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<br />! - <br /> <br />A propcl'1y ope rat illg PRV wi 11 reduce a high upstream pressure to a desired <br />lowe.' pressure on thc dOlmstrcum side of the valve. If high water usage <br />downst"CJm causes prcssures to drop at the dischurge sidc of the PRV, it will <br />open slightly to boost pressun:s to the downstreJm setting. In this way, a <br />PRV call lIluintain a constant pressure at its discharge side while reducing <br />high, fluctuating, upstream pressures. <br /> <br />Pressure reducing valves would also be installed at the ends of branch trans- <br /> <br /> <br />mission lines prior to entering the distribution systems of NCO:,A members. <br /> <br />The use of PRV's will allow the NCDWA system to match or boost the pressure in <br /> <br /> <br />existing systems. For those members who \~ish to have the NCDWA system supply <br /> <br /> <br />water to a storage tank, throttling altitude valves ~ould be used to control <br /> <br />the level of the tank. Another transmission line valve which would be necessary <br /> <br />to operate the NCDIIA system is the automatic air and vacuum relief valve. This <br /> <br /> <br />valve is necessary to release air when filling the transmission line with <br /> <br />water and to prevent the pi pe from co 11 aI's i ng due to vacuum if a br"eak in the <br /> <br />line occurs. Installation of air and vacuum relief valves is recommended at <br /> <br /> <br />hilltops and river crossings. <br /> <br />Butterfly valves are necessary at regular intervals along the transmission <br />line in order to isolate short sections of line if a break should occur. The <br />spacing of these valvcs in a transmission line can range from several valves <br />per mi le to one valve in several m; les. The spacing is dependent on the type <br />of terrain Jnd the degree of control desircd for the operJtion of the system. <br />Gate valves could be substituted for' the bulterfly vulves, but they would be <br />morc difficult to operate under the high pressures and large flows of wuter <br />which would be truns;lIitted through the NCD,:A system. <br /> <br />17 <br />