The major factor in preventing CUI is to keep liquid from intruding into the insulation. Water decreases the effectiveness of the insulation and leads to corrosion of pipe or equipment. Poor conditions caused by wet insulation can be aggravated by weathering, vibration or abuse from people.
There are five factors in preventing CUI: (1) insulation selection; (2) equipment design; (3) protective paints and coatings; (4) weather barriers; and (5) maintenance practices. If an area is subject to spills or high humidity, special consideration must be given to selecting the insulation. Some insulations leave the system less sensitive to defects in weatherproofing or paint films because the insulations are nonabsorbent and chemically nonreactive.
Unfortunately, the insulation picked is normally based on installed costs versus energy saved, and maintenance or corrosion costs are not considered. The following should be considered:
Insulations such as calcium silicate, glass fiber and, to some extent, cellular plastic foams absorb and retain liquids and vapors. Additional flashing is required where spills, leaks or drippings may occur, or where washing and hosing are carried out. The only fully nonabsorbent insulation is cellular glass. Cellular glass should be used where corrosive or flammable liquids are present.
The proper design of insulation for pressure vessels, tanks and piping includes consideration of the support and connection of the material. Details can be found in a handbook from Midwest Insulation Contractor's Association. According to plant operators, weather barriers for insulation are frequently broken either because inappropriate details were originally given for equipment or not enough space was allotted around the insulation. Improvement in design can be accomplished by handling the insulation specifications early during the vessel design and by "simplifying" the surface to be insulated.
The coating system must protect for long periods against water or corrosives. Highly permeable coatings allow corrosion to start behind the coating even in the absence of breaks or pinholes.
An epoxy or epoxy-phenolic should be applied in two coats over an abrasive blast cleaned surface. Inspection of the surface preparation is critical at welds.
In selecting coatings, consider temperature and abrasion resistance, and a service rating for water (or corrosive-chemical) immersion. It is difficult to visually inspect coatings under insulation to find points that need touching up. Unless corrosion or insulation failure causes reinsulating an entire insulation setup, recoating is done every 10-15 years.
The weather/vapor jacket of the insulation provides the primary barrier to water. This covering is the only part of the system that can be inspected quickly and repaired economically. It must not only keep liquids out but also allow for evaporation of any liquid that manages to get into the insulation system. For weatherproofing, a rating of two perms, measured according to ASTM Standard E 398, is acceptable. Also, it should be durable, offer flame-spread resistance, and be economical. The material must be maintained periodically (usually, two to five years) to remain effective.
Further, routine maintenance is needed to catch defects due to deterioration or abuse. If the system is opened in any way for maintenance or inspection, it should be closed promptly after work is completed. One plant reported that openings made in a metal vessel's insulation for acoustic emission tests were never closed. Severe corrosion occurred.
Extensive use of a nonbreathing metallic jacket is believed to contribute greatly to corrosion of warm equipment. Without a permeable jacket, water is trapped. Water in the insulation reaches a point where it is vaporized. Vapor travels to the jacket and condenses; the cycle repeats itself.