There is nothing inherently wrong with a 30 ft/sec velocity in pipe. The
higher the velocity, the higher the friction loss, but if you can get
the needed flow and pressure, that's all that matters. There have been
claims floating around for years that friction loss calculations are
inaccurate when the velocity is high, but that is not true. In the past,
some manufacturers of check valves and water meters put a 32 ft/sec
limit on flows through their devices, but that was to prevent damage.
Factory Mutual had a design velocity limit for small branch line piping
in grids, but that was done in an attempt to prevent silly designs (e.g.
300 ft. long 1 inch branch lines) that saved a few pennies but the
system could not tolerate even one extra head operating on a branch
line. That created so much confusion (people were being told to replace
piping in existing systems that had high velocity because they wouldn't
control a fire!) that the limit was eliminated.
I actually heard someone explain the velocity limit by saying that the
water couldn't "make the turn" to flow out of an open sprinkler and
would just flow past it!
Joe
Mark Pedroia wrote:
From: McCormick, Mark <[EMAIL PROTECTED]>
To: Los Angeles Area Fire Marshals Association <[EMAIL PROTECTED]>
Sent: Tue Apr 01 21:48:50 2008
Subject: [laafma] Pipe Velocity in 3/4" Water Service Lateral
We adopted a sprinkler ordinance in January for SFD’s and now are dealing
with some of the vacant lots in town that have a ¾” copper water service
lateral with a 1” meter.
Our Water Department (regulates from the corporation stop to the customer
side of the meter) has a maximum velocity standard of 15 feet per second.
When you flow 26 gpm through a ¾ inch service lateral the velocity is in the
30 fps range.
I am proposing we stay with NFPA 13D, the guiding design standard for
residential sprinklers in SFD’s. 13D does not seem to have a velocity
requirement and does allow ¾” pipe as long as hydraulically the pressure and
flow is available.
New subdivisions will have a 1.5” lateral so we won’t have the problem in
the future.
Has anyone ran into the velocity issue for residential 13D systems?
If so, were you successful in getting around the hurdle?
We do not believe exceeding the velocity in ¾” copper for a test and
possible fire will be an issue.
Thanks for any information you have.
Mark
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