Water Pressure, small inlet vs. large outlet

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Discussion Overview

The discussion revolves around the dynamics of water flow in a sewer box with an 8" inlet pipe and an 18" outlet pipe, particularly under conditions of inundation from storm water entering through a large grate. Participants explore the implications of pressure and elevation on the flow of water, considering whether backflow into the inlet pipe occurs when the system is overwhelmed by incoming water.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the incoming storm water through the grate would primarily discharge through the larger 18" outlet pipe, potentially negating any backflow into the 8" inlet pipe.
  • Another participant emphasizes that the pressures and elevations at the ends of the pipes are more critical than their diameters, indicating that flow direction could be influenced by these factors.
  • It is noted that both the 8" inlet and 18" outlet have equal elevations and that their far ends are lower than the grate inlet, which could affect the flow dynamics.
  • A later reply concurs that under the stated conditions, there would be some backup into the 8" inlet due to the pressure from the incoming water.

Areas of Agreement / Disagreement

Participants express differing views on the influence of incoming water on backflow into the inlet pipe. While some suggest that the larger outlet would dominate, others indicate that pressure dynamics could lead to some backup into the inlet. The discussion remains unresolved regarding the extent and conditions under which backflow occurs.

Contextual Notes

Participants have not fully explored the specific conditions that might affect flow, such as the rate of incoming water, potential blockages, or the precise configuration of the pipes. The discussion relies on assumptions about pressure and elevation without detailed calculations.

FriscoGary
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If there is a "sewer box" (2' x 2' x 2') with a 8" drainage inlet pipe into it and it has a 18" drainage outlet pipe to
discharge water entering into the sewer box and the large 2' x 2' grate opening at the top...IF the entire sewer box is inundated with storm water, will the incoming water thru the top 2' x 2' grate allow water into (backflow) the 8" inlet pipe as water is also discharging out the 18" drainage outlet pipe? I would think the storm water entering from the grate opening would mainly discharge out the larger 18" discharge pipe and sort negate or
"push back" any water trying to drain into the box from the small 8" pipe. My question is will the volume of water from the top grate not allow any water from the small 8" inlet to enter into this system if it is inundated from the top. My thought was all the pressure from the top would negate or equalize any from the 8" pipe...any insight would help!
 
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The diameter of the inlet and outlet are less important than the pressures or elevations at the far ends.

In theory, pressure in the middle could cause flow in both directions, backing up the inlet, but whether this happens or not depends on more details than the sizes.
 
The elevations are equal for the 8" inlet and 18" outlet and their far ends are lower than the top of the "sewer box" grate inlet. The 8" inlet only has
pressure from the water draining into it and it is a level pipe.
 
FriscoGary said:
The elevations are equal for the 8" inlet and 18" outlet and their far ends are lower than the top of the "sewer box" grate inlet. The 8" inlet only has
pressure from the water draining into it and it is a level pipe.

In that case, yes you will get some backup into the inlet.
 

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