Head Loss Calculation: Discrepancies?

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Discussion Overview

The discussion revolves around the calculation of head loss in fluid dynamics, specifically comparing results obtained from a formula and a nomogram. Participants explore discrepancies in their calculations and seek clarification on standard values for head loss in pipes, particularly those larger than 15 mm.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes a significant discrepancy between head loss calculated using a formula and a nomogram, questioning if a fundamental misunderstanding exists.
  • Another participant inquires about the data inputs used for the calculations.
  • A participant suggests that nomograms are derived from equations for head loss and should yield similar results unless there is a units error.
  • After resolving earlier discrepancies, a participant raises a question about the common literature value of 100 Pa/m for head loss in pipes larger than 15 mm, noting their calculations yield higher values.
  • One participant explains that the 100 Pa/m figure is a design parameter based on experience and should not be taken as an absolute value.
  • Another participant introduces the concept of an average loss coefficient, suggesting it varies by pipe type.
  • A different participant questions the definition of average loss coefficient, asserting that only the friction factor should vary with pipe type, and that initial calculations often assume a smooth pipe.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of standard head loss values and the factors influencing head loss calculations. There is no consensus on the appropriateness of the 100 Pa/m figure or the role of average loss coefficients.

Contextual Notes

Participants mention potential issues with units and assumptions regarding pipe types, but these aspects remain unresolved within the discussion.

Who May Find This Useful

Individuals interested in fluid dynamics, engineering design, and those involved in calculations related to head loss in piping systems may find this discussion relevant.

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I was reading on this page...

http://www.cda.org.uk/megab2/build/pub125/sec4.htm#5.1

...and I tried calculating head loss using both the formula given, and the nomogram. Aren't these supposed to give the same result? Or am I missing something fundemental? Because they don't equal each other by a long shot...

An explanation would be very much appreciated :-)
 
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The nomograms will be made from the equations for head loss. Unless a type-o was made in the webpage, they should coincide pretty closely. Are you sure you don't have a units snafu somewhere?
 
I got them pretty much equal now, don't even know what must have gone wrong before. Damn units...

One more question for you guys:

I often see in literature that head loss is often set to 100 Pa/m pipe, at least for pipes bigger than 15 mm without further calculations, but almost all of my calculations show much higher values than this. Any idea why?
 
To my knowledge, a number like that is a design parameter (rule of thumb) developed after a lot of experience. It is a target to shoot for, not an absolute. This would help calculate the pipe diameter.

I really can't imagine someone saying that "well, my pipe size is over 15mm. I guess my pressure loss will be 100 Pa/m no matter what I am doing." That just doesn't make much sense.
 
The average losses should depend on an assumed pipe TYPE. Each type should have a different average loss coefficient.
 
Average loss coefficient? What is that? The only factor that would depend on the type of pipe is the friction factor. And then most people assume a smooth pipe on the first go around anyways.
 

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