We generally require a 5 psig safety factor to accommodate future expansion and intrinsic errors in hydrant tests (day of the week, hour of the day, etc.)
I think the answer to this question has a wide array of correct answers. If you are doing an FM Global job for instance they are going to require 10 psi buffer or 10 % below available pressure. This is a good rule of thumb to follow to allow for unforeseen changes/demand that a public water system may encounter after the system is installed. Of course you will have instances where this may not be possible and you have to take into account what type of water supply you are dealing with: tank and pump, private fire line, etc. I would be especially leery of a system model water flow provided by the AHJ. These are inaccurate most of the time. A live water flow from hydrants is the best and most accurate way to go.
There are so many variables between plans / calcs and install, that it would be tough to try to go with anything below 10%. That even concerns me on some projects.
1 - Do you have the seasonally adjusted lowest flow test for your site as req'd per NFPA 13, or do you just have a single point in time measurement of your water supply?
2 - Will your plan be installed EXACTLY as drawn so that there are no offsets or extra pipe installed that will impact the calculations?
3 - How accurate is/was your testing equipment for the water supply?
Just those few things there can eat up a lot of margin very quickly.
I agree with everything said here. I once had a boss that would say if you get anything positive you were ok. His reasoning behind it is the remote areas are a safety factor in themselves.
I have always heard this, but have never seen concrete definition on the amount of safety factor inherent in the remote area. If you can't provide concrete evidence of the margins, I would not want to be on a witness stand explaining why the system didn't work because we didn't design to the seasonally adjusted water supply and left it at 0 margin.
Georgia requires a 10 psi safety factor which doesn't present a problem because everyone is on the same page.
Personally I like even more... 15 psi is what I shoot for.
But "hitting the line" do you really save anything? Let's say you got a 100,000 sq. ft. building and with 1 1/4" grid you have a safety factor of 4.0 psi but using 1 1/2" you got a safety factor of 18.5 psi. How much money does it really cost to go the extra mile if it's a 1,000 head job that sold for $120,000?
Between 1 1/4" and 1 1/2" labor will be the same.
Hangers will cost 2 cents more so you got $20 extra there.
Heads and welded outlets will be the same as will fabrication.
The difference between 1 1/4" and 1 1/2" might be $0.15/foot and with 10,000 feet we're looking at $1,500?
For couplings we're $0.23 difference so with 500 couplings we're looking at $115.00.
All told we're looking at maybe $1,700 additional cost on a $120,000 project? Big deal.
The benefits:
1. I sleep a whole lot better at night. If someone were to run a flow test indicating 10 psi less than what was obtained a year previously I will still sleep good at night. At my age I need a good nights sleep.
2. If so some reason I missed something and the guys have to add in a dozen extra elbows I most likely won't have to worry about it. Sleep is everything.
3. Most of the time I can make up some of the $1,700 additional expense maybe by reducing the size of the riser or one of the cross mains without effecting the results to much.
If you've ever cut a system way back, take it to the line, and get caught the additional expense to "bring it up to standard" can be devastating. I've never had this happen to me but have watched it happen to others. It just ain't worth it.
As far as "there's plenty of safety factor built in" that isn't my call to make. If a building burns down, lawsuits and perhaps wrongful death, I would like to see that reasoning used in court. Lawyers would eat you for lunch and by the time they got done you'd be wishing you died in the fire.
I got a small 10,000 sq. ft. job I am working on right now. Toyed with the idea of 1" in all the branch lines (it's a light hazard occupancy) and if I did I would have 10.1 psi safety factor. I upped all the branch lines to 1 1/4" sch. 10 which gives me a safety factor of 33 psi. Additional cost? Less than $100. Big deal, cry me a river Mr. Owner of the company.
Recently I was asked by code official for 30% safety margin. By the way, when we talk about percentage, what is the proper way to calculate: delta P divided by available pressure or system demand pressure?
