SUSTAINED SUPPLY
BY WILLIAM F. ECKMAN
A sustained water supply may be the most important safety aspect of an interior structure fire attack.
Engine companies often depend on the water they bring with them for the initial attack. If the water tanks on the apparatus carry enough water, the fire is still confined to a limited area, and an aggressive attack is made, then the initial supply may be sufficient to gain control–in which case a sustained supply will not be needed for knockdown. On the other hand, an initial offensive attack usually involves entering the structure. When an attack team enters a well-involved room or area, only the water from the attack line keeps the crew from being burned. Unless a continuous water supply is available, crews may be trapped inside the building, overtaken by the fire.
Most engine companies carry only enough water to support a single initial handline attack for three to five minutes of flowing time. Even where pumper tankers are used, the water supply will not be enough to continue an aggressive attack for much longer than 10 minutes. If the critical flow rate exceeds 500 gpm before the first engine company arrives, attack teams should not enter the building until a sustained water supply has been established.
METHODS OF SUPPLY
There are only three ways to provide a sustained water supply:
1. When the residual pressure at the hydrant is sufficient and large-diameter hose is available, a direct supply from the hydrant can be used.
2. Pumpers at the hydrant (or the drafting source) and at intervals in the hoseline can be used to relay the water to the scene of the incident.
3. In suburban and rural areas, mobile water supply apparatus (operating in a water shuttle) can provide a reliable sustained water supply.
DIRECT SUPPLY
A direct line to a fire hydrant (supplied by hydrant pressure) is the quickest and most reliable source of water. The distance to the incident, size of the supply line, and residual pressure at the hydrant limit the maximum flow from a direct supply.
A typical water system in a suburban or rural community will have a static pressure of 50 to 75 psi throughout the distribution network. During periods of peak usage in the system, the residual pressure may drop to 40 or 45 psi, but for planning purposes, calculate available flow based on a peak-period residual pressure rating of 20 psi. With typical hydrant spacing, large-diameter hose will supply 500 gpm or more. When a higher flow is required, LDH is not readily available, or the supply line is more than 500 feet long, a pumper will have to be used at the hydrant to increase the pressure and overcome the higher friction loss in the line.
Using a direct supply line from the hydrant has a number of advantages:
A sustained supply can be provided before the initial supply in the tank is exhausted.
A water supply can be set up before the second engine arrives to connect to the source.
Operation is simpler because only one pump operator is involved.
RELAY OPERATION
Where the distance from the water source to the fire is more than 1,000 feet, a supply pumper should be used at the water source and a relay pumper inserted in the line every 1,000 feet due to the pressure limitations of the LDH and friction loss. The box below shows the number of lines required for a 1,000-gpm relay with pumpers spaced at 1,000-foot intervals.
Two problems, typical of relay operations, limit effectiveness:
1. It usually is not possible to get a relay organized and operating before the initial supply has been exhausted.
2. By the time the relay is established and water is moving, the relay may not be able to supply the needed fire flow.
A relay operation is most effective when large quantities of water are needed over a long period of time and the distance from the water source to the scene of the incident is less than 2,000 feet. If the distance is more than 2,000 feet and the appropriate hoselines are used, only three pumpers will be involved in the relay. Once it has been set up and water is moving, the relay can operate for extended periods with very little adjustment.
WATER SHUTTLE
Mobile water supply apparatus can be used to provide a reliable water supply over distances up to several miles from the water source to the scene of the incident. An attack pumper with a high-capacity tank (1,000 gallons) can serve as a nurse tanker. As each tanker arrives, it can transfer its water to the tank on the attack engine and go for another load of water while the attack continues. If the initial attack engine does not have a high-capacity tank, a portable tank can be set up from which the attack engine or a separate water supply pumper can draft while the tankers go for another load of water.
Using nurse tankers or portable tanks to provide a reserve at the fire scene while other tankers are used to shuttle water from the source make it possible to establish a dependable water supply. High-capacity water shuttles with multiple tanks, multiple fill sites, and a number of tankers can provide a fire flow as high as 2,500 gpm and sustain it indefinitely.
Among the advantages of using a water shuttle for a sustained water supply are the following:
A minimal delay is involved. Tankers should arrive with the initial engine company, and the water shuttle can begin immediately.
Tankers can provide the needed fire flow when the closest water source is several miles from the fire scene.
Due to the number of tankers involved, mechanical failure will reduce only the quantity of water being transported by the affected tanker; it will not interrupt the water supply completely, as would be the case when a relay pumper fails.
A water shuttle has enough flexibility to divert tankers to an alternate fill site if difficulties are experienced at the primary fill site.
Water shuttles are most effective when the distance between the water source and the fire scene is more than 2,000 feet and the needed flow is no more than 500 gpm.
WILLIAM F. ECKMAN spent 25 years in the volunteer fire service and served as a chief officer in Pennsylvania and Maryland. He is a fire protection consultant specializing in department evaluation and master planning with an emphasis on water supply. He has conducted numerous seminars and specialized training programs involving fire pumps and fire department water supply and authored Fire Department Water Supply Handbook (Fire Engineering Books, 1994).
