Industrial Insulation Jacketing
Insulation is rarely installed as a stand-alone item. Instead, insulation material is part of a system that includes the insulation; the securement; a vapor barrier, in the case of low-temperature applications; and an outer layer that might be referred to as the cover, jacket or lagging.
Each component plays an important part in the overall function of the system. The insulation material itself is the primary barrier to the flow of energy, while the securement holds the insulation in place. The function of the vapor barrier is to prevent the passage of moisture into the insulation. The most multi-functional component in the insulation system is the outer covering, referred to here as the jacket. It has a variety of functions, including protecting the insulation from mechanical damage, providing support, preventing moisture penetration and establishing the system emissivity and appearance. This article will examine the high points of insulation jackets.
To a casual observer, the most obvious characteristic of an insulation system is its appearance. Appearance is dominated by the characteristics of the jacket chosen for the system. In the chemical process industry (CPI), where most insulation is located outside, exposed to the elements, metal is the most commonly used material. For a metal to be useful in this situation, it must first have sufficient corrosion resistance to withstand exposure to both the elements and to the chemicals present in a typical chemical processing environment. Aluminum and stainless steel are the most commonly used jacket materials in the CPI because, among many other desirable characteristics, they have sufficient corrosion resistance to meet a basic low-maintenance, high-durability requirement. Both are available in a range of thickness, finishes and corrugations.
Aluminum has many advantages that make it attractive as a jacket material. Its low density and excellent formability allow it to be used in thicker sections that improve its damage resistance and make it more installer friendly. Metal jacketing that is used at low thickness such as 0.010 inch is difficult for the installer to handle because of the risk of injury from sharp edges. Thinner jacket materials are used safely every day; they just require more care than thicker materials.
Aluminum’s formability gives the designer choices in surface finish that include smooth, embossed and corrugated. Each of these serves a specific purpose. A smooth surface is easily cleaned and provides what many consider to be a more attractive appearance. A disadvantage of the smooth finish is its tendency to show imperfections such as mechanical damage to the surface. The rougher appearance of the stucco-embossed finish tends to hide minor imperfections and is often chosen for that reason. The downside of the embossed finish is its greater ability to hold surface contamination.
Another significant option when choosing aluminum is corrugation. Corrugation is available in a variety of sizes and adds stiffness to the jacket material. A concern is that when corrugated metal is used on horizontal surfaces, water collects on the top surface that might subsequently penetrate the jacket or cause corrosion or fouling of the jacket material.
A disadvantage of aluminum is its poor durability when exposed to fire. Because aluminum melts at 660 C, it does not take long for an aluminum jacket that is less than one-sixteenth of an inch thick to lose its integrity. When this happens, the fire has direct access to the underlying insulation material, with potentially disastrous results.
When fire is a concern, the jacket material will often be stainless steel. Stainless steel melts at a much higher temperature and will remain in place much longer than aluminum. This greater durability protects the contents of the insulated asset and prolongs the time of exposure before a pressure relief will occur. This is a significant benefit that can be taken advantage of when designing relief systems. Other jacket materials such as galvanized steel can be used in the same way as stainless steel; however, the zinc coating on galvanized steel will most likely melt in a fire and poses a risk of liquid metal embrittlement to any nearby stainless steel surface. This is one reason that stainless steel is more often specified for fire-resistant installations. Thermoplastic jacket materials such as polyvinyl chloride (PVC) have even less resistance to fire and should never be used on equipment where fire resistance is a consideration.
When metal jacketing first came into common use it was all metal with no liner installed. With time, it was discovered that if the insulation material became wet, the metal jacket would corrode from the inside. This problem was solved by the introduction of the bonded craft paper liner. A continuous paper liner is bonded to the inside surface of the jacket to prevent corrosion of the metal by acting as a barrier between the wet insulation and the vulnerable metal jacket.
Later, the use of thermoplastic sheet material, such as surlyn, replaced the paper. The primary benefit of this approach is the improved water resistance of the surlyn plastic. The paper deteriorates when exposed to water and will eventually no longer provide an effective barrier. Surlyn does not absorb a significant amount of moisture and thus provides more effective protection against corrosion of the jacket.
Stainless steel, both 304 and 316, is readily available, has a clear advantage in corrosion resistance over aluminum and, along with its better fire resistance, is generally chosen instead of aluminum. Aluminum has a cost advantage over stainless so there must be a functional reason to choose stainless instead of aluminum if a metal jacket is to be used. Although stainless is not usually used at a thickness as great as aluminum for weight and cost reasons, it can be. Stainless is also not made with an inner corrosion barrier since it is not typically subject to corrosion caused by wet insulation.
Other Types of Jackets
Metal is not the only material used as insulation jacket; thermoplastics and fabrics are also important jacket materials. There are a wide variety of fabric materials that are used in an equally wide variety of ways. A common application is to combine fabric with a mastic material to create a jacket over a complicated shape. Often this might be a valve or fitting. The advantage of this approach is that it is easier to cover an unusually shaped item than it is with metal. A disadvantage, especially in outdoor applications, is reduced durability when compared to metal jacket. Fabrics are also used to provide the outer surfaces of flexible, removable-reusable insulation. Clearly, without fabric, these would not be possible. The type of fabric specified for these applications depends on the use temperature and the type of environment to which the insulation will be exposed.
Thermoplastic jackets are made from a variety of thermoplastic materials that include PVC and Saran, among others. Most often these materials are used for low-temperature applications. They have poor resistance to fire and should not be used in situations where there is significant risk of fire. They are used as smooth sheet material and are often selected in applications where cleanliness is important because they typically have better release properties than metal and are thus more easily cleaned. The plastics also have good resistance to a wide variety of chemicals and are not damaged by water. They are not used as commonly in industrial applications as the metals.
The jacket material plays an important role in the overall thermal performance of the system because it establishes the system emissivity. NAIMA’s computer program, 3E Plus®, contains a list of materials and their emissivity, used by the program to calculate heat transfer. New, shiny aluminum has an emissivity of 0.04, which changes to 0.1 after the aluminum oxidizes in service. Stainless steel has an initial emissivity of 0.13 that rises to 0.3 after it becomes dull in service. By comparison, the thermoplastic PVC and the colored mastics that would be used with fabric have an emissivity of 0.9. How does this impact performance? For a 4-inch diameter steel pipe operating at 350 F in an exterior environment with no wind and an ambient temperature of 75 F, with an oxidized aluminum jacket over mineral fiber insulation the energy loss at 1.5 inches is 650,500 Btu/ft/yr and the surface temperature is 128 F. If the jacket is changed to PVC or fabric and mastic, the heat loss rises to 704,700 Btu/ft/yr but the surface temperature falls to 104 F at the same 1.5-inch thickness of mineral fiber. This shows that if the primary purpose of the insulation is personnel protection, then a high-emissivity jacket will allow a reduced insulation thickness.
What about low-temperature systems? Change the example to a process operating at –50 F with 50-percent relative humidity and an ambient dewpoint of 55 F with cellular glass. With oxidized aluminum jacket, the heat gain at the 2.5 inches of thickness required to prevent condensation is 220,200 Btu/ft/yr. Using PVC, the thickness required to prevent condensation drops to less than 1.5 inches, but at that thickness the heat gain rises to more than 332,300 Btu/ft/yr. To achieve the same heat gain as obtained with aluminum jacket, the thickness must be more than 3 inches. Again, that jacket choice has a clear influence on thermal performance and should be made with a good understanding of why the insulation is being used. At low temperatures, if condensation control is all that matters, then a high-emissivity jacket should be chosen.
This article has just scratched the surface of insulation jacketing for industrial applications. Clearly, there are more materials available than are described here, and many more factors that influence jacket selection than one could cover in a brief article. Jacket selection is not a trivial matter. It effects many aspects of system design and performance and should be made by a knowledgeable designer with well-defined design criteria for each project. A one-size-fits-all solution to jacket selection fails to take advantage of the wide variety of excellent materials available in today’s marketplace.