Variation in Fire Sprinkler Safety Factors

8/29/2017

NFPA 13 does not clearly identify a required safety factor for hydraulically designed fire sprinkler systems. What safety factors are commonly required by jurisdictions in your area?

Note: This is a Daily Discussion question submitted anonymously to us. Submit a Question or Discuss This Here.

5 Comments

Kristen E Nich

8/29/2017 05:47:43 am

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.

David

8/29/2017 08:16:39 am

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.

Patrick Turner, P.E.

8/29/2017 08:19:29 am

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.

Pete

8/29/2017 09:19:13 am

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.

Marc

8/29/2017 06:04:54 pm

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.

Free Signup

Our daily posts run during study season (June - October). Subscribe for posts on PE Problems and Daily Discussions, straight to your inbox, for free:

Standings

Complete Standings for the Weekly Exam Series:

Now Available:

2018 PE Prep Guide

Weekly Practice Exams

Filters

Archives

August 2018
July 2018
June 2018
May 2018
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
November 2016
October 2016
July 2016
June 2016
June 2015

Daily Discussions

Daily discussion questions are open-ended questions posed by members for the benefit of sharing expertise and learning from other perspectives. Join as a member here.

PE Problems

Visit July-October for daily Fire Protection PE Exam sample questions.

Solutions are posted the day after posting.

Comment with your solutions, questions or clarifications.

Please note that questions posted are unofficial and in accordance with NCEES rules are intended to be similar to actual exam questions, not actual exam questions themselves.

RSS Feed

Home - About - Blog - Contact - Exam Prep - Store

Copyright © 2018 MeyerFire, LLC
The views, opinions, and information found on this site represent solely the author shown and do not represent the opinions of any other party, nor does the presented material assume responsibility for its use. Fire protection and life safety systems constitute a critical component for public health and safety and you should consult with a licensed professional for proper design and code adherence. Some (not all) outbound links on this website, such as Amazon links, are affiliate-based where we receive a commission for orders placed elsewhere.