Author(s): Robert C. Elliot; David H. Axworthy
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Keywords: No Keywords
Abstract: Typical surge analyses of wastewater systems involve pump power failure and startup at lift stations discharging into force mains. The presence of solids in wastewater can entrain air, reducing the acoustic wavespeed because air is more compressible than water. It's commonly perceived that lower wavespeeds will reduce the severity of damaging waterhammer pressures created following pump power failure or other actions. However, this isn't always the case because each system has unique characteristics (pump operation, size and material of force main, pipe profile). As a result, it's difficult to predict without first performing hydraulic transient modeling the response of the system to the possible presence of entrained air. It is particularly important to consider entrained air when evaluating the effectiveness of proposed surge control strategies. A mitigation strategy that may appear reasonable based upon the assumption of non-air-entrained fluid, reducing damaging pressure transients to acceptable levels while maintaining a positive magnitude, may in fact lead to larger magnitude low pressures and possible vapor cavity formation when air entrainment is considered. Without sufficient surge mitigation measures in place, vapor cavity collapse and subsequent creation of large-magnitude pressure spikes may result that could damage a force main. This paper describes unique considerations that should be addressed when performing hydraulic transient analyses and recommending surge protection for wastewater systems compared with typical installations on raw/treated water systems. Pressurized surge tanks (air chambers), static seating pressure for sewage vacuum relief valves, flywheel difficulties with variable-frequency-drives, and standpipe design will be discussed. Recent project examples in Washington and California will be presented.
Year: 2009