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| | | Forum Newbie
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| | I am relatively new to collection systems design so bear with me. I am currently reviewing a utility plan proposing a lateral through a development with existing intermediate sewers connecting to it. As it leaves the property line, the design shows a connection from a 15" PVC to an 8" VCP sewer. They justified using the larger diameter sewer to meet its self-cleansing (2.0 ft/s) requirements for 15" sewer and because of the difficulty to meet slope requirements upstream for smaller pipes. Is this good engineering judgment and what kind of capacity or other types of problems can arise from this? Would it be costly to propose a pump station and use a force main to meet these requirements? |
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| | | Supreme Being
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| | No, they just don't want to put in a liftstation. |
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| | | Supreme Being
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Last Login: Today @ 4:42:29 PM
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| I am currently reviewing a utility plan proposing a lateral through a development with existing intermediate sewers connecting to it. As it leaves the property line, the design shows a connection from a 15" PVC to an 8" VCP sewer. They justified using the larger diameter sewer to meet its self-cleansing (2.0 ft/s) requirements for 15" sewer and because of the difficulty to meet slope requirements upstream for smaller pipes. Is this good engineering judgment and what kind of capacity or other types of problems can arise from this? Would it be costly to propose a pump station and use a force main to meet these requirements?
I am having trouble understanding and visualizing the scenario that you are describing. Are you saying that the proposal is for a 15" collection system in the new development that connects to an existing 8" vcp system? And, is there no waste flow "upstream" of the new development?
At any rate, I would expect that the velocity of flow in the 15" pipe would be slower than for a comparable 8" pipe, given the same slope. So, I can't see how increasing the pipe diameter would increase the "self-cleaning" capability of the system. The 2 ft/second flow velocity is commonly used as a minimum in gravity waste piping in order to keep solids in suspension. The trade-off between a force main/pumping station versus a gravity system would be a double-edged sword in that with a gravity system there is the potential for clogging that would require occasional cleaning, while the force main and pump station obviously would be expensive, and would also require maintenance along with the power coast to operate it. It may also require occasional cleaning.
Jeff Naumann
(310) 540-0045
FAX (310) 540-0337
http://www.jeffnaumannassociates.com/ |
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| | The lateral is actually within stadium (which is city-owned) property and no, there is no waste upstream from the stadium. It will be handling domestic waste from several service connections from the stadium. I was not clear with the 15" self-cleansing velocity requirement, I meant they wanted to meet the minimum slope requirement for the 15" (0.15%) versus an 8" (0.4%) so that's how they justified connecting to a smaller sewer. I was just wondering what problems (capacity, larger pipe becoming septic, etc.) can arise from connecting a 15" pipe to an 8" pipe and is this good engineering judgment? Existing service connections upstream from where the lateral leaves the property line, it appears, inhibit the allowance for a steeper slope. Would it be more feasible to run a force-main to meet grade? |
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| | | Supreme Being
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Last Login: Today @ 4:42:29 PM
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| The lateral is actually within stadium (which is city-owned) property and no, there is no waste upstream from the stadium. It will be handling domestic waste from several service connections from the stadium. I was not clear with the 15" self-cleansing velocity requirement, I meant they wanted to meet the minimum slope requirement for the 15" (0.15%) versus an 8" (0.4%) so that's how they justified connecting to a smaller sewer. I was just wondering what problems (capacity, larger pipe becoming septic, etc.) can arise from connecting a 15" pipe to an 8" pipe and is this good engineering judgment? Existing service connections upstream from where the lateral leaves the property line, it appears, inhibit the allowance for a steeper slope. Would it be more feasible to run a force-main to meet grade?
As long as the 15" pipe system is designed to flow properly, and not allow the settling of solids, it should be fine. Keep in mind that the stadium is used only intermittently so that continuous (or, near-continuous) flushing is not available. I assume that the 8" pipe is at a higher slope, so that it's capacity is actually more than the 15" pipe under typical conditions of flow.
Another reason to NOT install a pump station is the intermittent nature of the system, which would leave some volume of sewage standing in the wet well in between stadium events. This would lead to settling of solids and odors.
Jeff Naumann
(310) 540-0045
FAX (310) 540-0337
http://www.jeffnaumannassociates.com/ |
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