 comprises an inlet 11 for contaminated liquid, the influent. A scraper mechanism 6 moves in the direction as indicated by the arrows a and b for transporting the layer of sludge on the liquid in the direction of the space 8. The space 8 is separated from the flotation liquid in the tank 2 by means of a screen 7. The sludge is removed from the space 8 via conduit 12. Purified liquid is removed from the tank 2 via conduit 13. A screen 9 prevents the layer of sludge or sludge particles from finding their way into the conduit 13. The purified liquid flows to the conduit 13 over screen 10. Conduit 14 is connected to the tank 2, near a location where the liquid is already purified in large measure, and it connects the tank 2 to the recirculation pump 3. A pressurised gas is supplied to the saturation vessel 4 by means of device 39. Generally, the device 39 will be an air pump, which pumps air into the saturation vessel 4, but also other gases are conceivable. At the bottom of vessel 4, an amount of liquid 51 is present, which liquid is saturated with the gas that has been supplied under pressure by the device 39. The gas-saturated flotation liquid is transported to the pressure discharge device 5 via conduit 16, under the influence of the overpressure in the space 52 in the saturation vessel 4.
[0017] In Figure 1, several places are indicated where pressure discharge valves may be provided in the pressure discharge device 5. Thus, the pressure discharge valve 20 is disposed at the bottom side of the tank 2, pressure discharge valve 21 is disposed more at the front side of the tank 2, and pressure discharge valves 22 and 23 are not disposed on the tank 2, they are connected to the influent conduit 11. The gas bubbles that have formed after the pressure discharge valves 22 and 23 can thus adhere to dirt particles or flakes in the influent conduit 11 already before flowing into the tank 2.
[0018] The gas-saturated liquid, which is supplied to the pressure discharge valves 20, 21, 22 and 23 via the conduits 16, 17, 18a, 18b and 19b, is under a pressure of about 6 bar. The gas-saturated solution expands in the pressure discharge valves 20, 21, 22 and 23. As a result, the dissolved gas is released in the form of millions of small bubbles having dimensions in the order of 20 to 70 µm.
[0019] Whilst up to now devices in which a body having a predetermined shape and a fixed opening are moved relative to each other for adjusting the passage formed between said body and said opening have been used as the pressure discharge valves 20, 21, 22 and 23, the present invention employs valves.
[0020] One example of such a valve is shown in Figure 2. The valve 40 that is shown in Figure 2 comprises a substantially cylindrical housing 41, which is provided with an inlet opening 42. A conduit 43 can be connected to said inlet opening. A cylindrical, flexible hose member 44 is fitted between the housing 41 and the flanges 46 and 47. A space 48 is present between the housing 41 and the hose member 44, which space is in communication with the inlet 42.
[0021] The operation of the valve 40 is as follows. The valve 40 is fitted in a conduit 53 (schematically shown) by means of connecting pieces 46 and 47. A liquid flows through said conduit 53. The liquid flowing through the conduit 53 also flows through the cylindrical part 50 of the valve 40. Via a conduit 43, a fluid, a gas or a liquid is forced into the space 48 under pressure via the opening 42. The pressure of the fluid in the space 48 may be greater than the pressure in the liquid in the space 50. The flexible hose member 44 will bend in that case, for example to a position as illustrated by the dashed line 35. As a result, the fluid passage in the conduit 53 will be reduced from the area at the inlet or outlet 49 in the connecting piece 46 to the cross-sectional area at 54. When the pressure in the space 48 is sufficiently high, the valve 40 will shut off the flow of liquid through the conduit 53 altogether.
[0022] The diameter of the opening 49 may vary, for example between 10 and 30 mm, and the length are of the valve 40 may vary between and 120 and 220 mm, for example.
[0023] Advantageous in the present application is the fact that the passage 54 is bounded by the hose member 44, which extends in a flowing manner, as is illustrated in dashed lines 45. This minimises the risk of a dirt particle carried along with the flotation liquid from becoming trapped in a sharp corner, where the velocity of flow is low.
[0024] If a dirt particle carried along in the flotation liquid flowing through the conduit 53 should have dimensions such that the opening 54 is obstructed, a pressure difference will be created across the particle, which pressure difference will attempt to force the particle through the opening 54 yet. It is an advantage in this connection that the hose member 44 is flexible and that the shape of the opening 54 can adjust itself to the shape and the size of the particle that is sufficiently large to shut off the opening 54. The result is that the particle in question can pass the valve 40 yet. Only dirt particles of considerable dimensions, which are significantly larger than the dimensions of the opening 54, run the risk of being stopped by the valve 40.
[0025] Such stopping is noticeable in that the amount of liquid flowing through a conduit 53 will decrease. The amount of liquid that flows through a conduit 53 with a specific pressure in the space 48 is known. By measuring the amount of flotation liquid that flows through the conduits in question per unit time, for example by means of sensors 33, 34, 35, 36 and 37, it is possible to establish whether one of the valves 20, 21, 22 and 23 has been obstructed. The signals from the sensors 33-37 are carried to a control unit 38 by the signal lines 28, 29, 30, 31 and 32. The control unit 38 is furthermore arranged for supplying pressurised fluid to the valves 20, 21, 22 and 23 via the conduits 27, 26, 25 and 24, respectively.
[0026] Furthermore, the control device 38 is arranged, in a manner which is known per se, for converting the signals from the sensors 33-37 coming in via the signal lines 28-32 into a desired pressure of the fluid in the conduits 24-27.
[0027] If an obstruction occurs in a conduit, which is detected by one or more of the sensors 33-37, the reaction of the control device 38 will be to reduce the fluid pressure before the valve in the conduit 24-37 or 43 in question that may be obstructed. As a result of the pressure reduction, the hose member 44 in the valves in question will be curved less, as a result of which the opening 54 in the valves in question will be enlarged, so that the dirt particle in question can pass the valve in question yet.
[0028] Preferably, the control device 38 is so arranged that the pressure of the fluid in the conduits 24-27 is reduced at predetermined points in time. In the case of obstructions that have not been detected yet or in the case of obstructions in installations which do not comprise sensors 33-37, the valves in question will thus be cleared of accumulated dirt yet.
[0029] A special advantage of valves, such as the valve 40, used in a pressure discharge flotation device as described by way of example with reference to Figure 1, is that such valves, which are used as pressure discharge valves, only need to be adjusted once as far as the pressure of the fluid forced into a space 48 via the conduit 43 is concerned. with a specific pressure in the space 48 and a specific pressure of the gas-saturated flotation liquid in the conduit 53, the opening 54 will provide a pressure drop across the valve 40 as determined for said two pressures at all times. Preferably, the pressure in the space 48 is selected so that gas bubbles having a desired diameter are formed in the liquid which has passed the valve 40. The advantage of using a valve 40 as a pressure discharge valve is that when the flotation liquid being supplied has a specific pressure, the diameter of the gas bubbles formed after the pressure discharge is directly related to the pressure in the space 48.
[0030] As a result, it is only necessary to adjust the pressure of the fluid in the space 48 in order to adjust the desired diameter of the gas bubbles for the flotation. Complex mechanical adjustments and mechanisms for mechanically positioning a body and opening relative to each other in such a manner that gas bubbles having the correct diameter will be formed again after the installation has been out of operation are no longer necessary. With existing mechanical installations, it is not possible in practice to select the same adjustment automatically again in order to achieve the same result. With the valves that are used in accordance with the invention, on the other hand, it is possible to select the same adjustment every time in order to achieve the same result.
[0031] Hose valves consist of a length of flexible hose, through which a gas or a liquid can flow. In the present case, gas-saturated flotation liquid flows through the hose. The hose is surrounded by a housing, which is connected thereto, so as to form a space between the housing and the hose. A pressurised fluid can be supplied to the space between the housing and the hose. The fluid exerts a pressure on the outside of the hose, thus reducing the hose diameter. EPDM, NR, SPR, Neoprene CR, butyl rubber, FKM, silicone SI PUR etc. are examples of the material to be used for those. In the present application, the diameter of the hose will frequently range between 10 and 40 mm, depending on the flow rate. The wall thickness of the hose may vary between 4 and 8 mm.
1. A pressure discharge flotation device comprising a flotation tank fitted with an aerating
system, which aerating system comprises a device for pressurising a flotation liquid
and dissolving a gas therein, and at least one pressure discharge device for reducing
the pressure of the pressurised flotation liquid containing dissolved gas, which pressure
discharge device comprises at least one valve arranged in a conduit line of the flotation
liquid, which valve can be controlled by means of a fluid under a working pressure,
whilst pressure control means are present for controlling the working pressure of
said fluid, characterized in that the valve comprises a flexible hose member, which hose member forms at least part
of the wall of the conduit line at the location of the valve, and wherein the passage
of said hose member is adjustable through omnilateral deformation thereof by the fluid
under a working pressure.
2. A pressure discharge flotation device according to claim 1, characterized in that the passage of the flexible hose member is substantially identical to the passage
of the conduit line if the working pressure of the fluid equals zero.
3. A pressure discharge flotation device according to any one or more of the preceding
claims, characterized in that the pressure control means for controlling the working pressure of the fluid comprise
a pressure chamber which is cylindrically oriented round the hose member.
4. A pressure discharge flotation device according to any one or more of the preceding
claims, characterized in that the pressure control means comprise sensor means for sensing the flow of the pressurised
flotation liquid, which pressure control means are arranged for reducing the working
pressure of the fluid if the through-flow falls below a previously adjusted value.
5. A method for unblocking a conduit line for flotation liquid in a pressure discharge
flotation device, in which a valve whose passage can be adjusted is incorporated in
the conduit line, characterized in that the valve comprises a flexible hose member, whose passage can be adjusted through
omnilateral deformation of the hose member, and wherein said passage is so adjusted
in the case of an at least partial blockage of the conduit line that the internal
diameter of the hose member is at least as large as that of the conduit line.