Abstract
Polyurethane rigid foam is widely used as a thermal insula tion material in low temperature service because of its low thermal conduc tivity. This paper describes the results of laboratory studies on various aspects of this application such as maintenance of insulation efficiency, corrosion protection, and friability.
Comparisons between thermal insulation systems on 3-inch pipe in -320°F service showed that a system employing slightly over one inch thickness of polyurethane rigid foam was more efficient than a system employing six inches of cellular glass. Because of the thickness-area geometry of pipe insulation, low-conductivity polyurethane rigid foam offers greater insulation advantages on small pipes than on essentially flat surfaces.
Studies of unprotected polyurethane rigid foam installed on a vessel maintained at —109°F and exposed to surroundings at 74° F and 50 percent relative humidity for a period of 200 days. The one inch thick specimens had K-factors averaging 0.21 after 200 days; thicker specimens resulted in progressively smaller K-factor increase. These results are attrib uted to an expected decrease in water vapor permeability through polymer films at low temperatures, and the formation of a frost barrier at the 32° F level within the foam.
Some conditions of service can result in corrosion of metal in contact with insulation, and such situations should be identified and handled with protective measures. When the metal was foamed in place or embedded in foam, the effect of foam on stainless steel, aluminum, galvanized iron, and copper appeared protective rather than corrosive; mild steel seemed adversely affected by foam contact. When the metal formed the faces of sandwich panels, foams poured in place tended to protect mild steel for about seven months, but did not appear to protect aluminum. When aluminum foamed in place was exposed to an environment which pro moted condensation of water at the aluminum surface where it could leach corrosive constituents from the foam, significant corrosion was observed within 3 months. A wash primer coating provided complete protection, but coal tar and modified acrylic latex coatings failed to provide any significant protection.
The friability of thermal insulation materials is a factor in their dura bility and their potential for health hazards due to dust inhalation. Fria bility tests of various materials showed that commercially available rigid cellular plastics were less friable than cellular glass.
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