Polyurethane shoes act as an all-encompassing assembly that both insulates and supports a pipeline, particularly in cryogenic applications. As a result, the shoes can solve a variety of problems that typically occur under these low temperatures. With their insulating properties, they noticeably reduce the heat transfer between the process fluid inside the pipe and the pipe’s surroundings. This improved efficiency prevents condensation and icing on the pipe, thereby avoiding corrosion problems. They also eliminate temperature fluctuation at the support point of the pipe. In the installation process, polyurethane pipe supports do not need to be welded to the pipe, thereby eliminating the need for a costly post-weld heat treatment.
Other alternatives used to insulate pipes in cryogenic applications include permali, micarta or foam glass. Choosing polyurethane foam provides many benefits as insulation in low-temperature applications. For example, the polyurethane material is relatively soft compared to the other types of insulation, so it can be molded into shapes for a custom fit around the pipeline. It also can be tailored to meet various design specifications.
To demonstrate the importance of using insulated components in cryogenic applications, researchers conducted a test on an uninsulated cryogenic pipe on a small-scale model. The objective was to determine the amount of ice formation on the pipe.
To simulate a cryogenic condition for an extended period of time, liquid nitrogen from a tank was supplied through a control valve into the pipe at one end, and the nitrogen vapor was allowed to escape through a 0.5" tube from the other end. The test facility was in a workshop area where large doors allowed cross-ventilation, maintaining atmospheric conditions. A Type K thermocouple was set between the pipe and the clamp near the liquid nitrogen inlet to measure the pipe’s outer surface temperature throughout the test. Simultaneously, the ambient room temperature, relative humidity and dewpoints also were measured.
Liquid nitrogen was allowed to fill the stainless steel pipe completely first, followed by a steady flow of liquid nitrogen. By adjusting the control valve to a point when the liquid nitrogen ceased to exit through the vertical outlet tube, the pipe remained completely full of liquid nitrogen throughout the test. This flow process caused a small fraction of the flowing liquid nitrogen in the pipe to change phase to vapor by gaining latent heat of evaporation from the ambient air and exiting through the outlet tube. This effect maintained almost a constant pipe surface temperature over an extended period of time. Metering scales were fixed radially outward at three locations along the pipe (near the two ends and at the middle) to measure the ice formation depth as a function of time.
The variations of the ambient and pipe surface temperatures with time are shown in figure 2. In each plot, the day and night durations are shown by arrows. After the pipe was filled with liquid nitrogen, its surface temperature started falling exponentially with time. After nearly 20 hours, the pipe reached an almost steady state with small variations, following a trend similar to that of ambient temperature variations that occurred during day and night.
A Dual-Purpose SolutionThe large amount of ice that formed on the pipe during the test proves that a pipe used in a cryogenic application must be insulated either by a pre-insulated support or another form of insulation that is independent from the support. Polyurethane shoes offer advantages in both the installation and lifetime of the support. With polyurethane shoes, a company install a support that serves as both insulation and support for a pipeline.
There are some limitations to using polyurethane shoes. For example, low-density shoes cannot be used in applications that require a relatively high load capacity. The density, thickness and length of the polyurethane foam determine load capacity. While the polyurethane material can be made to sustain heavy loads, it might not provide the necessary insulation that is required for cryogenic temperatures at extremely high loads. Polyurethane shoes also cannot sustain harsh conditions, such as extremely high temperatures or corrosive environments.
Overall, however, polyurethane shoes are effective in cryogenic applications and are efficient in the range of purposes that they serve. The shoes eliminate the need for custom fabrication to insulate the pipe and pipe support during installation. When fabricating a polyurethane pipe support, only the portion around the process pipe must be insulated. The rest of the support can be fabricated from standard carbon steel materials because these components do not see the fluctuations in temperature that occur at the pipe surface. In applications ranging from chilled water and steam water pipelines to power plants, polyurethane shoes can prevent heat exchange between the pipe and the environment and help ensure an efficient, cost-effective cryogenic cooling operation.