Document pe8bYGYMdvwzk4No5nYp0rREj

57,6 CHAPTER 21 \ 1958 Guide' pumps is established by .adding the floy.'capacity of one pump to the, flow, capacity of;the other at-identical heads. ;If two pumps are placed in series, thmrelationship, of flow; ,to. head due to operation of both pumps is estab lished by adding the pump heads at identical flows. When/using pumps in series, they must be identical or similar since the maximum flow will be linufed by tbe snialler puhip. Gate valves should be provided to. enable isolation of either pump when only one is in operation. The Heat Source or Boiler t The most frequently used source of heat is the 30 p'si hot water heating boiler, constructed, of cast, iron or steel in accordance with the ASMEboiler construction code for low-pressure heating boilers: See Chapter 16, Heat ing Boilers; Furnaces, Space Heaters. Boilers, classed as low pressure Hot Water Heating Systems ;577 equipment; maintain minimum-pressure for all normal, operating tempera tures in order that air can be vented and cavitation at the pump suction arid boiling of system water prevented; and accomplish these Objectives1with as little addition of new water to the system as possible. ' 1,1 ' -: 1 ' : .> An expansion tank is the primary device used to accomplish'system pres sure control. The excess water volume in the system resulting-from1 in creased temperature is stored in the expansion tank during1 periods of high operating temperatures and returned to the system when1 the system water temperature is lower. The expansion tank must be designed' to store the required volume of water during maximum design operating temperatures heating boilers, may be used for hot water with pressures as high as 160 psi if the temperature of the water does not exceed 250 F and if they are con structed; according to. the ASME code for the pressure at which they are operated.14 The ASME code requires-installation of-a relief valve on each hot water heating'boiler to limit the pressure exerted on that boiler to the value for which it was designed and tested.14 Similar requirements exist for converters and electric water heaters. This operating pressure limit for the boiler is' an important factor, in the design of the hot water heating system. To gether with the size and characteristics of the expansion tank, it determines the permissible height from the boiler to the top element or pipe of the dis tribution system, as discussed under Expansion Tanks. In addition to the. pressure relief valve required by the ASME code, a hot water heating boiler should be equipped with a thermometer, located toward the top of the boiler, and a reliable pressure or altitude gauge with a range at least twice that of the maximum operating pressure or altitude allowable for the boiler. EXPANSION TANKS AND SYSTEM PRESSURE CONTROL . The objectives of system ..pressure control are ,to: maintain maximum: pressure at, all equipment in the system below the working pressure of such,. Fig. 16. Expansion of Water Above 46 F' !` r 1 Note: Net expansion of water equals actual expansion of- water less volumetric expansion of metal system.17 . without exceeding the maximum operating pressure allowable, and to main tain required minimum pressure when the system is cold. Some designers provide an automatic fill valve to maintain the minimum pressure in the system by supplying water to make up for losses due to leakage. Others prefer that any water added to the system should be supplied by a manual valve. ... , v . .^relief valve installed on hot water boilers in, accordance with the .4TM" code performs the function of limiting the maximum pressure at . boiler. Since it is a safety device, it should not be considered an operat es control. ..... 1 Systems are designated as open, Fig. 14, or closed, Fig. 15.. . .The ^ tank system is vented to atmosphere and,is generally limited installations having operating temperatures of 180. F. or less-because of at 1st ^ng and tank water evaporation problems. . The tank should be the6aS^ - ^ ^hpve the high point of the system and should be connected to suction side of the pump to prevent sub-atmospheric system' pressures