First - last week I put together a draft PDF cheatsheet for fire alarm design in elevators. Lots of great response to that tool. One major flub on my part - I didn't actually link to it. Here's an actual working hyperlink (fingers crossed).
K-Factor & Pressure Versus Area & Density
One of the hand calculations I do frequently when laying out sprinkler systems is comparing the k-factor, minimum pressures, and resulting flow for the sprinkler. It comes up all the time with residential-style, extended coverage, special application, and storage sprinklers.
Many hydraulic calculation programs do this comparison automatically. That being said, it is important to understand and compare the minimum flow from sprinklers for a hydraulic calculation.
Reducing unnecessary flow from a sprinkler reduces the total calculated flow from a system, which has major impacts on pipe sizing for some branch lines, cross mains, feed mains, and even the underground service size.
Driver #1: K-Factor and Minimum Pressure
There are two drivers for the actual minimum flow that must come from a fire sprinkler.
The first driver is the K-Factor and Minimum Pressure. This equation is
Q = k√P
Q = Flow (gpm)
k = Sprinkler k-Factor
P = Pressure (psi)
With a 5.6 k-factor and a minimum pressure of 7.0 psi (as is required by NFPA 13), we get a flow of 5.6 x √7 = 14.8 gpm
There's a wide array of k-factors available on the market, and a wide variety of minimum sprinkler pressures too. Extended coverage, residential, attic, storage, and ESFR all vary in required minimum pressures.
Driver #2: Area and Density (When Using the Density/Area Design Approach)
When reviewing cutsheets for sprinklers it's easy to take a k-factor and minimum pressure and assume that you then know the minimum requirements for a sprinkler and you're done. If you're using design criteria that only uses that approach, then you may actually be done.
However, if you're using the density/area approach of NFPA 13 then you also have to ensure the sprinkler is actually delivering the minimum density for the area its protecting.
It's easy to skip over this step. If you've ever laid out residential-style sprinklers, then you probably already know this.
Residential-style sprinklers can have small k-factors and relatively low minimum pressures to cover a reasonable floor area. However, these sprinklers can be used in NFPA 13, 13R, or NFPA 13D systems. 13R and 13D specifically can allow densities less than 0.10 gpm/sqft. The cutsheets often offer the minimum pressure for a given k-factor and floor area coverage, but the cutsheet may be assuming a 0.05 gpm/sqft density.
When we have higher densities (such as residential-style sprinklers in an NFPA 13 design), we have to consider this second driver for sprinkler flow.
The equation for density/area coverage is also straightforward:
Q = D x A
Q = Flow (gpm)
D = Minimum Density (gpm/sqft)
A = Area Covered by Sprinkler (sqft)
A sprinkler spaced at 15 ft x 15 ft with a minimum design density of 0.10 gpm/sqft requires a flow of 22.5 gpm.
With this, a k-5.6 sprinkler at 7 psi, spaced 15 x 15 feet with a 0.10 gpm/sqft density will actually need to flow 22.5 gpm.
Here's how this scenario looks when graphed:
The red line above represents the hydraulic pressure/flow relationship that a k-5.6 sprinkler offers. As the minimum pressure increases, the flow will increase. Similarly, as the flow needed through the sprinkler increases, the minimum pressure required to deliver that flow also increases.
For this scenario, the actual flow through the sprinkler must be the higher of the two amounts, or 22.5 gpm which will occur at 16.1 psi (see the blue lines above).
This means for a light-hazard, typical sprinkler we're demanding that the pressure at the sprinkler is over double what the code minimum is!
Will this difference break your calculation? No, it won't.
But let's look at another example where these decisions become a little more critical.
Take a Viking VK460 residential sidewall sprinkler. It's a 5.8 k-factor and has varying coverage areas with varying pressure and flow requirements.
Based on a 12 ft x 12 ft spacing, the minimum pressure required under the product data is 7.6 psi.
The Sprinkler-Driven minimum flow becomes Q = k√P = 5.8 x √7.6 = 16.0 gpm.
Assuming an NFPA 13 design, the Density-Area minimum flow becomes Q = 0.10 gpm/sqft x (12 ft x 12 ft) = 14.4 gpm.
In this scenario, the flow is Sprinkler-Driven. The actual flow through the sprinkler is driven by the k-factor and minimum pressure, and not the density/area point.
This same sprinkler, however, at a 16x20 spacing, looks a little different.
Based on a 16 ft x 20 ft spacing, the minimum pressure required under the product data is 20.1 psi.
The Sprinkler-Driven minimum flow becomes Q = k√P = 5.8 x √20.1 = 26.0 gpm.
Assuming an NFPA 13 design, the Density-Area minimum flow becomes Q = 0.10 gpm/sqft x (16 ft x 20 ft) = 32.0 gpm.
The demand through this sprinkler now becomes density-driven, and notice the actual pressure required to achieve this density is now 30.4 psi. If you have a poor water supply then these decisions can begin to really impact your hydraulic calculations.
Do you need to assess whether your sprinklers are driven by the k-factor and pressure or density/area? No - many hydraulic calculation programs cover this already.
These differences to become critical though with sprinkler selection, reducing the system demand, reducing the system pressure, and refining a design to end up with the most efficient system possible as an end result.
This Tool Available Now
If you're a Toolkit user, you can give this new tool a try today. Click here for online access to it.
This tool comparison tool allows different k-factor inputs, minimum pressures, density and areas with immediate graphed comparisons.
This tool will also be available for download with the latest Toolkit release here in a few weeks. More on that to come.
Thank you for reading and have a great, safe week!
Wow a lot has changed in a week. We're holding on fine here, but I hope you and your family are safe and healthy wherever you are.
Now back to more fun things like fire protection -
After last week's debut of fire sprinkler requirements for elevators, I had a couple emailed requests for a fire alarm version. I love the idea and put some time into reviewing and organizing the requirements on the fire alarm side.
This first iteration is a draft, and if you're well versed in this arena I'd love for you to take a look and let me know what you think. Feel free to email me directly at firstname.lastname@example.org, or comment on it here.
In the upcoming week I plan to incorporate ASME A17.1 and it's impact on the fire alarm side of accounting for elevators, hence the big [DRAFT] watermark on this PDF.
Click on the image below to get a PDF copy of the Fire Alarm Elevator Cheatsheet:
If you know anyone that could benefit from this content, please consider forwarding them a link.
Hope you have a safe and healthy rest of your week! Thanks for reading.
While it is a basic question, the code path is somewhat complex. When does an elevator require fire sprinkler protection?
Today I'm exploring the code requirements for elevator sprinkler protection under the International Building Code (IBC) and NFPA 13. Here's a free PDF cheatsheet for navigating these requirements. To download, just hover over the image and click print or export.
A special thanks to Philip Valdez who sent over the suggestion to put this one together. I hope you find it helpful!
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If you've found this helpful, consider sharing it with a friend or colleague. As always you can subscribe and get more free tools like this at www.meyerfire.com/subscribe.
Thanks & have a great rest of your week!
Last spring I created a beta test tool for soffit obstructions to sprinklers. It was fairly basic using the dimensional rules for a soffit against a wall for a standard-spray pendent or upright sprinkler.
Thanks to some feedback and more input on this tool, I'm happy to debut it with new features. I've added code references from the 2007 to 2019 editions of NFPA 13, the different style sprinklers, and an updated visual diagram.
This tool is useful when there's a dropped soffit against a wall to determine whether the sprinkler will throw sufficiently underneath the soffit.
In the coming weeks I'll break out a code path for determining when each of these tools are used. For now, if you're familiar with the NFPA 13 Sections for Obstructions Against Walls then you'll recognize this tool's quick usefulness.
This tool stems from the Figures (b) and (c) for Obstructions Against Walls found in NFPA 13 Section 18.104.22.168.2 for Standard Spray Sprinklers, 22.214.171.124.2 and 126.96.36.199.4 for Extended Coverage Sidewall and Pendent/Uprights, and Sections 188.8.131.52.2 and 184.108.40.206.4 for Residential Sidewalls and Pendent/Uprights.
I'll have it hosted for free use for a couple months before it transitions to the Toolkit package. For today it's live here and on thecloud access for all current Toolkit users. Interested in getting access to every tool? Get the Toolkit here.
Know someone that might be interested in this tool? Send them a link! It's greatly appreciated.
Have a great week!
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Joseph Meyer, PE, owns/operates his own Fire Protection Engineering practice in St. Louis, Missouri. See bio on About page.