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616 CHAPTER 27 1952 Guide (L. B. McMillan, Proc. National District Heating Association, Vol. 18, p. 138). Fio. 6. Chart fob Determining Economical Thickness of Pipe Insulation Pipe Insulation 617 size; thence vertically to the scale at the top right margin where the eco nomical thickness may be read off directly. A rapid method for determining the economical thickness of insulation by use of tables has been published.* UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes (see Chapter 28); ' Detailed data on commonly used forms of tunnels and conduit systems, have been pub lished by the National District Healing Association.* Pipes in tunnels are covered with sectional insulation to provide maxi mum thermal efficiency, and are also finished with good mechanical.pro tection in the form of metal or waterproofing membrane outer jackets. In some instances, where actual submersion of hot lines may occur, it has been found good practice to firmly secure the covering with corrosion Table 11. Thickness of Loose Insulation fob Use as Fill in Underground Conduit Systems Steam Pressure Pbio or Condition Steam Temperature Fahrenheit Degrees Hot Water, or 0 to 25 212 to 267 25 to 125 267 to 352 Above 125, or superheat 352 to 500 Minimum Thickness op Insulation in Inches Steam Lines Return Lines Pipes Less Pipes 4 In. Pipes Larger Pipes Less Pipes 4 In. than 4 In. to 10 In. than 12 In. . than 4 In. and Larger ik 2 2A IK m 2 2'A 3 IK i A 2K 3 3K IK IK Minimum Distance Between Steam Return i IK IK resistant wire, then sew on a wire-inserted asbestos fabric jacket with wire. This jacket is porous. The principle of withstanding submersion is that water may enter as water, then actually boil at the pipe surfaces and escape as steam without rupturing the insulation or jacket. Conduit systems are in more general use than tunnels. Pipes carried in conduits may be insulated with sectional insulation; however, the more usual practice is to fill the entire section of the conduit around the pipes with high quality, loose insulating material. The insulation must be kept dry at all times, and for this purpose effective waterproofing membranes enclose the insulation. A drainage system is also provided to divert water which may tend to enter the conduit. The economical thickness of insulation for underground work is difficult to determine accurately due to the many variables which have to be considered. As a result of theories4 previously developed, together with other experimental data which have been presented, the usual endeavor is to secure not less than 90 percent efficiency for underground piping. Table 11 can be used as a guide in arriving at the minimum thickness of loose insulation fills to use for laying out conduit systems. Other factors such as the number of pipes and their combination of sizes, as well as the. standard conduit sizes, are primary controlling factors in the amount and thickness of insulation for use. When sectional insulation is applied to lines in tunnels or conduits, usual practice is to apply the most efficient materials $ in. less in thick-