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:
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. 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. Do you get these free weekly articles? If not, subscribe here.
Last Week's Survey Results
Last week I sent a survey asking for "challenges associated with sprinkler identification and design selection." I really appreciate the input provided, there was really helpful and great feedback: common challenges people noted in the survey included sprinkler market availability, listing and approvals, field identification, adherence with product data, price, storage limitations, pressure requirements, and spacing requirements. Anticipation for the Big Launch I am very excited to say that I've been developing a live resource over the past couple years to address almost exactly those challenges. Stay tuned, as more details will be available about the launch in a few weeks. In the meantime, I'm also excited that the blog posts over next three weeks (starting with today) will feature tools designed to help streamline and speed workflow for inspectors, designers and engineers. Part I of III: The Cloud Ceiling Calculator This first week covers the relatively new allowances for cloud ceilings.
"Cloud" Ceilings where directly addressed in NFPA 13 beginning with the 2016 Edition
Cloud Ceilings include any ceiling installed in the same plane with horizontal openings to the structure above on all sides (NFPA 13-2016 3.3.5.1). The "cloud" is simply in reference to the appearance that the ceiling "floats". The new provisions in NFPA 13-2016 allows sprinklers to be omitted above cloud ceilings where the gap between clouds (or clouds and walls) meets a maximum allowable dimension based on the floor-to-cloud ceiling height. Backed by Research What I love about this new verbiage is not just that the NFPA 13 committee addressed a specific topic that many had asked about for some time, but that the development of the rules for this section are based on a commissioned project by the Fire Protection Research Foundation. So what is the guidance based on the research findings? Spaces above cloud ceilings do not require sprinklers where the openings have a combined total area of not more than 20 percent of the ceiling, construction feature, or plane used to determine the boundaries of the concealed space and the cloud ceiling arrangement meets Section 8.15.24.1 (NFPA 13-2016 8.15.1.2.1.3). Limitations I've already mentioned that the opening between all cloud ceilings can't be more than 20% of the total room area, but there's a few others that also apply:
Ceiling Spacing Calculator If these limitations can be met, sprinklers may be omitted above where the spacing below the ceiling complies with Table 8.15.24.1. The table addresses the maximum protection area based upon the research, and is a little less than intuitive. Here's a quick calculator that takes your parameters and gathers the appropriate maximum sprinkler protection area (click the link to see the full tool, with a schematic section of the ceiling arrangement): Enter your project parameters in the red highlighted cells to test your situation. Give it tool a try and let us know what you think in the comments section below. If you've found this interesting or helpful, consider getting more of these tools by joining our email group. Already subscribed? Share this on LinkedIn for others who might find it interesting.
Last week I explored when volumes of a system or portion of a system become important. Principally, volume in fire sprinkler or standpipe systems becomes important in overall capacity limitations of dry and pre-action systems and in draining portions of wet, dry, and pre-action systems. See the full article here.
This week I've created a basic quick-and-dirty volume calculator based on the length of pipe in a system (don't see the calculator below? View in your browser here): Simply select the type of pipe used and enter the approximate length of pipe for each pipe size in the system or portion of a system you're evaluating. I created this when looking at whether I break the 5-gallon volume threshold for portions of wet sprinkler systems, bottom legs of standpipes, or overall dry and pre-action system capacities. Email suggestions or tips for improvement to me at info@meyerfire.com. Want to see more like this? Subscribe to these free weekly articles here.
One of the most common and basic issues many of us encounter in fire sprinkler design or during on-site review is whether a sprinkler is considered to be obstructed. While the premise of the obstruction tables within NFPA 13 is fairly straightforward, there are a handful of variations in the tables that are dependent upon the edition of 13 being used, the sprinkler type, and in some cases the orientation of the sprinkler.
This reference tool below was built to quickly determine whether a ceiling-mounted element is considered an obstruction. It can be especially helpful during sprinkler layout or during site review where lugging the entire code volume might not be practical
Common examples of where obstructions are considered are with sprinklers adjacent to surface-mounted lights, soffits (not against a wall), mechanical equipment in walk-in coolers and freezers, signage, banners, lowered ceilings, thresholds above large openings, raised ceiling pockets, or exit lighting.
Give this a try and let us know what you think in the comment section below (the red highlighted cells are input values). Having trouble viewing? Click here to see the full tool. |
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+ Unsubscribe anytime AUTHORJoe Meyer, PE, is a Fire Protection Engineer out of St. Louis, Missouri who writes & develops resources for Fire Protection Professionals. See bio here: About FILTERS
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May 2024
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