TFS · Hydraulic & Fluid Applications · Problem 5PDFSolution in PDF ↓
TFS · Hydraulic & Fluid Applications · Problem 5
Problem & Solution
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Topic: Bernoulli / flow energy — 50GPM of cold water at 60 degrees enters a 100 foot tall building via 3-inch schedule 40 steel supply piping.
Method: Street pressure prior to the service entrance to the building is 85 PSIG.
Key values: 85 PSIG, 85 psi, 100 foot tall building via 3
Reference: search the reference handbook for Darcy Weiss back equation and th
✅ Answer: A
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Student questions asked in live office hours about this problem
OH 114
Q: Why can't we use friction loss values from the table instead of the Darcy equation? My calculated H_f was 6.2 feet versus your 3.3 feet—when is it safe to use table values for speed?
A: There's a real discrepancy between Darcy equation and steel pipe friction tables, likely due to surface roughness assumptions, but the relative error rarely affects exam answers. The tables tend to be more conservative; while your 6.2 ft is almost double the 3.3 ft, it's only a few percent difference on the total system scale, so either method should work fine as long as you understand which direction you're biasing the answer.