What others said. But there is no problem with negative suction pressure
until the pressure is low enough to start boiling the water. I don't know
how low that exactly is. Vapor pressure of water at the given temperature
vs. local atmospheric pressure in the vessel (pump). The pump will continue
to pump water until air is sucked in or the water boils. Air won't get
sucked in until the water level gets below the intake. Typically with tanks
you don't count the positive head in the first place. The -3 is when you
are at the bottom anyway so it's all friction loss in the piping and on
occasion static gain, but never loss.
I've pulled about -15' elevation from a draft through a fire truck before
with no problems. So this was about -10 psi. Granted I have a throttle and
lower the GPM to avoid boiling water (you can hear it, sounds like pebbles
in the pump).
While it will probably work IMHO I can't think of a good reason to go less
than -3. Either put the tank closer to the pump or upsize supply, but you
knew that.
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: Friday, October 31, 2008 1:10 PM
To: [email protected]
Subject: Re: Fire Pump Suction
That is a very legitimate concern. My guess is that you could very
easily have a problem, especially if your static head is less that
4.6 psi. In my experience, there are very few good uses for 4" pipe
prior to the system riser. With 6" pipe, your friction loss would be
about 2 psi.
At 01:32 PM 10/31/2008, you wrote:
>
>400 gpm fire pump supplied from tanks with suction flanges higher than pump
>suction flange. Approximately 100 feet of 4" piping (including fittings)
>from tanks to pump. NFPA 20 permits the gauge pressure at the pump suction
>flange to drop to -3 psi, with the pump flowing at 150% (600 gpm in this
>case). The friction loss in the piping for 600 gpm flow is about 7.6 psi.
Is
>there a concern for the pressure available at the pump suction, when the
>water level drops in the tanks? I have been told that it will, so checking
>the opinion of the forum.
>
>Tony
>
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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