NFPA 20 - suction/discharge pipe sizing

[skdesigner]

I am in need of a fire pump history lesson.

In the past two weeks, I have looked at two separate fire pump installations where 2,000gpm pumps had been installed with 8" suction and discharge piping, instead of the minimum 10" piping required by NFPA 20. The first time, I was quick to point it out as an error - but the second time, I started to wonder if it was more than just a coincidence.

Both pumps were installed in the early 80's, meaning that the design would have been governed by an early-to-mid 70's edition of NFPA 20. The oldest edition of 20 I have access to is '87, which lists minimum suction and discharge piping as 10" for a 2,000gpm pump.

Was there ever a time when NFPA 20 permitted 8" piping for 2,000gpm pumps?

Thanks in advance.

 

[chicopee]

Have you done a fire pump test to determine its performance?

 

[LittleInch]

Just go to the NFPA site and buy it or view / download for free.

If you go to NFPA20 you can choose your edition.

http://www.nfpa.org/codes-and-stand...ds/list-of-codes-and-standards/detail?code=20

Well sort of - the 1976 revision shows table 2-16 and 2000 gpm is 10" suction and discharge.

The 1974 edition is the "full" code and also shows it as 10"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[skdesigner]

Would you look at that. What a phenomenal organization NFPA is.

 

[mk3223]

It could be possible that the existing pumps didn't follow the NFPA requirement when implemented in 80's. I saw some old pumps in the facility have been converted to the firewater supply system in the similar way and accepted by AHJ.

 

[skdesigner]

mk3223: Indeed it might be possible. The installations were not only in different cities, but different parts of the country. Both being around the same age, I wondered if perhaps it wasn't just a co-incidence and NFPA 20 requirements had changed over the years.

I thank you all for the replies.

 

[MGXFP]

Just curious but are they listed fire pumps?

I have encountered this same scenario with water main end-suction pumps used as fire pumps.

 

[skdesigner]

Hi NewtonFP - they are absolutely listed, dedicated Fire Pumps. I was even able to unearth the original sprinkler contractor's hand-drawn installation plans from '81 showing a beauty of an isometric view of the pump installation. 8" in, 8" out, just like on site.

We witnessed a test of the first pump - flowed at 2000 gpm for about 3 minutes before the engine got so hot that coolant started boiling out of the tank, so it's safe to say that undersized suction/discharged piping is but one of the issues.

Quite the scene to witness, though.

 

[SprinklerDesigner2]

Of course those are minimum and not mandated sizes.

About three years ago I used a 1,250 gpm fire pump where I had to have a 12" suction in order to maintain a minimum 20 psi residual pressure at the "pump inlet flange" with the pump operating at 150%. This is downstream the backflow assembly. Long suction line, off my memory I think it was 800 feet, and calculations were showing we should have 23 psi @ 150% and during the test we got 24 psi residual.

For the C900 I used a C-Value of 140 which is more realistic given tests that have been conducted. Yeah, you can use the 150 for calculations but to be safe you better use 140 for something like this.

Also, while the pipe was 12" used reducers to install an 8" backflow because it had the lowest friction loss at 1,875 gpm. What I do on something like this I don't shop on price I want whatever provides the lowest pressure loss at 1,875.

Oh, and in Georgia if you don't have the 20 psi @ 150% you fail the acceptance test and how do you fix that, re-bore the four lane highway and install 800' of 12" to replace the 8" installed because "the book" said you could? After you tell the owner this you commit Hari-Kari to atone for the sin of not properly doing your job?