Abstract:
A corrugation has a radially inner wall circumferentially surrounding an axis. The inner wall defines an inner cavity configured to conduct a liquid axially through the inner cavity. A radially outer wall of the corrugation overlies the inner wall. The outer wall adjoins the inner wall so as to form a closed outer cavity bounded by the inner and outer walls and separated from the inner cavity by the inner wall. The inner wall has at least one opening providing fluid communication between the inner and outer cavities for the corrugation to serve as a reservoir for the liquid.

Description:
TECHNICAL FIELD 
     The present invention relates to corrugated pipes. 
     BACKGROUND 
       FIG. 1  shows a prior art flexible polyethylene corrugated pipe  10  used in an irrigation system. The pipe  10  is centered on an axis  13  and extends axially from a front open end  15  to a rear open end  17 . A bell-shaped section  20 , or “bell,” of the pipe  10  is located at the rear end  17 . The pipe  10  also includes annular corrugations  30  arranged in a linear series extending axially from the bell  20  to the front end  15 . A spigot  40  of the pipe  10  comprises the front-most corrugation  42  and an annular rubber gasket  44 . The spigot  40  can be inserted into the bell of another pipe (not shown) to join the pipes together with a sealed joint. 
     As shown in  FIG. 2A , each corrugation  30  has a cylindrical inner wall  50  defining an inner cavity  51  centered on the axis  13 . Additionally, each corrugation has an outer wall  60  adjoining the inner wall  50  to define a closed annular outer cavity  61  centered on the axis  13 . 
     During use, as illustrated in  FIG. 2A , water  66  is conducted axially through the inner cavity  51 . A blockage can occur downstream of the pipe  10  due to, for example, a downstream valve being shut off. As the water  66  continues to flow from upstream, while being blocked from exiting downstream, the water level in the inner cavity  51  rises, as illustrated in FIG.  2 B. The entire column of water flowing toward the pipe  10  has a speed and thus an inertia. The inertia can be significant, because it equals the speed of the water times the mass of the entire column of water flowing toward the pipe  10 . 
     At some point, the water  66  can entirely fill the inner cavity  51 , as illustrated in FIG.  2 C. At that moment, with suddenly no more space to contain further incoming water, the inertia is suddenly dissipated by the impact of the water  66  against the inner wall  50  of the pipe  10 . The impact, called water hammer, is manifested as a peak pressure within the inner cavity  51 . 
     The pipe  10  must be designed to withstand the peak pressure, so that the pipe  10  will not rupture and the joint will not leak. This requires making the pipe walls thicker than would be necessary if the peak pressure were lower, and thus increases cost and weight of the pipe  10 . 
     SUMMARY 
     An embodiment of the present invention is a corrugation. The corrugation has a radially inner wall circumferentially surrounding an axis and defining an inner cavity configured to conduct a liquid axially through the inner cavity. A radially outer wall of the corrugation overlies the inner wall. The outer wall adjoins the inner wall so as to form a closed outer cavity bounded by the inner and outer walls and separated from the inner cavity by the inner wall. The inner wall has at least one opening providing fluid communication between the inner and outer cavities for the corrugation to serve as a reservoir for the liquid. 
     Preferably, the inner wall is cylindrical and centered on the axis. The outer cavity is annular, centered on the axis, and fully surrounds the inner cavity. The corrugation is one of an axially extending series of such corrugations comprising a pipe. The corrugation has a predetermined installed orientation defined by a designated bottom end of the corrugation. The at least one opening comprises first and second openings, and, in the installed orientation of the corrugation, the second opening is located higher than the first opening. The corrugation is configured for a pressurized air pocket to be formed in the outer cavity by the liquid rising above the second opening. 
     Another embodiment of the invention is a pipe comprising a wall circumferentially surrounding an axis. The wall defines an axial-flow cavity for conducting a liquid axially through the axial-flow cavity. The pipe further comprises an axially extending series of reservoir structures. Each reservoir structure defines a closed reservoir cavity separated from the axial-flow cavity by the wall. The wall has, for each reservoir cavity, at least one opening providing fluid communication between the axial-flow cavity and the reservoir cavity. 
     In another embodiment, at least one lower opening in the wall provides fluid communication between the axial-flow cavity and the reservoir cavity, such that a radially outward flow of the liquid from the axial-flow cavity to the reservoir cavity occurs by the liquid in the axial-flow cavity rising above the lower opening. At least one upper opening in the wall is located above the at least one lower opening in a predetermined installed orientation of the pipe. The at least one upper opening provides fluid communication between the axial-flow cavity and the reservoir cavity, such that a pressurized air pocket is formed in the reservoir cavity by the liquid in the axial-flow cavity rising above the at least one upper opening. The pressurized air pocket opposes and slows the radially outward flow to the reservoir cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art corrugated pipe; 
         FIG. 2A  is a sectional view taken on line  2 A— 2 A of  FIG. 1 , depicting an initial water level in the pipe; 
         FIGS. 2B-2C  are views similar to  FIG. 2A , depicting higher water levels in the pipe; 
         FIG. 3  is a perspective view of a corrugated pipe embodying the present invention; 
         FIG. 4  is a sectional view of the pipe of  FIG. 3 ; 
         FIG. 5  is a sectional view of the pipe of  FIG. 4 , shown joined to another pipe; 
         FIG. 6A  is a sectional view taken on line  6 A— 6 A of  FIG. 3 , depicting an initial water level in the pipe; and 
         FIGS. 6B-6E  are views similar to  FIG. 6A , depicting higher water levels in the pipe. 
     
    
    
     DESCRIPTION 
     The apparatus  110  shown in  FIG. 3  has parts which, as described below, are examples of the elements recited in the claims. 
     The apparatus is a flexible polyethylene corrugated pipe  110  for conducting water in a system of pipes, such as an irrigation system. The pipe  110  is centered on an axis  113  and extends axially from a front open outer end  115  to a rear open outer end  117 . As shown in  FIG. 4 , a bell  120  of the pipe  110  is located at the rear end  117 . The pipe  110  also includes annular corrugations  130  arranged in a linear series extending axially from the bell  120  to the front end  115 . The corrugations  130  include first, second and third corrugations  131 ,  132  and  133  extending consecutively and contiguously axially rearward from the front end  115 . A spigot  140  of the pipe  110  comprises the first corrugation  131  and an annular rubber gasket  144 . 
     The corrugations  130  have similar features, described as follows with reference to the third corrugation  133 . Like the third corrugation  133 , each corrugation  130  includes a cylindrical inner wall  146  centered on the axis  113  and defining a cylindrical inner cavity  150 . Each corrugation  130  further includes an annular outer wall  156  with a generally U-shaped cross-section, centered on the axis  113  and overlying the inner wall  146 . The outer wall  156  adjoins the inner wall  146  along two axially opposite annular edges  158  of the outer wall  156 , to define a closed annular outer cavity  160  centered on the axis  113 . The outer cavity  160  fully surrounds the inner cavity  150 , and is separated from the inner cavity  150  by the inner wall  146 . Upper openings  162  and lower openings  164  ( FIG. 3 ) in the inner wall  146  provide fluid communication between the inner and outer cavities  150  and  160 . 
     The bell  120  is centered on the axis  113  and comprises three sections, as follows. A flare section  170  extends axially and radially inward from the rear end  117 . From the flare section  170 , a generally cylindrical section  172  extends axially inward to an annular back wall section  174 . The back wall section  174  extends axially and radially inward to the series of corrugations  130 . 
     The spigot  140  includes the first corrugation  131  and an annular rubber gasket  144 , both centered on the axis  113 . The outer wall  156  of the first corrugation  131  has an annular groove  180  extending radially inward and centered on the axis  113 . The gasket  144  is seated in the groove  180  and extends radially outward from the groove  180 . 
     As shown in  FIG. 5 , the spigot  140  is configured to be telescopically inserted into the bell  188  of another pipe  190  to join the pipes together. When the pipes  110  and  190  are joined, the gasket  144  is compressed between the first corrugation  131  and the bell  188  of the other pipe  190  to provide a sealed joint  192 . 
     The bell  188  of the other pipe  190  receives both the first and second corrugations  131  and  132 . Therefore, the first and second corrugations  131  and  132  must be short enough to fit within the bell  188  of the other pipe  110 . The third corrugation  133  is not received by the bell  188  and thus can be taller, as shown. The corrugations  130  ( FIG. 4 ) rearward of the third corrugation are similar to the third corrugation  133  in size and shape. 
     The pipe  110 , and each corrugation  130  of the pipe  110 , has a specific location designated as the bottom end  200 , shown in FIG.  6 A. The bottom end  200  serves as a reference from which vertical positions of the openings  162  and  164  are determined. Accordingly, the pipe  110  has a predetermined installed orientation in which the designated bottom end  200  faces down. In contrast, since the prior art pipe  10  ( FIG. 2A ) does not have openings in the inner wall  50 , it is symmetric about its axis  13  and does not need a predetermined installed orientation or designated bottom end. 
     As viewed in its installed orientation shown in  FIG. 6A , the lower openings  164  are located horizontally opposing each other at a level L L . Similarly, the upper openings  162  are located horizontally opposing each other at a level L U  higher than L L . The considerations for determining the optimal levels L U  and L L  for the openings  162  and  164  are explained through the example illustrated in  FIGS. 6A-6E . Although this example is described below with reference to the third corrugation  133 , the following explanation applies to all the corrugations  130 . 
     In the example of  FIGS. 6A-6E , the pipe  110  is one of a plurality of such pipes joined together in series to form a system of pipes, such as an irrigation system. The dimensions shown for the cross-sections in these and the other figures are for illustration purposes, and actual pipes that incorporate the present invention may vary relative to the dimensions shown. From upstream, a column of water  205  flows downstream to and through the pipe cavity  150 . 
     The level of the water  205  in the inner cavity  150  is herein referred to as the inner water level L i . As shown in  FIG. 6A , the inner water level L i , is below the lower openings  164 . Therefore, the water  205  cannot enter the outer cavity  160  through the openings  164 . Nevertheless, the outer cavity  160  in this example contains residual water  207  to an outer water level L o  equal to the level L L  of the lower openings  164 . This residual water  207  is water remaining after the water  205  in the inner cavity  150  was previously elevated and flowed through the openings  164  into the outer cavity  160 . 
     In this example, due to a blockage downstream from the pipe  110 , the inner water level L i , starts to rise. Each successive figure of  FIGS. 6B-6E  depicts a successively higher inner water level L i  due to the blockage. 
     In  FIG. 6B , the inner water level L i  has risen above the lower openings  164 , but remains below the upper openings  162 . As the inner water level L i  rises, the water  205  flows radially outward through the lower openings  164  to the outer cavity  160 . As the outer water level L o  rises in unison with the inner water level L i , air in the outer cavity  160  can flow radially inward through the upper openings  162  to the inner cavity  150 . This keeps the air pressure in the outer cavity  160  equal to the air pressure in the inner cavity  150 , which is atmospheric pressure. Therefore, as long as the inner water level L i  is below the upper openings  162 , the air in the outer cavity  160  is not pressurized, i.e., raised above atmospheric pressure, and the outer water level L o  equals the inner water level L i . 
       FIG. 6C  shows the pipe  110  soon after the water  205  has risen above the upper openings  162 , giving rise to an air pocket  210  in the outer cavity  160 . The air pocket  210  is pressurized by the hydrostatic pressure of the water  205  in the inner cavity  150 . As the inner water level L i  progressively rises, the pressurization progressively increases. Concurrently, the air pocket  210  is progressively compressed, thereby providing room for water influx. The pressurization of the air pocket  210  opposes and thus slows the radially outward flow of water  205  into the outer cavity  160 . Therefore, the outer water level L o  is lower than the inner water level L i . 
       FIG. 6D  shows the pipe  110  at the moment the water  205  has filled the entire inner cavity  150 . Although the inner cavity  150  is full, inertia of the entire water column flowing axially forward toward this pipe  110  continues to drive the water  205  into the pipe  110 . The water  205  is then forced radially outward through the openings  162  and  164  into the outer cavity  160 . The radially outward flow through the openings  162  and  164  enables the axially forward flow to continue despite the inner cavity  150  being full. Room for the radially outward flow is provided in the outer cavity  160  by the air pocket  210  being progressively pressurized and compressed. However, the progressively increasing pressure of the air pocket  210  opposes and thus gradually slows the radially outward flow of the water  205  into the outer cavity  160 . This, in turn, gradually slows the axially forward flow of water into the inner cavity  150 . The inertia is thus dissipated gradually. 
       FIG. 6E  shows the pipe  110  at the moment the axially forward and radially outward flows of water  205  have slowed to a stop. At this instant, the air pocket  210  reaches a maximally compressed volume. Simultaneously, the inner and outer cavities  150  and  160  reach a peak pressure as the inertia reaches zero. The peak pressure is lower than in the prior art pipe  10  (FIG.  2 C), thereby producing less or no water hammer, because the inertia has been dissipated gradually. 
     After the moment depicted in  FIG. 6E , the pressure in the air pocket  210  forces some of the water  205  in the outer cavity  160  back into the inner cavity  150 . This causes the water  205  in the inner cavity  150  to flow back upstream to a limited extent before stopping. The pressure in the inner cavity  150  is thus reduced to a level below the peak pressure. 
     In this example, the downstream blockage is eventually removed. The inner water level L i  recedes back to the level depicted in  FIG. 6A , below the level L L  of the lower openings  164 . Concurrently, the water  205  held in the outer cavity  160  empties back into the inner cavity  150  through the openings  162  and  164 . The outer water level L o  thus recedes back to the level L L  of the lower openings  164 . Since the section of the outer cavity  160  below level L L  is typically permanently filled with water (with variations due to evaporation), this section is not part of the reservoir volume. 
     The above example illustrates at least four effects provided by the invention. The first effect is the reservoir effect, illustrated in  FIG. 6B , in which the lower openings  164  enable the radially outward flow of the water  205  from the inner cavity  150  to the outer cavity  160 . This effect provides a reservoir that temporarily holds a portion of the water  205  whenever the inner water level L i  is above the level L L  of the lower openings  164 . The reservoir volume comprises the volume of the outer cavity  160  from the lower openings  164  to the bottom of the maximally compressed air pocket  210  (FIG.  6 E). The reservoir volume is provided by every corrugation along the length of the irrigation system, if those corrugations embody the invention. Therefore, the total volume of the reservoir can be substantial, and thus capable of taking up a large influx of water. The reservoir effect is operative from the time the water rises above the lower openings  164  until it stops flowing. 
     The second effect is the gradual slowing of the radially outward flow of the water  205  to the outer cavity  160 . In this gradual slowing effect, illustrated in  FIG. 6C , the gradually increasing pressure of the air pocket  210  gradually opposes and slows the radially outward flow of the water  205  into the outer cavity  160 . This effect is operative from the time the inner water level L i  surpasses the upper holes  162 , which is when the pressurized air pocket  210  is formed, until the water  205  stops flowing. Consequently, the air pocket  210  and the accompanying slowing effect start to occur after the reservoir effect has been operative for some period of time. 
     The third effect is the damping effect to reduce or eliminate water hammer. In this effect, illustrated in  FIG. 6D , the aforementioned gradual slowing of the radially outward flow from the inner cavity  150  gradually slows the axially forward flow into the inner cavity  150 . Since the inertia is dissipated gradually, water hammer is reduced or eliminated. Understandably, the inertia of the entire water column upstream of the pipe  110  can be substantial. Nevertheless, the present invention can dissipate the substantial inertia gradually, because the damping effect is provided by every corrugation along the length of the irrigation system, if those corrugations embody the invention. The damping effect is operative from the moment the water  205  just fills the inner cavity  150  until it stops flowing. 
     The fourth effect is tightening of the seal of the joint  192 , illustrated in FIG.  5 . This effect applies only to the spigot corrugation  131  and entails the increased pressure in the outer cavity  160  slightly expanding the outer wall  156 . This expansion increases the compression of the gasket  144  against the bell  120 . The seal is thus tightened when it is needs tightening most—when the cavities  150  and  160  are pressurized. This seal tightening effect is operative the entire time the water  205  in the inner cavity  150  is completely filled with water. 
     Determining the level L U  of the upper openings  162  in  FIG. 6A  is based on two considerations. On the one hand, the lower the upper openings  162 , the greater the damping effect. On the other hand, the higher the upper openings  162 , the smaller the air pocket  210  (FIG.  6 E), and thus the greater the reservoir effect. Preferably, the upper openings  162  are located at an angle Θ U  of 0-90 degrees, and more preferably about 35 to about 55 degrees, above the axis  113 , as illustrated by imaginary line  220 . 
     Determining the level L L  of the of lower openings  164  is also based on two considerations. On the one hand, the lower the lower openings  164 , the greater the reservoir effect. On the other hand, the higher the lower openings  164 , the less likely they are to be plugged with sediment in the water  205 , accumulated at the bottom of the cavity  150 . Therefore, a determination of the level L L  of the lower openings  164  is based on the level of the sediment or sludge that the pipe  110  is likely to encounter during use. Preferably, the lower openings  164  are located at an angle Θ L  of 0-90 degrees, and more preferably about 35 to about 55 degrees, below the axis  113 , as illustrated by imaginary line  222 . 
     Preferably, the angles Θ L  and Θ U  are approximately equal. The openings  162  and  164  are then spaced approximately symmetrically about the axis  113  such that rotating the pipe  110  by 180 degrees about the axis  113  yields an equivalent configuration. Therefore, in addition to the first bottom end  200  described above, the pipe  110  has a second designated bottom end  230  located opposite the first designated bottom end  200 . The pipe  110  thus has two predetermined installed orientations, comprising a first orientation in which the first bottom end  200  faces down and a second installed orientation in which the second bottom end  230  faces down. 
     More preferably, the angles Θ L  and Θ U  are both equal to about 45 degrees. The openings  162  and  164  are then spaced symmetrically about the axis  130  such that rotating the pipe  110  by 90 degrees about the axis  113  yields an equivalent configuration. The pipe  110  of such an embodiment thus has four designated bottom ends  200 ,  230 ,  232  and  234  located 90 degrees apart from each other. The pipe  110  then has four corresponding predetermined installed orientations. In each of the four installed orientations, one of the designated bottom ends  200 ,  230 ,  232  and  234  faces down. 
     Determining the size of the openings  162  and  164  is based on the anticipated rate of the axial flow. A faster rate requires a larger opening to more quickly equalize the pressure and water level between the inner and outer cavities  150  and  160 . 
     While the invention has been described with reference to a polyethylene pipe, other types of pipes may also benefit from the invention, the invention not being limited to a particular type of material. Furthermore, while specific corrugation shapes are shown, the invention is not limited to a particular shape or size of corrugation. In addition, an exemplary shape and size is shown for the bell and spigot of the pipe. However, other bell and spigot shapes and sizes may also be utilized with the invention. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.