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Fire Sprinkler Friction Loss: A Quick Calculator

3/20/2018

 
Estimating Pipe Sizes for Sprinkler Systems

Occasionally when designing fire sprinkler systems I'm interested in approximately sizing a specific run of pipe early in a project.

That point of interest is often an underground service entry, a main for coordination, or even standpipes. Prior to doing a complete set of hydraulic calculations, running a quick calc using the Hazen-Williams formula can help give an order of magnitude pressure loss that is helpful with initial sizing.

Here's the calculator I use for these estimates. Don't see the tool below? See it here.


​Example: Underground Service Main Sizing

Consider a new project with an Ordinary Hazard Group II fire sprinkler system.

What should the underground service size be? 

A 4-inch fire main can be permitted under special circumstances (see NFPA 13 2002 Section 15.1.3, 2007-2010 23.1.3, 2013-2016 24.1.3). A 6-inch fire main is common. Is an 8-inch necessary? If the length of the service main is 10 feet, my answer can often be quite different than if the service main is 1,000 feet.

For this exercise I often run a quick calculation to judge the pressure loss in this single pipe as opposed to running calculations for a full system, to get order of magnitude pressure loss. Let's assume a long service main length of 750 feet.

Hazen-Williams Formula

NFPA 13 stipulates the Hazen-WIlliams formula be used for pipe friction loss calculations for systems other than antifreeze (NFPA 13 2002 Section 14.4.2.1, 2007-2010 22.4.2.1, 2013-2016 23.4.2.1).

The Hazen-WIlliams formula, while generally considered conservative, only requires the flow, friction loss coefficient (or C-Factor), and the actual internal diameter of the pipe.

Estimating Flow for a Sprinkler System

For an Ordinary Hazard Group II example, I can roughly estimate the flow for the system simply based on density and area (assuming the density/area calculation approach). A density of 0.20 gpm/sqft over the most remote 1,500 sqft begins to look like:

Download this Tool
​Approximate Flow = Density x Area x Overflow Rate + Hose Allowance
Approximate Flow = (0.20 gpm/sqft) x (1,500 sqft) x (1.3) + (250 gpm)
Approximate Flow ​= 640 gpm
​
Why include the Overflow Rate? Naturally a fire sprinkler system is not going to be perfectly balanced.

While my most remote sprinkler can be calculated at exactly 7 psi and it's k-factor that throws exactly 0.20 gpm/sqft, the feed to that sprinkler will have friction loss. Due to that loss, the adjacent sprinkler will experience a slightly higher pressure than 7 psi and thus will throw slightly more water. This process repeats where sprinklers closer to the riser will provide more than the stipulated density.

For order-of-magnitude estimates, I've found that a 30% overflow will be generally close to the final flow result.

Pipe Schedule

The pipe thickness affects the actual internal diameter of the pipe, so I've included it here. I typically will use Schedule 40 pipe for sizes 2-inch and smaller (so that they may have threaded ends), but I've left the schedule type open to users as I know these preferences can vary.

C-Factors

The C-Factor relates to the friction-loss due to the surface of the interior of the pipe. NFPA 13 stipulates C-Factors for fire sprinkler systems depending upon the type of system and pipe material. These can be found in NFPA 13 2002 Table 14.4.4.5, 2007-2010 Table 22.4.4.7, 2013 Table 23.4.4.7.1, 2016 Table 23.4.4.8.1.

Note that important and impactful changes to the c-factors occurred in the 2013 edition for use of galvanized steel, which has been found to accelerate corrosion by focusing the corrosive action at specific weak points in pipe.

Fire Sprinkler Pipe

Friction Losses

With only a few inputs (Flow, Pipe Thickness, C-Factor, and Length of Pipe) you'll now have a comparison of pressure loss across a handful of pipe sizes. Punch in 640 gpm, a Global C-factor of 140 for underground pipe, and a 750 foot pipe length to test this example.

If there is plenty of water at high pressure available to the site, perhaps a 48 psi drop on the service entry could be tolerable and a 4-inch main could be used where it meets other NFPA 13 requirements.

For the vast majority of projects I cover this loss (48 psi) would not be acceptable. The 6-inch service main shows a pressure loss of under 7 psi, and an 8-inch shows under 2 psi loss. Depending on the water to the site, either of these begin to look much more reasonable.

The Friction Loss Calculator

This tool is designed to give quick-comparisons of pressure loss for a run of pipe and compare it against other pipe sizes.

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Landon Veitenheimer
3/21/2018 11:30:13 am

Awesome stuff!!! Thank you

Brian H
3/21/2018 01:44:06 pm

Great stuff. Very excited about future post.

Rob Stewart
3/22/2018 07:50:53 am

Very helpful. It's great that you are willing to share.

Jennifer link
3/23/2018 07:24:26 am

Awesome blog!!! Learning so much thank you so much!!

Michael link
3/29/2018 08:09:04 pm

Great spreadsheet. Great to use for standpipes, too.

Tom Taylor
3/30/2018 10:19:06 am

Nice work Joe!

Douglas Keefer link
1/15/2020 02:34:13 am

Look forward to using these calculation tools

Daniel Harper
2/26/2020 08:33:07 pm

Hi, Can you clarify if hose stream requirements and standpipe requirements are the same thing? If not would you include the standpipe flow requirement in the above calculation?

Joe Meyer
2/27/2020 09:22:43 am

Hi Daniel,

No, hose requirements for a fire sprinkler system is an allowance that gives firefighters the ability to pull some water off of the water supply during a fire and not severely impact the fire sprinkler system. It acts like a safety factor in the fire sprinkler system calculations by adding additional flow. These are outlined in NFPA 13.

Standpipe calculations is separate entirely. Those are for a standpipe system, typically governed by NFPA 14.

This tool is built as a check on friction loss across a single pipe. If you're looking to estimate the size of an underground main, for example, you're going to want to include the most flow across that pipe. If you have a sprinkler system light hazard and are including your hose allowance across the underground, then find your light hazard system flow + hose allowance and use that total flow for this calculator. Everything will be finalized with the hydraulic calculations, but this tool can be helpful for estimating.

Hope this helps!


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  • Blog
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    • NFPA 13 EDITION TRANSLATOR ('99-'22)*
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    • OBSTRUCTIONS AGAINST WALL*
    • PLUMBING FIXTURE COUNTS
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    • REMOTE AREA ANALYZER*
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    • SPRINKLER FLOW*
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