Figuring Flow Capabilities Of Large Diameter Hose
The Volunteers Corner
The decision to buy large diameter hose can be made rather easily. You already know that you wish to move water at a larger flow rate than you can now.
A decision that is more difficult—or at least more complicated—to make is the diameter hose to be purchased. To reach a rational decision on diameter, you should consider the fire flows required by various types of hazards and the distances these hazards are from water sources. You also have to consider the relationship between friction losses and hose diameters because these two factors affect the distance a pumper can supply a specific rate of flow—or what we call gpm.
Desired fire flows: A logical way to approach the problem is to develop a list of desired fire flows for various types of areas and for target hazards. You may well find that a major portion of your residential area can be handled with a fire flow of 500 gpm while a smaller section might require a 1000-gpm fire flow. A light industry area might call for a fire flow of up to 2000 gpm while a heavy industry sector could demand a fire flow of 6000 to 8000 gpm. You might have a target hazard—an industrial complex, a petrochemical plant, or a plastics plant—that requires a fire flow of from 8000 gpm to “we don’t have enough water for a major fire there.”
By looking at your fire records and your pre-fire plans, you can work up a list of fire flows for the various types of areas in your municipality.
The following table shows what rates of flow will cost in terms of friction loss with different hose sizes:
Determine distances: Next you have to determine how far you would have to lay large diameter hose to supply water to a fireground pumper in various sectors of your community and to specific target hazards—which could be a factory, a cluster of farm buildings or an 18-room mansion. You don’t have to measure the distance from the water source to every building. For example, if you block out a area of about a square mile, you can spot logical locations where a fireground pumper could cover a small area with hand lines. By eliminating areas where there are no buildings, you will have a reduced number of pumper locations and large diameter hose I lays to measure.
From these hose lay distances, you can determine the average, the median, and the longest stretches necessary. Then consider the fire flows required in these areas. If the average stretch is 1000 feet with a maximum desired fire flow of 750 gpm, then a look at the tables shows that 4-inch hose can provide a sufficient rate of flow for that distance. You wouldn’t need to buy 5-inch hose to handle the problem.
Another thing to consider when the distances are longer and the fire flow requirements greater is the use of a pumper relay with large diameter hose. The two tables below show the maximum lengths of single lines that will provide the indicated rates of flow. Back pressure, of course, would affect the maximum line length.
Fire Hose Flow Capabilities
(Number of feet water can be relayed through large diameter hose. Allowance made for 20 psi residual at pumper receiving flow.)
Evaluating needs: The 150-psi engine pressure table is recommended for pumper relays. When the source pumper and those in the line all pump at 150 psi, an effective relay can be established in minimum time and without confusion about engine pressures.
In making a decision on the size hose to buy, be realistic about your needs. Frequently, you will find that the solution for 90 percent of your problems results in the most efficient decision in terms of fireground operations, maintenance and cost effectiveness. You may eventually find that the solution for the other 10 percent of your problems puts an excessive burden on your operations and budget.
For pumper relays that are too long for your department to handle, you will have to use mutual aid, and this should be considered in selecting the hose size for your department. Helping each other could be the dominant factor in the hose diameter decision made in several neighboring departments.
