During construction or fire protection impairments, the need to supply standpipes and sprinklers does not simply go away because of the impairment or because the building is under construction. The question of fire protection water supply is often dismissed with “the fire department will just pump into the system.” While this may be the case in many circumstances, it is not always possible.
The volume of water and pressures that the local fire department can provide during an incident depend on several key factors:
- How much water is required
- Where the water needs to be pumped
- Capacity of the fire apparatus
- Proximity and adequacy of the water supply (e.g. hydrants, municipal supply)
In order to determine whether the fire department can supply the required fire protection water during an event, first an understanding of how much water is required is necessary. For the majority of buildings with sprinkler and standpipe systems, the standpipe demand will drive the overall fire protection water supply needs. As such, this discussion will focus on standpipe system demands, but it is important to note that in cases where the sprinkler system (or other fire protection system) demand exceeds the flow or pressure requirements of the standpipe system, then similar considerations would need to be evaluated for those systems. That said, the number of standpipes has a direct impact on the volume of water required. A single standpipe requires 500 gpm, two require 750 gpm, and three or more standpipes require 1,000 gpm. Current code requires these flows be provided at 100 psi minimum residual pressure at the hose connection outlet.
The next question is where does the water need to go? If the building in question is only three stories tall with two standpipes (750 gpm), it is likely that the local fire department can supply the standpipes with few problems. The difficulties begin as the building’s height increases. The taller the building, the greater pressure is required to overcome gravity’s influence on the water. An additional 0.433 pounds per square inch (psi) is needed for every foot of elevation. A building that is 150 feet tall requires 65 psi just to lift the water to the top, not including the friction loss through the piping or pressure required at the outlet. The presence of pressure restricting valves (PRVs) will only increase the required pressure at the Fire Department Connection (FDC), given the additional pressure loss attributed to the PRVs.
Once the in-building demands are defined, the capacity of the local fire apparatus needs to be understood. Most engines in New England are equipped with single-stage pumps ranging from 1,000 gpm to 1,500 gpm (although two-stage pumps or pumps with ratings over 2,000 gpm are not unheard of). In our area, engines in Boston and Cambridge predominantly have single-stage, 1,250 gpm pumps, with a few notable exceptions. This capacity is measured at 150 psi discharge pressure, “at draft.” This means the pump can pull from a static water supply, like a river, pond, or tank, and add 150 psi to the 1,250 gpm as it passes through. It also means that it can have a residual pressure of 0 from a fire hydrant, and still add 150 psi (though most fire departments will maintain at least a 20 psi residual pressure when working from a hydrant).
If the pressure is increased beyond 150 psi, the pump’s volume capacity begins to drop. At 200 psi, the pump will only flow 70% of its rated capacity (875 gpm for our example 1,250 pump). At 250 psi, it will only flow 50% (625 gpm). This is the pressure added to what is being supplied to the truck, so if the connected fire hydrant has a residual pressure of 100 psi at 625 gpm, the pump will provide 350 psi (100 psi + 250 psi) at 625 gpm at the pump outlet. That said, above 250 psi, many engines will start to see relief valves operate to protect the truck’s piping systems and hose lines. There is a practical maximum pressure that is available, and it is dependent upon how each individual engine was specified and built. Whether these flows and pressures are adequate will depend on the determined in-building demand.
The last factor discussed here is the proximity and adequacy of the water supply that will be connected to the engine. How far away are the fire hydrants, and can they supply the quantities and pressures that are needed? We often see “good” hydrants in large downtown areas – hydrants that can supply 1,200 gpm or 1,400 gpm with a 70 psi residual pressure, but can the hydrant nearest the FDC supply the quantities of water that are needed? Or is it at the end of a 6-inch main in a low-pressure part of the water distribution system? A hydrant that can provide 1,000 gpm but with only 20 psi residual can be problematic for systems that require high pressures.
If the hydrants are more than 100 feet away, the fire department will have to extend the hose lines to reach the FDC, which will lower the pressure available at the FDC. What size hose does the fire department use to connect to hydrants? A 25-foot length of 6-inch hose will provide far more volume – and have less friction loss – than a 100-foot length of 4-inch hose or two 3-inch hoses.
The answers to these and similar questions will impact whether “the fire department will just pump into the system,” and be capable of providing adequate water supply during an incident. Discussions with the local fire department, and an understanding of how their fire engines operate, is crucial to providing adequate fire protection during construction or impairments. A proactive and thoughtful approach can go a long way to mitigating the potential fire protection problems that could otherwise result during an incident response.
If you have questions on standpipes, impairment planning, or construction fire safety, please reach out to firstname.lastname@example.org for additional information.