TECHNICAL FIELD The present invention relates to filtration syste based on solid particulate filtration media, particularl ion-exchange particulate filter media and the lik supported on a septum. Such filtration systems, often known as "precoat filters" are widely used in a variety of applications, one of the most demanding of which is in the filtration of boiler water steams of nuclear powered stea electrical generating operations, i.e., in the condenser stage of nuclear power plants.

BACKGROUND OF THE INVENTION The occurrence of both particulate materials and dissolved contaminants, some of which may be ionic, represent a substantial burden on nuclear power generating facilities; the problems attributable to such constituents can include build-up of materials which pose hazards from corrosion, scaling, radioactivity, embrittlement and stress cracking of heat exchange tubes, and the like. Not only do such phenomena limit the service life of very expensive equipment, they also pose intolerable hazards of personal safety, environmental pollution and extreme waste disposal and storage difficulties.

Other contexts of industrial processing, in the chemical and petroleum processing industries, in the treatment of a diversity of industrial wastes and related processes also involve highly demanding requirements of filtration systems.

The standard of performance required in such highly demanding environments has come to indicate the use of a finely divided ion-exchange resin filtration system wherein the particulate filter medium is supported on a septum where it is intended to be retained by the flow stream of the liquid to be filtered.

Such a septum offering performance superior to prior septa is disclosed and claimed in applicant's related and co-pending application, Serial No. 07/395,512, filed

August 18, 1989, the disclosure of which is hereby incorporated by reference herein. The septum defined therein is a preferred structure in the context of the present invention, which relates to an improvement 5 thereon, although it should be noted that the improvement disclosed in the present invention may be adapted to use with other septa, albeit without the particular advantages of the preferred septum of the related application.

In the most usual circumstances, the septum is 10 employed as a plurality of cylindrical or tubular cartridge elements deployed in a pressure vessel. The particulate filter medium system is introduced into the vessel in a liquid carried and is deposited on the outer surfaces of the septum and is retained there by the liquid 15 flow, which passes through the particulate filter medium, through the septum, into a flow channel inside the tube and thence out of the filtration pressure vessel.

When the filtration effectiveness of the particulate filter medium drops, whether because the ion-exchange 20 capacity is exhausted or because of the accumulation of particulate material entrained by the particulate filter medium, it is usual to pause the filtration to backwash the filter, removing and collecting the particulate filter medium for cleaning and regeneration or for waste 25 disposal, applying a fresh particulate filter medium precoat, and then resuming filtration operations.

In operation, the particulate filter medium on the surface of the septum is often exposed to substantial turbulence in the liquid flow stream. The turbulence may 30 be localized in certain parts of the system or may be a general condition affecting all or substantially all the particulate material, depending on the design and operating characteristics of the pressure vessel. Turbulent flow conditions can often pose problems of

^• ?_; stripping the particulate filter medium, or a material proportion of the particulate filter medium, from the septum, resulting in a risk of material passing through

the septum and into the filtrate which the particulat filter medium is intended to remove.

A discussion of the turbulent flow problem is set out in Bhanot, U.S. Patent 4,904,380. In that patent, the effect of turbulence is addressed by the provision of a "flow straightener" element for reducing turbulence or a "flow distributor" element also addressed to reducing turbulence, or a combination of both.

The approach defined by Bhanot defines the problem to which the present invention is directed, but provides a solution which adds materially to the cost of the septum, which introduces additional elements into the system with the attendant risk of failure over time, and which reduces the effectiveness of backwash operations. As disclosed in the present application, applicant has developed a septum which has integral capacity to retain the particulate filter medium in place in the presence of turbulent flow conditions, at least to the extent necessary to maintain effective performance, and which does not impede backwashing when required to a significant extent.

SUMMARY OF THE INVENTION In the present invention, a septum is provided with an integral spiral winding of yarn in an open diamond winding pattern which overlays the normal septum base structure, having a thickness which approximates the desired thickness of the particulate filter medium precoat. The overlay may be applied to the entire length of the septum or only to those locations exposed to high turbulence. The open pattern of the overlay is sized to permit the particulate filter medium to rather freely penetrate the depth of the overlay portion of the septum to the base layer, and to fill the open interstices to the surface of the overlay, and beyond if desired, and to freely pass out of the overlay when the direction of flow is reversed during backwash operations.

It is preferred that the septum base layer be a closed spiral wound yarn pattern, and particularly

preferred that it be formed in accordance with the teachings of our related co-pending application Serial No. 07/395,512. Other types of structures may be employed in the base layer, but generally lack the advantages of the most preferred construction.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an exploded view, in perspective of a septum core assembly with end caps.

Fig. 2 shows a perspective view of the precoat septum with its integral resin retainer, wound on a septum core assembly of Fig. 1.

DETAILED DESCRIPTION In such contexts, it is possible to employ as the filter medium an ion exchange resin, a solid absorbant material, or a mixture of both, applied in a layer of suitable depth on a septum. A variety of such materials are well known and commonly employed, and are selected in accordance with the dictates of the particular demands of the filtration to be achieved. In industrial scale operations, the filtration is most often conducted in a suitably sized vessel, containing an inlet, an outlet, means to dispose a plurality of septa within the vessel, a particulate filter medium applied as a coating on the septa, and means to assure that fluid to be filtered must pass the filter medium and the septa in order to pass from inlet to outlet. Most often, the interior volume of the vessel is divided by a header plate adapted to sealably receive and engage one end of the septa elements, so that fluid passing into the vessel passes through the filter medium, the septa, and then into an outlet manifold which carries the filtrate out of the vessel.

In the context of the present invention, the terms septum and septa are employed to designate a barrier support layer, designed to receive and hold an applied coating of a layer of a solid particulate filter medium, to provide a barrier to the flow of the filter medium and its entrained debris our of the filtration vessel with the

filtrate, and to permit effective cleaning and removal o the fouled filter medium from the filter vessel b backwashing operations. A septum does not itself perfor any filtration functionality during the filtratio operation, and desirably imposes the minimum possibl hydraulic flow resistance to the filtrate.

The structure of the individual septa elements ar generally in the form of a hollow, tubular form, at leas one open end adapted to engage a receiver in the tub sheet (or header plate, as it is sometimes called) of th filter vessel. The present invention is applicable t septa formed with two open ends or with one open and on closed; both forms are common in the art. The septu element itself is a porous material, formed int cylindrical configuration, supported as necessary b structural reinforcing elements such as a tubula perforate hollow core, or self supporting, and engaging a one end the closed end cap structure and at the other end the open end cap structure appropriate to engage th receiver. The septum structure may be formed of metalli or polymeric materials, and may be a perforated shee material, a woven or non-woven fibrous fabric, or a spira wound yarn configuration. Sheet materials and fabri materials may be employed in wrapped or pleated form familiar to the art, and may be reinforced by a variety o support materials. Spiral wound yarns are normall applied to a tubular core element which serves as th winding form and as structural support for the septum i use. In the present invention, spiral wound yarn septa ar preferred, for their ease and economy of manufacture, th ready reproducibility of characteristics, and their lon service life in demanding circumstances.

Particularly preferred are the septa taught an disclosed in our related co-pending application Serial No.

07/395,512, filed August 18, 1989, based on septa forme by spiral winding of air-jet bulked continuou multifilament yarns.

All septa are exposed in use to turbulent flow conditions over at least certain areas of their surface during filtration operations and are to at least some degree susceptible to erosion and scouring of the particulate filter medium from the septum surface in such areas.

A number of techniques have been employed in the design of filter vessels, and in elements associated with septa, headers, inlet configurations, and the like to reduce the incidence of turbulence, or to reduce its magnitude when present. Such efforts have met with limited success, represent considerable additional expense, often increase pressure drop through the filter vessel, and introduce new and often unacceptable opportunities for failure of the filter system integrity. The most effective of such efforts in the design of low turbulence filter vessels or redesigned header and/or inlet designs are not practical for existing installations. In the present invention, an integral particulate filter medium retainer is employed in areas subjected to turbulence which operates to retain an acceptable coating of the particulate filter medium in place on the septum, so that incident turbulent flow is not able to remove the filter medium to a meaningful extent from the region of turbulent flow protected by the particulate filter medium retainer. In operation, the need for turbulence damping elements is eliminated.

The particulate filter medium retainer of the present invention is an integral spiral winding of a suitable yarn applied in an open pattern on the surface of the septum subject to high turbulence in use, or the substantial equivalent. The winding is applied in a thickness which approximates the intended thickness of the particulate filter medium coating to be applied, and the open pattern effectively serves to form a segmented series of "pockets" or open apertures in the winding of the particulate filter meάiurr. retainer adapted to receive the particulate filter

medium when applied. The applied coating fills ope pockets to the required depth for filtration, down to th septum surface, and since the yarn winding prevent turbulence from scouring or eroding at least the majorit of the depth of the coating away from such surfaces, filtration efficiency is sustained over the entire area o the septum.

It is preferred that the particulate filter mediu retainer be applied to the septum only in those areas o the surface subject to high levels of applied turbulenc during filtration operations. The particulate filte medium retainer may be applied to the entire septum i desired, but will show some undesirable features which ar better minimized if possible. The particulate filte medium retainer will increase the pressure drop across th septum to a degree, for example, both during filtratio and during backwash operations, so that it is generall desirable to minimize the area of coverage to include tha necessary for effective performance, i.e. areas of hig incident turbulence.

While benefits of the present invention will accru with any thickness of the particulate filter mediu retainer which does not interfere with the fit of th septum element into the filter vessel, or interfere wit filtration flow, and which is thick enough to retain at least some particulate filter medium in the scoured areas of the septum, it is generally desirable to provide thickness to the particulate filter medium retaine winding which approximates the thickness of the filte medium layer. In this fashion, any particulate filte medium which is applied in a thicker coating in areas o high turbulence, and is not confined by the particulate filter medium retainer, may in fact be eroded away by the scouring action of turbulent flow conditions, but only to the effective depth defined by the particulate filter medium retainer.

The particulate filter medium retainer is not dependent on any narrowly defined pattern dimensions, so

long as the pattern admits the particulate filter medium and allows it to fill the interstices from the septum surface to the outer limits of the retainer structure. If these criteria are met, it is generally preferred that the openness of the pattern afford the smallest practical openings, since the larger the openings, the more susceptible to the effects of turbulence the particulate filter medium will be. As a general rule, spacings of up to about 1 cm have proved to be generally effective and are reasonably convenient, although openings of up to as much as about 5 cm, but more often up to 2.5 cm, will still be surprisingly effective.

The particulate filter medium retainer may be any suitable material for the intended environment of use, and includes synthetic polymer fiber yarns and the like selected from the group consisting of polyolefins, polyamides, polyesters, polyacrylics, pol sulfides, polyimides, polyether-etherketones, poly benzimidazoles, carbon fibers, and blends and mixtures of such fibers. Those of ordinary skill in the art are fully familiar with the criteria for such selection of filter materials, and may select appropriate fiber materials in the context of the service requirements in use.

The basic structure of the septum will also be largely conventional and familiar to those of ordinary skill in the art. It is important that the structure be sufficient to receive and support the particulate filter medium retainer, but as those of ordinary skill will readily recognize, most of the septa employed in the art will ordinarily meet such criteria without change, or with readily achieved adaptations.

It is also preferred to employ the present invention with septa of spiral wound yarn configuration, and particularly those taught in our related application, Serial No. 07/395,512, filed August 18, 1989, discussed above. The preference is based on the added convenience available in the manufacture of a spiral wound particulate filter ediur. retainer as a continuation of the windinσ

operation employed in the manufacture of the septu structure itself. As those of ordinary skill in the art will readily understand, the spiral wind configuration to achieve the open pattern of the particulate filter mediu retainer is not a great departure in manufacturing technique or equipment from that employed in the formation of the basic septum, and is preferably a change in winding pattern and length with the same yarn used in the base septum winding. It is possible, and often convenient, that the diameter of the septum be reduced by a small amount, at least in the region to which the particulate filter medium retainer will be employed, to avoid interference within the filtration vessel. With the added annular reinforcement of the septum achieved by the application of the particulate filter medium retainer, it is often possible to reduce the depth of the septum winding without concern that backwash conditions may damage the septum.

As noted above, it is also possible to employ the particulate filter medium retainer of the present invention with other types of septa commonly used in the art, including metallic structures, ceramic structures, and the like. A variety of such structures are generally known to those of ordinary skill in the art, and may be employed when desired and appropriate to the particular environment of use and requirements of manufacture. The selection of the septum is not narrowly significant to the effectiveness of the particulate filter medium retainer of the present invention, although spiral wound types are preferred for their many known virtues and their particular convenience and effectiveness when employed with the particulate filter medium retainer of the present inventio .

It is preferred that the septum structure employed be based on the use of a tubular, perforated core to support the septum structure. Such cores are essential to spiral wound yarns, and are often employed to assure sufficient physical properties in the use of other types of septa.

As discussed below, particular advantages may be achieved in the case of spiral wound yarn septa by adapting certain parameters of such core structures.

Septa are either closed at one end, and open at the other, or open at both ends, depending on the requirements of the housing where they will be employed. The opening is to permit egress of filtrate after passage through the septum, and to admit the fluid employed for backwashing operation. One open end is adapted to be manifolded into the appropriate structure in the filtration vessel, such as a tube sheet or header plate or the like, in conventional fashion. In usual fashion, these functions are most often, and generally most conveniently achieved by the employment of end caps which engage the ends of the septum and the core.

Since the greatest turbulence to which the septum and its coating of particulate filter medium normally encounter are most frequently at one end of the septum, the employment of a core with an associated end cap permits an additional approach to the objective of assuring that the erosive effects of turbulence does not result in the loss of filtration effectiveness in such systems.

It has proved possible in the case of spiral wound septa, in particular, to employ tubular cores and/or end caps which are impermeable and cylindrical in the regions where the overlying septum structure is most susceptible to turbulence, the loss of the particulate filter medium, and the reduction or failure in filtration effectiveness and efficiency. The impermeable regions offer a winding surface to which the spiral winding may be applied at sufficient tension that a radial seal is formed over the impermeable span, which precludes liquid flow into said core through the areas of said septum exposed to turbulent flow.

In the most usual case, the annular, imperforate segment extends from the end cap, i.e., from the open end of said tubular core, along the length of the core for a

length of about 0.5 to about 15 cm, although more often length of about 0.75 to about 4 cm, and preferably abou 0.75 to 2 cm is preferred.

The employment of such an axial sealing arrangemen can be of particular benefit when, as is often the case, it permits the termination of the particulate filte medium retainer short of the end of the septum adjacent t the associated end cap. If the particulate filter mediu retainer is applied in a fashion which extends over th end cap, or to a point adjacent the end cap, with n spacing between the end cap and the beginning of th particulate filter medium retainer, there is a frequen failure of the septum to seat properly in the header, an a failure to achieve a proper and effective seal. It i accordingly preferred to terminate the particulate filte medium retainer at a point which is axially remote fro the end cap and the end of the septum by at least abou 0.5 cm, and preferably about 0.75 to 2 cm. Without th employment of the axial seal, the span of septum lef unprotected in this high turbulence location would b prone to substantial loss of the particulate filte medium, and a failure of filtration effectiveness.

The axial seal affords a sufficient protection tha substantial, or even complete, loss of the applie particulate filter medium in such region will not occasio a filtration failure, since the radial seal achieved in such regions with ordinary winding tension in the formation of the septum is sufficient to preclude flo along the axis of the septum to an open -region. Substantially all flow through the septum in such regions is blocked, and with it, the potential for filtration failure.

In most circumstances, it is preferred to employ both the particulate filter medium retainer and the axial seal together, although each alone will, in some cases, be sufficient.

It is preferred that the particulate filter medium retainer not be employed over the entire surface of the

septum, or in areas not exposed to significant turbulence in operation. Since the particulate filter medium retainer adds to the diameter of the septum structure, it may produce interference with the fitting of the septa or with flow properties within the filtration vessel. In addition, backwash may be more efficient if excessive coverage by the particulate filter medium retainer is avoided.

The present invention may be understood by reference to Figs. 1 and 2 which illustrate a preferred embodiment of the present invention, exemplifying the best mode for practice of the invention.

Referring to Fig. 1, an exploded perspective view of a septum core assembly is shown of core tube (1) and two open end caps (2) and (2') .

Core tube (1) is a spiral wound sheet of perforated material, preferably steel, and most preferably stainless steel, having a substantial number of perforations (20) distributed over the surface of the material. The spiral configuration is fixed by welding, shown at seam (21) . While core tube (1) is generally cylindrical, it may have axial, spiral or circumferential depressions formed in the surface to aid in flow from the septum winding to be applied to one of the perforations (20) in use. The core tube may be formed of other materials and in other specific configurations familiar to those of ordinary skill in the art, including formed polymers, other metals, and the like. The present invention may be employed with any core tube convenient, although that shown in Fig. 1 and described above is preferred.

The core assembly also includes two end caps (2) and (2'), as illustrated in Fig. 1. In the embodiment shown, both end caps (2) and (2') are open-ended and identical, although it should be understood that one or both may be substituted with different configurations to meet the requirements of a specific installation, and may have any specific configuration suited to the equipment and requirements of use, excepting the portion which mates

with the core tube (1) and which subsequently received th septum winding, as discussed below.

The end cap (2) is normally steel, preferabl stainless steel, and is made up of annular rim (3) an face (4) which together are adapted, in well know fashion, to sealably engage a seat assembly disposed o and forming a part of a tube sheet or header plate in th filtration vessel (not shown) . The seal may be augmente by gaskets and/or O-rings (also not shown) in customar fashion. These elements are familiar to those of ordinar skill in the art and form no part of the presen invention.

Projecting axially and inwardly from face (4) i cylindrical member (5) , provided with raised cylindrica portion (6) , defining an axial flow port (7) to provid ingress and egress of liquid during operation. The inne portion of cylindrical member (5) has an outside diamete substantially equal to the inside diameter of a terminal portion of core tube (1) , and is adapted to slidably fit within core tube (1) . The raised cylindrical portion (6) defines a shoulder (8) which serves as a stop for core tube (1) . The outside diameter of cylindrical portion (6) is preferably substantially equal to the outside diamete of core tube (1) . End caps (2) and (2') are slid into engagement with core tube (1) . They are preferably fixed in place by convenient means, preferably by tack welding or the like, in close abutment with shoulder (8) .

The completed septum core assembly is then provided with the septum winding (10) and the resin retainer winding (11), shown in Fig. 2. While the septum may be made of a variety of materials and forms, the spiral winding of a continuous multifilament bulked yarn is preferred, particularly yarn bulked by air jet entanglement, such as Taslanized® yarns and the like. Polypropylene yarns are preferred, although others maybe employed, as discussed above. The technique for winding of septum (10) , in a closed diamond pattern, is well known

in the art. Septum winding (10) is applied over the core assembly of Fig. 1 from end cap- (2) to end cap (2 ¹ ), where it will bear against faces (4) and (4'), and overlie the length of core tube (1) and the annular, imperforate cylindrical segments (6) and (6') . The winding tension forms a tight radial seal over segments (6) and (6'), through which no flow will pass in use. Segments (6) and (6') may have any convenient length sufficient to form the radial seal when the septum winding (10) is applied, from as little as about 0.5 cm, up to as much as about 15 cm, preferably about 0.75 cm to about 4 cm, and most preferably about 0.75 to 2 cm.

It should be noted that turbulent flow conditions will often occur only at one end of the septum. In such circumstances, a conventional end cap with no cylindrical segment (6) may be employed to avoid any needless reduction in effective filtration area. It is generally preferred to employ the radial seal over the cylindrical ' segment (6) on at least one open end of the septum. After septum winding (10) is applied, the winding operation is continued with a different winding pattern, and preferably over a shorter span of the length of the septum and preferably using the same yarn as that employed to form septum winding (10) . This second winding, in an open diamond spiral pattern, forms integral particulate filter medium retainer (11) . Such winding procedures are also well known in the art.

The retainer winding (11) should be applied over the areas of the septum subject to high turbulence, which are generally at one end, except that it should be terminated at or preferably slightly overlying the radial seal, and not extend completely to the end cap to avoid interference in seating of the septum. The winding (11) is preferably terminated at least 0.5 cm, preferably at least about 0.75 cm, from the end cap face (4) and annular rim (3) , and perhaps slightly more, leaving a portion (12) of septum winding (10) adjacent to end cap annular rim (3) and face

(4_ exposed. Exposed septum winding portion (12) adjacent

the end cap is fully protected from the consequences of turbulent flow by the annular seal formed on cylindrical segment (6) .

When the filter medium precoat is formed, the radial seal area (12) will experience substantially no flow, and will receive little or no coating of the filter medium.

On the other hand, since there is no flow, there will equally be no leakage in operation.

It is possible to substitute for the retainer winding (11) , particularly when a septum of a material other than the winding (10) is employed, a sleeve of extruded polymer mesh. Such materials are generally familiar to those of ordinary skill in the art. One to five plies of such mesh, preferably about three plies, will form a filter medium retainer comparable to the open diamond pattern winding (11) .