Evaluating Force Main Transients with SWMM5 and Other Programs
The question of whether SWMM5 and other hydraulic calculation programs commonly used to model wastewater collection systems can accurately simulate the flow conditions in pump station force mains was raised and discussed in the SWMM Users Group in 2008. Rises in pressure occur in the downstream force main following a pump turning on, and drops in pressure occur following pumps turning off. These changes are water hammer and result in pressure waves and flow rate oscillations in the force main.
Four one-dimensional hydraulic calculation programs were compared in the analysis of a theoretical force main problem. All of the programs are link and node type programs. The links may be channels, conduits, valves, gates, dams, weirs or pumps (depending on the program). The links are connected to junctions at either end. The junctions may be reservoirs, manholes, or closed conduit junctions such as pipe tees.
The programs evaluated include: SWMM5 by the United States Environmental Protection Agency, Water Hammer and Mass Oscillation (WHAMO) by the United States Army Corps of Engineers, the Transient Analysis Program (TAP) by Applied Science, Inc., and InfoWorks CS (Collection System) by Wallingford Software Ltd.
SWMM5 has a new force main feature that was tested. This feature did not exist in earlier versions of the SWMM program. Conduits may be specified as cylindrically shaped force mains that always flow full and have friction losses calculated using the Hazen-Williams or Darcy-Weisbach formulae.
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