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May 31, 2021

On episode 8 of the solocasts of fire code tech we are breaking into hydraulic calculations. Working through the basics and exploring the fundamentals of how to determine pressure and flow required for sprinkler systems. Tune in as we explore these questions and more on this week’s episode. Don’t forget to check out the Patreon at if you would like to check out more on the subject.


What are hydraulic calculations?

When do you need to have calculations?

What is the density area method?

What are the density area curves?

What are area adjustment factors?

What is fire department flow?



Hello. Welcome to the solo cast of fire code tech in these episodes. It's just gonna be me, your host, Gus Gagliardi. There's gonna be a range of topics, but I'm gonna talk about specific technologies, installation, standards, codes, and how they work as well as some other interesting topics that don't neatly fit inside of the context of a normal I.

Hello, welcome to episode eight of the solo cast of fire code tech. On this episode, we are talking about hydraulic calculations for sprinkler systems. I wanted to talk about fire pumps this week, but I realized that. I had to introduce the topics of hydraulic design and how to analyze a fire suppression system to see if you are designing for the appropriate amount of pressure and flow.

This is one of the most quintessential tasks for a fire protection engineer and something that's near and dear to my heart. So I wanted to go over the basics and address hydraulic calculations. When you need them, how to address the codes and standards around them. And also, uh, what are the basic steps for how to produce them?

We'll see how far we get in this episode. But my goal is to take you through the entirety of a, how to do a hydraulic calculation and the different varietals of hydraulic calculations. So. Yeah, let's get started. Don't forget if you like these solo cast episodes, you can go to code tech and get two of these episodes a month for the price of a cup of coffee.

Also, don't forget to subscribe and give us a good review on iTunes if you would do so I'd greatly appreciate it and follow us on social. So you don't miss when these great shows come out. So I wanted to start off with, um, what are hydraulic calculations? So hydraulics is the study of, of fluids and the dynamic motion of fluids.

And the reason why you would have hydraulic calculations is to justify, um, sprinkler piping, configuration, and, uh, sizing and the different. Size of sprinkler orifice slash head type. And so when you need these sys, when you need this topic is when you have already determined that you have. A need for a fire suppression system in new construction or in existing construction, where you are making a significant modification to a sprinkler system, which requires a, um, reanalysis of the pipe sizing.

And performance of the system is just a, a note for this topic. There's a couple different ways you can design a sprinkler system. There's the hydraulically designed systems, which are the most frequently produced kind in modern construction. And then there's the pipe schedule system, which is how many legacy fire sprinkler systems were designed before.

Um, hydraulic calculations were as common. And so, uh, the definition of a hydraulically designed system, and this is in chapter three of NFPA 13, and this particular definition is out of the 2019 version is a calculated sprinkler system in which pipe sizes are selected on a pressure loss basis to provide a prescribed water density in gallons per minute, per square.

Or a prescribed minimum discharge flow per sprinkler. And then it says distributed with a reasonable degree of uniformity over a specified area. And so what this says to me is that. There is a prescribed flow and pressure that is, um, required for the system. And the hydraulic calculations are provided in order to justify that the system will be able to discharge this amount of water at the requisite, uh, pressure and flow.

And so one of the predominant components that you need. An essential component of, uh, hydraulic calculation is a water supply. And so you would obtain a water supply through a flow test, or you would obtain data points about the information of what a water supply is by a flow test. Um, a flow test would be a great topic to do, uh, show over and I'm sure I'll get into that, but.

Salient points at this podcast. Let's say we have three data points from a flow test, a static pressure, uh, residual pressure and a residual flow. And so these can be obtained through N FPA 2 91. and then you can plot these out on hydraulic, uh, graph paper or paper with a 1.85, um, scale. And you can take a look at the distribution of pressures and flows of the water supply that's available.

And so why is this important? Um, understanding what is the available capacity of the water supply system that your fire suppression is, um, using as its source of water is important to know, because if you are above this curve that is made by plotting the, uh, static pressure, the residual pressure and the residual flow, then.

You were not able to use the sprinkler system in its design manner. You're gonna need either a fire pump or a fire protection water storage tank. . And so this is a little bit hard to explain because it's a very visual, a, a graph of hydraulic water supply is a very visual thing, but I just wanted to mention, um, that, and so a pipe schedule system is basically just.

Based on a set of sizes. And so the definition from chapter three is a sprinkler system in which the pipe sizing is selected from a schedule that is determined by the occupancy classification and in which a given number of sprinklers are allowed to be supplied from a specific size of pipe. So a great place to look if you're wondering about information on hydraulic calculations.

And I haven't said this yet, but all this information about how to calculate a fire sprinkler system is through the lens of, you know, codes and standards in the us. So international building code, um, and then followed by N FPA 13 and then so big chapters in NFPA 13 for. Uh, hydraulic calculations are the definitions chapter and also plans in calculations.

Um, uh, in the 2013 version that's chapter 23. And so. Place where you can see more information about pipe schedules and how many sprinklers equals what pipe size is in, um, 23, where they talk about for the specific hazard, how in the specific type of pipe, how many sprinklers can you get on each, um, size of line?

So it starts off at two sprinklers on one inch line. And then if you have more than two sprinklers, You have three sprinklers, you have to have on a line, you have to have that on. And this applies to lines or mains. Um, you have to have it on, on, uh, inch and a quarter. And so this table goes up and up and up.

And so, uh, another good point to talk about is hazard determination and. Uh, what does hazard determination look like? So, um, let's go over a couple definitions for the different hazard types and when you would need these, uh, there are a couple different types of. Of, uh, preliminary hydraulic analysis or hydraulic analysis.

And, um, one of the most common ones that I've seen is the density area method. And one of the first steps in calculating, uh, fire sprinkler system for the density area method is determining the hazard classification. and so we have light hazard, ordinary hazard one and two, and then extra hazard one and two.

And so as you can imagine, light hazard is the least hazardous, which requires the least amount of water and extra hazard. Two is the most hazardous and requires the most amount of water. And so this is, uh, density area is the most simple method of, um, hydraulic calculations. There's also, uh, more specific methods for, um, storage, sprinklers, like ESFR sprinklers or CMSA sprinklers that don't follow the standard conventional rules.

And so taking a look at at those that their, their specific that the core tenets of what it means to address the hydraulics of it, which is verifying the pressure and flow is adequate is the same, but the way that you select how many sprinklers you flow and, um, some of the other factors can change. So, so let's move forward with the discussion of a typical area density approach.

So. I'm much more used to the, uh, approach and how it's laid out in, uh, NFPA 13 in the earlier versions of NFPA 13, uh, as of 2019, they've totally reconfigured the standard. I'm not as comfortable with that, uh, methodology. Maybe I'll do a comparison in the future, but for. I'm still taking a look at it through the lens of, um, the 2013 edition.

So right now we're looking at chapter 11. Which talks about the design, uh, density and area, uh, method and, and how to, uh, model this approach. Um, the 2019 edition of the standard lays out the, uh, design process a little bit more linear in the way that the book approaches it. Yeah. Hazard determination is one of the first steps in the process.

And so. It says classification shall be as followed light hazard, ordinary hazard, extra hazard, special occupancy provisions. Uh, they give a caveat for storage applications and how those are not covered as a part of this, um, design approach. And so it gives a couple of options for how you would determine this, this occupancy hazard, fire control approach, and special design approaches of chapter 11 storage design approaches of chapter 12 through 20 special occupancy approaches of chapter 22.

So what this is telling you is it can use, you can use the standard density area approach of chapter. If you have storage designed, so that's high piled storage, or, um, appreciable amounts of storage in excess of a quantity that's allowed in NFPA 13, you would use chapter 12 through 20, or if you have a really special case occupancy, say you have, uh, sprinkler system for a Marine, um, scenario.

So you have a, a river boat with. Fire sprinkler system. Then you may wanna look in, uh, chapter 22, or if you have a really kind of unique design approach, then you wanna look in chapter 22. And so really at this point in the process, you wanna take a look at where your lead in for your sprinkler system is gonna come into the building and then you wanna, you know, analyze where you're most.

Most hazardous area of your building is this is gonna help you determine where your remote area is gonna be and help you, um, figure out where the most demanding pressure and flow area of your building is. And so. Um, there's different. There's, you know, of course, many scenarios in which you'll have to do multiple hydraulic calculations in order to justify your piping and, and sprinkler head configuration in multiple areas.

But, um, let's just, uh, for instance, say that you have a square building and you come into the Southwest corner of the building and the Northeast corner of the building is your most hazardous area. And it's an ordinary hazard. Area based on your hazard category, you need to determine the minimum amount of area you can calculate, uh, with your required density in order to determine the flow needed.

If you look at the charts on 11.2 0.3, water demand requirements, hydraulic calculation methods, you'll see a very interesting table. Um, figure 11.2 0.3 0.1 0.1 density area curves. This is where the term, the density area method comes from. And so you have, uh, a linear line sloping downwards, and they have one for light hazard.

They have one for ordinary and, uh, you know, one and two extra one and two. And so from here, you can get your first sense of. Some different factors for density. And when I say density, I mean, um, density of water per square foot. And so, um, there is a line that slips downward. And so I would take a look at ordinary one for the process.

Um, we're speaking of, and so there's an area of sprinkler operation and the beginning of the line starts at, you could take a look at 0.1 GPM. Um, over 4,000 square feet and then add the slide slopes all the way down to the second point is 0.15 GPM add 1500 square. And so basically there's an acceptable range of densities and areas allowed for ordinary hazard.

One that you can use to calculate your system demand flow. And so, so you choose your remote area and now we have to choose the density and the area of operation. So, um, you could select 4,000 square feet at 0.1 GPM. I've never in my whole history of my, um, design experience, seen somebody select, uh, towards the top of this line.

Um, they always select near the bottom of what's allowed, uh, 0.15 over 1500, which, uh, the reason why you would do this there's that you could. Essentially be more conservative or less conservative, or you could say more cost effective. Um, and so 0.1, five times, or let's just go with 0.1 times 4,000 is 400 gallons per minute.

And then 0.1, five times 1500 is 225 gallons per minute. So you're, you're flowing. Uh, you know, another, uh, 175 GPM for the same classification of hazard. Um, it just doesn't happen, um, because more flow equals, um, needing bigger pipe sizes, bigger sprinklers, and more cost along the way. So in the common convention, you're gonna, you're gonna pick the lowest point of the curve unless there are some, um, design guidelines in place that are going to, um, make it.

Essentially required to select somewhere else in that range. And so. If it's, so there's other, some area adjustment factors you might have to use. If you have a dry system, you might need to add a multiplication factor to increase the area. You have to take a look at by 0.3. If you have a roof that is severely sloped, you might have to add a, uh, a factor that increases the area in which you need to analyze the sprinkler system by, you know, I think it's, uh, from.

Point one to 0.2, I forget exactly how much. Um, and so that is a factor that could also change, um, the area in which you're looking at. And so, conversely, there are some reductions that you can apply if you have, uh, light or ordinary hazard occupancies with specific ceiling Heights, uh, flat ceiling Heights.

And so you can get a quick response re. Um, up to, it looks like 40%. Um, and you see this used, uh, very commonly, uh, cuz for an office space with the, uh, 10 foot or eight foot ceiling, you would be able to take this exception and it would be advantageous because you could. Um, lower the water required and dusty, um, piping and sprinklers could also be reduced in, um, you could be beneficially compensated for this reduction in flow.

And so after this, you, um, In regards to flow. So everything we talked about so far has been in regards to flow. We haven't yet started to talk about pressure and how to analyze pressure the system. So as a, as a quick recap, we've determined hazard, we've determined remote area, and we have taken a look at the, um, Area and density required.

And, uh, we've adjusted for allowance factors like, uh, dry pipe and, uh, you know, uh, and we've adjusted for ceiling slope and, and, uh, let's just say it's, uh, since it's ordinary hazard one, let's say it is just a, uh, small utility shed, um, with the minimal combustible loading in. Don't think we're gonna make it all the way through, uh, to pressure in this session.

So let's just say. Um, talk about the last factor which would for flow, which would be, um, hose allowance for, uh, extra fire department connections. And so, um, you have different hose, stream allowance and duration requirements for the, the differing hazards. And so, um, it ranges from a hundred to 500 gallons per minute.

And so two 50 for ordinary hazard. And so you would have to add this. So if you're doing preliminary hydraulics and you know that you have an ordinary hazard, uh, occupancy, and you want to get a sense of how much flow you're gonna have, you can, you know, take your 1500 square feet. Uh, you can put a safety factor on that.

If you would like, um, you'd say like times 1.3 and say 1800 square feet. Um, multiply that by a 0.15, um, GPM per square foot that would give you 270 GPM. And then you would add an additional 200 to 50 GPM for the host stream allowance. This would give you an approximation of what kind of flow you would be expecting for an ordinary hazard occupancy.

Um, just through experience and through repetitions I've seen, uh, flows for ordinary hazard can range through about like 500 to maybe the upper of what you might expect would be like. 800 or 900 GPM if you have ordinary two. And so this would be in line with what I've seen and what I would expect anyways.

Well, that wraps up, uh, flow for the most part. Doesn't cover all of that. All of what could be considered of course, but, um, we're coming up on a lengthy one here. And so we're gonna sign off. Thank you guys so much for listening and don't forget to subscribe and, uh, if you're interested, go check out the Patriot.

All right. Thanks guys. Thanks for listening everybody. Be sure to share the episode with a friend, if you enjoyed it, don't forget that fire protection and life safety is serious business. The views and opinions expressed on this podcast are by no means a professional consultation or a codes and standards interpretation.

Be sure to contact a licensed professional. If you are getting involved with fire protection and or life. Thanks again, and we'll see you next time.