Filter assembly with replaceable filter element

Abstract

A filter assembly is provided having a filter chamber and a filter element therein that is replaceable without contamination of filtered fluid with unfiltered fluid even though the filter chamber is not emptied of unfiltered fluid comprising, in combination, a filter housing; a filter chamber in the housing; an inlet for unfiltered fluid and an outlet for filtered fluid in the housing; a filter element removably disposed in the filter chamber across the line of fluid flow from the inlet to the outlet so that fluid flow from the inlet to the outlet normally proceeds through the filter; a weir spaced from and downstream of the filter element, compelling filtered fluid from the filter flowing towards the outlet to proceed upwardly through the space between the weir and the filter and then by overflow across the weir; and a weir follower operatively associated with the base of the filter element and movable along the weir in fluid-tight relation to the base and the weir, to carry filtered fluid in the space between the weir and the filter element on the downstream side of the filter element to the top of the weir while a used filter element is being removed, and to displace unfiltered fluid in the filter chamber from contact with the downstream side of a clean filter element while the clean filter element is being installed. The filter assembly is particularly suited for use as a return line filter mounted in a tank.

Description

Filter assemblies are normally so constructed that it is possible to remove and replace a filter element when the element either fails or becomes so loaded with contaminants that the system is starved for filtered fluid downstream of the filter. Removability is ensured by placing the filter element in a filter chamber or bowl, to which access is provided either by removing the bowl or by removing the top of the bowl or chamber. When the filter is removed, the unfiltered fluid in the chamber before the filter becomes mixed with the filtered fluid in the chamber downstream of the filter, and before it is possible to place a fresh filter element in the assembly, it is necessary to clean out the filter chamber or bowl of unfiltered fluid, in order to prevent contamination of the downstream filtered fluid surface or side of the filter. This is a considerable nuisance, since it wastes fluid, and is time-consuming.

It is also a particular problem when the filter chamber or bowl is in an inaccessible location, such as, for example, when mounted in a tank or reservoir, as, for example in the case of a tank-mounted return line filter. Such a filter filters the fluid entering the tank, which filtered fluid is then held in storage in the tank, for supply of filtered fluid to the system. If any portion of the unfiltered fluid in the return line is allowed to enter the tank, the entire tank becomes contaminated, and must be filtered again before it can be used. In such a situation, it is essential that the replacement of the filter element be possible without contamination of the downstream filtered fluid with upstream unfiltered fluid. In tank-mounted return line filters, this normally has meant the removal of the entire filter assembly, bowl and all, and other precautions, which increases the cost and time for filter element replacement.

Most in-tank filter assemblies are arranged for filtering fluid flow from the inside-out of a cylindrical filter element. In this way the contaminants removed by the filter are inside the filter element, and when the filter element is removed from the filter bowl or chamber, the likelihood of contamination of the downstream filtered fluid with contaminants is reduced.

This however considerably complicates the relief valve system for the filter assembly. It is essential in all return line filters to provide a bypass path for relief of differential fluid pressure across the filter element, when the filter element becomes so loaded with contaminants that the differential pressure across the filter increases to the point where backpressure affects system function or filter element collapse is imminent. The provision of a relief valve for a filter element with inside-out flow usually takes the form of a reciprocably-mounted filter element which is itself the relief valve poppet, and which is spring-mounted to close off the bypass path, but which is moved outwardly from the valve seat when the differential fluid pressure increases above a predetermined minimum.

Filter elements are heavy, particularly when loaded with contaminants, and if a reciprocable filter element is subjected to vibration and/or external shock, the filter element can oscillate, and open and close the bypass path, even though the minimum differential pressure at which the bypass path should be opened has not yet been reached.

It is also difficult to build a filter for inside-out flow that withstands flow fatigue. System return lines have very high flow differences (unlike system pressure lines), and the resistance of a filter element to differential fluid pressure is greatly diminished after it has experienced a number of high flow surges. If the filter element is corrugated, the corrugations are expanded outwardly into the supporting sheath, stretching and reshaping the corrugated form, and this flexing of the element eventually destroys the element.

It would be desirable, therefore, for return line filters mounted in tanks, to utilize conventional outside-in flow, with conventional relief or check valves, but this is very difficult to do, because of the contamination problem of downstream filtered fluid with unfiltered fluid when the filtered element is being changed.

What is needed is a mechanical structure which makes it possible to remove a filter element from a filter bowl or chamber without emptying the filter bowl or chamber of unfiltered fluid, which nonetheless prevents such unfiltered fluid from passing downstream of the filter into the downstream filtered fluid line, and which also makes it possible to install a clean filter element in a filter bowl or chamber containing unfiltered fluid without permitting the escape of unfiltered fluid to the downstream side of the new filter cartridge or element being installed.

In accordance with the invention, a filter assembly is provided having a filter chamber and a filter element that is replaceable without employing the filter chamber of unfiltered fluid. This is made possible by providing for attachment at the base of the filter element a weir follower that is movable along a weir in fluid-tight relation both to the base of the filter element and to the weir. The weir is spaced from and downstream of the filter element, and the weir follower moves across this space, sealing it off from the remainder of the filter chamber. When the contaminated filter element is being removed, the weir follower carries filtered fluid in the space downstream of the filter element to the top of the weir, overflowing into the tank, while the disappearance of the volumetric displacement of the removed filter element causes the level of contaminated or unfiltered fluid upstream of the filter to fall well below the top of the weir. When the clean filter element is being installed, the weir follower prevents unfiltered fluid in the filter chamber from contacting the downstream side on the interior of the filter element. The weir follower compels filtered fluid in the filter chamber to move downwardly and outwardly to the upstream side of the element. This makes it possible to protect both the filtered fluid line and the space downstream of the filter between the filter and the weir from intrusion by unfiltered fluid during both removal and installation of a filter element. After installation of the filter element the weir compels filtered fluid in the space between the filter element and the weir proceeding towards the outlet to proceed through the space by upflow, and then overflow across the weir.

Accordingly, the filter assembly of the invention comprises, in combination, a filter housing; a filter chamber in the housing; an inlet for unfiltered fluid and an outlet for filtered fluid in the housing; a filter element removably disposed in the filter chamber across the line of fluid flow from the inlet to the outlet, so that fluid flow normally proceeds through the filter; 100 110, each of which is apertured at 111 for reception of the cap screws 112, which pass through the bores 111 of the lugs 110 and are secured in threaded sockets 113 in the ring 109. A leak-tight seal between the lip 104 of the cannister 103 and the ring 109 is provided for by the gasket ring 117.

The filter head 102 has an inlet port 118. The outlet port 119 is the aperture in cap 105 at the other end of the filter cannister 103.

The cap 107 carries at one side, off center, a magnetic pressure indicator 120 of conventional type, such as described in U.S. Pat. No. 2,942,572 to David B. Pall, patented June 28, 1960, and from two to eight relief valves 121 of the conventional poppet type. The pressure indicator is installed in a side bore 122 of the cap 107, with sealing provided by the O-ring seal 123. Upstream unfiltered fluid pressure is sensed at the inner face of the magnetic piston 124 via the passage 125, leading to chamber 106, upstream of the filter element, and downstream fluid pressure is sensed at the outer face of the piston 124, via the passage 126 to the filtered fluid space 141 on the interior, i.e., downstream, of the filter element.

The relief valves 121 are press-fitted into sockets in the head 137, and are in flow connection on their upstream side via passages 132 with space 106, and on their downstream side with chamber 133 of the cap 107. A filter element adapter 128 is threaded into the socket 134 of the head 137 in fluid flow connection at its open interior with the chamber 133 leading to the outlet port 119 via the open interiors of the adapter 128 and weir 151. In response to a pressure differential across the valves 121 between the inlet port 118 and outlet port 119 that exceeds a predetermined minimum, the valves 121 open, and thus permit bypass flow via passages 132, 133 into and through the adapter 128 and weir 151 to the outlet 119. This predetermined minimum is how ever greater than the differential pressure sensed by the differential pressure indicator at passages 125, 126 at which the pressure indicator 120 gives a signal.

Thus, the differential pressure indicator indicates when the filter element becomes loaded sufficiently that opening of the relief valves 121 is imminent, and before they are actually opened, making it possible to change the filter element before unfiltered bypass flow begins. In the event that the filter element be not changed, and differential pressure continues to increase, one to eight of the relief valves 121 open, and the bypass passages 132, 133 are opened to prevent collapse of the filter element or system instability from excess back pressure or pressure relief arising from high viscosity oil on cold start-up.

The head cap 107 also has a through bore 180 opening at one end to the exterior of the cap at threaded port 181 which is closed off by fill cap 182, and opening at the other end to the chamber 106. This bore serves as a fill passage, to add liquid to the tank in which the assembly is installed, and all such liquid since it is admitted upstream of the filter is filtered as it passes into the tank.

The element adapter 128 attached to the head cap 107 at socket 134 thereof supports one end of the filter element 140, and actually projects into the open interior 141 of the filter element. There is an external recess 142 of this portion of the element adapter 128. The end cap 143 of the filter element 140 carries a weir follower 155 projecting inwardly, which defines a recess 144, in which is captured an O-ring 145, which ensures a fluid-tight connection between the filter end cap and the element adapter, and thus prevents bypass of unfiltered fluid into the space 141 in the interior of the filter element 140.

The filter element 140 carries between end caps 143 and 146 a perforated core 147, and a corrugated microporous multilayer filter sheet 148 having a microporous layer of bonded nonwoven inorganic fibers attached to a substrate, with an average pore diameter within the range from 3 to 25μ. While this filter assembly does not include it, an external sheath can be provided, to protect the outer surface of the filter element.

The element adapter 128 centers the filter element in the filter cannister, with a space 150 therebetween, and spaces the core of the filter element from the weir 151, with the space 152 therebetween. In this case, the weir 151 is in the form of a standpipe, disposed in and securely attached to the outlet 119 in the base cap 105 of the filter cannister.

Securely attached to the inside wall of the weir, opposite each other at 153, at the top of the weir, are two spring detents 155, in the form of resilient wire with an outward projection 158.

The end cap 146 carries a weir follower 158 which is movable reciprocably along the outside of the weir and engages the exterior of the weir 151 in a fluid-tight seal by way of the O-ring 161 captured in the recess 162.

The adapter 128 includes a groove 142, shaped to intercept O-ring 145 of the weir follower 155, and thus when the cap 107 is being removed from the cannister 103 the filter element is retained to the cap, and carried with it. This makes it possible to remove the filter element at the same time as the cap 107 is being withdrawn.

Since the filter element is sealed to the adapter 128 at its upper end cap 143 and to the weir at its lower end cap 146, it will be apparent that normal fluid flow from the inlet 118 to the outlet 119 of the filter housing 100 will proceed from the inlet port 118 into the space 106, then into space 150, and then through the filter 140 into the space 152 between the weir and the inside of the filter core. The filtered fluid proceeds by overflow over the top of the weir 151 into the open interior 159 of the weir, and then proceeds directly to the outlet 119 from the cannister 103.

Attached to the lower face of the base cap 105 of the filter cannister is a spider 166, which has a central aperture 167 in its base 168 in which is securely fastened a bolt 169. The end portion of the bolt is threaded, and receives a cap nut 170.

The cap nut and bolt secure across the outlet 119 from the filter cannister a fluid diffuser distributor 171 of stainless steel wire mesh sheet. The sheet is held between retaining plates 172, 173, and ensures that flow from the outlet is distributed relatively uniformly at a velocity of approximately 2 ft/sec. This inhibits aeration and eliminates turbulent flow in the tank downstream of the filter assembly (not shown).

In order to replace the filter element, all that is necessary is to remove the cap 107 of the filter head 102. The cap, which is externally threaded at 127, is retained in the threaded socket 137 of the filter head, and a fluid-tight seal at the interior portion of the socket is ensured by the O-ring seal 139. Accordingly, the cap 107 is simply unscrewed from its socket, carrying with it the adapter 128, and with the adapter the filter element 140, the end cap 143 being held by O-ring 145 in groove 142. The filter element 140 and weir follower 158 move together to the top of the weir 151. Since the follower 158 seals against the weir 151 in a fluid-tight seal, filtered fluid above the weir follower in space 152 is carried up with the filter element 140, and continues to flow by overflow into the weir 151, as the filter element 140 and weir follower 158 proceed up through the cannister, while the unfiltered fluid in the cannister flows into the space beneath, formerly occupied by the filter element 140 and space 152. The volume thus released for unfiltered fluid is adequate, by the time the filter element 140 has reached the top of the weir 151, to receive all of the unfiltered fluid in the space 150, as well as in the space 106 above the filter element in the filter head 102, and this unfiltered fluid is below the top of the weir and thus does not flow into the filtered fluid line downstream of the filter.

When the weir follower 158 reaches the top of the weir 151, the filter element can be withdrawn and replaced.

A fresh filter element is installed by placing the projecting portion of the adapter 128 into the weir follower 155 of end cap 143, and then placing weir follower 158 of the lower end cap 146 over the weir 151. The assembly is then pushed down along the weir until the threaded portion of the cap 107 reaches the threaded portion of the socket. The cap can then be screwed into the socket, and the closure completed.

As this is being done, the unfiltered fluid is displaced outwardly and upwardly around the bottom and outside of the filter element 140 into the space 150 between the filter element and the filter cannister. Some of this fluid may proceed through the filter into the space 152, now re-created between the filter and the weir, thus reducing the volume of fluid to be accommodated on the upstream side of the filter, in space 150. Unfiltered fluid cannot enter the space 152 between the filter elements and the weir except by passage through the filter, because of the weir follower 158, and its sealing engagement with the weir. Thus, the filter element can be installed in the cannister without danger of contamination of the filtered fluid line with unfiltered fluid. The level of unfiltered fluid in the cannister will be below the top of the filter and the weir, when installation is complete.

After the filter and the cap have been fully installed, and the cap 107 sealed to the head, flow can again begin.

The flow distributor at the outlet end of the filter bowl can be removed and replaced when required, simply by removal of the cap nut.

The drawings all show embodiments having internal weirs for filtered flow from outside-in of the filter element.

If flow through the filter is from inside-out, then the weir can be concentrically disposed outside the filter element, and the weir follower will be outside the filter element as well, between the filter element and the weir. Then, unfiltered liquid is confined inside the weir, rather than outside. The filtered fluid is of course collected and distributed from outside the weir, but otherwise the assembly is exactly the same.

The drawings all show embodiments having cylindrical filter elements. The invention is also applicable to flat or curved sheet filter elements, such as a corrugated filter sheet. In this case, the filter chamber is in two parts, with the weir a compartmenting divider wall separating the two parts. The filter is disposed on one side of the weir, with the weir follower and space therebetween. Again the weir flow proceeds through the filter to the space between the filter and weir, over the top of the weir. Filtered fluid is collected on the other side of the weir and the weir and weir follower protect the downstream side of the filter from contamination with unfiltered fluid, when the filter is removed.

The filter assemblies of the invention are useful with any filter element having a filter sheet in tubular or cylindrical form, provided with end caps to close off the interior space enclosed by the filter sheet, and give control of the flow of filtrate. One or both of the end caps can be provided with flow passages for filtrate flow. The caps can be of any desired material, appropriate to the system and the need, and are bonded to the cylinder ends in a leak-proof seal by appropriate bonding agents. Such filter elements are conventional, and well known to those skilled in this art, and form no part of the instant invention. Melt-sealed end caps, as disclosed in U.S. Pat. No. 3,457,339, patented July 22, 1969, to David B. Pall, et al, may be used.

The filter sheets can be formed of any porous sheet material having pores extending from surface to surface. One or several sheets of the same or varying porosity, and one or more of which may act as prefilters, can be employed, in close juxtaposition, or even bonded together, or also spaced apart. Sheets such as open-weave wire or plastic mesh may be added. Paper, which can, if desired, be resin-impregnated, is a preferred base material, since it yields an effective, versatile and inexpensive fluid-permeable filter medium. The invention is, however, applicable to sheet materials formed of any type of fiber, including not only cellulose fibers but also synthetic resin fibers and fibers of other cellulose derivatives including, for example, fibers of polyvinyl chloride, polyethylene, polypropylene, polyvinylidene chloride, cellulose acetate, cellulose acetate propionate, viscose rayon, polyacrylonitrile, polymers of terephthalic acid and ethylene glycol, polyamides, and protein fibers of various sorts, such as zein and the alginates, glass, asbestos, potassium titanate, mineral wool, polystyrene, rubber, casein, hemp, jute, linen, cotton, silk, wool, and mohair. Also useful, in addition to papers, are textile and wire fabrics, and woven and nonwoven fibrous layers of all kinds, such as felts, mats and bats made of fibrous materials of any of the types listed above, and woven wire mesh.

The sheet material should in most cases be sufficiently rigid to be self-supporting when folded in cylindrical form, but if it is not, a core and/or external sheath of rigid metal, plastic or similar rigid material can be provided, as a support.

Also, the filter sheet material of which the filter elements of the invention are made can be, if desired, impregnated with a synthetic resin or cellulose derivative to increase its strength and resistance to wear by the fluid being filtered. The impregnating agent can be any material useful in the impregnation of papers and textile materials. Such materials are well known in the paper and textile arts, and form no part of the instant invention. The impregnating agents can be in liquid form, capable of undergoing solidification as by polymerization, cross-linking, or the like. They can also be in solid form, and applied to the base from a solution in an inert solvent, or as melts. Representative impregnating resins include phenol-formaldehyde resins, urea-formaldehyde resins, melamine-formaldehyde resins, polyester resins, and polyepoxide resins.

The end caps capping the filter tube or cylinder can be of any desired material, such as metal or plastic. The end cap should be rigid, and attached to the tube or cylinder in a leak-tight seal. The end cap can be formed by molding or casting in the desired shape, integral or in one piece with the means movable along the weir, if this be part of the end cap, as in FIGS. 4 to 6.

The core and/or sheath support sections and end caps can be formed of any desired material sufficiently rigid to provide adequate support for the filter sheet with which it is to be used. Metal core supports are preferred, such as core supports of stainless steel or aluminum, which are readily stamped in any desired cross-sectional configuration, and which will retain that configuration and provide the desired rigidity. Stainless steel, which provides greater resistance to certain highly reactive fluids, is preferred. Steel, copper, magnesium, beryllium, titanium, nickel, iron and various alloys thereof are typical additional metals which can be used.

The core and/or sheath supports and end caps also can be formed of rigid synthetic polymeric materials and cellulose derivatives, such as, for example, glass, ceramics, phenol-formaldehyde resins, polytetrafluoroethylene, polychlorotrifluoroethylene, urea-formaldehyde resins, melamine-formaldehyde resins, polyvinyl chloride, polyvinylidene chloride, polystyrene, epoxy resins, polyoxymethylene, polypropylene, polyethylene, polyvinyl butyral, cellulose acetate, ethyl cellulose and cellulose acetate propionate. Cores made of such materials can be formed into core sections by molding from powders of the material, or by stamping or shaping of sheets of the material. This may be easier than molding the entire core support, in the case of complex cross-sectional configurations and when complex end connections for the core support are necessary. The plurality of sections composing such core supports of the invention can be bonded together by application of heat and pressure, or a suitable adhesive, or by application of a solvent for the material which will make possible fusion of the adjacent edges of the sections at the seams.

A suitable core is described in U.S. Pat. No. 3,246,766, to David B. Pall.

Claims

1. A filter assembly having a filter chamber and a filter element therein that is replaceable without contamination of filtered fluid with unfiltered fluid in the filter chamber even though the filter chamber is not emptied of unfiltered fluid when a filter element is removed and installed, comprising, in combination, a filter housing; a filter chamber in the housing; an inlet for unfiltered fluid and an outlet for filtered fluid in the housing; a filter element removably disposed in the filter chamber across the line of fluid flow from the inlet to the outlet so that fluid flow from the inlet to the outlet normally proceeds through the filter; a weir spaced from and downstream of the filter element, compelling filtered fluid from the filter flowing towards the outlet to proceed upwardly through the space between the weir and the filter and then by overflow across the weir; and a weir follower operatively associated with the base of the filter element and removably attached to one end of the filter element and mounted on the weir in a manner to be retained thereon while a used filter element is being removed, and while a clean filter element is being installed, and slidably movable along the weir in fluid-tight relation to the base and the weir, to carry filtered fluid in the space between the weir and the filter element on the downstream side of the filter element to the top of the weir while a used filter element is being removed, and to displace unfiltered fluid in the filter chamber from contact with the downstream side of a clean filter element while the clean filter element is being installed.

2. A filter assembly according to claim 1, in which the weir follower is removably attached to one end of the filter element and is slidably mounted on the weir in a manner to be retained thereon while a used filter element is being removed, and while a clean filter element is

being installed. 3. A filter assembly according to claim 2, 1 in which the filter element is cylindrical with end caps, and the weir follower is removably attached to one end cap.

4. A filter assembly according to claim 1, in which the weir follower is removably attached to the weir and fixed to the filter element in a manner to be retained thereon while a used filter element is being removed, and while a clean filter element is being installed.

5. A filter assembly according to claim 4, in which the filter element is cylindrical with end

caps, and a weir follower is fixed to each end cap. 6. A filter assembly according to claim 1, in which the filter element is tubular with an open center and end caps having central apertures, and the weir and the weir follower are disposed concentrically within the open center of the

filter element tube. 7. A filter assembly according to claim 6, in which the top of the weir extends high enough and the filter chamber volume below the top of the weir is large enough that the top of the weir is above the level of unfiltered fluid remaining in the filter chamber after the filter element has been removed, so that the unfiltered fluid left behind in the chamber does not flow over the top of the weir into the

outlet for filtered fluid. 8. A filter assembly according to claim 6, in which the filter housing has a projection extending into the filter chamber above the weir, extending into and sealing off one end of the filter element at the end cap, with the weir follower sealing off the

other end of the filter element at the other end cap. 9. A filter assembly according to claim 1, in which the weir is an upstanding wall defining an inner wall of the filter chamber on the downstream side of the filter element with access to the outlet for filtered fluid being by overflow

across the top of the weir. 10. A filter assembly according to claim 9, in which the filter element is tubular with an open center and end caps having central apertures, and the weir is a baffle disposed concentrically within the filter element upstream of the filtered fluid outlet of the housing for filtered flow in the direction from the outside of the filter

element towards the inside. 11. A filter assembly according to claim 1, in which the filter element is tubular and the weir is a central standpipe for flow in the direction from the outside of the filter element towards the inside.

12. A filter assembly according to claim 1, in which the filter element is tubular with an open center and end caps having central apertures and the weir follower is an annulus projecting inwardly from an end cap, slidable along the outside of the weir, having a recess facing the weir outside, capturing a sealing element in the recess, and sealing

against the outside of the weir at the sealing element. 13. A filter assembly according to claim 1, in which the filter element is tubular with an open center and end caps having central apertures and the weir follower is an annulus projecting outwardly from the weir, slidable along the outside of the weir, having a recess facing the filter element end cap, capturing a sealing element in the recess, and removably sealing against the filter element end cap, and sliding with the filter element to the top of the weir when the filter element is being removed, and to the button bottom of the weir when the filter element is

being installed. 14. A filter assembly according to claim 1, having a detent at the top of the weir, to engage and retain the weir follower upon removal of the used filter element, the detent being releasable after installation of a fresh filter element, to permit the weir follower to proceed with the filter element down the weir to the operating position.

. A tank for storage of filtered fluid having a feed outlet line and a return inlet line for circulation of fluid to and from the tank, and, disposed in the return line upstream of the tank, a filter assembly

according to claim 1. 16. A tank according to claim 15, in which the filter assembly has a port for entry of fluid for replenishing fluid in the tank, and filtering such fluid before entry into the tank. 17. A filter assembly having a filter chamber and a filter element therein that is replaceable without contamination of filtered fluid with unfiltered fluid in the filter chamber even though the filter chamber is not emptied of unfiltered fluid when a filter element is removed and installed, comprising, in combination, a filter housing; a filter chamber in the housing; an inlet for unfiltered fluid and an outlet for filtered fluid in the housing; a filter element removably disposed in the filter chamber across the line of fluid flow from the inlet to the outlet so that fluid flow from the inlet to the outlet normally proceeds through the filter; the filter element having a top end and a base end and being removably attached in fluid-tight relation at the top end to the filter housing; a weir spaced from and downstream of the filter element, compelling filtered fluid from the filter flowing towards the outlet to proceed upwardly through the space between the weir and the filter and then by overflow across the weir; and a weir follower operatively associated with the base of the filter element and movable along the weir in fluid-tight relation to the base and the weir, to carry filtered fluid in the space between the weir and the filter element on the downstream side of the filter element to the top of the weir while a used filter element is being removed, and to displace unfiltered fluid in the filter chamber from contact with the downstream side of a clean filter element while the clean filter element is being installed.

18. A filter assembly according to claim 17, in which the weir follower is removably attached to one end of the filter element and is slidably mounted on the weir in a manner to be retained thereon while a used filter element is being removed, and while a clean filter element is being installed.

19. A filter assembly according to claim 18, in which the filter element is cylindrical with end caps, and the weir follower is removably attached to one end cap.

20. A filter assembly according to claim 17, in which the weir follower is removably attached to the weir and fixed to the filter element in a manner to be retained thereon while a used filter element is being removed, and while a clean filter element is being installed.

21. A filter assembly according to claim 20, in which the filter element is cylindrical with end caps, and a weir follower is fixed to each end cap.

22. A filter assembly according to claim 17, in which the filter element is tubular with an open center and end caps having central apertures, and the weir and the weir follower are disposed concentrically within the open center of the filter element tube.

23. A filter assembly according to claim 22, in which the top of the weir extends high enough and the filter chamber volume below the top of the weir is large enough that the top of the weir is above the level of unfiltered fluid remaining in the filter chamber after the filter element has been removed, so that the unfiltered fluid left behind in the chamber does not flow over the top of the weir into the outlet for filtered fluid.

24. A filter assembly according to claim 22, in which the filter housing has a projection extending into the filter chamber above the weir, extending into and sealing off one end of the filter element at the end cap, with the weir follower sealing off the other end of the filter element at the other end cap.

25. A filter assembly according to claim 17, in which the weir is an upstanding wall defining an inner wall of the filter chamber on the downstream side of the filter element with access to the outlet for filtered fluid being by overflow across the top of the weir.

26. A filter assembly according to claim 25, in which the filter element is tubular with an open center and end caps having central apertures, and the weir is a baffle disposed concentrically within the filter element upstream of the filtered fluid outlet of the housing for filtered flow in the direction from the outside of the filter element towards the inside.

27. A filter assembly according to claim 17, in which the filter element is tubular and the weir is a central standpipe for flow in the direction from the outside of the filter element towards the inside. 28. A filter assembly according to claim 17, in which the filter element is tubular with an open center and end caps having central apertures and the weir follower is an annulus projecting inwardly from an end cap, slidable along the outside of the weir, having a recess facing the weir outside, capturing a sealing element in the recess, and sealing against the outside of the weir at the sealing element.

29. A filter assembly according to claim 17, in which the filter element is tubular with an open center and end caps having central apertures and the weir follower is an annulus projecting outwardly from the weir, slidable along the outside of the weir, having a recess facing the filter element end cap, capturing a sealing element in the recess, and removably sealing against the filter element end cap, and sliding with the filter element to the top of the weir when the filter element is being removed, and to the bottom of the weir when the filter element is being installed.

30. A filter assembly according to claim 29, having a detent at the top of the weir, to engage and retain the weir follower upon removal of the used filter element, the detent being releasable after installation of a fresh filter element, to permit the weir follower to proceed with the filter element down the weir to the operating position.

31. A tank for storage of filtered fluid having a feed outlet line and a return inlet line for circulation of fluid to and from the tank, and, disposed in the return line upstream of the tank, a filter assembly according to claim 17.

32. A tank according to claim 31, in which the filter assembly has a port for entry of fluid for replenishing fluid in the tank, and filtering such fluid before entry into the tank.