Launder Design Program

This program is used to design the pipe that is used for transporting water out of the sedimentation tank and into the exit channel. The launder is located between the top of the lamella and the surface of the water. There are orifices located on the top of the launder. The launder program uses equations found in the fluids functions and sedimentation programs.

Launder Design Program Algorithm

Launder AutoCAD Drawing Program

Algorithm

Given the maximum flow rate through the treatment plant, and the number of sedimentation tanks in the plant, the flow rate is equally divided between the number of sedimentation tanks. Previously, the length of the launder was equal to the length of the sedimentation tank, minus two thicknesses of the channel wall, minus the width of the inlet channel and the width of the exit channel. When the launder was this length, it did not reach all the way across the sedimentation tank. Instead, it stopped just short of the wall. The length of the launder is now being extended to reach all the way across. Increasing the length will help to optimize the distribution of water through the orifices so the distribution is more even.



To determine the number of orifices in the launder, the length of the launder is divided by the specified orifice spacing (center to center), found in the Design Assumptions. Note that there are two rows of orifices on a single manifold. This number is rounded down to the nearest whole number.

The number of launder pipes per sedimentation bay (N.SedLaunders) is an expert input. From that number the flow rate through each orifice is calculated.

Using an iterative program found in the Fluids Functions (ND.Manifold) and the available pipe sizes, the nominal diameter of the manifold is determined and defined as ND.SedLaunderEst. This equation is a function of different parameters established in the Design Assumptions.
The total head loss in the launder is then calculated using the HL.Manifold equation found in the Fluids Functions. This equation is defined below with the given inputs.

Next, the head loss through the orifice is the given head loss, HL.SedLaunderBod, minus the calculated head loss through the whole launder. This is defined as HL.SedLaunderOrificeEst.
Finally, the diameter of the orifice holes is calculated using the D.Circle equation from the fluids functions. This number is rounded to the nearest available drill size. The equation is as follows:

Flow through the Launder

The only complication from extending the length of the launder has to do with the coupling. The coupling had been previously been rigid and made of PVC. However, if the launder is the same length as the actual tank it is in (extending slightly into the channel wall due to the placement of the coupling), there would be no way to insert the launder into its coupling. A solution to this problem is changing the coupling. Instead of using a rigid coupling, the plant will utilize a flexible coupling made of rubber. The flexible coupling will allow the launder to be "tilted" and easily put into the coupling.

Since the launder does not extend across the entire length of the sedimentation tank, the open end on the inlet channel side must be capped. Otherwise water would flow through the open end of the pipe instead of flowing through the orifices. This is done with a PVC pipe cap. The dimensions for this cap are derived from the PipeSizes matrix in the Pipe Database that defines the dimensions for a cap based on Nominal Pipe Size. 

Removing the Launder and Draining the Sedimentation Tank

Control pieces that fit into this coupling have been designed to stop the flow between the sedimentation tank and the exit channel when it is necessary to drain the tank.

To stop the flow, a PVC cap is glued to a short stub of PVC pipe. The assembly fits into the end of the coupling on the exit channel side. The size of the pipe is the same size as the launder pipe.

 
To re-fill the sedimentation tank with clean water from the exit channel, a uniquely designed control piece is fitted into the launder coupling. The launder is removed from its place when the tank is drained. Then, a control piece consisting of a short pipe covered by a cap with a single orifice is fitted in its place. The size of this orifice is calculated so that the flow rate of water into the empty sedimentation tank is not higher than the flow rate of water out of the other sedimentation tanks so that the plant water level does not drop.

The height used in the equation is the difference in height of the water in the exit channel and the height of the center of the orifice, calculated as:

Once the tank is refilled, the control piece is removed and the launder replaced.