From my Tumblr Account: battalionbean
I recently was working on a new General Order and the title question was raised and sparked a lot of dinner table conversation. I found a great article about FDC Operations by my good friends at Fire Engineering.http://www.fireengineering.com/articles/print/volume-164/issue-3/features/fire-department-connections-start-to-finish.html.
I have cut and pasted the relevant part. I did remove the pictures as they would copy correctly. Enjoy the discussion and answer.
Chief Bean
In recent years, the fire service has seen the introduction of sexless or quick-connect type fittings on system FDCs typically with four- or five-inch hose. The local fire authority would make the selection, since compatibility with hose fittings is required. There are, however, some things that need to be considered when using these fittings and large-diameter hose (LDH) for supplying system FDCs. These considerations should be part of any discussion on changing over to LDH connections for existing systems or using them for new system installations.
LDH was introduced to the fire service as a means to provide large volumes of water over long distances with reduced friction loss compared with traditional 2½- or three-inch supply lines. This advantage of large volume and reduced friction loss did not contemplate appreciable pressure loss caused by elevation, such as in standpipes for high-rise buildings. LDH supplies volume to pumper apparatus, which then produce the necessary operating pressures for firefighting operations. Fire sprinkler and standpipe systems rely on specific pressures at certain volumes to achieve effective sprinkler coverage or hose stream reach and penetration. Successful usage of LDH to move large volumes of water is also dependent on the pumping apparatus’ design and piping. The installation of an adapter to convert an existing 2½-inch discharge into a five-inch discharge creates a chokepoint at the pump discharge where water velocity will be greatly increased and result in additional friction loss as flow volume increases.
You must also consider the hose. Since LDH is generally classified as supply hose, its service pressure is 200 psi, whereas three-inch hose is available with service pressure listings from 300 to 600 psi. When looking at the requirements for properly supplying standpipe systems in vertical buildings, particularly multizone systems, you will encounter minimum pressures well beyond 200 psi. Although the friction loss in the 100 feet of hose between the pumper apparatus and the FDC will be less in the LDH, do not overlook the fact that pumping 175 psi through the LDH and supplying upward of 1,000 gpm is wholly inadequate for the companies operating off the standpipe system if it will take 250 psi and 750 gpm to actually reach them and support their hose streams. The use of three or more three-inch hoselines into the FDC will easily deliver beyond that 750 gpm; more importantly, it will deliver it with the necessary pressure to overcome the elevation loss (gravity) in reaching those crews. Large-demand sprinkler systems or standpipes that require flows of more than 750 gpm are still better served by multiple (three or more) hoselines through a three- or four-way FDC.
When you consider the needs to properly supply a multizone building (discussed below), very few pumping apparatus are equipped to properly support two or more LDH discharges. The most basic engine apparatus will have at least six discharges for 2½-inch hose couplings, which can feed three lines into each FDC on two different zones. With this popularization of LDH, many agencies began to remove three-inch hose from their apparatus and use LDH for their supply lines. This is not a question of using LDH as a supply between hydrants and apparatus; it’s about recognizing that while you may be “supplying” the FDC, you are actually on the discharge side of the pump and you should be using “attack”-rated hose to connect to the FDC. Although it is not necessary in these cases to carry 1,000 feet of three-inch hose, consider your response area and its building makeup. Fire departments still must maintain some complement of three-inch hose on their attack apparatus to properly provide for the vital function of supporting an FDC.
At the height of a firefight in a standpiped building, the interior crews are relying on the water supply through the FDC for their protection and safety. In a building equipped with a single large-diameter FDC, there is one and only one hoseline between the fire pumping apparatus and the building. If the hoseline is damaged by falling debris, driven over by an errant vehicle, or ruptured by pressurization, that singular hoseline becomes lost and instantly endangers the interior crews. Several minutes would be needed to stretch and reconnect a new hoseline while interior crews scramble for cover and safety in deteriorating conditions. When multiple hoselines, even two, are in place and are supporting the FDC, it provides a safety factor if one of the hoselines is lost or damaged. Interior crews should use the water remaining in the additional FDC lines to retreat to a protected stairwell with hoseline protection.
The codes have traditionally required that FDCs be positioned between 18 and 42 inches above the adjacent grade level. The lower measurement takes into consideration accessibility during snowfalls and snow drifting; the upper measurement considers ease of reach and connection but also the weight of water-filled hoselines pulling at their fittings and kinking. The impact of water weight and pull on couplings is dramatically compounded in LDH, as a single foot of five-inch hose weighs approximately 34½ pounds when filled with water. The higher an FDC is mounted, the more hose length that must be supported by the fitting before the hose makes ground contact—at 42 inches high, about 121 pounds of water would be hanging on the connection (photo 11).
This also increases kinking of the hoseline, which in turn reduces and may eliminate the hydraulic efficiency of a LDH. A charged three-inch line has enough rigidity so that it would be relatively self-supporting at even 42 inches above-grade, but a charged five-inch hoseline has no self-supporting characteristics and will immediately create a kink and pull at the coupling-to-FDC. Some agencies have been stipulating the installation of a pipe elbow to alleviate this condition (photo 12), but we must still remember that height is working to our disadvantage here and the codes have not contemplated the use of LDH for FDCs. A similar effect can occur on FDCs that are low to the ground (photo 13). When you include the size of the hose coupling, you force a kink to occur under the connection that, once again, robs hydraulic throughput and defeats any advantage of LDH.
Early LDH couplings lacked a physical means for retaining their connection, and they were just friction twisted together with as little as a ¼- to ½-turn action. Today, we have large-diameter connections with a ¼-turn twist action and a safety latch to physically hold them connected. There is no question about the value of or need for this safety latch, and we can regularly inspect and maintain our hose. When using these connections on an FDC, we have much less control over the care and maintenance of this safety latch; if it becomes broken or damaged, we have no reliable means to restore it in an emergency. This increases the possibility that the hose connection will fail or separate under pressure and deprive interior crews of water. When using traditional hose sizes, we rely on hose threads that make up their connection over several turns vs. a fractional turn. If a threaded hose connection is found damaged, responding personnel have several options for making a rapid field repair and achieving a timely hose connection to allow for support through the FDC. A failed or damaged large-diameter connection effectively renders it useless for the duration of the fire emergency and must be repaired by a service contractor.
