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Insulation System Design for Resisting CUI

WEDNESDAY, MAY 9, 2018

By Gary Whittaker, PE, Valentus Specialty Chemicals


Corrosion under insulation (CUI) of metallic piping and equipment has been a problem in the chemical industry for as long as thermal insulation and fire proofing materials have been in use. In simple terms, CUI occurs when a corrosive liquid — most often water — is present on the surface of an insulated metallic component.

Both the American Petroleum Institute (API) and the National Association of Corrosion Engineers (NACE) have published standards that define the CUI temperature range for aqueous corrosion as 25 to 350 degrees Fahrenheit. There is not a single solution to preventing CUI; prevention requires a systems designer to consider the nature of the substrate, the process operating temperatures and the external environment. Each of these factors influences the selection of insulation type, jacket (or lagging) type and protective coating type.

The complexity of the equipment to be insulated is also an important factor. The more nozzles or supports penetrating the insulation, the more opportunity there is for moisture and process fluid leaks to penetrate it as well. Thermal insulation is not an install-and-forget system. Good design is critical, but maintenance is equally important. The best systems will not perform to maximum potential if not properly maintained.

When designing a new system, the first question is, What is the operating temperature range? In the case of continuously operating systems, the temperature will be the same for most of the operating time, but temperature excursions always occur. In batch systems, the temperature is typically not continuous and cycles among multiple temperatures.

Once the design temperatures are established, the next question is, Why are we insulating? In most cases, the answer is to save energy, but in other cases, insulating may be for process stability, personnel protection or a variety of other reasons. In the case of lower operating temperatures, the reasons always include energy conservation but also include ice and condensation control. Temperature matters because it has a large influence on the insulation material selected and is the primary determinant of insulation thickness. 

These first two questions address factors directly related to the asset being insulated, but the answers are not sufficient by themselves to make complete design decisions. Equally important is the operating environment. In addition to the reason for insulating and the operating temperature range, the ambient temperature conditions, wind speed and type of materials to be used all factor into the choice of insulation thickness.

The ambient environment plays a major role in CUI. Seacoast environments with high humidity and airborne chloride levels or heavy industrial environments are more likely to produce CUI than rural or desert locations. In environments that tend to be corrosive, both insulation and jacket must be especially resistant to corrosion.

Closed-cell materials are good choices for corrosive environments. When in good condition, these materials do not absorb liquid, maintaining a dry barrier between leaks and the surface of the pipe or equipment. A commonly used closed-cell form of insulation is cellular glass. Unlike fibrous materials, such as fiberglass blanket or mineral fiber, cellular glass provides long-term resistance to liquid absorption.

Figure 1 shows corrosion that occurred beneath fiberglass blanket insulation at an insulation support ring. The granular material used - calcium silicate - is also not resistant to moisture absorption. Both the fibrous and granular materials have many desirable insulation properties and can be used in environments where CUI is possible, but only if proper design precautions are taken.

Figure 1. CUI at an insulation support ring. Note the remnants of fiberglass, adhered to the surface above the ring, and the complete absence of corrosion below the ring where water could not collect.

When designing a CUI-resistant system, even when chosen insulation materials are resistant to liquid absorption, it is important to choose a substrate coating system that also resists CUI. The American Petroleum Institute publishes recommended practice API 583, “Corrosion Under Insulation and Fireproofing,” which provides information on the causes and mitigation of CUI. The National Association of Corrosion Engineers publishes SP0198, “Control of Corrosion Under Insulation and Fireproofing,” which provides information on the use of coatings to prevent CUI. In both documents, coatings play a key role in preventing CUI.

After the insulation and coating selections are made, the system must be properly installed and — equally important — properly maintained. Insulation jacket systems deteriorate over time for a variety of reasons:

  • The most common source of damage for piping systems is walking on them.
  • Damage is also caused when insulation is removed for equipment maintenance and not replaced, leading to moisture intrusion.
  • Flange and valve leaks are the usual sources of process chemical contamination. It is essential that the insulation be properly repaired when damage of this sort occurs, or CUI is likely to follow.

Figure 2 shows a low-temperature column with missing jacket that had not been replaced. Note, also, that in this case no vapor barrier was installed between the cellular glass insulation and the jacket, which contributed to the complete failure of the system. The use of an appropriate insulation standard and proper inspection at the time of the original installation would have prevented this very costly failure. Re-insulation of a column like this is a multi-million-dollar project.

Figure 2. Distillation column showing unrepaired jacket damage and missing vapor barrier.

Proper design and installation of the insulation jacket and securement system is critically important to the proper function and corrosion resistance of all insulation systems. Most producer companies in the chemical process industry (CPI) have a set of insulation engineering standards that control how insulation is installed on their equipment. In 1993, an organization called The Process Industry Practices (PIP) was founded by a group of chemical companies and their engineering contractors to create a comprehensive set of common engineering standards that are used throughout the CPI. PIP today has over 120 members, including most major chemical producers and engineering companies operating in North America.

Included in the PIP standards are a comprehensive set of insulation standards that can be used to specify all aspects of an insulation installation. Perhaps the most important part of the PIP insulation standards, from a CUI perspective, is the installation details. The details are drawings and notes that describe exactly how the insulation system should be installed. Included are details describing jacket installation to ensure proper water shedding and the use of flashing around penetrations to prevent water ingress.

The use of and adherence to the requirements of a standards system like PIP are essential to a successful insulation installation. Equally important is inspection of the insulation project, while in progress, by a qualified inspector to ensure that the requirements of the standards are being met. The PIP insulation practices include an insulation inspection checklist that helps the inspector make a complete inspection. The column seen in Figure 2 was insulated before the PIP practices were available.

Project Example

After many years of operation, a carbon steel distillation column was de-insulated and found to have CUI damage at the insulation support rings (Figure 1). The column was prepared and coated with the Valentus VSC1100 and VSC1200 system while it remained in operation. The operating temperature was at the upper end of the temperature range in which this system could be used to control CUI, so special attention was paid to designing an insulation system that would provide maximum resistance to moisture intrusion.

First, because of the complex nature of this distillation column, a flexible, moisture-absorption-resistant insulation — Aspen Pyrogel — was chosen. It was not only easier and less costly to use than rigid closed-cell materials but provided almost equivalent absorption resistance.

For the jacket, an elastomeric sheet material made by Ulva Insulation Systems was selected. Ulvashield is secured adhesively rather than mechanically, as is done with more rigid jacket materials like aluminum or stainless-steel sheet metal. The advantage of adhesive attachment is that the jacket is sealed to every penetration and at every seam. This seal, combined with the flexibility of the elastomer, provides a jacket that blocks entry of all external moisture. While more costly than conventional jacket systems, Ulvashield made sense in this case because of the known history of CUI and the great difficulty and cost associated with working on a column that could not be taken out of service. It offered the highest probability of long-term success to support the coating system.

Following the Ulvashield installation guide was critical, and the manufacturer was available, as needed, to provide field support during installation. This is highly recommended if the installer does not have previous Ulvashield experience. Figure 3 shows the Ulvashield jacket installed on the top of the column.

Figure 3. Ulvashield jacket installed on top head of column. Note adhesive seal of jacket to top platform supports where they penetrate the jacket. This seal will prevent future moisture penetration.

Conclusion

Controlling CUI requires a systems approach. A suitable coating must be chosen to protect the substrate. Equally important is the design of the insulation system. Appropriate absorption-resistant insulation should be chosen when there is a significant risk of CUI, and it should be installed in compliance with the requirements of a recognized insulation standard such as the PIP insulation practices.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

ABOUT THE AUTHOR
Gary Whittaker, PE, Valentus Specialty Chemicals

Gary Whittaker retired in February 2018 from his roles as an Eastman Engineering fellow and a fellow of the Materials Technology Institute in the chemical process industry and is now a senior fellow, Materials Engineering, at Valentus Specialty Chemicals. He was employed by the Eastman Chemical Company since 1981 after earning BS and MS degrees in materials engineering at Virginia Tech. Whittaker is a licensed engineer in the state of Tennessee.

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