By James Burnham
When a fire scene is outside of a district equipped with a water supply system and hydrants, the delivery of the needed fire flow to control and extinguish the fire is a challenge that must be addressed by the incident commander.
There may be a nearby suction source; perhaps a supply line to a distant source is needed; or, if the distance is too great, water may need to be shuttled to the scene in mobile apparatus.
Do you find that you are struggling to maintain the necessary water flow on your fires? Are you running out of water at the most inopportune times? Most rural departments also face these challenges with only minimal staffing in the early stages until additional help arrives. Does the task of setting up your water supply rob you of personnel who should be conducting fire suppression?
Several items need to be taken into consideration when deciding how to supply water:
- 1. What fire flow (gallons per minute) is needed and for how long?
- 2. Where is the nearest viable water source?
- 3. What water delivery method should be used?
- 4. What resources are needed to deliver the needed flow rate?
Determining Needed Flow Rate
National Fire Protection Association (NFPA) 1142, Standard on Water Supplies for Suburban and Rural Firefighting, provides some good guidelines on the amount of water needed and the flow rates. This standard provides a methodology for calculating these numbers. Is the fireground the place to be doing these calculations? Of course not. Do the calculations for some representative structures in your response district and apply these numbers to similar groups of buildings. NFPA 1142 appears complicated on the surface, but if you do a few sample calculations, you will find that most of your buildings fit into a small number of categories.
Basically, most dwellings fall into Occupancy Hazard 7 and Construction Classification Number 1. Do one calculation for single-story ranches, another for 2½-story wood frame dwellings, etc., depending on the types of construction you encounter in your response district. You will find that for average-sized single-family dwellings, the required flow rate is 250 gpm and the total water supply needed is around 2,500 gallons. For larger dwellings, the flow rate will generally be in the 500-gpm range and the total water required is 10,000 gallons or less. These types of structures represent the bread-and-butter operations of most rural fire departments. If you are one of the unlucky, you will have commercial buildings or business districts where the buildings are larger and fire exposures exist to neighboring buildings. In these isolated cases, your estimated flow requirements may be 1,000 gpm or greater.
Another means of estimating required fire flow is to examine past incidents where your department has had successful saves (discount anywhere there was no hope of saving the structure on arrival). What size and how many lines were you flowing? Two 1¾-inch lines (150 gpm each)? One 1¾- and one 2½-inch line (400 gpm total)? If you look at your past experience, you likely will find that the NFPA calculations are reasonably accurate.
Determining Water Delivery Method and Resources
A direct supply line from a nearby source is always the most efficient method, particularly for the higher flow rates. Know the hose loads of your apparatus and any incoming mutual-aid apparatus. Four-inch hose can effectively deliver 1,000 gpm through a 1,000-foot hoselay or 500 gpm through a 4,000-foot hoselay using a single pumper at the source. Since this supply will take some time to set up, initial attack will be done with tank water. The sooner you can transition to a direct supply line, the better.
If there are no nearby sources, a water shuttle will likely be used. Using a portable tank, a single 2,000-gallon-capacity apparatus can maintain a flow rate of about 200 gpm over a shuttle distance of one mile (one way) with quick fill and quick dump. Each additional mile reduces this flow rate by 50 gpm.
For shuttling operations based on the above 2,000-gallon capacity apparatus, a single piece of apparatus is estimated to deliver the following flow rates/distances:
- One mile or less one way: 200 gpm.
- One to two miles one way: 150 gpm.
- Two to three miles one way: 100 gpm.
- Three miles or more: less than 100 gpm.
So if your needed flow is 500 gpm and your water source is one mile away, you will need three apparatus in the shuttle. Larger apparatus tanks will increase the flow rate. Any instance where the water is pumped off the apparatus may reduce the flow rate 50 percent.
For the structures the size of typical single-family residences, fire flow rates of 250 to 500 gpm are needed. The total gallons of water needed are in the 2,000- to 10,000-gallon range. Look at the number of gallons that you are arriving with and how long it may last. When can you expect the second-due tanker/tender to arrive? You may find that you can maintain the required flow rate for the required time without ever setting up an actual water shuttle.
Also look at the staffing you can expect at the time of the initial response. Setting up a water shuttle operation requires personnel to set up portable tanks, suction, and operate apparatus. Do you have the personnel to do this, or should you be focusing their efforts on putting the water that you carry on the fire?
Set up training evolutions that focus on maintaining a continuous supply of 250 gpm or 500 gpm rather than trying to achieve a 1,000-gpm supply for your bread-and-butter operations. For single family residences, more property will be saved with a 250-gpm continuous supply than a 1,000-gpm supply after a delay of 20 minutes with no water.
Train also on the skills necessary for delivering the 1,000 gpm for when it is needed, but do not attempt to set up this water delivery system for every fire.
Establish some department guidelines to assist officers with estimating the required fire flow. Use photos to demonstrate examples of smaller vs. larger single-family homes and the flow requirements. Similarly, use examples of where exposures exist or for commercial buildings. Give your officers the ability to distinguish between the lower flow rate and higher flow rate buildings.
Water Shuttle Methods
Traditionally, many departments have used the portable tank method for water supply in nearly all instances where a direct supply line from a source is not available or practical. Personnel set up a tank, a pumper drafts from the tank supplying the attack apparatus, and apparatus dump their water into the portable tank. This method requires considerable staff and several pieces of apparatus to set up and operate. When it is set up properly, flows of 1,000 gpm can usually be achieved using a single portable tank.
Some departments use the nurse tanker method for water supply if the large flow rates are not needed. In this case, the first arriving tanker sets up a supply line to the attack engine and begins pumping water to the engine. As additional tankers arrive, they pump off their tanks into the nurse tanker, which continues to maintain the supply to the attack engine. For the lower flow rates, this method can be quite effective in maintaining an uninterrupted flow of water.
Some considerations to include in specifications for rural attack engines follow:
- Gated pump intakes are needed to allow transition to drafting from a portable tank without interrupting water flow.
- Consider using smaller-diameter suction hose for portable tank drafting operations when lower flows are needed. Three-inch hose can supply 250 to 300 gpm, and four-inch hose can supply 500 to 600 gpm. Your goal should be to make it a one-person set up.
Some considerations to include in specifications for tankers follow:
- Tanker fill and dump rate should be at least 1,000 gpm for portable tank operations.
- The nurse tanker method requires that your tankers be equipped with a pump for offloading. There is an advantage to specifying a large pump, enabling the tanker to offload more quickly. A 500-gpm pump will limit your ability to maintain water flow from the nurse tanker (photos 1 and 2).
- The size of the tank-to-pump piping will limit the flow rate that can be pumped from a tanker. A minimum four-inch may be needed.
Rural water supply is not “one size fits all.” Rural areas vary considerably in their geography and makeup of structures. Each district will need to evaluate its needs to adequately protect its structures/occupancies. Smaller, well-separated structures require lower flow rates. Larger structures and those with exposures to surrounding structures require the larger flow rates. Train for the types of fires that you can expect to encounter. Training only for “the big one” may even have the inadvertent effect of causing an otherwise small fire to become the big one.
JAMES BURNHAM is the deputy chief and training officer with the Shelburne (MA) Fire District. He is a Massachusetts-registered fire protection engineer and was employed by FM Global as a senior engineering group manager. He has a bachelor’s degree in engineering and an associate degree in fire protection and safety.