To start, it helps to understand where the curves came from. Check out the cached link below (I'll try to update the link if I can find the article). Basically, using pipe schedule systems and varying end head pressures, the densities & areas were developed.
NFPA 13 SPRINKLER SYSTEM DESIGN DENSITY CURVES – WHERE DID THEY COME FROM?
I know that you can use the lower density when applying the 3000 sq ft minimum for unsprinklered combustible concealed spaces and in Extra Hazard applications, getting the density below .25 allows for 130 sq ft spacing vs 100 sq. ft. to name a couple. Some systems have plenty of flow, just not a lot of pressure so lowering the starting pressure can have a huge impact.
In EH applications you can also use the high temp area reduction with the lower density gained by going up the curve.
I used to work for an Insurance company in the US (Kemper) that was mostly using 3000 sq ft for the recommended design of all installations.
There were several reasons for that:
1. It is a bit more conservative and lead to the design of a slightly bigger water supply. Which is good as it would give some flexibility in the future if there are some changes in occupancy: If you design your system for OH Gr1 with 0.12 over 3000, your pump will be sized for a sprinkler flow around 400 gpm (to take into consideration friction losses). This means that if your occupancy move to OH Gr 2, you will find a demand point on your curves for OH Gr 2 that could still be fulfilled with the same water supply (0.2/1500 would require around "éà-"(à gpm). Some more flexibility for changes.
2. If you take 2500 or 3000 sq ft, this area of application is common from LH to EH Gr 2, and is also applicable to storage occupancies). 1500 is only applicable to LH or OH.
Again, if you have a lot of margin between your demand point and your water supply, you can assess the system without the needs for a new hydraulic calculation.
3. FM Global used for a long time an area of application of 2000 or 2500 sq ft. If you change insurance company, it is easier to check adequacy with FM Global data sheet requirements for example.
4. Many countries outside US (Europe) are using 260 m2 for occupancies and 300 m2 for storage (i.e. 2800 sq ft and 3225 sq ft). If you have plants all over the world, it might be easier to compare protection design between countries for homogeneity on the fire protection approach.
5. As indicated by Dan, you normally end up with a lower pressure demand (higher flow, lower pressure) which could be beneficial for the water supply, especially with city grid fed by elevated tanks where the flow is not the issue, but the pressure may be.
The only bad aspect when using larger areas, is that you need a bigger flow (and then a bigger tank capacity if you have an independent water supply).
What we are using now in my company, when we make recommendations, is the following (to allow as much flexibility as possible for possible future changes):
For new installations design (including the water supply):
- either 2500 or 3000 sq ft (more for specific application like turbine halls where we end up with 5000 sq ft).
For new installations with an existing water supply:
- what is feasible with the existing water supply. Again, if possible, 2500 or 3000. If limited tank capacity / pump nominal flow, we go down to 2000 sq ft.
We almost never recommend 1500 sq ft, unless it is the only possible solution, to accept an existing situation.
Note that if there is an existing installation adequately designed over 1500 sq ft, we don't ask to make modification. It is only for new installations.
To be honest, at the end, the difference of cost for the installation is not that huge.
And it could save a lot of money afterwards.
The best reason to use something other than 1500 sqft is because you have to. If you calculate your remote area of 1500 square feet, and you wind up at 1560, you can get some relief from the density curves. Maybe it's 0.095/1560. You need to check the 1.2sqrt 1560 and make sure your rectangular area still works, but the smaller density means a lower start pressure for the sprinkler. Maybe.
The other rational reason would be for modification to existing pipe schedule systems. You may find that altering the area and density works better for the system.
You will always flow less total water at the bottom of the curve. For a new system, this is where you should be. But the curves have stayed in to provide design flexibility. Use it to your advantage where it makes sense.
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