According to a "wallet card" I have, 1.75" hose with 1.5" couplings is as follows, based on 100' lengths of hose:
175 gpm 8 psi
200 gpm 8 psi
No, you don't whip out the pencil and calculator on the fireground.
You whip them out when you're doing your pre-plans.
You whip them out when you're reviewing your SOPs and want to make sure you have the right combination of hose diameter and pump pressure to feed the nozzle you've put on the end of the preconnect.
You whip them out when you're at drills or doing annual hose testing to verify you're achieving the GPM that math says you should. If you're significantly less, and you've double checked your math, you start looking for maintenance issues whether it's a clogged fog nozzle, hose whose lining has delaminated, or something ****ed up on your pump or guages.
You whip em out to show somebody why the 1.75" line with 100psi fog nozzle ain't a good idea to use off the standpipe.
You whip em out when you've received bids back on fire hose, and need to compare the relative quality of the lines (along with stuff like weight, bending radius, etc).
BTW, go with the manufacturer's friction loss specs all the time.
Most of the "formulas" and "tables" you see in the IFSTA and
other books are based on hose from 40 years ago. They are accurate for rubber lined, double jacketed cotton fire hose or whatever the table says it's for.
Modern fire hose usually has slipperier lining materials like polyurethanes instead of rubber; they may have the liner impregnated into the inner jacket altering the interior size larger than the nominal hose size, and the hose jackets may be more flexible and expand when pressurized to larger than nominal.
Angus Hi-Vol is one well known example -- it's 4" and 5" both gain 2/10ths of an inch when charged, and 2/10ths is a huge difference in this application.
Ponn Conquest is a nice attack line that combines the slipperier polyurethane liner, embeds it as part of the inner liner instead of a seperate layer, and according to some also expands under pressure -- whatever they do, the net effect of everything is their 1.75" performs like most mid-price range 2".
So yep, on the fireground, you can use the system of when the firemen lift off the ground, turn the throttle to the left method.
But there is an awful lot of planning & engineering you should be doing off the fireground that a solid understanding of hydraulics and fire hose construction is very important. Because getting that stuff right helps reduce problems the night of the big one.
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