Pumps & Systems, December 2007
Industrial processes that manufacture, transfer, and distribute a variety of liquefied natural gases, from propane to oxygen to ammonia, require components to protect both the piping system and the pipe supports from the extremely cold temperatures. Pipe supports and piping that are not insulated may become brittle and crack, posing high risk to the piping and structural components.
Additionally, when pipes that are not properly insulated carry liquefied natural gas (LNG), the pipe attracts moisture from the environment, which potentially leads to condensation, icing, or corrosion of the pipe. Utilizing an effective piping insulation is essential to preventing damage to the piping components and to protecting the pipe from the surrounding moisture.
A dual-purpose solution to both insulate and support the pipe would be to utilize a pre-insulated pipe support. These supports are fabricated with carbon steel bases and stainless steel anchors to prohibit movement of the pipeline while serving to insulate the piping with polyurethane foam insulation.
Rigid polyurethane foam functions as an ideal insulation material in pre-insulated pipe supports for cryogenic lines. Polyurethane is highly suited for pre-insulated supports because it can be tailored to meet various load requirements of varying applications. The combination of high strength to support a range of loads and low thermal conductivity to minimize temperature fluctuation at the support point of the pipe makes rigid polyurethane foam an attractive solution.
For these particular polyurethanes, thermal conductivity increases with density. As a result, the higher the foam density of the pre-insulated support, the higher the thermal conductivity will be. In effect, polyurethane pre-insulated supports are ideal for cryogenic lines because they can be customized for each pipeline's unique requirements.
One particular polyurethane pipe support that is almost entirely specialized per customer specifications is the polyurethane injection molded anchor. Users may desire to eliminate pipe movement, prevent the transfer of thermal energy between the pipe and support, and simultaneously remove gaps between existing/previous insulation and the supports.
This particular pipe support is effective in solving a wide variety of needs in a single unit. It may be the solution to just one or many issues that a user would like to resolve on the support system of the pipeline.
The injection molded anchor is a pre-insulated support that is designed and fabricated according to the pipe dimensions on which it will be installed. This advantage increases its efficiency to accurately insulate and support the pipeline.
The injection molded anchor is designed with essentially three major components: a carbon steel outer cover and base, a stainless steel inner pipe, and polyurethane foam. The outer cover and base incorporates ring stops at either end to engage the insulation and keep it from moving. Similarly, the stainless steel inner pipe also uses center rings to engage the insulation and, in turn, prohibit movement of the pipeline axially, vertically and transversely.
Customized end caps are used during the polyurethane foam injection molding procedure to correctly size the insulation and to center the inner pipe within the carbon steel base. When the end caps are removed, the molds are engineered so that the insulation is formed in offset layers to avoid a direct air path on the pipe in the field. This system of layered insulation prevents condensation, frost, and ice buildup on the cryogenic pipeline.
By using injection foam technology, a tight fit exists between the stainless steel rings of the pipe and the polyurethane foam. This technique eliminates gaps that might otherwise exist between the pipeline insulation and the pipe support. Consequently, the injection molded anchor is able to withstand a great deal of force acting upon the line since it is comprised of polyurethane foam and steel bonded into one solid unit.
When designing injection molded anchors, examination of the loads and subsequent stresses that will be acting upon the support is important. When a structure is under load, it may have areas of stress concentration. In cryogenic situations, these areas of concentrated stress become brittle and may develop cracks, which could eventually lead to catastrophic failure. Thus, selecting the proper type of steels and insulation is necessary to avoid such a failure.
The area of particular concern on an injection molded anchor is the inner surface of the outer retaining rings on the carbon steel outer cover. A finite element analysis (FEA) was performed on an anchor with a pipe size of 42-in to determine whether carbon steel is an appropriate choice of material for this structure. This particular injection molded anchor was modeled with an axial load of 600-kN, a vertical load of 175-kN, and a lateral load of 70-kN acting upon the support. The stresses in the carbon steel base and outer ring are shown in Figure 1 below.
The stresses were not significant in this main area of concern. The remaining areas of high stress revealed in the analysis were as anticipated, near the base and the center ring of the inner pipe. This is a result of significant vertical loads acting upon the injection molded anchor. However, the stress analysis did not reveal any areas of excessive stress that would cause a risk of damage to the pipeline. Thus, carbon steel used in manufacturing the outer shell and base is still an optimum material for this application.