Design criteria for and extra hazard room of 96 sq.meter

[ajitjose]

I am new to sprinkler design.
Can you guide me how to do a hydraulic calculation for an area less than the minimum required area?
For example, for Extra Hazard group 2, min density and area =232m2 @ 16.3 mm/min. Consider my room is 96 m2. What will be the minimum flow for an area of 96 m2 ?
The below is from NFPA. Can anyone explain to me? I could not follow it. As per my understanding, Step 3 will always be greater than 0 and I didn't understand the rest of it.

23.4.4.2.4* Where the available floor area for a specific area/
density design criteria, including any extension of area as required
by 11.1.2 and Section 12.3, is less than the required
minimum design area, the design area shall be permitted to
only include those sprinklers within the available design area.

A.23.4.4.2.4 The following steps outline the procedure for
calculation in accordance with 23.4.4.2.4:
(1) Calculate the hydraulic design discharge including those
sprinklers within the available floor area.
(2) Calculate the minimum required discharge by multiplying
the required design density times the required minimum
design area.
(3) Subtract the discharge calculated in Step 1 from the discharge
calculate in Step 2.
(4) Where the discharge calculated in Step 3 is greater than 0,
the hydraulic design discharge is recalculated including
an additional flow equal to that calculated in Step 3. The
additional flow is added at the point of connection of the
branch line to the cross main furthest from the source.
(5) Where the discharge calculated in Step 3 is less than or
equal to 0, the hydraulic design discharge is as calculated
in Step 1.

Regards,

Ajit Jose

 

[TravisMack]

There are several instances where you could have excess flow from an area. Remember, no mechanical system is 100% efficient.

Let's say you are calculating a typical retail space for 0.2/1500. I have NEVER seen the required flow be exactly 300 gpm. There is over-discharge inherent in any system.

So, you could have a 1033 sq ft area at EH1 density that has a significant over-discharge to get a minimum of 600 gpm so you don't have to figure additional flow. If your flow in the area is less than 600 gpm, then you must add the difference at the end of the main in the area.

I am assuming EH1 and high temp or K11.2 sprinklers so that you can reduce the design area to 2000 sq ft.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[UFT12]

So, in short, if your calculated flow (which by definition incorporates always an overflow) over an area less that what is mandated by nfpa 13, is larger than the density x application area (ch11), you re done. If it is less you need to include the difference on the main to reach the minimum mandated by the code.

But I have a question here too. How do we practically include the extra flow to reach minimum required? Do we just consider the far end of main open and add a flow requirement of the difference with whatever pressure is calculated on that node? Similar like adding hose demand at the base of riser?

 

[ajitjose]

Thank you Travis. Can you please look at below.

step 1: Design discharge= 1033 x 0.4= 413.2 gpm
step 2: Minimum req. discharge= 2000 x 0.4= 800 gpm ( Considering k= 11.2)
step 3: step 2 - step 1= 800-413= 386.8 gpm
step 4: 413+386.8= 800 gpm
The additional 386.8 gpm is added at the point of connection of the last branch line on the cross main.
If I have only 4 sprinklers on the last branch line. I need to consider the above additional flow at this point.
In such a case, the pipe size on the last branch would be at least. 3 inch( 80 mm ) while the other branches lines are smaller.
In addition to this, for a room of small size such as this, the cross main size would be quite big.

I hope I have done the steps correctly. Eventually if we need to calculate with 800 gpm itself, I don't understand the use of all these steps given by NFPA and what is the point of adding the flow at the last branch.

I am just asking from the procedural point of view set forth by NFPA and trying to understand the practical approach to the problem.

I have another question.
If my room volume= 96 m2 x 3 m Height= 288m3= 76081 gallons
Now, if as per design 800 gpm is discharged for 60 minutes then the volume of water @ 60 min= 48000 gallons. Won't this discharge flood the entire room.
I mean I don't get the logic.

Please help.

 

[UFT12]

You didn't include the water of the fire fighter hoses. In general, the actual number of sprinklers operating is typically way less than the calculated. In addition, the primary mechanism of sprinklers fighting a fire is cooling through evaporation. There is going to be substantial amounts evaporated. For a small fire you may not expect a lot of evaporation but the duration of discharge is going to be a far cry from 60, 90 or even 120 min the standard mandates. From the other hand, in case where the system is needed to remain on for the maximum times, it wouldn't be prudent to worry about the water amount rather not having the building collapsing.

 

[TravisMack]

The demand is added like a hose allowance at the end of the main in your area.

1033 sq ft * 0.4 is THEORETICAL. If you calc it out based on actual discharges and pipe sizes, then you will have something in excess of your 413.2 gpm. You may be closer to 500 or 600 gpm. So, you are only adding the difference in it. But, you add it at the main. All of the branch lines will still be the same size.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[ajitjose]

Initially, I simply could not believe it that such a small room would require such a big cross main but I understand your explanation. Most people I spoke to just multiply the actual room area times the minimum density and had been doing the wrong designs all this time.

So, in reality we don't have to do all these steps. We just take the minimum design area @ density irrespective of the room size however small or big it may be. Then, theoretically, I have to size the pipes of that room to discharge @the calculated demand. In my case, I will have to design piping of 16 sprinklers to discharge @800 gpm (plus the hose demand)

Speaking of hose demand, what does the combined inside & outside hose stream allowance mean? Does it mean hydrant and indoor hose rack usage?

 

[TravisMack]

I think you still have some misunderstandings of what is going on. You are not calculating your 16 sprinklers to get 800 gpm.

Design your 16 sprinklers to provide your density per the occupancy required. Then, if you are less than the minimum of density x area, you add the difference at the end of the main in that area.

You still have to include your hose allowance where ever it is required.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[ajitjose]

Thank you Travis.
So, can we assume another hazard of EH1 in the same facility, Assume k=11.2.
Now, demand = 2000 X .30= 600 GPM. For the sake of discussion consider actual hydraulic calculation gave 750 GPM.

As explained to me earlier, the actual demand for the earlier discussed area would come to around 600gpm. Consider I get the same in hydraulic calculation.

What is my consideration for the maximum demand?
Case 1- 1033sq.ft, EH2 which has 600 GPM demand
Or
Case 2- 2500 sq.ft, EH1 which has 750 GPM demand

Or Even though case 2 has higher value, we should consider case 1 as the maximum demand @ 800 GPM because it's EH2.

Which is correct?

Regards,

Ajit

 

[TravisMack]

Your EH2 will have the higher flow, but which has the higher pressure? If you are sizing for a pump capacity or tank size, then you go with the higher flow. However, after you add the extra required in Chap 23 for the EH2 area, that is going to be 800 gpm.

For pump pressure, you take which gives you the greater pressure requirement.

Also, with the 11.2k, you get the same benefit as high temp sprinklers. So, your design area of both would be 2000 sq ft. I think you are trying to do something that is beyond your experience and expertise level. At this point, you should probably hire a consultant that can assist you on your specific project. There just seems to be a breakdown in understanding here, and it may just be a language / cultural misunderstanding. I apologize if the misunderstanding is on my end.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

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[ajitjose]

Thank you Travis for the clarification.
You are right when you say its beyond my experience and expertise. I want to go beyond fire Detection which I currently do. I am trying to learn sprinkler systems by reading already designed systems, NFPA 13, and asking some designers whom I know. The information I get is bits and pieces and that's why I am missing out on major basics.
Can you give me a guide as to where I can get a systematic learning experience for getting by basics right?

Regards,
Ajit Jose

 

[cdafd]

Ajit

What country/ state are you located in

 

[ajitjose]

I am currently located in Saudi Arabia but I am originally from India.

Regards,
Ajit Jose

 

[TravisMack]

Sadly, there is not a great place that is done online. Always keep asking questions in here and we will try to help you out as much as possible. Some things just require "showing" more than explaining in a forum can do.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[ajitjose]

Thank you very much Travis for the support and understanding.

Regards,
Ajit Jose

 

[stookeyfpe]

If you're looking for distance learning courses, Oklahoma State University has several that may fit your need:

https://ceatpd.okstate.edu/content/fire-protection-and-loss-control

Full disclosure: I'm a graduate of the program so I have a bias.

 

[ajitjose]

Thank you.
That's good. If you are a graduate you can tell me if the online courses are good.
For starters, should I consider taking the below course.
Link
Basic Principles Of Automatic Fire Sprinkler Protection
Required Textbook:
Fire Detection and Suppression Systems, 4th edition, ISBN: 978-0-87939-398-4

Regards,
Ajit Jose

 

[stookeyfpe]

ajitjose (Electrical)(OP)6 Dec 17 06:00
Thank you.
That's good. If you are a graduate you can tell me if the online courses are good.
For starters, should I consider taking the below course.

I can't advise you on the quality of the online classes because when I was in college we didn't have on-line classes. But I passed (with high marks) one of three classes on water based fire protection systems at Oklahoma State University and an earlier edition of this book was used. You should also consider taking the course on fire protection hydraulics - it's very rigourous.