I thought it sounded unusual too John. If the suction gauge pressure is 0, then the Net Positive Suction Head is about 34 feet. A lot of pumps have Required NPSH of ~15 feet, so the requirements in NFPA 20 are conservative. The allowance to go as low as -3 PSI for suction tanks with the bottom at or above the pump is to account for the velocity head, which would be: h=3/.433=6.928 ft. v=SQRT(2gh)=(2*32.2*6.928)^.5=21.12 ft/sec. Brad
p.s. sorry I missed your session Cecil but I was busy coming unglued -----Original Message----- From: Steve Leyton [mailto:[email protected]] Sent: Friday, June 13, 2014 11:08 AM To: [email protected] Subject: RE: 2 hour fire water supply What Bilbo said. Hydraulically, if you have a tank with a water column height of ... let's say 10' above pump suction, that makes for about 4.3 psi of positive suction head. What we typically do is ignore that and simply size the pump for the demand, thus building a little more safety factor into the pump's total discharge head. However, if you need a minimum positive suction pressure to meet the demand, then everything about your design should be based on that lowest HGL (hydraulic grade line). But when you get into a scenario that depends on a certain rate of recovery to meet demand over the course of the required duration, then you're talking about a break tank by definition. I don't mean to complicate the issue because it's not the question you asked, but if your AHJ is sharp, they'll note the potential requirements for redundancy of fill valves and the required rate of recovery and could make you calculate and/or test to those standards. Steve Leyton -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, June 13, 2014 8:50 AM To: [email protected] Subject: Re: 2 hour fire water supply Karen, We are required to use the "lowest gradient elevation" as the highest point for calculations (see NFPA 22). The lowest tank gradient is usually the point where the water may drop before refill begins. from never-ending meetings in Las Vegas, Cecil Bilbo Sent from my Verizon Wireless Phone ----- Reply message ----- From: "John F Hoffman" <[email protected]> To: "[email protected]" <[email protected]> Subject: 2 hour fire water supply Date: Fri, Jun 13, 2014 8:10 am So the tank is needed for head pressure? Sounds unusual. Normally the pump is sized to operate off of essentially zero suction from the tank. If the design basis of this pump needs the head pressure from the tank, and if the fill rate equals the discharge rate, all you need is a 10-inch diameter tank that is tall enough. It is more likely that the tank was sized from the sprinkler design. John Hoffman, PE Facility Engineering Services KCP, LLC -- Burns & McDonnell Engineering National Nuclear Security Administration's KC Plant Operated by Honeywell FM&T, LLC office: 816-488-7213 [email protected] -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of Karen Lesko Sent: Friday, June 13, 2014 9:05 AM To: [email protected] Subject: RE: 2 hour fire water supply I don't know what the design basis was, but it looks like we need the automatic fill to maintain the head pressure, so probably not. On Fri, Jun 13, 2014 at 8:20 AM, Brad Casterline <[email protected]> wrote: > Karen, > Sorry about the superfluous talk about fire- I was watching it all > play out in my mind (fast forwarded because 2 hours is a long time)- > but this is interesting in that the fill rate is equal to the > discharge rate, so it is no different than a city water flow test, > except you need a fire pump for pressure. > Was the water storage tank capacity arrived at based on system demand > for 2 hours, with no consideration of automatic fill? > Brad > -----Original Message----- > From: Brad Casterline [mailto:[email protected]] > Sent: Thursday, June 12, 2014 9:14 PM > To: [email protected] > Subject: Re: 2 hour fire water supply > I see accidental fires, Sprinklered or not, going through three phases: > erratic growth, somewhat steady burning, then some sort of decay. If > you look at what might be the lowest water level in the tank over two > hours would be you could get an idea of what the minimum pressures at > the sprinklers would be. >> On Jun 12, 2014, at 9:01 PM, Brad Casterline >> <[email protected]> > wrote: >> >> Probably not since there is a difference between the total capacity >> and > the available head at any one instant in time? >> >>> On Jun 12, 2014, at 8:40 PM, Karen Lesko <[email protected]> wrote: >>> >>> When evaluating fire water supply to ensure there is a 2 hour >>> supply, do > I >>> have to take the head pressure loss of the fire water tank level >>> drop > into >>> account over the 2 hours? The tank has makeup from wells that have > normal >>> and back-up power, and the well makeup into the fire water tank >>> equals > the >>> output of the fire pump. >>> >>> Thank you in advance, >>> >>> Karen Lesko >>> _______________________________________________ >>> Sprinklerforum mailing list >>> [email protected] >>> > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkl > er.org >> _______________________________________________ >> Sprinklerforum mailing list >> [email protected] >> > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkl > er.org _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkl > er.org _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkl > er.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
