Why Does Pressure Decrease with More Outlets in a Fluid System?

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SUMMARY

The discussion centers on the relationship between pressure and flow rate in a fluid system using a positive displacement pump delivering 30 gallons per minute (gpm) through a 1.5-inch pipe with multiple outlets. When one outlet is used, the pressure is at P1, but when two outlets are utilized, the pressure drops to P2, indicating that the pressure in the pipe decreases as more outlets are opened. This phenomenon can be explained by the principles of fluid dynamics, specifically the Bernoulli equation, which suggests that while flow rate remains constant, the distribution of pressure changes due to increased outlet area. The user also explores pipe losses in a 6mm diameter steel tube at 33m/s, indicating complexities in calculating pressure drops beyond standard charts.

PREREQUISITES
  • Understanding of fluid dynamics principles, particularly the Bernoulli equation
  • Knowledge of positive displacement pumps and their operational characteristics
  • Familiarity with pipe flow calculations and head loss due to friction
  • Experience with pressure measurement techniques and gauge readings
NEXT STEPS
  • Study the Bernoulli equation in detail, focusing on its application to multiple outlet systems
  • Research methods for calculating head loss in pipes, including the Darcy-Weisbach equation
  • Examine the effects of pipe diameter and flow velocity on pressure drops in fluid systems
  • Explore advanced fluid dynamics topics, such as turbulent vs. laminar flow and their impact on pressure
USEFUL FOR

Fluid mechanics engineers, hydraulic system designers, and anyone involved in optimizing fluid flow in piping systems will benefit from this discussion.

NakorOranges
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Hey, so my situation is a positive displacement pump churning out 30gpm. there's a bunch of pipe losses that I can deal with, and then a long 1.5" pipe with 8 possible outlets (you can screw on a hose). When one hose is attached the pressure is a we will say P1 (read from a gauge on the 1.5" pipe). When two hoses are attached P2 < P1. So the pressure in the pipe goes down. This makes sense when you think about it, less pressure required to push all that liquid out since you have twice the area.

My problem is trying to explain this with the bernoulli equation. Shouldnt the energy (head or stagnation pressure if you want) in the pipe stay constant in these two situations? But since I am sure there is 30gpm going through either one hose, or 15gpm each going through 2, there is always 30gpm going through the 1.5" pipe. That means the pressure has to stay constant for the total energy to stay constant right?

Whats happening here??

Update: At this point I am of the thought the pump just pushes harder. Now I am at the stage of trying to get the pipe losses in a 6mm diameter steel tube with water going through at 33m/s. This goes past my moodly chart, and therefore the answers seem to give giant pressure drops...
 
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Update: At this point I am of the thought the pump just pushes harder. Now I am at the stage of trying to get the pipe losses in a 6mm diameter steel tube with water going through at 33m/s. This goes past my moodly chart, and therefore the answers seem to give giant pressure drops...
 

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