What is the correct pressure to pump a 1-3/4” handline?

The best answer would be…it depends. The pump pressure required is going to be ENTIRELY dependent on the flow YOU want to deliver. A common error some departments make is they assume just because they change to TFT automatics and 1 3/4″ hose line, that they automatically are now delivering 200 GPM. Not so… UNLESS they are pumping the pressure (at the pump) necessary to deliver that flow. An automatic constant pressure nozzle uses the same flow hydraulics and calculations as any other type of nozzle. The main difference is that we KNOW what the nozzle pressure is going to be. Therefore, we eliminated the one variable in the equation that we did not know before. Using the standard formula of pump pressure (PP) is equal to the nozzle pressure (which we know is going to be approximately 100) plus the total amount of pressure losses due to hose size, length, elevation, device losses, etc. Because we are working with basic “fire ground 2 a.m.” type hydraulics, we can simplify the equation to be:

PP=NP+FL (FL=Hose Friction Losses) (the other losses are of less consequence)

Now it becomes a much simpler question. Let’s say we want to deliver 150 GPM through the 1 3/4″ hose line. Using a slide chart, table, hose manufacturers’ data, or other source for hose losses, we find that a flow of 150 GPM corresponds to a hose loss of about 35 psi PER 100 feet of hose. If the hose line is 150 ft. long, then we would take 1.5 times 35, or 52.5 psi. If it is 200 ft. long, then it is TIMES 2, or 70 psi hose loss.

What’s the nozzle pressure? Of course, we know it is going to be about 100 psi, so we add that to the hose loss to get our required pump pressure…. for 150 GPM…

150 ft. = 150 psi pump pressure
200 ft. = 170 psi pump pressure

NOTE: I typically round all calculated numbers to the nearest 10 PSI. There are so many variations in hose types, and interior finishes that to “split hairs” any further is simply not practical on the fire ground. In a classroom maybe, but when pressure gauges typically flutter and bounce 5-10 psi, what is the point of trying to obtain 152.50 psi?

Finally, what happens when we have the same 150 ft. reconnect and we increase the pump pressure to 200 PSI? Since the nozzle pressure remains constant, we now have 100 psi for hose loss. Divide by 1.5 and we get the loss per 100 feet or 66 psi. Again referring to your hose friction loss chart, this equals a flow rate of 200 GPM.

If you are using a Mid Force Dual pressure nozzle, rather than a single pressure nozzle, the nozzle operating pressure should be substituted. This is the reason that with the same hose length and pump pressure, the Mid Force will deliver more actual GPM because there is about 30 psi (100-70) additional pressure which is used to overcome additional hose losses.