Lower Velocity Where Hazen-Williams Not Valid?

5/11/2020

I am interested if anyone is aware of any research or article that provides guidance on the lowest water velocity that is acceptable for performing a C-factor calculation for a 6” pipe?

I am curious if there is a lower flow velocity where the Hazen-Williams equation may not be valid.

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7 Comments

Dan Wilder

5/11/2020 09:08:52 am

Actually, HW would be your "Go-To" until either higher velocities or different viscosity's are being used. Below are some good articles to read.

https://sprinklerage.com/understanding-darcy-weisbach-equation/

https://www.vikinggroupinc.com/sites/default/files/documents/Calculating%20Friction%20Loss.pdf

Mike

5/11/2020 10:28:19 am

Why would you want lower velocities? As understand it, H-W is for laminar flow and lower velocities would be laminar flow.

Anyone here validate that?

Ivonn

5/11/2020 11:24:01 am

I agree with you, H-W is for laminar flow, so if He/She had low velocity won't be a problem

Ivonn

5/11/2020 10:59:45 am

In NFPA 13 Ed.2019 include a new statement in the section 27.2.1.4 says : the velocity of water flow shall not be limited when hydraulic calculations are performed using the Hazen - Williams.

Hazen-Williams and Darcy–Weisbach are empirical equations, if you do a comparative of pressure loss with the same pipe and same velocity there are differences in the lost, I did several examples with a hydronics software and the lost are always greater with hazen-williams I think maybe for that NFPA 13 said there are no problem with the velocity.

Hazen-Williams empirical test are with laminar flow for that in the oldest time FM Global and other AHJ sugesst 20-30 ft/s to avoid turbulence,but like a said in this point there is not limitation in NFPA or FM Global.

5/11/2020 04:15:33 pm

As far as my knowledge, none of the NFPA provides any limitation in velocity criteria for fire water system, sometime rather these limitation comes from piping materials as guidance...e.g. Metallic pipe, 3.0 m/s or non-metallic pipes 5.0 m/s...

In terms of NFPA 13, it does not provide a specific velocity limitation for the use of the Hazen-Williams formula. This is, in
part, due to an expectation that excessive friction loss values
will result in increasing pipe sizes, thereby serving as an inherent limit on velocity. However, the fact that NFPA 13 does not provide a specific limit should not be taken as an endorsement that the formula can be used for any velocity of water flow. The formula was empirically determined using “normal” conditions.
When the velocity in the pipe exceeds that which was
used to determine the formula, the formula might no longer
be valid. There has been some research performed (Huggins
1996) in which results using the Hazen-Williams formula and
the Darcy-Weisbach formula were compared, and the conclusion was that a specific velocity limit applied to all pipe sizes is not appropriate.

Mark

5/11/2020 11:46:49 am

Maybe I am not reading the question correctly but the C Factor is defined in NFPA 13 based on pipe material?

Mike

5/12/2020 01:00:49 am

C factor is basically a coefficient of roughness that can change over time. Easily determined in existing systems if all other factors are known.

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