Layout Example: Mechanical Room, Part I

MeyerFire University | FX108.72C

By Joe Meyer, PE

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• One-Page Summary [PDF]
• Notes Page [PDF]

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: A.3.3.41.1(5) Examples of Obstructed Construction: Beam and Girder Construction
• NFPA 13 – 2022: A.4.3.3 Ordinary Hazard Examples
• NFPA 13 – 2022: Table 10.2.4.2.1 Spacing Criteria for Standard Spray Ordinary Hazard Pendents & Uprights 

FX174 SERIES

1. What are the Steps to layout sprinklers?
2. Layout Example: Conference Room
3. Layout Example: Mechanical Room, Part I
4. Layout Example: Mechanical Room, Part II
5. ​Why does sprinkler spacing matter?
6. Layout Example: Ambulance Bay, Part I
7. Layout Example: Ambulance Bay, Part II
8. Layout Example: Ambulance Bay, Part III
9. Exercise: Layout for Conference Room
10. Exercise: Layout for a Fire Pump Room
11. Exercise: Layout for a Covered Patio​ 

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Example Sprinkler Layout for Mechanical Room, Part I 

INTRODUCTION

This is Part 1 of a 2-part series. We’re going to spend some time going step-by-step in a sprinkler layout for an open mechanical room. Now I want to go through this in detail so you get the full methodology and get to see the full thought process step by step. So, in this Part I, we’re going to cover the first seven steps of the layout process – essentially making our selections and finding all of the applicable rules that will govern the layout. In Part II, we’re going to use those rules and position the sprinklers and pipe to get a code-compliant fit. 

So – onto Part I.

So, in this series we introduce the steps needed to layout sprinklers. In our last segment, we covered a conference room as an example, but what about a different scenario? What if we don't have a ceiling? In this take, we're gonna layout sprinklers for a mechanical room from start to finish. 

So – this example. 

We have a larger mechanical room for a new construction building. The building is type two B. This mechanical room does not have a ceiling. It has exposed steel beams with a metal deck and concrete floor above. The beams appear to frame into the side of the girders with the information we have. 

The building itself is gonna be protected in accordance with NFPA 13. 

The mechanical room that we have here is on the first floor of a two-story hospital. How do we go about laying out sprinklers for this space? 

Well, let's use our same step by step process that we did before and see what we come up with. 

#1 WHERE REQUIRED

First, step number one, let's determine where sprinkler protection is required. 

Here in this example, we have one large volume. There's equipment, there's potentially going to be some boxes and things thrown here and there. There's a whole lot of mechanical equipment in this room. It's accessible. There's combustibles. 

NFPA 13 does not offer any exceptions to protect the space. The space needs sprinkler protection. Since it's one giant volume without a ceiling, we don't have to evaluate the above ceiling space and below ceiling space. It's all just one and it's required, so it needs to be protected. That's our step number one. 

#2 HAZARD CLASSIFICATION

Step #2: Hazard Classification. 

This room is gonna have mechanical equipment, water service, but not a whole lot of much else. There's no areas where there would be storage. It looks pretty consistent with a typical mechanical room. 

So that said, we go to NFPA 13 Chapter 4 in the 2022 edition, Classification of Hazards, and we could start at light hazard and read through those examples in the annex. If you've been in sprinkler design for just a little bit, you know that's not gonna be a match here. The next spot we would check is Ordinary Hazard Group 1. The next higher classification. Let's go to the examples in the annex. And yes, there we have it, mechanical rooms. It's identified as an example of Ordinary Hazard Group 1 in the annex of NFPA 13. 

So, there's nothing here that we're given on the plan that seems out of line with a typical mechanical room. So, let's proceed with Hazard Classification of Ordinary Hazard Group 1. 

#3 OBSTRUCTED OR UNOBSTRUCTED

Step #3: Obstructed or Unobstructed?

Let’s take a look at the reflected ceiling plan we have here. So, we have solid steel beams, here that are shown to be W12x14 and W10x12. The girders, which are picking up the weight of those beams, are W16x26.

We have steel columns here too.

Now, if you’ve already taken our structural series and our Unobstructed and Obstructed Construction series, then you’re all over these details already. But just as a quick refresher, for solid steel beams, the “W” designation is the type of steel beam, which means wide flange, the first number is the approximate depth of the beam, in inches, and the second number is the weight of the beam, in pounds per linear foot.

That’s the same convention we use for metric, except the first number is the approximate depth in millimeters.

The critical dimension that we need to know from the designation is that first number, the depth of the beam.

So, getting back to what is probably our most important step in this process for this example, are we Obstructed or Unobstructed?

For that, let's go back to NFPA 13, go to the definitions chapter, which is Chapter 3.

Does the construction impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control a fire? Well, my gut says yes, but I'm not gonna rely on that. So, let's go to the Annex to find an appropriate example.

What do you know, our first example of Obstructed Construction says Beam and Girder Construction. The key point here is that we have steel beams, which are 3-ft to 7-1/2-feet (900 mm to 2.3 m) apart, on center, and are supported or framed into girders. 

What is our beam spacing? Well, let’s run dimensions on those. Closer to the top, or Grid line “A”, our beam spacing is about 6-1/2 feet or 2.0 meters. Closer to the bottom, our spacing is 5-1/2 feet, 1.7 meters. In both of these cases, our beam spacing is within the range that's described as Obstructed Construction. 

So, we meet one of the examples in the Annex straight-up, we’re considering our situation here to be Obstructed Construction. 

#4 COMBUSTIBLE OR NONCOMBUSTIBLE

Step #4: Combustible or Noncombustible?

This one is pretty straightforward. We have solid steel beams, a metal deck above and concrete above that. We’re going to be considered noncombustible.

#5 PIPE SCHEDULE OR HYDAULICALLY CALCULATED

Step #5: Pipe Schedule or Hydraulically Calculated? 

Another quick step here. This is new construction, so we’re only allowed to hydraulically calculate the system. Again, even if it’s an existing pipe schedule system, there are reasons to still calculate it and see if the water supply can handle our system demand.  

But for our case here, hydraulically calculated. 

Now we’re not into Extra Hazard or Storage, so the design density will not affect the layout rules. I can skip that part of this step. 

#6 SELECT SPRINKLER TYPE AND ORIENTATION

Step #6: Select sprinkler type and orientation. 

Here, like our last case, I'm gonna keep things pretty straightforward with standard spray sprinkler. If I try standard and I don't like the quantity of sprinklers, or let's say this room size repeated many times over and over and there is an opportunity for savings, or I'm already using extended coverage sprinklers elsewhere on the project, then sure, I'd consider them. As this plays out, this might actually be a good example for extended cover sprinklers if we wanted to save two or three sprinklers. 

But here, starting out, I have no reason to go with extended coverage and I personally feel comfortable giving standard spray sprinklers a try. 

What about the sprinkler orientation? Well, we don't have a ceiling. Could I use pendant sprinklers? I could, but there are reasons why I wouldn't. 

First is at the height of the sprinkler to the deck above is measured from the sprinkler deflector to the deck. For a pendent sprinkler, the deflector is gonna be on the bottom of the sprinkler. For an upright sprinkler, of course, that deflector is on the top of the sprinkler. So, there is a maximum limit on what distance it can be from the deflector to the deck. 

When we have an upright sprinkler, we're easily able to have our pipe below the sprinkler. We essentially have more freedom and flexibility on where we locate our pipe when we use an upright. Because that maximum distance from the deflector to the deck is relatively fixed. So, if the deflector is on top of the sprinkler, that means our pipes below the sprinkler and we've got a little bit more flexibility on where that pipe can be. 

If we were to use a pendent, we'd have a whole lot less space to work with for that pipe. Also, especially when we're thinking about open structure areas that may someday have equipment moving around, maybe hydraulic jacks or possibly even forklifts, upright sprinklers have a more natural protection to protect their bulb or that feasible link than even pendent sprinklers do. And that's that pipe itself. When the pipe is below the sprinklers and the sprinklers on top, it effectively gives just a little bit more protection for that sprinkler. It's kind of an added benefit. 

Now, Joe, come on. That's obvious. It's open structure, of course. I'm gonna use an upright. Just say that and be done with it. 

Well, you're right, it's usually that simple. However, I just wanted to point out that if the deflector distances work, you could go with a pendent sprinkler. That'll be more important later. When we have situations where we might be running pipe through concrete tees and wanna use pendents, or where we have storage situations where we could benefit from the use of a pendent storage sprinkler, or let's say that because of the storage rules or because our pipe is so large that it would obstruct the sprinkler and we have to use a pendent sprinkler. Those could be cases that even though we have upland structure, we go with a pendent sprinkler. I just wanna point that out and so that seed as to why we make those decisions about sprinkler orientation. We could go with pendent sprinkler, but most of the time when we have open structure, we're gonna end up with uprights. 

So long story short, upright is the best sprinkler orientation here. 

#7 FIND THE LIMITS

Step #7: Find the limits for sprinkler spacing. 

We’re under NFPA 13, so let’s go there. Starting at the Table of Contents we go to Chapter 10 for Standard Pendent, Upright, and Sidewall Spray Sprinklers. That’s us. 

Chapter 10 – Installation Requirements for Standard Pendent, Upright, and Sidewall Spray Sprinklers 
    10.2 covers Standard Pendent and Upright Spray Sprinklers 
       10.2.4 Protection Areas per Sprinkler 

Here, we work our way all the way down to Table 10.2.4.2.1(b), which gives us our Protection Areas and our Maximum Spacing for sprinklers in our scenario that’s specifically for Ordinary Hazard. Note here that we’re using Standard Spray Sprinklers.  

So, in this table, in all cases, regardless of construction type, obstructed or unobstructed, we have a maximum protection area of 130 sqft (12 sq.meters) and 15-ft or 4.6 meter maximum spacing between sprinklers. 

Joe – why did you make me figure out combustible and noncombustible and obstructed vs. unobstructed when it didn’t affect our spacing? 

Well, one, you’re right. However, like we discussed back in the series on Obstructed Construction, that determination does more than just affect the spacing of the sprinkler. It affects the height of the sprinkler. 

Now, in our last example we worked through, we had a suspended ceiling. So we didn't pay much attention to the height of the sprinkler. We knew we were gonna have a pendent sprinkler and who was gonna protect the space below the ceiling. That was about the extent of the discussion around the height of the sprinkler. 

Here, the height of the sprinkler becomes very important. But what is that limit? What height can the sprinkler actually be? 

If we stay in Chapter 10, but scroll down to Section 10.2.6 Deflector Position (again this is for Standard Pendent and Upright Spray Sprinklers), we get to Section 10.2.6.1 Distance Below Ceilings. 

For Obstructed Construction, which is what we have here, Section 10.2.6.1.2 spells out our requirements. Now we only have to follow one of these approaches. We do not need to meet all of these rules. So, let's go through the different approaches. 

Approach #1  

The sprinkler deflector must be within 1-6 inches (25-150 mm) below the bottom of the structural member, and a maximum distance of 22-inches (550 mm) below the ceiling or roof deck. This is giving us two parameters. One is that we need to be below the structural member (which is the bottom of the steel beam for us), but still within 6-inches of the bottom of thast beam. We can’t go any lower with this sprinkler deflector. Some of our beams here are 10-inches deep (255 mm), some of our beams are 12 inches deep (305 mm), and the girders are deeper, those are 16-inches deep (405 mm). If we’re spacing across a beam, sprinkler to sprinkler, then this rule is telling us that we need the deflector to be 1 to 6 inches (25-150 mm) below the bottom of those beams. If we're spacing across the beams, then that's the range, the height that we need to be. 

The second rule that comes into play here is that we can’t be more than 22-inches (550 mm) below the deck. That is still true even if we have really deep beams.  

Now, I like this rule, I like this approach and I think we can follow it just fine for this room. I'm gonna tentatively plan on using this approach. We'll call it approach number one because our beams just aren't terribly deep and I think we can make it work.

But let’s go through the other rules just to see how those would pan out. 

Approach #2

We can have deflectors at, or above the bottom of the structural members, if the deflector is within 22-inches (550 mm) below the roof deck, when the sprinkler is installed in accordance with 10.2.7.2 That section talks about sprinklers near obstructions. It’s basically saying we can move sprinklers up higher than the bottom of the structure, if we account for the beams and effectively throw underneath the beams. 

If we have really deep beams, like say 22-inch (560 mm), or 24-inch (610 mm) deep beams, then yeah, we’re going to need to start locating deflectors higher than the bottom of those beams. That’s not our situation here, so I still like Approach #1 over this second option. 

Approach #3 

Install sprinklers in each bay of obstructed construction, with the deflectors 1-12 inches (25-300 mm) below the ceiling. 

Now unless our spacing were to magically work out where each bay is just below our maximum spacing limits, I’m not gonna go down this path. This one is less-flexible than the others. I still like approach #1. 

Approach #4

This one deals with composite wood joists, so that’s not us here. I can skip over this one. 

And the last one, Approach #5

Deflectors under concrete tee construction – well that’s not us either. Can skip out on this one too. 

So, of all the options that are allowed here, I like Approach #1 and that we read at the beginning, keep the deflector 1 to 6 inches (25 to 150 millimeters) below the bottom of the beam, up to a maximum of 22 inches (550 millimeters) below the deck. Let's hold onto those limits and see if we can make it work. 

So, of all the rules that are imposed here, I like the Approach #1 that we came read at the beginning. Keep the deflector 1-6 inches (25-150 mm) below the bottom of the beam, up to a maximum of 22-inches (550 mm) below the deck. Let’s hold onto that limit and see if we can make it work. 

So now we have our limits. That was step #7. Now onto step #8, position the sprinklers. 

So now we have our limits. Onto the last step. 

So, at this point we’re going to wrap up as Part I of our 2-part series. 

We’ve gone through the first seven steps of the layout process, only saving the actual sprinkler layout and pipe layout for Part II. 

Now we’ve determined where sprinklers are required, we’ve determined the hazard classification, Obstructed Construction, Noncombustible Construction, that we have a hydraulically-calculated system, that we’ve selected Standard Spray, upright, sprinklers, and we’ve determined the criteria for the sprinkler coverage area, the maximum sprinkler spacing distance, and the appropriate height of the sprinklers within the space. 

All that’s left is to lay the thing out. That’s Part II and is next in the series. 

I’m Joe Meyer, this is MeyerFire University.

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