The pressures surges generated during backwashing by two microfilter units at the Coles Run Filtration Facility in Augusta County, Virginia, were evaluated in order to limit maximum surge pressures to 10 – 20 psi (23 – 46 ft) over the steady-state operating pressures and to prevent vapor pressure conditions. For the pressure surge analysis, the system was evaluated from the source reservoir, through the 10-inch diameter and 1500 ft feed line, to the upstream sides of the feed valves at the microfilter units. The steady-state pressure at the feed valves, when each unit is operating at 425 gpm, is about 115 ft (50 psi). A complete backwashing sequence was modeled, with total flows varying from 425 to 1275 gpm through the two microfilter units over a period of about two minutes.The analysis predicted that a maximum surge of 333 ft (144 psi) over the steady-state pressures would occur as a result of backwashing valve operations and a maximum surge of 392 ft (170 psi) would occur as a result of failure during backwash. Minimum pressures were predicted to drop as low as vapor pressure. Therefore, a surge tank was recommended to be installed on the upstream side of the microfilter units. Two surge tank alternatives were provided: a 375 gallon surge tank to limit maximum pressure surges to 14 psi (32 ft) or a 940 gallon surge tank to limit surges to 9 psi (20 ft) over steady-state.

 

A 1,040 gallon surge tank was subsequently installed and pressure measurements have shown that it is successfully controlling pressure surges as predicted.