Check out NFPA 20 - 5.14.3.2, allows -3 psi. So with 315' of 8" you're about
-1.7psi with 8" and with 6" about -6.6psi. So 8" is reasonable with no lift
or elevation on the tank. 6" would need elevation.
Also a clarification on what Mark said. "It is only after the tank has
nearly emptied that you'd have a negative pressure condition. At that
point, you'll be in a siphon condition."
Some negative pressure is allowed. But also consider you have much bigger
problems than air if the tank has run nearly empty during a fire. You have
at least 30 minute but in reality 60 minutes or more. If you drain the tank
I'd bet the fire overwhelmed the sprinklers in the first place and the FD
didn't supplement and FD didn't put the fire out. 15 minutes is probably
all that is needed in MOST (but not all) situations.
True a centrifugal pump won't pump air. You need to start without air in
the line, thus the equipment for positive head to start.
And the pump doesn't siphon until air is drawn in. Perhaps he meant
something else by siphon? Siphoning doesn't start just because you have
negative suction pressure. You can draft well below the pump inlet. The
theoretical maximum below the pump is the atmospheric pressure converted to
feet. Twenty something feet of lift is certainly within reality. Yes you
lose pressure on the discharge side by being negative. But again if you get
near the bottom of the tank you have bigger problems.
This is why fire trucks can draft out of a pond without parking in the pond.
The key is to not let air in. This is the reason for the vortex plate to
break up the drain vortex (swirl you see that sucks in air when you drain a
tub) from allowing air in before the actual water elevation reaches the
inlet elevation.
While we follow the code it is very conservative in this area. Remember all
code limits are based on 150%. As long as you start with no air in the
suction line, and don't allow air in (through either slow draft or vortex
plates) a centrifugal pump will draft just fine.
Chris Cahill, P.E.
Fire Protection Engineer
Sentry Fire Protection, Inc.
763-658-4483
763-658-4921 fax
Email: [EMAIL PROTECTED]
Mail: P.O. Box 69
Waverly, MN 55390
Location: 4439 Hwy 12 SW
Waverly, MN 55390
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Todd Williams
- FPDC
Sent: Sunday, August 03, 2008 7:07 AM
To: [email protected]
Subject: RE: effective tank volume
The other detail is that I have to give them an elevation for the
bottom of the tank. In addition, there is about 315 equivalent feet
of pipe from the tank to the pump. the building requires a 500 gpm
pump and I plan on calling for an 8" suction line. I was also
thinking about insisting that the center line of the suction line be
6 ft above the centerline of the pump for just that reason.
At 11:56 PM 8/2/2008, you wrote:
>Yes - but remember that you have positive pressure when the tank is
>filled. (Normally, a horizontal, split-case pump is taking suction
>from a tank whose water elevation - when filled - is above the
>centerline of the pump). It is only after the tank has nearly
>emptied that you'd have a negative pressure condition. At that
>point, you'll be in a siphon condition.
>
>Mark Sornsin, PE
>Ulteig engineers, Inc.
>Fargo, ND
>
>-----Original Message-----
>From: [EMAIL PROTECTED]
>[mailto:[EMAIL PROTECTED] On Behalf Of Russell
>Sent: Friday, August 01, 2008 3:57 PM
>To: [email protected]
>Subject: RE: effective tank volume
>
>Would not anything below the centerline of the pump impellor be a negative
>pressure? Split-case centrifugal pumps have to have a positive pressure on
>the suction side. Or have I misunderstood the situation?
>
>Russell Rewis
>Brown Automatic Sprinklers, Inc.
>107C Hemlock Street
>Valdosta, Georgia 31601
>229-244-8130
>[EMAIL PROTECTED]
>-----Original Message-----
>From: [EMAIL PROTECTED]
>[mailto:[EMAIL PROTECTED] On Behalf Of Ed Kramer
>Sent: Friday, August 01, 2008 2:14 PM
>To: [email protected]
>Subject: RE: effective tank volume
>
>I recently completed a similar project. I agree with Mark, the available
>water is that part of the tank below the inlet of the overflow and above
the
>bottom of the vortex plate. Also, reference NFPA 22 section 4.1.3 (03
>edition).
>
>Ed Kramer
>Littleton, CO
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:sprinklerforum-
> > [EMAIL PROTECTED] On Behalf Of Todd Williams - FPDC
> > Sent: Friday, August 01, 2008 7:05 AM
> > To: [email protected]
> > Subject: effective tank volume
> >
> > I am working on a project where the water supply for the facility
> > will be fed from concrete tank(s). The absolute volume (completely
> > full) of the tank is 10098 gallons. A 6" suction line with elbow &
> > anti-vortex plate will need to be installed. According to NFPA 22,
> > the face of the suction elbow has to be a minimum of 6" above the
> > bottom of the tank. The take-out for the elbow is 8" and the top of
> > the pipe will be 4" (+/-) above that. I am trying to calculate the
> > effective or usable volume in the tank based on what my minimum
> > height above the floor can be relied on for suction. Would it be the
> > 6" to the bottom of the elbow? 14" to the center line of the suction
> > pipe? 18" to the top of the suction pipe? The tank is 10 feet tall,
> > so this could have a significant impact on available water. (BTW, I
> > am already assuming that the tank will only be filled 9" to 12" from
> > the top; would that be reasonable?)
> >
> > Todd G. Williams, PE
> > Fire Protection Design/Consulting
> > Stonington, Connecticut
> > www.fpdc.com
> > 860.535.2080
> > _______________________________________________
> > Sprinklerforum mailing list
> > [email protected]
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> >
> > To Unsubscribe, send an email to:Sprinklerforum-
> > [EMAIL PROTECTED]
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>
>_______________________________________________
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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