Pipe design help. Know Q and ΔP, need to find D

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Discussion Overview

The discussion revolves around determining the appropriate diameter for a pipe given specific flow rate (Q) and pressure drop (ΔP) conditions. Participants explore calculations related to pipe sizing, pressure drops, and the implications of connecting to an existing system. The scope includes technical calculations and conceptual considerations regarding fluid dynamics in piping systems.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks verification of their calculations for pipe diameter based on known parameters, including flow rate and pressure drop.
  • Another participant questions the feasibility of achieving a velocity of 46 ft/s in a typical piping system, suggesting it is unusually high for the given flow rate.
  • A clarification is provided that the pressure reading of 32 psi is from an existing pipe, with the new pipe discharging into an open container at 0 psi, indicating the need for a significant flow rate to maintain water levels.
  • Concerns are raised about the downstream effects of tapping into the existing line, suggesting that the overall system pressure may be affected by the new flow rate.
  • One participant confirms the accuracy of the pressure drop calculations for a 6" ID pipe, noting a calculated drop of approximately 27 psi at 4000 GPM, which aligns closely with the original poster's findings.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the proposed pipe sizing and its impact on the existing system. While some calculations are confirmed as reasonable, there is no consensus on the overall feasibility of the proposed setup or the downstream effects.

Contextual Notes

Participants highlight potential limitations related to the assumptions made about system pressure and flow dynamics, particularly regarding the impact of tapping into an existing pipe and the resulting changes in pressure.

coolkid
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Hi all,

Need help determining what size diameter for some pipe. I just want someone to check my calculations, hopefully they're okay. I attached a quick sketch of the set up.

Here are the knowns:
Qmin = 4000gpm
ΔP = 32psi
L = 65ft
e(roughness of commercial steel pipe) = 0.00015ft

water properties:
ν = 1.08x10-5 ft^2/s
ρ = 1.94 slug/ft^3

Is it safe to assume since the discharge is below the source we can ignore the height change it and just pretend its a horizontal pipe?

Take a look at my calculations in the excel sheet I attached. The first tab is an estimation for the diameter, the 2nd tab is sort of a recheck, estimating Flow given diameter.

Thanks!
 

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Are you trying to size a pipe to drop the pressure 32 psi to maintain a flow rate of 4000 gpm?

46 ft/s pipe velocity is, generally speaking, way too high for a typical piping system. Rarely do you see anyone putting 4000 gpm through a 6" pipe, I can't really think of any reason to do so.
 
No, the pressure where we will be taping an existing pipe reads 32psi. The new pipe will be discharging into an open container, 0psi. The reason we are adding this water pipe is because there is a container that is constantly too low, so I am proposing we need approximately 4000gpm of water from this new pipe supplying the tank.
 
What's downstream of this? Tapping this line will change the whole system.
 
Travis_King said:
What's downstream of this? Tapping this line will change the whole system.
This is a good point. With 4000 GPM coming out through this pipe, the pressure you have available may be lower than the 32 psi you have now.

Regardless, I checked your pressure drop calculations and they look about right. I get ~ 27 psi pressure drop through a 6" ID pipe at 4000 GPM, so I think your calculations should be pretty close.
 

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