Document aDE1ZM1Xn3E8oBMkZmzam7349

592 CHAPTER 28 <1949. Guide : Fig; 6. Chart -fob Determining Economical Thickness of Pipe Insolation >;v pipe Insulation A 593; size; thence vertically to-the scale at the top right margin where the economical thickness may be read off. directly. A rapid method for determining the economical thickness of insulation by use of tables has been published4. " UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types,-sizes and shapes (See Chapter 29); Detailed data on commonly used forms of tunnels and conduit systems, have been published by the National District Heating Association?. Pipes in tunnels are covered, with sectional, insulation , to provide maximum thermal efficiency, and are also finished with good mechanical protection in the .form ;qf. 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 op Loose Insulation for Use as Fill in Underground Conduit Systems , Steam Pressure Psig or Condition Steam Temperature Fahrenheit Degrees Hot Water, or 0 to 25 25 to 125 Above 125, or superheat 212 to 267 267 to 352 352 to 500 orMinimum Thickness Insulation in Inches Steam Lines Return Lines * Pipes Less Pipes 4 In. Fipes Larger Pipes Less ` Pipes 4 In. than 4 In. to 10 In. than 12 In. than 4 In. ana Larger lX 2 2A IX DA 2 2'A 3 DA DA m3 3A DA DA Minimum Distance Between Steam Return 1 IX DA 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 fin 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 theories' previously developed, together with other experimental data which have been presented, the usual endeavor is to secure not less than 90 per cent 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-.