Abstract:
The present invention relates generally to an apparatus and method for preventing disbondment in an insulated piping system that is used for conveying high temperature fluids. More specifically, an external slip wrap is shown, capable of surrounding the outer protective jacket of the insulated piping system at a location along the piping before an elbow shaped or angular change in direction. The slip wrap comprises a loosely received outer sleeve which surrounds the outer protective jacket of the piping without being bonded thereto, thereby allowing the insulated and jacketed pipe to move axially relative to the slip wrap for a selected distance once the pipe is buried in the ground.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to an apparatus and method for preventing disbondment in a bonded foam insulated piping system of the type used for conveying high temperature fluids. 
         [0003]    2. Description of the Prior Art 
         [0004]    Insulated pipelines are needed in a variety of situations. For example, distributed HVAC (heating, ventilation, and air conditioning) applications utilize chilled water for cooling and steam for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam are distributed to other locations. For example, on a school campus the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings. A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulted pipelines is used to carry the steam from the boiler to the other locations and back to the boiler. Oftentimes, the temperature inside the pipe is either higher or lower than the ambient temperature surrounding the pipe. It is necessary for the pipes to be insulated in order to retain the internal temperature of the fluids and keep heating and cooling losses at a minimum. The insulated pipelines are usually located underground. 
         [0005]    Insulated pipe of the type under consideration is conventional and commercially available. There are predominately two types of piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate bonded foam systems. The present application is directed toward the bonded foam type system. These systems utilize a steel pipe to convey fluid, and often the fluid is a different temperature as compared to the ambient environment. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyisocyanurate foam. In the case of high temperature piping systems, the insulating foam serves to keep heat loss from the starting location of the pipeline to the ending location at a minimum. Around the outside of the foam is a thin jacket of thermoplastic material, such as high density polyethylene (HDPE). The plastic jacket protects the foam from mechanical damage and also provides a watertight seal to prevent corrosion of the steel pipe. Although steel is commonly used for the inner pipe which carries the media to be piped, copper, aluminum or other metals as well as fiberglass, PVC, and similar materials may be utilized, as well. 
         [0006]    The most important engineering criteria for a foam system of the type under consideration is that it must be treated as a bonded system. In other words, the foam is bonded to both the carrier pipe and the outer jacket. In such a case, the bonded system acts as a monolithic unit moving underground. Higher temperatures can act adversely upon the bonded foam system, however. The hot fluid in the steel carrier pipe causes the carrier pipe to thermally expand. At temperatures of 400° F. this expansion is on the order of 2.8 inches per 100 feet of pipe. This expansion is not a problem as long as the system remains bonded and the carrier pipe, foam and jacket move together as one unit underground. This movement is controlled by the expansion force of the steel carrier pipe, but it is the bond strength of the foam to the pipe and jacket that is important in keeping the system moving together. This monolithic movement of the system occurs along each incremental length of a particular run, and as long as total movement is not greater than 4 to 6 inches and the system remains bonded, no undue stress is subjected at any one point of the jacket. If the system were to disbond, however, the surrounding soil would fix the jacket in place and the carrier pipe would still thermally expand thereby pushing through and destroying the jacket at the first change of direction. 
         [0007]    Generally speaking, the proper choice of insulating materials can counteract many of the thermal expansion effects discussed above. It has been well established by industry case history that the polyurethane foam bond for systems running at 250° F. to 300° F. is strong enough to keep the entire system acting as a bonded system. However, for systems running above these temperatures a higher temperature rated foam, such as polyisocyanurate foam, is generally required. Even in systems utilizing “high temperature” polyisocyanurate foam, the higher heat can, in some instances, begin to fry the foam at the foam/carrier pipe interface, thereby bringing into question the strength of the foam bond to the steel carrier pipe. 
         [0008]    Various approaches have been taken to control this undesirable expansion in insulated pipe systems of the type under consideration. For example, expansion “bolster” materials are supplied in the form of resilient pads which can be used at elbows or expansion loops. These pads are placed adjacent to the piping and create a cushion which acts as a stress relief area in critical areas, such as angles and elbows. 
         [0009]    Despite the advances seen in the high temperature piping industry, a need continues to exist for improved systems for preventing disbondment in bonded foam insulated piping systems. 
         [0010]    A need also exists for such an improved system which utilizes many of the conventionally available materials and manufacturing techniques commonly used in the industry. 
         [0011]    A need also exists for such a system which is simple in design and economical to implement. 
       SUMMARY OF THE INVENTION 
       [0012]    The method and apparatus of the present invention provide an improved insulated piping system for conveying high temperature fluids. The insulated piping system has a first and second length of insulated and jacketed pipe, each having a joining end to be joined to an end of the other length, and each pipe length comprises an inner carrier pipe having an interior surface and an exterior surface. An envelope of foamed insulation surrounds the inner pipe exterior surface, and an outer protective jacket surrounds the envelope of insulation. The joining ends of adjacent pipe lengths are welded together to form fixed joints, whereby the adjacent pipe lengths provide a continuous length of fluid conduit for conveying high temperature fluids. 
         [0013]    In addition, a discrete length of an external slip wrap is placed at a selected location along the length of the piping system, generally in a location at which the piping encounters an angular change of direction, such as at an elbow or expansion loop. The slip wrap comprises a loosely received outer sleeve for the piping which surrounds the outer protective jacket without being bonded thereto, whereby the insulated and jacketed pipe can move axially relative to the slip wrap for a selected distance once the pipe is buried in the ground. Preferably, the external slip wrap is a thin sleeve formed of a flexible plastic type material, such as a polyolefin material having characteristic coefficient of friction which allows the jacketed pipe to slide within the sleeve. In one preferred form of the invention, the external slip wrap may be formed of polyethylene. 
         [0014]    The location of the slip wrap along the length of piping is selected to in order to prevent disbondment of the foam insulation from the inner carrier pipe. Prevention of disbondment is possible by allowing relative movement of the pipeline relative to the surrounding earth, thereby eliminating the separation of the envelope of foamed insulation from the exterior surface of the inner metal pipes as the temperature of the inner metal pipes increase and the pipeline expands. In a typical case, the lengths of insulated piping are part of a pipeline conveying steam, hot water or other hot fluids at a temperature in the range of above about 200° F. 
         [0015]    Additional objects, features and advantages will be apparent in the written description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a simplified representation of a typical distributed HVAC system utilizing chilled water for cooling and steam for heating. 
           [0017]      FIG. 2  is a schematic representation of an expansion loop in a pre-insulated pipeline prior to thermal expansion. 
           [0018]      FIG. 3  is a schematic view of the pipeline of  FIG. 2  under the influence of thermal expansion forces. 
           [0019]      FIG. 4  is a simplified side view of a bonded foam insulated pipe showing the inner carrier pipe, surrounding layer of foam, and outer polyolefin jacket and showing the external slip wrap of the present invention surrounding the polyolefin jacket. 
           [0020]      FIG. 5  is an end view, partly in section, of an underground bonded foam insulated pipe showing the external slip wrap of the present invention surrounding the pipe at a selected location along the length thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Turning first to  FIGS. 1-3 , there is illustrated a typical environment in which the pre-insulated piping systems of the invention might be employed.  FIG. 1  shows a school campus having a number of isolated buildings  31 ,  33  connected by an underground insulated pipeline  35  carrying steam which at points includes right angle loops or elbows  37 . The U-shaped bend  37  is provided for the purpose of allowing the pipe to expand and contract without producing an unacceptable level of stress in the pipe, pipe fittings, or attachment points for the pipe. 
         [0022]      FIGS. 2 and 3  are schematic views of the standard piping installation of the type under consideration designated generally as  39 . The installation  39  includes a number of coaxially oriented lengths of pipe, such as length  55  (shown broken away in  FIG. 2 ). The installation may also include a number of angled fittings such as the right angle elbows (generally shown as  41 ) in  FIG. 2 . Each length of pipe includes an inner pipe  11 , typically formed of steel, an envelope of foamed insulation  15  surrounding the inner pipe and outer protective jacket  13  surrounding the envelope of insulation. The joining ends of adjacent pipe lengths are affixed, as by being welded together, to form fixed joints, whereby the adjacent pipe lengths provide a continuous fluid conduit for conveying high temperature fluids. The jacket  13  ( FIG. 1 ) is typically formed of high density polyethylene (HDPE) or a similar polyolefin type material. The following references, among others, teach the manufacture of such prior art systems: U.S. Pat. No. 3,793,411; U.S. Pat. No. 4,084,842; and U.S. Pat. No. 4,221,405, all to Stonitsch et al. 
         [0023]    The piping systems of the type illustrated in  FIGS. 2 and 3  are sometimes utilized to convey fluids at high temperature and/or pressures. For example, a typical steam line might be conveying fluid at, for example, 400° F. The temperature differentials which exists between the piping system materials and the fluid being conveyed can cause a force (“F” in  FIG. 2 ) to be applied along the coaxially aligned pipes lengths. As mentioned earlier, these U-shaped bends  39  in a piping system are provided for the purpose of allowing the pipe to expand and contract without producing an unacceptable level of stress in the pipe, pipe fittings, or attachment points for the pipe. However, the greatest amount of stress in the pipe is located just prior to the first angle in the expansion loop, shown generally at  43  in  FIG. 3 . 
         [0024]    In the piping system illustrated in  FIG. 2 , the longitudinal runs of pipe in the system are displacing as a unit and moving axially in the surrounding soil. This movement does not damage the jacketing or the foam of the system because they are both incrementally being pulled along throughout the entire length of the straight run. Because of this monolithic movement no one individual section of the jacket is over stressed and thereby ruptured, and no one individual section of the foam is required to support the entire force of the thermal expansion of the pipe. The bond distributes these forces along each incremental length of the entire run. It will be understood, however, that should the forces become great enough, disbondment of the foam from the carrier pipe can occur. If the carrier is free to move independently from the foam and jacket (disbondment has occurred) then the surrounding soil will fix the jacket in place and the carrier pipe will burst through the foam and jacket in areas shown generally as  43  in  FIG. 3 . Failure of the surrounding insulated layers allows water or other contaminants to contact the steel pipe, leading to increased corrosion and joint failure in some cases. 
         [0025]    The present invention is intended to provide a solution for possible disbondment problems for foam bonded piping systems that are operating at temperatures generally above about 200° F. At temperatures that begin to exceed 250° F., foams have been developed that are stable structurally to handle these higher temperatures, but the bond strength of the foams at these temperatures may come into question. The invention is intended to prevent the potential problems that might occur if the foam bond strength is not sufficient to cause the systems to expand as one monolithic item. 
         [0026]    The reference in this discussion to pipe “lengths” is intended to refer to standard available factory pre-insulated piping of the type previously described having an inner metal pipe surrounded by an envelope of foamed insulation, which in turn, is contained within a polyolefin jacket. As referred to briefly above, typical commercial practice involves the use of steel, copper, aluminum or alloy conveying pipes, open or closed cell polyurethane, polyisocyanurate, polystyrene or the like, foamed rigid insulation and polypropylene, polybutylene, polyethylene, polyvinylchloride and similar protective jackets. 
         [0027]    The present invention is an improvement to presently available pre-insulated piping of the type which is commercially available and familiar to those in the relevant industries. Prior art pipe lengths of this general type are commercially available as standard factory type product. For example, such products are available from Thermacor Process, LP of Fort Worth, Tex., assignee of the present invention. One typical example is sold commercially as the HT-406 High Temp Steel Piping System. The published specifications for systems are as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 Carrier Pipe- 
                   
               
               
                 diameter less than about 2″ 
                 A53 ERW Grade B, Std. Wt. 
               
               
                   
                 Black Steel 
               
               
                 diameter greater than about 2″ 
                 A106 SML, Std. Wt. Black 
               
               
                   
                 Steel 
               
               
                 HDPE Jacket- 
               
               
                 Compatible with ASTM D3350 
               
               
                 Specific Gravity (ASTM D792) 
                 0.941 min. 
               
               
                 Tensile Strength (ASTM D638) 
                 3100 psi min. 
               
               
                 Elongation Ultimate (ASTM D638) 
                 400% min. 
               
               
                 Compressive Strength (ASTM 
                 2700 psi min. 
               
               
                 D695) 
               
               
                 Impact Strength (ASTM D256) 
                 2.0 ft. lb/in. North Min. 
               
               
                 Rockwell Hardness (ASTM D785) 
                 D60 (Shore) min. 
               
               
                 Polyisocyanurate Insulation- 
               
               
                 Density 
                 &gt;2.4 lbs/ft 3   
               
               
                 “K” Factor 
                 ≦0.14 @ 70° F., ≦0.24 @ 406° F. 
               
               
                 Compressive Strength 
                 &gt;30 psi 
               
               
                 Closed Cell Content 
                 ≧90% 
               
               
                 Minimum Thickness 
                 ≧2.5″ @ 366° F., ≧3.0″ @ 406° F. 
               
               
                   
               
             
          
         
       
     
         [0028]    The present invention addresses the problem of foam disbondment by helping insure that the inner carrier pipe and outer layer of bonded foam continue to move as a unit as the inner pipe expands. This object is accomplished by providing an “external slip wrap” which surrounds the outer protective jacket of the piping system at selected locations. The external slip wrap is a sleeve formed of a flexible polyolefin material having a desired characteristic coefficient of friction. Since the external wrap is not bonded to the protective jacket, the insulated and jacketed pipe can move axially relative to the slip wrap in the earth for a selected distance once the pipe is buried in the ground. It is important to note, the external slip wrap is not intended to further insulate or waterproof the piping system, as that is already handled by the foam and outer protective jacket respectively. Instead, the function of the external slip wrap is to allow movement of the insulated pipe by providing a slidable environment that normally would not exist when the surrounding earth is holding the protective jacket in place. 
         [0029]    The external slip wrap of the invention is designated generally as  17  in  FIG. 5 . The wrap  17  is intended to be used in any coaxially aligned piping system of the type previously described and has particular application where the lengths encounter an angular bend or turn, such as the elbow  41  ( FIG. 2 ). The slip wrap  17  is particularly advantageous in countering the harmful effects of coaxial stresses which are often encountered in a “high temperature” insulated piping system. The term “high temperature” is intended to encompass any temperature above ambient which would tend to cause the type of damage to the surrounding insulating layers of the piping system discussed with respect to  FIGS. 1-3  above. Typically, such temperatures will be above about 200° F., and in some cases temperatures of 400° and higher will be encountered. 
         [0030]    The preferred external slip wrap  17  of the invention is a thin sleeve formed of a flexible material, such as a suitable polyethylene material. As shown in  FIG. 4 , it fits as a sleeve around the protective jacket  13  of the pipeline  35  and is typically installed at a location which precedes a turn in a U-shaped expansion loop  37  in the piping system. As has been described, each pipe length comprises an inner pipe  11 , an envelope of foamed insulation  15  surrounding the inner pipe and an outer protective jacket  13  surrounding the envelope of insulation. In the particular embodiment of the invention illustrated in  FIG. 5 , the surrounding foam insulation layers  15  are typically polyurethane closed cell foam insulation for systems of up to about 250° F. and polyisocyanurate foam insulation for systems above 250° F. The surrounding jacket  13  is a polyolefin, preferably HDPE. The pipe lengths  11  can be standard factory type product of the kind described above and available from Thermacor Process, LP of Fort Worth, Tex. 
         [0031]      FIG. 5  is intended to be a simplified view of what the external slip wrap would look like in position at a selected location surrounding the insulated pipeline with a layer of earth surrounding the complete assembly. Note that the force of the earth generally compresses the polyethylene sleeve  17  downwardly onto the top and bottom of the pipeline (generally at  13 ). 
         [0032]    In the particular system illustrated in  FIGS. 4 and 5 , the external slip wrap  17  surrounds the inner pipe  11  for about six feet prior to the first angle in the expansion loop in the pipe system. The greatest amount of stress is now thought to occur at the location immediately before the U-shaped expansion loop  37 , as shown by location  43 . The pressure force “F” causes the pipeline to bend inwardly, as drawn in phantom in  FIG. 3 . Disbondment of the foam insulation  15  from the inner carrier pipe  11  may occur in some situations. 
         [0033]    However, because the external slip wrap allows the insulated and jacketed pipe to move axially relative to the wrap for a selected distance once the pipe is buried in the ground, the outer jacket remains intact and the integrity of the foam insulation is not disrupted. Since the insulating layer remains intact, water or other contaminants are prevented from reaching the inner steel pipe, thereby extending the useful life of the pipeline. 
         [0034]    An invention has been provided with several advantages. The external slip wrap of the invention alleviates problems previously encountered with high temperature piping systems where elbows and other angled fittings caused the pipe to be subjected to damaging stresses. The system incorporates several existing, commercially available materials or components, thereby simplifying manufacture and assembly. The particular application of the slip wrap of the system compensates for relative movement of the inner steel pipe which could disrupt the continuity of the surrounding insulating layer at an elbow or other fitting. The coupling is simple in design and economical to implement in a variety of industrial applications.