Pumps, water and most importantly... time!

Today for our drill we decided to answer a question that has been posed so many times... how fast does my tank water run out when I am pumping attack pressure... 150psi???

To set-up our experiment we took out our first line engine which carries 2000 gallons when full and performed this test using the line we pull on the majority of entry fires, the 200' 1.75" crosslay.

The first test we went with the operating pressure which hydraulic calculations tell us: C(2) x Q(2)/100 x L/100 or 15.5 x 1.5(2) x 200/100 ... which comes out to a friction loss of 69.75 in your hose.  So with a tip pressure of 100 psi for the fog nozzle, 150 GPM desired flow and 70 psi of friction loss we set our panel pressure to 170 in hopes of 150GPM.  In theory we would have 13 minutes of flow time with this set-up.  We achieved 8 minutes and 40 seconds.... or about 230GPM with this number... huh...

Take 2... 150 panel pressure... flow should go down right?  wrong... went up to 240 GPM (keep in mind that a pump achieves maximum efficiency at 150psi) time was 8 minutes 10 seconds... of flow before cavitation.

Take 3... 125 panel pressure... we made 10 minutes and 4 seconds before cavitating... or right at 200 GPM.

And Take 4... 105 psi panel pressure, 13 minutes of flow time... and according to the math 2000gallons/13 minutes is right shy of 150GPM...

What this means for now is when we are teaching drivers we are telling them to set pump panel pressure to 150psi so you get 150GPM of flow and roughly 12 to 13 minutes of time with your tank water... this has now been proven wrong.  Due to newer hose and pump efficiency and other things we can now start operating at lower discharge pressure and make the life of our hose crew significantly easier.  Yeah!  We will now be setting our initial attack pressure to 110psi panel pressure and then bump to 150psi panel pressure for an involved fire when we don't pull a 2.5" line.

Go out and try this with your pumpers (especially if you rely on tankers and not hydrants and figure out your own operating pressures)

McC