Determine Engine Pressures With One Simple Formula
DEPARTMENTS
The Volunteers Corner
Determining engine pressures for those unusual master stream situations should not be much of a problem for pump operators if they remember one simple formula.
Fortunately it is easy to solve in your head. It is the formula for friction loss in 2 ½-inch lines:
Working a problem: Let’s see how the friction loss formula can help you determine the engine pressure needed for supplying a 1 ½ – inch straight tip (solid bore nozzle) on a deluge set supplied by two 2 ¼-inch lines, each of them 300 feet long.
You know that the standard 80-psi nozzle pressure will result in a flow of about 600 gpm through a 1 ½ -inch tip. However, you don’t remember the friction loss, so you go to square one—the friction loss formula. The Q in the formula is the gpm divided by 100.
Because the pump is supplying the 600 gpm to the deluge set through two 2 ⅛-inch lines of equal length, each line is flowing 300 gpm and the friction loss is that created by the flow of 300 gpm—not 600 gpm.
To determine the friction loss for 300 gpm, we go to the formula cited above and determine that the value of Q for this problem is 300 divided by 100, or 3. In substituting 3 for Q in the formula, first square the 3 to substitute for Q2 so the formula now looks like this:
Finding engine pressure: There are 300 feet of hose in each line, so you now multiply 21 by 3 and get 63 psi for friction loss in the parallel line layout. Remember, when figuring friction losses in parallel lines (no matter how many) of equal length and size, you figure the friction loss for only one line.
To the 63 psi friction loss in the line, add 80 psi for nozzle pressure and 10 psi for friction loss in the deluge set, which gives an engine pressure of 153 psi which you round off to 155 psi.
If the master stream device was considerably higher than the pump, you would add ⅛ psi per foot of elevation—or 5 psi for each floor above the ground if the deluge set were on an upper floor or a fiat roof. The loss created by the elevation of the nozzle above the level of the pump is called back pressure. If the nozzle was considerably lower than the pump, then you would subtract the back pressure.
Back pressure must be included in your determination of the engine pressure when the pump is supplying a stream on an aerial ladder or elevating platform. Friction losses in ladder pipes and turret pipes are considered to be 10 psi.
If your master stream device had a fog tip, then you would allow 100 psi for nozzle pressure.
When feeding aerial devices, you also must add the friction loss in the supply line or pipe up the ladder or the pipe to the platform.
Losses in larger lines: What happens when you want to figure the friction losses for larger lines? You still start at square one— the friction loss formula for 2 ½-inch hose— and determine what the friction loss would be for the gpm in a 2l/2-inch line. For 3-inch hose, multiply the friction loss in 2 ½-inch hose by 0.4. For example, in the problem above, the friction loss for 300 gpm flowing through a 2 ½-inch line was 21. Multiply 21 by 0.4 and you get a friction loss of 8.4 (round it off to 8) for 3-inch hose.
For 3½ -inch hose, divide the 2½ -inch hose friction loss by 6. For 4-inch hose, divide the 2 ½-inch friction loss by 11.
If you wished to flow 500 gpm through 4inch hose, by substituting 5 for Q (Q being the gpm divided by 100), the friction loss in 2 ½-inch hose would be:
Now divide 55 by 11 (the factor for 4-inch hose) and you find that the friction loss for 500 gpm in 4-inch hose is 5 psi.
That can give you an idea why more and more large-diameter hose is being used to supply master stream devices. Metropolitan fire departments are learning what rural departments learned many years ago—that large-diameter hose is the answer to friction problems when large flows are necessary.
Using automatic nozzles: When the master stream device has an automatic nozzle, the pump operator still has to know the friction loss in the line so he can supply the flow desired by the fireground commander. If 600 gpm or 800 gpm is desired, then the pump operator can determine the friction loss by using the friction loss formula as we have explained.
Large-flow automatic nozzles will develop a good looking stream at 400 gpm as well as 800 or 1000 gpm. However, fires are not quenched by good looks. Fires demand a stream of sufficient gallonage to overcome the heat they develop. .Therefore, the pump operator must develop a stream with the gpm sought by the fireground commander.
Like we said, that brings the pump operator back to square one—use of the friction loss formula if he doesn’t remember the friction loss
