GB2086745A

GB2086745A - Gas diffuser

Info

Publication number: GB2086745A
Application number: GB8114356A
Authority: GB
Original assignee: Hawker Siddeley Water Engineering Ltd
Current assignee: VTW Anlagen UK Ltd
Priority date: 1980-10-09
Filing date: 1981-05-11
Publication date: 1982-05-19
Also published as: GB2086745B

Abstract

A gas diffuser for use in aerobic sewage treatment includes a valve device operable to seal the interior of the diffuser from a gas supply pipe whenever the pressure of gas in the pipe is below a predetermined level, increase in the gas pressure above the predetermined level causing the valve to open. The amount by which the valve opens is dependent upon the pressure of gas in the supply pipe. In one embodiment the valve comprises an inlet port one end of which is closed by a ball held there by gravity until lifted by the pressure of gas applied to the inlet port. The sides of the bore in which the ball rises with increasing gas pressure may diverge along the length of the bore away from the inlet port. In an alternative arrangement the walls of the bore are slotted. Alternatively, the valve member may comprises an inlet port coupled to a member having flexible walls which normally close the inlet port and which flex to open the port when the pressure applied to the inlet port exceeds the predetermined level. <IMAGE>

Description

SPECIFICATION Diffuser arrangements DESCRIPTION The invention concerns the gasification of liquids, particularly their gasification by diffuser arrangements placed therein.

The purification of sewage and industrial waste waters is often carried out by biological processes one of which provides that the purification is effected by aerobic bacteria.

The growth of such bacteria is encouraged by injecting a gas containing oxygen (for example air) into the waters. The bacteria feed on the pollution content of the sewage (or other waste waters) and turn the pollution mass into an "activated sludge".

One of the most important aspects of such a system is the means used to inject oxygen into the sewage which, if the injection is to be efficient, must be able to pass oxygen to the liquid in which the oxygen dissolves, circulate the oxygenated liquid to the micro-organisms, and keep the sludge well mixed and held in suspension.

One known arrangement for aerating sewage and other industrial waste waters for its purification provides that air is diffused into the sewage via a plurality of porous diffusers.

The diffusers are mounted at spaced locations along an air supply pipe each being located above an aperture in the pipe. Air passes from the pipe to each diffuser and bubbles up therefrom into the sewage.

In the known arrangements the diffusers generally comprise a plate mounted directly on the air supply pipe and a porous dome diffuser coupled to the plate. One method of coupling the dome and plate comprises the use of an orifice bolt which passes through the dome to engage the plate. In the known arrangement at least the lower end of the orifice bolt is hollowed out and is provided with a side aperture feeding into the volume under the dome. In this way air from the pipe passes up through the bolt and into the dome.

It has been found necessary in such arrangement to restrict the air flow to each dome to achieve a uniform air pressure distribution amongst those domes over the various lengths of air supply pipes which may be used. This is done by providing a fixed sized aperture in the side of the orifice bolt.

Until recently diffuser arrangements were required to supply a range of gas flow rates the ratio of the largest to smallest of which is 3 : 1 andwere designed to provide an appropriate range of air flows uniformly to each of the diffusers. More recent requirements for diffuser arrangements include those in which the range of gas flow rates, rather than being in the ratio of 3 : 1, need to be in the ratio of 10 : 1 or greater. The pressure loss across the aperture in the orifice bolt at the higher flow rates is substantial leading to a waste of energy in driving the compressors supplying the system.

One object of the invention is to provide a diffuser arrangement including a valve which is operable to accept a wide range of gas flow rates therethrough with a substantial reduction in the rate of increase of pressure drop across the valve with increasing gas flow rates.

According to the invention there is provided a gas diffuser arrangement including a valve having an inlet port to which gas is applied to flow therethrough, the valve including means operable to vary the area through which gas may flow in dependence upon the variation in the pressure of gas applied to the inlet port such that the area through which gas may flow increases in proportion to the pressure of gas applied to the inlet port over a range of gas flow rates through the valve.

The valve may comprise a member having a bore one end of which communicates with the inlet port, the effective diameter of the bore being greater than that of the inlet port and increasing along the length of the bore away from said one end, and a ball member within the bore which may adopt a position closing the inlet port. Alternatively the walls of the member including the bore may be slotted, in which case the effective diameter of the bore may remain constant along its length.

With the above noted arrangements the ball member normally rests in a position sealing the interior of the diffuser from the inlet port until the pressure of gas applied to the valve arrangement is sufficient to move the ball away from this position. As the pressure of gas applied to the arrangement increases the ball moves further from its rest position so increasing the area through which gas may pass from the valve to the interior of the diffuser dome.

Preferably the valve arrangement extends into the dome by an amount such that its upper end is spaced from the dome less than the diameter of the ball: alternatively the upper end of the valve may be provided with means preventing the ball escaping from the bore.

In another arrangement the valve comprises a member with flexible walls, the walls normally lying in a position in which they close the inlet port but which may flex to open that port and enable the flow of gas therethrough when the pressure applied to the inlet port exceeds a predetermined level.

Various embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a part sectional side view of a diffuser arrangement embodying the invention; Figures 2 and 3 show alternative forms of a valve included in the arrangement of Fig. 1; and Figure 4 graphically illustrates the effect of operation of the valve included in the diffuser arrangement of Fig. 1.

Fig. 1 shows a gas supply pipe 10 having coupled thereto a porous alundum dome 11 by an orifice bolt 12 having its head bearing on the upper surface of the dome 11 via washers 13. Bolt 12 extends through dome 11 and into pipe 10, its lower threaded portion 14 engaging a threaded portion of a plate 17. The central, lower part of bolt 12 is hollow and is provided with an orifice 15 such that gas may pass from pipe 10 to the interior 1 6 of the dome 11. Bolt 12 acts to clamp dome 11 to plate 17 and the lower edges of the dome contact the plate 17 via the intermediary of a rubber dome joint 18. Plate 17 is normally glued to the surface of the pipe 10.

The arrangement so far described is that of a standard diffuser which has a pressure drop to gas volume flow rate (dP:V) chacacteristic as shown by line 50 in Fig. 4. It will be seen that this characterstic gives an acceptable pressure drop and good gas distribution in a diffuser system containing hundreds of domes, over the range of volume flow rates normally encountered in use todate (3 : 1 maximum), but that with the increasingly necessary requirement for ranges of flow rates extending over a greater range (8 : 1, 10 : 1) the pressure drop is too high at high flow rates. It would be possible to increase the size of the orifice in the orifice bolt (or to provide an additional fixed orifice) to give an alternative characteristic such as is shown by line 51 in Fig. 4, and to provide an acceptably low pressure drop at high gas flow rates.Such an arrangement however has the disadvantage that at low gas flow rates the pressure drop is too low and the air distribution amongst the various diffusers is adversely affected.

In addition to the conventional arrangement so far described, the present invention provides the diffuser with the valve arrangement comprising a valve body formed by a hollow tubular inlet port 20 through the wall of the pipe 10 as shown, leading via a valve seat 21 to a variable area flow section 22 surrounding a ball 24. A sleeve 23 is located at the top end of the valve arrangement; this sleeve could be formed as an integral part of the valve body by extending the variable area flow section 22.

The ball 24 is of stainless steel and will close the inlet port 20 at low pressures under the influence of gravity thus ensuring good air distribution at low flow rates over a multitude of diffusers. Other suitable materials may of course be used.

In operation air is pumped to the pipe 10 and begins to flow through the bolt 12 and the orifice 15 into the interior 16 of the dome 11. Air bubbles into the sewage after passing through the porous material of the dome 11.

When an increase in air flow rate is required the pressure in the pipe 10 increases until a stage is reached at which the pressure (acting through the inlet port 20) is sufficient to lift ball 24 from its rest position. As the ball lifts the air can flow round it between the ball and the walls of the variable flow section 22.

Increasing gas supply pressure has the effect of lifting the ball further away from the port 20 and due to the conical shape of the variable flow section 22 the area through which air may flow from pipe 10 into volume 16 (that is to say the area between the ball 24 and the walls of variable flow section 22) increases such that the pressure drop across the valve increases only slightly with increasing gas flow rates.

It is to be noted that the upper edge of the sleeve 23 is spaced from the under surface of dome 11 by a distance which is less than the diameter of the ball 24 so ensuring that the ball remains within the valve. Although described with a standard orifice bolt 12 having an orifice 15 feeding gas to the volume 16 it will be seen that in certain applications all the air supply to volume 16 may be made via the valve.

Fig. 2 shows a modified form of valve embodying the invention to include a cupshaped upper part 30 the base of which is pierced by an inlet 31 extending through a lower threaded part 32 by which the valve may be threadably engaged in an appropriately threaded aperture in plate 17. The walls 33 of the cup-shaped housing are slotted at 34 as shown. A ball 35(similar to the ball 24 of the arrangement of Fig. 1) is located in the cup 30 and normally rests, under the influence of gravity, in a position in which it closes the inlet 31. An increase in pressure applied to the inlet 31 above a predetermined minimum pressure level lifts the ball and opens the inlet port. Gas then flows through the valve.

As the desired rate of gas flow increases the ball 35 rises higher in the housing 30 exposing more of the slotted parts of the wall 33.

In this way the area through which air may flow increases and the pressure differential across the valve remains substantially constant for a great range of air flow rates.

Fig. 3 illustrates the upper part only of a valve formed with flexible walls 40. The walls 40 are normally biased to the position shown that is to say they shut off an inlet 41. With an increase in air pressure applied to the inlet 41 the walls are pushed apart enabling air to flow through the valve. As air pressure contin ues to rise at the inlet 41 the walls are pushed further apart increasing the area through which air may flow thereby maintain ing a substantially constant pressure drop across the valve for a wide range of air flow rates.

Fig. 4 illustrates the effect of the valves described in Figs. 1, 2 and 3 graphically. In this figure a plot, using linear scales, is shown of the pressure drop dP against gas flow rate V through the valve. The line 50 shows the increase in pressure drop across a fixed orifice bolt with increased gas flow rates through the valves, and line 51 shows the equivalent plot when two fixed orifices are used.

With the form of valve shown in Figs. 1, 2 or 3 the pressure drop rises as shown by line 52 running initially along line 50 (the valve 19 being closed and air entering volume 16 via orifice bolt 12) until the pressure applied to the inlet of valve 19 is sufficient to open it.

The pressure drop across the valve thereafter runs parallel with line 51 for increasing gas flow rates.

We have by experiment determined that a number of features of the valve arrangement described with respect to Figs. 1 and 2 are critical if the desired low, steady and predictable pressure drop across the valve over the widest range of envisaged flow rates is to be maintained. The particular numerical values of these features will vary with different applications for the diffuser arrangements however the following criteria should be borne in mind.

The smaller the diameter of the entry port 20 or 31 the greater the rate of increase in pressure drop for increasing gas flow rates.

The cross-sectional area of the inlet port 20 or 31 should not be less than 0.03% of the inner flat area of the diffuser arrangement for satisfactory 'Pressure Drop' versus 'Flow Rate' characteristics.

The included angle of the taper of the surfaces 21 and 30 on which the ball rests when there is no flow through the device should not be so small as to cause the ball to jam.

The diameter of the ball should preferably not be less than 2% of the equivalent diameter of the inner flat area of the diffuser and should preferably be 80% or more of the widest diameter of the tapers 22 and 30. If the diameter of the ball is too small it will oscillate from side to side in unpredictable ways when lifted, increasing the pressure drop across the valve.

Many modifications to the particular form of valves described with reference, in particular, to Figs. 1 and 2 may be made without departing from the scope of the invention. In Fig. 1 the valve arrangement is described as comprising a port 20 forming part of the plate 17 on which the dome is mounted and a sleeve 23. In Fig. 2 the valve body comprises a single member having a lower threaded portion which may pass through and engage in the plate 17. Clearly modifications may be made to arrive at a particular design thought particularly useful in any application. For example the bore in which the ball moves may be of increasing diameter along the length away from the base of the valve.

Various described embodiments of the invention enable an energy saving to be made in diffuser systems by means of the variable orifice valve which is designed to perform with an almost constant, minimum, pressure drop across it over a wide range of air flow rates. The valves also function as non-return valves limiting the water which might otherwise enter the air supply pipe upon failure of air flow therethrough. Furthermore the valves perform satisfactorily without maintenance over a long period.

If these variable orifice valves are fitted into existing diffuser installations the energy saving permits the standard compressor supplying air to the diffuser system to deliver a larger volume of air for a given power requirement than has heretofore been possible while still providing a good air distribution at low air volumes.

The variable orifice valves described herein may also be applied to other systems of installations that convey gases or liquids requiring pressure equalisation of the media over a wide range of fluid flow rates and plant positions. For such diverse applications the form of valve may be varied by varying any one or more of the criteria noted above.

Claims

1. A gas diffuser arrangement including a valve having an inlet port to which gas is applied to flow therethrough, a valve including means operable to vary the area through which gas may flow in dependence upon the variation in the pressure of gas applied to the inlet port such that the area through which gas may flow increases in proportion to the pressure of gas supplied to the inlet port over a range of gas flow rates through the valve.

2. A diffuser according to Claim 1, wherein the valve comprises a first member having a bore one end of which communicates with the inlet port the diameter of the bore being greater than that of the inlet port, and a ball member within the bore operable to adopt a rest position closing the inlet port.

3. A diffuser according to Claim 2 in which the bore is in the form of an inverted truncated cone the narrower end of which communicates with the inlet port and in which the ball is operable to rest under the influence of gravity at said narrower end, being lifted from that position to allow air to flow into the bore when the pressure of air supplied to the inlet port exceeds a predetermined minimum pressure level.

4. A diffuser arrangement according to Claim 3, wherein said ball sits on a shoulder formed in the bore or by the end of the inlet port.

5. An arrangement according to Claim 4, wherein the shoulder is tapered, the angle of taper o the shoulder being different to the angle of taper of the walls of the bore.

6. An arrangement according to any one of Claims 2 to 5 in which the walls of the bore are slotted.

7. A diffuser arrangement according to any one of Claims 1 to 6 and including a plate on which a diffuser dome is mounted and wherein the valve arrangement extends through that plate.

8. A diffuser arrangement according to Claim 7, wherein the valve arrangement comprises an inlet port formed integrally with said plate to extend above and below said plate and the member including the bore comprises a sleeve mounted coaxially with the inlet port within the volume of the diffuser.

9. A diffuser arrangement according to Claim 7, wherein the valve arrangement comprises a one piece member the first part of which is formed with an inlet port and is externally threaded for threadable engagement with an aperture in the plate and a second part of which comprises said member having said bore.

10. A diffuser arrangement according to Claim 1, wherein said valve arrangement comprises a flexible member the walls of which normally close the inlet port and which may flex to open that port and enable gas to flow therethrough when the pressure applied to the inlet port exceeds a predetermined minimum pressure level.

11. A diffuser arrangement according to any one of claims 1 to 10 and including a fixed orifice bolt.

12. A diffuser system including a diffuser arrangement in accordance with any one of Claims 1 to 11.

13. A diffuser arrangement substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

14. A diffuser arrangement substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

15. A diffuser arrangement substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

16. A gas valve arrangement including means operable to close the inlet port at low applied pressures such that there is an adequate pressure drop at low gas flow rates to ensure good gas distribution over a multitude of diffusers.

17. A gas valve arrangement including an inlet port to which gas is applied to flow therethrough, the valve arrangement including means operable to vary the area through which gas may flow in dependence upon the variation in the pressure of gas applied to the inlet port such that the drop in pressure across the valve arrangement increases at a limited rate over a wide range of gas flow rates through the valve.