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.

Posted anonymously for discussion. Discuss This | Submit Your Question | Subscribe

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.

Leave a Reply.

Home
Our Cause

The Blog
The Forum

PE Exam Prep
The Toolkit
MeyerFire University

Pricing
Login
Support
Contact Us

MeyerFire.com is a startup community built to help fire protection professionals shine.
Our goal is to improve fire protection practices worldwide. We promote the industry by creating helpful tools and resources, and by bringing together industry professionals to share their expertise.

MeyerFire, LLC is a NICET Recognized Training Provider and International Code Council Preferred Education Provider.
All text, images, and media Copyright © 2023 MeyerFire, LLC

We respect your privacy and personal data. See our Privacy Policy and Terms of Service. The views, opinions, and information found on this site represent solely the author and do not represent the opinions of any other party, nor does the presented material assume responsibility for its use. Fire protection and life safety systems constitute a critical component for public health and safety and you should consult with a licensed professional for proper design and code adherence.

Discussions are solely for the purpose of peer review and the exchange of ideas. All comments are reviewed. Comments which do not contribute, are not relevant, are spam, or are disrespectful in nature may be removed. Information presented and opinions expressed should not be relied upon as a replacement for consulting services. Some (not all) outbound links on this website, such as Amazon links, are affiliate-based where we receive a small commission for orders placed elsewhere.

Lower Velocity Where Hazen-Williams Not Valid?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES