GB2132504A - Filtering apparatus - Google Patents

Abstract

Filtering apparatus for removing toxic particles from a fluid includes a sealed vessel (20) having a fluid inlet (30) and a fluid outlet (28) between which is an array of elongate filter elements (42). The filter elements are so disposed that when viewed in the direction of their length the axis of each filter element is spaced from that of each adjacent filter-element by a distance R whereby the axes lie at the apexes of a plurality of equilateral triangles (see Fig. 4). The axes of six of the filter elements are equispaced about a central pitch circle whose radius is R and at the centre of which is the axis of a seventh filter element. A portion 22 of the wall of the vessel is removable to permit access to the filter elements and each filter element has means to locate a machine for remotely exchanging the filter elements. <IMAGE>

Description

SPECIFICATION Filtering apparatus The present invention relates to filtering apparatus and is concerned with that type of apparatus which includes a container within which is a filter assembly which is intended to remove toxic particles from a fluid and of which, as a result of the toxicity, the individual filter elements are constructed and arranged so that they may be exchanged for new filter elements by remote handling techniques. The invention may find application in removal of any toxic particles from any fluid but is particularly concerned with the filtering of air in nuclear power stations to remove particles of radioactive dust.

Nuclear power stations commonly have vaults in which radioactive material, such as exhausted fuel material, is stored and it is common to circulate air through these vaults both to cool them and to prevent the build up of high levels of air-born radioactive dust. The air must of course be effectively filtered before discharging it to atmosphere and this is conventionally performed in a pressurised container containing a plurality of stainless steel filter elements connected in parallel. A large number of these filter elements is required if they are to cope with the desired throughput of air and their construction and disposition must be such as to permit their removal and replacement by remote handling techniques when they are clogged by radioactive material.

Conventionally a number of filter elements are positioned on a pitch circle at the centre of which is a mounting and locating block for an automatic filter element exchanging machine which can index around the circle to exchange each filter element in turn. It is not possible to include many filter elements in such a circular configuration since otherwise the diameter of the pitch circle would be such that the filter assembly would take up too much space and it is therefore usual to provide the filter elements on a plurality of spaced pitch circles. This arrangement occupies a considerable amount of space and in addition necessitates repositioning of the filter element exchanging machine to the centre of each pitch circle during the exchange process. In addition a mounting and locating block must be provided at the centre of each pitch circle.

Accordingly it is an object of the present invention to provide a filtering apparatus of the type referred to above in which the filter elements are disposed in a configuration which occupies the minimum volume and which permits the filter elements to be more easily exchanged by an automatic filter exchanging apparatus.

According to the present invention filtering apparatus for removing toxic particles from a fluid includes a sealed vessel having a fluid inlet and a fluid outlet between which is an array of elongate filter elements, the filter elements being so disposed that when viewed in the direction of their length the axis of each filter element is spaced from that of each adjacent filter element by a distance R whereby the axes lie at the apexes of a plurality of equilateral triangles, the axes of six of the filter elements being equispaced about a central pitch circle whose radius is R and at the centre of which is the axis of a seventh filter element, a portion of the wall of the vessel being removable to permit access to the filter elements and each filter element affording means to locate a machine for automatically exchanging the filter elements.

Thus in the simplest form of the invention there are only seven filter elements, the axes of six of them being equispaced about a pitch circle whose radius is R and at the centre of which is the axis of a seventh filter element. It is however preferred that there are more than seven filter elements and all those over and above the basic seven elements are positioned so that their axes lie at the apexes of a plurality of equilateral triangles. The position of the further filter elements may be generated by producing additional pitch circles of radius R centres on each of the six filter elements and the additional filter elements will then be equispaced about the further pitch circles.

In the most preferred embodiment of the invention there is a total of nineteen filter elements, the axes of twelve of them lying outside the said central pitch circle and being associated together in threes on a respective further pitch circle of diameter R centred about one of the axes of the said six filter elements, two of each of said three associated axes lying on two said further pitch circles.

Thus in the filtering apparatus in accordance with the present invention the filter elements are arranged in a single homogeneous array rather than being disposed in a plurality of separate arrays. This results in the filter elements taking up very much less space than in the prior construction referred to above which is of considerable importance in installations such as nuclear power stations. In addition, the equispaced disposition of the filter elements results in a symmetrical fluid flow and thus an equal loading of the filter elements with the result that none of them tends to become clogged before the others. Of even greater importance is that filter elements disposed in accordance with the present invention may be exchanged by a remote handling technique more easily than in the known construction.Thus an automatic filter element exchanging machine may be centred with respect to the central filter element at the centre of the central pitch circle, for instance by aligning it with the aperture which is usually closed by the removable portion of the vessel wall, and a slave unit may then be located on each of the six filter elements defining the central pitch circle in turn and then index about that filter element as a central pivot which will enable the machine to exchange each filter element without having to completely re-align it one or more times.Thus disposing the axes of the filter elements at the apexes of a plurality of equilateral triangles results in all the filter elements being disposed on the circumference of one or more overlapping pitch circles and this configuration lends itself readily to a simple and rapid exchange of the filter elements by an automatic filter element exchanging machine.

Although nineteen is the preferred number of filter elements it will be appreciated that one or more of the outer twelve elements may be removed though this would result in no advantage since the filtering capacity would be reduced without significantly reducing the space occupied by the array of filter elements. Further filter elements may of course be added and the resulting configuration would possess all the advantages referred to above.

When the filter elements are being exchanged it will not be necessary for the exchanging machine to be located with respect to every filter but nevertheless every filter is provided with means for locating the exchanging machine since this enables all the filters to be identical and saves having to provide two different types of filter.

Preferably each filter element affords a circular section tapered recess centred about the axis of the said filter element which constitutes the locating means, the tapered recess being directed towards and diverging towards the removable portion of the wall of the vessel. The exchanging machine may thus be provided with a conical locating member whose shape corresponds to that of the tapered recesses and may thus engage in the tapered recesses in known manner to precisely locate the exchanging head of the exchanging machine with respect to the filter elements.

The filter elements may be secured in position in any convenient manner which ensures that the fluid to be filtered must pass through them and cannot bypass them. In the preferred construction the vessel has a plate extending across its interior and provided with a plurality of threaded holes, each filter element having an externally threaded portion and being accommodated in a respective hole and affording means by which it may be engaged by the machine for automatically exchanging the filter elements and rotated by it to screw it into or unscrew it from its associated hole.

Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example only with reference to the accompanying drawings in which:~ Figure 1 is a partly cut away side elevation of a filtering apparatus in accordance with the invention; Figure 2 is a sectional end elevation of the apparatus shown in Figure 1; Figure 3 is a scrap sectional elevation on an enlarged scale showing a single filter element; and Figure 4 is a diagrammatic representation of the array of filter elements showing their relative disposition.

The filtering apparatus comprises an eliptical sectional steel pressure vessel 20 whose major axis is 1800 mm, whose minor axis is 425 mm, whose height is 2300 mm and whose wall thickness is 12 mm. The pressure vessel is sealed at its upper end by a removable lid 22 which is provided with a sealing ring 24 and is reinforced about its periphery by a plurality of t-shaped strenthening beams 26. The vessel is provided with an outlet pipe 28 communicating with its interior and with an inlet 30 which communicates with an inlet passage 32 extending down the exterior of one side of the vessel which in turn communicates with the interior of the vessel via an aperture 34 at its lower end. Situated adjacent the aperture 34 is a baffle plate 36 which is constructed to diffuse the inflowing fluid flow across the entire cross-section of the vessel.

Extending across the interior of the vessel adjacent its upper end and upstream of the outlet 28 is a steel plate 38 which is welded to the wall of the vessel at 40 and through which nineteen elongate filter elements 42 extend in a configuration which will be described below.

Each filter element is of a type known per se and comprises a tubular filtering medium of stainless steel fibres which is retained in position on a stainless steel core by a stainless steel mesh and closed by a bottom closure. It is 1000 mm long and axially symmetrical, in this case substantially cylindrical. Each filter element has an external screw thread at its upper end which mates with a corresponding screw thread on the periphery of a respective hole in the plate 38. Each filter element is of generally tubular form and has a flange 44 at its upper end which defines an upwardly divergent frustoconical aperture 46 and which affords two holes 48 for engagement by a tommy bar to enable the filter element to be screwed into its hole or unscrewed from it. Each filter element is provided with a sealing ring 50 which seals its flange to the plate 38.

The relative disposition of the filter elements 42 in the plate 38 is illustrated diagrammatically in Figure 4. As will be seen, there are a total of nineteen filter elements and these are numbered 1 to 19 respectively. A central filter element 1 is symmetrically disposed with the vessel so as to be on its longitudinal axis. Six further filter elements 2 to 7 inclusive are equiangularly disposed about the periphery of a pitch circle centred on the filter element 1 and having a radius R, in this case about 400 mm. It will be appreciated that this automatically results in each of the filter elements 2 to 7 being spaced a distance R not only from the filter element 1 but also from the two adjacent filter elements on either side of it. Twelve further filter elements 8 to 19 are disposed outside the central pitch circle on which the elements 2 to 7 lie, these further elements being so disposed that each adjacent pair of them lie at the apexes of a respective equilateral triangle at the third apex of which one of the elements 2 to 7 lies. Thus all the filter elements are disposed such that their axes lie on the apexes of a plurality of contiguous equilateral triangles.It will be seen from Figure 4 that the positions of the elements 8 to 1 9 may be generated by producing six pitch circles, each of diameter R and centred on one of the elements 2 to 7, the position of one half of the further elements being at the points of intersection of these six pitch circles and the postion of the other half of the further elements being situated midway between adjacent pairs of the points so produced and a distance R away from each of them.

It will be seen from Figure 4 that this configuration of the filter elements represents the optimum as regards occupying the minimum of space, and in addition the fact that all the filter elements of the entire array are equally spaced apart means that the fluid flow through each of them is uniform and thus no one filter is subjected to a greater loading than any other.

In use, the apparatus is situated in a nuclear power station with its inlet 30 connected to a duct communicating with vaults containing spent nuclear fuel material. Air is constantly flushed through these vaults and then passed into the apparatus which removes radioactive dust particles from it which are retained by the filter elements 42. The purified air is discharged to atmosphere through the outlet 28.

When the filter elements are fully loaded with radioactive material the air flow through it is ceased and the interior of the vessel is flushed through with fresh air which is passed in through an additional pipe 52. After the interior of the vessel has been purged of all suspended dust particles, the cover 22 is removed after withdrawing a number of bolts (not shown) connecting the cover to the vessel. An automatic filter element exchanging machine is then brought into engagement with and centred with respect to the upper flange of the vessel and is thus aligned with the filter element 1. An indexing unit, having a downwardly extending conical locating member is then located on one of the filters 2 to 7 and then indexed round all the filters located on the pitch circle centred on that filter element and replaces them one by one with fresh filter elements. The indexing unit is then centred on the next of the filter elements 2 to 7 and the process is repeated. As the process of replacing filter elements proceeds it will be appreciated that certain of the filter elements lying on any one pitch circle will already have been replaced since they also lie on a further pitch circle and allowance is of course made for this. Once all the filter elements have been replaced the replacing machine is retracted, the cover 22 replaced and operation of the filtering apparatus continues as before.

Claims (5)

1. Filtering apparatus for removing toxic particles from a fluid including a sealed vessel having a fluid inlet and a fluid outlet between which is an array of elongate filter elements, the filter elements being so disposed that when viewed in the direction of their length the axis of each filter element is spaced from that of each adjacent filter element by a distance R whereby the axes lie at the apexes of a plurality of equilateral triangles, the axes of six of the filter elements being equispaced about a central pitch circle whose radius is R and at the centre of which is the axis of a seventh filter element, a portion of the wall of the vessel being removable to permit access to the filter elements and each filter element affording means to locate a machine for automatically exchanging the filter elements.

2. Apparatus as claimed in Claim 1 including nineteen filter elements, the axes of twelve of them lying outside the said central pitch circle and being associated together in threes on a respective further pitch circle of diameter R centred about one of the axes of the said six filter elements, two of each of said three associated axes lying on two said further pitch circles.

3. Apparatus as claimed in Claim 1 or Claim 2 in which each filter element affords a circular section tapered recess centred about the axis of the said filter element which constitutes the locating means, the tapered recess being directed towards and diverging towards the removable portion of the wall of the vessel.

4. Apparatus as claimed in any one of the preceding claims in which the vessel has a plate extending across its interior and provided with a plurality of threaded holes, each filter element having an externally threaded portion and being accommodated in a respective hole and affording means by which it may be engaged by the machine for automatically exchanging the filter elements and rotated by it to screw it into or unscrew it from its associated hole.

5. Filtering apparatus substantially as specifically herein described with reference to the accompanying drawings.