Calculate flow rate in parallel pipes

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SUMMARY

The discussion focuses on calculating the flow rate in a system of parallel pipes, specifically a 30" pipe splitting into three 24" pipes and converging back into a 30" outlet. When one of the parallel paths is closed, the flow rate can be estimated using the principle of continuity, which states that the total flow into the parallel pipes equals the total flow out. The analogy to Ohm's Law is employed to understand the relationship between flow rate and pressure drop, emphasizing that identical pipes will maintain the same pressure drop and flow velocity. The key takeaway is that closing one path increases the flow velocity in the remaining pipes, affecting the overall flow rate.

PREREQUISITES
  • Understanding of fluid dynamics principles, particularly the continuity equation.
  • Familiarity with the concept of pressure drop in fluid systems.
  • Basic knowledge of Ohm's Law and its application to fluid flow.
  • Ability to manipulate and solve equations involving flow rates and pipe dimensions.
NEXT STEPS
  • Research the continuity equation in fluid dynamics.
  • Learn about calculating pressure drop in pipes using the Darcy-Weisbach equation.
  • Study the effects of pipe diameter and flow velocity on flow rate in parallel systems.
  • Explore practical applications of fluid flow analogies in electrical circuits.
USEFUL FOR

Engineers, fluid dynamics specialists, and anyone involved in the design or analysis of piping systems, particularly those working with parallel flow configurations.

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.
 
I have encountered a vertically oriented hydraulic cylinder that is designed to actuate and slice heavy cabling into sections with a blade. The cylinder is quite small (around 1.5 inches in diameter) and has an equally small stroke. The cylinder is single acting (i.e. it is pressurized from the bottom, and vented to atmosphere with a spring return, roughly 200lbs of force on the spring). The system operates at roughly 2500 psi. Interestingly, the cylinder has a pin that passes through its...

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