Pressure Drop Due to Combining Flow in a Water Pipe System

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Discussion Overview

The discussion revolves around determining the pressure drop in a water pipe system due to fittings, specifically Tees and Ys, when two flows combine into one. Participants explore methods and values related to pressure loss calculations in this context.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about calculating pressure drop due to fittings when two flows enter and one exits through a Tee or Y.
  • Another participant suggests looking into Hazen Williams values for pressure drop calculations.
  • A different participant mentions using the Darcy-Weisbach/Colebrook method, which involves "K" values for resistance coefficients, but expresses uncertainty about its applicability to fittings.
  • One participant offers to provide foot equivalents for resistance but indicates limitations in providing an equation.
  • Specific fittings are listed by one participant, including sizes and flow configurations, requesting further information on resistance values.
  • Another participant proposes approximating the pressure loss for a Tee as twice that of a 90° elbow and references Crane's for K values related to fittings.
  • Foot values for specific fittings are provided, including those for 10" and 6" Tees and Ys, along with a reference to NFPA standards.
  • A participant seeks clarification on whether the provided values are equivalent hydraulic radii and their application in the Hazen Williams equation.
  • Another participant confirms that the friction coefficient is applied to the equivalent foot value of the fitting to calculate pressure and flow rates.

Areas of Agreement / Disagreement

Participants express varying methods and values for calculating pressure drop, and while some values are provided, there is no consensus on a single approach or method applicable to all scenarios discussed.

Contextual Notes

Some participants express uncertainty about the applicability of certain methods to fittings, and there are references to specific standards and tables that may not be universally accessible or applicable.

metalhead10
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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!
 
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.
 

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