Document dadE3Dn5DLqxNMjxn0w98p6jG

668 CHAPTER 28 1956 Guide data on commonly used forms of'tunnels aiid conduit systems, have been published by the National District Heating Association.s ; Another form of underground insulation consists of the direct burial of the pipe in a trench with a complete covering of about 4 in. of a granular bitumen which', upon heating, forms a semiplastic, waterproof covering having a very good insulating value. The pipe must be supported on a solid insulating substance such as insulating blocks or asbestos cement pipe sections filled with an insulating cement. The expansion of steam and hot water pipes, using loop expansion joints, offers no difficulty as the plastic nature of the warm material allows free movement of the pipes. Care must be exercised in installing this material in accordance with the manufacturer's recommendations. 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. Table 11. Thickness of Loose Insulation fob Use as Fill in Underground Conduit Systems Steam Pressure Pbio os Condition Steam Temperature Fahrenheit Degrees 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 Hot Water, or 0 to 25 212 to 267 m 2 2'A DA m 25 to 125 267 to 352 2 2J4 3 DA DA Above 125, or superheat 352 to 500 2A 3 3A DA DA Minimum Between Steam and Return i da DA In some instances, where actual submersion of hot lines may occur, it has been found good practice to fasten the covering securely with corrosion resistant wire, and 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 ^ept 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 Pipe and Industrial Insulation