Document Edm4d4Vek6epMMn18X27J4bZ0

578 CHAPTER 21 1958 Guide caused by: pump . operation;15 It should have an internal overflow, drain. Provision must be made to prevent water from freezing in the tank, the tank .vent, and the pipe leading: to the tank. These provisions are also required by the ASME code. The minimum tank volume should be equal to %0o of the total system water volume. The closed system utilizes an air-tight tank which provides a means of pressurizing the system for operation, over a wide rangek>f conditions.. As the excess water due to thermal expansion moves into the expansion tank, it compresses the air trapped therein and increases the pressure on the system. . If. the tank, or the amount of air in it, is too .small,, the pressure on the system will exceed the maximum allowable, causing the relief valve to waste water from the system. When the system cools, the pressure will drop to a value less than minimum, making the venting of air impossible or drawing air into the system if automatic air vent valves are located at a high point of the piping. If the tank is too large, it will cost more and require more space. The size of a closed expansion tank is determined by: volume of the water in the system; range of water temperatures normal to operation of the sys tem; pressure of the air in the expansion tank when the fill water first enters the tank; relationship of the height of the boiler which usually, but not al ways, is the item in the system with the lowest working pressure; the ex pansion tank, and the high point of the system; head of the circulating pump; location of the circulating pump with respect to the expansion tank connection, and the boiler. The expansion tank size for a closed system may be determined from the following ASME formula. This formula should be used only for operating temperatures between 160 F and 280 F. (0.00041 t - 0.0466) F. P. _ P. P P,, (4) where Ft = minimum volume of the expansion tank, gallons. F. = system volume, gallons. t = maximum average operating temperature, Fahrenheit. A widely used formula recommended for temperatures below 160 F is: Pi P,, where . E = net1* expansion of the water in the system when heated from minimum temperatures to maximum temperature, gallons (See Fig. 16). ' p. = pressure in the expansion tank when the water first enters, usually atmos pheric pressure, feet of water, absolute. ` , Pi = initial fill or minimum pressure, feet of water, absolute, at tank. P0 = maximum operating pressure at tank, feet of water, absolute. The volume of the water in a system should be determined from water capacities of heating units as given by manufacturers and the volume of pipe or tube from tables such as Table 4. ^ ' The required minimum pressure, Pi, and the maximum pressure in th? expansion tank, P0 , may change for specific systems, depending on to? Hot Water Heating Systems 579 effect of relative pump and tank location on system pressure changes caused by pump operation. Refer to diagrams A, B, C, and D of Fig. 17 and fol lowing discussion. - The degree of system pressure change caused by pump operation is es tablished by the pump head. Whether the pump' head will be added to, or subtracted from the system static pressure at'the time of pump operation is determined by the relative pump'and tank location. This is so because the junction of the tank with the system is a point of no pressure change re gardless of whether or not the pump operates. Consequently, when thetank is located close to the pump suction, the-pump suction pressure is unchanged when the pump starts and the pump head appears as an in crease of pressure on the system. Conversely, when the tank is located close to the pump discharge, the pump discharge pressure is unchanged \yATERTable 4. Volume of IN Standard Pipe and Tube Standabp Steel Pipe LTtpe Coppbb Tube Sub Inches H K % k i Hi iK 2 2K 3 3K 4 5 6 8 10 12 Schedule No _ . 40 -- 40 40 40 40 40 40 40 40 40 40 40 30 30 30 DiamIentseirdeInches _ 0.622 -- 0.824 1.049 1.380 1.610 2.067 2.469 3.068 - 3.548 4.026 5.047 6.065 8.071 10.136 12.090 GaLllionnsFtper InsideInDchiaems eter _ 0.0157 -- ,0.0277 0.0449 0.0779 0.106 0.174 0.249 0.384 0.514 0.661 1.04 1.50 2.66 4.19 5.96 ; 0.430 0.545 0.666 0.785 1.025 1.265 1.505 1.985 2.465 2.945 3.425 3.905 4.875 5.845 7.725 9.625 11.565 GaLlloinnsFpt er 0.0075 0.0121 0.0181 0.0251 0.0429 0.0653 0.0924 0.161 0.248 0.354 0.479 0.622 0.970 1.39 2.43 3.78 5.46 when the pump starts and the pump head appears as a decrease in pressure at the pump suction and on the system. When the pump discharges away from the boiler and expansion tank (A, v!g. 17) all pressure changes created by pump operation are additive. A gage located at any point in the system will show an increase in the pressure on sorting the pump equal to the friction pressure drop between the gage anc* TMe boiler inlet (tank). Since the tank is located on the boiler, boiler pressures are unaffected by pump operation. The minimum pressure, Pi, required, is equal to height of the system above the expansion tank, plus any pressure required to assure positive air venting and prevent boiling; the maximum water temperature exceeds 212 F. The maximum pres> depends upon the pressure rating of that item of equipment hav- val *owes*i rated working pressure, usually the boiler or boiler relief ajve, and the vertical distance between it and the expansion tank. This jregement is recommended where high head pumps are used. 17) n n PumP discharges into the boiler and expansion tank, (B, Fig. Raeel Pressure changes induced by pump operation are subtractive. A |fg > located at any point in the system, will show a decrease in pressure on mg the pump equal to the pipe friction pressure drop between the gage