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.
NFPA 13 stipulates the Hazen-WIlliams formula be used for pipe friction loss calculations for systems other than antifreeze (NFPA 13 2002 Section 126.96.36.199, 2007-2010 188.8.131.52, 2013-2016 184.108.40.206).
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:
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.
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.
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 220.127.116.11, 2007-2010 Table 18.104.22.168, 2013 Table 22.214.171.124.1, 2016 Table 126.96.36.199.1.
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.
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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