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?
At 02:53 PM 11/29/2008, you wrote:
I think that it's important to work with the water provider and establish
the flow, and response time for the pumps. Unlike traditional NFPA 20 fire
pumps which will only start after the jockey pump supply capacity is
exceeded, this system may in fact be a staged system.(Example: A small
100gpm pump for domestic demand, and a larger 1500 or more for fire flow.)
We have a local system with 2 -50 Gpm pumps, 2-100 Gpm pumps, 2-250 Gpm
pumps, and 2-1500 Gpm pumps all set up leed/lag. So at night maybe only one
50Gpm pump is running, but during the day anything up to 500 Gpm may be on
based upon demand, and when we flow a hyd. The first and then second fire
flow pumps kick in. Usually in less than 3 Minutes. So what's the flow test
done at? Well without the pumps, there is maybe a 14 or 15 Psi flow
available at some locations.(Not usually where we're working.) The entire
town is "PUMP" dependant, so we usually do our flow test with only one fire
flow pump on line, because everything up to 500 from the other pumps may
already be required for system supply. (I should mention that the 50 and 100
Gpm pumps kick off as a larger pump kicks on, but the 2-250 pumps run with
the fire flow pumps.)All pumps have a 85Psi start, 105 Psi off, with a
minimum run timer to prevent short cycle, and an emergency generator is
available to run the full load demand.
But back to my original thought, Ignoring a pump is not always the more
responsible thing to do, especially when doing so means you're going to
install yet another fire pump, and possibly a holding tank. Which do you
think will be most reliably maintained? The water system pump where any
failure gets immediate attention from angry consumers or your new fire pump
out in the pump house, out of sight and out of mind?
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: Friday, November 28, 2008 10:21 AM
To: [email protected]
Subject: RE: variable public water supply
The system is a combination of gravity tanks and pumps. This particular
location is close to the highest point in town and pumps are required to
much more that flush a low-flow toilet.
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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