Again I was playing devil's advocate to an extent...when in the discussion
did you say additional sprinklers would open? I guess that calls for what
you mean by additional sprinklers. The additional because at 2/3 of the
heads you are not putting out .3 and only when you get to 3/4 the pump kicks
on and all is well. 2/3 - 3/4 is not a critical addition of sprinklers
IMHO. If you mean of the full design area plus additional sprinkler or
lacking density that is a problem. But if that were the case you wouldn't
have posted in the first place.
Yes, I think you might explore the critical transition points for interest
but in the end what are you going to do about it? OK maybe changing a few
pieces of pipe size is a good idea but what if a fire pump is the only real
solution for this middle ground. Or what about changing most of the pipe
sizes and the costs.
If I understand correctly you work as an independent designer. Changing
things like this could cause real pain to your customers and their
customers. There is nothing wrong with causing pain, just make sure it's
worth it because you believe you have a real problem not an academic one.
Remember to do your exploration in supply calcs not demand calcs. And doing
so how are you going to factor in the over density with only a few heads
flowing. Does this negate the under density later on? Can you predict how
long the under density lasts? Isn't that critical? Most testing I
understand is done with exact densities at each head not reflective of the
real world and over pressure and GPM along the system.
I've never heard of testing or discussions in the area you are concerned
with. I submit the things I don't know or haven't heard of could fill a
book or two or three. I also submit the 100 years of sprinklers and this
isn't the only time it's been crossed, most likely unknown to the designer
or installer.
Seriously, I don't think you have a problem if you are designed to the full
area at the right density with hoses. Think about this, when is the last
time anyone took the GPM of a design (including hoses) and subtracted that
from the city tower and modified the flow curve and designed to it to
account for the drop in available elevation pressure. Doesn't 13 say the
supply should meet the demand for the full time. We do this when an onsite
tank is the supply in the way of not counting the head of the full tank but
only the pump rating as the design pressure. Or what about a jockey and
fire pump settings that get you in the same bind. Hypothetical the jockey
is probably oversized and runs the first head fine but the second or third
are 80% of the required density until the fourth starts because the FP is
set to come on too low.
Flow testing and the coordination between water systems and sprinklers are
the biggest disaster we have in the codes. This is the root of this
discussion.
Basically I do agree with you but I don't think there is enough information
to really come up with an answer. I guess this sounds like a MS or PHD
exercise. I played the other side for the sake of discussion and to help me
think through the problem for when I face it.
For what it's worth I did an exercise like this last year but it had to do
with a respected PE's design that called for a deluge system to create a
rated wall. What I was worried about was the deluge running with the extra
flow from overhead wet system when the fire wasn't in the remote area.
Basically a supply calc with the deluge running and the overhead close to
the riser. Turned out it wasn't a problem but until I ran 10 calcs or so I
wasn't convinced.
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
Sent: Monday, December 01, 2008 10:58 AM
To: [email protected]
Subject: RE: variable public water supply
This goes beyond the Code and looks at due diligence. If an initial
water supply could create a situation where the available water could
not provide the minimum flow and additional sprinklers open up
because of it, is this a problem that could make the sprinkler system
inadequate? I would think it should be addressed. As far as the shape
of the design area, I would follow the 1.2 rule used for the full area.
At 11:14 AM 12/1/2008, you wrote:
>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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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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