# Pressure Drop Due to Combining Flow in a Water Pipe System

## Main Question or Discussion Point

Hello,

I am wondering how to determine the pressure drop due to fittings such as Tees and "Y's" when there are two flows coming in and one coming out. So for example there is a Tee where there are two flows coming in opposite directions (180 degrees from one another) and they combine and exit through the branch. It is the same story for the Y. Any help or information would be greatly appreciated.

Thank you.

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Are you looking for the Hazen Williams values?

Any values/method would be useful. I have been using the Darcy-Weisbach/Colebrook method which uses "K" values for representative resistance coefficients for fittings. The Hazen Williams uses a friction factor but from what I can tell it is just for straight pipe and I can't find anything for fittings. I could be wrong. Let me know what you think.

If you have size I can give you foot equivalents for r but if you need an equation I can't help. The table in NFPA 14 is based on size alone.

Ok, so I have the following fittings:

10" Tee
6" Tee
10" Y
6" Y

They are all equal flow with them all going 2 into 1.

If you could scan that table that would be great.

Thanks again!

FredGarvin
For a tee, you can think of it as 2 times the loss of a 90° elbow to get an approximation. Crane's does list a tee with flow out of the branch as having a $$K= 60*f_t$$ where $$f_t$$ is a friction coefficient based on clean pipe and the size of the pipe.

A Y will be tougher but I would still model it as two elbows of varying angles.

A 10" Tee has a foot value of 50.
A 6" Tee has a foot value of 30.
A 10" Y has a foot value of 16
and a 6" Y has a foot value of 9.

ref: NFPA 14.5.10.1 (2006 ed.)

Ok so just to confirm, those values are "R" values which are equivalent hydraulic radiuses? Like does the table call them something else or just specificially "R" and that they are used in the Hazen Williams equation?

You're there. The friction coefficient for the radius of the pipe is applied to the 'equivalent foot value' of the fitting to obtain pressure and flow rates.