Document N9L8k5oqKE8J0ejJpQMDXJaD

570 CHAPTER 21 1957 Guide caused by pump operation.16 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 %00 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 range of 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 fora 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. where y _ (0.00041 t - 0.0466) V. ` p. _ p. Pi Pc (4) Vt = minimum volume of the expansion tank, gallons. V. = system volume, gallons. t = maximum average operating temperature, Fahrenheit. A widely used formula recommended for temperatures below 160 F is: F, E P. _ P. Pi Po (5) 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. Po = 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, Pt, and the maximum pressure in the expansion tank, P0, may change for specific systems, depending on the Hot Water Heating Systems 571 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 the tank 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 Table 4. Volume of Water in Standard Pipe and Tube Standard Steel Pips LType Copper Tube Size Inches Schedule No Inside Diameter Inches Gallons per Lin Ft Inside Diameter Inches Gallons per Lin Ft H % H l Vm4 2 2 3 m 4 5 6 8 10 12 _ ___- ___ 0.430 0.0075 40 0.622 0.0157 0.545 0.0121 ___ -- 0.666 0.0181 40 0.824 0.0277 0.785 0.0251 40 1.049 0.0449 1.025 0.0429 40 1.380 0.0779 1.265 0.0653 40 1.610 0.106 1.505 0.0924 40 2.067 0.174 1.985 0.161 40 2.469 0.249 2.465 0.248 40 3.068 0.384 2.945 0.354 40 3.548 0.514 3.425 0.479 40 4.026 0.661 3.905 0.622 40 5.047 1.04 4.875 0.970 40 6.065 1.50 5.845 1.39 30 8.071 2.66 7.725 2.43 30 10.136 4.19 9.625 3.78 30 12.090 5.96 11.565 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, Fig. 17) all pressure changes created by pump operation are additive. A gage located at any point in the system will show ah increase in the pressure on starting the pump equal to the friction pressure drop between the gage and the 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 My pressure required to assure positive air venting and prevent boiling; rf the maximum water temperature exceeds 212 F. The maximum pres^ure, P0, depends upon the pressure rating of that item of equipment havuig the lowest rated working pressure, usually the boiler or boiler relief valve, and the vertical distance between it and the expansion tank. This arrangement is recommended where high head pumps are used. When the pump discharges into the boiler and expansion tank, (B, Fig. 1') all pressure changes induced by pump operation are subtractive. A Sage, located at any point in the system, will show a decrease in pressure on starting the pump equal to the pipe friction pressure drop between the gage