ELBOW FITTINGS FOR DOUBLE CONTAINMENT PIPE ASSEMBLIES

Abstract: A double containment pipe assembly having curved portions formed from elbow fittings (10, 12) extending between straight portions of an inner and outer concentric pipe assembly. The elbows or curved fittings (10, 12) occupy the same angular sector of a circle and are concentric to each other. The spacing between the inner and outer curved surfaces of the fittings of the elbows (10, 12) is consistent and the same throughout the radius of curvature of the elbows. This provides maximum protection against contact of the inner and outer fittings (10, 12) during thermal expansion and contraction of the pipe. Straight portions of pipe (18, 20) can be secured to opposite faces (14, 16) of the fittings for ease of field fabrication of the double containment pipe assembly. The straight portions may be butt-welded or joined by butt-and-wrap techniques to the end faces (14, 16) of the fittings or nested in or over sockets (14', 16') provided at the end faces.

Description: ELBOW FITTINGS FOR DOUBLE CONTAINMENT PIPE ASSEMBLIES BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to double containment pipe fittings and their assembly. Description of the Prior Art: The general concept of providing a double or dual containment pipe system wherein an inner carrier pipe is concentrically located within an outer containment pipe to deliver dangerous or hazardous fluids is well known and an accepted commercial practice. Historical applications for such systems have been found in the nuclear, gas petroleum production and refining and chemical processing industries. The inner pipe is used to transport the hazardous or toxic fluid while the outer pipe is present to confine any leaks. Thus, it is also known to provide the annulus between the concentric pipes with various types of detectors and/or drainage apparatus to handle leakage. Examples of double containment pipe assemblies are shown in U.S. Patent Nos. 4,786,088; 4,886,305 and 4,930,544. With the advent of stricter governmental regulation concerning the piping of petroleum products and hazardous chemicals, the anticipated increased use of various types of pipes in double containment applications is a certainty. As such, the structural design of these fittings and method of installing and repair of double containment pipes containing such fittings is necessary. The present invention provides novel double containment fittings for forming a double containment pipe joint. The products that have been developed to date have not completely addressed some of the unique problems that arise when a pipe is placed within another pipe. One such problem is the ease of maintaining, modifying or repairing such systems. To date, all above ground pressure systems have been installed without the ability to assemble or disassemble the piping or components in modular sections. Therefore, if a repair is to be made, the original system manufacturer/ contractor must be called in to facilitate the repair. In many designs, a repair of a certain section would not be feasible or even possible due to the location of the system with respect to adjacent equipment or building parts. What would be very helpful to most facility owners is the ability to have a modular system that is capable of being readily disassembled. A system capable of being readily disassembled would give the facility engineer the ability to have the section requiring modification or repair sent to a shop type environment readily suitable and equipped to facilitate the change or repair. Another such problem has to do with the ability of a system to withstand the effects of inner and outer piping and components that are subjected to different amounts of thermal expansion and contraction. It is the norm, rather than the exception, that the inner and outer pipes of a pressure rated double containment piping system are subjected to different amounts of thermal expansion. This situation may arise in several different ways. The most common way involves the situation whereby a hot fluid is transported through the inner pipe. Under this circumstance, the external environment (external that is to the outside diameter of the secondary containment piping) is normally at a lower temperature than the hot fluid. Since there is either an insulating dead air space between the two pipes, or other insulating material, the inner piping temperature becomes close to that of the fluid, while the outside piping remains closer to that of the external ambient environment. Therefore, the materials normally grow to different lengths due to their being at different temperatures. When this does occur, there are thermal strains that are imposed on interconnecting parts and on parts such as interstitial supports that create a contact point between the inner and outer piping. The most obvious place where there is a problem potential is at the interconnecting points. However, any place where loads can be transmitted back and forth between the two pipe systems can result in a problem. An example would involve inner and outer elbows where contact is caused between the two due to differential thermal expansion. When this occurs, both the inner and outer elbows will impose stresses and strains upon each other. With the exception of the development described in this application, all interconnecting parts designed for this situation are constructed of a singular material, with such a design that the residual stresses and subsequent strain on the materials can lead to a failure of such parts. These existing parts then become the point in the overall system at which failure is most likely to occur. This is compounded by the fact that the components are truly single containment at that point, without having a containment area to prevent the hazardous fluids from reaching the environment. Therefore, exactly where it would be the least desirable location for failure to occur is the most likely place for failure to occur. Additionally, a double containment piping system that is constructed of an interconnecting part as described above, actually increases the chance that failure to the external environment can occur as compared to a singular containment piping system designed with proper treatment of the thermal expansion of its components. Another problem with existing systems has to do with the ability to combine different inner and outer pipe materials in an efficient manner. Different materials can mean materials that are of a different class, (such as metallic-thermoplastic, metallic-reinforced thermosetting plastic, or thermoplastic-reinforced thermosetting plastic) , or materials that are within the same class but constitute a different material, (such as within the thermoplastic family, combining a fluoropolymer within a polyolefin, or a polyolefin within another polyolefin) . The reason that it is desirable to combine materials typically has to do with economics. It is desirable in many situations to combine an expensive material that is capable of handling a chemical on a full time basis within a less expensive material capable of withstanding the corrosive effects of a chemical for a limited period of time. Another major economic reason has to do with the use of a material for the outside piping capable of withstanding the corrosive effects of atmospheric conditions, thus eliminating the need for expensive coatings, cathodic protection, etc. A typical example of this would be in combining a metallic material within a nonmetallic outside material for the reasons just described. Yet another reason involves the selection of a combination of materials due to structural requirements. (E.g., the placement of a non-metal within a metal piping due to heavy burial loads.) Piping elbows have been used ever since the first pipe systems were used, and are considered to be a basic component of the piping system. Standard elbow patterns have been created and established that are used in the majority of all piping applications. For butt-welded systems, the standard elbow patterns involve short radius elbows (radius of the elbow is approximately equal to the nominal diameter of the pipe) , long radius elbows (radius of the elbow is approximately equal to 1-1/2 times the nominal diameter of the pipe) and for sanitary applications elbows that have a radius equal to approximately four times the nominal diameter. While these elbows have long served a purpose for singular piping systems, the use of standard size elbows in double containment piping systems present many potential problems. One such potential problem has to do with certain combinations where the selection of standard elbow patterns result in dissimilar radii. If the difference is large enough, the elbow may not be capable of being readily fabricated into an inner and outer arrangement, while maintaining the concentricity of the pipes leading into the elbow. Even if the elbows do fit, they will likely not be positioned in a perfectly concentric arrangement when placed in the outer elbow. That will mean that the inner elbow will be positioned in a less than ideal space relationship with respect to the outer. The allowable room for movement will therefore be less than the ideal relationship, where the inner elbow is perfectly positioned at any one given pipe within the outer. If the elbows contact each other due to a differential movement of the elbows, severe problems can occur. The inner and outer elbows will be subjected to a displacement strain and will experience a concentration in stress. The outer elbow may fail under this strain, creating a breach in the secondary containment. The inner elbow may also subsequently occur, resulting in a breach of the primary containment and a subsequent double failure. Even if the elbow itself doesn't fail, failure can result at joints, or even in straight pipe sections due to excessive bending and torsional loads. These can also lead to a double failure. There are also issues of the evenness of heat transfer and the resistance to potential annular flow. Each of these engineering aspects are somewhat compromised by the use of combinations of "standard" inner and outer elbows as well. SUMMARY OF THE INVENTION The elbow system employed in the present invention has many benefits over the current practice of using existing standard fittings for attachment to their respective pipes, as shown for example in U.S. Patent Nos. 4,786,088, 4,886,305 and 4,930,544. Perhaps the most important is that, because the matching inner and outer elbows are designed so that they maintain their concentricity throughout the bend, there is maximum allowable space for the elbow in both the direction of thermal expansion and contraction. In most piping systems, the pipes are subjected to both thermal expansion and contraction due to process and ambient cyclic conditions. Double containment piping is further complicated because both pipes are subject to different movements in a simultaneous fashion. Therefore, it is the differential movement in a double containment piping system that sets the needed space. The configuration that has been determined to be the most efficient maximizes the allowable space in both directions, and it is equal throughout. The likely points of contact are all at the same optimum distance from the centerline of the elbows. Further, due to this configuration, -an equal amount of heat transfer is maintained throughout the elbow section. Finally, the flow of air or other fluid through the annulus or space between the inner or outer pipes is maximized and thus, the frictional losses (resistance to flow) will be minimized. This is vitally important because flow is poor to begin with through double containment piping. If resistances to flow is high, it will require a high pressure to cause flow. A high pressure might collapse the primary piping due to exceeding its collapse pressure. Therefore, the resistances to flow in the annulus should be minimized in each component in order to cut down on the required flushing pressure to be used during flushing and cleaning procedures. A standard radius pattern can be used as the basis for the primary and secondary containment fittings. This will allow some conformity to conventional elbows, although the conventional elbow styles are not intended to produce adequate flow conditions in a double containment piping system. The elbow concept of the present invention also has benefits beyond its technical advantages. It also allows the double containment piping assembly to be assembled and welded (bonded) together in a much more efficient and easy manner. All elbows could be field assembled and provide modular sections which can be individually repaired, replaced and inspected. In accordance with the present invention, the angle of the sector of both the inner or primary pipe and the outer or containment pipe of both fittings should be equal. The annular space between the fittings should be consistent or the same throughout the curvature and length of the fittings. Either end of the inner and/or outer fittings or both ends of one of the fittings can be provided with a straight portion extending beyond the end of the other fitting. This will save several steps in the overall fabrication of the double containment pipe assembly in that it allows the ability to use the same length of inner and outer pipes between elbows, or between elbows and other fittings. Because of the concentric nature of the primary and outer or containment elbow fittings, along with the consistent annular spacing between the primary and containment portions of the elbow fittings, potential problems due to thermal expansion and contraction of the inner and outer pipes is minimized, as potential contact of the pipes is minimized; but heat transfer throughout the elbow section is maximized and the flow through the annulus or space between the inner and outer pipes is maximized because the resistance to flow is minimized. Further, the design permits utilization of different materials for the inner and outer pipes and allows for complete field fabrication. BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will become more apparent from the following description and claims, and from the accompanying drawings, wherein: FIGURE 1 illustrates the concentric elbow pipe fittings used to form the double containment pipe assembly of the present invention, with portions indicated in phantom lines comprising optional straight line extensions of the fittings enabling ease of field or shop fabrication of the elbow fittings in the double containment pipe assembly; FIGURE 2 illustrates another form of the concentric elbow pipe fittings; FIGURE 3 is a cross-sectional view illustrating the use of a pair of elbow pipe fittings of FIGURE 1 in a double containment pipe assembly of the present invention; FIGURE 4 is a cross-sectional view illustrating the use of a pair of elbow pipe fittings of FIGURE 2 in a double containment pipe assembly of the present invention; FIGURE 5 is a cross-sectional view illustrating a concentric elbow pipe fitting wherein the inner pipe has straight line extensions at either end; and FIGURE 6 is a cross-sectional view illustrating the f the elbow fitting of FIGURE 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, wherein like numerals indicate like elements throughout the several views, the elbow fittings of the present invention are indicated by the numeral 10 and 12 and are used as a connection between straight portions of a double containment pipe assembly (not shown) . The outer or containment pipe elbow 10 and the inner or primary elbow 12 have the same radius of curvature or occupy the same angle θ of the sector of a circle and are concentrically located with the inner or primary pipe in the outer or containment pipe 12 so that the annular space between the outer surface of the inner pipe 12 and the inner surface of the outer pipe 10 is constant, or the same throughout the radius of curvature or sector. The end face 14 of the inner pipe 12 and the end face 16 of the outer pipe 10 are adapted to be connected to the inner and outer corresponding straight sections of a double containment pipe assembly, respectively. This can be accomplished in a number of manners, but, the most usual way is to butt weld the straight section of each pipe to the end face 14,16, respectively, as illustrated in FIGURES 3 and 6. If desired, the inner and outer pipes 10',12' (FIG. 2) may be formed with sockets or flanges 14 ' , 16 ' along their end face extending in the direction of the pipe to be joined, for nesting engagement with the inner and outer pipes, as illustrated in FIGURE 4. In some instances, this will ease the fabrication of the double containment pipe assembly, particularly in the field. Optionally, either end of the inner or outer pipe fitting elbow 12,10, respectively, may be formed with a straight extension portion 18 and 20, respectively. E.g., in FIGURES 5 and 6, the inner pipe fitting elbow has straight extensions 18. It is desirable that the straight portions of the inner section be at one end and the straight portions of the outer section at the other, or vice versa, (e.g., see straight sections 18',20' of the elbow fittings of FIG. 2) , or the straight sections be at both ends of the inner or outer pipes (see FIGURES 5 and 6, for example) , so that during fabrication of the double containment pipe assembly, equal lengths of the straight pipe sections can be used for the inner and outer pipes between the elbows and other containment fittings, as illustrated in FIGURES 3 and 6. This enables standardization to be achieved for the system. It may also greatly enhance the ability of the systems to be inspected and examined prior to and during testing. However, the inner/outer pipe can have or be provided with the straight portions at both of its ends, if desired, and the standardized system could have one length of inner and one length of outer pipe. By providing separate inner and outer fittings for the elbow, the material can be matched to the inner and outer pipe material of the straight length sections between elbow fittings. Further, the portions of the double containment assembly between elbow fittings can be modular in that, if a leak occurs, it is only necessary to repair and replace one section of the pipe assembly. Utilizing concentric elbow fittings enables the maximum spacing to be achieved between the fittings so as to provide for thermal expansion and contraction without potential contact between the fittings which may cause disruption or cracking, and by utilizing such a system, improved flow through the system is achieved along with more uniform heat transfer.

Claims: CLAIMS : 1. A double containment pipe assembly having one or more curved sections defined by an inner and outer concentric pipe fitting joined to straight sections of concentric inner and outer pipes supporting said inner and outer pipe fittings in a separate and spaced-apart relationship and defining an unobstructed space throughout the annulus between said inner and outer pipe fittings, said curved sections being formed from separate inner and outer elbow fittings occupying a sector of a circle having the same angle, said separate curved sections being concentric throughout their curvature defining a substantially uniform spacing between said curved sections throughout their curvature. 2. The double containment pipe assembly of claim 1 wherein one of said inner and outer pipe fittings of said elbow has a straight portion extending from one end face thereof. 3. The double containment pipe assembly of claim 2 wherein both end faces of said one pipe fittings include a straight portion. 4. The double containment pipe assembly of claim 2 wherein both said inner and outer pipe fittings of said elbow include a straight portion. 5. The double containment pipe assembly of claim 4 wherein the straight portions on the inner pipe and outer pipe are on opposite ends of said elbow. 6. The double containment pipe assembly of claim 5 wherein the end faces of at least one of said inner and outer elbow fittings includes a socket member for receiving a straight length of pipe. 7. An elbow fitting for a double containment pipe assembly having at least one inner section and at least one outer section of straight pipe, comprising an outer fitting defining a curved section, and a separate inner fitting defining a curved section and located within the outer fitting and defining an annulus between the inner and outer fittings, each fitting being coupled to a respective section of straight pipe and defining an unobstructed space throughout the annulus between the inner and outer fittings to prevent any contact or transmission of forces between the inner and outer fittings when there is differential thermal expansion, the curved section of the inner fitting being concentric with the curved section of the outer fitting forming a substantially uniform spacing between the curved sections of the inner and outer fittings. 8. An elbow fitting as defined in claim 7, wherein at least one of the inner and outer fittings includes a straight portion extending from at least one end of the fitting for coupling to a section of straight pipe. 9. An elbow fitting as defined in claim 8, wherein the inner fitting includes a straight portion on one end and the outer fitting includes a straight portion on the other end relative to the straight portion of the inner fitting. 10. An elbow fitting as defined in claim 7, wherein at least one of the inner and outer elbow fittings includes a socket member on one end for coupling to a section of straight pipe.

Inventors: ZIU CHRISTOPHER G, ZIU, CHRISTOPHER, G.
Applicants: DOUBLE CONTAINMENT SYSTEMS
Application year: 1992
Main classification: F16L9
IPC codes: F16L39