Why does this pipe network problem give different answers for different flow equations?

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foo9008
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Homework Statement


in this network of pipe , the author make the attempts to solve it using ( Q1 = Q2 + Q3) , so delta Q = Q1 -(Q2 +Q3) , the value of head of junction that he gt is 25.2 , with delta Q = (3x10^-3) ( not shown in the working)

Homework Equations

The Attempt at a Solution


however, when i tried to solve it using ( Q1 + Q2 = Q3) , which means ( Q1 +Q2 - Q3 ) , thae ans that i gt is hj = 28 , with delta Q = 0.07 [/B]

P/s : we have to find the delta Q = 0 for the ans so that water from inlet = water from outlet
so , question has two ans depending on the flow of water ? or i am wrong ?
 
Attachments
  • Diagram of a branched pipe system with Reservoir A, Reservoir B and Reservoir C connected through Junction J, with a table below listing elevations and pipe lengths and diameters.
    178.PNG
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  • Diagram of a three-reservoir pipe network: Reservoir A at 30 m above datum feeds Pipe 1 to Junction J, Pipe 2 runs to Reservoir B at 18 m, and Pipe 3 runs to Reservoir C at 9 m, with the pipes labelled D and L; below are calculations of Hazen K values for each pipe.
    179.PNG
    19.7 KB · Views: 470
  • Diagram of a three-pipe network with Reservoir A at 30 m, Reservoir B at 18 m, Reservoir C at 9 m and a datum line, joined at Junction J, with handwritten head loss calculations for each pipe.
    180.PNG
    21.7 KB · Views: 499
  • Handwritten-style worked step titled Step 3, Compute Discharges into Junction J, using hf = kQ^1.85: Q1 = (hf1/K1)^1/1.85 = (10/3.942)^1/1.85 = 1.654 m3/s (inflow); Q2 = (2/30.552)^1/1.85 = 0.229 m3/s (outflow); Q3 = (11/15.812)^1/1.85 = 0.821 m3/s (outflow).
    181.PNG
    12.9 KB · Views: 497
  • Worked steps from a pipe network calculation: Step 4 gives deficit ΔQ = Q1 − (Q2 + Q3), then = (1.654) − (0.229 + 0.821), then = 0.604 m³/s; Step 5 says repeat Steps 2 to 4 for junction J until ΔQ ≈ 0.
    182.PNG
    12.5 KB · Views: 524

Discussion

is it wrong to have 2 answer for such question ,
i find it work by using ( Q1 = Q2 + Q3) and ( Q1 + Q2 = Q3)
 
i have done this question in 2 ways , which is ( Q1 - (Q2 + Q3) ) and ( Q1 +Q2 - Q3 ) which is correct ? or both are correct ? please refer to post 1
 
pls ignore the question i posted above ...
now , i have new question with the same figure..
from the diagram , we know that the junction is lower than the pipe at B . why the author want to make an assumption that the junction is located at 20m from the datum ? is it correct to do so ?
 
can i start a new thread and delete this thread ? how to delete this thread ??
 

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