IE45397B1 - Improvements in or relating to apparatus for back-washing granular-bed filters - Google Patents

Description

The present invention relates to apparatus for back-washing granularbed filters, particularly but not exclusively, sand filters.

Rapid gravity sand filters are cleaned using an upflow wash of air, water, or air plus water combined. One common form of sand filter uses parallel K lateral pipes or ducts inserted into the side of a main duct or header.

Such lateral pipes may either be perforated or be provided with special nozzles. During air flow through the lateral pipes to the filter the holes or nozzles are designed so that the lateral pipes remain partly filled with air and partly filled with water,·-- When air and water are supplied simultaneously it is essential that the air-water interface remains at an approximate level in the lateral pipes and that wave formation is minimised.

In.extreme cases waves may form plugs in which the water closes the gap between the invert and crown of therlateral pipe. Such a wave is then rapidly propelled along the length of the lateral pipe.

The invention provides apparatus for back washing a granular-bed filter S 3 9 7 with air and/or liquid comprising a main header duct or pipe and a plurality of lateral pipes or ducts, wherein each lateral pipe or duct communicates with the main header duct or pipe by means of a liquid inlet and an air inlet which is separate from the liquid inlet. Thus, the air flow and liquid flow to the lateral pipes are separately controlled and are not affected by disturbances in the air/water interface in the header duct.

Preferably, a portion of each lateral pipe or duct extends into the main duct or pipe, said portion including a leg, an elbow bend from which the leg depends, end a separate orifice to admit air, saio orifice being, advantageously, positioned in either the top of said portion of the lateral pipe or duct cr on part of the elbow beno above the operating liquid level in the lateral pipe or duct.

Preferably, air-guide means are provided for guiding air from the air Orifice along the respective lateral pipe or duct in such a way that, in use, it does not impinge upon the air liquid interface in the lateral pipe or duct.

Preferably, the dimensions cf the air orifice are smaller than the air-guide means within the lateral pipe or duct.

Thus, for example ths air-guids means may be a tube fixed on the underside of the crown of the 1*serai pipe or duct passing through the elbow bend, the tube being provided with an orifice at the inlet end, so that within the length of the tube, in use, the initial velocity occurring in the orifice is diffused across the full cross section of the tube.

The invention further provides a granular bed filter provided with apparatus as described above.

The invention will now be described with reference to embodiments shown by way of example in the accompanying drawing, wherein: Fig. 1 is a cross-sectional view of a main header duct and a lateral pipe, Fig. 2 is a cross-sectional view of a modified lateral pipe, and Fi9· 3 is a cross-sectional view of a double lateral pipe.

In Fig. 1 a main header duct 1 containing water 2 has a plurality of lateral pipes 3 (only one is shown). Each lateral pipe 3 has an elbow bend 4 and a verical leg 5 extending down below the level 6 of the water 2 in the header duct 1. Tubes 7 extending upwardly from the lateral pipe 3 terminate in filter nozzles 8. Alternatively, the lateral pipe 3 may just be provided with air escape holes in the crown of the lateral pipes.

The underside of the crown of the lateral pipe 3 is provided with an air guide tube 9 extending out through the elbow bend 4 and having an air orifice 10. The cross sectional area of the air orifice 10 is considerably smaller than that of the tube 9. Thus, whilst air is flowing there should be a sufficient head loss to depress the water level in the header duct 1 to a convenient distance, for example half way, down the vertical legs beneath the elbow bend 4.

The orifice 10 and air guide tube 9 of Fig. 1 may be replaced by an air orifice 11 and plate or elbow 12 as shown in Fig. 2.

Each lateral pipe may extend from one side, or from opposite sides of the header duct. In the latter case, instead of using two lateral pipes each having an elbow bend and vertical leg, a double lateral pipe may be formed from a T-shaped pipe as shown in Fig. 3, the arms 13, 14 of the cross-piece extending from opposite sides of the header duct (not shown) and the leg 15 of the T extending downwardly into the water 2 and having an air inlet associated with each arm 13, 14, or a single air inlet 16 common to both arms 13 and 14 of each lateral A plate or tube 17 acts as air-guide means. Λ In use, when no water is flowing, the depression between the water levels in the lateral pipe 3 and the water level in the header duct 1 is purely a function of the pressure loss at orifice 10. When water flows there is an entry loss at the bottom of the vertical leg 5 plus a loss at the elbow bend 4. The depression is then less by the sum of the latter two. The height of the vertical legs 5 (or 15) and the characteristics of the air orifice 10 30 (or 11, or 16) are chosen to maintain a satisfactory water seal at all operational flow rates.

The above described embodiments provide a distribution system from a header duct 1 into the lateral pipes which is substantially unaffected by a small changes in water level in the header duct due either to pressure recovery (that is to say the recovery of pressure which occurs when a mass of fluid a a 3 0 7 in motion slows down) in the water entering the duct or to wave formation. Also, it is possible to lay the lateral pipes with less accuracy than would be otherwise possible, since in the absence of the elbow bend 4 and vertical leg 5, the lateral pipes must be laid within a tolerance band equal to the pipe diameter and in practice to within about % of the diameter, whereas in the embodiments described above an air flow will be achieved in all lateral pipes at any range of levels, provided that the range of levels is less than the vertical distance between the level 6 and orifice 10 which is much larger than J of the diameter of the lateral. Thircly, there is greater flexibility of operation, particularly where it is desired to use air and water simultaneously.

Claims (11)

1. Apparatus for back washing a granular-bed filter with air and/or liquid comprising a main header duct or pipe and a plurality of lateral pipes or ducts, wherein each lateral pipe or duct communicates with the main header duct or pipe by means of a liquid inlet and an air inlet which is separate from the liquid inlet.

2. Apparatus as claimed in claim 1, wherein a portion of each lateral pipe or duct extends into tne main duct or pipe, said portion including a leg, an elbow bend from which the leg depends, and a separate orifice to admit air.

3. Apparatus as claimed in claim 2, wherein the air orifice is positioned in the top of said portion of the lateral pipe or duct.

4. Apparatus as claimed in claim 2, wherein the air orifice is positioned on part of the elbow bend above the operating liquid level in the lateral pipe or duct.

5. Apparatus as claimed in claim 2, 3 or 4, wherein air-guide means are provided for guiding air from the air orifices along the respective lateral pipe or duct in such a way that, in use,air does not impinge upon an air/liquid interface in the lateral pipe or duct.

6. Apparatus as claimed in claim 5, wherein the dimensions of the air orifice are smaller than the air-guide means within the lateral pipe or duct.

7. Apparatus as claimed in claim 5 or 6, wherein the air-guide means is a tube fixed on the underside of the crown of the lateral pipe or duct passing through the elbow bend, the tube being provided with an orifice at the inlet end, so that within the length of the tube, in use, the initial velocity occurring in the orifice is diffused across the full cross section of the tube.

8. Apparatus as claimed in claim 1, wherein the lateral pipes or ducts are formed as T-shaped pipes horizontal arms of which extend from opposite sides of the header duct or pipe, at least one orifice being provided in the junction 45337 above the operating liquid level in the junction,

9. Apparatus as claimed in claim 8 further comprising air-guide means for guiding air from the at least one orifice down the arms leading from the junction so that, in use, the air does not impinge upon an air/liquid 5 interface in the horizontal arms.

10. Apparatus for cleaning a granular bed filter with air and/or liquid substantially as herein described with reference to any one of the embodiments shown in the accompany:ng drawing.

11. A granular bed filter provided with apparatus as claimed in any 10 one of the preceding claims.