THE VOLUNTEERS CORNER
Setback Buildings with Long Driveways
SOME of our toughest fires involve buildings situated a couple of hundred feet or more from a road. Once a pumper is driven down a long driveway or county lane within operating range of a fire, it often cannot be moved. It’s there to stay until the fire is out.
There’s an advantage in getting that first engine with a large booster tank up to the burning building quickly because the water carried on it may be all that is needed if it is applied soon enough. With the help of a 350 to 500-gallon booster tank, it is not much of a problem to get water on a fire fast with a couple of 1 ½inch lines. Hut there are the formidable blazes that demand more water than any booster tank holds. The problem is to supply water to keep the pump in continuous operation.
Long driveways, such as are common in suburban and rural areas, are not built for maneuvering fire engines. So we have to prepare for the worst on the way in if we are to make a solid initial attack with the assurance of a plentiful backup supply of water.
With a response of at least two engine companies, it Is a simple matter for the first piece to drop one or two lines at the driveway entrance and stretch to the fire. This is where a divided hose bed is an advantage because then it is no problem to lay out parallel lines down the driveway.
The second engine picks up the lay with double male or double female couplings and stretches to a hydrant, a nearby pond or other water source. While this company is stretching, the first engine is getting water on the fire from its booster tank.
Thus we can buy a few minutes of time until the second pumper is ready to supply water to the first. With a little practice, this evolution can be so smooth that the supply lines are charged before the booster tank runs dry and by the time a 2 ½ -inch line, if needed, is stretched to position.
‘Hie advantage of parallel lines is evident when you think of the limited volume of water that can be put through a 2 1/2-inch line. The maximum flow feasible in a single line is about 325 gpm with a friction loss of 25 psi. This means that the longest practical hose lay is 1,000 feet. This gives us a friction loss of 250 psi. Adding a desirable 20-psi residual pressure for the pump near the fire, this requires the source pumper to operate at 270 psi.
If we limit the flow to 200 gpm, we cut the friction loss to 10 psi. Then we can use 2,500 feet of hose, giving us a total friction loss of 250 psi plus a 20-psi residual for the receiving pumper, again making an engine pressure of 270 psi for the source pumper.
A flow of 250 gpm with a 15-psi friction loss will limit us to supplying 1,600 feet of hose. This gives us a friction loss total of 240 psi plus a 20-psi residual for an engine pressure of 260 psi.
The advantage of parallel supply lines is that the friction loss, residual pressure and engine pressure figures remain the same when the total amount of water delivered to the pumper at the fire is doubled. Naturally the capacity of the second pump is a governing factor which must be taken into account.
Figuring an average of 250 gpm for each 2 1/2-inch parallel line, the pump at the fire will be receiving 500 gpm. This means you can use 1 ⅛-inch tips on two 2 ½-inch lines on the fire. Or you can use 1-inch tips on two 2 ½-inch lines (400 gpm) and two ½-inch tips on two 1 ½inch lines (100 gpm).
Why do we keep the maximum engine pressure down to 270 psi? Well, most hose is tested annually at 250 psi and it doesn’t make sense to go much above the pressure you know old hose has taken unless you absolutely have to.
By making the stretch from the water source to the fire in two sections, we have made the initial attack in the quickest possible time. We have avoided tangling up engines trying to maneuver in long, narrow driveways; we have stretched up to 2,500 feet of hose quickly; we have supplied a respectable amount of water for a suburban or rural fire; and we have bought valuable time to enable other engine companies to set up and operate if necessary.