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Domestic Allowance

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TravisMack

TravisMack

Mechanical
Sep 15, 2003
1,752
This is truly a question for a friend :)

When doing a large apartment complex per NFPA 13R (say 16 buildings where each are appropriate for 13R) that is supplied by a common fire loop, what do you do for the domestic allowance? This particular site had a domestic booster pump placed on the site for the domestic/fire loop. The sprinkler demand is 80 gpm +/- for the 4 residential sprinklers.

Since this is a domestic/fire loop 13R states that you have to figure a domestic allowance. My conservative nature says that you should figure the domestic load for each building on site and include that in the loop with your fire sprinkler flow. The reasoning is that if this is a fire in building #1, people will likely still be doing dishes, using bathrooms, showers, etc in most of the other buildings. So, that pump is still going to be supplying that demand.

However, NFPA 13R states that you just figure the individual building where you have a combined fireline for the building. My thought is that would be like a single small condo or something where you have the domestic and fire split in the building. This particular project has separate fire lines and domestic water lines to each building.

The main problem on this project is that the sprinkler guy only figured the sprinkler demand for the individual building and used the pump at the left hand side of the curve. There was like 50 psi or so at the street, then boosted with 80 psi @ 500 gpm pump. Well, since the sprinkler demand was so low, he basically had 130 psi to deal with. Now, the AHJ has come back and stated that he wants the domestic load included. The domestic load based on the table in NFPA 13R (A9.6) will be about 400 gpm for this complex. So, they end up losing about 20 psi of total supply pressure. As you can imagine, the system is installed (prior to permits - VERY BAD IDEA) and now none of it works. Due to the very high pressures they thought were available, the lead-ins were installed as 1" pipe, and all 1" and 3/4" cpvc.

So, basically, how would you guys interpret the domestic load that is needed?

Travis Mack
MFP Design, LLC
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are you saying that there is an underground loop which feeds multiple apartment buildings, and each individual building has a combined fire/domestic riser after a common pump? If so, the common pump would be suffering from the operating dishwashing/toilet flushes/etc. in the other units as one unit had sprinklers operating.

However, NFPA 13R states,
"Domestic demand for the building being calculated shall be included as part of the overall system demand for systems with co mmon domestic/fire mains where no provisions are made to prevent the domestic waterflow upon sprinkler system activation."

Now, I have no idea what provisions could be made to prevent domestic waterflow in the other units upon sprinkler system activation, but if it was my company I'd be looking into them right about now...
 
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Each individual building has a separate fire line and a separate domestic line. The site has a single pump that feeds an 8" loop that serves all of the buildings downstream of the pump. That is why I say that the other buildings will have the toilets/dishwasher/etc going on so those domestic loads need to be applied.

For your other question, there are things called a domestic shut off valve that will only allow flow to the sprinkler system upon activation. Tyco makes these. I think they come in 1" and 2" sizes.



Travis Mack
MFP Design, LLC
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Well, that sounds like one way to recover without having to destroy too much finished construction, no?
I also submitted this question to the NFSA so we'll see what they say.

 
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No. Those would only shut off flow to the individual building if you have a combined lead-in. That is not how this system is set up. There is no way to shut down the rest of the complex, nor should there be, based on a fire in a single building.

My feeling is that it was a bad design by the site utility contractor, but that is another story. There was not correlation to Appendix B of the IFC either.

Travis Mack
MFP Design, LLC
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I just reread this again and it technically says "building" singular
like you were implying in the original post

NFPA 13R 2013 "Domestic demand for the building being calculated shall be included as part of the overall system demand for systems with common domestic/fire mains where no provisions are made to prevent the domestic waterflow upon sprinkler system activation."

It kind of makes sense because this demand itself is being taken out of the small 1" and 3/4" pipe while the other buildings are taking out of a common underground supply, like any underground in a typical city block.

If there's nothing in NFPA 20 that says you have to account for multiple simultaneous domestic allowances downstream a fire pump which supplies multiple risers/buildings then you might could make an argument that you only have to account for one building and the AHJ is asking for something beyond the word of the standard, therefore go above his head or ask for a change order based on the AHJ going above and beyond what is required (and have the owner take it up with the AHJ if he's an owner with influence in small town or something)

Just throwing ideas out there

 
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Yeah. That is the argument that the design guy wants to use if needed. My gut is that is only applying to something where you have a single building fed from a city main that has a common line. But, I get overly conservative in these things at times.

Travis Mack
MFP Design, LLC
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Fpst

I want to say normally domestic is a concern when there is one line going into the building, and it splits one going to domestic and one going to the sprinkler system.
 
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Yeah. My concern is that the booster pump was sized by the MEP guys to handle what they thought was domestic and fire requirements. But, they figured them independently.

I know we only anticipate a fire to be in one building at a time, but don't you expect all buildings to have a domestic demand at one time? My buddy is concerned because fingers are already being pointed at him.

There are other complicating issues that there are fire hydrants downstream of this pump and they are no where near able to deliver 1500 gpm at 20 psi at a minimum.

My feeling is that a project like this should have the booster pump sized for the domestic demand and also the largest fire demand. This was not done. I can't see how it is the fire sprinkler contractor's responsibility to verify the site and plumbing engineers do their job correct.

He likely should have questioned harder in the beginning. I mentioned these details in the beginning of this project. I think he let others tell him how it should be and didn't hold firm. Nothing is in writing where he stated these requirements to his customer. Now there are potential major problems on this site.

I was hoping some one would say, "Hey, Travis, you big idiot. Of course you only factor the demand of one building in this situation." I've done these in the past and I always figure total domestic load of the site when it is arranged like this. But I know intend to be overly conservative at times.

Travis Mack
MFP Design, LLC
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I didn't realize the site/contractor provided the fire pump, if that's the case, and they didn't install it per nfpa 20, it should be on them to correct it by buying the correct pump, then your buddy is in the clear. what am I missing?
 
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It is not even technically a fire pump. It is a domestic booster pump, basically as you find at any city pumping station.

It's one of these where when things go wrong, crap rolls downhill and he is at the bottom of the hill. This is also a lesson why you document everything in writing.

Travis Mack
MFP Design, LLC
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if that is the case, then could he not treat it like a normal "non fire sprinkler device" as if the booster pump were right below a water tower? then he goes out to one of these hydrants downstream the pump, figures out a clever way to flow really close to whatever his most demanding building requires, and takes a flow test. Then, since it's a loop put your "supply" at the tee leading into the building. All other buildings and their domestic allowances would be ignored since they're before your supply point, and treated as separate calculations...

All of this being approached that it's not a "fire pump", but a "domestic pump" which does not have any place in a sprinkler hydraulic calculation (trying to get creative, maybe I'm wrong haha)
 
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I proposed a similar solution. Put a 24 hour recorder on one of the hydrants installed downstream. Leave this on for a week and you learn the peak time to do a flow test.

If you perform a flow test at that time, then you should be taking into account all domestic demands and you only need to see if your system is below that supply curve.

It is a simple method to take out all theory and get to real world solutions. But, if the domestic load of all buildings on the loop didn't need to be considered the entire problem goes away.

Travis Mack
MFP Design, LLC
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Hate to say...,

But I have always interpreted that as domestic demand. Each building is to be accounted for. My normal total is around 450gpm.
This is one of those "assumed to understand" areas. If there is a demand present on the supply, it counts. The standard has no variance on that statement.

I wish you luck.

R/
Matt
 
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Yeah,

And the annex doesn't help. It says All units downstream... etc.., etc..

Don't think this one is winnable.


R/
Matt
 
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Wouldn't one solution be to simply calculate the fixture load using 2012 International Plumbing Code Table 604.3? At least it's a legally enforced document when adopted by a governmental jurisdiction.
 
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I would be very interested in seeing how the certified results compare to 13R's guess.
According to 13R, they be flushing some toilets...

R/
Matt
 
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The NFSA EOD gave their informal opinion on this,

"What you are describing here is a private water service since it is supplied from a domestic booster pump and feeds both the domestic and fire lines for the complex. Private water services are outside of the scope of NFPA 13R. The applicable portion of the pipe that would fall under NFPA 13R ends where the separate fire line meets the loop. If the building is supplied from a common domestic/fire main then the domestic demand must be added into the fire sprinkler calculation. Since this the fire line and domestic water lines are both separately piped, this is not a common domestic/fire main and therefore you would not include the domestic load of the building into the fire sprinkler hydraulic calculation.


The private water main will need to consider both the domestic and fire demands as it is the supply for both. In municipal systems, this is overseen by the water authority for the jurisdiction. The private water main owners will need to determine the available flow and pressure of the water based on the supply and equipment being utilized. However, using NFPA 13R to make those adjustments would not be appropriate and should be determined by some other approach."
 
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