Know Pumper’s Capabilities With Mounted Deluge Gun
DEPARTMENTS
Dick Sylvia’s Volunteers Corner
The increasingly popular deluge gun mounted atop a pumper adds a blitz attack with a master stream to the options of the first-in engine company. To make this quick application of a large rate of flow of water practical, however, the engine company members must be familiar with the capabilities of their apparatus.
Before mounting a deluge gun on a pumper, a test of the rate of flow from the booster tank to the pump should be made. All you have to do is flow tank water through one or two straight tips (depending on their diameter) and measure the nozzle pressure with a Pitot gage to determine the gpm flowing For this test, the pump operator should power the pump just short of cavitation so that the maximum flow available from the booster tank is used by the pump.
Now you know the rate of flow limitation of your booster tank and the plumbing to the pump. NFPA Standard 1901 requires that the flow rate from tank to pump be at least 250 gpm for 300 to 750-gallon tanks and 500 gpm for tanks larger than 750 gallons. To avoid disappointment, test your pumper before mounting a deluge gun on it.
Learn limitations: If a gun is already on your apparatus, make the maximum tankto-pump test, using the gun with an appropriate size tip. You will then determine the limitations of your pumper when using tank water. There is, of course, no flow problem when taking water from a hydrant or from draft other than the usual problems associated with such operations. What we are concerned about here is the quick application of a master stream with tank water for a rapid knockdown of those fires that are serious but within the darkening capability of a reasonable master stream — say 500 gpm or slightly more.
The size of the piping from the booster tank to the pump, the size of the tank drop valve and even the baffling in the tank can all affect the rate of flow of water from the tank to the pump. It is possible to install a second pipe from tank to pump or to replace existing piping with larger piping to increase tank flows.
One of the common misconceptions among pump operators is that because the deluge gun has two or three inlets, it is necessary to run two hose lines from the pump to the gun to get a large flow of water The accompanying table on friction losses should clear up this misconception.
Friction Losses In 10 Feet of Hose
Friction loss explained: An 8 to 10-foot length of hose is plenty long enough to service a deluge gun mounted on almost any pumper. What you have to remember is that, for example, although a flow of 500 gpm creates a friction loss of 55 psi in 2½-inch hose, no more than 10 feet of hose is needed, so the friction loss is 1/10 of the 55 psi for 100 feet of hose, or 5.5 psi. At 1000 gpm, the friction loss for 10 feet of 3-inch hose is only 7.7 psi.
What this means is that you really need to connect only one short length of hose to the gun before putting it into operation. If the blitz is unsuccessful and the master stream is to be continued, then supply lines can be laid to your pumper by another engine or another engine can [jump directly to your deluge gun
Water supply SOP: It should be a standard operating procedure in your department for a second engine to get supply lines to any pumper that makes a blitz attack with tank water. In rural areas, a large tanker can come alongside the operating pumper and provide water to extend the initial application With a 500-gpm master stream operating, a 3000-gallon tanker can permit six more minutes of operation and a 4000-gallon tanker, eight more minutes
When you mount a portable deluge gun on your apparatus, make certain that it is adequately sec ured The table on nozzle reaction shows the nozzle reaction in pounds that will be created at various nozzle discharge rates For fog nozzles, the nozzle reaction is about halt the gpm. Essentially, it is the same reaction problem encountered in ladder pipe streams Instead of the force affecting the stability of the ladder, it affects the security of the mounting of the portable deluge set.
Nozzle Reaction (R = 1.57d2P)
In a classical blitz attack, at least 250 gpm — and usually at least 500 gpm when using a master stream — should darken down the fire within 30 to 60 seconds. If you can position your pumper to permit a good shot at the fire and if fire conditions are suitable, the blitz attack with the mounted deluge gun can solve your major problem — knocking down the fire
However, you must not only be able to size up the fire properly, but also must know the capability of your equipment if you are to be successful.
