Backwash Method and Apparatus

This invention relates generally to a method and apparatus for cleaning a plurality of filter elements that are vertically positioned within a filter vessel. More specifically, the invention relates to a method and apparatus for cleaning such filter elements positioned within a filter vessel that has a tube sheet dividing the filter vessel into a lower filter compartment and an upper plenum compartment.

The method and apparatus in accordance with the invention is disclosed in conjunction with a specific filter apparatus that utilizes precoats on filter elements. It is contemplated that the invention may be similarly utilized in conjunction with other types of filter apparatus that do not utilize precoats on filter elements.

It is known that liquids may be very effectively purified by passing them through a filter screen which has been precoated with a layer of particles, referred to herein as "precoat layer", such as ion exchange resin particles. Such precoat layer may remove dissolved solids and undissolved solids when the precoat layer contains active materials, such as ion exchange particles. This method is disclosed in U.S. Pat. No. 3,250,703, issued May 10, 1966, and assigned to the same assignee of this application. Periodically it becomes necessary to clean the filter because the precoat layer's adsorbent or absorbent capacity has been exhausted, or the filter's resistance to flow has increased due to an accumulation of particulate in or on the precoat layer, or within the filter elements, or both.

The filters normally employed are filter elements that have cylindrical cores with apertures to allow flow to pass through the walls of the cylinder. The core may be made of perforated metal, molded plastic, sintered powdered metal or plastic, laminated layers of woven wire metal or plastic, or other constructions providing adequate tensile and compressive strength and permeability. The filter elements are typically positioned within a filter vessel and are secured to a tube sheet that divides the filter vessel into a filter compartment and a plenum compartment. There are generally two types of such filter apparatus that employ vertically positioned elements; one has the plenum compartment above the filter compartment (top tube sheet filter), an example of such type is disclosed in U.S. Pat. No. 3,637,079. The other type has the plenum compartment below the filter compartment (bottom tube sheet filter), an example of such type is disclosed in U.S. Pat. No. 3,666,097. The present invention is particularly directed to cleaning the filter elements in the former type of filter apparatus.

It is the well known practice to clean the filter elements in a bottom tube sheet filter vessel by use of a series of steps that include a series of backwash cycles to force a flow of cleaning liquid through the filter elements in the reverse direction of the service flow at high velocities. The high velocity may be imparted to the cleaning liquid by use of gas from a pressurized source. Such a backwash cycle is commonly known as a gas or air surge backwash cycle. The volume of cleaning liquid that is forced through the filter elements during each backwash cycle is approximately equal to the volume of cleaning liquid that is in the plenum compartment above the lower ends of the tube sheet fittings associated with the filter elements. In existing filter apparatus where the volume of the plenum area is fixed, and in newly designed filter apparatus where limitations are placed on the vessel length or tube sheet fitting length, the volume of cleaning liquid flow during each cycle is accordingly limited.

In U.S. Pat. No. 4,405,466, assigned to the same assignee as the present application, a backwash method and apparatus is disclosed for a bottom tube sheet type of filter. The backwash system directs an external volume of backwash liquid into the plenum compartment and through the filter elements under gas pressure. The volume of cleaning liquid directed through the filter elements during each backwash cycle is in the range of 1.0 to 3.0 times the total element core void volume of the filter elements.

The heretofore proposed backwash systems for top tube sheet filters have been limited to backwash flow rates that have proven to be inadequate for small retention rated filter elements. Further, in many applications capacities of waste treatment systems required to process liquids discharged during backwashing is limited, making numerous repeats of the backwash cycle impractical.

Such backwash systems typically start with a liquid filled vessel, except for a pressurized gas dome in a portion of the upper plenum compartment. The energy available for backwashing is limited to the energy stored in the pressurized gas. This energy is expended in overcoming: (1) the inertia of the entire mass of liquid in the vessel and (2) the entire system's resistance to flow. It is inherent in such backwash systems that the majority of the limited energy is wasted in the movement of the liquid in the lower filter compartment which provides no backwash benefits. The magnitude of this wasted energy may be gauged by considering that during a backwash, the pressure drop through the bottom drain valve and piping may be 10 to 15 times that through the filter elements, and the mass of liquid in the lower filter compartment may be about 3 times that in the upper plenum compartment at the start of a backwash cycle.

Accordingly, there is a need for an improved backwash method and apparatus for cleaning the filter elements in a top tube sheet filter that more effectively directs the energy to the backwash liquid to create a driving force for the movement of the backwash liquid in a rapidly moving piston-like flow of liquid surge downwardly through the core of the filter elements and radially outward through the filter surfaces thereof.

In accordance with the present invention, a method is provided for cleaning a plurality of filter elements, which are vertically positioned within a filter vessel having a tube sheet dividing the filter vessel into an upper plenum compartment and a lower filter compartment. The filter elements extend downwardly into the filter compartment and have upper end portions that are in fluid communication with the plenum compartment. Briefly stated, the method includes the following steps: draining at least a portion of the liquid from the filter vessel; establishing a fixed volume of backwash liquid in the plenum compartment immediately above the tube sheet; directing a gas into the plenum compartment and the filter compartment so as to establish a pressurized condition therein; and reducing the pressure in the filter compartment so as to cause the fixed volume of backwash liquid to move downward through the cores of the filter elements and surge radially outward through the filter surfaces thereof. The preceding steps of the method may be repeated a plurality of times.

The apparatus of a preferred form of the invention includes a filter vessel having a tube sheet dividing the vessel into an upper plenum compartment and a lower filter compartment. A plurality of vertical precoat filter elements extend downward from the tube sheet into the filter compartment and have upper portions that communicate with the plenum compartment through openings formed in the tube sheet. The filter vessel includes a drain for draining liquid and slurries from the bottom of the vessel. The upper end portions of the filter elements extend upwardly into the plenum compartment in a manner that maintains a fixed volume of backwash liquid immediately above the tube sheet, which allows the liquid to flow from the plenum compartment into the filter elements when the filter compartment pressure is less than the pressure in the plenum compartment. A source of pressurized gas communicates with the plenum compartment and the filter compartment so as to establish a pressurized condition therein. A vent means is provided to rapidly reduce the pressure in the filter compartment so as to cause the fixed volume of backwash liquid to move downwardly through the cores of the filter elements and surge radially outward through the filter surfaces thereof.

In one preferred form of the apparatus, the upper end portions of the filter elements are formed so as to define a one hundred and eighty degree elbow. In another preferred form of the apparatus, the upper portions of the filter elements extend upwardly into the plenum compartment a distance substantially equal to the depth of the fixed volume of backwash liquid above the tube sheet and a cap member is supported in covering relationship and spaced from the upper end portions of the filter elements. The cap members have lower end portions spaced from the tube sheet.

The invention, both as to its organization and method of operation, taken together with the objects and advantages thereof, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a prior art filter apparatus designed to employ a prior art backwash method;

FIG. 2 is a schematic representation of a filter apparatus embodying the features of the present invention and designed to employ the backwash method of the present invention; and

FIG. 3 is an enlarged view of a portion of the plenum compartment showing an alternative embodiment of the apparatus of the present invention.

The method and apparatus of the present invention are described below in conjunction with the filter apparatus schematically illustrated in FIGS. 2 and 3. It being understood that the present invention contemplates the use of alternative apparatus in practicing the teachings of the invention.

Referring to FIG. 1, a prior art filter apparatus 10 is schematically shown. Filter apparatus 10 includes a filter vessel 12 having an inlet feed line 14 and an outlet filtrate line 16. The vessel 12 is divided into an upper plenum compartment 18 and a lower filter compartment 20 by a tube sheet 22, suitably secured to the interior of the vessel 12 by welding or the like. Mounted vertically within the filter compartment 20 are a plurality of precoat filter elements 24 through which an influent stream must pass before entering the plenum compartment 18 and being discharged from the vessel through outlet line 16. The vessel is provided with a vent line 26 in communication with an upper portion of the plenum compartment 18 and a drain level line 28 in communication with an intermediate portion of plenum compartment 18. Lines 14, 16, 26 and 28 are provided with suitable valves.

When the precoat on the filter elements 24 becomes exhausted, the service cycle is stopped and a backwash cycle is initiated. At the start of the backwash cycle, the liquid level in the vessel is drained down to the level of line 29 so as to create a fixed volume of backwash liquid above tube sheet 22 in plenum compartment 18. The upper portion of compartment 18 is pressurized by directing a gas thereinto through a pressurized gas supply line 30. Opening of the bottom drain line 32 causes the backwash liquid in plenum compartment 18 to move downwardly through the cores of the filter elements and radially outward into the filter compartment. The backwash liquid, the liquid in the filter compartment, and the expelled filter cake and precoat layer are directed from the vessel through line 32. Much of the energy available from the pressurized gas to enhance the velocity of the backwash liquid is expended in overcoming the inertia of the liquid in the filter compartment and the system's resistance to flow.

Referring to FIG. 2, the filter apparatus in accordance with a preferred embodiment of the invention is indicated generally by the reference numeral 50. The apparatus 50 includes a filter vessel 52 having an inlet or feed line 54, controlled by a feed valve 55, and an outlet or filtrate line 56, controlled by an outlet valve 57. The filter vessel is a generally cylindrical vessel made of steel or the like, and having a top 58 suitably shaped for the design pressure and a convex o cone shaped bottom 60. The vessel is divided into a lower, influent or filter compartment 62 and an upper, filtrate or plenum compartment 64 by a tube sheet plate 66, suitably secured to the interior of the vessel 52.

Mounted vertically within the filter compartment 62 are a plurality of filter cartridges or elements 68 (only one of which being shown), through which the influent stream must pass before entering the plenum compartment 64 and being discharged from the vessel through the outlet line 56. The filter elements 68 are typically cylindrical in shape with closed ends 70. The filter elements 68 have suitable mounting means (not shown) for securing the elements to the tube sheet 66 at a corresponding opening 72 formed therein.

A plurality of filter elements 68 are suitably spaced within vessel 52 and suitably spaced from the sidewalls of the vessel. The top 58 is provided with a vent line 74 that is controlled by a vent valve 75. The bottom 60 is provided with a drain line 76 that is controlled by a drain valve 77.

The filter apparatus 50 of the present invention will normally be used to treat a stream of liquid, such as condensate water in the steam generating system of an electrical power plant. As those skilled in the art will understand, however, the filter apparatus 50 may be used to treat other liquids.

In preparing the filter apparatus for operation, the initial step is to precoat the filter elements 68. In order to accomplish this, the filter vessel 52 is filled with a low-impurity water, such as demineralized water. A slurry of precoat medium and demineralized water is prepared in a precoat system (not shown). The precoat slurry is directed through the inlet line 54 into the filter compartment 62 of the filter vessel 52. The pressure of the incoming slurry forces the demineralized water in the filter compartment 62 through the filter elements 68 and out the filter vessel 52 via the plenum compartment 64 and the outlet line 56 and back to the precoat system. The precoat material is retained on the outer surface of the filter element 68. After virtually all of the precoat material is deposited on the elements 68, the precoating cycle is complete. The thickness of the precoat layer varies, but it is preferably in the range of about one-quarter inch to no more than fifty percent of the distance between adjacent elements. The filter apparatus 50 is now ready to treat contaminated liquids.

In operation of filter apparatus 50 during the service cycle, only valves 55 and 57 are opened, all others are closed. In this manner, untreated liquid enters the filter vessel 52 through inlet line 54 to the filter compartment 62. The pressure of the incoming untreated liquid forces it through the precoat layer, the filter elements 68, and the plenum compartment 64 into the outlet line 56 to a suitable service line (not shown). As the untreated water passes through the precoat layer and filter elements, undissolved solids are removed. Dissolved solids are also removed, if the precoat layer includes adsorbents and/or absorbents. As the service cycle continues, the ability of the precoat layer and filter elements to remove contaminants diminishes, and the resistance to liquid flow increases due to the accumulation of solids on and within the precoat layer, and within the filter elements.

As is well-known in the art, the precoat material may contain various reactive materials, such as ion exchange resins, activated carbon etc., or non-reactive materials, or combinations thereof.

Eventually the precoat becomes exhausted or the resistance to liquid flow becomes impractical. At such time, the filtering or service cycle is stopped. It is now necessary to clean the filter elements 68, by removing the filter cake and the precoat layer therefrom, before the filter elements 68 are precoated and the filter apparatus 50 returned to service. The present invention is particularly directed to a method and apparatus to optimize the effectiveness of this cleaning cycle.

Referring again to FIG. 2, a preferred embodiment of the cleaning or backwash method in accordance with the principles of the invention will now be described. The upper end portions 80 of the filter elements 68 extend through openings 72 in tube sheet 66. The upper end portions may be formed as a one hundred and eighty degree elbow having an open end 82 spaced a short distance above the surface of tube sheet 66. Optionally, the uppermost part of the elbow may be fitted with a small vent to avoid siphoning, if the plenum is overfilled. For reasons that will become more apparent, the configuration of the upper end portions 80 are such that the elevation of the surface portion indicated at 84 above tube sheet 66 is substantially equal to the preselected depth of the fixed volume of backwash liquid to be established during each backwash cycle. The elevation is indicated by broken line 86 in FIG. 2. A backwash water inlet line 87, in communication with a source of backwash water (not shown), communicates with plenum compartment 64 at an elevation preferably below line 86. A backwash water inlet valve 88 controls the flow of backwash water into plenum compartment 64.

A gas inlet line 90 is provided to selectively supply a pressurized gas, from a source of pressurized gas (not shown), into plenum compartment 64 and filter compartment 62. Gas inlet line 90 communicates with plenum compartment gas inlet line 92 and filter compartment gas inlet line 94. Lines 92 and 94 respectively have control valves 96 and 98 associated therewith. Inlet line 92 preferably communicates with plenum compartment 64 above line 86 and inlet line 94 preferably communicates with an upper portion of the filter compartment 62. Vent lines 100 and 102 are provided to respectively vent plenum compartment 64 and filter compartment 62. Vent line 100 has a control valve 104 associated therewith and vent line 102 has a control valve 106 associated therewith.

The discussion of the operation of the backwash method and apparatus of the present invention that hereinbelow follows describes the operation of the system and discloses certain novel relationships and operating conditions which form a part of the invention.

At the initiation of the cleaning or backwash cycle, the filter compartment 62, the plenum compartment 64 and the vessel 52 are full with liquid. The backwash cycle is initiated by opening vent valve 75 and drain valve 77 to partially or fully drain the liquid from the filter compartment 62 of vessel 52. During the drain step, backwash liquid may be periodically or continuously introduced at a low flow rate through line 87 into plenum compartment 64 and then through the filter elements 68 in the reverse direction to normal flow to remove the spent resin and crud from the filter elements.

Upon completion of the drain step, valves 75 and 77 remain open and backwash liquid is directed into the plenum compartment 64 through line 87 at a flow rate not to exceed the ability of the elements to pass liquid with the liquid level at elevation 86, by opening valve 88 until a fixed volume of backwash liquid is established above tube sheet 66 at a depth indicated by line 86. The valves 75, 77 and 88 are then closed and valve 98 is open to direct pressurized gas (preferably air) into compartment 62 and then to plenum 64. At this stage of the backwash cycle, the filter elements 68 and the filter compartment 62 are free of liquid and the pressure in compartment 62 and the upper portion of compartment 64 are equal and preferably in the range of from about 60 to 300 psi.

The driving force for movement of the volume of backwash liquid is created upon closing valve 98 and quickly opening the valves 77 and 106. Optionally, valve 96 may be opened to sustain driving force. In so doing the pressure in filter compartment 62 drops below the pressure in plenum compartment 64 creating a pressure differential therebetween. Since compartment 64 and the filter elements 68 are free of liquid, an unimpeded piston like flow of the backwash liquid from above the tube sheet 66 surges down the core of each of the filter elements. As the rapidly moving piston of liquid reaches the bottom of each filter element, the kinetic energy of the liquid is converted to a high pressure wave which reflects up the column of liquid in each filter element. As the wave moves upwards, instantaneous high radial flow surges are created at the filter surface of each filter element.

Following the kinetic surge step, backwash liquid flow is sustained, unimpeded by liquid in the filter chamber.

The aforementioned kinetic surge step may be repeated a sufficient number of times to thoroughly clean the filter elements 68. During subsequent kinetic surge steps, it may be desirable to allow the filter compartment 62 to partially fill with backwash liquid immediately prior to initiation of the next backwash step. By so doing, turbulence is created within compartment 62 which helps to break up large chunks of materials and facilitates the removal thereof from vessel 60.

After the filter elements 68 have been thoroughly cleaned, the filter compartment 62 is drained for the last time and the hereinabove described precoating procedure is initiated.

In accordance with a presently preferred embodiment, the fixed volume of backwash liquid is in the range of 1.0 to 3.0 times the total core void volume of the filter elements.

An alternative embodiment of the configuration of the upper end portions of the filter elements is shown in FIG. 3. The upper ends of the filter elements 68 are suitably received by tube sheet fittings 110 that are rolled or welded into the tube sheet 66 at opening 72. The fittings 110 extend upwardly into plenum compartment 64 a distance substantially equal to the height of the preselected fixed volume of backwash liquid above the tube sheet 66. A seal cap 112 is supported in covering relationship and spaced from the upper portions of the fittings 110. The lower end portion of the cap is spaced a short distance above the upper surface of the tube sheet. A small vent hole (not shown) may be placed in each cap 112 to eliminate trapped air during the service cycle and to prevent siphoning if the plenum compartment 64 is overfilled during the backwash cycle. The vent hole may optionally have a suitable check valve device associated therewith.

The caps 112 may be supported by a suitable grid assembly 114. Assembly 114 is attached to the tube sheet 66 by a series of studs 116. Although not shown, the caps 112 could also be attached to the tube sheet fittings by a pair of spoked hubs that are welded or screwed to the tube sheet fitting.

In accordance with a presently preferred embodiment, a 2 inch thick tube sheet 66 is provided with tube sheet fittings 110 having a 1 3/16 inch outer diameter and 141/2 inches in length. This length is designed to provide approximately 1.1 core volumes of backwash water for each 55 inch filter element. The seal cap has a 23/8 inch outer diameter and is thirteen inches in length.

The above described method and apparatus in accordance with the present invention solves a long known problem of in-situ cleaning of the filter elements of top tube sheet filters. The method and apparatus of the invention provides a means of effectively directing the energy to the backwash liquid to create a driving force for the movement of the backwash liquid in a rapidly moving piston-like flow of liquid surge downwardly through the core of the filter elements and radially outward through the filter surfaces thereof.

Many modifications and variations of the present invention as hereinabove set forth will occur to those skilled in the art, and it is intended to cover in the appended claims all such modifications and variations as fall within the true spirit and scope of the invention.