Coverage Area of Sprinkler within Remote Area?

2/21/2022

When calculating a sprinkler system using the area/density method, I have heard several schools of thought:

All sprinklers within the remote area are to be calculated with the maximum sprinkler area coverage installed within that area (all 100 sqft, as an example).
Each sprinkler within the remote area can be calculated with its individual coverage area within the listing limits (94 sqft, 98 sqft, 100 sqft, etc.)
I have also seen, where software doesn't allow otherwise, all sprinklers within the area given an average area coverage within the calculation by adding the coverage area of each sprinkler and dividing by the number of sprinklers.

I tend to go with the most demanding as I don't worry about bidding projects most of the time, but in short, I was wondering what the consensus was concerning validity of each of these methods.

Thanks in advance.

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

19 Comments

Dan Wilder

2/21/2022 07:52:47 am

1 & 2 would be the "most" correct and interchangeable, with #3 be applicable in some specific circumstances but still allowable.
My personal approach is more conservative with taking the max available sprinkler spacing, but allowing for area reductions just because of the ability to allow the field to change spacing.

For a Light/Ordinary hazard example:
You can provide a full 1500 FT² remote area with each sprinkler discharging at the max area x density which will have the most safety factor for things like water supply and total fitting changes but it comes at the cost of pipe size, install labor.

Providing a 900 FT² at actual density x area is also acceptable allowing the smallest pipe sizing, but if there are changes in the field (additional fittings or sprinkler spacing changes), then you may run into an issue with a calculation that no longer under the curve requiring pipe size increase after the first installation.

Taking a calculation to a safety of 0.01% (after water supply reduction) is also perfectly acceptable in either case, but again, changes to install can wreak havoc on the final product.

I do know some contractors that have argued that even with the design encroaching into the safety factor in the as built phase, it is still under the total available water supply.

Mike L

2/21/2022 08:00:47 am

Assuming you're doing a demand calculation, the purpose of the calc is to determine the minimum water supply required to ensure all sprinklers are providing the water density for the hazard classification given the system layout.

As an example, for light hazard classification using K-5.6 sprinklers, each sprinkler must be able to provide 0.10 gpm per square foot of coverage. NFPA gives the method to determine sprinkler coverage area.

Most software needs a starting point, the end-head condition. If we assume the most hydraulically demanding sprinkler is providing the required density over the maximum coverage area, then the system demand with have a high required water pressure and flow. This may be acceptable for a new system as it gives a big safety factor for future modifications.

However, if you are working with an existing system with a limited water supply, the calc may not work. In this case, you may want to use actual sprinkler coverage area and ensure each sprinkler is providing minimum density.

Glenn Berger

2/21/2022 08:10:21 am

Item 1 is standard practice.

Item 2 can sometimes be considered when trying to reduce pipe sizing to sprinklers not directly connected to the branch line (elbowed over). Note calcs may need to be rechecked with the change in pipe sizing.

Item 3 - I do not believe that any of the current (up to date) programs still uses this method.

Pierre

2/21/2022 08:24:49 am

What say NFPA about that ?

Anthony

2/21/2022 08:25:19 am

With in the D/A method there is a clear method to determining coverage area per sprinkler ie:S x L as defined in section 8.5 (NFPA 13 2016).

In many hydraulic calculation programs you can enter a specific discharge minimum per sprinkler head which is perfectly acceptable. HOWEVER that head discharge condition will be over ridden if and raised if the head down stream requires more water/pressure.

Say you have a branch line where the last/end head requires 22.5gpm and the rest of the heads up stream only require 19.6 gpm. In this case all the sprinklers on that line will discharge minimum 22.5 gpm. Thus a savvy sprinkler designer will endeavor to keep the heads on the end of the line discharging as little water as possible to reduce pipe sizing and errant friction losses.

So you can enter each head's minimum discharge but it has limited application unless you really understand the math behind the calc.

Garbage in Garbage out.

I strongly urge anyone that does sprinkler calcs to yearly do at least one simple tree calc by hand. If just to stay fresh on the math and physical principals being modeled in a sprinkler demand calc.

Jesse

2/21/2022 08:26:10 am

Item 1 would be standard.

Manually would also work to find the ceiling level density of each sprinkler. Calculations can tell us the pressure that each sprinkler is operating at. Sq. root of pressure x K-factor / coverage area can give us density provided by each sprinkler individually/

Jay

2/21/2022 08:37:35 am

I always use #2.
This helps identify which sprinkler is most demanding (hydraulically remote) and therefore where changes can be made to minimize overdischarge.
NFPA 13 says:
28.2.4.7.1
Each sprinkler in the design area and the remainder of the hydraulically designed system shall discharge at a flow rate at least equal to the stipulated minimum water application rate (density) multiplied by the area of sprinkler operation.
I use HydraCad, which allows you to measure and/or assign an area of coverage for each sprinkler.

2/21/2022 10:44:51 am

Thanks.

Sorry but i don't find chap. 28.2.4.7.1 ! Can you give me more détails ? NFPA 13 2019 ?

2/21/2022 01:34:41 pm

2022 edition of NFPA 13

Franck

2/21/2022 10:59:37 am

I will explain you how I review hydraulic calculation as a loss prevention engineer working for an Insurance Company.
This will explain you why option 1 is conservative but works, and when option 2 may not work easily and may become a nightmare. Especially if you do calculations manually as suggested by Anthony (definitely a good practice to better understand).

1. I check that the selected remote area is correct
2. I check that the number of sprinklers by branch line is correct
3 I check if there are various area per sprinkler.
If not, option 1 is the right one. Easy to check - take the pressure at end head and confirm you have the right density.

If yes, then take the pressure at end head and confirm you have the right density (remember the 7 psi minimum pressure). Then take sprinklers with a greater coverage area and check if the available pressure given by hydraulic calculation from the end head is also enough to have the right density.
If not, this means that the end head needs a higher pressure.

Let’s take an example.
K5.6, area end head is 100 sq ft and design density is 0.15 gpm/sq ft.
This requires a minimum pressure of 8.1 psi
Now, imagine that because of friction losses, this ends up with a pressure of 11.2 psi but the coverage area is 120 sq ft. This still works as the resulting density would be greater than 0.15. But if the pressure is only 10 psi, then the resulting density would be less than 0.15.

Normally, when coverage areas are very close, there is no issue. But if the end sprinkler area is much smaller than the second sprinkler coverage area, you might have big surprises!

With option 1, no coverage area would be greater than your end sprinkler, so no issue (but you will end up with an artificially calculated remote area bigger than reality).

This is one of the reasons why FM global is not thinking in term of design density for storage occupancies, but number of operating sprinklers @ a minimum pressure.

Carlos E. Jara Fire Protection P.E. (Ca. license #1056)

2/23/2022 11:29:10 am

Franck, the required minimum pressure is 7.1psi, not 8.1

Alex

2/21/2022 02:40:18 pm

I have always used the square footage the sprinkler is covering. Like mentioned above, software will determine the most remote sprinkler and work backwards. Therefore, a sprinkler in a closet might require far less pressure than it will receive. If you use software like Revit (HydroCalc), you can input Sq.Ft. per head. Old old software like Hass, you are stuck with a single Sq.Ft.

Casey Milhorn

2/21/2022 02:42:18 pm

Method #1 is correct when using extended coverage, residential, or other heads with specific areas and pressures. Also acceptable for CMDA standard spray heads but kind of a lazy way to do it.

Method #2 is the correct way (in my opinion) on CMDA heads and as someone else mentioned, it helps you identify under utilized heads, driving head, etc.....

At the end of the day, any head other than the driving head is going to flow what its going to flow based on the driving head. So a lot of the time its going to be a moot point. I always liked to find the maxed spaced head, put that number in there for all of them, and then find the driving head and start adjusting to actual head areas to see if the driving head jumped and where to, and then keep doing that until it either stopped jumping, or went back to the original driving head. I teach my designers to think about their design area before they lay out the first head. That can save you a lot of headaches if you plan ahead.

Cliff Schulze

2/22/2022 09:41:15 am

I always use method 2. This comes in handy in trying to see what heads are driving up the flow and thus the pressure.

Brian Cockburn

2/22/2022 10:42:55 am

I usually use Option #2, but I will use #3 if each sprinkler area is similar. For example, if there were three sprinklers covering 94, 98, and 100 sq.ft respectively; I might calculate them each at 100 and call it a day.

Option #1 is typically what is required for extended coverage sprinklers (at least it's what I do).

Pete H

2/23/2022 07:14:03 am

Would agree with the consensus that 1 works for extended coverage, 2 works for standard coverage.

#3 only really works for standard coverage upright/pendents in a small room that fall into 10.2.4.1.2.1 (where the area of the small room can be divided by the number of sprinklers in the room for the protection area of each sprinkler.)

Brad

2/23/2022 01:35:04 pm

Might I ask the question from a fire district plan review perspective? If I am reviewing a fire sprinkler plan and let say these are the design parameters used:

Design area: 1500 sqft
Density: .10
K-Factor: 5.6
Coverage per sprinkler: 200 sqft

In my opinion, all fire sprinkler heads located in the design area should provide a minimum of 20 gpm at 12.75 psi.

I also believe that there should not be a fire sprinkler head spaced over 200 sqft per head, unless additional hydraulic calculations are provided supporting spacing above the 200 sqft.

If you have a design area and the fire sprinkelrs are only spaced at 100 sqft per head and they are hydraulically calculated to provide the minimum gpm and psi for 100 sqft spacing, then how would you be able to space a fire sprinkler in that building at anything over 100 sqft?

Am I wrong in this type of thinking?

2/23/2022 02:40:23 pm

Great question Brad. If during initial review you discovered a head in the calc area spaced at 225 sqft lets say, when the designer had 200 sqft max in the calc, then yes. I would ask them to revise their calc. If you happened to find some heads outside of the calc area spaced more than 200 sqft, then personally no I would not ask for more calculations to be done, or the original revised.
The thing to remember is that the calc should take place in the hydraulically most remote 1500 sqft area. This could mean it captures 8 heads at 200 sqft, it could mean it captures 7 heads at 225 sqft, but the end result is going to be very similar. You will see around 160 gallons of demand, plus delta flow. The point is that when you have a smaller head area per head, you end up with more heads in your calc area, and when you have a larger coverage area per head, you end up with less in principal. It mostly equals out. The only issue is that the end head pressure will be higher with the larger coverage area but this should be minimal.
BUT, at the end of the day most fires are extinguished or controlled by 1 to 2 sprinkler heads. Most fires don't happen in the hydraulically most remote areas. The calculation procedure itself has safety margins built in and on top of that, and many fire/code officials ask for additional safety margin. That all being said, many people get caught up on the details and want to talk about 1 or 2 psi, when a good compliance program will do more to save lives and buildings than worrying about the what ifs of design. A valve being closed, an unsprinklered addition/room, a major occupancy change from mercantile to storage, and on and on. Its like measuring with a micrometer and then cutting with an axe.
Sorry, got on my soapbox. Long story short, in your scenario you are proposing, no that wouldn't concern me unless it was obvious that the designer was trying to pull a fast one and deliberately under spacing heads in their calc area while heads outside their calc area were maxed out in spacing.

2/24/2022 07:17:05 am

Casey, thanks for your response and explanation. It is always good to get feedback from others within the same field!

Coverage Area of Sprinkler within Remote Area?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES