I like this test/solution choices. But today will not be indicative of
future. Remember in theory new pipe performs better than the c-factor used
for new pipe. The changes in the c-factor may become an issue. A few hand
calcs should give you an idea where you are at now and where you will be in
the future. It will tell you WHAT but won't tell you WHEN though.
Read Joe's post, suggests -12 to -13 would be OK as far as capitation so it
seems that and my personal experience NFPA 20 is very conservative.
I'd also add is this really an existing condition? If it was approved by
the AHJ years ago and you discovered this during annual testing it may or
may not be existing. It's only existing if the AHJ (EOR, owner) approved
knowing the pump was bigger than necessary and proactively considered this
negative pressure problem, if it exists. If it was installed a few months
ago and it now acceptance time it may be there but it's not existing.
With what we know at this point I'd not be very inclined to accept as is.
Change the pump or changes the necessary pipe sizes AND run some calcs on
the supply side to see how fast the tank drains with varying locations and
sizes of the calc'ed area. The use judgment.
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
- FPDC
Sent: Friday, November 07, 2008 7:19 AM
To: [email protected]
Subject: RE: Fire Pump Suction
It's nice to see that I don't get all of this stuff to figure out. I
wonder where the EoR was on this project? I also love the term "had
access". Is this a pump he mis-ordered for another job and got stuck with?
If this were my crisis, I would run a test increasing the flow in 100
gpm increments until I got to the -3 suction. That would give me what
I can run the pump to flow wise. Assuming you hit the -3 before the
600 gpm, you essentially have three choices: install the correct
pump, change out the suction line or come up with a way to live with
the existing situation. This would probably include discussions with
the owner, pump rep and the AHJ. Best of luck.
I am working on a hospital with a 250 gpm pump that is trying to feed
3 standpipes. I'll let you know when this pump becomes available (just
kidding)
At 11:58 PM 11/6/2008, you wrote:
>The concern is the size of the suction pipe. The pump selected was 250 gpm,
>to match the 222 gpm, 40 psi demand. But the contractor had access to a 400
>gpm, 70 psi pump which got installed instead. An overkill, but that is what
>happened. The suction piping from the tank to the pump is 4". The engineer
>is pointing to NFPA 20, 2003 section 5.14.3.2 which requires gauge pressure
>at pump suction flange to be no less than -3 psi. Although the flow is not
>mentioned in this particular section, from section 5.14.3.1 it can be
>deduced that it is 150% of the rated flow. Now flowing 600 gpm through the
>4" suction piping and avoiding cavitation is the problem.
>
>Tony
>
>-----Original Message-----
>From: [EMAIL PROTECTED]
>[mailto:[EMAIL PROTECTED] On Behalf Of Chris Cahill
>Sent: November 6, 2008 1:59 PM
>To: [email protected]
>Subject: RE: Fire Pump Suction
>
>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
>
>
>
>_______________________________________________
>Sprinklerforum mailing list
>http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
>For Technical Assistance, send an email to: [EMAIL PROTECTED]
>
>To Unsubscribe, send an email to:[EMAIL PROTECTED]
>(Put the word unsubscribe in the subject field)
>
>_______________________________________________
>Sprinklerforum mailing list
>http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
>For Technical Assistance, send an email to: [EMAIL PROTECTED]
>
>To Unsubscribe, send an email to:[EMAIL PROTECTED]
>(Put the word unsubscribe in the subject field)
>
>_______________________________________________
>Sprinklerforum mailing list
>http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
>For Technical Assistance, send an email to: [EMAIL PROTECTED]
>
>To Unsubscribe, send an email to:[EMAIL PROTECTED]
>(Put the word unsubscribe in the subject field)
>
>_______________________________________________
>Sprinklerforum mailing list
>http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
>For Technical Assistance, send an email to: [EMAIL PROTECTED]
>
>To Unsubscribe, send an email to:[EMAIL PROTECTED]
>(Put the word unsubscribe in the subject field)
Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
_______________________________________________
Sprinklerforum mailing list
http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
For Technical Assistance, send an email to: [EMAIL PROTECTED]
To Unsubscribe, send an email to:[EMAIL PROTECTED]
(Put the word unsubscribe in the subject field)
_______________________________________________
Sprinklerforum mailing list
http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum
For Technical Assistance, send an email to: [EMAIL PROTECTED]
To Unsubscribe, send an email to:[EMAIL PROTECTED]
(Put the word unsubscribe in the subject field)
