FLUID FILTERING SYSTEMS

Abstract: A fluid filtering system which includes a fluid filtering unit having a localised cleaning arrangement. The fluid filtering unit comprises a filter element through which the fluid being filtered is passed and a means for cleaning the filter element without interruption of the filtering operation of the unit. The cleaning means includes a cleaning head which ejects fluid under pressure and which is moveable relative to the filter element to bring about localised cleaning of the element. Preferably, the filter element is moved, by an hydraulic arrangement to bring about localised cleaning over the whole of the filter element.

Description: This invention relates to fluid filtering systems and units therefor and in particular, to the cleaning of the filtering element of such units. In commonly used types of fluid filtering systems two similar filter units are arranged in parallel so that when one unit is in service the other one is out of service, enabling the latter to be cleaned, and vice versa. The cleaning of a filter unit out of service is often effected by back-flushing of the filter element with a high pressure fluid, suitable means for doing this being provided in each filter unit. The high pressure fluid used in the cleaning may be derived from the clean side of the system or from an independent source, and in the latter case the high pressure fluid may be different from the fluid being filtered. Systems having two or more filter units in which one unit only is in service at any one time are common and can be cleaned in a similar manner. In these types of systems the out of service filter units are at different stages in the cleaning cycles, and will come into service in strict rotation. The provision of back-flushing means in each filter unit has proved to be costly, space consuming and leads to an increase of weight of the associated fluid filtration system. This is due to the duplication of equipment associated with this form of fluid filtration system. In certain applications, the increase in weight and/or the space consumption will be considered to be a major problem and this is certainly true in the aerospace and aircraft industries where weight and space are important. The present invention is concerned with providing an efficient self cleaning filtering system which operates with a single filter unit, cleaning of which does not involve interrupting the flow of fluid being filtered. It is known from GB-A-495112 to provide a self cleaning filtering unit. This unit has a cleaning head which is movable relative to the filter element of the unit and which produces a flow of cleaning fluid in a direction reverse to that of the fluid being filtered, so as to progressively clean the filter element. However, the dross removed from the filter element of this unit is collected in a sump. The sump is space consuming and has to be emptied at intervals. In seeking to mitigate these drawbacks, the present invention is characterised in that the cleaning means further comprises a collecting duct co-operable with the cleaning head to collect used cleaning fluid and the filtered out solid washed from the filter element, said collecting duct being provided on the opposite side of the filter element to the cleaning head and being movable in alignment with the cleaning head relative to the filter element. As this type of filter unit, generally, does not have to be removed from service in order to effect cleaning of the filter element duplication of equipment is avoided. The cleaning head advantageously includes a jet arrangement having a fluid ejection system and in this case the cleaning head is preferably a rotating or rotary jet system. The jet arrangement is preferably mounted with respect to the filter unit so that the complete length of the filter element can be cleaned. Preferably, cleaning is effected by moving the filter element , so as to bring about relative movement between the filter element on the one hand and the cleaning head and collecting duct on the other hand and in a preferred embodiment a hydraulic dash-pot arrangement is used to the effect movement of the filter element. The collecting duct need not form a fluid tight seal with the filter element, and therefore some of the fluid is extracted from the filtered fluid passing through the filter unit escapes into the collecting duct and is removed. If the cleaning fluid is extracted from the filtered fluid on the clean side of the filter unit, the fluid passing from the collecting duct may be recycled, so as to recover the cleaning fluid. The filter unit may be operated on a continual cycle basis. However, it is preferably provided with sensing means responsive to the degree of contamination of the filter element so that, when the filter is no longer in a condition suitable for further filtering, the cleaning means if brought into operation. Such a sensing means preferably measures the pressure drop across the filter element caused by the contamination thereof, so that when this reaches a predetermined level the cleaning means is brought into operation. Preferably, the sensing means comprises a simple pressure balance which measures the pressure drop. Alternatively electronic sensors may be used and these can measure the pressure at any point in the system. Preferably the filter unit has a cylindrical filter element and the cleaning means is positioned relative to the filter element so that it is caused automatically to move relative thereto during cleaning. During cleaning discharge of the cleaning fluid through the filter element in the direction opposite to normal filtering flow therethrough will occur, dislodging the contaminant particles from the filter element. The filter element may be made from any suitable material. The specific type of material used will be dictated by the use to which the unit is to be put and the nature of the filtering solutions etc. If the unit is to be used to filter out large particles for example certain applications associated with the mining industry, a wire mesh grill filter element could suffice. The advantages offered by the invention include the reduction in cost as a result of the use of only one filter unit its associated cleaning means and attendant simplification of the filtration systems as a whole together with its automatic operation. Normally, it will only be necessary to inspect/maintain the filter element of this type of unit during the routine inspections of the associated equipment. The invention will now be described, by way of examples, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a preferred filter unit of the invention; Figure 2 is a schematic representation of the fluid flow lines in the filter unit shown in Figure 1, during use thereof; Figure 3 is a schematic representation of the dashpot mechanism use with the filter unit of Figure 1; Figure 4 is a schematic representation of the filter unit as shown in Figure 1 adapted for use with a suction system; and Figure 5 is a schematic representation of the filter unit as shown in Figure 1 adapted for use with a pressure system. Referring to Figures 1 and 2, a filter unit 1 comprises a casing 2; a wire mesh filter element 3 connected to a dash-pot mechanism 4 for causing the filter element to move; and a rotary jet system 5 mounted so as to cause cleaning fluid to pass through the filter element in the opposite direction to the normal direction of flow of the fluid being filtered and having a collecting duct 6 to collect the used cleaning fluid. The filter unit 1 is provided with a fluid inlet I and an outlet O, as well as a cleaning fluid inlet line C to the rotary jet system and a dump line D from the collecting duct 6. The rotary jets eject fluid at a pressure greater than the fluid being filtered, so as to ensure effective cleaning of the filter element. These rotary jets are eccentrically mounted in the head (not shown) and the ejection of the fluid provides the force required to rotate the head. In operation speeds of rotation of 2000r.p.m. (revolutions/minute) are typical, thereby ensuring a efficient cleaning process across the whole surface of the filter element. Figure 2 shows a rotary jet 10 with a nozzle 11 and the relationship of the collecting duct 6 and the filter element 3 with the rotary jet system 5. In operation of the unit the cleaning fluid is ejected through the nozzle 11 and against the filter element 3, so that it passes through and removes the contaminant from the filter element. The fluid is then collected in the collecting duct 6 and passed along the line 7 to dump. The dash-pot mechanism 4, as shown in Figure 3, is of a type commonly used in industry and therefore only a brief explanation will be given. The mechanism comprises a hollow shaft 20, which can be divided into two sections 21,22 having different external diameters and being separated by a collar 23; and a casing 24 which is in sealing contact with the collar 23. A first space 25 is defined by the upper surface of collar 23, the adjacent casing wall 27 and the external surface of section 21 of the shaft 20 and a second space 26 is defined by the lower surface of the collar 23, the adjacent casing walls 28 and the external surface of section 22 of the shaft 20. The cleaning fluid supply line C is directly connected to the space 26 and a pressure sensor valve 29 is incorporated in the line by means of a first bypass 30 taken from the cleaning fluid line prior to an on/off switch 31. In addition, a second bypass 32 directly connects the cleaning fluid supply line to the space 25, and this is taken from a point in the line subsequent to the switch 31. In operation, the switch 31 is opened and cleaning fluid is allowed to flow to the rotary jets and along the bypass line 32 to the space 25. As a result of the difference in external diameters of sections 21 and 22 of the shaft 20 a pressure differential is caused between spaces 25 and 26. This pressure differential causes the shaft 20 to move in the direction A, thereby moving the filter element 3 relative to the rotary jet system 5. The degree of travel of the shaft and hence the filter element is controlled by the engagement of lugs 33 provided on the end of the shaft 20 remote from the fluid filter with a shaft 34 of the valve 29. Normally, the system would be designed so that in operation the movement of the filter element 3 relative to the rotary jet system 5 would take about ten seconds before the lugs 33 engage the shaft 34. When the lugs 33 engage the shaft, the valve 29 is tripped resulting in the closure of the cleaning fluid supply switch 31. The cleaning fluid supplied to the space 25 is thereby cut off, and the pressure in this section of the line returns to the pressure in the fluid filter housing 2. The pressure differential between the spaces 25 and 26 is therefore reversed and the shaft 20 is caused to move in the reverse direction until the filter element is in the normal working position. In operation the filter unit will work more efficiently if a drive is given to force the fluid to be filtered through the filter element. The drive can be supplied by a pump, and, to avoid unnecessary cost, the suction side of the pump performing a separate function can be used. Figure 4 shows a flow diagram for such a suction system having a filtering fluid inlet I, and an outlet O, with the component parts of the dash-pot mechanism and filtering unit being as previously described. The apparatus further comprises a dump valve 50 having a fluid mover 51 in the line 7; a restrictor 52 in the by-pass line 32, and a restrictor 53 in the by-pass line 30. The pressure sensor valve is hydraulically operated and comprises a diaphragm chamber 54 having a pliable member 55 and a rod 56; a snap action valve 57 having a shaft 34 for engagement with the dash-pot mechanism 4; a shaft 58 which is associated with the rod 56 of the diaphragm chamber 54, and a pressure storage device 59. The pressure sensor valve is connected to the on/off switch 31 in the cleaning fluid line and the dump valve 50, so that when the degree of contamination has reached the required level and the valve is tripped the dump valve and on/off switch are opened. The fluid mover 51 is provided in the dump line to ensure the level of dump of the cleaning fluid is adequate, thereby preventing back up of fluid in the filter unit. The restrictors 52 and 53 are provided in the relevant by-pass lines to control the speed of travel of the shaft 20 of the dash-pot mechanism during cleaning. Figure 5 shows a filter system which can be used in conjunction with a separate pump. i.e. one not performing a separate function and the configuration of this system is very similar to that with the suction system. In this system, restrictors 52 and 53 are again provided in the by-pass lines 30 and 32 to control the travel of the dash-pot mechanism. No fluid mover is, however, required in the dump line as the pressure of the system is sufficient to prevent cleaning fluid back up. The pressure sensor valve, dump valve and the on/off switch 31 are connected in a similar manner as with the suction system. High pressures are being used in this system and therefore a diaphragm valve may not be suitable and the more robust ram and piston configuration 60 has been fitted. It is necessary to ensure that when the filter has been cleaned the ram and piston is reset. To do this the pressure drop across the filter element is measured between the filtering fluid inlet and the cleaning fluid inlet at a point subsequent to the on/off switch 31, the ram piston will therefore be reset during the cleaning cycle. The measurement of the pressure drop is possible between these two points as the pressure in the cleaning fluid supply line subsequent to the switch 31 will reduce to the pressure of the filter chamber by virtue of the rotary head arrangement venting in the filter unit.

Claims: 1. A fluid filtering unit including a filter element (3) through which fluid being filtered must pass and means for cleaning the filter element in situ without interruption of the filtration process, the cleaning means comprising a cleaning head (5) which is movable relative to the filter element and from which, in use, cleaning fluid issues at a pressure greater than and in the reverse direction to that of the fluid being filtered so as to cause localised cleaning of the filter element characterised in that the cleaning means further comprises a collecting duct (6) co-operable with the cleaning head to collect used cleaning fluid and the filtered out solid washed from the filter element, said collecting duct being provided on the opposite side of the filter element to the cleaning head and being movable in alignment with the cleaning head relative to the filter element. 2. A fluid filtering unit as claimed in claim 1, characterised in that the cleaning head (5) includes a jet arrangement having a fluid ejection system. 3. A fluid filtering unit as claimed in claim 2, characterised in that the cleaning head (5) is a rotating or a rotary jet system. 4. A fluid filtering unit as claimed in any one of the preceding claims, characterised in that the cleaning head (5) is mounted with respect to the filter unit so that the entire length of the filter element (3) 5. A fluid filtering unit as claimed in any one of the preceding claims, characterized in that the cleaning head (5) and th collecting duct (6) are stationary while the filter element (3) is movable and in that cleaning is effected by moving the filter element (3), so as to bring about relative movement between the filter element (3) on the one hand and the cleaning head (5) and collecting duct (6) on the other hand. 6. A fluid filtering unit as claimed in claim 5, characterized by a hydraulic dash-pot arrangement (4) for effecting the movement of the filter element. 7. A fluid filtering unit as claimed in any one of the preceding claims, characterised by sensing means responsive to the degree of contamination of the filter element so that, when the filter is no longer in a condition suitable for further filtering, the cleaning means is brought into operation. 8. A fluid filtering unit as claimed in claim 7, characterised in that the sensing means comprises a simple pressure balance for measuring the pressure drop. 9. A fluid filtering unit as claimed in claim 7, characterised in that the sensing means comprises electrical pressure measuring sensors.

Inventors: WHYTE, JON FRANK ROSS
Applicants: MECO MINING MACHINERY LIMITED
Application year: 1988
Main classification: B01D29
IPC codes: B01D29, B01D35