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714 CHAPTER 27 1957 Guide is applied with the canvas lap pasted. Outdoors, it is necessary to provide a weatherproof finish. Fittings and bends are insulated with portions of standard preformed insulation, blanket insulations, or insulating cements. Fitting insulation should be carefully specified -to be compatible with the pipe insulation. Insulation on lines carrying cold water, brine, or other cold fluids must be protected to prevent the infiltration of water vapor into the insulation. Flat, curved, and irregular surfaces such as boilers, breechings, tanks, and vessels, are normally insulated with blocks or lagging, or with blanket forms of insulation. The insulation is secured in a variety of ways de pending upon the form of insulation and contour of the surface to be insulated. One specification includes wiring the blocks in place, stretching a hexagonal mesh netting tightly over the insulation, and fastening se curely with wires. Over the netting a coat of mineral wool or asbestos cement is applied with a second coat of asbestos cement trowelled to a smooth even finish. For outdoor installations weather protection is then applied. For insulation over surfaces at temperatures below ambient, water vapor protection must be applied for both indoor or outdoor applica tions. The application of insulation to air conditioning ducts is accomplished in a variety of ways depending largely on ambient conditions and tempera ture differentials between the air in the room and conditioned air. Boards or blankets are normally used in a single layer 1 in. thick. Boards are fastened to the duct surfaces with spot or solid application of adhesive, and further reinforced with wires, bands, spindle anchors, or sheet-metal screws. Although, where the service is not too severe, it is permissible merely to point the joints with a mastic compound, in most cases the in sulations should be protected against water vapor permeation. Where blankets are used the insulation can be wrapped around the duct, and held in place with tape over all joints or with spirally wrapped twine. Again in most cases the insulation should be provided with an outer vapor barrier protection. UNDERGROUND PIPE INSULATION Underground steam distribution pipes may be installed in protective structures of various types, sizes and shapes (see Chapter 28). Detailed data on commonly used forms of tunnels and conduit systems, have been published by the National District Heating Association* 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- The pipe must be supported on a solid insulating substance such as in sulating blocks or asbestos cement pipe sections filled with an insulating cement. The expansion of steam and hot water pipes, using loop expan sion 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 materia] 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 pr" tection in the form of metal or waterproofing membrane outer jacketsIn some instances, where actual submersion of hot lines may occur, it b88 been found good practice to fasten the covering securely with corrosion resistant wire, and then sew on a wire-inserted asbestos fabric jacket wiu> Pipe and Industrial Insulation 715 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 theories6 previously developed, together with Table 10. Thickness op Loose Insulation for Use as Fill in Underground Conduit Systems Pressure ftia ob 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 or Insulation in Inches Steam tupw Return Links Pipes Less t>n 4 In. Pipes * In. Pipes Larger Pipes Less to 10 Is. than 12 Ik than 4 In Pipes 4 In. awl larger m 2 2 2K 2M 3 IK lK IK iK zyt 3 3K IK IK Minimum Distance Between Steam AND Return 1 IK IK otner experimental data which have been presented, the usual endeavor is to secure not less than 90 percent efficiency for underground piping. Table 10 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 pipes 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. 6. The data in Fig. 6. 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 underground P*Pe system estimates. ?at Lss from Copper Piping, by R. H. Heilman (Heating, Piping and Air ConZoning, September 1933, p. 458). t From Technical Data Sheet, August 1955, (Johns-Manville Corp.). bv of Determining the Economical Thickness of Pipe Insulation, tleyW. Smith (ASHVE Journal Section, Heating, Piping and Air CondiT??' Oetober m7< P- H8). Handbook of the National District Heating Association, Third