Launder Design Program

          The launder design program is used to design the pipe that transports 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 that are used to generate a more even flow of water. Once the water enters the launder pipe, it travels to the exit channel. 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 total number of sedimentation tanks, the flow rate is equally divided between them. 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. It was not possible to extend the launder previously, because when using a rigid coupling, the launder had to be pulled out of the coupling (horizontally towards the inlet channel) to remove it from the tank.The launder now reaches all the way across the sedimentation tank, extending from inlet channel to exit channel, because a new, flexible coupling is being used. The increased length makes it possible to build a ledge to support the launder (attached to the inlet channel wall), and it also helps to optimize the distribution of water through the orifices so the distribution is more even. 
 

          Before I could determine the length of the launder, it was necessary to decide on the new setup for the launder in the sedimentation tank (above). Before, there was a rigid or PVC coupling located somewhere in the exit channel wall, and the launder was inserted into that. In order to extend the length of the launder all the way across the length of the sedimentation tank, a flexible coupling must be used. Because this rubber coupling bends, it will allow the launder to be tilted/slanted and then removed, hence making a longer launder length workable. However, a PVC coupling must also be used in this setup. The new setup involves the the rigid coupling being placed inside of the channel wall flush with the outside of the wall. Then a length of PVC pipe called a nipple will be inserted into PVC coupling. On the other side of the nipple, a flexible coupling can be found. Into the opposite end of this coupling is where the launder will be placed.

          In order to determine the length of the launder, it was necessary to consider two cases of rigid couplings: when the coupling extends past the channel wall and when the coupling is flush with or inside the channel wall. The width of the exit channel is comprised of multiple parts; the W.ExitChannel and also T.ChannelWall, shown in the above image. The coupling itself is placed in the part of the channel with width T.ChannelWall (the wall of the channel). If the length of the coupling is greater than this variable, the length of the coupling is equal to the length of the sedimentation tank, minus the width of the inlet channel, minus the width of the exit channel, minus the length of the PVC coupling, minus the length of the flexible coupling, plus the socket depth of the flexible coupling, minus the thickness of the cap (the cap is at the inlet channel side). Below is a drawing of case number one. The flexible coupling is shown in light yellow, while the rigid coupling, which extends into the exit channel, is shown in turquoise.

          In the second case where the coupling length is less than the thickness of the wall, the flexible coupling would not be flush with the PVC coupling but rather would be flush with the channel wall. The length of the launder in this case would be the length of the sedimentation tank, minus the width of the inlet channel, minus the width of the exit channel, minus two thicknesses of the channel wall, minus the length of the flexible coupling, plus the socket depth of the flexible coupling.

 
          When determining the length of the launder, it was important to keep in mind that there is a cap on the end of the launder. This has to be accounted for when deciding exactly how long the launder should be because the cap adds length to the whole "system." In order to determine how much length the cap added, it was necessary to know the thickness of the cap. The thickness of the cap can be calculated by subtracting the inner diameter of the cap (or the outer diameter of the launder) from the outer diameter of the cap. Thickness is constant throughout the entire cap so this calculation will indeed produce the horizontal length added by the cap. The final product is shown above.

          To ascertain 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 previously been rigid and made of PVC. However, if the launder is extended across the entire tank, there would be no way to insert the launder into its coupling. A solution to this problem is adapting the coupling. Instead of using only a rigid coupling, the plant will also utilize a flexible coupling made of rubber. The flexible coupling will allow the launder to be "tilted" and easily put into the coupling.

          The "open" side of the launder, that is, the side in the inlet channel, must be capped to ensure that no water seeps in around the edge of the side of the pipe that is up against the wall. 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 Pipe Database using the nominal pipe sizes.

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.

Supporting the Launder

          Another change to the design of the launder is how the launder is actually supported. In each plant thus far, engineers have used whatever materials they could find to hold the side of the launder up that is not in the coupling, or the side near the inlet channel. Instead of leaving this as somewhat of an undefined "variable," the design will be changed and a support will be added. This support will be essentially a concrete "box" underneath the launder. The length of this support will be determined by the diameter of the launder, as it needs to be just slightly longer that this diameter in order to ensure the entire launder pipe fits and is stable. Its height will be determined by the height of the lamina, which is located directly below the launder. The ledge has to be "short" enough so that it does not interfere/come in contact with the lamina.