THE VOLUNTEERS CORNER
PROBABLY the most unsatisfactory fire-ground evolution is the setting up of a pumper relay. It requires a good deal of time and work—and the results are often unsatisfactory.
First of all, we shall assume that a 2 1/2-inch line will be fed because few departments have a large number of feet of bigger hose. Secondly, we shall limit the volume of water pumped to 200 gpm. We arrive at this figure because of the fact that rural engines usually carry large amounts of hose and an engine generally dumps its entire load in a relay.
This means that the distances between engines run from 1500 to as much as 2000 feet. At 200 gpm, friction loss alone will run from 150 to 200 psi. We like to have a residual pressure of 20 psi at each pumper. This means required engine pressures of 170 to 220 psi.
Of course, an engine can pump at 300 psi. But what are we doing? Hose, according to the old National Board standards, should be tested at 250 psi. If we pump at a pressure greater than that, we are putting the first few lengths of hose through a pressure test greater than the annual test.
The starting pressure for each engine is often the stumbling block to a quick relay operation. It shouldn’t be. Either within a department or within a mutual aid group, an initial engine pressure should be selected long in advance of any relay operation. A 200-psi engine pressure will often be acceptable (up to 1800-foot lays).
However, this figure will depend on the maximum amount of hose expected to be laid between pumpers and the assumption that 200 gpm will be delivered at the fire. This means that if 1800 feet is the maximum hose lay, then 180 psi will be used to overcome friction, leaving 20-psi pressure at the receiving pump.
If a department experiences shorter distances between pumpers or if it is satisfied with the delivery of less water, then the initial pump pressure can be reduced. For instance, if the maximum hose load in a department or mutual aid group is 1600 feet, then the initial pump pressure can be reduced to 180 psi for 200 gpm. This will still leave a 20-psi residual pressure.
Now the secret of rapid delivery of the first water is that every pumper in the layout except the one at the fire uses the standard initial engine pressure. The pump at the fire supplies the proper pressure for the hose and nozzle it feeds.
After water is being delivered at the fire, then an officer can check individual pump pressures and refine them so that no pump is working harder than necessary. This step, however, need not be undertaken at once. If there is only 1200 feet between a couple of pumpers under the 200-psi system, it means only that the residual will be about 80 psi. This has the same effect as pumping from a hydrant with that residual pressure. Most pump operators would be delighted to have that incoming pressure. The only reason for cutting it is to even out the work load among the engines.
Inasmuch as relay stretches are usually from fire to water source, it sometimes happens that the source engine may have only 500 or 600 feet of hose going to the next pumper. Would the source pump start at 200 psi? This is where we would expect the operator to use his head and cut the initial pressure. The other pumps in the line would not be affected by this change. Even if the source pump operator knew little about hydraulics, he could easily see that he had stretched less than half his hose load. As a matter of logic, if he cut the 200-psi pressure in half, he would be on the right track.
As hoses are attached to pump suctions, a hose gate should be opened. And if the suction is gated, it also should be opened. This will prevent the suction gate from being jammed by the water pressure, and the opening of a hose gate on the pressure side will provide a vent for any possible excess pressure in the pump. If necessary, water can be dumped out the pump through an open hose gate to relieve pressure that might lock the gate to which hose in the next pumper in line has been belatedly attached.
The open gate also provides a good vent for the air being pushed out of the line by oncoming water. If you do not use an open gate, but open a gate charging the line to the next pumper, try opening the bleeder for this gate until the air is out of the line. In this way, less air will have to course through the entire length of the relay.
It there is any possibility of delay in attaching your incoming line, slap a hose clamp on it at once. Then you won’t be embarrassed by a coupling gushing water.
The source pumper should start pumping as soon as possible, and all others follow. Each engine should be shifted into pump gear before the water arrives so that all the operator has to do is to increase the engine rpm for relay pressure.
Once the water reaches the engine at the fire, it can be employed in the same manner as it would if the engine were at a hydrant. Remember that this engine cannot pump more water than it receives.
However, the standard 200 gpm can be used for one 2 1/2-inch line with a 1-inch tip, or it can be divided into several 1 1/2-inch lines, or a combination of a smaller tip on a 2 1/2-inch line with one or two 1 1/2-inch lines with 50-gpm fog nozzles.