Abstract:
A self-cleaning strainer which may be installed upstream of a heat exchanger to prevent debris greater than a critical size from reaching the heat exchanger. The strainer is preferably mounted just upstream of a heat exchanger in order to minimize the possibility of debris reaching the heat exchanger. The strainer comprises a screen element which can be advanced across the flow path of the fluid between two rollers. The screen element is placed so that the fluid must flow across the screen element face to continue through the process pipe; the screen may be perpendicular or parallel to the fluid flow axis of the strainer body or at an intermediate angle to it. Flow blocking members are used so as to direct the flow to pass through the screen element.

Description:
This application claims the benefit of U.S. Provisional application 60/411,670, filed Sep. 18, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to heat exchanger equipment and to processes employing such equipment and more particularly to straining devices which are placed upstream of heat exchangers and other fluid equipment. 
     BACKGROUND 
     One of the most problems associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop by reducing the flow area for the fluid flowing on the inside of the exchanger. 
     There are a large number of techniques suitable for reducing fouling which can take the form of structural features within the heat exchanger body itself. Significant fouling reduction can however also be achieved by removing debris from the process stream upstream of the heat exchanger. In fact, the presence of debris in various streams that are fed into heat exchangers as well as fluid streams which flow through other devices can cause significant problems that, in some cases, can not be remedied, even by the most effective fouling mitigation technique within the heat exchanger or other device. Fouling can result in problems such as hydraulic limitations, poor heat exchanger thermal performance and premature tube failures causing unplanned unit shutdown. In addition, frequent opening and closing of heat exchangers can lead to poor reliability as a result of wear and tear on the heat exchanger and possible damage to heat exchanger components. 
     In many petrochemical processes, straining of debris upstream of the heat exchanger is provided by a bucket-type strainer. Unfortunately, because these devices are cumbersome and require frequent cleaning, they are often eliminated from the fluid flow circuits. As a result, many flow streams, although they may have been designed to include a straining function, often do not have one in practice. In some cases, straining functionality may even be left out of the process design because of expense or because of an understanding of the realities of the difficulties in using bucket-type straining devices. Even if one of these straining devices is included in the fluid flow, the strainer must usually be bypassed during cleaning and large debris can therefore pass towards the heat exchanger when cleaning is underway. This problem can be avoided through the use of at least two strainers, connected in parallel, in the process but such a solution adds significant expense. Strainers require isolation, draining, and steam-cleaning before they can be taken apart for cleaning. This is a tedious and time-consuming process. 
     While various strainer types that provide automatic cleaning as debris builds up within the straining device exist, these devices are generally very expensive, relatively ineffective or both. In addition, since automatic strainers require motors, electrical power is required and the drive mechanism and motor reliability can become concerns. Finally, automatic strainers require a third fluid stream to remove the debris. This stream and the supporting hardware and piping create additional maintenance and upkeep requirements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a self-cleaning strainer comprises a movable screen element that is attached to two rollers and placed in the fluid flow path to intercept debris in the fluid comprises: 
     (a) a housing into which the flow stream passes from the flow pipe for filtration and from which it passes after filtration; 
     (b) a screen element; 
     (c) a source roller attached to a first end of the screen element; and 
     (d) a take-up roller attached to a second end of the screen element. 
     The screen element extends across the interior of the housing in the path of the flow stream to define (i) a flow region upstream of the screen and a flow region downstream of the screen, (ii) an active portion of the screen element which is in the path of the flow stream and through which the flow stream may pass, and (iii) a non-active portion, and means for rotating the source roller and the take up roller to move the screen element from the source roller to the take-up roller so as to periodically replace the active portion of the screen element with a previously non-active portion of the screen element. 
     The screen element may be placed perpendicular to the fluid flow direction in the housing so that the fluid flow in a straight line flow path directly through the screen element to continue on through the equipment. Alternatively, the screen element may be placed at another angle to the flow. It may be placed parallel to the general flow axis (but still in the path of the fluid flow), with blocking members to force the flow direction at the inlet to turn in order to pass through the screen. As a result, the fluid passes through the screen element to intercept any debris particles caught up in the fluid. 
     The rollers contain a length of screen element with its face in the flow channel, that may be rolled from one roller to another over time. The rollers may be operated manually or by an electric motor. Various options are available for triggering the rotation of the rollers to feed new screen element length into the flow channel. In addition to manual rotation as determined by an operator, automated rolling may occur based upon, for example, elapsed time and/or a specific level of debris buildup as measured by an increase in the pressure drop across the screen element. Other automatic advance triggers are also possible. 
     The present invention provides many advantages including a significant reduction of debris within various fluid flow systems particularly those that include a heat exchanging function. In the case of processes involving heat exchangers, the removal of debris upstream of the heat exchanger provides a very significant amount of fouling reduction and the strainer may be used either with or without other fouling mitigation techniques within the heat exchanger itself. The present strainer may be employed in a great many applications only one of which is processes that include heat exchangers. The present strainer may be used in connection with any application which involves any fluid and which benefits from the removal of debris particles from the fluid flow in order to improve process performance, preserve process equipment or otherwise. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic diagram providing a side view, as viewed from the flow direction, of the fouling mitigation device of the present invention in a first embodiment in which the filtering screen element is placed in position perpendicular to the flow direction through the device. 
     FIG. 2 is a schematic diagram of cross-section Y—Y of FIG. 1 providing a front view of the fouling mitigation device. 
     FIG. 3 a  is an exploded front view of the filtering screen in one embodiment of the present invention showing a screen lock which controls the advance of the filtering screen. 
     FIG. 3 b  is a plan view of the filtering screen in one embodiment of the present invention showing the guide elements. 
     FIG. 4 is a schematic diagram illustrating one embodiment of the screen lock. 
     FIG. 5 is a schematic diagram illustrating a second embodiment of the pin-and-hole feature. 
     FIG. 6 is a schematic diagram providing side and front views, as viewed from the flow direction, of the present fouling mitigation device in a second embodiment in which the filtering screen element is placed in position such that it is parallel to the flow direction through the device. 
     FIG. 7 is a schematic diagram of cross-section A—A of FIG. 6 providing a plan view. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 illustrate the fouling mitigation device  100  of the present invention, referred to for convenience a the strainer, in a form in housing  140  in which the filtering screen  110  is placed in position perpendicular to the direction of the flow axis occurring through and through housing  140  and in a substantially vertically upright position. FIG. 1 provides a side view of the device and FIG. 2 provides an end view of the device. Filtering screen  110  comprises a length of screen material which is wound on source roller  150  and on take up roller  160 . Although these Figures show source roller  150  and take up roller  160  on a vertical centerline, each may be alternatively displaced from this centerline by some amount in order to optimize performance. Filtering screen  110  is preferably manufactured from a wire mesh or perforated sheet which may be stainless steel or some other metal which is resistant to corrosion or some non-metallic material if it is compatible with the flowing fluid. In one embodiment, for example, filtering screen  110  may measure 20 cm wide, 3 mm thick and 36 m long. The length of filtering screen  110  is preferably sufficient to permit continued use without requiring installation of a new filtering screen  110  or cleaning of the existing filtering screen  110  for a period of five years or more, to coincide with normal turnaround schedules although the time required between required changes of filtering screen  110  will depend on many factors such as the actual length of the installed filtering screen  110 , the amount and type of debris in the fluid stream, the frequency and factors at and upon which the filtering screen  110  is advanced and other factors. 
     By rolling filtering screen  110  from source roller  150  to take up roller  160  over time, cleaning of the actual element which captures debris is unnecessary because this element is replaced with a new portion of filtering screen when the filtering element becomes dirty to the point where its effectiveness is reduced and/or flow is significantly reduced. 
     Fouling mitigation device  100  consists of a cruciform housing  140  which contains source roller  150 , take up roller  160  and filtering screen  110 . Filtering screen  110  is rolled from source roller  150  to take up roller  160  within the chamber. Rolling of the filtering screen may occur manually as determined by an operator using crank  175  or some other mechanism which causes take up roller  160  to wind additional filtering screen  110  from source roller  150  onto take up roller  160 . A crank may also be employed in connection with source roller  150  in order to permit an operator to manually move filtering screen  110  in both directions. Reverse movement may be used, after removing fouling mitigation device  100  from the fluid stream, for the purpose of cleaning filtering screen  110 . Alternatively, the screen may be rolled automatically from source roller  150  to take up roller  160  as a result of a specified level of debris on the active portion (i.e. the portion of filtering screen  100  presently within pipe  130 ) of screen  110  as determined by pressure drop changes across the active portion of the screen. 
     Automatic rolling from source roller  150  to take up roller  160  can take place irrespective of a change in pressure drop but instead at a predetermined rolling rate per unit time. In this case, rolling occurs on a periodic basis over time but preferably only with respect to a portion of the active screen length. Thus, for example, if the active screen length present in the pipe is approximately 15 cm (corresponding to an approximate 15 cm pipe cross section diameter), rolling may occur once a day with a roll amount of 25 mm. As such, in this example, the active portion of filtering screen  110  will be completely replaced each six days but on a staggered basis. Alternatively, the complete active portion of filtering screen  110  may be replaced all at once at some fixed periodic rate. For example, filtering screen  110  may be advanced a full 15 cm once every six days in the case of a 15 cm pipe cross section. The present strainer is not necessarily limited to use with pipes having a circular cross-section; it may easily be adapted to fluid flow structures with non-circular cross-sections. 
     As can be seen in FIGS. 1 and 2, pipe  130  passes through the cruciform housing  140  forming the body of strainer  100  which extends from top cap  165  at one end to debris flushing valve  145  at the other end. Strainer  100  is placed in the fluid flow in a vertical position so that debris particles having a density which is greater than that of the fluid fall off filtering screen  110  under gravity into debris collection area  155 . Debris may be flushed periodically from debris collection area  155  at the bottom of fouling mitigation device  100  by opening debris flushing valve  145 . Process fluid can pass through bottom opening  125  into the lower chamber of fouling mitigation device  100  but lower blocking member  190  which engages with stationary screen support member  175  at the rear face of screen  110  prevents the fluid which has not passed through filtering screen  110  from entering the flow region downstream of screen  110  and from there into pipe  130 . Bottom opening  125  preferably comprises an opening of approximately 25% of the diameter of pipe  130  in its lateral length along the flow direction of pipe  130 . This size is big enough to permit debris of larger particle size to fall into debris collection area  155  but not so large as to create excessive re-circulation flow between pipe  130  and fluid volume surrounding take up roller  160 . 
     When a new filtering screen  110  is installed, the majority of filtering screen  110  is present on source roller  150 . Filtering screen  110  is fed through the cross section of pipe  130  and then a leader portion of filtering screen  110  is rolled onto take up roller  160 . Bearings  147  are preferably used in connection with both source roller  150  and take up roller  160  so as to ensure smooth rotational operation and to avoid the possibility of the shafts becoming jammed by mud-like sedimentation that could occur in many processes. 
     The moving mechanical parts of strainer  100  may be accommodated in removable cartridge  210  within housing  140 . Cartridge  210  may be slidably inserted and removed into and out of the upper portion of housing  140  which is permanently placed forming a cruciform relationship with pipe  130  forming the filtering area at the intersection of two intersecting cylindrical bodies, housing  140  and  130 . In a preferred embodiment, removable cartridge  210  houses bearings  147 , rollers  150  and  160 , filtering screen  110 , blocking members  180  and  190  and blocking members  220 . Seal  230  surrounding the actuating shaft for roller  160  (here, the shaft of crank  175 ) prevents leakage past the shaft. 
     In addition to bottom opening  125 , strainer  100  also includes a top opening  135  which permits process fluid which has flowed through filtering screen  110  to enter the upper chamber strainer  100  from the flow regions downstream of the screen. Process in chamber  100  is prevented from re-entering the flow region upstream of filtering screen  110  by upper blocking member  180  which extends from the wall of housing  140  into sliding contact with the front face of screen  110 . Flanges  195  are typically included so that the strainer can be connected to existing piping and removed for maintenance. Support element  175  in sliding contact with the rear face of screen  110  provides additional structural support to filtering screen  110  and suitably comprises a perforated plate through which the filtered process liquid can flow. 
     Strainer  100  need not be interposed at a ninety-degree angle to axis of housing  140  and pipe  130  as shown in FIGS. 1 and 2. As an alternative, it may be interposed at a slanted angle relative to the axis (in the flow direction) of housing  140  and pipe  130 . One advantage of doing so is to provide additional active surface area for the portion of filtering screen  110  that is deployed to trap debris as opposed to the case in which pipe  130  and fouling mitigation device  100  are deployed in the ninety-degree cruciform arrangement. As shown in FIGS. 6 and 7 below it may also be interposed parallel to the axis of pipe  130 . 
     A close up view of filtering screen  110  is provided in FIG. 3 a . As shown in FIG. 3 a , filtering screen  110  may include, on one or both edges, metal band  320  which contains evenly spaced holes  340 . Holes  340  are arranged such that a pin (FIG. 4) located at or near the top of the lower chamber of strainer  100  and near the lower surface of pipe  130  may be selectively engaged within at least one of the holes  340 . The pin-and-hole feature of this embodiment serves to assist in controlling the proper advancement of filtering screen  110  as well as holding filtering screen  110  in place during normal operation. 
     Depending upon the particular application, holes  340  within metal band  320  may be spaced apart a length which is equal to one complete advancement of filtering screen  110  in which the complete “active” portion of filtering screen  110  is replaced by a complete new “active” portion of filtering screen  110 . For example, if the diameter of pipe  130  is equal to approximately 15 cm and so is the “active” portion of filtering screen  110 , holes  340  may be spaced apart approximately the same distance to ensure complete replacement of the “active portion” during a normal advancement of filtering screen  110 . Alternatively, if incremental advances of filtering screen  110  are desired (i.e., less than the complete “active” portion is replaced in connection with one advancement) holes  340  may be spaced apart some length which is less than the length of one active portion. For example, holes  340  may be spaced apart 5 cm for a 15 cm active portion allowing three advancements to completely replace the active portion of filtering screen  110 . In FIG. 3 b , it can be seen that removable cartridge  210  also preferably includes a pair of guides  350  which accept metal bands  320  of filtering screen  110  to lend additional strength to screen  110 . 
     The pin-and-hole screen lock feature may be employed with either automatic advancement implementations (in which the pin is automatically retracted from hole, filtering screen  110  is advanced and the pin is then replaced in another hole  340 ) or in manual advancement implementations (in which an operator manually retracts the pin from the hole, advances filtering screen  110  and then replaces the pin in the next hole  340 ). 
     FIG. 4 illustrates a screen lock actuation mechanism that may be employed in connection with the pin-and-hole feature described above as one example of how a pin-and-hole feature could be implemented. In this form, pin  410  is movable so that it may selectively be placed in a forward position to pass through hole  340 . When filtering screen  110  needs to be advanced, pin  410  may be moved to a rearward position in which it is not engaged within a hole  340 , so filtering screen  110  is freely movable. The pin actuation mechanism consists of piston  810  which is positioned at its extreme left position when filtering screen  110  is clean. Piston  810  is forced into this position by the force of coil spring  820  which pushes against piston  810  in cylinder  850  in the direction of piston receptacle  840  and which, in turn, pushes pin  410  leftward so that it protrudes into hole  340 . As debris collects on screen  110 , the downstream pressure decreases and piston  810  moves to the right as a result of the relatively higher upstream pressure transmitted to the interior of cylinder  850 . At some point, enough debris collects on screen  110  to result in enough of a decrease in downstream pressure to cause pin  410  to retract completely out of hole  340  thus permitting the free lateral movement of filtering screen  110 . If piston  810  becomes stuck, pin  410  may be manually retracted from hole  340  by pulling piston  810  back through the use of knob  830 . Drain valve  860  may be included so as to permit the removal of sedimentation which may enter into and settle in the piston mechanism. Piston mechanism also includes O-rings  845  which are placed on the piston to ensure that fluid does not leak from one side of piston to the other. O-ring seals  855  are placed on the stem of piston  810  where it exits cylinder  850  to prevent leakage from cylinder  850 . 
     In another version of the pin mechanism which is described in connection with FIG. 5, the overall pin mechanism includes a diaphragm to move the piston to actuate the screen pin. Pin  410  comprises an arm which extends toward pivot  450  and a substantially vertical arm  455  extends downward away from pivot  450  and terminates at main pivot  460 . Pin  410  may pass through guide  430  to ensure that pin  410  stays straight and lines up properly with holes  340 . A third arm  465  extends generally parallel to pin  410  and terminates at yet another pivot  470 . Finally, a substantially vertical arm  475  extends upward from pivot  470  and terminates where it contacts diaphragm  440 . Diaphragm  440  is preferably comprised of stainless steel and may have a thickness of approximately 1.5 mm with a diameter of approximately 10 cm but other materials and other sizes may be used so long as it has the required mechanical properties to actuate the piston and the pin mechanism. Support element  480  is preferably included to attach pivot point  460  to the frame of removable cartridge  210 . 
     When debris and other solid materials build up over time on the surface of filtering screen  110  that faces the fluid flow (the front surface), a pressure differential between the front surface of filtering screen  110  and the rear surface of filtering screen  110  is created. Diaphragm  440 , due to its flexible nature, will move either in the direction inward towards the bottom chamber of fouling mitigation device  100  or inward toward the interior of pipe  130 , depending on the sense and magnitude of the pressure differential. 
     As debris builds up on the front surface of filtering screen  110 , the pressure differential across filtering screen  110  and thus across diaphragm  440  causes diaphragm  440  to move upward into the interior of pipe  130 . This, in turn, causes pin  410  to move in the direction away from filtering screen  110  thus removing pin  410  from hole  340 . When this action is combined with either automatic or manual advancement of filtering screen  110 , the combination ensures that filtering screen  110  can be advanced at the point when a predefined pressure differential exists across filtering screen  110 . The pressure differential which is required to move pin  410  can be controlled specifically by a particular combination of diaphragm sizing, diaphragm materials, diaphragm placement, pivots, and arm and pin sizing and placement. In an embodiment in which automatic advancement of filtering screen  110  is employed, automated advancement (in addition to pin movement) may also be triggered based upon a predefined pressure differential across filtering screen  110 . Backing element  445  may be included and placed on the low-pressure side surface of diaphragm  440  to prevent damage to diaphragm  440  in the case of extremely high pressure differentials. Backing element  445  should be placed to allow a large enough gap between it and diaphragm  440  so as to allow for normal deflection of diaphragm  440 . 
     Selective pin  410  withdrawal from holes  340  may also be accomplished by use of an electric transducer coupled to an actuator. The transducer may be configured to measure the pressure difference across filtering screen  110 . A resulting signal which indicates the value of such pressure difference may be fed to a control system and actuator which, in turn, causes pin  410  to be retracted from hole  340  and the automatic advancement of filtering screen  110  as desired. 
     FIG. 6 shows an embodiment of strainer in which the face of filtering screen  110  is placed parallel to the flow axis of the strainer housing and of the flow pipe to which it is connected (as opposed to perpendicular to the flow as in FIG.  1 ). In order for the actual fluid to pass through the screen  110 , the flow is caused to make a ninety-degree turn through the use of blocking members within housing  140 . The face of screen  110  is placed parallel to the flow axis of the strainer body and of pipe  130  and blocking member  710  is placed within pipe  130  so as to ensure that the flow passes through filtering screen  110  as opposed to around it. Filtering screen  110  in this embodiment is a flat screen element that is essentially located on the central plane of pipe  130 . Blocking member  710  is configured so that any cross-sectional area of pipe  130  that is not covered by filtering screen  110  is blocked by blocking element  710 . Blocking member  710  may be constructed as a single piece or as a multiple pieces placed in housing  140  so preclude by-passing of flow around filtering screen  110 . 
     FIG. 6 shows that strainer  600  comprises many of the same components as are present in the first embodiment of the present invention in which filtering screen  110  is placed perpendicular to the flow axis. Strainer  600  includes heavy debris collection area  655  for collecting debris that falls from filtering screen  110 . A large gap area  630  is preferably included in the area between pipe  130  and the housing holding take-up roller  160  to allow debris to fall into heavy debris collection area  655 . Debris flushing valve  645  may be used to flush debris from fouling mitigation device  600  as necessary. As can be seen from the end view presented on FIG. 6, rather than pipe  130  and housing  140  forming a cruciform shape as in FIG. 1, in the second embodiment, source roller  150 , take up roller  160  and pipe  130  are spatially related as three laterally spaced circular cylindrical bodies. As can be further seen, filtering screen  110  passes from source roller  150  through pipe  130  and onto take up roller  160 . Filtering screen  110  is disposed in pipe  130  so that only the relatively small leading edge of filtering screen  110  faces the flow direction; the width of filtering screen running parallel with flow direction may be as large as desired to provide the required filtering surface area through which the liquid may pass. This results in a less frequent need to advance filtering screen  110  since debris will be spread over a larger surface area of filtering screen  110 . This is in contrast to the first embodiment of the present invention in which the surface area of the face of filtering screen  110  is limited to the cross-sectional diameter of pipe  130 .