PPST Plant Leveling Tank

Plant Leveling Tank

The plant leveling tank will handle the outflow from both the floc blanket tank and the plate settler tank. Using one leveling tank and identical inlet and outlet piping for the two tanks ensures identical flow (assuming that differences in the loss coefficients through the tanks themselves are negligible). We decided to make the head loss through the leveling tank the dominant head loss through the system, which then sets the flow rate through the sedimentation tanks. Our design for the outflow system was a surface piercing 7.62cm (3") pipe with an orifice cut into the pipe below the water level. It was desired that the flow rate through the sedimentation tank would remain constant at 24 L/min no matter the flow rate coming into the flocculator (as long as it was at least 24 L/min). In order to ensure this the head loss out of the plant leveling tank (which controls flow in the sedimentation tank) had to be significantly greater than the head loss over the alternative exit in the flocculator. We chose a factor of safety of 18, thus the head loss out of the plant leveling tank would be 18 times the head loss out of the flocculator. the depth of the orifice hole beneath the water surface was determined to be 13.673cm (18 times the head loss over the flocculator outlet weir). Then using the orifice equation, with the flow set at 24 L/min, this head was used to determine the orifice size (2.223cm or 7/8"). The orifice equation used was the same as used for the flocculator outlet but converted to a circular perimeter from a rectangle.