Head Loss Calculation: Discrepancies?

[TSN79]

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 :-)

 

[russ_watters]

What data are you plugging in?

 

[FredGarvin]

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?

 

[TSN79]

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?

 

[FredGarvin]

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.

 

[civil_dude]

The average losses should depend on an assumed pipe TYPE. Each type should have a different average loss coefficient.

 

[FredGarvin]

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.