Steady Upstream for Loose Control - Custom-built proportional integral loops overcome PLC software limitations

摘要

The Norumbega Reservoir used to be an essential part of the water transmission system for the metropolitan Boston area. Now the Norumbega Covered Storage Tank (NCST) facility replaces it as one of the largest drinking water storage tanks in the world. Outflow from the NCST fluctuates widely throughout the day. The cause stems from a 405-MGD water transmission system supplying the Boston area that suffers a large diurnal demand variation. To keep the transmission system and the upstream treatment processes running steadily, the system should make as few changes as possible in the upstream supply valve's position. To accomplish this, the level of the 115-million-gallon water storage tank becomes the process variable in a custom-built timed, reverse-acting, proportional/integral closed-loop level controller. This controller modulates remote upstream water supply valves through a minimal amount of movements, allowing the loose control needed to satisfy the hydraulic system's varied needs.