Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

VoDich (Mechanical) (OP)
3 Aug 11 14:07
How does a FPE calculate the fire water demand for a building?  For example, a Light Hazard building with 3200 s.f.  Using a wet-pipe system and Density/Area Curves in NFPA 13, I'm coming up with 150 gpm (0.10*1500) as a minimum water demand for this building.  Assuming hose streams are not required for this example.

Is it safe to tell a FPE to design a sprinkler system for this building using 150 gpm and not go over this number as the pressure at the higher flow is too low?  I'm guessing that the FPE would use a computer program such as Haas or something to run the numbers.  Does Haas has its own way to calculate water demand for the building?

I have seen a submitted hydraulic calculations where the sprinkler flow is 375.91 gpm for the building using 0.10 gpm/s.f.  Not quite sure why the flow is so high and if it's possible to lower the flow.

Thanks.
Helpful Member!  TravisMack (Mechanical)
3 Aug 11 14:25
That is a theoretical output.  A sprinkler system is basically a machine.  Have you ever seen a machine with 100% efficiency?

If you calculate 1500 sq ft, and the heads are spaced at 200 sq ft / head, then you are going to calc a minimum of 8 heads.  8 sprinklers requiring a minimum of 20 gpm is going to be 160 gpm.  Now, the heads will likely not discharge exactly 20 gpm, due to friction losses in the piping network, so you get even more over discharge.

I have seen a dry system in an attic with demands in the 500 gpm range and it is still a light hazard design criteria.

It is all dependent on the actual layout of sprinklers in the facility.

And, I am not even an FPE smile

Travis Mack
MFP Design, LLC
www.mfpdesign.com
 

NewtonFP (Automotive)
3 Aug 11 15:15
Not an FPE here either,

It is possible to lower the flow by adjusting the orifice size, piping arrangement, pipe sizes etc. You will only be able to calculate the minimum required flow. The flow by way of calculations will always be higher than the minimum.

A dry system in a steeply sloped attic will absolutely require lots of water and pressure when compared to an office building with 8'-0" AFF ceilings.

As TM said; it all depends on the building, layout and hazard class.
Helpful Member!  VoDich (Mechanical) (OP)
3 Aug 11 16:42
Travis: So if the FPE knows that the city is only capable of supplying say 250gpm at 40 psig at the fire riser.  Wouldn't it make sense to design a sprinkler system to use a maximum of 250 gpm?  Why would you design a system for a much higher flow rate that would require a fire pump?

From what Travis and Newton are saying, it seems that the flow rate can be controlled by the FPE by the number of sprinkler heads and piping arrangement?

Thanks for the responses, I'm just trying to get a better understanding of what is going on.
Helpful Member!  TravisMack (Mechanical)
3 Aug 11 16:56
If you design a system for higher flows, you are putting in a water storage tank.  A pump can only provide pressure.  If you don't have water, you need a tank.

The flow rate can be controlled to an extent.  However, if you have a really cut up space then you are going to have a higher water demand.  For example, if I have a bunch of small offices in my design area, I am likely going to have more flow than if I have a wide open space and can maximize sprinkler layout.  So, in that sense, the architect has the biggest impact on sprinkler flow.

Granted, I can chose smaller orifice sprinklers to choke down flow.  I can modify pipe sizes.  I can increase orifice size to decrease pressure.  If I have ceilings less than 20', I can use the allowed QR reduction to reduce my design area.  But, each project is going to have to be designed to provide coverage per NFPA 13.  You can't always make it work with the municipal supply, especially if you have a poor water supply.

Now, the FPE on this particular project may not have made the most effective design.  We don't know that.  But, there are many variables that come into play when putting the system together.  If there is that great a concern on this project, get a second professional to review the plan for possible modifications to reduce the sprinkler system demand.  You may find out that what the original FPE did is actually the best layout.  Or, you may find he did a rather inefficient layout.

Travis Mack
MFP Design, LLC
www.mfpdesign.com
 

VoDich (Mechanical) (OP)
3 Aug 11 17:57
Thanks Travis.  That makes a little more sense with your bunch of small offices vs wide open space example regarding the flow rate.

So what do you tell your civil engineer when they ask you what your building's fire water flow rate is during the design?  We don't have a FPE to run the cals until the project is bid out.  By then, adding a fire pump is a huge cost impact to the project.  Is there a better estimation than using the Density/Area Curves in NFPA 13?
cdafd (Specifier/Regulator)
3 Aug 11 18:22
either work with a sprinkler designer up front or hire the FPE up front, since it sounds like one will be invovled anyway!!!!!!
TravisMack (Mechanical)
3 Aug 11 19:58
Be careful that you are not being asked site fire flows which are totally different than sprinkler demands.

As far as demands, it totally depends on the structure and type of system.  If I have a wood attic, dry system and 4:12 roof pitch, I would easily say 500-600 gallons.  A wet system for ceiling sprinklers only, light hazard, 10' ceilings, probably no more than 120-150 + hose.  But, you can mess that up with using extended coverage sprinklers and get to a 200+ gpm demand plus hose allowances.

Ordinary Hazard Grp II with no QR reductions, around 400 gpm + hose.  

But, these are just airball numbers that may be way off depending on the sprinkler system layout. As you can see, there are several variables.  In just a light hazard office building, you can range from 120 - 600 gallons depending on the variables.

But, as CDAFD said, you need to have some one experienced in these matters to handle this up front for you.  Otherwise, a fire pump after the fact becomes an unfortunate reality.  On jobs where I have had to give a number for reference early in the design in the past, I tell them to bring me a 6" line and give me a minimum of 70 psi at the base of the riser at 500 gpm.

Travis Mack
MFP Design, LLC
www.mfpdesign.com
 

Helpful Member!  pipesnpumps (Mechanical)
3 Aug 11 21:33
It doesn't take an FPE to calc a sprinkler flowrate.  NICETs excel at sprinkler design and usually have vastly greater experience in hydraulic calcs.  Maybe if you are friends with one they will help you out.

For what its worth I think you should look at using quick response reduction, and you should roughly lay out the heads on the floor plan based on max. distance of say 12 ft from walls or 24 ft between heads.  Draw a rectangle around the most hydraulically remote 1,500 sq ft of floor area(or less if you are taking quick response reduction).  Count the number of heads and calculate the flow using 24 gpm per head.  

When you get that total flowrate (a theoretical minimum flow), add on about ~30% on for overage.  Example, say it is 9 heads x 24 gpm/head x 1.3 = 281 gpm.   If your rooms are shaped such that your heads are bunched close together and not covering much floor area, then doing the above method will account for that..  The 30% for overage will leave pressure drop margin for the sprinkler designer to use.

You can reduce the overage by installing larger branch lines and feed mains; this also reduces friction losses so somewhat reduces pressure demand.  You can reduce pressure demand by installing K 8.0 sprinkler heads throughout instead of K 5.0's.  You can reduce the pressure demand by spacing the sprinklers closer. (less flow per head means less pressure required to force the lower flowrate through the head).

You really should completely read the calculation section of NFPA 13 if you want to do any of this, else hire a PE who actually knows sprinklers (hard to find!)..   If it looks like it is close for having a booster pump (or pump+tank), then detailed hydraulics should be done.  For this small system (non-gridded) they can be done by hand quite easily, no software required.



 

Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.  

Helpful Member!(2)  stookeyfpe (Specifier/Regulator)
4 Aug 11 2:31
Ahh, the old sprinkler demand versus fire flow question. This is debated several times every year.

Here is a helpful thread my friend Travis contributed to:

http://www.eng-tips.com/viewthread.cfm?qid=170610

This subject needs a white paper. I think I'll work on crafting something.
VoDich (Mechanical) (OP)
4 Aug 11 14:14
stookeyfpe: Actually, I just wanted to know the building's sprinkler demand.  I may have used the wrong terminology that got confused with site fire flow.  It is a very informative thread however.  Did Travis get his steak at the steak house?

pipesnpumps: Thanks for the tips, I'll try your quick calcs next time.  One thing that confuses me is that you mentioned using 24 gpm per sprinkler head, but a couple of paragraphs down, you mentioned reducing pressure demand by spacing the sprinklers closer and "(less flow per head means less pressure required to force the lower flowrate through the head)".  Where is this less flow per head coming from when you mentioned 24 gpm per head?  Doesn't spacing the sprinklers closer mean having more heads within that 1500 s.f., which means higher total flow?

Travis: Do you have the section number in NFPA that talks about wood framed attic or metal framed attic requiring different demand?  And a section number for the roof pitch?  I'm seeing section 11.2.3.2.4 "Sloped Ceilings in Non-Storage Applications" where system area of operation shall be increased by 30%.  Nothing on roof pitch.

Where in the system are inside and outside hose streams connected to?  What is the required pressure for these hose streams?  I'm guessing the outside hose stream is to feed the fire truck which then boosts the pressure to fight site or building fires?  If so, couldn't hose stream be designed to be on a separate system in certain situation rather than adding to building's sprinkler demand?

Thanks!!
NewtonFP (Automotive)
4 Aug 11 15:25
Right below in 11.2.3.2.5 dry systems must increase the remote area 30%. So for a dry system in a steep sloped attic gives me 1500*1.3*1.3=2535 minimum square foot remote area.

My NFPA 13 (2010 edition) says in 11.2.3.2.4 that if the ceiling slope exceeds 2/12 then the remote area must be increased 30%.
TravisMack (Mechanical)
4 Aug 11 15:40
VoDich:

I think it is about time that NFPA 13 becomes your sole source of reading material.  When I started in the industry, my boss required that I read NFPA 13 every night for about the first 6 months I worked there.  Granted, it was a lot smaller back then, but it taught me generally where to find things in the standard and a good basis for all of my future work.

Anyway, for spacing in all light hazard areas, refer to:
Table 8.6.2.2.1(a) Protection Areas and Maximum Spacing of Standard Pendent and Upright Spray Sprinklers for Light Hazard

For required increase in design area for sloped ceilings (roof deck):
11.2.3.2.4 Sloped Ceilings

For required increase due to dry pipe / preaction:
11.2.3.2.5* Dry Pipe and Double Interlock Preaction Systems.

So, if you have a light hazard, combustible attic with structural members less than 36" on center and a slope of 4:12 or greater that is protected by a dry system, you have a base design area of 1500 sq ft.  You then increase that 30% for the dry system to a design area of 1950 sq ft.  You then increase that 30% for the slope to a design area of 2535 sq ft.

In an attic such as that, (based on doing these a lot), you are going to average about 80-90 sq ft per sprinkler. (Remember, the max spacing for that attic is 120 sq ft per sprinkler). At 90 sq ft on average per sprinkler, you are looking at 29 heads to be calculated.  They will have a minimum discharge of 14.82 gpm (7 psi).  Right there, you have 430 gpm demand without any over-discharge from the sprinklers.  A dry system can be expected to have about a 20-30% inefficiencies (over discharge, tree system, etc..) , for a demand of +/- 550 gpm.

So, that is how your small little light hazard office can easily give you a crazy sprinkler system demand.

Now, get a copy of NFPA 13 to take home every night and read it front to back at least 5 times.  It is a great cure for insomnia, but if you are going to be in this industry, you need to have a good handle on the requirements set forth in NFPA 13 and where to find them when you can't recall the exact information.
 

Travis Mack
MFP Design, LLC
www.mfpdesign.com
 

pipesnpumps (Mechanical)
4 Aug 11 16:25

VoDich,
A sprinkler only has to be sized for the floor area it protects.
As an example, if it protects 225 sf @ 0.1 gpm/sf = 22.5 gpm out of that head.   This requires a pressure of:  P = (Q/K)^2 = (22.5/5.6)^2 = 16.1 psig at the head.

If the same K5.6 heads were spaced at 12'x12' then it is 144 sf @ 0.1 gpm/sf = 14.4 gpm   P = (Q/K)^2 = (14.4/5.6)^2 = 6.61 psig BUT a sprinkler head is required to have 7 psig minimum.  So the last head would be calc'd at 7 psig, and the 2nd head up the branch line would then probably have >7 psig due to friction losses..

Likewise you can see that using k8.0 heads will also reduce pressure requirements.  This is all basic stuff, kindergarten material for a NICET.  Greek to most PEs..

 

Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.  

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close