Testing a Single Lateral of an Underdrain System

Abstract

To evaluate the performance of the laterals of an underdrain system manufactured by Johnson Screens, a test-stand basin was built at St. Anthony Falls Laboratory. Two laterals were placed in the basin of which one was a blind lateral, i.e. with no orifice holes, for the leakage test. The laterals were 8-inch conduits with 9.25-inch diameter vee-wire wrap around them. Eleven series of tests were conducted on the laterals, of which five test series were to assess the hydraulic performance under the backwash mode, three test series to assess the hydraulic performance under the filter mode, and three test series to determine the amount of leakage. Under the backwash mode, the flow rate from every two rings of orifices was measured using a flow-capturing apparatus. The flow rate varied from 53 GPM at the upstream point to 64 GPM at the downstream point. Two tests series were also conducted and flow velocities around the lateral were measured to determine the turbulence intensity along the lateral. The average turbulence intensities estimated at the upstream, downstream and the midpoint of the lateral were comparable. Under the filter mode, the flow rate from the orifice rings could not be measured, therefore, only the approach flow velocities were measured. The average turbulence intensities estimated at the upstream, downstream and the midpoint of the lateral were again comparable. For the leakage test, the amount of leakage exceeded the threshold during the first two series. Subsequently, the tested lateral was modified and improved by the Johnson Screens crew members. The final product was tested and the amount of leakage under 35 psi pressure with a duration of four minutes decreased to 5 GPM.