A large facility may have a complex fluid loop, with several sub-branches off the main supply and return lines. Each loop should be provided with valves on the supply and return lines. These serve two purposes. First, they act as isolation valves for each major segment of the system, in the event piping or valves need repairs. The branch loop under repair can be valved off, so the remainder of the loop can stay in operation. Second, the valves allow the system to be balanced so that flow through each loop will ensure a uniform flow rate and pressure. Only then will the system use the least possible energy to move the operating fluid.
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Excerpt from Energy Conservation Projects.