Document BKKvKZLX3ew20GwLGyBQdJOE

336 Chapter 17 1945 Guide Fig. 5. Chart for Determining Economical Thickness of Pipe Insulation Pipe Insulation ;337 margin representing the given number of hours of operation per year; theii proceed vertically to the line representing the given value of heat; thence horizontally to the right, to the line representing the given tem perature difference; thence vertically to the line representing the con- , ductivity of the given material; thence horizontally, to the left, to the line representing the given discount on that material; thence vertically to the curve representing the required per cent return on the investment; thence horizontally to the right, to the curve representing the given pipe size; thence vertically to the scale at the top right margin where the economical thickness may be read off directly. UNDERGROUND PIPE INSULATION Underground steam distribution lines are carried in protective struc tures of various types, sizes and shapes (See Chapter 43). 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 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 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 theories previously developed, together with other experimental data which have been presented, rile usual endeavor is to secure not less than 90 per cent efficiency for underground piping. Table 13 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 ness than that determined by the use of Fig. 5. The data in Fig. 5 are based on conditions of insulation exposed to the. air, whereas normal ground temperature is substituted for air temperature in determining the tem perature difference for use with the chart when applying it for under ground pipe line estimates. *Handbook of the National District Heating Association, Second Edition, 1932. Vo,`T2^2f from Buried in the Ground, by J. R. Allen (A.S.H.V.E. Transactions