Vibrating screen for fine screening of liquids

[57] ABSTRACT

A vibrating screen is provided for screening with cloths having 60 to 100 mesh openings. A rotary vibrator mechanism producing a minimum stroke of five-sixteenths inch is located above the screen cloth and forward of the longitudinal center of the screen box to obtain a high acceleration and unequal elliptical motion at the ends of the box. The screening surface has a first horizontal section and a second section sloped downwardly at 5° to obtain maximum dewatering on the first section and maximum retention on the second section. Adjustable flow control vanes are included in the feed flume.

Claims, 10 Drawing Figures

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Patented May 30, 1972

3,666,095

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Patented May 30, 1972

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Patented May 30, 1972

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Patented May 30, 1972

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Patented May 30, 1972

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3,666,095

VIBRATING SCREEN FOR FINE SCREENING OF LIQUIDS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to new and useful improvements in flat deck vibrating screens and particularly those screens suitable for the fine screening of oil well drilling muds. Drilling mud is a water solution of certain clays and other additives but the components may vary widely to perform various functions. Some of the primary functions of this mud are to act as a lubricant, a sealant to maintain hole pressure, a coolant, and a hydraulic carrier of the cuttings from the bottom of the hole to the surface during the rotary drilling of oil wells. Due to its expense, mud is generally reconditioned and recirculated. In the early history of drilling, cuttings were removed from the mud by settling. As oil wells went deeper and the drilling activity was transferred to the Gulf Coast and State of California, U.S.A., the solids in the mud broke down in size, remained in suspension, and were recirculated. Settling devices then had to be abandoned in favor of screening devices, desanders, desilters and centrifuges, which thus became an essential part of any rotary drilling rig.

The volume and type of solids present in a mud system directly influence its density as well as its viscosity and gel strength. The sooner the drilled solids; i.e., cuttings, cavings, sand, and shale, are removed from a mud, the less chance they have of being recirculated and being broken down through erosion and abrasion to create viscosity and gel problems, as well as increasing wear in the pump and other equipment. The latter factors greatly influence mud and well costs, drilling rates, hydraulics, etc. Since the clays and other chemicals go into solution, it is desired that as much as possible of the solids in the mud be removed regardless of their size. Solids control thus becomes one of the most important phases of mud control. If solids can be removed mechanically, as by screening, it usually proves much cheaper than trying to control them with water, barite and chemicals. The shale shaker or vibrating screen is where solids control begins and the finer the screen cloth openings, the greater the advantage. Maintenance costs of pumps and other equipment in the circuit are also greatly reduced. Thus, the importance of fine screening, using a screen cloth of 60 to 100 mesh, is being recognized. The mesh of a screen cloth is defined as the number of openings per lineal inch as measured from the centers of the wires and the size of the clear openings of fine mesh cloth is usually given in microns where one micron equals one one-thousandth of a millimeter or approximately 0.0000394 inches.

Fine screening presented new problems as compared to the previously used shale shakers. The coarse screen cloths are made of relatively heavy wires (0.0154 inches and larger) and have an open area of approximately 50 percent; whereas the 80 mesh screen cloth is made from much finer wire (0.0055 inches and having a 177 micron opening) and has an open area of only 31.4 percent. This means that to handle the same volume of fluid with the same efficiency the screen with 80 mesh cloth must have nearly twice as much surface area. The finer diameter wire on the 80 mesh screen cloth also makes the cloth quite fragile; so reasonable screen life becomes much more difficult to obtain. Coarse screening, as done previously, permits the fines and water to pass quickly and easily through the screen cloth; so a steeply inclined, relatively short deck was satisfactory. However, with the finer screening there is a much higher percentage of fines in the mud nearer the 80 mesh size and, with the irregular and wedge shape of these particles, they tend to build up on the cloth and to blind the openings. With the viscosity of drilling mud being considerably higher than water and with the natural surface tension that any liquid has on a fine screen, the problem becomes even more difficult.

2. Description of the Prior Art

In general two types of screening apparatus have been previously used:

1. the rotary or so-called barrel shaker, and

2. the flat deck vibrating screen or shale shaker. Both types use the same screen cloth sizes, but the vibrating screen type is more efficient, particularly in handling high flow rates and viscous muds.

One type of the conventional shale shaker consisted of a single screen deck in a shallow screen box with a downward slope of about 10° with a two-bearing unbalanced weight vibrator having an eccentric central section of its shaft mounted transversely across the top of the screen box at the center of its length. The vibrator was operated at high speed (1,750 RPM) and small stroke (one-sixteenth inch) to produce a relatively low rate of acceleration and an elliptical vibrating action at the ends of the screen box. A V-belt drive from a separately mounted electric motor on the supporting structure was connected to the vibrator shaft to drive the vibrator. The screen box was mounted on a four-point suspension system, using leaf springs, coil springs or rubber shear mountings to a separate supporting structure. The feed box and collection tank were usually independently mounted to suit the individual installation. Generally, the units were installed in pairs as a dual unit to provide adequate capacity. With these screens the finest screen cloth was 30 mesh, but more commonly only 10 to 14 mesh screens were used. Common sizes were 3 feet wide by 4 feet long, or four feet by 5 feet. Thus, these units were only suitable for removing large cuttings and cavings.

SUMMARY OF THE INVENTION

After considerable testing to overcome the difficulties encountered and to provide a more efficient screen with reasonable screen cloth life, the applicants developed this special, improved vibrating screen for fine screening. It was discovered that a vibrator with relatively high acceleration and a stroke of vibration of at least five-sixteenths inch at a speed of at least 1,130 RPM achieved these objectives. Stroke is the total amount of movement, or the diameter of the circular motion, and is equal to twice the amplitude measured from the mean position to the point of maximum displacement. This arrangement gave the necessary motion to lift the particles and to have them impact the cloth. Also knowing that the vibrator, being located above the single deck, would produce elliptical motion to a particle at the ends of the box with the major axes of these ellipses intersecting at some point over the vibrator, the location of the vibrator was selected slightly beyond the longitudinal center of the box.

The above motion is utilized to its greatest advantage by providing a first deck section nearer the feed end which is horizontal and a second deck section which is sloped at 50° downhill. It was discovered that the vertical component of the vibratory motion at the feed end puts the slurry in suspension; whereas the fine solids collect in a layer closer to the screen surface. There they are conveyed forward by the horizontal component of the motion plus the initial hydraulic velocity from the liquid flowing on to the screen cloth from the feed box. As the fines move forward, they scrub the cloth and help to keep it open, thus permitting the falling slurry to pass through the unobstructed openings more easily. In between the first and second sections a total nominal drop of 3 inches is provided which is divided by a sloped, connecting flow plate set at an intermediate level. This changes the direction of the material as it drops and tends to break up the larger globules so they may be separated into smaller fractions and more easily release further moisture. On the second sloped section the material is somewhat drier and contains more of the solids, as much of the free moisture has already passed through the first section. The vertical component of the vibratory motion is greater on this second section so tosses the material to present more surfaces to further drain off the remaining liquid through the screen cloth. The horizontal component of the motion is directed against the flow so assists in retaining the material on the deck to effect further separation. The 5° downslope, however, gives sufficient conveyability by gravitational action to carry the remaining oversize material off the end of the screen. As the material moves down the second section, it was

3,666,095 further discovered that there is a definite agglomeration effect by the rolling of the larger solids that tends to pick up and trap fines smaller than the screen cloth openings. This is a particular advantage not obtained in coarse screening and further increases the efficiency of this new screen. In one test approxi- ⁵mately 60 percent of the fine sands of 200 - 325 mesh size were so removed from the sample.

A new feed box was designed to hydraulically reduce the velocity and even out fluctuations of the fluid entering the box as much as possible while spreading the flow out transversely ¹⁰to the full screen width. This feed box is arranged with a short horizontal flume section projecting out over the feed end of the screen cloth and at a height of about 9 inches above the cloth so as to have some potential energy in the entering fluid to assist in driving the free moisture through the screen cloth. The flume section is also provided with adjustable flow control vanes across its width to distribute the fluid uniformly over the feed end of the screen and to help straighten the flow to a direction parallel with the flow of material on the screen. ₂0

All of the above design improvements combine to successfully accomplish the objectives of designing a vibrating screen capable of the 80 mesh screening of drilling mud and to obtain reasonable screen cloth life and little mechanical maintenance problems. 25

It is an object of this invention to provide an improved vibrating screen for the fine screening of liquids with 60 to 100 mesh screen cloth with 234 to 149 micron openings respectively.

It is a further object to provide a vibrating screen that will give increased capacity under fine screening of drilling mud and similar liquids.

It is another object to provide a fine screening unit that will have improved operating life.

A further object of this improved fine screening unit is to improve drilling mud properties and solids control by screening out a greater percentage of the cuttings and solids.

Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view of the improved vibrating screen.

FIG. 2 is a schematic plan view of the screen shown in FIG. 1, with parts broken away.

FIG. 3 is a schematic end elevational view of the screen shown in FIG. 1 from the free end.

FIG. 4 is a schematic transverse sectional view taken along line 4—4 of FIG. 1.

FIG. 5 is a longitudinal sectional view taken along line 5—5 of FIG. 2.

FIG. 6 is an enlarged transverse sectional view of FIG. 4 showing the screen cloth supports and tensioning system.

FIG. 7 is an enlarged perspective view of the rubber cushioning strips between the screen cloth and its longitudinal support bar.

FIG. 8 is an end elevational view of the spring controlled counterweight assembly on the vibrator shaft shown in FIG. 4.

FIG. 9 is a diagrammatic illustration of the relative motions at each end of the screen box, plus the center, in relation to the sideplate of the screen box and the vibrator location under an actual operating screen of the applicants’ design.

FIG. 10 is a partial plan view looking in the direction of arrows 10—10 of FIG. 5 and showing the members joining the first and second screen deck sections at one side of the deck.

DETAILED DESCRIPTION OF THE INVENTION

In the embodiment of the invention shown in FIGS. 1 and 2, this vibrating screen 12 for the fine screening of oil well drilling mud and similar slurries is a portable, self-contained unit having a combined tank and base 15, a screen box 16, a screen deck 17 and a drive 18. The combined tank and base 15 is essentially L-shaped in its side elevation, as shown in FIG. 1. The screen deck 17 is approximately twice as long as it is wide; so the screen is substantially rectangular in its plan view, as shown in FIG. 2. The screen deck is also made up of two sections in which a feed end section 19 (FIG. 5) is horizontal and a discharge end section 20 is inclined downhill approximately 5°. The screen box 16 is spring supported on the base 15 and carries the screen deck 17 and a vibrator assembly 21. The drive 18, as shown in FIG. 4, consists of a Vbelt 22 connecting a pulley 23 attached to one end of a vibrator shaft 24, and a pulley 25 attached to an electric motor 26 mounted on the top of the screen box 16. Since the motor 26 is supported directly on the screen box, whipping of the V-belt 22 is reduced and its life is increased.

The tank and base 15 is composed of two vertical sideplates 27, two transverse tubes 28, a feed box 29 (FIG. 3) and a collecting tank 30. Each sideplate 27 is a reinforced weldment having horizontal stiffeners 31 and two reinforced spring support brackets 32. The upper edge 33 of the sideplate is sloped downwardly from the feed end to the discharge to conform generally to the average slope of the screen deck 17. Two tubes 28 act as transverse structural ties between the sideplates 27 at each end of the base near the bottom of the sideplates. These extend outwardly beyond the sideplates and the horizontal stiffeners to permit a hoisting sling to be used over each tube end 28α.

At the feed end of the screen, a feed box 29 (FIGS. 1 and 3) is attached to the sideplates 27 just above one of the transverse tubes 28 and comprises a lower portion 29α and an upper portion 29ft. The lower portion is triangular in transverse cross-section and is arranged to receive material from an inlet pipe 35 and direct it upwardly to the generally rectangular portion 29b which communicates with a flume 37 that extends over the top of the feed end section 19 of the screen deck. This feed box provides a relatively large volume to reduce the velocity of the liquid coming in the inlet and to evenly distribute this across the full width of the screen.

In the feed flume (FIGS. 2, 3 and 5) adjustable feed vanes 40 are provided to further direct and control the flow. Each of these consists of a pivoted vertical vane plate 41 attached to a threaded rod 42. The plates are approximately as wide as the spacing between the threaded rods and as high as the sides 43 ⁵ of the flume. When set perpendicular to the flow, they can prevent flow but when open they present a wide surface to control the direction of flow. Each of the threaded rods is secured to a bottom plate 44 of the flume and to a transverse ₅θ angle 45 across the open top of the flume. Nuts 46 retain the rods in the angle and normally hold the vane plates 41 at any desired angle with respect to the flow of the liquid after they have been manually positioned. The bottom plate 44 of the flume is located so that there is a vertical drop to a screen ₅₅ cloth surface 47 approximately equal to the height of the vane plates in order to have some potential energy to assist in the initial phases of screening by having this force tend to push the water through the screen cloth quickly and thus obtain greater initial dewatering action.

The collecting tank 30 (FIGS. 1 and 4) is integrally formed in the base 15 to provide a watertight hopper for collecting the undersize material and water flowing through a screen cloth 47. In the lower, central portions of each sideplate 27, discharge openings 48 are provided for withdrawing the un65 dersize material. Vertical, manually operated slide gates 49 are provided in guides 50 in these openings to control the flow. A formed bottom plate 51, sloping from the feed end to discharge openings 48 and then flanged vertically at the discharge end, is welded to the sideplates 27 to provide for the 70 collection of the undersize material and water and to permit it to flow to the discharge openings 48.

The screen box 16 (FIGS. 1, 2 and 4) consists essentially of two vertical side plates 55 which are approximately trapezoidal in outline. A spring support bracket 56 is welded to 75 each sideplate at a distance approximately one-sixth of the

3,666,095 sideplate’s overall length from each end. Coil springs 57 are located between the screen box brackets 56 and the base brackets 32 to provide for isolation of the vibrating screen box 16 from the base 15. The screen deck 17, which will be described later, is bolted to these sideplates 55 to tie them together transversely into a rigid structure. A motor support plate 58 is fastened to the top of one sideplate 55 slightly forward of its longitudinal centerline. A horizontal transverse tube 52 connects the motor support plate 58 to a counterweight 53 mounted on the opposite sideplate. Counterweight 53 balances the weight of the drive on the screen box to obtain more uniform motion. A similar transverse tubular tie member 54 also connects the tops of sideplates 55 at the feed end. As shown in FIG. 4, the vibrator assembly 21, also to be described later, is bolted to each sideplate 55 with fasteners 59 so that its axis is approximately 2 inches forward of the longitudinal center of the sideplate and approximately 11 inches vertically above the extended plane of the top of the feed section 19 of the screen deck 17. The drive extension 61 of vibrator shaft 24 extends beyond a counterweight assembly 62 on the same side of the screen box as the motor support plate 58. An automatic lubricator 63 of known design is used to supply grease to vibrator 21 through grease pipe 64 as shown in FIGS. 1 and 3. The screen box 16 with the screen deck 17, drive 18, vibrator assembly 21 and screen cloth 47 all move as a unit with a vibrating motion produced by the vibrator 21.

The screen deck 17 (FIGS. 2,4 and 5) consists of a horizontal feed section 19 followed by a discharge section 20 downwardly inclined at 5° in the direction of flow. To eliminate undesirable whipping and to obtain better support for and longer fatigue life in the screen cloth 47, the screen deck 17 is made very rigid and the surface on which the screen cloth is supported is made as smooth as practicable. Each deck section consists of two longitudinal angles 65 and a series of six parallel longitudinal flat bars 66 on edge vertically and spaced about 6 inches apart transversely across the width of the screen. The feed end for the first section 19 has a transverse endplate 67, which is a channel-shaped plate whose ends are secured to sideplates 55 and whose top flange extends up under the bottom plate 44 of the feed flume. Spacer plates 68 and 69 are welded between the longitudinal bars 66 and the angles 65 to stiffen the deck at this feed end and are notched out at their juncture with bars 66 so that rubber extrusions 79 on bars 66 can pass beyond the plates 68 and 69. At the end of section 19 an angle shaped endplate 70 extends transversely across the deck and joins longitudinal angles 65 and bars 66. To rigidly join the discharge section 20 with the feed section 19 (FIG. 10), a vertically bent plate deflector 71 is welded to each end of plate 70, each deflector plate having a flange 71α extending inwardly toward the center of the screen and toward the discharge. This is further stiffened by a second plate 72 set at 5° from the vertical and connected to the endplate 70 and to the outstanding flange 71α of plate 71. A horizontal gusset plate 73 connects the tops of plates 71 and 72 to form a short box section at each end of plate 70. Another transverse plate 74, set parallel to section 20, is welded to the vertical leg of plate 70 about 1 ½ inches below the top of angles 65 of deck section 19 and about 1 ½ inches above the corresponding angles 65 on deck section 20 in between the deflector plates 71 and extending beyond the stiffener plates 72 to form the beginning of deck section 20. Thus plate 74 forms a connecting flow plate for the flow of liquid between the two deck sections 19 and 20 plus providing the slight vertical drop in making the transition from one section to the other and further stiffening the joint by making plate 70 into a channel-shaped member. At the end of the discharge section 20 a transverse formed plate 75 is welded to the longitudinal angles 65 and bars 66. The lower flange of plate 75 is bent downwardly and toward the feed end approximately 30° (FIG. 1) to provide a self-draining plate overlapping the vertical end flange of bottom plate 51 of the tank so the undersize material must flow to the tank outlets 48. On the outside of the upstanding leg of plate 75 and near its top, a second formed plate 76 is welded to provide a chute surface for the oversize particles from the top of the screen deck to pass over to the discharge point. This plate is sloped approximately 55° from the horizontal and has a lower flange extending vertically downward.

The decks 19 and 20 are also provided with transverse tubular support members 77 at the longitudinal center of each section welded to the vertical legs of angles 65. To add further stiffness to deck section 19 and the connecting joint of sections 19 and 20, two angle members 78 are welded diagonally between the transverse plate 70 and the longitudinal angles 65 under the bars 66 as shown in FIG. 2.

As seen in FIG. 4, the longitudinal bars 66 are arranged vertically so their top edges are progressively higher toward the longitudinal centerline of the screen. This convex surface provides a camber in the deck so that when the screen cloth 47 is placed on the deck, the cloth can be tensioned and not have any uneven pockets or ripples. However, contrary to normal practice used previously, this camber is greatly reduced to maintain the even distribution of the feed and keep the flow parallel to the longitudinal axis over the entire screen length. In one embodiment having a nominal 4 foot width of screen, the longitudinal bar on the transverse centerline is approximately seven-eighths inch higher than at the ends as compared to 1 13/16 inches used previously. The special U-shaped closed cell Buna-N vinyl synthetic rubber extrusion 79, as shown in FIG. 7, having a relatively soft hardness and a density between 15 to 30 pounds per cubic foot, fits over the top edges of each bar 66. This material is known as Type R-l8 83H as made by Rubatex Corp., Bedford, Virginia, U.S.A. The top surface of this extrusion is convexly curved and is approximately 1 inch wide with a lower groove to tightly clamp over the top edge of the associated bars 66. These extrusions 79 are also cut slightly longer than the bars over which they are to fit; then compressed lengthwise and cemented to the bars 66 with a silicone adhesive, such as General Electric Company’s Type RTV Silastic Cement. Great care must be exercised in the welding and finishing of the top surfaces of the screen deck so that they are smooth and free of surface irregularities which would otherwise produce early failure of the fine mesh screen cloth 47 under the vibrating action.

The vibrator assembly 21, as shown in FIG. 4, will now be described. This assembly consists of a tubular housing 80, with a flanged end-bell section 81 on each of its ends, which is bolted to the screen box sideplates 55. Within each end-bell 81 there is a flanged, cartridge-type, sealed spherical roller bearing block 82. The automatic lubricator 63 supplies grease to bearing blocks 82 through grease pipe 64 to ensure long life under the high acceleration operating conditions of vibrator 21. These bearing blocks support the vibrator shaft 24, which has a concentric extension 83 on each of its ends beyond the bearing blocks for mounting the counterweight assemblies 62. The end 61 of the vibrator shaft is further extended to carry the V-belt pulley 23 for the drive 18.

Each of the counterweight assemblies 62, as shown in FIG. 8, consists of a hub 84 secured to the shaft extension 83 with key 85. The hub has a downwardly projecting flange 86 that contains two holes 87, spaced from the centerline of the shaft for mounting pivoting counterweights 88 and two additional holes 89 for attaching connecting links 90. The counterweights 88 are somewhat kidney-shaped plates and are pivotally connected to hub flange 86 with a pin 91. Additional counterweight plates 92 may be bolted to the counterweights 88 by bolts 93 and 94 to alter the counterweight effect. The upper surface of the hub 84 has a spring seat 95 on which is mounted a coil spring 96. A spring cap 97 is placed over the top of spring 96. This cap 97 has a projecting ear 98 on each side, parallel to the projecting flange 86 of the hub. A hole 99 is located in each ear 98. A capscrew 100 passes through hole 99 and screws into the upper end of a connecting member 101. The lower end of each member 101 has a clevis formed therein for receiving the upper end of link 90. A pin 102 connects the upper end of connecting link 90 to the clevis and to

3,666,095 the projecting ear 98, while the lower end of this connecting link 90 is attached to the hub flange 86 by means of pin 103.

In the rest position and during starting and stopping, when the vibrator shaft is not up to its normal speed, the spring pressure is such that the counterweights remain in their closed 5 position as shown solid in FIG. 8. The counterweights are then dynamically balanced and no vibration occurs. After the speed has passed the critical speed of resonance, the higher speed will then begin to act on the counterweights 88 like a fly-ball governor on a steam engine. The weights will start to rotate away from the axis of shaft 24 under the centrifugal action caused by the rotating flange 86 so that they no longer remain dynamically balanced. At the normal operating speed, the counterweights will be fully open, as shown dotted in FIG. 8, and their center of gravity will have moved away from the '³center of the vibrator shaft 24. Thus the counterweights become dynamically unbalanced and the vibrator produces a continuous uniform vibration on the screen box.

The resulting unequal elliptical motions at the ends of the screen box 16 produced by the non-symmetrical location of the vibrator 21 is shown in FIG. 9 for one embodiment. The vibrator assembly 21 is displaced forward of the longitudinal centerline of the screen box sideplate 55 by the dimension E, approximately two inches, and is above the screen deck 17. At 25 the feed end the ellipse of the vibrating motion is inclined forwardly and upwardly toward the vibrator by the angle X, approximately 52° from the horizontal. At the discharge end the ellipse is inclined backwardly toward the vibrator by the angle Y, approximately 58° from the horizontal. The second screen 30 deck section 20 is inclined downwardly by the angle Z or about 5°. The motion at the vibrator is circular, having a diameter of five-sixteenths inch and thus a stroke of the same amount. The speed of vibrator shaft 24 is about 1,130 RPM to produce a relatively high rate of acceleration by the vibrator.. 35 Since the angles of the ellipses and their resultants, F„ and D_s, are unequal, the vertical component of the motion at the feed end, F_r, is slightly less than that of the vertical component at the discharge end, D_r. The horizontal component of the feed end, F„, is in the direction of flow; whereas the horizontal 40 component at the discharge end, D„, is opposite to the flow. At the feed end the vertical component, F_r, is also slightly larger than the horizontal component, F_H, and combined with the relatively large stroke; thus providing more lifting action than conveying to thoroughly agitate the material for rapid ⁴⁵separation. This action tends to put the slurry in suspension and the fines to settle in a layer closer to the screen surface. Since the horizontal component acts in the direction of flow, it aids the initial hydraulic velocity of the liquid flowing on to the feed section to provide adequate conveying action of the fines toward the second section 20 of the screen deck. This layer of moving fines also tends to scrub the screen cloth and keep it open, permitting the slurry to fall back to the screen cloth and pass through the unobstructed openings more easily. At the -transition from the first section 19 to the second section 20, the vertical drop, broken up by the connecting flow plate 74, changes the direction of fall of the material and the impact tends to break up any large particles. On the second section 20 the material is somewhat drier and contains more of the solids, gQ The larger vertical component at the discharge end, D_F, tosses the material over to present more surfaces to further drain off the remaining liquid through the screen cloth. The horizontal component, D_H, being directed against the flow, also assists in retaining the material on the deck longer to complete the 65 separation. The 5° downslope, however, gives sufficient conveyability by gravity action to carry the remaining oversize material over the end of the screen. The longer length of the combined screen sections thus gives ample retention time to remove the maximum amount of mud solution suitable for 70 recirculation while still separating the undesirable solids.

In assembly, the base 15 is placed into position with springs 57 on their spring brackets 32. The screen box 16 is then mounted over the springs so that the box spring brackets 56 line up properly with the springs and the horizontal feed end 75 section 19 of the screen deck is under the feed box flume. As seen in FIGS. 4 and 6, screen cloth 47 is carefully placed over the screen decks 19 and 20. Tension plates 110 are then placed over the bent binder edge 111 of the sides of the screen cloth. Tension bolts 112 are placed through the holes in tension plates 110 with a washer 113 under each bolt head and a sponge rubber seal block 114 between the inside of the tension plates 110 and sideplates 55 as sealing members. The hole in block 114 is smaller than the diameter of the tension bolt so that it tightly grips the bolt on assembly to prevent passage of any liquid along the bolt. On the outside of the sideplates, tubular spacers 115 are welded to the box over the tension bolt holes with their outer face parallel to the main surface of the tension plate 110. As the bolts 112 are pushed through the tubular spacers, a washer 116 is added, then spring 117, a second washer 118 and finally a nut 119. After all of the tension plates and their bolting assemblies are installed, the bolts are carefully tensioned from the center of the screen out to each end, carefully smoothing the wire cloth 47 to eliminate any wrinkles or unevenness. Each tension bolt is then tightened until the spring coils are fully closed. As the screen cloth stretches in use, the springs take up the slack. The combination of the washer 113 and rubber block 114 with the positioning of the flanges of the tension plates 110, when the bolts are fully tensioned, provide a watertight seal for the liquids within the screen box from escaping through these tension bolt holes. The deflector plate 71 at each side of the screen, between screen decks 19 and 20, also tends to redirect the liquid at the sides from entering in behind the ends of the tension plates in the second screen deck section 20 to provide further water tightness.

Although the best mode contemplated for carrying out the present invention has been shown and described herein, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.