Principles of Effective Relays With Large-Diameter Hose
The Volunteers Corner
The use of large-diameter hose to get water to the fireground has made a drastic change in rural fire fighting.
With 4 and 5-inch hose, rural fire departments are getting enough water on the fireground to make aggressive attacks on fires that years ago called for defensive tactics. With the ability to put 600 to 1000 gpm or more on the fireground, rural fire departments consider mounting a blitz attack instead of merely protecting exposures.
Obviously, if a sizable flow of water is to be sent over a long distance, then relay pumping is necessary. This meaqs that a rural department must have enough pumpers available for a relay. To get enough pumpers for a long, large-volume relay, mutual aid is usually needed, and in many areas automatic mutual aid puts pumpers from outside departments on the road with the initial alarm for large structures.
Area agreement: The decision to use large-diameter hose in relays has to be the result of an agreement involving a number of fire departments in a large rural area. If your neighbors prefer tanker shuttles, then they will not have the large-diameter hose needed for your relays and your best bet is to accept their decision and go the tanker shuttle route.
However, if the choice of pumper relays with large-diameter hose is feasible, then all fire departments in the area should work out an agreement on the size hose to use, automatic response of pumpers to alarms at target hazards, and standard operating procedures for relays. Consideration also should be given to conducting mutual aid drills so that all departments in the area know what they are expected to do in developing large-flow relays.
The first users of large-diameter lightweight hose a couple of decades ago used 3V2-inch hose. Then 4-inch hose soon became the standard. However, in the last few years, 5-inch hose has been bought by an increasing number of fire departments. Four-inch hose, for practical purposes, is limited to 1000 gpm at a friction loss of about 20 psi per 100 feet. At 1500 gpm, the friction loss is about 40 psi. Five-inch hose can handle 1000 gpm at about 7 psi friction loss and 1500 gpm at 15 psi friction loss. For 2000 gpm, the friction loss in 5-inch hose is about 25 psi.
Making hose decision: Which size of large-diameter hose you buy should depend on the distances your target hazards are from water sources and how large a fire flow they require. The available space on apparatus to carry the amount of hose needed also enters into the decision on hose size. Cost is also a factor. One way to compare the cost of 4-inch hose with that of 5-inch hose is to base it on the cost per gpm potential instead of cost per foot. Then, all the departments that work together should decide which size is best for use in their area.
Any pumper relay program with largediameter hose should include inline relay valves. Designed on the principle of a fourway hydrant valve, these relay valves allow water to pass through them as though they weren’t in the line when a relay pumper is not available. When a pumper arrives, it hooks up to the relay valve without disturbing the flow of water and with the turning of a valve, the pumper then increases the pressure of the water it now receives (allowing more volume to flow) just as it would if it had been hooked up to the line when it was first laid.
Each pumper in the relay that is receiving water also must use a relay relief valve. This valve can be set to dump water coming to the pump when the pressure exceeds anything from 5 to 10 or 15 psi. This prevents damage to the pump if any pumper closer to the fire suddenly shuts down or nozzles are closed. These suction side relief valves are necessary to prevent rupturing hose.
Operating the relay: Starting a relay is simplified if a standard engine pressure is established for all relay operations in the area. For example, the source pumper and every pumper relaying water might pump at 150 psi. With a flow of 1000 gpm and allowing a 10 psi residual pressure at the next pumper in line, the 140 psi available for friction loss would allow 700 feet between pumpers with 4-inch hose and 2000 feet between pumpers with 5-inch hose. With 1500 gpm flowing, pumpers with 5-inch hose could be 900 feet apart.
This standard engine pressure gets water from the source to the fireground as rapidly as possible because it eliminates both extensive calculations and guesswork. After the relay operation has started to deliver water, then a water officer can go down the line and refine the pumping pressures if necessary. The main thing is to get a steady flow of water to the fireground as quickly as possible.
Your relay plans should include information on apparatus that carry unusually large amounts of hose and the plan should include inserting a relay pumper in the line when necessary.
For a few target hazards, the responsible department can develope a pre-fire plan that includes specific points where relay pumpers should be positioned. Thus, responding pumpers can be given their locations by the dispatcher. This can eliminate confusion in establishing a long relay.
Although large-diameter hose can supply large volumes of water, you have to remember that you can’t apply more water to the fire than you receive on the fireground. However, you can divided up this water as you wish. If the supply is 1000 gpm, you can use it all in one deluge set, or you can use one 600-gpm master stream and two 200-gpm hand lines. Just don’t try to supply more streams than your water supply allows.