NOTES & SUMMARY
CODE & STANDARD REFERENCES
The Difference between Hazard Classification and Design Criteria?
This is the last topic we’ll cover in our series on sprinkler occupancy hazard classifications.
If you’ve been doing this as a series, well, so far we’ve covered what an occupancy hazard classification is, why it’s important, who is responsible for figuring it out, we’ve completed a few workshops where we assigned hazard classifications, how to go about protecting areas with multiple hazard classifications, and now today we’re wrapping this up with the difference between hazard classifications and sprinkler design criteria.
Now again here as part of this series we’re really focusing on the terms and requirements of NFPA 13. Different standards will go about this in their own way.
So, if you remember back to the first part of the series, we said that the fire sprinkler occupancy hazard classification gets called a lot of different things.
Sometimes they're called sprinkler occupancy, hazard classes, hazard classifications, or other things.
Sometimes they're mistakenly called sprinkler design criteria.
If that’s the case, then what’s the difference between an occupancy hazard classification and design criteria?
Well first, sprinkler occupancy hazard classification, like we talked about before, is the way we describe a fuel load and fire severity. Examples of hazard classifications include Light Hazard, Ordinary Hazard Group 1 or 2, Extra Hazard Group 1 or 2, or a Special Occupancy Hazard, which we'll get into later like storage.
This classification then has implications for all the downstream decisions we make for fire sprinkler systems. This would include sprinkler spacing, the type of sprinklers, the amount of water that goes into the calculations, the amount of water that goes into the design, and the hydraulic calculations, and a lot more.
A sprinkler design criteria on the other hand, is specifically how we intend to hydraulically calculate a fire sprinkler system. It's only determined after we know our hazard classification.
So, as an example, let’s assume we have a large mechanical room.
The hazard classification, or more properly the occupancy hazard classification for this could be Ordinary Hazard Group 1.
The sprinkler design criteria would then be 0.15 gpm over the most remote 1,500 sqft (or 6.1 mm/min) over the most remote 139 sq.m.).
HOW TO GET DESIGN CRITERIA
How did I come up with those numbers? How did I come up with that sprinkler design criteria?
Well, once we have our occupancy hazard classification, once that's determined, getting the sprinkler design criteria is pretty straightforward.
NFPA 13 has a table for water demand that is based on the hazard classification.
Now for many years, this has been a graph, where as a designer we can choose a point along the “curve” so to speak to take our hazard classification and turn that into design criteria.
Using Figure 220.127.116.11.1 of the 2022 edition, or what used to be Figure 18.104.22.168.1 in older editions of NFPA 13, we can take our hazard classification of that “Ordinary Hazard Group 1” and we pick a point along that curve, it’s typically at the bottom, and we get both the required sprinkler density, and the “area of sprinkler operation”.
In this graph, the bottom x-axis has imperial values, and the top has metric values. Same with the left, imperial right metric. So, an Ordinary Hazard Group 1 mechanical room, like in our example, would be 0.15 gpm/sqft over 1,500 sqft, or 6.1 mm/min over 140 sq.m. (the old graphs used to say 139 sq.m).
That’s how we go about taking a hazard classification and translating that into design criteria.
Now, if you're joining along and don't have a long history of sprinkler experience, we'll cover hydraulic calculations of sprinkler systems in a whole lot more detail. We'll talk about those numbers, what they mean down the road. Today, not too important. We just want to know that our hazard classifications directly translate to a sprinkler density and area that we will use later for hydraulic calculations.
Starting in the 2022 edition of NFPA 13, we are only given a table for the sprinkler density area where we can only select one of two criteria.
There was a lot of consternation over this and it was debated at length, but essentially the reason was that the committee wanted to simplify the density area method to the densities in the areas that people were usually using in the field anyways.
So, for a new design or a new installation now, we no longer go to the curves – we go right to Figure 22.214.171.124.1, look up the hazard classification that we have, and we gather our design criteria.
TYPES OF DESIGN CRITERIA
Now, I've used the density area method as an example here, but not all design criterias just use a density in an area. There are multiple types of design criteria.
These include the Density/Area method (which is what we just used), but it could also be something like the Room Design Method, or Special Design Approaches.
The Density/Area method uses a water density, or basically a flow rate of water over a specific floor area, like we did earlier.
The Room Design Method takes into account all of the sprinkler flowing in the most-demanding room. It makes use of fire-resistance-rated construction to limit the fire growth beyond that room, so it works in conjunction, passive and active work together to limit the fire spread to only the largest to only that room.
Then there’s also Special Design Approaches. The include Residential Sprinklers, Exposure Protection, Water Curtains, Combustible Sloped Roofs, Sprinkler-Protected Glazing, and Manufacturer’s tested criteria.
Last, we also have Design Criteria for Storage Occupancies. Now, NFPA 13 addresses these in the storage chapters that’s later-on in the book, in Chapters 20 through 27 if you're looking at the 2019 or 2022 edition. Out of these chapters we can get storage design criteria that might include calculating something like 12 sprinklers that’s at a specific pressure, across three different branch lines. So, instead of a density and an area, instead for storage criteria, it might include something like 12 K-14.0 Upright ESFR Sprinklers at 50 psi (or 3.4 bar) each. That's design criteria for storage occupancies.
For non-storage and non-residential occupancies, the most common approach we use to calculate a fire sprinkler system is the Density/Area method. That’s the most common. When we go that path, it’s relatively easy to take the hazard classification and turn that into a density and an area just by looking at the table in NFPA 13.
So, what is the difference between a hazard classification and design criteria?
Well, an occupancy hazard classification describes a fire severity and a fuel load. It’s a categorization. One example of that would be Ordinary Hazard Group 1.
Design Criteria, on the other hand, is a quantitative way to describe how the system is going to be calculated. It’s determined after we know the hazard classification. One example is 0.15gpm/sqft over the most remote 1,500 sqft (or 6.1 mm/min over the most remote 140 sq. Meters).
That wraps it up for this series. I’ll see you on the next one.
I’m Joe Meyer, this is MeyerFire University.
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