Just to play devils advocate where does the code say you need to consider
anything else but the hypothetical .3/2000 for 600 gpm? The code also says
you have to include the hose demands.
And if you do this exercise for less than the total heads what does the area
look like? Do you start with one head and add heads on the BL until you
reach the number prescribed by the 2000 sq.ft. then add from the next line?
If you try .3/1500 the heads per line would be different. The full heads on
the BL would be the most demanding pressure wise usually, but what about the
same number of heads across several lines. What about rings starting with
1, 4, 9 then 16?
You get into an almost infinite number of permutations all falling on
different points on the curve with an infinite number of hose steam
possibilities. And which curve anyway, the 500 year drought and theoretical
peak demands assuming the sustained growth of the city for said 500 years.
Infinite defined by the realistic time I have to run all these calcs. There
is no reason a curve can't have a step up in it at the gpm the pumps kick
on. In theory your LH demand could have a lower pressure available than a
higher demand. I don't see a problem.
This is definitely an interesting discussion but may be academic. Consider
the failure rates of sprinklers and the causes. Also consider I suggest 95%
of flow tests are done wrong and not on the conservative side. And yet we
don't have a lot of failures from this. If the valve is open and no one
shut it early the failure rates get really, really small (true not 0). Take
out the wrong design for the hazard and we're even closer to 0. Also
consider for many operations nowhere near the designed number goes off.
Further consider the local FD supplement. Out in the middle of nowhere it
may be more important. Really bad sprinklers designs and installations put
out a lot of fire. This paragraph is not an excuse to design bad systems
but a suggestion there is a point of diminishing returns as we tweak the
code and our engineering capabilities. Show me attics burning with the <99'
code and I can go along with changing the attic requirements, but I Duross.
But if I'm the one I don't care about the statistics when I call my
insurance company because one of my protected building burned down.
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 Thom McMahon
Sent: Saturday, November 29, 2008 3:01 PM
To: [email protected]
Subject: RE: variable public water supply
It sounds like more study for the actual system demand is required,
otherwise your just guessing at the 15 vs. 20 flowing. You'll need to verify
all start stop settings with the civil engineer for the water purveyor, and
then look at the system demand, from the flow recorders. What time will the
system supply/demand be the greatest? This is when your pump will kick in
fastest. When is the supply/demand the lowest? This is when the pump will
take longest to kick in, possibly even over the 660 Gpm your system demands
if when you did your flow test the domestic demand on the system exceeded
160 Gpm, and your lowest demand is less than 10 Gpm, then it's possible your
system will never kick the pump in.
Thom McMahon, SET
Firetech, Inc.
2560 Copper Ridge Dr
P.O. Box 882136
Steamboat Springs, CO 80488
Tel: 970-879-7952
Fax: 970-879-7926
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Todd Williams
Sent: Saturday, November 29, 2008 1:46 PM
To: [email protected]
Subject: RE: variable public water supply
In this case, the new pump is variable speed and is designed to maintain a
constant pressure. At the hydrant we were reading the pressures on, it
kicked in at 45 psi and boosted it to maintain 65 psi out to the capacity of
the pump.
My thought is that ignoring the pump at sprinkler flows up to where the pump
kicked in (approx 500 gpm in this case) is correct. If the system demand was
over the 500 gpm, what would be the effectiveness of the sprinkler system at
the flow goes from 0 to whatever the demand is when the curve is not 1.85
linear? From the time of first sprinkler opening, up to the 500 gpm, we will
be operating on a different flow curve than what we will over the 500 gpm.
Our designs are usually based on a maximum flow requirement only and assumes
the linearity of the flow curve. Say you have a demand of .30/2000 (ignore
hose streams in this case), or roughly 660 gpm. The sprinklers require 33gpm
each. The design says that we have to calculate 20 sprinklers flowing.
Should the design for the first 15 sprinklers (.30/1500) be based on the
lesser curve and the .30/2000 be based on the curve with the pump? Two sets
of calcs? How do you justify this to the septic system engineer that speced
out the sprinkler system?
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