If I understand your concern you are worried the tank will run out before 30
minutes because of overflow due to a supply calc vs. a demand calc. You are
worried about calc'ed world vs. real world. Fine but in what real world
will the actual design area operate. Odds are you won't be in the remote
area which increases overflow meaning less than 33 minutes. Odd are less
than the calc'ed number of heads will flow decreasing the demand meaning
more than 33 minutes. Odds are 2 heads (or less) will flow and you won't
have anywhere near 222 gpm coming out. This probability is the driving
force showing in reality the tank will last more than 33 minutes.
Overflow is good from a fire suppression standpoint anyway. More water per
time equals higher probability of suppression and quicker time. It's also
not linear, you get more bang for the buck with increase in density. I
don't know of specific figures, just going from experience. If you put the
fire out in 10 minutes and the tank runs dry in 20 is there a problem?
IMHO the system failed if the full remote area operates in the first place.
Or at least a serious investigation is warranted to determine why. If you
still need the sprinklers after about 10-15 minutes there is a problem. The
extra heads and 30 minutes already have a deal of safety in them.
The actual produced is always a balance between flow and available pressure.
So yes if the entire remote area opens you will in reality drain the tank
faster than 33 minutes. Could be much quicker depending on the shape of the
pump curve and the overflow. But assuming you are light hazard based on the
very little information provided I don't see a problem. If you are really
curios start running system supply calc's for different areas and you should
see the tank will never last 33 minutes with a full sized calc'ed area and
much longer when less than the full area.
Now maybe you are 45 minutes from the closest FD and this becomes an
engineering question to consider. Some day the code may differentiate
between available levels of fire service but not yet.
But hey maybe I missed the point of your question.
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 A.P.Silva
Sent: Thursday, November 06, 2008 2:11 PM
To: [email protected]
Subject: RE: Fire Pump Suction
The maximum sprinkler demand as per hydraulic calcs. is 222 gpm. The water
supply is by means of storage tanks having a total volume of 7448 gallons.
Just barely enough for approx. 33 minimute at maximum demand. The contractor
has installed a 400 gpm fire pump. NFPA 20 requires gauge pressure at the
fire pump suction flange to be 0 psi or higher when pump is operating at
150% of rated flow, which in this case is 600 gpm. Is this realistic? I
know, if the suction pressure is 0 psi or higher for 600 gpm flow, it will
work for all cases. However, I would expect a flow between the supply and
demand with all design sprinklers flowing. Anyone with experience with
pumps, care to comment?
Tony
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Joe Hankins
Sent: November 4, 2008 8:56 AM
To: [email protected]
Subject: Re: Fire Pump Suction
Boiling (cavitation) starts starts when the water pressure at the impeller
is reduced to the vapor pressure of water at the water temperature. You see
this addressed in the pump literature as "NPSH"
The term net positive suction pressure (NPSH) is defined as the difference
between the suction pressure and the vapor pressure of water and must be a
positive number to avoid cavitation. At 68 degrees F water temperature, the
vapor pressure is 0.35 psi. So, as long as the inlet pressure is greater
than 0.35 psi absolute or -14.3 gauge, no cavitation.
In practical terms, so long as the suction pressure is above -12 to -13 psi,
you should be fine. When a typical centrifugal fire pump starts to
cavitate, you'll know it. (sounds like the pump is full of rocks), and
you're not going to damage the pump unless you let it cavitate for an
extended period of time. The problem, of course, is that the pump
performance deteriorates significantly.
Joe
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