NOTES & SUMMARY
CODE & STANDARD REFERENCES
Why is an Occupancy Hazard Classification Important?
Ok. Here we go Joe – give us a guilt-laiden lecture on why I should care about some fancy term from NFPA 13.
And to my answer, I'll oblige. I will give you the guilt-laiden saga here.
However, I will say as dramatic as I can make this out to being as much as I can go on my tangents and soapbox, I still don't know that I could ever undersell how important an appropriate hazard classification is for a fire sprinkler system.
Now, in our last segment, we covered what a sprinkler occupancy hazard classification is. There's three words, occupancy hazard classification. In this segment, I'll probably refer to it at different times as just hazard classification. It kinda rolls off the tongue a little bit easier, but anyways, it's the way that we categorize the fuel load and the potential fire intensity for an area. It could be a room. It could be a space within a room. It could be multiple rooms. It's loosely associated with how intense a fire can be for a given area.
But as we're recovering today, why is this important?
IMPORTANCE HIGHLIGHTED IN NFPA 13 COMMENTARY
First, let's start off with our official declaration of importance.
In the handbook of NFPA 13, the commentary specifically mentions that the classification of the occupancy or the commodity, which commodity we’ll cover later in storage, is the first major decision that is made in the design of a fire sprinkler system.
It can have a huge impact on the effectiveness of the system during a fire. That's not even my drama. That's the commentary itself saying that. The classification has a huge impact. They use that term huge on the effectiveness of the system during a fire.
It then goes on to say that decisions improperly done can result in a compromised sprinkler system that could “allow for unabated horizontal fire spread.”
Commentary section in A.4.3 says that proper classification of a fire hazard is critical to the overall success of the sprinkler system. That's a lot. That's commentary that is even emphasizing here with some drama how important this is.
But why, why does it have such a critical effect?
EFFECTS OF AN OCCUPANCY HAZARD CLASSIFICATION
The occupancy hazard classification influences many of the later guidelines we have about a fire sprinkler system.
Sprinkler spacing, just as an example, is driven by the occupancy hazard classification.
Let's look at a quick example. The allowable coverage area for standard spray pendant sprinkler, and an ordinary hazard area is less or more restrictive than what is allowed for a light hazard area. In Ordinary Hazard Group 2, wee’re allowed a sprinkler coverage area of 130 square feet (that's 12.1 square meters). For a hydraulically calculated noncombustible construction light hazard area, we’re allowed up to 225 square feet (or 20.9 square meters) per sprinkler.
That's a big difference. That can be a huge cost difference, a difference in design, a difference in install and labor time and all of that.
IMPLICATIONS FOR THE FIRE
But what does this mean to the actual fire?
Well, it means that a sprinkler in a light hazard space can end up being further away from a fire source. It can react a little bit slower because it takes more time for heat to go up from that fire, traveling further out to the sprinkler.
When we spread out that heat, it's not just going directly to the sprinkler, it's going in a 360-degree direction. Or even if you have a fire that's in the very corner of a room, it's still fanning out. So that heat spreads out. So not only does that heat have to travel further to reach a sprinkler, but the heat that goes across the sprinkler is slightly less intense.
Again, it takes longer for that sprinkler to activate.
And that's just one aspect that we're thinking about here because the design density of a sprinkler system also depends on our occupancy hazard classification. So, a light hazard system is required to discharge 0.10 gpm/sqft (or 4.1 mm/min), whereas an Ordinary Hazard Group 1 system is required to discharge 0.15 gpm/sqft (or 6.1 mm/min). That's 50% more water. That can be a huge difference.
Now our principle theory for fire sprinkler systems, assuming a normal fuel load of common materials, is that if we have something that burns bigger that we need to apply more water to suppress it, the more intense the fire is, the more water we'll need the water from sprinklers cools the fire.
Now, you know, if you've made Kraft macaroni and cheese, that it takes forever for that water to get up to a boil. That's because water has a very high, specific heat capacity. It can absorb a ton of heat, applying water to a space that's in a fire condition, cools down the temperature in that area.
The spray from a sprinkler also pre-wets adjacent materials. So, it's harder for a fire to spread to a new fuel source if that fuel is wet. When we have sprinklers that are closer together, like in this example, there's more overlap between sprinklers and more of that pre-wetting occurs.
WATER VS. FIRE
Now, if we miscategorize this one big decision for a hazard classification, then we might be putting too little water on too big of a fire. And that's at the end of the day, what's really at stake here. That's what the root of the issue is. The amount of water that we're gonna design for needs to match how big that fire's gonna be.
We need to be sure based on all the history of fire loss and all the standard is telling us and all that's in our best engineering judgment, that the hazard classification is appropriate for the space that we're looking at.
If it's not, if it's not a match, if it's not appropriate, then we could be installing a brand-new system that is already outmatched by the fuel that's inside the building. We're not even giving the sprinkler system a fair fight.
A PROJECT STORY
I've had personal experience with this and I'll share the story because I think it illustrates the point and the importance of the hazard classification a little bit. And no, fortunately it does not involve a major building fire from a mis-categorization in design that has happened historically but fortunately I've never been part of something like that.
So, this really isn't a worst-case scenario, but it's a scenario that I think highlights some of the misunderstanding around this. So, at the time I was working as a freelance fire protection engineer and my client was a sprinkler contractor, there was a new construction, single-story building. The bulk of the building was this one large space, and it was called an indoor utility lab space. Now, if you remember from the last video, you'll know what I think about labs they're wildly vague, and they can actually go into different hazard classifications.
Anyways, the project had plans and specifications that went out for bid and the consulting engineer on that project had issued floor plans with hatched areas that called out this giant lab space as “Light Hazard Dry System.”
Now this space was large. It was over 25,000 square feet (2,400 square meters). So, this was going to be a very large dry system. I shouldn't say very large. There's obviously people that get up to 40,000 square feet and bigger systems. So I don't wanna say it was massive, but it's not a small system. It was a pretty large building. It was mostly pre-engineered metal building. And in the center, it had a peak of 54 feet or 16.4 meters. And that sloped all the way down to eaves at 35 feet or 10.6 meters. So, this was a large space, both in area and in height. It was a big volume.
One last wrinkle on this job, just so you're caught all the way up to speed, is that the water supply was not great. We had a static pressure in the mid-50s in PSI (around 3.7 bar), and a drop in residual pressure that happened pretty quickly after that. So not a whole lot of water pressure to work with either.
WHAT IS AN “INDOOR UTILITY LAB”?
Now the term indoor utility lab is somewhat vague term. What is this space actually going to be used for? Is it more like a teaching space, more like a classroom, or is this something else?
Now, the best thing to do in the situation for a sprinkler contractor, and I recommend this all the time, is to submit your question in writing before bids are due. It's not always possible. We get rushed. There's rush bids. GCs get stuff out late. Projects or issues on fast track. I get it. But if you can, get it in writing and get it submitted pre-bid. These are pre-bid RFIs or Request for Information and those are submitted before all of the bids are in so that the scope of work can be clarified for all contractors that are bidding the job. If I know one thing about contractors, it's that they want to bid apples to apples. They don't want a competitor assuming oranges, and I'm assuming apples, and I price it too high and lose the job. That's bad. We want to bid apples to apples. And the pre-bid RFIs is the best way to do that.
So, in this case, that's what happened here. This indoor utility lab, all it had listed was dry system, light hazard. That's all the information we had. We submitted a pre-bid RFI about the issue, asking what the space will actually be used for, and whether that light hazard designation is really appropriate for the space.
Just as an example, if that's a lab with flammable and combustible liquids, well, that could throw us into NFP 30 or NFPA 45, and we can get up to Ordinary Hazard Group 2 or even extra hazard situations. So, we wanted to get that clarified before things went to bid so that we didn't assume something that was less than what was needed.
The response we received on the pre-bid RFI was that other bidders, other sprinkler contractors had not asked that question and we should just base our bid on light hazard for now. Any changes that happen after that could be discussed after the bid award. Part of me thinks that the architect and the GC and everybody thought the question would just go away and maybe it wasn't that much of an issue, or maybe another contractor got the award and our question was moved anyways because somebody else was moving on with it.
So as the story goes, time passed, my client was awarded the contract for the new sprinkler system install in this new building and we all met afterwards to discuss the use of that space. Basically, to try and get that pre-bid RFI that, well, should have been answered before bid to try to get that clarified and answer so that we could design the system.
Well, it turns out that that lab space was gonna be used for large utility trucks to be driven in and parked inside where utility workers would be trained on how to use large utility trucks and equipment. Essentially, they were gonna drive in on these large utility trucks and be hoisted up in buckets to practice what an electrical lineman would see from the utility side.
So, this really was very much not a classroom. And in my opinion, it was much closer to the hazard of a parking garage with those commercial utility vehicles that were gonna be fueled and parked inside that space. So, in my opinion, we were at least looking at Ordinary Hazard Group 1, maybe even Ordinary Hazard Group 2, but definitely not a light hazard.
Now this was still a few years ago and I had to look it up for this project specifically, the project was in 2019, but this was still before NFPA 13 ever increased parking garages up to Ordinary Hazard Group 2. So had this project occurred today and we would've had the NFPA 13 2022 edition published, then this would at least have been part of the discussion to also consider Ordinary Hazard Group 2. So, if you're not familiar with that, the 2022 edition of NFPA 13 increased the hazard category for parking garages from Ordinary Hazard Group 1 to Ordinary Hazard Group 2.
So back to the story. So what big deal, Joe? You just add more sprinklers, calc it at OH1 and move on. Ordinary Hazard 1, just live with it, move on. Stop making this an issue.
Well, for this project, it was a really big deal. So, we didn't have a lot of pressure to work with. That was one of the issues. We had a very tall roof, so we were gonna have a lot of elevation loss. We were gonna basically lose a lot of pressure because that building's so tall and the higher you go, the higher pressure you have to have in order to get that water all the way up to the top. And we had two other issues that were working against us, and you might have guessed it – we had a sloped roof that’s over 2/12 AND we had a dry system.
DRY AND SLOPE INCREASES
Now we'll cover both of these things in a lot more detail in future segments. We'll spend a lot of time on them.
But when we have a dry system, we have to increase our hydraulically remote area. This is for hydraulic calculations. We have to increase that remote area by 30% without adjusting the density. That means a 1500 square foot area (or a 139 sqm area) gets increased by 30%. So that 1500 becomes 1950 square feet, or that 139 square meters becomes 181 square meters.
Also, when we have a slope ceiling or slope roof, we have to increase our hydraulically remote area by another 30% also without adjusting the density. These are cumulative. You don't just choose one or the other. They actually get multiplied together. So now we take that 1950 square feet (or 181 sqm) and with that second 30% increase that gets us up to 2535 square feet (or 236 square meters).
Now, when we go from Light Hazard to Ordinary Hazard Group 1, the density that we use increases. Light Hazard goes from 0.10 gpm/sqft (which is 4.1 mm/min) to 0.15 gpm/sqft (or 6.1 mm/min), which we introduced earlier. We introduced that concept earlier.
So, a quick rundown of this situation, and this is our Sprinkler Estimator tool we have right on the website shows that with this exact scenario, we're estimating that for a Light Hzard, looped system that is dry, with the height that we have and the penalties that we have for slope and dry system, we end up with a system demand at 46 psi at 405 gpm. That's 3.16 bar at 1530 liters per minute.
So, this accounts for the elevation loss, which you can see here is about 24 psi. We're using k8 sprinklers and 2-inch branch lines with 6-inch looped mains. So, we've got large branch lines, we've got large looped mains already. This doesn't include the safety factor that we needed on the job, which was 5 psi for the project.
So, these pipes were about as large as we can reasonably get without adding a fire pump. When we ran our full set of hydraulic calculations, just to verify all this, and dial everything right in. Well, this is pretty much where it ended up. We did the exact layout, every swing joint, every detail included. And it was right here. We could make it work, but it was close. We met our safety factor. We were under our system demand was under what the available water supply was so we were good.
ORDINARY GROUP 1
However, when we considered this space at Ordinary Hazard Group 1, things start to look a little bit different.
Now, remember, this is where we thought the occupancy hazard classification should be not Light Hazard. We thought it should be Ordinary Hazard Group 1.
This is that same estimate, but instead designed at Ordinary Hazard Group 1.
Well, what changed? So, we said earlier that hazard, hazard classification has a lot of trickle down effects or a lot of requirements that are affected downstream.
So here on our input, the density is higher. Our hose allowance is higher. Our spacing becomes more condensed at 130 sqft per sprinkler (12.1 sqm per sprinkler), but everything else is kept the same.
Now look at our results. So, for a Light Hazard system, our system demand is 45.9 psi at 405 gpm. That’s 3.16 bar at 1,530 L/min.
But now in Ordinary Hazard Group 1, our system demand becomes 54.1 psi at 755 gpm. That's 3.73 bar at 2,860 L/min.
So, our pressure went up by 8.2 pounds or (0.57 bar) and that's at a flow that's 350 gpm (or 1,320 L/min). So, we have a higher demand pressure at a higher demand flow.
So, it's hard for me just in my mind to just stick that in my head and be able to tell the difference. So, let's take these two demand points and plot them out on a log graph.
So, our x-axis here is at the bottom where we have flow. Here, it's in gpm. We're just gonna stick with imperial units for this one.
On our y-axis on the left, we have pressure, which here is in psi, pounds per square inch.
Now our available water supply curve is what I'm drawing in now. This is all the information that we had from the project documents. And we actually did a prior job just down the street a few years before and the flow test from that job was pretty close to this one. So, we had some reasonable confidence that the available water supply was roughly accurate here.
Now, here is the system demand for the Light Hazard system. You'll see that our demand point falls underneath the available water supply. We're okay. We're good. The water supply can handle the system.
Now, here is the system demand point for Ordinary Hazard Group 1 system. This is really the point we felt we should be. As you can see, because the system demand is greater than the available water supply, we've got a problem. The water supply can't support the system without a fire pump, which would be boosting the pressure on the available water supply. So, our demand point would be under the fire pump and the water supply pressure.
So now back to our job at hand and it is our job at hand because you're in on the adventure now. My client has been awarded the job. This was bid at Light Hazard.
The building has no fire pump, nobody planned on one. If there were gonna be a fire pump, then we would need space for the pump. We would need exterior access and we need an emergency generator for it. So huge, huge cost implications. I mean just rough order of magnitude, we're talking about the fire pump, the cost of installing the fire pump, the generator, I mean, combined, we're looking somewhere between $120,000 and $200,000 added to the job. And that's in, I don't know this was 2019 dollars.
CRUX OF THE ISSUE
So, we finally met with the consulting engineer who was full disclosure, a mechanical engineer. And I don't want to take an underhanded swipe at mechanical engineers. I like him. I've got 'em in the family. I'm friends with some mechanical engineers even. Don't hold it against me. But in this particular situation, he was not exactly a fire protection person, rather he was a full-time mechanical engineer in both title and practice.
So, I'm gonna black out the image here as if he's in the witness protection program.
And he's not a bad person and we certainly all make mistakes. I've made many mistakes. I wanna full disclosure. I'm not perfect, but I think this project and this situation and the kinda attitude that we got, kind of shows some misunderstanding of what an occupancy hazard classification is. And I just wanna highlight and talk through that here ‘cause I think there's something to learn and take away from it.
So, the question that came back to us in this meeting was “so we would need a fire pump if we're under Ordinary Hazard Group 1, correct?” We said, “Yeah. Here's the calculations. Here's what we're dealing with.
So, then the question was,” well, why does it have to be Ordinary Hazard Group 1?”
Well, our answer was “because the annex of NFP 13 has examples of the different classifications and this lab space has hazards that are similar to those in Ordinary Hazard Group 1 examples.” You look at the annex, we tend to match more of that OH1 list.
So then the question was, well, what if you call it a Light Hazard space?
And at that point I said, okay, “well it's not really just a label that we slap on the room. This decision affects the design of the system and this spacing the amount of sprinklers and the amount of water and all of that.
And they said, “Okay, okay, but I'm okay with Light Hazard and the owner is okay with Light Hazard and then why can't we just call it Light Hazard?”
And it was at that point in the conversation that was really the crux of the issue is the crux of the point that I wanna make today.
BOOKS VERSUS REAL LIFE
Sometimes it's easy to get lost in the words and get lost in the standards and just kind of distance ourselves from the fact that the words in the book actually have meaning in real life. We can't, even if we're a licensed professional engineer, just make up what we want the space to be classified as. We don't just get to choose how aggressive a hazard is going to burn. If it burns, it burns. We don't exactly get to step in on some random Tuesday morning at like 2:15 AM and say, “Hey fire, you're not supposed to burn like Ordinary Hazard, you're supposed to burn like Light Hazard.”
The fire itself doesn't know any different. Fire's dumb. If it burns, it burns. We don't get to intervene at that point.
So, when we put these designations in classifying area as one occupancy class or another, we're trying to match the appropriate designation that kind of best exemplifies that area. We want to make an appropriate classification so that our system has a chance to fight that fire.
ACTUALLY SUPPRESSING A FIRE
Well, again, at the end of the day, the system is actually trying to suppress a fire. It's actually putting some amount of water, some quantifiable amount of water over some amount of area to make sure the building doesn't come down and that everyone gets out alive. That's actually our end goal here.
In a similar way, I got asked at a different time. Well, can't you just let the campus water department know in advance when you're gonna run the flow test so they can pump up the water tower and fill it to the top beforehand?
That's the same issue here. The fire itself is dumb. It doesn't know any different, and it's not gonna text me 72 hours in advance of igniting so that we can pump up that water tower to the highest level. We don't. These are automatic fire sprinkler systems. They're supposed to automatically suppress a fire, and that's what we're setting them up to do.
So why is an occupancy hazard classification important? And as we'll cover later, why are commodity classifications important?
Well, it's because this one decision sets the stage for how we design the rest of the system. The capability of a sprinkler system, the ability of the system to actually go and fight a fire depends on actually choosing the appropriate occupancy classification for an area. This decision will set up our system for potential success or potential failure.
Thanks for hanging around my very animated soapbox today. Obviously, I feel a little bit passionate about this area and have some scabs to share. I hope you're not offended by my approach here, but I kind of wanna share some of the internal dialogue and some of the misunderstandings around this, so that you have some perspective to chew on and come to your own conclusions about this. But in short, can't really stress how important this one decision is and how we categorize a hazard so that our systems are set up for success in the future.
I’m Joe Meyer, this is MeyerFire University.
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