I am not sure what is the minimum required by most AHJs, but my company standard is 10 psi. On an existing system sometimes we will allow less if most of the existing piping is to remain.
I have also heard of people using 10 percent as the factor of safety, but at least on many projects I've done, that would be less than 10 psi so we aim for 10 psi and require it in our specs.
Our state has mandated a 10 psi safety factor.
With as-built plans being required we had always tried to get a 15-20 psi safety factor depending on the site and job conditions. since additional fittings and piping can be added in the field and affect your calculations.
NFPA 13 does not have a specific Safety Factor requirement because NFPA 13 has numerous safety factors built into the code. For instance, a standard spray upright or pendent is required to have 7 psi minimum, and is required to be no more than 7'-6" off of a wall horizontally (except for small room rule which allows up to 9'-0" off one wall in one direction only with other stipulations as well). At Oklahoma State University, they have a Sprinkler Lab and they tested a standard spray upright at 7 psi (put a gauge on the bull head side of a tee just before the upright and throttled the control valve supplying the sprinkler until the gauge read 7 psi). They then had a circular "ruler" on the floor from the center of the sprinkler and at 7 psi, the sprinkler sprayed up to 12 feet.
My point is that NFPA 13 has numerous safety factors built into the code and therefore does not have a specific pressure safety factor other than that the demand point is below the supply curve.
Many AHJ's do not have a specific safety factor requirement either (although I do know of one here in the OKC Metro area that requires a 10psi safety factor). However, I do see, quite regularly, that the Architectural/Engineering Firm reviewing the subcontractor's submittals specify their own safety margin requirements whether the AHJ has them or not. They are usually either 10 percent or 10 psi. I have seen as much as 20 percent required by an A/E.
Thanks for your post. I am still amazed at the result of the study. I have a light hazard job, wood truss, that I will be lucky if I can find straight shot through the truss. It' looks like they put one half of the building offset or reversed. Need to survey more thoriughly. May need to add some 90s or 45 easier on the calcs. Cant get a hold of designer, till Friday.
My plans state saftey margin +18.843 or 31.3%
While safety factor is not required by NFPA 13, having a 5-10 psi is a good engineering practice to account for fluctuation and deterioration of the available water supply.
In addition to the safety factor, it is a good idea to contact the local water utility company to determine low hydraulic gradient of the system, and account for it in hydraulic calculations.
While it may be a conservative approach (and may trigger installation of a fire pump), it ensures that your sprinkler system will operate as designed even in worse case condition.
The 10 PSI seems to be common. I do a lot of freelance design all over the country and I've seen quite a range of requirements. Portland, OR degrades your flow test by 80%. Colorado requires a 10% minimum up to 100 PSI then a flat 10 PSI. Montgomery County, MD requires 20 PSI. As a rule, the 10% seems to be the most prevalent. I agree that NFPA is extremely conservative in it's requirements. I have observed several fire tests and the sprinklers usually out-perform expectations. There are a lot of factors that need to be considered when performing calculations. If an area has a dense population, and is built out, I'll use more of the available flow than an area that is relatively new and still has potential impact on the water supply. Potential for field revisions, upgrades such as tenant build-outs, all weigh in for consideration in the design of a fire protection system.
Explosion Protection & Prevention
Fire Detection And Alarm Systems
Flammable & Combustible LIquids
Information Sources For Analysis
Means Of Egress
Passive Building Systems
PE Prep Guide
PE Prep Series
PE Sample Problems
Smoke Management Systems
Special Hazard Systems
Types Of Analysis
UFC 3 600 01
Water Based Fire Suppression