Friction Factor-Simple Pipe Network
Friction Factor-Simple Pipe Network
(OP)
Hi all, I was wondering if anyone can inform me why it is for a simple pipe network problem such as the Hardy Cross method, why it is common practice to use an estimated/given value for friction factor and when and where it would not be fitting to do so?
Lastly, would it be possible to alter the Hardy Cross method so it would not be dependent upon friction factor from the outset?
Kind Regards
j20zi
Lastly, would it be possible to alter the Hardy Cross method so it would not be dependent upon friction factor from the outset?
Kind Regards
j20zi





RE: Friction Factor-Simple Pipe Network
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Friction Factor-Simple Pipe Network
What you're essentially saying is that, we can assume friction factor during turbulent flow i.e Re>4000 because it does not have a great effect on Darcy's equation?
And going back to the last question, how could we amend Hardy Cross, so it is not dependent on friction factor from the start?
RE: Friction Factor-Simple Pipe Network
As to the Moody diagram, there are no modern pipes that consistently put you to the right of the fully-turbulent line so with modern pipes the stuff that assumes constant friction factors is never really satisfied.
David Simpson, PE
MuleShoe Engineering
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RE: Friction Factor-Simple Pipe Network
However, you can simplify the process by selecting a single representative value for the FF/RC for the entire network. This is commonly done with the Hazen-Williams and Mannings equations when the pipes are all the same material. At worst, you might have several different values in your model based on pipe types. It's really not that big a deal with these two equations.
Darcy presents a more difficult situation because finding the mathematically correct friction factor (as opposed to physically correct) for each pipe involves an iteration (Colebrook-White) inside another iteration (Hardy Cross in your case). That's why selecting a single representative value (or a handful of representative values) is sometimes done when dealing with Darcy and networks. It's also easy to do. You should already know the range of flows, pipe sizes, materials, etc to at least a reasonable level, so pick a few reasonable flow rates, a few pipe sizes, etc and determine a few friction factors. This should provide enough information that you can confidently choose one or a couple friction factors for your pipes.
Personally, I rarely use Darcy and never for modeling water systems. Based on how the equations behave and some literature I have seen that showed how the equations compare to the Moody Diagram, I use Hazen-Williams for municipal distribution (typically <= 24-inch) and Mannings for things like large diameter concrete pipe networks (e.g. irrigation backbones). I usually reserve Darcy for things like pumping station manifold piping and non-water flows.
One other thing, there are better and more robust solution methods than Hardy Cross. If you like free software, I suggest you take a look at EPANET: http://www.epa.gov/nrmrl/wswrd/dw/epanet.html. I have used about a dozen different programs to model water systems, including two Hardy Cross programs I wrote for the HP-41CX calculator (one was a substantial rewrite of the program in HP's HP-41 Hydraulics Solutions book and the other was written specifically for a particular system that I had to analyze for specific regulatory conditions). EPANET doesn't have all the bells and whistles that some programs have but it also doesn't cost hundreds or thousands of dollars.
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RE: Friction Factor-Simple Pipe Network
It dosnt take long to implement, is very roboust and does not require a lot of computational power.
Best regards
Morten
RE: Friction Factor-Simple Pipe Network
http://www.eng-tips.com/viewthread.cfm?qid=335355
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Friction Factor-Simple Pipe Network
PE manuafacturers with sophisticated instrumentation can produce pipe near to minimum wall thickness without too much trouble. After all PE pipe is just a polymer with energy squeezed into it. Reduce the amount of polymer and the energy to melt same equals greater profit.
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