Many different types of insulation materials are used on above-ambient pipes and equipment in industrial facilities. It is important to understand that if the facility owner keeps water from intruding into the insulation system in the first place, the facility is not likely to suffer from CUI.
The first of these is hydrophobic (or waterproof) insulation. Several types of commercially available insulation materials have a chemical hydrophobe added in sufficient quantity to make them truly water repellent. These include perlite block and pipe, aerogel blankets, and certain designated hydrophobic microporous insulations (those specifically coated with a hydrophobe). Mineral fiber insulation materials use the same type of hydrophobe, but in lesser percentages than the other materials, and they can still absorb water. Hence, while mineral fiber insulation is somewhat water repellent, it is a wicking material and cannot really be considered water repellent in the way that the other three materials can.
The hydrophobe typically used for some of these hydrophobic insulations is an organic silicone emulsion. It can be added during material manufacturing process. In the case of aerogel insulation, the material is made hydrophobic by virtue of the manufacturing process, by which organic methyl groups are added to the inorganic silica aerogel material. In all cases, this hydrophobic treatment will remain functional apparently up to a temperature range of 400° to 600°F. In that temperature range, the organics, that make the insulation hydrophobic, start to decompose and the insulation becomes less hydrophobic. Therefore, service temperature is the major limitation of the hydrophobe within hydrophobic insulation, regardless of insulation type.
There is an ASTM test for insulation hydrophobicity after heat aging. ASTM C610, the standard for expanded perlite block and pipe material, includes a water-absorption test for material first heat aged in a 600°F oven. The maximum allowable water absorption, after subsequent immersion in water for 48 hours, is 50 percent by weight. Nevertheless, when this material does absorb water and remains wet for a prolonged period of time, the chemical bonding agent is susceptible to failure, possibly leading to physical degradation of the material. However, a major benefit of this behavior is that the bonding agent is an excellent chemical inhibitor against CUI. Expanded perlite has this advantage and hence can be considered a tool in the CUI prevention toolbox for two reasons: One is its hydrophobicity and the other is that it contains a chemical inhibitor against corrosion.
As discussed above, aerogel blanket insulation and hydrophobic microporous insulations are hydrophobic in the service temperature range where the organic material, that make the insulation hydro-phobic, does not thermally decompose. Once the organics decompose, if these insulations are later exposed to water, they will then absorb the water. Later, if dried out, thermal conductivity will have permanently increased due to damage to the tiny pores that make these types of insulation so thermally effective when new. At that point, the insulation will no longer be as effective a thermal insulation as when new. Nevertheless, these two types of insulation can be valuable tools in the CUI prevention toolbox for service temperatures from ambient to the range of 400° to 600°F. For use above that temperature range where CUI prevention is a goal, the insulation manufacturers of these materials should be consulted to ensure that conditions are avoided where excessive loss of hydrophobic treatment and subsequent absorption of water might occur.
How about the effectiveness of closed cell inorganic insulation? There is only one: cellular glass insulation. Indeed, it holds very little water due to its closed cell structure and the fact that water does not pass between the cell walls. While not exactly hydrophobic, it will not absorb water. This behavior can be an important potential contributor to preventing CUI. In “Corrosion Under Insulation: Prevention Measures” (Insulation Outlook, October 2007), Dr. Hira S. Ahluwalia recommends the use of cellular glass. However, since cellular glass is fragile, it is susceptible to vibration-induced damage and can suffer from boiling water trapped between the pipe and the insulation. Therefore, its effectiveness can be limited. Further, as with many types of insulation, the boiling of water is damaging to the cellular glass structure. One point worth noting, since stress relief cracking of cellular glass typically begins to occur at service temperatures above 450° to 500°F, the manufacturer should be consulted for the best method for insulating these systems. While cellular glass insulation has some limitations in above-ambient applications, it can be considered an effective tool against CUI for applications up to 450° to 500°F.
What about corrosion inhibitors? It was already mentioned that expanded perlite contains an excellent corrosion inhibitor. Some types of calcium silicate also contains a considerable quantity of chemical inhibitor—not as a bonding agent but as an additive specifically intended to be a chemical inhibitor and thereby to prevent CUI. If calcium silicate insulation with a chemical inhibitor absorbs water, the chemical inhibitor dissolves and inhibits against corrosion.
In general, high-compressive-strength insulation provides better resistance to external loads than low-compressive-strength insulation does. These materials provide better support for the metal jacketing, limiting its compression, denting, and opening of gaps.
Expanded perlite and calcium silicate insulations both have high compressive strengths. The compressive strength for expanded perlite, per ASTM C610, is a minimum of 60 pounds per square inch (psi). For calcium silicate, the compressive strength is a minimum of 100 psi, per ASTM C533. That is the highest value of any commercially available block and pipe insulation.
On an insulated pipe or surface that is subjected to external loads, such as foot traffic, the high compressive strengths of perlite and calcium silicate will provide extra support to the metal jacketing system, helping prevent the jacketing from “fish-mouthing” at the overlaps. Fish-mouthing of the metal jacketing will allow for rainwater intrusion. By virtue of providing better support of the metal jacketing, calcium silicate and perlite insulations are considered valuable tools for CUI prevention, with calcium silicate being the strongest material available. As mentioned above, both of these materials contain a corrosion chemical inhibitor. Furthermore, expanded perlite has been included above due to its hydrophobicity.
Summary
There really is no single “silver bullet” that will prevent CUI in all circumstances and all applications. However, there are a number of different tools that can be used, each of which brings numerous features and benefits. By combining several of these tools, the facility owner can reduce the instances of CUI to the point of prevention. This may require spending more money up front on the new facility, specifying and selecting the protective jacketing system and insulation materials more carefully and thoroughly, and spending time and money maintaining the insulation system. If it is done, however, it will reduce the overall operating cost for the facility. As always, “an ounce of prevention is worth a pound of cure.”
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