Calculate flow rate in parallel pipes

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Discussion Overview

The discussion revolves around calculating the flow rate in a system of pipes where a larger pipe splits into multiple parallel paths and then converges back into a single outlet. Participants explore the implications of closing one of the parallel paths on the flow rate at the outlet, considering concepts from fluid dynamics and analogies to electrical circuits.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents a scenario with a 30" pipe splitting into three 24" parallel pipes and seeks to estimate the new flow rate when one path is closed.
  • Another participant suggests using Ohm's Law as an analogy to understand the flow dynamics in the parallel pipes.
  • A participant questions the assumption that the flow rate remains the same when one path is closed, indicating uncertainty about the relationship between flow rates in parallel systems.
  • One participant emphasizes the continuity relation, stating that the total flow into the parallel pipes must equal the total flow out, and notes that closing one pipe would increase the flow velocity in the remaining pipes.
  • There is a reiteration that the pressure drop across the pipes remains the same due to their identical dimensions, which should simplify the analysis.
  • Participants discuss the need to write out equations using dummy variables to represent unknowns, reinforcing the analogy to electrical circuits.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding flow rates when altering the configuration of the pipes. While some agree on the principles of continuity and pressure drop, there remains uncertainty about the exact flow rates and how they change with the closure of one path.

Contextual Notes

Limitations include the assumption that all pipes are identical and the dependence on the continuity equation. The discussion does not resolve how the flow rate specifically changes when one path is closed.

Who May Find This Useful

This discussion may be useful for individuals interested in fluid dynamics, engineering applications involving pipe flow, or those exploring analogies between fluid and electrical systems.

steves1080
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So I have a 30" pipe that splits into 3 parallel paths, each 24" in dia. The pipes converge into a 30" outlet and then flows at about 40,000 gallons per minute at full head pressure. Assuming the same head pressure and one path closed off (i.e. now only flowing through 2 parallel paths), how can I determine my new flow rate from the outlet? Testing this would be a real hassle. I'm just looking for a good estimate, not necessarily right on the money. Thanks
 
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Hint: Ohm's Law.

You want to know the current for two identical resistors in parallel given the current for three and the same voltage across them.
 
Good analogy. But do I know the current? I know the "current" for 3 open parallel paths, but if I eliminate one path, I can't exactly assume it's the same flow rate as before - right?
 
You know the total flow into the parallel pipes must equal the total flow out. This is the continuity relation. If you block one pipe but don't change the flow rate into the network, the the flow velocity must increase.

You must also have the same pressure drop (or resistance to flow, to continue the electrical analogy) from the entrance of the parallel pipes to their exit, but having all of these pipes the same diameter and length makes it easy to satisfy this requirement. If the flow velocity is the same in each pipe, then the pressure drop thru each pipe should also be the same, as long as the pipes are identical.
 
steves1080 said:
Good analogy. But do I know the current? I know the "current" for 3 open parallel paths, but if I eliminate one path, I can't exactly assume it's the same flow rate as before - right?
That is correct - the same pressure difference can produce a different flow rate like the same voltage can produce different currents. Your main advantage is that you know the pipes are identical so the resistance to flow will be identical and that the pressure difference (voltage) is the same.

Write out the equations - use dummy variables for everything you don't know.
Just pretend it's an electric circuit.
 

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