The 2010 NFPA 20 Handbook commentary to 4.6.5.1 states "Further, some water authorities prohibit flowing a quantity of water that causes residual pressures to drop below 20 psi."
In South Carolina, the subject pressure requirement is found in this section of the "State Primary Drinking Water Regulation" found at http://www.scdhec.gov/environment/water/reg.htm#dw: R.61-58.7(E)(7) The minimum pressure in the distribution system under normal operating conditions shall be twenty-five (25) pounds per square inch at a customer's service connection. A minimum pressure of twenty (20) pounds per square inch shall be maintained at all service connections during unusually heavy flows (i.e., fire or flushing). In college I remember learning that 20 psi was to prevent infiltration so if there was any leakage it was to the outside not into the pipe [which might be in saturated soil under a head of water...?] My old 1985 Water Supply and Pollution Control (4th ed.) textbook states [on p.119] under the hydraulic design section that water distribution systems "...are usually designed so that reasonably uniform pressures prevail. A pressure of 30 psi is normally considered to be the minimum desirable in any area, with the exception that during a serious fire it may be permissible to allow the pressure to drop to about 20 psi. Main feeders should be designed for pressures between 40 psi and 75 psi whenever possible." I hope this helps. Respectfully, David W. S. Blackwell, II, PE, CFPE Engineer III Office of State Fire Marshal SC Department of Labor, Licensing, & Regulation, 141 Monticello Trail, Columbia, SC 29203 Telephone: 803.896.9800 [Office] 803.896.9833 [Direct] Fax: 803.896.9806 [Office] Email: [email protected] Website: http://www.scfiremarshal.llronline.com/ Please note that you can sign up to receive automatic information on SC Division of Fire and Life Safety current events, policies, laws and procedures by visiting our Web site at: http://www.scfiremarshal.llronline.com/ -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Sprinkler Academy - C Bilbo Sent: Wednesday, August 24, 2011 9:45 PM To: FORUM Subject: RE: Minimum pressures in water supply; Why 20 psi? Folks, The 20 psi minimum is set by every state EPA (that I found) to ensure compliance with the Federal Clean Water Act. If the pressure in mains goes below 20 psi, the water provider must issue a "boil order". This ensures that any substantial drop in pressure did not result in "back siphonage" as eluded to in an earlier post. If a water purveyor fails to issue a boil order after the pressure in a main goes under 20psi, the State EPA will issue substantial fines to that purveyor or municipality for non-compliance with that State's EPA Clean Water Regulations. To reiterate, they are not concerend with cavitation, rather contamination. Hope this is helpful, Cecil Bilbo SprinklerGeek Academy of Fire Sprinkler Technology Champaign, IL 217.607.0325 www.sprinkleracademy.com<http://www.sprinkleracademy.com> [email protected]<mailto:[email protected]> OUR STUDENTS SAVE LIVES!! ---------------------------------------- > Subject: RE: Minimum pressures in water supply; Why 20 psi? > Date: Wed, 24 Aug 2011 19:09:30 -0500 > From: [email protected] > To: [email protected]; [email protected] > > The 20 psi is from the AWWA requirements. The concern is cross contamination > when the residual pressure drops below 20 psi and the potential for > contaminants entering the water system at fittings. > > The FD SOPs are based on the purveyor's polices in not pulling down the > system. The fire department pumper intake gauges are plus/minus up to 15%, > therefore that gauge cannot be trusted with a high level of accuracy. In > adverse conditions (need for more water) the operator can pull the system > down to 5 psi as long as the pump is not caviating. > > Regards, > > Tim Frankenberg > Fire Product Manager > Potter Electric Signal Company > > Deputy Chief/Fire Marshal > Washington (MO) FD > > > -----Original Message----- > From: [email protected] on behalf of David de Vries > Sent: Wed 8/24/2011 9:11 AM > To: [email protected] > Subject: Minimum pressures in water supply; Why 20 psi? > > Does anyone know where the 20 psi minimum for undergound piping originated? I > have seen it in some state water regs, but have never been satisfied that it > is a sound, scientifically-based number. > > I have also seen fire dept SOP's set an alert on suction pressure at the > pumper to 5 psi. Depending on the hose lay, that might equate to 20 (or 10) > psi in the main. > > Dave > > David A. de Vries, P.E., CSP > Firetech Engineering Incorporated > > > --- On Wed, 8/24/11, Letterman, Todd <[email protected]> wrote: > > > From: Letterman, Todd <[email protected]> > Subject: RE: Minimum pressures in water supply > To: [email protected] > Date: Wednesday, August 24, 2011, 8:27 AM > > > I agree with all who have responded and I'm not sure if there is hydrant > at the entrance to this place or along the street that could rob your > suction pressure. If there is a hydrant coming into the property, then > they are most likely going to start there hose lay at that point, which > may rob all the theoretical values so far. You are not going to have > much suction pressure at a 100%. If a fire takes it to that demand but > there is really nothing stating that you cant go lower than 20 unless > the water authority or other entity states other wise. It was never > stated if it is going to be a Electric or Diesel Driver or have you > gotten that far yet? > > Todd Letterman > Fire Prevention Engineer CFPS > Riverside County Fire Department "Fire never sleeps" > (951) 955-5273 FAX (951) 955-4886 > [email protected] > > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Todd > Williams > Sent: Wednesday, August 24, 2011 5:21 AM > To: [email protected] > Subject: Minimum pressures in water supply > > I am working on a new school project, where the building is 65 ft above > the water supply in the road and 760 ft horizontally. The water supply > is 54-46-1130. There is a pump planned, which I have to size. The total > pressure loss in the underground between the supply and the pump house > is approximately 30 psi. I know the general consensus (and some water > company requirements) is that the water pressure not be drawn down below > 20 psi. A 500 gpm at 60 psi pump would be a good fit for this project, > but the suction pressures at the pump would end up being below 20 psi. > The supply in the public mains would remain above 20 (due to the > elevation), but would be below that in the private main at the pump > room. Any thought if this would be acceptable? > > Todd G. Williams, PE > Fire Protection Design/Consulting > Stonington, CT > 860.535.2080 > www.fpdc.com > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://fireball.firesprinkler.org/mailman/private/sprinklerforum/attachments/20110824/0eebc788/attachment.html> > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://fireball.firesprinkler.org/mailman/private/sprinklerforum/attachments/20110824/80796028/attachment.html> > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum > _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum
