Method and device for fine filtering

Abstract

A method for use of an oil-hydraulic fine filter of the type where the outlet side of the fine filter is designed to operate without overpressure, where the working fluid that flows from the fine filter in a depressurized state is pumped on by a second pump, and where the second pump is driven by the same driving device as the feed pump of the fine filter.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to International Application No. PCT/NO02/00190, filed on May 30, 2002, which claimed priority to Norwegian Application No. 20012862, filed on Jun. 8, 2001, the disclosures of which are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] This invention regards a method of ensuring stable fine filtering of liquids in e.g. oil-hydraulic installations. The invention also regards a device for carrying out the method.

[0003] It is well known that the presence of contaminants in the form of e.g. metal and mineral particles in the working fluid in oil-hydraulic installations is may reduce the service life of the components of the installation significantly.

[0004] Such installations therefore normally comprise at least one filter designed to filter out contaminants. A simple and inexpensive solution is to provide the suction side of the pump with a relatively coarse suction strainer and the return line to the reservoir of the installation with a return filter. However, with this filter positioning, contaminants that are generated in the hydraulic pumps will not be captured before they flow into valves and actuators. In particular when using more complex equipment such as servo valves, a high pressure full flow filter is normally placed between the valves and pump(s) of the installation.

[0005] An ordinary full flow filter is not capable of filtering out the smallest of the contaminants in question. It has therefore been necessary in order to overcome this situation, to carry out relatively frequent oil changes or perform fine filtration, also termed deep filtration, of the working fluid.

[0006] Fine filters of the type mentioned here are, if used correctly, capable of filtering out sufficiently fine contaminants to allow the working fluid to be considered of satisfactory value in terms of contaminants, after treatment. Fine filters may be equipped with a water separator.

[0007] In order to obtain a satisfactory result from fine filtering through filters of the type in question, the fluid fed to the fine filter must lie within a relatively narrow pressure range, so as to keep the pressure drop across the filter constant. The fine filter works best without overpressure on the outlet side of the filter. Thus the filtered working fluid flows away from the filter by means of gravity. The flow rate through the filter must be constant and preferably in a laminar form.

[0008] According to prior art, two principal methods are employed in connection with the use of fine filters. By a first method, working fluid is pumped via a separate pump from the main reservoir of the installation to a fine filter. The excess and filtered fluid is returned to the reservoir in a depressurized state. By a second method, a certain volume of working fluid is drained to the fine filter from the pressure loop of the installation by means of e.g. a constant flow valve.

[0009] The working fluid may flow back to the reservoir in a depressurized state; however it is more common to use a separate reservoir for fine filtered fluid, from where the fluid is later returned by means of a separate pump to the main reservoir. The pumps used with a separate reservoir may be level controlled in order to allow automatic control of the treatment plant.

[0010] It is also known to use a reversible pump that will feed working fluid to the fine filter when rotating in one direction, and transfer the working fluid between the reservoirs when rotating in the opposite direction. Inevitably of necessity, this latter method will prolong the filtration period.

[0011] The possible positioning of the fine filter in relation to the remaining components of the hydraulic installation is limited to a considerable degree by the fact that the return side of the fine filter is unpressurized. Installations according to prior art are not suited for use in mobile installations such as diggers and equipment on wheels.

[0012] In fine filter installations where the filtered working fluid is returned to a separate reservoir in a depressurized state, experience has shown that deposits of contaminants and accumulation of water may occur after a period of use. The cleaning required results in periodic shutdowns of the installation.

[0013] The object of the invention is to remedy the disadvantages of prior art.

[0014] The object is achieved in accordance with the invention by the characteristics stated in the description below and in the appended claims.

SUMMARY OF THE INVENTION

[0015] A double pump, a tandem pump, a pump with a feed part and a main part, alternatively another pump equipped with two separate pump parts, are driven by one drive motor. The suction side of the first pump, the feed pump, is connected to the main reservoir of the installation, and in operation pumps is working fluid from the reservoir to a fine filter. Any excess volume from the delivery side of the first pump flows through a pressure control valve to the suction side of the second pump, where the fine filtered working fluid from the outlet of the fine filter also flows in.

[0016] Excess volume here means the difference between the volume delivered by the first pump, and the volume that may pass though the fine filter without causing the pressure drop across the filter to become higher than specified.

[0017] Thus the pressure control valve maintains a constant pressure drop across the filter. As the fine filter gets clogged, the flow of working fluid through the pressure control valve will increase.

[0018] In one embodiment, working fluid flows back to the main reservoir from the delivery side of the second pump at a high velocity, so as to prevent any deposits and water accumulation from occurring. In an alternative embodiment, working fluid from the delivery side of the second pump is passed to a downstream hydraulic installation that may also include other pumps, filters, valves and actuators.

[0019] It may be advantageous always to feed a limited amount of working fluid from the delivery side of the first pump to the suction side of the second pump in order to prevent the second pump from running dry and/or cavitating.

[0020] The following describes a non-limiting example of a preferred method illustrated in the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows a schematic circuit diagram for a fine filter treatment plant in which cleaned working fluid is led back to the reservoir; and

[0022] FIG. 2 shows a schematic circuit diagram of the installation in FIG. 1, but here, the outlet/delivery side of the second pump is connected to a downstream hydraulic installation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0023] In the drawings, reference number 1 denotes a fine filter treatment plant for working fluid.

[0024] Working fluid flows from a main reservoir 2 via a pipe 4 to the inlet/suction side of a first pump 6. The pump 6 and a second pump 8 are driven by a motor 10. Working fluid flows from the delivery side of the first pump 6 via pipe connections 12, 14 and 16 to a pressure reducing valve 18 and to the inlet side of a fine filter 20 via a check valve 22. From the outlet of the fine filter 20 and the outlet of the pressure reducing valve 18, working fluid flows through pipes 24 and 26 to the suction side of the second pump 8.

[0025] A limited amount of working fluid flows through the connection 28 from the delivery side of the first pump 6 to the suction side of the second pump 8 for reasons of lubrication and cooling.

[0026] From the delivery side of the second pump 8, working fluid flows back to the reservoir 2 via a pipe 30, possibly through a check valve 32.

[0027] The fine filter 20 may be equipped with a differential pressure indicator 34 and a flow meter 34a. All the components of the installation are of a type that is known per se.

[0028] When the pump 6 feeds a greater volume of working fluid to the fine filter 20 through the pipes 4,12 and 16 than that for which the fine filter 20 is dimensioned, the pressure reducing valve 18 will maintain an accurate inlet pressure to the fine filter 20 through opening the pipes 14 and 26 to flow. This means that the fine filter 20 can be utilised in the best possible manner with regard to both treatment capability and service life.

[0029] Working fluid flows to the suction side of the second pump 8 from the fine filter 20 through the pipe 24 in a manner such that the outlet side of the fine filter is not subjected to overpressure, and from the pressure relief valve 18 through the pipe 26. In the second pump 8, the pressure is increased in order to enable the fluid to be used in a subsequent operation, whereby the velocity of the working fluid may be increased when flowing back to the reservoir 2 via the pipe 30, by means of e.g. a restriction (not shown) in the pipe.

[0030] In an alternative embodiment, see FIG. 2, the pipe 30 is connected to a downstream hydraulic installation 40 that may include one or more of the following components: a pump 42 with a motor 44, a main filter 46, a check valve 48, a pressure relief valve 50 and valves and actuators 52. All these of a type that is known per se.

[0031] Thus the fine filter treatment plant 1 can take care of the entire supply of working fluid to the hydraulic installation 40. Alternatively, the pump 42 may through use of the motor 44 also pump working fluid to the hydraulic installation 40. The main filter 46 is a full flow pressure filter protected by the bypass coupled check valve 48. The pressure relief valve 50 prevents overloading of valves and actuators 52.

[0032] By using the method according to the invention, one drive unit will be sufficient for the pumps of the installation, both in the case of pressurized installations and unpressurized installations. Returning the working fluid to the reservoir 2 at a substantial velocity results in good circulation in the reservoir, whereby deposits are prevented.

[0033] Furthermore, it is an advantage that fine filtering according to the proposed method is relatively insensitive to sudden movement and thus well suited for mobile equipment.

[0034] A further advantage is the freedom of placement of the fine filter in relation to the remaining components of the hydraulic installation, and the fact that there is no requirement for a separate filter reservoir.

Claims

1. A method for use of an oil-hydraulic fine filter (20) of the type where the outlet side of the fine filter (20) is designed to operate without overpressure, characterized in that working fluid that flows from the fine filter (20) in a depressurized state is pumped on by a second pump (8), where the second pump (8) is driven by the same driving device (1) as the first pump/feed pump (6) of the fine filter (20) and that the first pump's/feed pump's (6) outlet communicates with the second pump's (8) in through a pressure reducing valve (18).

2. A method in accordance with claim 1, characterized in that working fluid from the second pump (8) flows to the reservoir (2).

3. A method in accordance with claim 1, characterized in that working fluid from the second pump (8) flows to a hydraulic installation (40).

4. A method in accordance with one or more of the preceding claims, characterized in that working fluid flows from the delivery side of the feed pump (6) via a pressure relief valve (18) to the suction side of the second pump (8).

5. A method in accordance with one or more of the preceding claims, characterized in that a limited amount of working fluid flows from the delivery side of the feed pump (6) to the suction side of the second pump (8).

6. A device by an oil-hydraulic fine filter (20) of the type where the outlet side of the fine filter (20) is designed to operate without overpressure, characterized in that the outlet side of the fine filter (20) is connected to the suction side of the second pump (8), and where the second pump (8) is operated together with the feed pump (6) of the fine filter (20), by the same driving device (1), and where the first pump's/feed pump's (6) outlet is connected with the second pump's (8) inlet through a pressure reducing valve (18).