Main Drain Flows as a % of static PSI

[ContractorDave]

When performing the main drain flow test, at what percentage of the static psi should a low residual start to raise red flags or is this being too simplistic? I realize you don't want to bottom out but you'll get different results with different supply and drain pipe arrangements given similar static pressures. I've poured over my 13 and 25 again today trying to find a reference I recall from somewhere about how the main drain residual flows should be within a specific range. If it's too low you might have a design problem. If it's too high you may have to up-size your main drain to get adequate flows. Thoughts on both calculated and scheduled systems would be helpful.

Regards
Dave

 

[Cidona]

My understanding is that the pressure reading of the main drain flow are only to be used to compare to the previous main drain results and the original results to prove there has not been a deteriation of the supply since the original installation.

 

[LCREP]

See NFPA 25, 08 edition, section 13.2.5.2

13.2.5* Main Drain Test. A main drain test shall be conducted annually at each water-based fire protection system riser to determine whether there has been a change in the condition of the water supply piping and control valves. (See also 13.3.3.4.)

13.2.5.1 Systems where the sole water supply is through a backflow preventer and/or pressure reducing valves, the main drain test of at least one system downstream of the device shall be conducted on a quarterly basis.

13.2.5.2 When there is a 10 percent reduction in full flow pressure when compared to the original acceptance test or previously performed tests, the cause of the reduction shall be identified and corrected if necessary.


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Fire Sprinklers Save Firefighters’ Lives Too!

 

[stookeyfpe]

I've never considered a main drain test as a prediction of the water supply. The purpose of a main drain test is to confirm that the connection between the sprinkler riser, the underground fire protection water main and the water supply is completely open. That's all a main drain test will do.

To evaluate a water supply requires three variables: Static P, Residual P and the flow (Q). I've had engineers tell me the static P is 90 PSI and so everything should be good. I generally yell at the phone after I hang up.

If I'm missing something or you guys have some really cool trick that Bournelli or Hazen-Williams missed please let me know.

 

[Cidona]

Probably should have clarified my phrasing but I think we're about saying the same thing. I was not insinuating that a satisfactory main drain test is to be analysised as a complete picture of the water supply rather that it was to be used to compare to previuos years results to detemine if there is an obstruction or such (partly closed valve, rock in the pipework...) in the water supply to the system that was not there when the previuos test was performed. As LCREP quoated from NFPA 25 13.2.5.2, a 10% reduction is given as a benchmark to start investigating what is causing that reduction.

 

[LCREP]

The main drain test is an indication of"a water supply", it may be adequate it may not be adequate it all depends on a lot of things. Do you recall this post for a few weeks ago?

http://www.eng-tips.com/viewthread.cfm?qid=235867&page=1

The 1994 flow test was 73 static / 68 residual @ 839 gpm
the 2009 flow test was 44/28/731

Main Drain Test Results

1997 - 54/45
1998 - 47/42
1999 - 49/40
2000 - 55/40
2001 - 47/40
2002 - 52/42
2003 - 45/40
2004 - 45/40
2005 - 50/40
2006 - 55/40
2007 - 50/40
2008 - 55/40

We never found out if the sprinkler system had a hydraulic calculation placard posted. BUT if it did and let just say it said the demand was 65 psi @ 750 gpm.

Do you think the main drain test results from above indicate we may have a problem with the water supply?? How much water does a main drain test flow? Well it depends on a lot size of main drain piping, water pressure, size of water main, fire pump etc. etc. BUT based on some testing we have done with an ultra sonic flow meter and varying water supplies a main drain can flow between 150-400gpm.

So with the above demand posted, water flow tests and main drain test results would you investigate this as a potential problem??

Just another good reason for the contractor to post a data placard on teh riser!

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[ContractorDave]

But, as must often be the case, when doing first time inspections on systems that have no history or data card, or were scheduled, you don't have the luxury to compare anything. When doing these I typically note that the test results are now to be considered base line for future inspections. One might suggest that this is ALL you can do, but there is inevitably going to arise situations where your flows vs static are going to be cause for concern, and then it would be incumbent upon you to ... well, foremost to note the concern I suppose. And of course personal experience will play a part in how you will interpret any data. What I was asking was if there was a reference in NFPA to percentage of static that residual flows might / should be cause for said concern. Perhaps I mis-read the 13.2.5.2 LCREP was so kind as to reference.

 

[ContractorDave]

But, as must often be the case, when doing first time inspections on systems that have no history or data card, or were scheduled, you don't have the luxury to compare anything. When doing these I typically note that the test results are now to be considered base line for future inspections. One might suggest that this is ALL you can do, but there is inevitably going to arise situations where your flows vs static are going to be cause for concern, and then it would be incumbent upon you to ... well, foremost to note the concern I suppose. And of course personal experience will play a part in how you will interpret any data. What I was asking was if there was a reference in NFPA to percentage of static that residual flows might / should be cause for said concern. Perhaps I mis-read the 13.2.5.2 LCREP was so kind as to reference.

 

[ContractorDave]

Sorry for the double (DOH! Triple) Post
eng.tips submission errors