Minimum pressure needed at fire riser... 7

[madvb]

Hi,

What is a typical pressure loss through a fire sprinkler system for a single story office building (4000 sf) with about 200 feet of piping? We have been doing a deferred approval so that a sprinkler contractor will design the system and get approvals, so I do not know how much loss through the system. I believe the furthest sprinkler head requires a minimum of 7 psig. I'm not sure what a typical pressure loss is going through the fire riser and all. I'm trying to determine if civil giving me 20 psig at the building is going to be enough for the sprinkler system without having to oversized the piping to reduce pressure loss. What is a typical recommended pressure at the building/fire riser?

Thanks.

 

[TravisMack]

It really depends on a lot of variables. Do you have wood construction? If so, is the attic protected with a wet or dry system? If wood attic, what is the roof slope? Is a backflow preventer required?

You typically see sprinklers in an office at up to 14'x14' spacing. That is 12 psi for the sprinkler. Assuming 10' ceiling = +/- 4.5 psi loss. You have another 5 or so psi for the backflow preventer. Right there, you are at 20 psi +/- and that doesn't account for any friction losses in the system.

For a typical light hazard wet system, single story, you can do it comfortably with +/- 50 psi. If it is a dry system in the attic where the roof pitch is 4:12 or greater, you are going to want a larger volume of water and about 50-60 psi.


Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[madvb]

wow... I didn't realize that those variables you mentioned would affect pressure calculations. How is the building construction type (wood vs metal) affect calculations? Does one require more sprinkler heads, thus needing more fittings? I honestly have not looked at this section of NFPA as we always depend on the sprinkler contractor.

Is the 12 psi the minimum required per NFPA at the sprinkler head for an office building? Not sure where I saw 7 psi... maybe for residential?

This one I'm working on is a wood framed building with a 4:12 roof slope. Dry pipe system. Backflow preventer is at the point of connection (city water). So I'm losing about 5-10 psi going through a dry pipe riser (about 300 gpm through a 4" or 6" line)? Looks like civil giving 20 psi at the building is not going to be enough. Guess another diesel fire pump with enclosure is going in this project.

 

[SprinklerDesigner2]

The 7.0 psi is the minimum pressure required on a sprinkler not to be confused with what might be needed.

p=(q/k)^2

p=pressure required in psi, q=water om gpm and k is the constant.

Standard 1/2" orifice sprinklers have a k-factor of 5.6 and at 7.0 psi at the sprinkler they will discharge q=k*p^.5 or 14.8 gpm.

The standard density for an office is .10 gpm so as long as the head does not cover more than 148.0 sq ft the 7.0 psi will work.

But what if your spacing is 14'x14' for 196 sq ft? The density of .10 will require 19.6 gpm be discharged which will require 12.3 psi. It is important to recognize 14.8 (7.0 psi) gpm is the minimum required even if in a small 3'x2' closet.

You can have two identical buildings in different sections of town and it is likely the pipe sizes will change and sometimes radically. If city water for the first was 110 static, 90 residual @ 1,360 gpm. For the second building the water supply was 40 static, 28 residual @ 690 gpm. Given the two water supplies, one very good the other very marginal, the two systems will be totally different in regards to head spacing, number of sprinklers and size of pipe. Given the good nature of one water supply and the marginal nature of the second the system in the building with the marginal supply could easily (most likely in fact) cost 50% more than the one with the better supply.

All sprinkler systems are unique. One never looks exactly like another unless it is in identical buildings with identical water supplies.

Whether the building is made of wood or steel will dictate whether or not you have sprinklers in an attic if designed per NFPA #13. Combustible spaces require sprinklers while non-combustible spaces do not. If a residential building, designed per NFPA #13R sprinklers are not generally required in attic spaces.

In laying out sprinklers there's literally thousands of different "rules" and it takes years to learn most of them but if you read the forum you'll learn nobody ever knows all of them. So often to me it seems the more you know the more questions you have.

 

[TravisMack]

By having a wood attic with slope of 4:12, you will probably have an average of 80-100 sq ft per sprinkler in the attic. The design area will be 1500 sq ft + 30% for the dry system + 30% for the roof slope. This leads to a minimum design area of 2535 sq ft. At 100 sq ft per sprinkler, you are going to have 26 sprinklers in your design area. Since they are going to be spaced less than 148 sq ft, you will have a minimum of 14.8 gpm from each sprinkler. That gives you 384 gpm. With a dry system, you will end up with about 20% over discharge so your base of riser demand could be about 460 gpm. The pressure needed would be about 60 psi to keep respectable pipe sizes.

Again, this is just based on a bunch of estimations from past experience with dry systems.

Now, you could do the entire thing as an anti-freeze system and get your design area down to 1170 sq ft because it is essentially a wet system. You could use 4.2k sprinklers to cut down on over-discharge and have a demand of +/- 150 gpm plus outside hose allowance of 100 gpm. However, if the capacity of the system is >40 gallons, then you need to use the Darcy Weisbach(sp?) calculation method. You will also likely need a reduced pressure backflow preventer. But, if you end up needing a water storage tank for the project this would be a lot less stored water.

As you can see, there are many variables as to how you could approach the system. But, 20 psi at the base of the riser is not going to be close.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[pipesnpumps]

madvb, 20 psi as the minimum pressure allowed in the underground piping by environmental and AWWA regs.

You need to learn more about NFPA 13 to be doing engineered performance based plans. Buy an NFPA 13 handbook and read it.

Tennessee requires preliminary hydraulic calcs for the entire system on the drawings or submitted separately. The calcs should go from the test hydrant through to the hydraulically most remote area. The reason they require this is a) they rightly consider sprinkler systems to be engineered systems, and b) they learned they can't trust engineers like to properly do these calcs on their own.

Hatching a drawing and saying "light hazard per NFPA 13" won't cut it and at best results in huge headaches (fire pump added after bid phase). Worst case is the state gets involved and someone loses their PE license.


If you have 20 psi at the base of the riser, you will need to carefully size the fire pump. The residual pressure in the underground line must be >20 psig while your fire pump is flowing at 150% of its rating.

 

[firepe]

Along the same lines of what pipesnpumps is saying, I will take it one step further.

You are NOT qualified to be making these decisions. I suggest you consult a qualified fire protection engineer or contractor for assistance.

 

[pipesnpumps]

I somewhat agree firepe, but a qualified PE of any related discipline (civil, mechanical) is completely capable of doing sprinkler work in typical occupancy hazard approach situations. Obviously they need to RTFM (read the fricking manual) and then work under someone who can teach them the ins and outs before they are qualified.

It could be worse, he could have not even logged on to ask the question.. Just like every other yahoo that submits mickey mouse drawings. This is at least a first step in the right direction.

 

[firepe]

pipesnpumps,

We are saying the same thing here. An FPE does not have to be an engineer that attended Univ of Maryland and received a Fire protection degree, but rather, an engineer that took and passed the FPE PE exam. That would be most preferable, but I would also agree there are many other non FPE engineers out there that did not specifically take the FPE exam, but are still qualified to consult on fire protection. When I state "qualified" I mean qualified, not claims to be qualified.

As for my advice to the original post, madvb, that is why I stated what I did. The fact that they reached out to this forum is perfect. They need help, and should not attempt this alone.

 

[ChrisConley]

Totally unrelated to fire, but with 20 psi at your riser you're going to have problems with your plumbing system as well if you have any flush valves in the system (typically 25 psi minimum).

 

[madvb]

Thank you for all these very informative responses. We do have a set of NFPA in the office. We don't have a fire protection engineer here, and that's why we have been doing the deferred approvals and leave it to the contractor to get the right people to design the sprinkler system. For my part, I want to make sure that I have provided enough flow and pressure at the building.... just wasn't sure what a typical demand is at the building. Now I know...50-60 psi!

I'm sure there will be one of those projects where they say they can only give you 40 psi.... what then? Oversize your pipe a little? I guess we can always pay a FPE to run the numbers for us. Do you all have a set pressure in mind and will not go below that number no matter what? The cost of adding a packaged diesel fire pump with an enclosure isn't cheap.

SprinklerDesigner2: Thanks for running those quick calcs for me as I have overlooked some of the items you mentioned.

 

[madvb]

ChrisConley - I have always asked my civil guys to give me separate lines for domestic water and fire water. They usually give me enough pressure for domestic water as the flow is low (~25 gpm).

 

[SprinklerDesigner2]

I'm sure there will be one of those projects where they say they can only give you 40 psi.... what then? Oversize your pipe a little?

Most importantly you have several people, firepe and pipesnpumps among them, advising to consult with someone that has training which is the best advice you can receive.

Perhaps oversize is misleading a bit. To oversize implies there's a standard size when there isn't. But the general rule is the lower the available water pressure the larger the pipe gets which results in a higher price in material and labor to install.

Need to recognize the lower the pressure the the more difficult it becomes to compensate.

Assume a project was guessed to have 80 psi and it turns out you only have 60. It's going to cost some, perhaps have to increase a 2 1/2" main to a 3" main. But the job most likely won't be a disaster even though there's a 20 psi difference. But at lower pressures, assume someone figured 40 psi and there's only 30, it generally is going to get much more difficult, not to mention expensive, to make up that 10 psi. Instead of having to increase a main from 2 1/2" to 3" you might very well have to increase it from 4" to 6" or even 8" and that'll cost you dearly. Given an office for example it would probably be possible to provide sprinklers with as little as 16 or 17 lbs (assume 8' ceilings and a 3 psi loss through a backflow preventor) but you ain't going to be liking the price one bit. With 7.0 psi end head pressure, 3.5 psi elevation and 3.0 psi backflow you're at 13.5 psi without friction loss and if all you have is 16 or 17 psi available you can't have any friction loss. Possible to do, run all mains 6", use 3" on the branch lines and 2" drops to pendent heads and you might just barely do it without any margin for safety but the owner is not going to be liking the price. Also recognize in some jurisdictions, Georgia for example, a 10 psi "safety factor" is required which means you will need 26 or 27 psi.

Recognize fire pumps do not make water they just boost the pressure.

When we talk of pressure we're not talking about static or gauge pressure with no water running. By itself this means very little if anything. The important pressure is what is available in the line after subtracting the water required for sprinkler and fire department hose streams.

Sprinkler systems can be like spider webs; just the smallest movement in the most remote corner can cause the entire web to move. As Travis said there's lots of variables to consider.

 

[MGXFP]

Also there is always an absolute minimum pressure when designing a sprinkler system.

Even with, say, 6" mains and 4" branch lines there will be some minimum pressure needed to operate the thing for the performance criteria and it probably won't be 20 PSI.

Almost everything can be overcome except for a lousy water supply.