Thermal Insulation Basics
Mechanical Thermal Insulation Systems
Mechanical thermal insulation systems—materials and accessories that insulate components of mechanical equipment and piping—are designed, specified, and installed at industrial facilities for a range of purposes. These systems reduce heat loss or gain for energy conservation; control the temperature of process equipment; control surface temperatures to protect personnel; reduce emissions of greenhouse
gases; prevent or reduce condensation on surfaces; and provide fire protection, among other functions. While these objectives have remained unchanged, several developments over the past 5 to 10 years provide additional tools (e.g., materials, accessories, standards) to help engineers design an insulation system for a particular application.
This article discusses the basics of insulation values, a method to determine the economic thickness for insulation based on software developed by the North American Insulation Manufacturers Association (NAIMA), and finally, recent findings and tools to help prevent the occurrence of corrosion under insulation (CUI).
Insulation 101: A Quick Review
Materials used for thermal insulation differ from other materials in that they reduce all 3 modes of heat transfer (conduction, convection, and radiation) when a temperature gradient is present. Several material properties define a material’s ability to thermally insulate:1
- The K-value, or thermal conductivity, defines the rate of heat flow through a homogeneous material induced by a temperature gradient. Generally, a K-value of less than 0.5 Btu-in./hr-ft2-°F at room temperature distinguishes a thermal insulation material from a material that is not thermally insulating. Thermal conductivity is independent of material thickness.
- The C-value quantifies the thermal conductance of a material. Unlike the K-value, this property does depend on material thickness. As defined by ASTM International, thermal conductance is the rate of steady-state heat flow through a unit area of a material induced by a temperature difference between the 2 surfaces. The C-value can be calculated by dividing the K-value by the material thickness.
- The U-value, or thermal transmittance, characterizes the thermal performance of a system as opposed to a homogeneous material. To calculate the U-value of mechanical insulation, heat transfer through the homogeneous insulation as well as through any breaches and gaps between it and a different insulation material must be taken into account. Many of the standard thermal conductivity test methods
treat the insulation material as homogeneous, although in real applications, rigid materials have joints and even cracks. Thus, the U-value is typically higher than what it would be if the insulation behaved as a homogeneous material. Reference 2 provides guidance on calculating U-values.
Thermal insulation materials are generally classified as fibrous, granular, or cellular, and these categories are further divided based on material type (e.g., fiberglass, mineral wool, calcium silicate, perlite, cellular glass, polystyrene, polyisocyanurate, etc.). These materials come in a variety of forms, including rigid boards, blocks, and sheets; flexible sheets; flexible blankets; and both rigid and
When selecting a thermal insulation material for a particular application, many criteria must be considered, including thermal conductivity, thermal expansion and contraction, combustion characteristics, compressive properties, water vapor permeability, wicking, susceptibility to water absorption, and corrosivity of the environment. ASTM C1696, Standard Guide for Industrial Thermal Insulation Systems, provides
guidance on selecting insulation materials and accessories.3
Design firms and manufacturers have their own insulation thickness tables for hot-service (i.e., above-ambient temperature) piping and equipment. Many of these are based on oil and gas prices before the year 2000, and do not reflect the increase in energy costs since that time. Thus, chemical process industries (CPI) manufacturers and their design engineering firms should calculate their own economic thickness
values to determine the energy savings of incorporating thermal insulation into a particular application.4
Economic thickness refers to the thickness at which the energy cost savings achieved by the insulation offset the costs of the thermal insulation system (materials, installation, and maintenance). Software provided by NAIMA (3E Plus®) is one tool available for performing this calculation. This software calculates the thermal performance of insulated and uninsulated piping and equipment, translates energy
losses into costs, and calculates the economic thickness based on several variables including the fuel cost.
To determine an economic thickness, the design criteria for the insulation system must be identified. For example, if the insulation is used for hot process temperature control, typically a fluid temperature must be maintained (i.e., the heat loss per unit pipe length needs to be limited to some maximum value). With a known pipe length and maximum allowed heat loss, the allowable heat loss per pipe length can be
calculated and entered into the software to calculate the economic thickness. For process temperature control, each pipe size and process temperature combination needs to be considered separately.
If the insulation is used for personnel protection, a maximum allowable surface temperature must be specified (instead of the maximum heat loss per pipe length as needed when process control is the design criterion). While the cost of energy is critical in calculating the economic thickness for process-control insulation, it does not affect the economic thickness for personnel protection, which is based on some
maximum surface temperature that is not affected by the price of energy.
Corrosion Under Insulation
Corrosion under insulation (CUI) is, of course, an age-old problem, probably as old as insulated iron pipes.5 To provide guidance on how to minimize the occurrence of CUI, the National Association of Corrosion Engineers (NACE) published a standard for the control of CUI for thermal insulation.6 The original standard, issued in 1998, was updated in 2010 based on knowledge gained since the
original publication. This document provides good overall guidance to the specifier as well as to the facility owner and operator.
Also in the past few years, the American Petroleum Institute (API) has developed a draft recommended practice for minimizing CUI. This new document, which has reportedly been in review by API’s legal department since 2012, appears to be a very comprehensive guide to minimizing the occurrences of CUI. Hopefully, it will soon be released by API to the engineering public. In the meantime, CUI continues to be a
problem at industrial facilities, including petroleum refineries.
1. Hart, G. H., “K-Value, U-Value, R-Value, C-Value: Understanding the Value in All These Values,” Insulation Outlook, National Insulation Association, Reston, VA, www.insulationoutlook.com/ io/article.cfm?id=I0090302 (Mar. 2009).
2. ASTM International, “Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical Systems by Use of Computer Programs,” ASTM C680, ASTM International, West Conshohocken, PA (2010).
3. ASTM International, “Standard Guide for Industrial Thermal Insulation Systems,” ASTM Cl696, ASTM International, West Conshohocken, PA (2013).
4. Hart, G. H., “Multiple Choice, Part 1: Selecting Proper Insulation Thickness Helps Increase Energy Efficiency.” Insulation Outlook, National Insulation Association, Reston, VA, w ww.insulation.org/ articles/article.cfm?id=1007l 105 (Nov. 2007); Hart, G. H., “Multiple Choice, Part Two: How to Choose Economic Thickness of Insulation,” Insulation Outlook, National Insulation Association, Reston. VA,
www.insulation.org/articles/article. cfm?id=IO071202 (Dec. 2007).
5. Center for Chemical Process Safety, “Process Safety Beacon: Corrosion Under Insulation,” Chem. Eng. Progress, 110 (1), p. 16 (Jan. 2014); Richardson, K., “New Models to Prevent Corrosion Under Insulation,” Chem. Eng. Progress, 110 (I), p. 17 (Jan. 2014).
6. National Association of Corrosion Engineers, “Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systems Approach,” SP0I98-2010, NACE International, Houston, TX (2010).
Article excerpted with permission from Chemical Engineering Progress (CEP), June 2014. Copyright 2014 American Institute of Chemical Engineers (AIChE).