Our department is discussing lowering our pump pressure on our transverse beds. We are currently using 360 liter per minute at 700 Kpa nozzles, with 200 ft of 1 1/2 with a preset of 1040 kpa ( Nfpa). The debate we are having is with new combat hose lines there is no friction loss or little loss on the hose and none on appliances. So we are debating lower our pump pressure preset to 700 Kpa.
Start talking PSI and I will get back to you ;)
Before y'all ask, I did some quick calculations, and got this...
Originally Posted by Crompton
360lpm = 94gpm
700Kpa = 100psi
1040Kpa = 150psi
(All references above and below are US gallons, not Imperial)
Alright, it sounds like you're currently got this: 1040Kpa (150psi) pump discharge pressure, through standard 1.5" line, which we'll assume for the sake of this discussion is 200' (60m) long. Based on 24psi (165Kpa) per 100' section, that's 48psi (330Kpa) FL for the line. That would give you roughly 100psi (710Kpa) at the nozzle. Sounds good so far.
Now, keep in mind at ALL hose has FL, even though it might be insignificant at times. There are a lot of synthetic-style hoses out there, and one that we use is Ponn Conquest. One interesting thing about Conquest in that a flow as low as 360Kpa (100gpm), there is little advertised difference in FL. That leads me to another question: why are your flows so low? Most of the time, the lowest flow that I see is 473lpm (125gpm), but 568lpm (150gpm) for interior fire attack is more common.
Theoretically, if you went with Ponn Conquest hose, and left all other variables the same, and you were to reduce your PDP to 700Kpa (100psi), you'd only end up with 370kpa (53psi) at the nozzle, which is very dangerous unless you're using a smoothbore nozzle.
If I had to make a suggestion, it would be to go with low-friction-loss 1.75" line with a target of 568lpm (150gpm) at the nozzle. Imagine only 110kpa (16psi) of loss per 30m (100') of hose. Do the math, and you're looking at 220kpa (32psi) of FL through a 200' line. That's more water at a lower PDP. In fact, on a 700kpa (100psi) nozzle, you'd only need a PDP of 920kpa (133psi). Sounds like a win/win to me.
Hopefully all of this makes sense, let us know your thoughts.
Any time ANYONE makes the statement that there is little or NO friction loss in a hose line it should raise a great big red flag. This is particularly true of cross-lays where the final fitting is a Chicksan Swivel. I would do some testing with the new hose and a solid stream nozzle. (3/4" or smaller for 1 1/2" hose) If you have access to a pitot gauge, then connect the 3/4" nozzle (even if you need to adapt a 2 1/2" by using a double female 1 1/2 and an increaser) and supply it by slowly increasing the engine pressure until the pitot reads 35 psi. (238 K Pa) This will be flowing 100 gpm (384 lpm). Subtract the 35 psi from the engine pressure to find the true friction loss in the line and piping.
Wait a minute. 94gpm? What are you doing with that, watering the lawn?