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The absolute, utter worthlessness of newspaper numbers

September 9, 2005

Being just generally interested in machinery, I was thrilled to see an illustration in the Chicago Tribune about the huge pumps in New Orleans.  Hoping to understand them better, I poured a cup of tea and sat down to disappointment…

The first illustration is of a flooded pumping station.  Looks like it has three outputs and the caption says, “Each station can pump about 29 billion gallons per day.”  OK, that’s about 9.67bn gallons per pump per day, which works out to 6.72m gallons per pump, per minute.  I’m not buyin’ it.

The next illustration shows a pump impeller about three feet across.  To move that much water through a 3-foot tube, the impeller would have to turn at a speed far beyond where cavitation would occur and efficiency would drop.  Designers of submarines and ocean-going ships would be beating a path to the pump manufacturer’s door. 

OK, so how much can the pumps really move?  (I wanted to get some idea of the horsepower involved.)  The second illustration says each pump moves up to .25m gallons of water per minute.  That’s 1/26th of the amount in the first caption, but still a hell of a lot through a 3-foot tube – about 80 feet per second or 54 miles per hour.  Minumum horsepower (just lifting the water the 30 feet in the illustration and ignoring turbulence, friction, etc.) is roughly 1,800.  There are electric motors in that range and more, but by this time you wonder if the writer has any clue.

Something is not right here.  Not only do the illustrations contradict each other, moving water that fast gets wasteful of energy.  Did the paper lose track of the differences among the capacity of one pump, all the pumps in a single pump house, and the combined pump houses in the city?

Hey Chicago Tribune, why even print numbers if they just don’t add up? Why not just say; “The pumps are really big!  See the big pretty pumps?  Can you say, ‘pump?’”

One problem the pumps now face is the amount of debris in the water, including, well, human remains.  I wonder if a steam-condensation PDX pump (which has no moving parts) could be adapted to the purpose?

Notes:


  • These are literally back-of-envelope calculations.  If I made a mistake of more than ten percent, let me know and I’ll correct.

  • The title of this post was originally “…newspaper diagrams” but discussing it over breakfast this morning with MrsDoF I realized there was nothing wrong with the illustration itself.  The artist did an admirable job showing how the pump worked as simply as possible.  The problem lies in the numbers themselves – hence the name change.
  • Maybe Cajun will stop by and help us out.  He knows all about high-powered equipment and he lives in Louisiana.
Categories: News
  1. September 10, 2005 at 16:15 | #1

    Here’s a link to a Corps of Engineers history of New Orleans drainage:
    National Register Evaluation of New Orleans Drainage System, Orleans Parish, Louisiana

    According to that document, in 1992, the total system capacity (all the pumping stations in the city on line) was 47,000 cubic feet per second.  That makes that 29 billion gallons per day a pretty accurate number.

    Some of those pumps are 12 FEET in diameter.  Here’s another link:

    No. 1 Pumping Station, New Orleans, La., June 11 1974

    These pumps did 247,500 gallons per minute using 600 (!) horsepower.  That’s 356 MILLION gallons a day…

  2. September 10, 2005 at 18:36 | #2

    THANKS, Cajun!  Folks, that first link is an entire downloadable book covering all aspects of the subject – several lunch hours worth.  All the chapters are in my laptop now.

    The second link is a 10-page white paper on early NOLA, Baldwin Wood, the pump he invented, and the No. 1 pumping station.  As my kids can tell you, I eat this kind of stuff up. 

    From early perusal, it seems my initial assessment of the Chicago Tribune’s illustration was correct – they didn’t correctly show the scale or relationships of the pumps.  Still more evidence that “journalism” as a college major prepares you to entertain but not necessarily to inform.

  3. homebru
    September 12, 2005 at 08:55 | #3

    Pump geeks.  Who knew that there were pump geeks?

  4. September 12, 2005 at 13:11 | #4

    Homebru—

    Pump geek?

    man, I’d like to have a certificate attesting to that fact?

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