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Does anyone know why when figuring friction loss in wyed lines (all wyes equal) you only figure for one of the wyed lines? I get asked this all the time and have not been able to find the reasoning.


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It's more difficult to explain than to demonstrate with inline pressure gauges, and I hope my attempt at an explanation helps answer your question.  After the line is wyed, the two lines are running in parallel, therefore you can't add the two pressure losses together.  So, if you have a single line divided by a wye and supplying two lines (with equal nozzles), you could think of the wye outlets like pump outlets.  The pressure at each pump outlet will be the same as the main pump pressure; and as such the pressure at the wye outlets will supply each line at the same pressure.  The friction loss factors then apply to each line separately and is calculated from the pressure at the wye, because this will be the same down both lines.  So if the pressure loss through each hose line is 50psi (same nozzles and line size/length), then the pressure at the wye needs to be 50psi more than required at the nozzle.  If it was 100psi more each outlet of the wye would be 100psi more, and hence each line would also be 100psi more.  But, the flow rate to the wye will be the total required for both lines before it divides.  

The thing I find most misunderstood is that the single line supplying the wye is subject to friction loss based on the combined flow of each nozzle operating from the wye.  So, if you have two 125gpm nozzles coming from the wye, the line supplying the wye has the friction loss calculated on a flow rate of 250gpm, which will be 4 x more than 125gpm. 

Just reading this back to myself, and it looks terribly confusing.  To demonstrate the reasons why and how this works using inline pressure and flow gauges is the best way to get the head around it.  I really hope I've helped in some small way, but I'm not holding my breath.


Mike D

Thank you Mike,

Great explaination. I may do a training using pressure gauges, I hadn't thought about that.

Thanks Again

Bob Shovald

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