Document NGbnybpKJ3Onb5adKJDQjRY98

712 CHAPTER 27 1957 Guide INSULATION OF PIPES TO PREVENT FREEZING If the surrounding air temperature remains sufficiently low for an ample period of time, insulation cannot prevent the freezing of still water, or of water flowing at such a velocity that the quantity of heat carried in the water is not sufficient to take care of the resulting heat losses that will cause the temperature of the water to be lowered to the freezing point. Insulation can materially prolong the time required for the water to give up its heat, and if the velocity of the water flowing in the pipe is main tained at a sufficiently high rate, freezing may be prevented. Table 9 may be used for making estimates of the thickness of insula tion necessary to take care of still water in' pipes , at various water and surrounding air temperature conditions. Because of the damage and service interruptions which may result from frozen water in pipes, it is essential that an efficient insulation be utilized. This table is based on the use of a material having a conductivity of 0.30. The initial water tem perature is assumed to be. 10 deg above, and the surrounding air temperature 50 deg below, the freezing point of water (temperature difference, 60 F). Table 9. Data fob Estimating Requirements to Prevent Freezing of Water in Pipes with Surrounding Air at --18 F Nominal I Number of Hours to Cool 42 F 1 Water Flow Required at 42 F to Fife Size (Inches) g s u Water to Freezing Point II Prevent Freezing. Pounds per fl Linear Foot of Pipe per Hour | u 1- 21M 3 4 5 8 10 12 Thickness of Insulation in Inches (Conductivity, k m 0.30) 2 0.42 0.83 1.40 1.94 3.25 4.55 5.92 . 7.35 10.05 13.00 15.80 3 0.50 1.02 1.74 2.48 4.27 6.02 7.96 9.88 13.90 18.10 22.20 4 0.57 1.16 2.02 2.90 5.08 7.20 9.69 12:20 17.25 22.70 28.10 i | 1 2 0.54 0.68 0.84 0.95 1.24 1.47 1.73 1.98 2.46 2.96 3.43 3 0.45 0.55 0.68 0.75 0.94 1.11 1.29 1.46 1.78 2.12 2.45 4 0.40 0.48 0.58 0.64 0.79 0.93 1.06 1.19 1.43 1.70 1.93 The last column of Table 9 gives the minimum quantity of water at initial temperature of 42 F which should be supplied every hour for each linear foot of pipe, in order to prevent the temperature of the water from being lowered to the freezing point. The weights given in this column should be multiplied by the total length of the exposed pipe line expressed in feet. As an additional factor of safety, and in-order to provide against temporary reductions in flow occasioned by reduced pressure, it is' ad visable to double the rates of flow listed in the table. It must be empha sized that the flow rates and periods of time designated apply only for the conditions stated. To estimate for other service conditions, the following method of procedure may be used. If water enters the pipe at any temperature other than 42 F, the time re quired to cool it to the freezing point will be equal to (t'-- 32)/10 times that given in the table, or the rate of flow of water may be changed to 10/(t -- 32| times the indicated flow rate in the last columns of Table 9. . However, n the water enters the pipe at 34 F, it will be cooled to 32 F in one-fifth < Pipe and Industrial Insulation 713 the time given in the table. It will then be necessary to increase the rate of flow so that five times the specified quantity of Water will have to be sup plied in order to prevent freezing. If the minimum air temperature is -- 38 F (temperature difference 80 F) instead of --18 F, the time required to cool the water to the freezing point will be 00/80 of the time given in the table, or the necessary quantity of water to be supplied will be 80/60 of that given. In making calculations to arrive at the values given in Table 9, the loss of heat stored in the insulation, the effect of a varying temperature difference due to the cooling of pipe and water, and the resistance of the outer surface of the insulation to the transfer of heat to the air, have all been neglected. When these factors enter into the computations it is necessary to enlarge the factor of safety. Also as stated, the time shown in the table is that required to lower the water to the freezing point. A longer period would be required to freeze the water, but the danger point is reached when freezing starts. The flow of water will stop and the entire line will be in danger as soon as the water freezes across the section of the pipe at any point. If it is necessary to calculate the time required for the water to cool to 32 F, the following equation may be used ff _ (h -- 32) "(!*') <3' Cp* = Capacity of pipe lb water/lin. ft. = -- X 62.4 144 9 = i r* U lOge -- Ti (4) H = hours for water to cool to 32 F. fo 8=5 inside radius of pipe, inches. *1 -- initial water temperature. May be obtained from manufacturers catalogs and handbooks. See Equation 1 for definitions of symbols. T k|^ea 'Tater must remain stationary longer than the times designated in hnt 6 9' t"e.onl3r safe way to insure against freezing is to install a steam or : Water pipe parallel to the cold.water pipe or to place an electric re- -ce "eater along the side of the exposed water line. The heating the bm 5TM wafcer hue are then insulated so that the heat losses from ara' visystem are not excessive, and the heating effect is concentrated gainst the water pipe where it is needed. For this form of protection 2 ot an efficient insulation may be applied. APPLICATION OF INDUSTRIAL INSULATION horfma^l ?iipeS are nor?nally insulated with half-sectional insulation split and1n^a t an<^ furnished with factory applied jackets forming hinge m nl ' Large. Piping is insulated with segmental blocks, wired or banded fi'em aCf ' or .^ii'h blanket insulations, particularly where removal for luent servicing of the pipe is necessary. Indoors, sectional insulation