WHEN BIGGER ISN’T BETTER
The use of large-diameter hose in place of traditional 2 1/2and three-inch supply lines is becoming more common in the fire service. Increased flow capacity and low friction loss make large-diameter hose an excellent choice for water-supply operations, particularly where long-distance lays or large fire Hows are required. However, large-diameter hose may not be suitable in certain fireground evolutions. The fire service needs to be aware of the limitations of large-diameter hose and operate accordingly. Proper use of large-diameter hose, and all equipment, is essential for safe and efficient fireground operations and maximizing the service life of equipment.
At a recent fire in a 10-story condominium building, the first-due engine company was supplying the fire department standpipe connection with a five-inch supply line. The l()th-floor room and contents fire was successfully extinguished without incident, but questions were raised regarding the acceptability of using large-diameter hose for this purpose. The primary concern was whether water could be delivered to the upper floors of the building at a pressure suitable for firefighting without damaging the large-diameter hose.
To address this concern, two separate issues need to be investigated: (I) the capabilities/limitations of large-diameter hose and (2) the pressures required for effective standpipe operations.
URGE-DIAMETER HOSE
NFPA (National Fire Protection Association) 1961. Standard on Fire Hose, defines large-diameter hose as “hose of 3^-inch size and larger designed to move large volumes of water.” This standard further defines large-diameter supply hose as “hose that may be used at operating pressures not to exceed 185 psi.” The service test pressure for this hose is 200 psi. Attack hose has a maximum operating pressure of 275 psi and a corresponding service test pressure of at least 300 psi.
Large-diameter hose may be supply hose or attack hose depending on its rated operating pressure and must be labeled accordingly. Based on the operating and service test pressures above, attack hose can be readily used in place of supply hose, but supply hose cannot necessarily be used in place of attack hose. This conclusion is supported by NFPA 1962, Standard for the Care, Use, and Service Testing of Fire Hose Including Couplings and Nozzles. It requires that a pressure and volume relief device be provided on the discharge side of the pump when large-diameter supply hose is being used to supply attack lines, manifolds, standpipes, and sprinkler systems. This relief device is in addition to the standard relief valve on fire department pumpers, and the maximum setting must not be greater than the service test pressure of the hose. Therefore, if largediameter supply hose is being used in accordance with this standard, it would not be possible to supply the hose at a pressure greater than 2CK) psi.
STANDPIPE OPERATIONS
Unfortunately, there is no standard practice for conducting firefighting operations in buildings equipped with standpipes. The types of nozzles and the size and configuration of handlines used for standpipe operations vary among departments, depending on preferences, operational experiences, and the standpipe design requirements of local building codes.
The design requirements for standpipe systems have been a subject of much debate, particularly the issue of pressure at the standpipe hose outlets. Prior to the 1993 edition, NFPA 14, Standard on Standpipe Systems, required a residual pressure of 65 psi at the highest standpipe outlet with 250 gpm flowing. This requirement was based on the use of a “standard fire line,” i.e. 100 feet of 2½inch hose with a smooth-bore nozzle, which requires a pressure of 50 psi at the nozzle.
The 1993 edition of NFPA 14 recognizes that many departments now use smaller hoselines with combination or fog nozzles that require nozzle pressures of 100 psi. Table A-5-7 of NFPA 14 lists the flow rate and minimum operating pressure required at the standpipe hose outlet for a number of hose and nozzle configurations. These configurations are reproduced as Cases 1 through 7 of the table on page 79, which lists flow rates and minimum operating pressures for I 1 hose and nozzle configurations.
Many other combinations are in use throughout the country. For example, some departments use a 1 ½-inch combination nozzle on 150 feet of 1 ½-inch hose connected to the standpipe outlet with a 2!4-inch gated wye. This configuration allows a second line to be placed in service without shutting down the supply to the first line. Cases 8 and 9 of the table represent these configurations.
In many situations, an additional line may be placed in service on the floor above the fire to combat vertical fire extension. Cases 10 and 11 of the table are two configurations that could be employed in this scenario.
To determine the pressure required at the base of the standpipe riser, friction losses in the standpipe and fittings and losses due to elevation must be considered. As an iliustration, the table on page 79 lists the pressure required at the base of the standpipe riser for each floor of a hypothetical 10-story building for each hose configuration. These pressures are the result of hydraulic calculations based on the following assumptions:
- The hypothetical 10-story building has a dry standpipe sized in accordance with the pipe schedule of NFPA 14 (i.e., four-inch
- diameter for the top 10 floors and six inches for all floors below).
- The standpipe hose outlets are 2M-inch hose (angle) valves located at the intermediate landing between each floor.
- The first floor slab is at an elevation of 0 feet.
- The fire department connection is at an elevation of three feet.
- The first floor hose outlet is at an elevation of five feet.
- There is a 10-foot elevation difference between each subsequent hose outlet.
The losses in the pipe and fittings, like friction loss in hose, depend on several factors, including the size of the pipe, the type of pipe, and the quantity of water flowing through it. The Hazen-Williams friction loss formula, as described in NFPA 13, was used to calculate the friction losses in the pipe and fittings. For each floor, water must flow from the fire department Siamese connection; through a check valve and 90-degree standard elbow; up the standpipe; through a four-inch x 2^-inch tee; and finally, through the 2^-inch angle valve. (Note: The calculations do not consider losses through the fire department Siamese connection.)
Fire department pump operators use various rules of thumb to determine losses due to elevation. One such rule is to add 0.5 psi for every foot of elevation. This rule relies on the operator’s estimating the correct elevation. Another rule of thumb is to allow five psi for every floor above grade level. This is similar to the first rule, assuming 10 feet of elevation for every floor. For the calculations performed to develop the table that follows, a value of 0.433 psi per foot of elevation was used, as is common practice in the design of hydraulically calculated sprinkler and standpipe systems.
EVALUATION
The yellow’-shaded areas of the table indicate hose configurations and elevations that exceed the maximum operating pressure (185 psi) of large-diameter supply hose. The red numbers indicate hose configurations and elevations where the service test pressure (200 psi) of large-diameter supply hose is exceeded. If the large-diameter supply hose is being used in accordance with NFPA 1962, it would not be possible to supply sufficient pressure for these cases, since the relief device would operate at 200 psi.
The rated operating pressure of largediameter supply hose is exceeded for eight of the I 1 hose configurations listed in the table at or below the 10th floor (Cases 3, 4, 6, 7, 8, 9, 10, and 11). Of these eig^it cases, all but two (Cases 4 and 6) exceed the design service test pressure, resulting in operation of the relief valve.
Three hose configurations (Cases 1, 2, and 5) are within acceptable pressure limits up to the 10th floor. However, considering only the additional losses due to elevation in another taller building. Cases 1 and 5 would exceed the rated operating pressure at the 13th floor. The service test pressure (and relief valve setting) would be exceeded at the 16th floor. Only the hose configuration in Case 2, a 21-inch smooth-bore nozzle w ith a tip on 150 feet of 2!^-inch hose, has a significant margin between the required operating pressures and the rated operating pressure of large-diameter supply hose. Taking into account the additional losses due to elevation (0.433 psi/ft.) and friction in the standpipe (2.8 psi/100 ft.), it would be possible to supply Case 2 up to the 18th floor without exceeding the rated operating pressure of large-diameter supply hose and up to the 26th floor before the design service test pressure is exceeded.
The pressures reflected in the table are conservative. The calculations were performed assuming new pipe and the absolute minimum number of fittings. In reality, pipes may be partially obstructed due to their gradual deterioration with age or even vandalism. If the standpipe is part of a combination sprinkler/standpipe system, the water would likely pass through a tee, rather than an elbow, at the base of the riser, which would result in additional losses. Also, the calculations do not consider losses through the fire department Siamese connection. Finally, the method used to determine losses due to elevation yields conservative results when compared with the rules of thumb previously discussed, particularly for higher elevations.
It should also be emphasized that these calculations are based on a four-inch-diametcr, dry standpipe. As with friction loss in hose, as the standpipe size increases, the friction loss decreases, resulting in lower required pressures at the base of the riser. Similarly, a wet standpipe will reduce friction losses [a “wet” pipe is assigned a higher “C” factor (roughness coefficient)], also resulting in lower required pressures. The important concept to understand is that, regardless of the size or type of standpipe, a point will be reached at which the pressure required at the base of the riser exceeds the rated operating pressure of large-diameter supply hose.
CONCLUSION
Large-diameter supply hose provides great advantages over traditional 2½and threeinch supply hose for water supply and relay operations, but it has its limitations and cannot always be used as a direct substitution for traditional supply hose. Fire departments must be aware of the rated operating pressure of their large-diameter supply hose and operate accordingly.
While large-diameter supply hose may be used to supply a standpipe at excessive pressures on one or more occasions without incident, the hose will weaken over time. Eventually, this can lead to a catastrophic failure, such as rupturing of the hose, which could injure personnel in the immediate vicinity of the hose or injure or kill an interior hose crew that loses it water supply.
RECOMMENDATIONS
If a department uses or desires to use largediameter supply hose to supply standpipe risers, an engineering analysis of the hose configuration used for standpipe operations, the type of standpipe systems, and the elevation of the highest standpipe outlet to be supplied should be conducted. This analysis should determine the maximum pressure required at the base of the worst-case standpipe riser and whether it is feasible to use large-diameter supply hose to supply that standpipe.
Fire departments that carry only largediameter supply hose have three basic options for addressing standpipe operations.
The first, and simplest, is to carry several lengths of traditional 2½or three-inch supply hose to connect the pumper to the fire department Siamese.
The second option is to use large-diameter hose rated for operating pressures up to 275 psi (large-diameter attack hose).
- The third, and probably the most involved, option is to determine the maximum pressure that can be delivered at the highest standpipe outlet and modify the
hose configuration used for standpipe operations accordingly.
In any case, to prevent personnel injury and maximize the service life of equipment,
the pressure limitations of large-diameter supply hose must not be exceeded.