Patent Publication Number: US-6216745-B1

Title: Vacuum insulated pipe

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to insulated piping, and, more particularly, to prefabricated vacuum-insulated pipe sections and a method of joining them. 
     Thermally insulated pipes have a wide variety of industrial applications. For example, insulated piping is used to transport cryogenic liquids, that is, liquids having a boiling point at a temperature below −150° F. at atmospheric pressure, between storage tanks or between a storage tank and a use device. Other examples include utilization in chemical plants or petroleum refining. 
     A major source of thermal leakage in insulated pipe systems is at the junction between two pipe sections. It is therefore preferable to eliminate such pipe joints. This would require, however, that the insulating piping be custom-manufactured to fulfill the length requirements of specific applications. Such an arrangement would be prohibitively expensive. As a result, efforts have been directed towards developing prefabricated insulated pipe sections that may be connected with joints that suffer from minimal thermal leakage. An appropriate number of such sections may be delivered and joined in the field to create efficient piping systems of desired lengths. 
     One type of prefabricated insulated pipe features sections constructed of copper pipe surrounded by foam insulation. The foam insulation is surrounded and protected by a PVC plastic casing. Such foam preinsulated pipe is offered under the names X-50 by Process and Cryogenic Services, Inc. of San Jose, Calif. and INSULTEK by Insultek Piping Systems of Manlius, N.Y. 
     Sections of foam preinsulated pipe are typically joined by brazing or butt-welding. Alternatively, the ends of the sections may be threaded. Foam insulation is next wrapped, placed, foamed or fitted around the joint. A “clam shell” is then placed over the foam insulation so as to protect it and secure it in place. The claim shell is typically constructed of stamped thin metal or molded plastic and features two halves that are riveted, glued or otherwise joined together when the clam shell is in position. 
     While the above foam preinsulated pipe system offers the advantage of low cost, higher performance levels are required by a number of present-day applications. The efficiency of the foam preinsulated pipe is typically 1435 BTU/hr overall heat loss per 100 feet of 1 inch pipe when liquid nitrogen is carried. In addition, greater service life is desired. Foam preinsulated pipe often needs extensive repair or replacement after a short service life to maintain insulating performance at original levels. 
     Another type of prefabricated pipe features vacuum-insulated sections. This system is offered by MVE, Inc. of New Prague, Minn. under the name VIP. A VIP pipe section is constructed of an inner pipe surrounded by an outer pipe. The inner pipe and outer pipe are concentrically positioned so that an annular insulation space is formed therebetween. The ends of the outer pipe are welded to the inner pipe so that the annular space is sealed. Either the inner pipe or the outer pipe is provided with a bellows in its middle portion so that the integrity of the welds is preserved when the pipes expand or contract by differing amounts due to temperature changes. 
     The insulation space of the VIP pipe is evacuated and filled with a multiplicity of layers of thin fabric formed of fine fibers of glass, cellulose or other fibrous material characterized by low heat conductivity. Positioned between the thin fabric layers are reflective barrier sheets formed of highly reflective material, such as a aluminum foil. When utilized in cryogenic liquid applications, the insulation arrangement of VIP pipe minimizes conduction and radiation heat gain to the cryogen within the inner pipes. 
     While VIP pipe sections may be connected in a manner similar to foam-insulated pipes, that is, with insulated clam shells, they are optimally joined by a bayonet connector arrangement such as is disclosed in U.S. Pat. No. 4,491,347 to Gustafson. The &#39;347 patent discloses pipe sections that have interfitting metal male and female end portions. The material of the female portion has a coefficient of expansion that is greater than that of the male portion. As a result, when cryogenic liquid flows through the pipe sections, the female portion contracts into sealing relationship with the male portion. 
     Vacuum-insulated pipe such as the VIP is at the opposite end of the spectrum from foam preinsulated pipe in terms of both performance and cost. More specifically, the efficiency of the VIP system is much higher, with an overall heat loss of only 160 BTU/hr for 100 feet of 1 inch pipe when liquid nitrogen is carried. In addition, vacuum-insulated pipes such as the VIP typically retain their original insulating capabilities for fifteen to twenty years. The complexity of vacuum-insulated pipe with bayoneted ends, however, makes piping systems such as the VIP expensive to produce. 
     A further disadvantage associated with existing vacuum-insulated piping is that when the expansion/contraction bellows comprises a portion of the outer pipe, it is exposed to the weather and may otherwise be damaged. Alternatively, if the bellows comprises a portion of the inner pipe, it is difficult to repair and replace. 
     Accordingly, it is an object of the present invention to provide an insulated pipe that may be produced and installed in prefabricated sections. 
     It is another object of the present invention to provide an insulated pipe that offers high insulative efficiency at a reasonable cost. 
     It is another object of the present invention to provide an insulated pipe that has a long service life. 
     It is still another object of the present invention to provide an insulated pipe that is durable and that may be easily repaired. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an insulated piping system for transferring temperature-sensitive fluids such as cryogenic liquids. The system consists of prefabricated sections of pipe. Each section of pipe includes an inner pipe and an outer pipe concentrically positioned about the inner pipe. The end portions of the inner and outer pipes are joined by a pair of bellow members so that an annular space is defined therebetween. The annular space is partially evacuated. Insulating material covers a portion of the inner pipe. Preferably, this insulating material is wrapped in layers about the inner pipe with inert insulating granules placed between the layers of insulation. A number of spacers may be attached to the insulation. The outer pipe features a port to permit evacuation of the annular space in the field. 
     The inner pipes of the prefabricated pipe sections are joined by brazing or butt-welding. Alternatively, the inner pipes may be joined via threaded ends. The transition bellow members on adjacent pipe sections and the joint are covered by a clam shell that is filled with insulation material. This protects the bellow members and their welds from weather or other sources of damage. In addition, the insulation within the clam shell is protected from damage due to humidity. 
     The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional side elevation showing a section of the vacuum insulated pipe of the present invention with the insulation material on the inner pipe partially broken away; 
     FIG. 2 is a partially broken away side elevation of one of the transition elements of the insulated pipe of FIG. 1; 
     FIG. 3 is a partial sectional side elevation view of two sections of vacuum insulated pipe of the type shown in FIG. 1 joined in accordance with the present invention; 
     FIG. 4 is an exploded perspective view of the clam shell housing of the joint of FIG. 3 prior to installation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a section of insulated pipe constructed in accordance with the present invention is indicated generally at  10 . The section features concentrically positioned inner pipe  12  and outer pipe  14  which are constructed with materials and thicknesses tailored to a given application. Examples of suitable materials include, but are not limited to, ferrous or non-ferrous metals such as copper or stainless steel. The outer pipe typically features a wall that is thicker than the inner pipe to provide for additional resistance to corrosion and vacuum support. 
     A pair of bellow members  16  and  18  are connected between the inner and outer pipes. Bellow members  16  and  18  may be constructed of metal. As shown in FIG. 2, each bellow member features a pair of non-corrugated end portions  22  and  24  and a corrugated middle portion  26 . The diameter of one of the end portions  22  is sized to be slightly smaller than that of the outer pipe  14  while the other end portion  24  features a diameter that is slightly larger than that of the inner pipe  12 . As shown in FIG. 1, the bellow members are welded to the end portions of the inner and outer pipes so that a sealed annular space  28  is defined therebetween. 
     In addition to sealing the annular space, the bellow members perform two other functions. First, they allow for independent expansion and contraction of the inner pipe relative to the outer pipe without disturbing the integrity of the welds. For example, the inner pipe may contract extensively compared to the outer pipe when cryogenic liquid (at a temperature below −150° F.) passes through it. Under such circumstances, each bellow member would contract by a corresponding amount so that the resulting stresses in the welds and on the outer pipe are minimal. The placement of the bellow members between the inner and outer pipes allows each section of pipe to handle thermal expansion and contraction for fluids in the range of −320° F. to +320° F. 
     The other important function of the bellow members is that their corrugated portions lengthen the path of heat transfer between the inner and outer pipes. This increases the insulating efficiency of the pipe segment as heat transfer between the inner and outer pipes is significantly reduced. As a result, heat originating from the exposure of the outer pipe to ambient conditions is not efficiently transferred to the cryogenic liquid flowing through the inner pipe. Conversely, heat from a hot liquid flowing through the inner pipe is not easily transferred through the bellow members to the outer pipe. 
     As shown in FIG. 1, a portion of the inner pipe  12  is covered with insulation material  29 . For example, the inner pipe may be wrapped with multiple layers of a blanket insulation. Alternatively, the inner pipe may be wrapped with super-insulation consisting of alternating layers of thin fabric and reflective material. In either instance, it is preferable to sift in AEROGEL, or similar inert insulating granules, indicated at  33  in the broken away portion of insulation  29  in FIG. 1, between the layers to provide for enhanced insulating performance at poor vacuum levels. The volume of insulating granules as compared to the volume of insulation can be varied, depending upon the application of the pipe, from 1% to 75%. 
     As illustrated in FIG. 1, low conductivity spacers  32  are clipped or glued onto the insulation for longer pipe sections (over approximately six feet). These spacers may be constructed of materials such as ceramic or fiberglass. 
     As the final step in manufacturing the pipe segment  10 , air is evacuated through capped port  30  so that the annular space  28  features a vacuum-like pressure. Alternatively, a hole may be drilled in the outer pipe. Once the air is evacuated through the hole, it may be sealed with an O-ring plug. The capped port  30  or the plugged hole allow the annular space of the pipe to be re-evacuated in the field. That is, the pipe is field-serviceable. By increasing the amount of AEROGEL between the insulation layers, the vacuum requirements may be less stringent. Reducing the vacuum requirements of the pipe provides the benefits of reduced manufacturing costs and increased service life. 
     FIG. 3 illustrates how two sections of pipe, such as the one shown in FIG. 1, may be efficiently joined in accordance with the present invention. Opposing ends of adjacent pipe sections  40  and  42  are placed in abutting relation and are brazed, butt-welded or otherwise joined together to form joint  47 . Alternatively, the pipe sections may be provided with threaded ends that engage. 
     Once the pipe sections are joined, a clam shell, indicated in general at  50  in FIG.  4  and in phantom in FIG. 3, is used to cover the joint and the bellow member of each pipe. The clam shell is constructed of molded plastic or a stamped thin metal and features a female half  52  and a male half  54  that engage. Once engaged, the halves may be secured together by rivets or adhesive. The clam shell is joined to the outer pipes of sections  40  and  42 , just beyond their bellow members, by a silicon sealant and adhesive or similar substance. It should be noted that while a clam shell is illustrated, alternative covers that are known in the art could be used. 
     The assembled clam shell is then filled with an insulation such as foam, fiber, glass-foam or powder through port  60 . The choice of insulating material depends upon the application of the piping system. For example, foam is effective except in situations where the liquid being handled is extremely cold (as with cryogenics) or extremely hot. In the event of the latter, foam will melt and degrade. Powder or fiber would perform better under such conditions. In situations where cryogenic liquid is being handled, the use of foam could present a safety hazard as the possibility exists that oxygen would be liquified against the inner pipe and remain in the foam. This could form a fire hazard. The use of rock wool or glass-foam-both inert in oxygen-produces a cryogenically-safe joint. 
     Once the clam shell is filled with the appropriate insulating material, port  60  is sealed via cap portions  62  and  64 . As an alternative to port  60 , a hole may be placed through the clam shell and later sealed after the insulation is inserted. As yet another alternative to enhance performance, blanket insulation could be wrapped around the joint prior to installation of the clam shell. The clam shell may be lined with metal foil or plastic prior to assembly to provide a vapor barrier  65  for the insulation. 
     The placement of the joint and the bellow members within the clam shell offers a number of benefits. The expansion and contraction of the bellow members takes place within the clam shell. Movement of the outer pipes with respect to the clam shell is minimized so that the sealing of the clam shell is maintained. This prevents the leakage of water or other liquids into the interior of the clam shell, the occurrence of which may severely degrade the performance of the insulation. 
     The bellow members, and the welds connecting them to the inner and outer pipes, are also protected by the clam shell from weather or damage from another source. In addition, the clam shell itself will be protected from damage that could occur if the outer pipe expanded and contracted excessively. 
     It has been found that the performance of the piping system of the present invention is superior to traditional foam-insulated piping in terms of insulating capability and service life. More specifically, the thermal conductivity of the insulation of the piping of the present invention may be up to fourteen times lower than foam. As a result, the heat loss for the piping system of the present invention is typically 255 BTU/hr overall loss per 100 feet of 1 inch pipe (as compared to 1435 BTU/hr for foam-insulated pipe) when liquid nitrogen is carried. 
     One implication of the improved performance of the pipe of the present invention is that much longer unrefrigerated (or unheated) piping runs are possible. The increased efficiency also allows the pipe of the present invention to feature a smaller cross section than a comparable foam-insulated pipe. More specifically, a foam-insulated pipe typically has more than a six inch diameter while an insulated pipe with better performance constructed in accordance with the present invention typically has a two inch diameter. 
     The service life of the piping system of the present invention is significantly higher than that of foam-insulated systems. In fact, it is estimated that the service life of the piping of the present invention may be three to five times as long. 
     The piping system of the present invention offers all of the above advantages at a cost that is significantly lower than that of traditional vacuum-pipe systems that are joined via bayoneted ends or welded and evacuated field joints. 
     It should be noted that while piping with a round cross section is illustrated in the figures, the present invention could be applied to piping systems with alternative cross section shapes (such as square, oval, etc.) 
     While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.